REVIEW

Oxygen-deficient perovskites for oxygen evolution reactionin alkaline media: a review

Ahmed Badreldin1 & Aya E. Abusrafa1 & Ahmed Abdel-Wahab1

Received: 10 August 2020 /Accepted: 3 September 2020# The Author(s) 2020

AbstractOxygen vacancies in complex metal oxides and specifically in perovskites are demonstrated to significantly enhance theirelectrocatalytic activities due to facilitating a degree of control in the material’s intrinsic properties. The reported enhancementin intrinsic OER activity of oxygen-deficient perovskites surfaces has inspired their fabrication via a myriad of schemes. Oxygenvacancies in perovskites are amongst the most favorable anionic or Schottky defects to be induced due to their low formationenergies. This review discusses recent efforts for inducing oxygen vacancies in a multitude of perovskites, including facile andenvironmentally benign synthesis strategies, characterization techniques, and detailed insight into the intrinsic mechanisticmodulation of perovskite electrocatalysts. Experimental, analytical, and computational techniques dedicated to the understandingof the improvement of OER activities upon oxygen vacancy induction are summarized in this work. The identification andutilization of intrinsic activity descriptors for the modulation of configurational structure, improvement in bulk charge transport,and favorable inflection of the electronic structure are also discussed. It is our foresight that the approaches, challenges, andprospects discussed herein will aid researchers in rationally designing highly active and stable perovskites that can outperformnoble metal-based OER electrocatalysts.

1 Introduction

The global energy crisis associated with the increasing de-mand for energy as well as the rapid depletion of fossil fuelsand carbon dioxide (CO2) emissions associated with their useis one of the grand challenges facing humanity today. In fact,the International Energy Agency reported that the global en-ergy demand is expected to rise to 26 TW by 2040 comparedwith 18 TW in 2013, corresponding to 44 Gt/year of CO2emissions [1]. This has motivated the global community tomaximize deployment of renewable energy sources in orderto meet the ever-growing energy demand in a sustainablemanner [2, 3]. Hydrogen (H2) being a carbon-free energycarrier is an ideal sustainable energy source that can potential-ly replace non-renewable carbon-based fossil fuels due to itshigh specific energy [4–6]. Currently, the primary technolo-gies that are used to produce hydrogen include partial oxida-tion, steam reforming of methane, and coal gasification.

However, these technologies involve the generation of signif-icant amounts of greenhouse gasses, which is the main causefor global warming [7, 8].

Amongst various progressive technologies, electrochemi-cal water splitting driven by electricity produced from renew-able energy sources, such as solar or wind energy, hasemerged as an effective approach to sustainably producehigh-purity hydrogen with zero carbon emissions [1, 9].Photochemical and photoelectrochemical water splitting ma-terials have also seen an afresh interest in research over thepast decade. However, they face limitations due to reactor-design predicaments [10, 11]. Water splitting is composed oftwo half-reactions, including the hydrogen evolution reaction(HER), at the cathode (2H+ (aq) + 2e− → H2(g) acidic,2H2O + 2e

−→H2 + 2OH− alkaline) and the oxygen evolution

reaction (OER) at the anode (H2O(l)→½ O2(g) + 2H+ (aq) +

2e− acidic, 2OH−→½ O2 +H2O + 2e− alkaline) [7]. Under

standard conditions, water-splitting electrolyzers require apredicted thermodynamic potential of 1.23 V to drive theHER and OER half-reactions in alkaline and acidic media[12]. However, water splitting is kinetically an uphill reactionand entails sluggish reaction kinetics especially for the OER.This inflicts a considerable overpotential requirement (the dif-ference between the thermodynamic potential and

* Ahmed Abdel-Wahabahmed.wahab@qatar.tamu.edu

1 Chemical Engineering Program, Texas A&M University at Qatar,P.O. Box 23874, Doha, Qatar

https://doi.org/10.1007/s42247-020-00123-z

/ Published online: 21 September 2020

Emergent Materials (2020) 3:567–590

http://crossmark.crossref.org/dialog/?doi=10.1007/s42247-020-00123-z&domain=pdfhttp://orcid.org/0000-0003-4413-1427mailto:ahmed.wahab@qatar.tamu.edu

experimentally measured potential) to overcome the kineticenergy barriers and generate appreciable current densities[2]. Practically, the applied potentials required to drive watersplitting processes are around 1.8–2.2 V, which consumesapproximately 50% excess potential than the thermodynamicequilibrium value. The latter results in low energy conversionefficiencies which hampered the widespread application ofwater-splitting technology.

Achieving high electrocatalytic activities hence improvedconversion efficiencies inwater-splitting electrolyzers that canbe realized by determining the origin of overpotential throughfundamentally understanding the correlation between the elec-trocatalytic activity and the water-splitting reactions mecha-nism. The OER is the limiting reaction in water splitting andrequires high overpotential because it involves 4-electrontransfer steps [13, 14]. An additional kinetic energy barrier isinvolved with every reaction step where the slowest step isregarded as the rate-determining step (RDS), and it can im-pose a significant increase in the overpotential required todrive the overall water-splitting reaction.

Alkaline OER proceeds via the oxidation of an adsorbedhydroxyl anion on the metal active site (M) to form an M-OHintermediate (Eq. (1)). Then, protons and electrons are re-moved from M-OH to form M-O (Eq. (2)). Subsequently,oxygen formation follows two possible pathways. In oneroute, M-O reacts with a hydroxyl anion to form hydroperox-ide M-OOH followed by another proton-coupled electrontransfer process to produce molecular O2 (Eqs. (3) and (4))[15–18]. In another route, a direct combination of two M-Ointermediates produces O2 (Eq. (5)) [19].

OH−→M−OHþ e− ð1ÞM−OHþ OH−→M−Oþ H2Oþ e− ð2ÞM−Oþ OH−→M−OOHþ e− ð3ÞM−OOHþ OH−→O2 þ H2Oþ e− ð4ÞM−OþM−O→O2 ð5Þ

In practice, efficient and cost-effective electrolytic watersplitting requires highly active and stable electrocatalysts thatcan minimize the required energy input (overpotential) [20].Current state-of-the-art water-splitting technologies utilize no-ble metal-based electrocatalysts, including platinum (Pt) [21]and ruthenium dioxide (RuO2)/iridium dioxide (IrO2), whichshow remarkable activities towards HER and OER, respec-tively, at low overpotentials [22]. However, the large-scalecommercial use of noble-based electrocatalysts has been re-stricted due to their prohibitive high cost, scarcity, and long-term instability [21, 23]. Therefore, significant research effortshave been directed towards developing robust, highly active,cost-effective, and stable electrocatalysts based on earth-abundant elements for water splitting (OER and HER).

Perovskite oxides, a class of transition metal oxides, haveemerged as promising electrocatalysts for alkaline OER due totheir unique structural, electronic, ionic, and electrocatalyticcharacteristics [24–26]. The general chemical formula for pe-rovskite oxides is ABO3, where A is typically occupied bycation with 12-fold coordination and B is occupied by a cationwith 6-fold coordination with oxygen anions [27–32]. Otherperovskite oxides with more complex structures include lay-ered perovskites (An + 1BnO3n+ 1), A-site ordered double pe-rovskite oxides (AA′B2O6), and B-site ordered double perov-skite oxides (A2BB′O6) [33]. Compared with other electrocat-alytic materials, perovskite oxides are inexpensive becausethey are based on earth-abundant elements, easy to synthesize,and environmentally benign [27, 33, 34]. Moreover, depend-ing on the type of cations occupying the A and B lattice sites, aplethora of possible perovskite oxides with various stoichiom-etries can be generated withmodulable crystal structures, elec-trocatalytic activities, electronic, and ionic properties [35].Yet, despite their promise, perovskite oxides still require fur-ther enhancements to outperform noble-based metal catalystsdue to their relatively low electronic conductivity.

Several experimental and theoretical studies demonstratedthat the catalytic activity of perovskite oxides can be intrinsi-cally modulated through the inductive alteration of oxygennonstoichiometry to generate oxygen-deficient structures fora myriad of applications [35–39]. Table 1 summarizes variouselectrocatalysts made of oxygen-defected perovskite oxidesthat were reported in the literature for OER. Oxygen defectscan bring about substantial electrocatalytic performance im-provements by (1) tuning the electronic structure and chargetransfer, (2) promoting the exsolution of secondary active sur-face phases, and/or (3) providing moderate adsorption/desorption behavior of reaction intermediates during watersplitting. When the A and B positions are replaced, perovskiteoxides maintain their structural stability through undergoingcrystal lattice deformations caused by A-site and B-site ionicradii mismatch along a specific direction to construct crystal-lographic structures (e.g., tetragonal, orthogonal, or trigonal).Thus, oxygen vacancies can be generated on perovskite ox-ides to a certain extent without rendering a collapsed structure[33].

Reportedly, oxygen vacancies promote the eg-filling status ofthe B-cation to alter the electron configuration of themetal cationwith a high-spin–state electron occupying the eg orbital [40].Compared with the π-bonding t2g orbital, the σ-bonding eg or-bital strongly overlaps with the oxygen-containing adsorbates,which facilitates charge transfer between the transition metalcation and the reaction adsorbates (M-OH, M-O, M-OOH,H2O), hence accelerating the water splitting kinetics [62].Generally, optimal OER performance could be obtained by pe-rovskite oxides with an eg occupancy close to unity (~1.00) [1].Besides eg filling, oxygen defects can upshift the oxygen p-bandcenter relative to the Fermi level to promote hybridization and

568 emergent mater. (2020) 3:567–590

Table1

Summaryof

major

oxygen-deficient

perovskiteoxides

forOERelectrocatalysis

Synthesismethod

Material

Overpotentialb

efore

treatm

ent

Overpotentialafter

treatm

ent

Tafelslopebefore

treatm

ent

Tafelslopeafter

treatm

ent

Media

Ref.

Hydrogenreduction

NdB

aMn 2O5.5

500mVat5mA

cm−2

390mVat10

mA

cm−2

122mV

dec−

175

mV

dec−

1Alkaline

1M

KOH

[1]

Reductio

nReduced

La 0

.8Sr

0.2CoO

3−δ

470mVat

6.03

mA

cm−2

470mVat

39.94mA

cm−2

105.5mV

dec−

176.6

mV

dec−

1Alkaline

0.1M

KOH

[37]

Therm

alannealing

PrBaC

o 2O5.75PrBaC

o 2O5.5

–360mV(PrBaC

o 2O5.75)

420mV(PrBaC

o 2O5.5)

at10

mA

cm−2

–70

mV

dec−

1

(PrBaC

o 2O5.75)80

mV

dec−

1(PrBaC

o 2O5.5)

at10

mA

cm−2

Alkaline

1M

KOH

[38]

Hydrogenreduction

Ca 2Mn 2O5

–370mVat1mA

cm−2

197mV

dec−

1149mV

dec−

1Alkaline

0.1M

KOH

[40]

Hydrothermaltreatm

ent

Sr-dopedLa 1

−xSr

xFeO

3−δ

510mVat10

mA

cm−2

370mVat10

mA

cm−2

76.76mV

dec−

160.1

mV

dec−

1Alkaline

0.1M

KOH

[41]

Therm

alannealing

SrCoO

2.7

400mVat4.3mA

cm−2

400mVat28.4

mA

cm−2

58mV

dec−

131

mV

dec−

1Alkaline

1M

KOH

[42]

Therm

alannealing

Sr-doped

GdB

a 1−xSr xCo 2O6−δ

536mVat0.5mA

cm−2

420mVat0.5mA

cm−2

100mV

dec−

160

mV

dec−

10.1KOH

[43]

Hydrogenreduction

Reduced

Sr 2Fe

1.3Ni 0.2Mo 0

.5O6−δ

360mVat10

mA

cm−2

404mVat10

mA

cm−2

70mV

dec−

159

mV

dec−

1Alkaline

0.1M

KOH

[44]

Hydrogenreduction

Reduced

CaM

n 0.5Nb 0

.25O3−δ

300mVat0.5mA

cm−2

280mVat0.5mA

cm−2

208mV

dec−

198

mV

dec−

1Alkaline

0.1M

KOH

[45]

Hydrogentreatm

ent

Reduced

Pr 0.5Ba 0

.5MnO

3−δ

500mVat5mA

cm−2

340mVat5mA

cm−2

––

Alkaline

0.1M

KOH

[46]

Com

bustion,ballmilling,

calcination

NdB

a 0.75Ca 0

.25Co 1

.5Fe 0

.5O5+δ352mVat10

mA

cm−2

338mVat10

mA

cm−2

108mV

dec−

181

mV

dec−

1Alkaline

0.1M

KOH

[47]

Ballm

illing,calcination

Si-SrCoO

3−δ

488mVat10

mA

cm−2

417mVat10

mA

cm−2

76mV

dec−

166

mV

dec−

1Alkaline

0.1M

KOH

[48]

Therm

alannealing

BaT

iO3−x

–370mVat1.48

mA

cm−2

––

Alkaline

0.1M

NaO

H[49]

Therm

alannealing

La 0

.95FeO

3−δ

510mVat10

mA

cm−2

410mVat10

mA

cm−2

77mV

dec−

148

mV

dec−

1Alkaline

0.1M

KOH

[50]

Hydrogenreduction

Reduced

PrBa 0

.5Sr

0.5Co 1

.5Fe 0

.5O5+δ

440mVat10

mA

cm−2

390mVat10

mA

cm−2

92mV

dec−

172

mV

dec−

1Alkaline

1M

KOH

[51]

Therm

alannealing

La 0

.58Sr 0.4Fe 0

.8Co 0

.2O3+CB

–480mVat0.3mA

cm−2

–64

mV

dec−

1Alkaline

0.1M

KOH

[52]

Therm

alannealing,

sulfurization

S-dopedCaM

nO3

620mVat10

mA

cm−2

470mVat10

mA

cm−2

61mV

dec−

152

mV

dec−

1Alkaline

0.1M

KOH

[53]

Plasm

atreatm

ent

La 0

.7Sr 0.3CoO

3−δ

420mVat10

mA

cm−2

326mVat10

mA

cm−2

92.1

mV

dec−

170.8

mV

dec−

1Alkaline

1M

KOH

[54]

Hydrothermalsynthesis

La 0

.9CoO

3-δ

427.6mVat

10mA

cm−2

380mVat10

mA

cm−2

91.3

mV

dec−

182.5

mV

dec−

1Alkaline

0.1M

KOH

[55]

Ballm

illing,thermal

annealing

Nd 1

.5Ba 1

.5CoF

eMnO

9−δ

–395mVat10

mA

cm−2

–81

mV

dec−

1Alkaline

0.1M

KOH

[56]

569emergent mater. (2020) 3:567–590

high covalency of the O p orbitals and transition metal d orbitals[1, 63]. The O p-band should be neither too close nor too farrelative to the Fermi level [64]. Perovskites with an O p-bandvery near the Fermi level undergo surface degradation andleaching of the A-site elements and tend to become amorphouson the surface under OER conditions, thus providing poor dura-bility. On the other hand, when the O p-band is too far from theFermi level, the perovskites remain crystalline and provideprolonged stability however at very low current densities. At highcurrent densities, the B-site cations, typically recognized as OERactive sites, are leached [64–66]. The O p-band position can beoptimized via tuning the concentration of oxygen defects on theperovskite surface. The aforementioned configuration is general-ly reported to substantially increase the intrinsic OER activity bylowering the charge-transfer gap between the reaction adsorbatesand the metal active site, contributing to higher conductivity [1,64, 67]. Furthermore, in some cases, oxygen-deficient perovskiteoxides tend to have higher structural distortion compared withtheir pristine counterparts. Specifically, it was reported that lay-ered perovskite oxide NdBaMn2O5.5 (prepared via reductive an-nealing under hydrogen) exhibited a much higher distorted struc-ture compared with pristine Nd0.5Ba0.5MnO3-δ due to the Jahn-Teller effect and the ordering of the electronic orbitals [1]. Asimilar finding was reported on an oxygen-deficient Ca2Mn2O5perovskite (treated via hydrogen gas) where the Jahn-Teller ef-fect resulted in a more distorted structure and larger cell volumefor this oxygen defected perovskite structure than of the simpletwo unit stacked cells of CaMnO3 [40]. This distortion arisingupon removing lattice oxygen atoms from the aforementionedperovskite structures promotes the adsorption of key OER reac-tion recipes, H2O and OH

−, into the oxygen vacant sites [1, 33,40].

To this end, extensive efforts for inducing oxygen defects onperovskite oxides as a new class of highly active catalysts foralkaline electrochemical water splitting have been extensivelyreported in the literature. In this review, synthesis strategies re-ported to generate oxygen vacancies in a myriad of perovskiteoxides and their effect on surface morphology, crystal structure,surface chemistry, and hence electrocatalytic OER activity, aswell as commonly used characterization techniques to detectoxygen vacancies, are discussed. A detailed insight is providedon the role of oxygen vacancies on modulating the electronicstructure, adsorption/dissociation energies of reaction intermedi-ates, and their consequent effect on OER activities. Finally, fu-ture suggestions are discussed to propose new directions that canaid researchers towards the rational development of novel highperformance and stable materials for OER.

2 Synthesis methods

As previously discussed, oxygen defects can effectively en-hance the electrocatalytic OER activity of perovskite oxidesT

able1

(contin

ued)

Synthesismethod

Material

Overpotentialb

efore

treatm

ent

Overpotentialafter

treatm

ent

Tafelslopebefore

treatm

ent

Tafelslopeafter

treatm

ent

Media

Ref.

Ballm

illing,thermal

annealing

NdB

a 0.5Sr 0.5Co 1

.5Fe 0

.5O5+δ

–374mVat10

mA

cm−2

–84

mV

dec−

1Alkaline

0.1M

KOH

[56]

Therm

altreatm

ent

Hybrid

Ba 0

.35Sr

0.65Co 0

.8Fe 0

.2O3−δ

–260mVat10

mA

cm−2

––

Alkaline

0.1M

KOH

[57]

Therm

altreatm

ent

Sr 2Fe 2O6−δ

–420mVat10

mA

cm−2

–60

mV

dec−

1Alkaline

0.1M

KOH

[58]

Therm

altreatm

ent

LaM

n 0.75Co 0

.25O3−δ

630mVat1.5mA

cm−2

445mVat10

mA

cm−2

382mV

dec−

197

mV

dec−

1Alkaline

0.1M

KOH

[59]

Therm

altreatm

ent

LaN

i 0.85Mg 0

.15O3

550mVat10

mA

cm−2

450mVat10

mA

cm−2

165mV

dec−

1105mV

dec−

1Alkaline

0.1M

KOH

[60]

Therm

altreatm

ent

BaC

o 0.7Fe 0

.2Sn 0

.1O2.8

480mVat10

mA

cm−2

403mVat10

mA

cm−2

79mV

dec−

169

mV

dec−

1Alkaline

0.1M

KOH

[61]

570 emergent mater. (2020) 3:567–590

through modulating the surface morphology, electronic struc-ture, and thus their electrocatalytic properties. Variousmethods have been reported in the literature to preferentiallytune the degree of oxygen site defects in the perovskite latticevia in situ or post-synthesis treatments. This section will pro-vide a brief summary of synthesis approaches that have beencommonly adopted to induce oxygen vacancies in perovskiteoxides. These methods mainly involve gaseous hydrogen re-duction, thermal annealing, and elemental doping, whilst verylittle work has also been reported using other methods, such aplasma treatment, solid-state reduction, and wet-chemical re-duction. The degree of which the synthesis method and theprocess parameters affect the surface morphology, crystallin-ity, and electrocatalytic OER performance was highlighted.

2.1 Reduction

Many studies report the incorporation of oxygen vacancies inperovskite oxides by the facile but very effective hydrogenreduction method due to the high reproducibility of the hydro-gen gas [68, 69]. This method can generate oxygen defectseither at mild or elevated temperatures [70]. Typically, under asuitable reduction atmosphere, namely H2, due to the highreducibility of the H2 gas, the B-site metal cations of the pe-rovskite oxide are exsolved to form metallic nanoparticlesdistributed on the surface of the as-prepared pristine perov-skite oxides. Consequently, the reduced perovskite oxide willexhibit an enhanced electronic conductivity due to the highconductivity of the metallic particles, which in turn enhancesthe OER kinetics [44]. Moreover, since oxygen defects can begenerated at relatively mild/low-temperature conditions, thisavoids sintering or coalescence of the perovskite oxide duringthe treatment process. This also provides easier control of thereaction kinetics during the synthesis process [40]. However,under excessive reduction of annealing conditions, aggrega-tion of the perovskite oxide nanostructures could occur, lead-ing to a collapsed structure and degraded OER activity [51].

In a representative work by Kim et al., a single crystal-phase oxygen-defected Ca2Mn2O5 perovskite was synthe-sized using a low-temperature reduction approach.Ca2Mn2O5 was prepared through annealing pristineCaMnO3 (prepared via a sol−gel process) under 5% H2/Aratmosphere at 350 °C for 3 h [40]. Due to the mild reductionconditions used, no sintering or agglomeration was observedpost-annealing. Moreover, Mn4+ ions in CaMnO3 were re-duced to Mn3+ which could easily bond with OH−.Reduction annealing has also resulted in crystal structuraltransformations, where a single crystal phase orthorhombicCa2Mn2O5 coordinated with square pyramid subunits be-tween oxygen and manganese was formed, compared withCaMnO3, which had six-coordinated octahedral subunits con-nected by six oxygen atoms at the corner sites (Fig. 1a). Theunit cell structure of the oxygen-deficient Ca2Mn2O5 resulted

in the formulation of a molecular level porosity at its internalstructure on the oxygen-vacancy sites beyond its apparentstructural porosity, which facilitated the adsorption of OH−.The aforementioned modifications ultimately contributed to-wards enhancing the OER kinetics. OER activity measure-ments indicated that the onset potential in an alkaline0.1M KOH solution for defective Ca2Mn2O5 perovskite was1.5 V (vs RHE), compared with its pristine counterpartCaMnO3 (1.6 V).

Moreover, an oxygen-deficient Sr2Fe1.3Ni0.2Mo0.5O6 − δdouble perovskite was reported by Zhu et al. with enhancedOER activity, delivering a current density of 10 mA cm−2 at360mV compared with its pristine counterpart (404mV) in analkaline 0.1M KOH solution [44]. The defected perovskitewas synthesized via annealing pristine Sr2Fe1.3Ni0.2Mo0.5O6− δ under different atmospheres (Ar, 5% H2/Ar and H2) at700 °C for 3 h. The optimum reduction conditions, showingthe highest OER activity, were under 5% H2/Ar as shown inFig. 2b. During the annealing process, the metal cations werereduced resulting in mixed-valence states (Mo+5, Mo+6, Ni+3,Ni+2, Fe+2, Fe+3, and Fe+4). Moreover, the presence of metal-lic Fe0 and Ni0 was detected which improved the electronicconductivity of the catalyst due to the high conductivity of themetallic particles. Post-reduction, the as-synthesized pristineSr2Fe1.3Ni0.2Mo0.5O6 − δ perovskite tetrahedral lattice struc-ture expanded as a consequence of the partial reduction ofB-site cations. Furthermore, due to the high reducibility ofthe hydrogen gas, phase decomposition of the parent pristineperovskite occurred resulting in the exsolution of new Fe-Nia l l o y and S r 4F eMoO8 pha s e s on t h e r e du c edSr2Fe1.3Ni0.2Mo0.5O6 − δ. The surface area of the defected pe-rovskite increased from 2.6 to 4.0 m2 g−1 through formulatinga porous surface with more catalytic active sites as presentedin Fig. 2c and d. As reported, the enhanced OER activityenhancement in reduced Sr2Fe1.3Ni0.2Mo0.5O6 − δ is directlyrelated to the aforementioned modifications related to the for-mation of new active phases and more exposed active sites.

Besides gaseous hydrogen reduction, oxygen defects werealso created via solid-state reduction of (Sr1 − xBax)FeO2 pe-rovskite using solid reagents that can thermally release hydro-gen (CaH2, NaH) and wet-chemical reduction with H2O2aqueous solution on La0.8Sr0.2CoO3 − δ (LSC) [37].

2.2 Doping

The substitution of A and/or B site atoms using low valencydopants (e.g., La, Sr, Mg, Ba, Ca) is an effective approach tocreating oxygen vacancies. Upon removing a cation atomfrom the perovskite crystal structure, a negative charge is cre-ated resulting in an unbalanced atmosphere. To maintaincharge neutrality, an oxygen vacancy is formed by releasingan oxygen atom from the perovskite lattice [33]. The concen-tration of oxygen vacancies is highly dependent on the degree

571emergent mater. (2020) 3:567–590

of substitution and the type of elemental dopant used, whichdirectly affects the formation energy of the oxygen vacancies.Typically, low formation energy results in a higher degree ofoxygen defects. Sr and Ca are the most favorable and com-monly used alkaline-earth metals to create oxygen vacanciesin ABO3-type perovskite oxides [3, 70].

She et al. investigated the effect of dopant content on theOER electrocatalytic performance through synthesizing a se-ries of strontium (Sr)-doped La-based perovskite oxide cata-lysts with different compositions of La1 − xSrxFeO3 − δ (x = 0,0.2, 0.5, 0.8, and 1.0). Sr-doped samples were prepared byLSF-x solid precursors (La(NO3)3·6H2O, Sr(NO3)2Fe(NO3)3·9H2O) in a solution at 800 °C for 5 h (hydrothermalsynthesis) [41]. The concentration of oxygen vacancies onLa1 − xSrxFeO3 − δ increased as the concertation of the of Sr

+2

dopant increased, where the highest (O22−/O−) ratio was at x =

0.8. The high O22−/O− ratio indicates the formation of highly

oxidative oxygen species and is a direct measure of surfaceoxygen vacancies. Moreover, Fe with different oxidationstates (Fe4+, Fe3+, Fe2+) was detected on the surface and in-creased with increasing Sr+2 content. Structural analysis re-vealed that the substitution of La3+ by Sr2+ promoted the lat-tice structure to change from an orthorhombic structure to a

cubic structure. However, in the case where the Sr+2 dopantfraction was 1.0, a tetragonal lattice was adopted instead of acubic lattice. Amongst the five samples, La0.2Sr0.8FeO3 − δ(LSF-0.8) exhibited the highest activity towards OER in a0.1M KOH alkaline media, where the overpotential at10 mA cm−2 current density was 370 mV compared withundoped La FeO3 − δ (510 mV) (Fig. 2a). The cubic structureand the presence of abundant oxygen vacancies both contrib-uted towards enhancing the OER performance [41].

Kim et al. investigated the potential for improving the OERelectrocatalytic activity of NdBa0.75Ca0.25Co2O5 + d (NBCC)double perovskite oxide by doping multiple transition-metaloxides (Fe+2, Ni+2, Cu+2, and Mn+2) into the B-sites of theperovskite to generate oxygen vacancies. The catalysts weresynthesized through combusting solid precursors followed bymilling with zirconia balls in ethanol for 24 h, and then calci-nation for 4 h at 600 °C, and recalcination for additional 4 h at900 °C [47] . Amongs t t he p repa red ca t a ly s t s ,NdBa0.75Ca0.25Co1.5Fe0.5O5 + d (NBCCFe) showed thehighest electrocatalytic OER activity with onset potential of1.568 V at 10 mA cm−2 current density relative to 1.582,1.583, 1.591, and 1.599 V for NdBa0.75Ca0.25Co2O5 + d, Mn-doped NdBa0.75Ca0.25Co2O5 + d (NBCCMn), Ni-doped

Fig. 1 (a) Unit cell structures of CaMnO3 (left) and Ca2Mn2O5 (right).Reproduced fromRef. [40] with permission from the American ChemicalSociety, Copyright 2014. (b) Performance of SFNM samples treated

under different atmospheres. SEM images of (c) SFNM and (d) SFNM-5%H2/Ar at 700 °C. Reproduced fromRef. [44] with permission from theRoyal Society of Chemistry, Copyright 2017

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NdBa0.75Ca0.25Co2O5 + d (NBCCNi), and Cu-dopedNdBa0.75Ca0.25Co2O5 + d (NBCCCu), respectively. DopedNBCCFE, NBCCMn, and NBCCNi were crystallized withan orthorhombic structure that was very similar to NBCC.On the other hand, Cu doping resulted in the transformationof the orthorhombic NdBa0.75Ca0.25Co2O5 + d structure to atetrahedral structure due to the large difference between theionic radii and the electronic structure of Cu and Co.Based on morphological analysis, the agglomeratedNdBa0.75Ca0.25Co2O5 + d morphology was precluded upondoping as shown in the SEM images in Fig. 2b–f. Moreover,the metallic substitutions resulted in the generation of porousstructures where NBCCFe exhibited the highest exposed sur-face area (2.8 m2 g−1) compared with NBCC (1.9 m2 g−1),NBCCCu (1.8 m2 g−1), NBCCNi (1.9 m2 g−1), andNBCCMn (2.2 m2 g−1).

In contrast to thermal annealing/calcination, doping via hy-drothermal synthesis requires low processing temperatures(normally below 300 °C). This allows for precise control ofthe particle size, shape, and crystallinity of perovskite oxidevia tuning the synthesis process conditions. Moreover, due to

the mild temperature conditions used via this synthesis route,agglomerated free powders can be produced. Thus,electrocatalysts with high specific area and unique favorablemorphologies towards OER can be generated [3, 71].

2.3 Thermal treatment

Calcination/thermal annealing under low oxygen partial pres-sures or vacuum at elevated temperatures is a fairly simple anddirect approach to produce non-stoichiometric perovskite ox-ides [72]. However, catalysts produced using this method cancontain agglomerated large particles of various sizes and mor-phologies, as well as phase impurities [3]. Moreover, the highcalcination temperatures associated with this synthesis routecan result in sintering of the perovskite oxide catalyst whichmay result in a poor microstructure, reduction in surface areaand coarsening of the catalyst powders, hence low electro-chemical OER performance [73]. Therefore, the catalyst pow-der post-calcination is typically refined or ball-milled to sep-arate the particle aggregate and obtain a catalyst powder withenhanced particle size distribution [52].

Fig. 2 (a) Relationship betweenspecific activities and surfaceoxygen vacancies and surface Feoxidation states for the LSF-x(x = 0, 0.2, 0.5, 0.8, and 1) sam-ples. Reproduced from Ref. [41]with permission from theAmerican Chemical Society,Copyright 2018. Surface mor-phologies of the catalysts by FE-SEM: (b) NBCC, (c) NBCCFe,(d) NBCCMn, (e) NBCCNi, and(f) NBCCCu. Reproduced fromRef. [47] with permission fromthe Royal Society of Chemistry,Copyright 2018

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Miao et al. studied the role of oxygen vacancy concentra-tion in enhancing the electrocatalytic OER activity ofPrBaCo2O6 − δ perovskite. Two oxygen-deficient perovskiteswith different oxygen vacancy concentrations (δ) of 0.25(PrBaCo2O5.75) and 0.5 (PrBaCo2O5.5) were synthesized.The catalysts were produced by sintering PrBaCo2O6 − δ pow-ders (produced via a sol-gel method) in air at 1000 °C for 10 hfollowed by annealing in pure N2 at 250 and 600 °C for30 min to obtain PrBaCo2O5.75 and PrBaCo2O5.5.Interestingly, higher oxygen vacancies (δ = 0.5) have detri-mentally affected the intrinsic OER activity of the cobaltoxide-based perovskite. PrBaCo2O5.75 depicted faster OERkinetics in 1M KOH alkaline media with a low overpotentialof 360 mV to achieve a current density of 10 mA cm−2 ascompared with PrBaCo2O5.5 (420 mV). As the oxygen vacan-cy introduced, both PrBaCo2O5.75 and PrBaCo2O5.5 displayedsimilar particle morphologies as shown in Fig. 3a and b.However, at (δ = 0.5), as the oxygen atoms were dislodgedfrom the octahedral Co4+O6 in PrO1-δ layers, a pyramidalCo3+O5 was formed, resulting in an alternative arrangementof pyramidal Co3+O5 and octahedral Co

3+O6 within the PrO1− δ layers (Fig. 3c). Consequently, this ordered structure re-duced the cobalt oxidation states and caused a spin-state tran-sition from high-spin to low-spin states for the cobalt ions,which both significantly increase charge resistance hencehampering the OER kinetics [38].

In another work by Chen et al., an oxygen-deficientBaTiO3 − x perovskite denoted as (BTO-1300VAC) was syn-thesized using a sol-gel method followed by vacuum thermaltreatment at 1300 °C for 2 h [49]. For comparison purposes,BTO-950AIR was produced via calcinating pristine BaTiO3in air at 950 °C for 2 h. Moreover, benchmark IrO2 nanopar-ticles (denoted as Ir-550) were synthesized via a sol-gel meth-od followed by calcination in oxygen at 550 °C for 2 h todirectly compare its OER activity with the as-synthesized ox-ygen-deficient perovskite. The measured surface areas ofBTO-1300VAC and IrO2 were 24.7 and 21.1 m

2 g−1, respec-tively. Post-annealing, small agglomerations of the BaTiO3 − xparticles were observed due to the high-temperature condi-tions used in the calcination process (Fig. 3d). On the otherhand, BTO-950AIR formulated highly agglomerated particlesdue to the bonding reaction in air at 950 °C (Fig. 3e).Moreover, in contrast to BTO-950AIR and BaTiO3 whichexhibited pure tetrahedral phases (t-BaTiO3), mixture of te-tragonal and hexagonal BaTiO3 (h-BaTiO3) phases wasexsolved upon annealing. Results of electrochemical OERmeasurements in 0.1 NaOH electrolyte solution showed thatthe BTO-1300VAC sample generates larger current densitythan Ir-550 at relatively low potential (< 1.6 V). However,the measured current density for Ir-550 at high potentials (>1.6 V) rapidly increases and exceeds that for BTO-1300VAC.This was attributed to the higher conductivity and active sitedensity of IrO2 over perovskite oxides. BTO-950AIR showed

a very low electrocatalytic OER activity with an onset ofaround 1.8 V.

3 Characterization techniques for oxygenvacancies

As will be discussed in detail in Section 4, the presence ofoxygen vacancies results in notable changes in the perfor-mance of a perovskite electrocatalyst. Such enhancements inelectrocatalytic activities emerge from both structural andelectronic modulations to the material. For instance, the omis-sion of several surface and sub-surface oxygen vacancies canlead to structural instabilities and phase changes—as will bediscussed later. Crystallographic techniques, such as XRD,can indirectly probe the presence of oxygen vacancies instructurally complex perovskites upon slight phase expan-sions or compressions that result in a more stable configura-tion. Furthermore, it has been found that amongst the mainchanges that perovskites undergo upon the induction of oxy-gen vacancies, modulation in electronic and magnetic proper-ties as a result of metallic-oxygen species near the surface andcharge neutrality effects, respectively, are witnessed. This sec-tion will briefly overview some of the most prominent andadvanced characterization techniques employed in literatureto effectively identify the presence of oxygen-deficient perov-skites. It is worth noting that the techniques discussed havebeen utilized for the characterization of oxygen vacancies in amyriad of transition metal oxides (TMOs); however, this sec-tion will solely review relevant endeavors in identifyingoxygen-deficient perovskites.

The ordering of oxygen vacancies has been investigated inTMOs due to the observed influence that vacancy orderinghas on magnetic and transport properties. Advanced micros-copy using high resolution transmission electron microscopy(HRTEM) and scanning transmission electron microscopy(STEM) can be utilized to directly probe evident surface struc-tural distortions as a result of surface lattice realignment uponthe induction of oxygen vacancies. Klie et al. reported the useof high-angle annular dark-field (HAADF) STEM techniqueto convey patterned ordering of oxygen-deficientLa0.5Sr0.5CoO3-ẟ (LSCO) on SrTiO3 (STO) and LaAlO3(LAO) substrates in the [74] orientation [75]. Oxygen vacan-cies were indirectly found to be ordered in LCO thin filmsgrown on a (001) oriented STO substrate, whereby the con-trast is resulting from oxygen vacancy ordering [76]. This ispresented in Fig. 4a. Furthermore, Fig. 4b shows enlarged La-La distances as a result of the induced oxygen vacancy planes.

The induction of oxygen vacancies in pristinePr0.5Ba0.5MnO3 − ẟ (PBM) by direct annealing in H2 was re-ported by Ciucci et al., and clear changes to the crystallinephase of the reduced H-PBM are conveyed through XRDcomparison between both samples [46]. Phase transition from

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the cubic structure with randomly distributed Pr/Ba sites to theordered one can be depicted from Fig. 4c and d, respectively.Reduction of the pristine perovskite can also result in a low-ering of transition metal ions to a lower valence states and thedecomposition of the original coupled material into its constit-uent oxide and metals. However, the latter of this can be ruledout in the case of H-PBM based on XRD patterns not showingphases of the standalone components of the perovskite. XRDtechnique is one of a few characterization methods that wasreported in several studies to determine the presence oxygenvacancies in perovskites [77, 78]. Upon reduction—and con-sequent introduction of oxygen vacancies—peak shifts to

lower angles infer an increase in out-of-plane lattice parameter(c) and cell volume. This is attributed to atomic underbondingorbitals located in nonbinding orbitals of transition metals dueto the presence of large number of electrons upon oxygenvacancy formation [70].

Neutron powder diffraction (NPD) is another effective ap-proach for depicting oxygen vacancies in perovskites. Theneutrons utilized in NPD technique have higher penetrationdepth than the penetration depth of X-rays, which allows forbulk characterization and a higher sensitivity towards oxygenatoms. This proves effective for studying oxygen-deficientperovskites [79–82]. For instance, Chen et al. employed the

Fig. 3 SEM images of (a)PrBaCo2O5.75 and (b)PrBaCo2O5.5. (c) Schematiccrystal structures of PrBaCo2O5.75(left) and PrBaCo2O5.5.(right).Color codes: Pr, orange; Ba,green; Co, blue; O, red.Reproduced from Ref. [38] withpermission from the RoyalChemical Society, Copyright2019. SEM images for (d) BTO-1300VAC (1300 °C in vacuum)and (e) BTO-950AIR (950 °C inair) samples. Reproduced fromRef. [49] with permission fromElsevier, Copyright 2015

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Rietveld method in NPD to exemplify potential active sites inoxygen-deficient BaTiO3 [49]. It was also used to study theoxygen atom site occupancies in structural models as freevariables in the refined spectra. It was thus determined thattetragonal phases did not promote vacancy formation; howev-er, the oxygen site occupancies in hexagonal BaTiO3 phaserefined to 0.92(3)—representing BaTiO2.76. Such detailedphase analysis is essential for oxygen vacancy engineering,since as will be discussed later, not all phases have the sameenthalpic favorability towards vacancy formation and repre-sent different binding energies towards reaction intermediates.

X-ray photoelectron spectroscopy (XPS) is perhaps themost employed characterization method for indirectly deter-mining the presence of oxygen vacancies in the surface ofoxide-based materials. XPS works by determining the surfaceelectronic and chemical states of a material through effectivelymeasuring the contrast in outermost layer electrons and theiratomic cores’ binding energies. A reduced material with oxy-gen vacancies tends to always have surface oxygen vacanciessince reduction initially attacks surface lattice oxygen, andconsequently through the deconvolution of the O 1s spectraof a reduced material, the determination of vacant sites can beinferred. Due to charge neutrality restrictions, neighboring

cationic metallic sites to the oxygen vacancy site lower theirvalence oxidat ion sta tes . Yang et al . fabr icatedSr2Fe1.3Ni0.2Mo0.5O6 − ẟ (SFNM) with oxygen vacanciesthrough calcination in argon (Ar), 5% H2/Ar, and H2 atmo-spheres [44]. The resultant samples were denoted SFNM-Ar,SNFM-5%, and SNFM-H2, respectively. XPS results present-ed in Fig. 5 show the different degrees of oxygen vacanciesthe materials possess, which was directly correlated to theorder in OER performance. The deconvoluted peaks of theNi 2p, Fe 2p, and Mo 3d spectra in the modified SFNM sam-ples of Fig. 5a–c show a clear lowering of oxidation statesupon reduction in H2 and 5% H2/Ar atmospheres. This is inorder to cope with surface charge electroneutrality conditionsfor more thermodynamic surface-stable reduced structure.Furthermore, the reduction of B-site ions (Ni, Fe, and Mo)to a lower oxidation state leads to a decrease in the bindingenergy of the lattice oxygen, whilst the oxygen vacanciesgreatly enhance the binding energy of the lattice oxygen.Figure 5d shows two main deconvoluted peaks for the O 1sspectra of different SFNM samples. Essentially, a peak at529 eV corresponds to lattice oxygens, and the second peakat a binding energy of 531 eV corresponds to adsorbed oxygenspecies (i.e., O2

−, O−), which is an indirect identifier of oxygen

Fig. 4 High-resolution, high-angle annular dark field (HAADF) image ofa LCO thin film 15 nm thick, grown on a (001) oriented STO substrate,showing the contrast resulting from the O-vacancy ordering. The direc-tion of the modulation vector, q, is marked with an arrow. (a) Map of thein-plane distance between first La neighbors (Δx) in (b), along the

direction parallel to q. O-deficient planes are characterized by enlargedLa-La distances. Reproduced from Ref. [76] with permission from theAmerican Physical Society, Copyright 2014 . PXRD patterns of (c) as-synthesized PMB and (d) H-PBM. Reproduced from Ref. [46] with per-mission from The Royal Chemical Society, Copyright 2016

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vacant sites. The adsorbed oxygen electrophilic species couldattack regions of H2O molecules with the highest electrondensities—in effect enhancing OER kinetics.

The presence of oxygen vacancies in a perovskite leads tothe trapping of unpaired electrons, which can be detectedthrough electron paramagnetic resonance (EPR). EPR as acharacterization technique can shed light on both bulk andsurface unpaired electrons. However, a disadvantage in usingEPR as a sole method to determine the presence of oxygenvacancies lies in the fact that it innately is unable to differen-tiate between oxygen vacancies and other types of defects thatcan lead to the same unpairing effect of electrons. Eichel per-formed a thorough investigation of several transition metal-doped perovskites through the analysis of high-frequencyEPR [83]. The presence of oxygen deficiencies in all the stud-ies materials was confirmed by g-parameters close to 2.00. Forinstance, Mn-doped Bi0.5Na0.5TiO3 EPR analysis found a g-parameter of 2.0015. Numerous findings of g-parameters ap-proaching 2.00 for oxygen-deficient perovskites and transitionmetal oxides (TMOs) co-confirmed with other oxygen vacan-cy characterization techniques have been reported in recentyears [84–86]. Acquiring information pertaining to molecularvibrations and rotations via Raman spectroscopy is anothertool continued to be adopted for identifying oxygen-deficient materials and perovskites in general [87–90].

Changes in lattice molecular vibrational nodes due to novelbond configurations give rise to Raman shifts. Oxygen vacan-cies can thus result in observed Raman shifts or the creation ofnew peaks. Moreover, anionic vacancies tend to modify bandstructures—as can be seen from DFT band structure calcula-tions of oxygen-deficient materials. Such band structurechanges can lead to potential alternate paths for excited elec-tron generation and recombination—which occurs from light-directed electron excitation during photoluminescence (PL)readings [85, 91]. Peak position and intensity of the PL spectrashow easily identifiable changes upon such electronic modu-lations from oxygen vacancies [92].

The aforementioned analyses show a varying degree ofaccuracy and reliability in identifying oxygen vacancies inperovskites, and oxygen-deficient TMOs generally.Nevertheless, based on the most prominently utilized and ac-cepted techniques, XPS and STM are generally used to detectsurface chemistry changes and physical imaging of oxygenvacancies, respectively. Although the creation of surface ox-ygen vacancies is more important than bulk vacancies in pho-tocatalytic and electrocatalytic applications, some applicationsmay find bulk vacancies important. For the latter applications,both XPS and STM may not be very useful since they char-acteristically capture the first few layers of a material’s sur-face. It is generally a common practice to perform at least two

Fig. 5 XPS spectra of SFNM,SFNM-Ar, SFNM-5% H2/Ar,and SFNM-H2: (a) Ni 2p; (b) Fe2p; (c) Mo 3d; and (d) O 1s.Reproduced from Ref. [44] withpermission from The RoyalChemical Society, Copyright2017

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characterization techniques for an accepted claim of oxygenvacancies. Moreover, phase characterization techniques, suchas XRD, are only useful for identifying oxygen deficiencieswhen the target material is crystalline in nature. Therefore,factoring for the fact that every technique exhibits some formof limitation, it is considered best practice to use multiplecharacterization techniques on the same material to undoubt-edly prove the presence of oxygen vacancies.

4 Advantageous intrinsic effects of oxygenvacancies in perovskites

Induction of oxygen vacancies in perovskites has been report-ed to advantageously enhance the physicochemical propertiesdirectly related to lower overpotentials for electrochemicalOER through water splitting. Literature shows a clear sparkedincrease in interest towards oxygen-deficient perovskites—not only due to an apparent improvement in performance.Oxygen vacancies in perovskites are amongst the easiest an-ionic or Schottky defects to be induced and reasonably con-trolled due to their low formation energies. Other types ofdefects, such as interstitial or Frenkel defects, tend to be morechallenging to control. Furthermore, due to a growing interestin oxygen-deficient metal oxides, both experimental and com-putational studies have been devoted to mechanistically inves-tigate how oxygen vacancies alleviate an initially high OERoverpotential for a pristine sample. Quintessentially for perov-skites, three main areas were found to be modulated uponinduction of oxygen vacancies—namely, changes in the re-laxed configuration structure, improvements in bulk chargetransport, and favorable inflection of the electronic structure.

In their recent work, Tavassol et al. synthesized an oxygen-deficient Sr2 − xCaxFe2O6 − ẟ perovskite for tunable surface re-activity towards OER [93]. This was undertaken by changingthe A-site composition (A = Sr2 − xCax) and the oxygen vacan-cy content (ẟ). Figure 6a shows a clear direct proportionalitybetween the degree of oxygen vacancy (ẟ) and the reductionin OER overpotential. Similar findings have been reportedwith other oxygen-deficient perovskites. For instance,Fig. 6b also shows a direct correlation between the mostoxygen-deficient La1 − xSrxCoO3 − ẟ and the highest OER ac-tivity for the optimum material SrCo2.7 [42].

Researchers have utilized so-called “activity descriptors”for predicting the intrinsic behavior of transition metal oxides(TMOs) and their activity towards OER. Essentially, twomain descriptors that are related to oxygen vacancies havebeen frequently investigated to merit their utilization; theyare the eg occupancy and the difference between the metal d-band center and the oxygen p-band center (ΔEd − p) [94, 95].Both these descriptors can be thought of as modulators ofelectronic structure. In their recent and very thorough work,Kim et al. primarily focused on investigating the relationshipbetween oxygen vacancies, ΔEd − p, and apparentoverpotential for Sm0.5Sr0.5CoO3 − ẟ (SSC) [96]. The authorsweremotivated by the fact that previous studies solely focusedon the effect of ẟ on catalytic activity without much consider-ation of the electronic structure of cobalt in SSC. They per-formed a detailed study using X-ray absorption spectroscopy(XAS) characterization for recording changes to the oxidationstate of Co upon different degrees of oxygen vacancies (ẟ)were induced, and density functional theory (DFT) calcula-tions to reveal more accurate reaction pathways. Early TMs(i.e., V, Mn, and Cr) show higher Md compared with the

Fig. 6 (a) δ vs η (V) plot for the compounds synthesized in the series ofSr2 − xCaxFe2O6 − δ under air (red) and argon (black) atmospheres. Grayand light blue bars show activities associated with carbon background andLCO, respectively. η is calculated from the thermodynamic potential ofOER, 1.23 V vs RHE, at 2 mA cm−2. Dashed line is drawn to guide theeye. Reproduced from Ref. [93] with permission from the American

Chemical Society, Copyright 2019. (b) Correlation of oxygen evolutionactivity for La1 − xSrxCoO3 − ẟwith the vacancy parameter d. The vacancyparameter is indicative of the underlying electronic structure where va-cancies are generated when there is significant Co 3d and O 2p bandoverlap. Reproduced from Ref. [42] with permission from SpringerNature, Copyright 2016

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oxygen p-band center (Op), whilst late transition metals (i.e.,Ni, Co, and Cu) show lower Md compared with Op [97]. Thus,OER performance can be increased by increasingMd which ineffect decreases the ΔEd − p. This relationship is displayed inFig. 7a. The ẟ value was varied between 0 and 0.2 (with 0.1increments), and the content of oxygen vacancies in theoxygen-deficient SSC samples was determined usingiodometric titrations. DFT analyses confirmed that OER ac-tivity was improved upon vacancy introduction due to liftingthe metal d-band centers (Md) and reducing (ΔEd − p).

Figure 7b shows that the OER onset potential was found todecrease from 1.62 to 1.54 V (vs RHE) at 1 mA/cm2 with an

increase in oxygen vacancy content (ẟ). Furthermore, a DFTinvestigation pertaining to the placement of the oxygen vacan-cies was performed, whereby as can be seen from Fig. 7c, theoverpotential is lowest when the anionic vacancy is in the firstlayer. This is since ΔEd − p is lowest when oxygen vacanciesare induced in the top layer. The reaction coordinate diagramin Fig. 7d shows how the placement of oxygen vacancy in thetopmost layer lowers the thermodynamic barrier for each in-termediate step—namely, the rate-determining step (RDS).Similar findings of increased OER activity corresponding toa decrease in ΔEd − p have been reported over the years [98,99].

Fig. 7 (a) Schematic rigid band diagrams of the late transition metaloxide. (b) Variation in the Md energy levels and the energy leveldifferences between Md and Op (Ed − p) with the number of oxygendefects in the SSC. All band centers are relative to the Fermi level. (c)Overpotential and Ed − p values of the OER depending on the oxygen

vacancy position for the first, second, and third layers. (d) Calculatedfree energy profiles of the ORR and OER on the (100) surface of SSCin alkaline media, where the asterisk symbols in reaction coordinatesrepresent the adsorption state. Reproduced from Ref. [96] with permis-sion from the American Chemical Society, Copyrights 2020

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Suntivich et al. described the eg occupancy descriptor tostrongly affect the rate-determining step (RDS) in OER [62].The authors reported that Ba0.5Sr0.5Co0.8Fe0.2O3 − ẟ (BSCF)yielded an order of magnitude more prominent intrinsic activ-ity than benchmark IrO2 for OER in alkaline 0.1M KOH me-dia. The formulation of this optimized material was undertak-en by systematically examining ten different transition metaloxides (TMOs). A notable finding of an apparent volcano-plotpattern emerged relating the occupancy of the eg orbitals (i.e.,dz2, dx2–y2) in surface TM cations and occupancy of the 3delectrons. The peak of the volcano plot shows that an eg oc-cupancy close to unity, reflecting high covalency betweenTM-Oxygen bonds, corresponds to optimum OER activity.This is presented in Fig. 8a, whereby the performance ofBSCF is close to the apex of the volcano plot. Suntivich ar-gues that in the case where the eg occupancy is less than unity,

the formation of the peroxide ion from precedingoxyhydroxide would be the RDS. In contrary, if the eg occu-pancy is greater than unity, the formation of O-O bonds in theOOH*OER intermediate would be the RDS. The relevance ofthis comes in the finding that introduction of oxygen vacan-cies in perovskites actually brings the eg occupancy close tounity. For instance, Wu et al. found that oxygen-deficientCaMnO3 perovskite samples showed activity enhancementsup to a certain point—eg filling approaching 1 [36]. They alsorelated this enhancement to a dual improvement in intrinsicconductivity of the material. The optimum Yb-doped andoxygen-deficient Yb0.1Ca0.9MnO3 sample showed 100 timeshigher activity than the original undoped and un-reducedCaMnO3 sample. This was attributed to optimal Mn eg fillingstate as high as 0.81 and better conductivity. Similarly, theactivity of Mg-doped oxygen-deficient LaNiO3 (LNO)

Fig. 8 (a) The relation between the OER catalytic activity, defined by theoverpotentials at 50 μA cm−2ox of OER current, and the occupancy of theeg-symmetry electron of the transition metal (B in ABO3). Data symbolsvary with type of B ions (Cr, red; Mn, orange; Fe, beige; Co, green; Ni,blue; mixed compounds, purple), where x = 0, 0.25, and 0.5 for Fe. Errorbars represent SDs of at least three independent measurements. Thedashed volcano lines are shown for guidance only. Reproduced fromRef. [62] with permission from The American Association for theAdvancement of Science, Copyright 2011. (b) Schematic representation

of the octahedral and square pyramidal forms for Mn cations, withdifferent electron distributions in the corresponding crystal-field split d-orbit. (c) Dependence of OER activity on the occupancy of the eg orbit.(d) Relationship between OER and HER activity, both of which aredefined as the potential to deliver a current density of 0.5 mA/cm2 ofoxide. The error bar is the standard deviation calculated from three sep-arate experiments. The dashed volcano line is displayed only for refer-ence. Reproduced from Ref. [1] with permission from the AmericanChemical Society, Copyright 2018

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nanofibers fabricated through electrospinning was shown tooutperform the pristine LNO counterpart [60]. Typically, Ni2+

presence in LNO leads to an eg occupancy greater than unity.However, STXM-XANES analysis of the Mg-doped LNO(LNMO) sample showed an increase in the Ni3+ state, where-by an increased Ni3+/Ni2+ ratio could lead to an eg occupancyapproaching unity and enhancing OER activity. Moreover,Ciucci et al. reported the HER activity of oxygen-deficientNbBaMn2O6 − ẟ (NMB) fabricated via direct hydrogen reduc-tion treatment [1]. The optimum oxygen-deficientNbBaMn2O5.5 (NMB5.5) achieved a better overall water split-ting stable activity at large potentials (> 1.75 V) comparedwith benchmark IrO2 and RuO2 catalysts. Figure 8b–d showsNMB5.5 yielding the most favorable alkaline HER and OERactivity compared with other NMB fabricated perovskites,ascribed to near unity eg occupancy, along with favorableΔEd − p levels and structural distortions.

It has also been found that some activity descriptors, suchas eg occupancy and ΔEd − p levels, can be impacted throughmodulation of lattice strains, especially in perovskites.Although most studies tend to discuss oxygen deficienciesand lattice strains in perovskites independently, Liu et al.found a symbiotic relation between the two [78]. Theyexpressed these effects intrinsically through monitoring eg oc-cupancies in oxygen-deficient La0.7Sr0.3CoO3 − ẟ (LSC) thinfilms grown atop single crystal substrates (SrTiO3 (STO) andLaAlO3 (LAO), with in-plane tensile and compressive strain,respectively). Experimental results and DFT calculations il-lustrated that higher oxygen deficiencies in LSC/STO samplescorresponded to lower OER activities, compared with LSC/LAO with fewer vacancies and higher compression strains.This is an example of disadvantageous synergy between ten-sile strains in perovskites facilitating oxygen vacancies, whichleads to higher eg occupancies and ΔEd − p levels in the LSC/STO sample and results in diminished electrochemical activ-ity. On the contrary, Petrie et al. showed that 4.2% epitaxial/tensile strains in strontium cobaltite to enhance oxygen defi-ciencies at benign synthesis conditions and lead to an order-of-magnitude improvement in intrinsic OER activity [100].Such modest tensile strains allowed unconventionally low an-ionic concentrations not found in the unstrained bulk cobaltiteand lead to an increased Co3+ ratio which admits the eg occu-pancy closer to unity.

Based on the discussion thus far, an ideal OER perovskiteshould have an eg value approaching unity, but also high con-ductivity [62]. However, most pristine oxides do not haveboth ideal eg occupancies and good conductivities. Wu et al.related the 100 times increase in OER activity ofYb0.1Ca0.9MnO3 treated at 350 °C (CYMO-350), comparedwith pristine CMO, to both the enhancement of eg occupancyand conductivity [36]. This was achieved by sequential dop-ing and hydrogen treatment at elevated temperatures.Figure 9a, b represents the OER polarization curves and the

identified eg occupancy vs. conductivity of the representativesamples investigated. Similar findings have been numerouslyreported whereby the induction of oxygen vacancies has led toan apparent substantial increase in perovskite’s conductivity.For example, Shao et al. reported a silicon-incorporated pe-rovskite as an OER material, which showed a 12.8-fold in-crease in oxygen diffusivity and two orders of magnitude in-crease in conductivity—matching the attained 10-fold en-hancement in intrinsic OER activity [48]. Previously reportedpristine SrCoO3 − ẟ (SCO) with relatively high and stable OERperformance was modified with silicon. Pristine SCO showeda conductivity of 2 S cm−1, whilst a remarkable value of198 S cm−1 was recorded for the silicon modified oxygen-deficient sample (Si-SCO). The comparative alkaline (0.1 MKOH) OER polarization curve in Fig. 9c corresponds to Tafelslope values of 76 and 66 mV dec−1 for pristine SCO and Si-SCO, respectively. This is also a good indication of improvedOER kinetics since lower Tafel slopes imply significantly in-creased currents at only moderate increments of overpotential.Furthermore, Ciucci et al. developed a novel bifunctional pe-rovskite catalyst towards OER and ORR, whereby Nb wassubstituted in the Mn site of CaMnO3 (CMO) to yieldCMNO and inducing oxygen vacancies in the structure uponhydrogen treatment (H2-CMNO) [45]. Hydrogen treatmentwas found to not only affect the phase structure of CMNO,but it also enhanced the concentration of oxygen vacanciesbased on XPS analysis. This has contributed to more effectiveelectronic transport through conductivity enhancement be-tween CMO (3.33 × 10−3 S m−1) and H2-CMNO (2.19 ×10−2 S m−1). Figure 9d shows the substantial difference inmass activity as a performance parameter between pristineCMO, CMNO, and the highly oxygen-deficient H2-CMNO.Other works showing OER performance exceeding that ofbenchmark IrO2 have been reported over the years [101, 102].

Shao et al. systematically investigated structural distortionsof La1 − xSrxFeO3 − ẟ (LSF) upon varying degrees of Sr-dopingin LaFeO3 − ẟ (LF) which induced proportional increases inoxygen deficiencies based on XPS analysis [41]. The contentof Sr-doping was varied from x = 0, 0.2, 0.5, 0.8, and 1.0, withthe alkaline (0.1 M KOH) OER performance evaluated usingan RDE at 1600 rpm increasing in the order LF (LSF-0), SF(LSF-1.0), LSF-0.2, LSF-0.5, and LSF-0.8 corresponding toan increase in Sr-doping and oxygen deficiency character inthe doped perovskite. Based on XRD analysis performed,structural distortions and phase configurational changes occurupon progressive doping as can be observed from Fig. 10a.Furthermore, the corresponding electrochemical impedancespectra (Nyquist plot) in Fig. 10b shows highest resistivityin the LF and SF samples having orthorhombic and tetragonalstructures, respectively. Similarly, Cheng et al. correlatedstructural variations with Sr-doping in La1 − xSrxCoO3 (LSC)[103]. The authors found a relatively abrupt phase transforma-tion from a rhombohedral (LaCoO3) to a cubic structure

581emergent mater. (2020) 3:567–590

(La0.2Sr0.8CoO3), congruent to progressive alignment of theCo-O-Co bonds and an oxidation state of Co greater thanCo3+.

Density functional theory (DFT) was utilized in this workto calculate the Co-O-Co bond alignment. Figure 10c relatesthe structure type and Co-O-Co bond alignment to the Sr-doping content. Highest conductivity (300-time increase com-pared to the pristine undoped LaCoO3 sample), transition to acubic structure, second highest oxygen vacancy content, andlowest OER overpotential corresponded to the La0.4Sr0.6CoO3sample. This indicates the favorability of cubic Pm-3m struc-tures for intrinsic activity towards OER. Interestingly, Sunarsoand Zhou dedicated an investigation to structural changes inLaNiO3 − ẟ (LN) [104]. In their work, XPS analysis of oxygen

deficiencies was not undertaken—however, this leaves roomfor doping, which introduces oxygen vacancies and enhancesperformance as discussed earlier. Nevertheless, in this earlywork by the group, a direct relationship between OER/ORRactivity and LN structure was identified. LN samples that wereheated at 400, 600, and 800 °C yielded an apparent transitionfrom rhombohedral (R-3c) to cubic (Pm-3m) and showed adirect correlation with respect to OER/ORR activities—seeFig. 10d. Furthermore, the elongation of the Ni-O bond inthe cubic structure favors higher OER/ORR activity.

Computational analyses using varying density functionaltheory (DFT) approaches have proven invaluable in the inves-tigations of surface microengineering effects including, butnot limited to, effects of oxygen vacancies in perovskites.

Fig. 9 (a) IR-corrected OER polarization curves, and (b) the OER activ-ity increased with conductivity and eg electron-filling status optimized byhydrogen treatment. Inset: Jahn–Teller distortion promoted the formationof oxygen defects, resulting in an optimal Mn eg filling state and betterelectrical conductivity. Reproduced from Ref. [36] with permission fromWILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Copyright 2015.(c) OER kinetic currents (normalized to the geometric surface area of theelectrode, in mA cm−2geo) of SCO and Si–SCO collected in an O2-satu-rated 0.1M KOH electrolyte under ambient conditions. The contribution

from the conductive carbon as catalyst support is shown for reference.Inset shows the OER-specific activity (normalized to the BET surfacearea of the oxide catalyst, in mA cm−2oxide) of SCO and Si–SCO at1.60 V vs. RHE. Error bars are the standard deviations of triplicate mea-surements. Reproduced from Ref. [48] with permission from SpringerNature, Copyright 2020. (d) The mass activity of CMO, CMNO, andH2-CMNO in O2-saturated 0.1 M KOH solution at 1600 rpm.Reproduced from Ref. [45] with permission from the AmericanChemical Society, Copyright 2017

582 emergent mater. (2020) 3:567–590

The Kohn-Sham scheme is perhaps the most utilized “base-case” method used in DFT. The derivation of the non-interacting and spin density functional respective systemsare integrated within the Kohn-Sham scheme. The scheme’sequation (Eq. (6)) revolves around motion calculation ofinteracting electrons as individual moving particles [105].νKS(r) represents the Kohn-Sham potential, μi shows theone-electron orbital, and εi represents the total energy of oneorbital.

−1

2∇2 þ νKS rð Þ

� �μi ¼ εiμi ð6Þ

The different intermediates partaking in OER for perovskitesand other material types have been heavily investigated usingDFT. Ideally, OER intermediates (O*, OH*, and OOH*) shouldnot bind too strongly to the active site of a perovskite, to facilitatefor reasonable dynamics.Man et al. utilized this understanding todevelop and present a universal scaling relationship between thefree binding energy of OOH* and OH* in order to analyze thereaction free energy diagrams of all oxides in a general way[106]. This work gave rise to a simple and effective universaldescriptor for OER reactions on oxide surfaces, including perov-skites, and to an activity volcano plot. At the time, a fewmaterials

were at the apex of this activity volcano, representing a theoret-ical minimum overpotential, whilst remaining materials are bur-dened by either too weak or too strong O* intermediate bindingenergies. The activity descriptor (ΔGO*−ΔGOH*) remains in-valuable in all first principle design of materials, includingoxygen-deficient perovskites for the OER. DFT approaches,such as DFT+U, which introduces an artificial corrective poten-tial, are heavily used to study, fine-tune, and guide experimentalwork for improving OER performance in perovskites. For in-stance, Yao et al. employed DFT +U to investigate oxygen-deficient Mg-based A2Mn2O5 (A =Ca, Sr) [74]. They predictedSr2Mn2O5 to have lower OER overpotentials due to the largeionic radii of A-site elements caused by weakening the hybridi-zation between the Mn dz2 and OH-pσ orbitals. Furthermore,Zhao et al. computed the Gibbs free energy changes for theOER using pristine PrBaCo2O5+ δ (PBC, where Co as activecenter), PrBa0.5Sr0.5Co1.5Fe0.5O5+ δ (PBSCF, where Co and Feas active centers) [101]. Figure 11a shows how the undertakingof the reaction on the Fe active site of modified PBSCF resultedin preferable theoretical thermodynamic overpotentials. Chenget al. studied the effects of Co-O-Co bond alignment and Sr-doping amount (x) in La1− xSrxCoO3 (LSC) [103]. Shin et al.recently used first principles DFT calculations to probe how thehole compensations by electrons released from oxygen vacancies

Fig. 10 (a) Schematic structures of the LSF-x (x = 0, 0.2, 0.5, 0.8, and 1)powders and (b) electrochemical impedance spectra for the LSF-x (x = 0,0.2, 0.5, 0.8, and 1) samples obtained at 0.7 V vs Ag/AgCl. Reproducedfrom Ref. [41] with permission from the American Chemical Society,Copyright 2018. (c) Co–O–Co angle as a function of the Sr fraction in

LSC. Reproduced from Ref. [103] with permission from the AmericanChemical society, Copyright 2015. (d) Relation of ORR/OER with crys-tallographic structure of LN samples. Reproduced from Ref. [104] withpermission from the American Chemical Society, Copyright 2013

583emergent mater. (2020) 3:567–590

in La0.5Sr0.5FeO3− ẟ eventually lowers the conductivity and leadsto metal-to-semiconductor transition at low partial pressures ofO2 [108]. Density of states (DOS) calculations in Fig. 11b showhow the Fermi level, initially in the middle of the hybridizationband forO 2p and Fe 3d orbitals, increases. Two oxygen vacancystates appear, and as a result, the hybridized band becomes fullyoccupied. The excess electrons from the vacancies thus increasethe Fermi level and cause the transition from metallic to semi-conductor state, in this case leading to a decrease in conductivity(Fig. 11c). DFT under the general gradient approach (GGA),such as the Perdew-Burke-Ernzerhof (PBE) method, has provento be an effective strategy for initial screening of oxygen-deficient materials for electrolysis applications [109]. However,there are still opportunities for improvement by including orbitaldependent potentials and hybrid functionals (i.e., HSE06, PBE0)that can provide more accurate electronics analysis (i.e., bandgaps for semiconductors, band structure) compared with the ba-sic PBE approach at the cost of high computational resources[110, 111]. Incorporation of relatively recent methods, such asthe GW approximation for calculating self-energy of a mainbody, is highly expected to curb limiting computational resourcesespecially for large systems [112].

5 Disadvantageous effects of oxygenvacancies in perovskites

The previous section discussed changes in the intrinsic effectsthat promote an enhanced OER performance as perovskites be-come oxygen deficient to some degree. However, we will nowfocus on cases which do not fall under this general category.

Several reports have discussed important intrinsiccharacteristics—electric conductivity, phase transformations,stability—that are disadvantageously affected upon significantreduction of a perovskite. Recently, Yamada et al. reported phasetransformation and relationship of structure with stability for dif-f e r e n t d eg r e e s o f oxygen de f i c i e n c i e s (ẟ ) i nBa0.5Sr0.5Co0.8Fe0.2O3− ẟ (BSCF) perovskite oxides [95]. OERactivity was evaluated for a range of oxygen-deficient BSCFperovskites, namely heavily oxygen vacant (r-BSCF, ẟ = 0.51),pristine conventional (p-BSCF, ẟ = 0.38), and highly oxidized(o-BSCF, ẟ = 0.12) BSCF. Interestingly, overpotentials forheavily oxygen vacant and highly oxidized samples were inferiorto that of the moderate and conventional sample (ẟ = 0.38).Figure 12a reports the lattice parameter (a) increases linearly withrespect to the degree of oxygen vacancy (ẟ) in BSCF, indicatingan increase in the ionic radii of the B-site ions (Fe, Co) whichcauses structural instabilities [114, 115]. Figure 12b shows therelationship between ẟ and the recorded OER overpotential.Moreover, Du et al. reported excessive reduction via annealingin H2-reducing atmosphere led to the phase transformation ofCaMnO3 from Pnma to Pbam (CaMnO2.5) [113]. The oxygen-deficient and stable CaMnO2.75 sample—showing the lowerOER overpotential—was experimentally found to be the mostconductive (4.68 × 10−2 S m−2) compared with that of pristineCaMnO3 (1.56 × 10

−3 S m−2) and CaMnO2.5 (1.8 × 10−3 S m−2).

In the cases of BSCF and CaMnO2 perovskites discussedpreviously, a certain degree of vacancies is favorable, but thisis not necessary the case for the wide variety of perovskiteoxides. For instance, Hirai et al. showed that the OER activityof layered perovskite Sr2VFeAsO3 − ẟ was substantially im-proved above lattice oxygen vacancy content of ẟ = 0.5,

Fig. 11 (a) The computed Gibbs free energy changes for the wholesystem for the OER at η = 0 V/298.15 K on a PrBaCo2O5 + δ (PBC)/PrBa0.5Sr0.5Co1.5Fe0.5O5 + δ (PBSCF) surface; the B-site in PBC (Co)and PBSCF (Co/Fe) is treated as the catalytically active centers, all ad-sorbates are bonded to these redox centers; insets are atomic structures ofPBC slabs with adsorbates. In all schematics, each deep blue ball is Co orFe, fluorescent yellow ball is Pr, green ball is Ba or Sr, red ball is O, andlight pink ball is H. Reproduced from Ref. [101] with permission fromSpringer Nature, Copyright 2017. (b) Comparison of electronic density ofstates (DOS) for the 2nd structure of (a) La0.5Sr0.5FeO3, (b)La0.5Sr0.5FeO2.875, and (c) La0.5–Sr0.5FeO2.750. Black lines, total DOS;

red lines, projected DOS for O; gold lines, projected DOS for Fe. Filled(empty) regions, occupied (unoccupied) states; vertical dotted lines, va-lence band maxima; black arrows, oxygen vacancy states. (c) Relativeelectrical conductivity (s/sd¼0, red triangle) and relative hole concentra-tion (p/pd¼0, blue square) as a function of oxygen non-stoichiometry,with values at stoichiometric state (d¼ 0) as reference state. Blue dottedline, trend of the hole concentration. Electrical conductivity is obtainedfrom the experimental data [107], and hole concentration is estimated byintegrating the unoccupied state density of the hybridized band.Reproduced from Ref. [108] with permission from The Royal ChemicalSociety, Copyrights 2020

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manifesting near 300 mV reduction in overpotential and 80times higher specific activity at 1.7 V vs. RHE [116].Chronoamperometric (CA) tests revealed that formation ofoxygen vacancies in Sr2VFeAsO3 − ẟ does not cause structuralinstabilities. DFT calculations for ẟ > 0.5 show that the oxy-gen vacant sites become available and promote OH- adsor-bates through more favorable O-O bond formation, whichfurther lowers enthalpic limitations for the OER.

DFT calculations were employed in order to generate adensity of states (DOS) plot shown in Fig. 12c and indicatethe creation of new electronic states near the Fermi level,which are known to enhance conductivity and allow favorablebinding energies with OER intermediates. Furthermore, thesame DOS plot predicts a 1.56 eV bandgap for the pristineperovskite sample, indicating the expected semiconductor be-havior. The creation of states in only spin-up near the Fermilevel in the DOS of CaMnO2.75 and an apparent gap for thespin-down states indicate a highly spin polarized systemexhibiting metallicity, which enhances conductivity. Miaoet al. explored the role of oxygen vacancies in the OER ofdouble perovskite PrBaCo2O6-ẟ [38]. Confirming previousreports of the disadvantageous effects excessive oxygen va-cancies can cause for OER performance of perovskites, theauthors found that intrinsic activity decreases for this cobalt

perovskite. Oxygen vacancies were found to form an orderedalignment of PrO1 − ẟ layers with an alternative configurationof pyramidal Co3+O5 and octahedral Co

3+O6, resulting in spinstate transition of Co3+ ions in the tetrahedral formation fromhigh-spin (HS: t42ge

2g) to low-spin (LS: t

52ge

0g) states. This

transition follows a reduction in the eg occupancy away fromideal unity and a decrease in the conductivity. Furthermore, anapparent weakening in the Co-O bond covalency results inhigher instabilities and reduction in the alkaline (1 M KOH)OER performance. Deficiencies in the Co-O bond covalencyin ABO3-type oxides caused by lattice distortions can severelylimit electrochemical performance [117]. Figure 12d showsthat as the oxygen vacancy content (ẟ) increases from 0.25to 0.50, both the overpotential at 10 mA cm−2 increases from360 to 420 mV and a corresponding increase in the Tafelslopes from 70 to 80 mV dec−1, respectively.

6 Conclusion and future outlook

This review gives an executive, yet thorough, examination inthe development of oxygen-deficient perovskites. A discus-sion pertaining to the major contributing factors of perfor-mance enhancements through electronic modulation has been

Fig. 12 (a) Lattice constant (a) asa function of oxygen deficiency(δ) for BSCF. The line representsthe least square fitting result and(b) OER overpotential (η) for p-BSCF, r-BSCF, and o-BSCF.Reproduced from Ref. [95] withpermission from The RoyalChemical Society, Copyright2019. (c) DFT calculation of thedensity of states (DOS) of pristineCaMnO3, CaMnO2.75, andCaMnO2.5. Reproduced fromRef. [113] with permission fromthe American Chemical Society,Copyright 2014. (d) Polarizationcurves for OER activities of dou-ble perovskite PrBaCo2O6 − ẟ inO2-saturated 1.0 M KOH.Reproduced from Ref. [38] withpermission from The RoyalChemical Society, Copyright2019

585emergent mater. (2020) 3:567–590

undertaken. Attaining eg occupancy close to unity, minimiz-ingΔEd− p, enhancing conductivity, and microengineering thesurface for optimum and moderate binding energies for theOER intermediates (O, OH, OOH) via the induction of anoxygen-deficient perovskite surface has been presented. Theimportance of careful tuning of oxygen vacancies in order toprevent structural instabilities was discussed.

Further research is needed in order to enhance the stability ofperovskites generally, which are known to undergoamorphization during OER operations through the leaching ofA/B-site cations, as is the case with BSCF materials [118]. Onthat note, propagated investigation of highly oxygen-deficientlayered perovskites is encouraged, due to their innate stabilitiesat high ẟ values and oxygen vacancy oriented OER mechanism.The apparent lower stabilities exhibited in perovskites for long-term OER operation compared with Ni- and Fe-based materialsare hindering some interest in them. However, this presents agreat area of potential development for these materials.Furthermore, the relatively costly and multi-process steps in-volved in the fabrication and fine-tuning of oxygen vacanciesin oxygen-deficient perovskites presently bottlenecks their poten-tial utilization for large scale applications. Development of moreenvironmentally benign and one-step syntheses for perovskites isexpected to revolutionize development and applications of theseelectrocatalysts. The modification of perovskites through othertypes of defects and/or heterojunction with novel supports, suchas conductive MXenes, may prove fruitful in enhancing theirelectrocatalytic performance.

The utilization of computational techniques, such as DFT,to accurately screen and predict favorable and highly activeperovskite surfaces is of invaluable importance. DFT employ-ment to further explore and understand the reaction mecha-nisms on perovskite surface during OER will lead to morerationally designedmaterials with more surface functionalitiesand properties. However, consideration should be taken whenpurely relying on DFT analysis for predicting performance ofTMO electrocatalysts. Currently, functionals and pseudo-potentials within conventional DFT can predict well the per-formance of 3d metals, but they do not work well for certain4d and 5d TMOs (e.g., Ca1 − xSrxRuO3, SrIrO3) [119, 120]. Insuch systems, direct observation of the electronic structure(bulk-sensitive photoelectron spectroscopy) are conducted tounderstand their electronic structure. Furthermore, implemen-tation of machine learning (ML) approaches for quick predic-tion and selection of effective catalytic surfaces has been onthe rise [121]. For instance, Li et al. have recently presented anadaptive ML framework for OER perovskites, which navigat-ed through numerous potential double perovskites (AA’B2O6)and identified feasible surfaces for fabrication [80, 113].Appropriating ML techniques to identify the most prominentactivity descriptors on surface-modulated perovskites willsurely lead to a high-throughput propagation of next-generation perovskite electrocatalysts.

We believe that it is of paramount importance to rationalizethe design of perovskites, and electrocatalytic materials ingeneral, for an effective and feasible integration of such ma-terials as viable industrial electrocatalysts in water splitting.Tuning activity descriptors relevant to the performance ofoxygen-deficient perovskite electro-anodes, coupled with re-cent synthesis techniques for surface-modulated structures,offer potential for further improvement of catalytic activity.Bridging the gap between DFT, ML, and experimental workhas been highlighted due to the immense potential it bringstowards efficient and systematic approach for discovery ofhighly active and stable catalysts.

Acknowledgments This study was made possible by a grant from theQatar National Research Fund under its National Priorities ResearchProgram award number NPRP12S-0131-190024. The paper’s contentsare solely the responsibility of the authors and do not necessarily repre-sent the official views of the Qatar National Research Fund.

Funding Open Access funding provided by the Qatar National Library.

Compliance with ethical standards

Conflict of interest The authors declare that there is no conflict of inter-est involved in this submission.

Open Access This article is licensed under a Creative CommonsAttribution 4.0 International License, which permits use, sharing, adap-tation, distribution and reproduction in any medium or format, as long asyou give appropriate credit to the original author(s) and the source, pro-vide a link to the Creative Commons licence, and indicate if changes weremade. The images or other third party material in this article are includedin the article's Creative Commons licence, unless indicated otherwise in acredit line to the material. If material is not included in the article'sCreative Commons licence and your intended use is not permitted bystatutory regulation or exceeds the permitted use, you will need to obtainpermission directly from the copyright holder. To view a copy of thislicence, visit http://creativecommons.org/licenses/by/4.0/.

References

1. J. Wang, Y. Gao, D. Chen, J. Liu, Z. Zhang, Z. Shao, F. Ciucci,Water splitting with an enhanced bifunctional double perovskite.ACS Catal. 8, 364–371 (2018)

2. Y. Yan, B.Y. Xia, B. Zhao, X. Wang, A review on noble-metal-free bifunctional heterogeneous catalysts for overall electrochem-ical water splitting. J. Mater. Chem. A 4, 17587–17603 (2016)

3. H. Wang, M. Zhou, P. Choudhury, H. Luo, Perovskite oxides asbifunctional oxygen electrocatalysts for oxygen evolution/reduction reactions—a mini review. Appl. Mater. Today 16, 56–71 (2019)

4. J. Turner, G. Sverdrup, M.K. Mann, P.C. Maness, B. Kroposki,M. Ghirardi, R.J. Evans, D. Blake, Renewable hydrogen produc-tion. Int. J. Energy Res. 32, 379–407 (2008)

5. C. Hu, L. Zhang, J. Gong, Recent progress made in the mecha-nism comprehension and design of electrocatalysts for alkalinewater splitting. Energy Environ. Sci. 12, 2620–2645 (2019)

586 emergent mater. (2020) 3:567–590

https://doi.org/

6. Z. Zong, Z. Qian, Z. Tang, Z. Liu, Y. Tian, S. Wang, Hydrogenevolution and oxygen reduction reactions catalyzed by core-shelled Fe@Ru nanoparticles embedded in porous dodecahedroncarbon. J. Alloys Compd. 784, 447–455 (2019)

7. J. Wang, X. Yue, Y. Yang, S. Sirisomboonchai, P. Wang, X. Ma,A. Abudula, G. Guan, Earth-abundant transition-metal-based bi-functional catalysts for overall electrochemical water splitting: areview. J. Alloys Compd. 819, 153346 (2020)

8. P. Ebrahimi, A. Kumar, M. Khraisheh, A review of recent ad-vances in water-gas shift catalysis for hydrogen production.Emergent Mater (2020). https://doi.org/10.1007/s42247-020-00116-y

9. L.M. Gandía, R. Oroz, A. Ursúa, P. Sanchis, P.M. Diéguez,Renewable hydrogen production: performance of an alkaline wa-ter electrolyzer working under emulated wind conditions. EnergyFuel 21, 1699–1706 (2007)

10. P. Sharma, P. Doiphode, O. Bhorade, A. Yengantiwar, Bismuthvanadate thin films for efficient photoelectrochemical water split-ting. Emergent Mater 3, 187–194 (2020)

11. Z. Wu, N. Ambrožová, E. Eftekhari, N. Aravindakshan, W.Wang, Q. Wang, S. Zhang, K. Kočí, Q. Li, Photocatalytic H2generation from aqueous ammonia solution using TiO2nanowires-intercalated reduced graphene oxide composite mem-brane under low power UV light. Emergent Mater 2, 303–311(2019)

12. J. Song, C. Wei, Z.-F. Huang, C. Liu, L. Zeng, X. Wang, Z.J. Xu,A review on fundamentals for designing oxygen evolutionelectrocatalysts. Chem. Soc. Rev. 49, 2196–2214 (2020)

13. Y. Guo, T. Park, J.W. Yi, J. Henzie, J. Kim, Z. Wang, B. Jiang, Y.Bando, Y. Sugahara, J. Tang, Nanoarchitectonics for transition-metal-sulfide-based electrocatalysts for water splitting. Adv.Mater. 31, 1807134 (2019)

14. J. Joo, T. Kim, J. Lee, S.I. Choi, K. Lee, Morphology-controlledmetal sulfides and phosphides for electrochemical water splitting.Adv. Mater. 31, 1806682 (2019)

15. J.O.M. Bockris, T. Otagawa, Mechanism of oxygen evolution onperovskites. J. Phys. Chem. 87, 2960–2971 (1983)

16. X. Li, X. Hao, A. Abudula, G. Guan, Nanostructured catalysts forelectrochemical water splitting: current state and prospects. J.Mater. Chem. A 4, 11973–12000 (2016)

17. F. Song, L. Bai, A. Moysiadou, S. Lee, C. Hu, L. Liardet, X. Hu,Transition metal oxides as electrocatalysts for the oxygen evolu-tion reaction in alkaline solutions: an application-inspired renais-sance. J. Am. Chem. Soc. 140, 7748–7759 (2018)

18. Y. Matsumoto, E. Sato, Electrocatalytic properties of transitionmetal oxides for oxygen evolution reaction. Mater. Chem. Phys.14, 397–426 (1986)

19. E. Fabbri, A. Habereder, K. Waltar, R. Kötz, T.J. Schmidt,Developments and perspectives of oxide-based catalysts for theoxygen evolution reaction. Catalysis Sci. Technol. 4, 3800–3821(2014)

20. M.Q. Yang, J. Wang, H. Wu, G.W. Ho, Noble metal-freenanocatalysts with vacancies for electrochemical water splitting.Small 14, 1703323 (2018)

21. Z.W. Seh, J. Kibsgaard, C.F. Dickens, I. Chorkendorff, J.K.Nørskov, T.F. Jaramillo, Combining theory and experiment inelectrocatalysis: Insights into materials design. Science 355,eaad4998 (2017)

22. M. Escudero-Escribano, A.F. Pedersen, E.A. Paoli, R. Frydendal,D. Friebel, P. Malacrida, J. Rossmeisl, I.E.L. Stephens, I.Chorkendorff, Importance of surface IrOx in stabilizing RuO2for oxygen evolution. J. Phys. Chem. B 122, 947–955 (2018)

23. H. Cao, X. Gong, T. Liu, F. Xiao, X. Lv, J. Zhou, L. Gai,Hierarchical Co-N microballs with heterostructure exhibiting su-perior electrochemical properties for water splitting and reductionof I3−. J. Alloys Compd. 797, 341–347 (2019)

24. J. Bao, X. Zhang, B. Fan, J. Zhang, M. Zhou, W. Yang, X. Hu, H.Wang, B. Pan, Y. Xie, Ultrathin spinel-structured nanosheets richin oxygen deficiencies for enhanced electrocatalytic water oxida-tion. Angew. Chem. Int. Ed. 54, 7399–7404 (2015)

25. S. Malkhandi, B. Yang, A.K. Manohar, A. Manivannan, G.K.S.Prakash, S.R. Narayanan, Electrocatalytic properties of nanocrys-talline calcium-doped lanthanum cobalt oxide for bifunctional ox-ygen electrodes. J. Phys. Chem. Lett. 3, 967–972 (2012)

26. J.B. Goodenough, Electronic and ionic transport properties andother physical aspects of perovskites. Rep. Prog. Phys. 67,1915–1915 (2004) 1993

27. X. Xu, W. Wang, W. Zhou, Z. Shao, Recent advances in novelnanostructuring methods of perovskite electrocatalysts for energy-related applications. Small Methods 2, 1800071–1800071 (2018)

28. I. Hussain, H.P. Tran, J. Jaksik, J. Moore, N. Islam, M.J. Uddin,Functional materials, device architecture, and flexibility of perov-skite solar cell. Emergent Mater 1, 133–154 (2018)

29. O. Malinkiewicz, M. Imaizumi, S.B. Sapkota, T. Ohshima, S. Öz,Radiation effects on the performance of flexible perovskite solarcells for space applications. Emergent Mater 3, 9–14 (2020)

30. C. Sun, R. Hui, J. Roller, Cathode materials for solid oxide fuelcells: a review. J. Solid State Electrochem. 14, 1125–1144 (2010)

31. E. Zhao, Z. Jia, L. Zhao, Y. Xiong, C. Sun, M.E. Brito, Onedimensional La0.8Sr0.2Co0.2Fe0.8O3−δ/Ce0.8Gd0.2O1.9 nano-composite cathodes for intermediate temperature solid oxide fuelcells. J. Power Sources 219, 133–139 (2012)

32. W. Yang