Electrocatalytic hydrogen evolution reaction activitycomparable to platinum exhibited by theNi/Ni(OH)2/graphite electrodeManjeet Chhetria, Salman Sultana, and C. N. R. Raoa,1

aNew Chemistry Unit, International Centre for Materials Science and Sheikh Saqr Laboratory, Jawaharlal Nehru Centre for Advanced Scientific Research,Bangalore 560064, India

Contributed by C. N. R. Rao, July 11, 2017 (sent for review June 9, 2017; reviewed by Michael L. Klein and Richard N. Zare)

Electrochemical dual-pulse plating with sequential galvanostaticand potentiostatic pulses has been used to fabricate an electro-catalytically active Ni/Ni(OH)2/graphite electrode. This electrodedesign strategy to generate the Ni/Ni(OH)2 interface on graphitefrom Ni deposits is promising for electrochemical applications andhas been used by us for hydrogen generation. The synergetic ef-fect of nickel, colloidal nickel hydroxide islands, and the enhancedsurface area of the graphite substrate facilitating HO–H cleavagefollowed by H(ad) recombination, results in the high current den-sity [200 mA/cm2 at an overpotential of 0.3 V comparable to plat-inum (0.44 V)]. The easy method of fabrication of the electrode,which is also inexpensive, prompts us to explore its use in fabri-cation of solar-driven electrolysis.

hydrogen evolution reaction | dual-pulse plating | nickel/nickelhydroxide interface | graphite rod electrode

To render electrochemical generation of H2 from water eco-friendly, we could use electricity from solar photovoltaic

devices. A major limitation would still be the use of Pt as thecatalyst. In the last few years, there has been great interest inreplacing Pt by inexpensive, readily available catalysts. Severalcatalysts have been studied in recent times for the electro-chemical hydrogen evolution reaction (HER) including transi-tion metal-based heterostructures (1–6) and certain metal-freecatalysts (7–11). Of these, Ni-based catalysts such as Ni2P (12,13), NiFeP (14), NiFe layered double hydroxide (15–17), and Ni/NiO/carbon nanotube (18) seem to be more promising for watersplitting. It has been shown recently that activation of a Ni-carbon–based catalyst through the application of an electro-chemical potential results in HER activity comparable to Pt inacidic medium (6). We have been investigating the use of Nialong with Ni(OH)2 as a potential catalyst for the purpose,since Ni(OH)2 clusters with Pt and other transition metals (19–23) generally exhibit good HER activity and Ni itself is onlynext to Pt in activity. With this purpose, we have used the dual-pulse–plating (PP) method (24) to generate the Ni/Ni(OH)2interface embedded in graphene sheets on a graphite electrode.Amazingly, the Ni/Ni(OH)2/graphite electrode prepared by usgives a current density of ∼200 mA/cm2 [at −0.30 V vs. reversiblehydrogen electrode (RHE)] and an overpotential of ∼190 mV re-quired to sustain a current density of 20 mA/cm2 over long periods.For a current density of 200 mA/cm2, this electrode beats the ac-tivity of the Pt wire by a factor of ∼1.5 in terms of the overpotential.The outstanding performance of the Ni/Ni(OH)2/graphite

electrode is due to the dual-PP method adopted by us to giverise to colloidal hydroxide inclusions in the electrodeposits (25–30) (Methods). While fabricating the Ni/Ni(OH)2 interface, thegalvanostatic pulses shift the cathodic potential in the negativedirection to such an extent that the ensuing water splittingyielding hydrogen is followed by the simultaneous incorporationof colloidal nickel hydroxide (Movie S1 and Fig. S1) (3). Thus,the use of dual PP affords in obtaining Ni and Ni(OH)2 from thesequential galvanostatic and potentiostatic conditions. While

only Ni deposition is expected in the potentiostatic pulse, bothNi and Ni(OH)2 get deposited in the galvanostatic pulse due tovariable potentials. The reduction of hydronium ions at thecatholyte leads to the codeposition of Ni(OH)2 along with Ni.The origin of Ni(OH)2 generation has been explained earlier(25, 29, 31). Compared with direct current deposition (DP),cycling potentiostatic and galvanostatic pulses in PP changesthe morphology of nickel deposits from pure nickel to layeredNi/colloidal Ni(OH)2 deposits on the graphite substrate to agreater extent (Fig. S1 A and B and Methods). The fresh de-posits of Ni on graphite referred to as “fresh Ni-Gr” get con-verted into “active Ni-Gr” during electrochemical HER in linearsweep voltammetry (LSV). Fresh Ni-Gr consists of walnut-shaped particles distributed throughout the electrode surfaceas observed in field emission scanning electron microscopy(FESEM) image (Fig. 1A), and this morphology is lost afteractivation of the electrode at HER-4. The active Ni-Gr elec-trode surface consists of Ni/Ni(OH)2 embedded in a sea ofgraphite sheets (Fig. 1B and Fig. S1C) as confirmed by energy-dispersive X-ray analysis (EDAX) (Inset of Fig. 1 A and B),high-resolution TEM (HRTEM) images, inductively coupledplasma spectrometry–optical emission spectrometry (ICP-OES)analysis, and the dimethylglyoxime (DMG) test. HRTEM im-ages (Fig. 1 C and D) of fresh and active electrodes revealthe presence of Ni/Ni(OH)2 interfaces throughout the catalyst.An analysis of the composition and morphology is providedin the supporting information (Figs. S2–S4). X-ray photo-electron spectroscopy (XPS) substantiates the generation ofcolloidal Ni(OH)2 embedded within the fresh Ni deposit andgraphene sheets.

Significance

The successful utilization of solar energy to economically pro-duce green fuel should involve facile and inexpensive meansfor electrolysis of water. To do so, it is necessary to replace theplatinum catalyst with an in situ electrode fabrication processinvolving active catalyst with readily available materials. Wehave been successful in synthesizing an inexpensive Ni/Ni(OH)2/graphite electrode whose performance is as good as Pt. By asuitable choice of the relative proportion of Ni and Ni(OH)2, weobtain high current density at low overpotentials. The sequentialgalvanostatic and potentiostatic pulses used for the electrode-position of Ni on the graphite rod provide control over themorphology and composition and the improved electrochemicalperformance.

Author contributions: M.C., S.S., and C.N.R.R. designed research; M.C. performed research;M.C., S.S., and C.N.R.R. analyzed data; and M.C. and C.N.R.R. wrote the paper.

Reviewers: M.L.K., Temple University; and R.N.Z., Stanford University.

The authors declare no conflict of interest.1To whom correspondence should be addressed. Email: cnrrao@jncasr.ac.in.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1710443114/-/DCSupplemental.

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We have studied the catalytic activity of the Ni/Ni(OH)2/graphite electrode by means of LSV plots (Fig. 2A) in com-parison with a Pt wire. The overpotentials required for

obtaining current densities of 100 and 200 mA/cm2 are 270 and299 mV for active Ni-Gr, while for Pt it was 243 and 442 mV,respectively (Fig. 2B), making active Ni-Gr a competitive, ready-to-use electrode in commercial electrolyzers. The tafel slopes areclose to 116 mV/dec (Fig. S5A), suggesting adsorption of hydro-nium ions to be the rate-determining step (32). Due to the burst ofgas bubbles (Fig. 2C and Movie S2), the LSV curve is noisy and wehave not attempted to smoothen the plot. To check the efficacyof our method of electrode fabrication, a comparative studyof the HER activity was carried out with an electrode fabricatedthrough conventional DP of Ni. The results showed much highercurrent density (12 times increment at 300 mV) with the electrodeprepared by the PP method (Fig. S5B). This significant differenceis ascribed to the absence of the active Ni/Ni(OH)2 interfaceembedded within the graphene sheets in DP-fabricated Ni-Gr.Presence of the polished graphite electrode is crucial for thesuccessful fabrication of active Ni-Gr electrode as confirmed bycontrolled experiments using a conducting carbon fiber (Fig. S6).We propose that, during HER in acidic medium, dissolution of thenickel deposit gives rise to or expose Ni/Ni(OH)2 interfaces, whichthen catalyze hydrogen evolution. Formation of Ni(OH)2 gets en-hanced with successive HER tests due to electrogeneration of thebase in the catholyte by the reduction of hydrogen ions.We have examined the stability of active Ni-Gr by cyclic

voltammetry (Fig. 2D), as well as chronopotentiometric (Fig.3A) and chronoamperometric studies (Fig. S5C). An activityretention of ∼96% was observed up to 200 cyclic voltammetry(CV) cycles between −0.18 and −0.28 V at a scan rate of5 mV/s (Fig. 2D). Active Ni-Gr can sustain a current densityof 20 mA/cm2 for 24 h requiring overpotential of only 190 mV(Fig. 3A). Since the adsorption of hydronium ions is the rate-determining step, we performed electrochemical impedancespectroscopic studies at onset potential to estimate the re-sistance involved in charge transfer (Rct) between the elec-trode and the electrolyte. Fig. 3B shows the Nyquist plot for

Fig. 1. Morphology study of the electrode. (A) FESEM image of Ni-deposited graphite electrodes. Inset shows the magnified image and EDAXdata for the deposit. (B) The FESEM images of active Ni-Gr electrode. TheInset shows cross-sectional view of the electrode surface. The elementalcomposition is also depicted. The active electrode consists of Ni/Ni(OH)2embedded in the graphite sheets. C and D are HRTEM images of active Ni-Grelectrode illustrating Ni, Ni(OH)2, and graphite interfaces.

A B

C D

Fig. 2. Electrochemical hydrogen evolution activity test. (A) Hydrogen evolution activity (HER) tests of the electrode by linear sweep voltammetry (LSV). HER-1to HER-4 represents successive LSV runs. (B) The comparison of HER activity of Pt wire (0.5-mm diameter × 30.25-mm length; CH Instruments, Pt counterelectrode CHI 115) and active Ni-Gr. (C) Photograph of hydrogen evolution (bubbles) during the HER tests through LSV. (D) Stability test of “active Ni-Grelectrode” before and after 200 CV cycles between −0.18 and −0.28 V vs. RHE at a scan rate of 5 mV/s. For HER testing, 0.5 M HCl was the electrolyte, and LSVwas run at a scan rate of 5 mV/s. Pt wire and Ag/AgCl were used as counter and reference electrodes, respectively. The potentials were reported with respectto RHE according to following equation: E(RHE) = E (vs. Ag/AgCl) + E°(Ag/AgCl) + 0.059*pH.

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the Ni-Gr and the equivalent circuit used to fit the data in theInset, giving the value of Rct as 58 Ω.XPS studies show a drastic change occurs in relative pro-

portion of Ni and Ni(OH)2 on the active Ni-Gr electrode sur-face during successive LSV runs in HER tests. We see a greaterfraction of Ni(OH)2 on the surface of active Ni-Gr electrode(Fig. 4) as corroborated by EDAX analysis (Inset of Fig. 1 Aand B). X-ray absorption near-edge structure (XANES) regionof the XAS spectra at the Ni-K edge shows a gradual evolutionof Ni(OH)2 with increasing HER cycles (Fig. 5A). The co-ordination environment of Ni changes as evident from the in-tensity of the white line. The relative composition of Ni andNi(OH)2 on the electrodes estimated by a linear combination fit(LCF) method (Fig. 5 B–D) gives the optimum ratio to be14.3:64.3 at the Ni/Ni(OH)2 interface responsible for the burst ofhydrogen evolution activity observed in our experiment (Table 1).Atomic force microscopy (AFM)-assisted topographic analysis

of active Ni-Gr and fresh Ni-Gr electrodes provide insight toHER activity (Fig. 6). The 3D topography of these electrodes iscontrastingly different in terms of surface roughness (Fig. 6 Cand D, and Figs. S7 and S8). Spikes and corrugations on the activeNi-Gr electrode surface give rise to excess surface area of theactivated graphite surface. The sharp edges favor the increasedelectronic charge density during LSV, thus aiding the high activity.

ConclusionsIn summary, we have discovered that nanoscale Ni/Ni(OH)2/graphite is an outstanding catalyst with high HER activitycomparable to that of Pt exhibiting high current density over arange of overpotentials. The design and electrode fabricationstrategy to generate the highly catalytic Ni/Ni(OH)2 interfaceon graphite from Ni deposits (Fig. S9, the coaction effect ofnickel, colloidal nickel hydroxide islands, and the enhancedsurface area of the graphite substrate facilitating HO–H cleavage

followed by H(ad) recombination, results in the high currentdensity) used by us are unique and unprecedented. To the bestof our knowledge, dual-PP synthesis has not been studied forimproving the HER activity on graphite. There is a need for anoptimum ratio between Ni and Ni(OH)2 for high activity. Insitu growth of the catalyst on the graphite surface eliminatescumbersome electrode fabrication procedures. The catalyst canadvantageously be reused, thus making the process economi-cal. The current density of ∼200 mA/cm2 (at −0.3 V vs. RHE)and a retention of activity (overpotential of ∼190 mV requiredto sustain a current density of 20 mA/cm2) for long term (24 h) isnoteworthy. In comparison with Pt wire electrode, Ni/Ni(OH)2/graphite requires 144 mV less overpotential to produce a currentdensity of 200 mA/cm2. Based on the results of the present study,

B

A

Rsol W

Rct

Cdl

Fig. 3. Electrochemical properties: (A) Chronopotentiometric V-t graphshowing the overpotential required to sustain a current density of 20 mA/cm2

over long time. (B) Nyquist plot of active Ni-Gr at onset potential and theequivalent circuit.

A

B

Fig. 4. Analysis of oxidation state by XPS. A and B are the Ni-2p corelevel XPS plots for fresh Ni-Gr and the active Ni-Gr electrodes, re-spectively. The peaks at 853.01, 855.77, and 862.07 eV can be assignedto 3p3/2 and peaks at 870.47, 873.61, and 881.23 eV to 3p1/2 for Ni,Ni(OH)2, and Ni(OH)2 satellite peaks, respectively. The XPS analysis hintsto two inferences: First, generation of colloidal Ni(OH)2 embeddedwithin the fresh Ni deposit and graphene sheets during the synthesis,and second, drastic change in relative proportion of Ni and Ni(OH)2 onthe active electrode surface during HER tests. We see greater fraction ofNi(OH)2 on the surface of active Ni-Gr electrode, also corroborated byEDAX analysis (Inset of Fig. 1 A and B) and XANES analysis.

Table 1. LCF data parameters of fresh and active Ni-Gr

Sample Ni metal, % Ni(OH)2, % R factor χ2 Reduced χ2

Fresh Ni-Gr 82.9 17.1 0.0011842 0.0324 0.000040HER-1 75.5 24.5 0.0006258 0.0170 0.000022HER-2 73.1 26.9 0.0006483 0.0185 0.000023HER-3 59.2 40.8 0.0022793 0.0615 0.000083HER-4* 14.3 64.3 0.007684 0.3161 0.000398

*The percentage sum is not equal to 100 since 21.3% is contributed by Ni2+

in aqueous solution. This is similar to Ni(OH)2 environment with one hydro-gen less. Hence the presence of Ni2+ surrounded by OH− ion is inferred. Theexact coordination number and nature of coordination shell are beyond thescope of discussion for this study.

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it would be desirable to examine the fabrication of acid/alkalineprototype electrolyzers operating at a low voltage and combiningthem with solar driven electrolyzers. It must be noted that PP is

not only a reproducible, inexpensive, and easy method, but it alsoavoids the pitfalls of the drop-casting method of electrodefabrication technique.

curve curve

A B

C D

Fig. 5. Analysis of oxidation state and relative proportion of Ni and Ni(OH)2. (A) Normalized XAFS spectra of Ni-K edge of the electrodes as a function of successiveLSV runs depicted as HER numbers showing the evolution of Ni(OH)2/Ni interface. (B–D) The linear combination fitted normalized XAFS analysis of the electrodes todetermine the percentage of Ni metal and Ni(OH)2 with increasing HER numbers. All probable Ni species viz. Ni foil, NiO, Ni(OH)2, and Ni2+ surrounded by watermolecules were taken as reference material for fitting purpose. The collected data at Ni-K edge for all these samples along with the electrodes were then analyzed byLCFmethod. Fittingwas done in Athena software. The fitting rangewas selected up tomaximum limit possible, that is, from−20 to 200 eV. In all cases, it was observedthat the principal contributions are from Ni foil and Ni(OH)2 species, and hence NiO and “Ni2+ in water” data were not included.

Fig. 6. Surface roughness and topographic analysis: atomic force microscopy (AFM) image of (A) fresh Ni-Gr surface, (B) active Ni-Gr electrode surface. TheInset shows the height profile of the electrode surface along white line in the AFM image. Three-dimensional topographic AFM images (20 × 20 μm2 in size) offresh Ni-Gr and active Ni-Gr electrodes are given in C and D. The increased surface area and the induced surface mesoporosity (by H2 evolution as a function ofHER numbers) along with difference in the population of catalytically active interface are evident from the 3D images.

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MethodsThe essentials of dual PP are as follows. The ends of 5-cm graphite rodswere cut into square shape. One side was polished with different gradesof silicon carbide paper and sequentially cleaned/degreased by soni-cating in water and acetone. Leaving an area of 0.6 cm2, the rest ofgraphite rod was electrically insulated (Fig. S1). The 0.3 M nickel acetatesolution in 5 mol% N-methylformamide–water mixture was used asan electrolyte for Ni deposition using Ni strip as counter electrode,Ag/AgCl as the reference, and the polished/cleaned graphite rod asworking electrode. Nickel electrodeposition was done through succes-sive potentiostatic/galvanostatic pulses (P/G PP with CHI760E). Forpotentiostatic pulse, voltage ranged between −0.9 and −1.2 V withrespective hold times as 10 and 100 s while in the galvanostatic pulse

current densities were fixed at 34 and 8.4 mA/cm2. Five segments con-

sisting of P-G-P-G-P were used during electrodeposition (Fig. S1 andMovie S1). The activation of Ni-Gr electrode was observed in successiveLSV cycles during hydrogen evolution activity test in 0.5 M HCl with theappearance of fresh Ni-Gr turning black (Fig. S4C).

ACKNOWLEDGMENTS.We thank the Jawaharlal Nehru Centre for AdvancedScientific Research for managing the X-ray absorption fine-structure mea-surement. We acknowledge Synchrotron SOLEIL for provision of synchrotronradiation facilities at beamline SAMBA (Proposal 20160052). M.C. thanks theUniversity Grants Commission of India for a fellowship. We thank theDepartment of Science and Technology, India (SR/NM/Z-07/2015), forfinancial support.

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