Low-Voltage Electrolytic Hydrogen Production Derived from EfficientWater and Ethanol Oxidation on Fluorine-Modified FeOOH AnodeGao-Feng Chen, Yaru Luo, Liang-Xin Ding,* and Haihui Wang*

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China

*S Supporting Information

ABSTRACT: Highly active, earth-abundant anode catalysts are urgently requiredfor the development of electrolytic devices for hydrogen generation. However, thereaction efficiencies of most developed electrocatalysts have been intrinsicallylimited due to their insufficient adsorption of reactants leading to high energyintermediates. Here, we establish that electronegative fluorine can moderate thebinding energy between the Fe sites (FeOOH) and reactants (OH− or C2H5O

−),resulting in more optimized adsorption, and can enhance the positive chargedensities on the Fe sites to facilitate oxygen evolution reaction (OER) and ethanoloxidation. Consequently, a low electrolytic voltage (1.43 V to achieve 10 mAcm−2) for H2 production was obtained by integrating the efficiently anodicbehaviors of OER and ethanol oxidation. The results reported herein point to fluorine moderation as a promising pathway fordeveloping optimal electrocatalysts and contribute to ongoing efforts of mimicking water splitting.

KEYWORDS: FeOOH, F modification, oxygen evolution reaction, ethanol oxidation, H2 production

Alkaline water electrolysis for hydrogen production, as aclean and sustainable energy technology, will be certain to

play a pivotal role in the future hydrogen economy.1 One majorroadblock for alkaline water electrolysis is the sluggish kineticsof the oxygen evolution reaction (OER), a four-electronoxidation process (4OH− ↔ O2 + 2H2O + 4e−), at the anode,which requires a high overpotential to achieve a considerablecatalytic current density.2 Over the decades, extensive studieshave been stimulated to pursue efficient catalysts (such asRuO2, IrO2, metal oxides, sulfides, phosphides, phosphates, andperovskites) to decrease the anodic overpotential.3−5 Inaddition to some progress, these studies concluded that thesluggish OER kinetics are ascribed to the unfavorable formationenergies of the reactants in reaction sites.6

FeOOH was demonstrated as the most active OERelectrocatalyst among single first-row transition metals.7 Itsutilization as an OER catalyst is typically hindered by its poorOER kinetics caused by the too strong adsorption of reactants.According to the Sabatier principle/volcano relation, the mostactive electrocatalyst should be one with surface−reactantinteractions that are neither too strong nor too weak.8,9

Therefore, to optimize the binding energies, recent studies haveincorporated FeOOH into Ni/Co-based materials that wereproved to show too weak adsorption for OER reactants.10,11

For example, it was verified that Fe plays an important role inthe optimization of multicomponent catalysis, and an activitytrend of Ni(Fe)OxHy > Co(Fe)OxHy > FeOxHy > CoOxHy >NiOxHy suggested that the moderate adsorption for inter-mediates led to more catalytically active sites for OER.12

Despite these achievements, a question remains: what is the keyactive site in OER catalysis? In other words, does theincorporation of Fe cations into NiOOH/CoOOH enhance

the OER activity of Ni/Co, Fe, or both, or does theincorporation of Ni/Co cations into FeOOH enhance theactivity of Fe, Ni/Co, or both?13,14 Therefore, the developmentof another approach to optimal adsorption energies forreactants at the Fe site would be significant for the furtherutilization of FeOOH in OER catalysis and should also provideclear guidance for the design of more efficient catalysts.Fluorine (F) has the strongest electronegativity among all

elements. Its doping may cause variation of adsorption ofreactants on FeOOH to improve the electrochemical perform-ance. In this work, we focus on the influence of F incorporationonto the FeOOH to alter the adsorption energies of OERreactants and the electrocatalytic activities. A comparative studyof three Fe-site OER catalysts in alkaline media were performedby combining detailed electrochemical evaluation and densityfunctional theory calculations. (Pure β-FeOOH, F-modified β-FeOOH, and Fe1.9F4.75·0.95H2O were grown on carbon clothdirectly as three Fe-sites anodes. β-FeOOH was chosen asexample study for F-modified effect in this work, because itssynthetic procedure is simple and energy-efficient. Fe1.9F4.75·0.95H2O as a complete fluoride material was also used forcomparison; however, the study was limited by its poorstability.) For affirming the synthesis of the three Fe-siteselectrodes, scanning electron microscopy (SEM), transmissionelectron microscopy (TEM), powder X-ray diffraction(PXRD), X-ray photoelectron spectroscopy (XPS), and energydispersive spectroscopy (EDS) measurements were employedfor characterization (detailed descriptions in the Supporting

Received: September 27, 2017Revised: November 19, 2017Published: December 8, 2017

Letter

pubs.acs.org/acscatalysisCite This: ACS Catal. 2018, 8, 526−530

© 2017 American Chemical Society 526 DOI: 10.1021/acscatal.7b03319ACS Catal. 2018, 8, 526−530

Information, Figures S1−5, Tables S1 and S2). For example,Figure 1a shows that the β-FeOOH is grown on carbon cloth

fibers in a nanorod morphology. The TEM images of β-FeOOH (Figure 1b) show that the nanorods are approximately60 nm in diameter and 200 nm in length. Its high-resolutionTEM image shows clear lattice fringes corresponding to the(110) planes of the crystalline β-FeOOH. For the F-modified

β-FeOOH, its surface morphology (Figure 1c,d) shows littlechange after surface modification with F. The TEM images ofF-modified β-FeOOH (Figure 1e) confirm that the entiresurface of the β-FeOOH nanorod is wrapped in thin F modifiedlayers. From a more nuanced view, the presence of abundantsurface defects in the lattice fringes confirms the F-modifiedeffects. The high-resolution TEM (inset of Figure 1e), STEM-EDS linear scan (Figure S6a-b), and point scans at the centerand side of nanorod (Figure S6c-d) reveal that the wrappedlayer has a low-crystallization structure consisting of Fe, O, andF elements. On the basis of the elemental mapping images(Figure 1f, S6e), F is certainly verified on the β-FeOOHsurface.The influence of F incorporation into FeOOH with electron

interactions was validated by XPS in the Fe 2p and O 1sregions. As shown in Figure 2a, the peak at 712.2 eVcorresponds to Fe 2p3/2, and the peak at 726.3 eV correspondsto Fe 2p1/2.15 Compared with the peaks for β-FeOOH, the Fe2p3/2 and 2p1/2 peaks of F-modified β-FeOOH shift to higherbinding energies by 0.8 and 1.3 eV, respectively. The O 1sspectra of β-FeOOH and F-modified β-FeOOH are shown inFigure 2b. Compared with the peak for β-FeOOH, the O 1speak assigning to Fe−O of F modified β-FeOOH positivelyshifts by 0.2 eV. Therefore, the changed binding energies basedon the XPS results can be attributed to strong electroninteractions between FeOOH and F in F-modified β-FeOOH.The above results were also confirmed by the XPS spectra ofFe1.9F4.75·0.95H2O (Figure 2c,d). Upon fluorination, two Fe 2ppeaks features appeared at higher binding energies (713.9 and727.3 eV) compared with those of typical Fe3+ oxides.16

In F-modified β-FeOOH, the electronic state of the Fe siteshould be altered by the strong electronic interactions betweenFeOOH and F, which was further studied by density functionaltheory (DFT) calculations. The optimized structural surfaces ofβ-FeOOH(110), FeOOH(110)+OH−, FeOOH(110)+-C2H5O

−, F-modified β-FeOOH(110), F-modified β-FeOO-H(110)+OH−, F-modified β-FeOOH(110)+C2H5O

− are pre-

Figure 1. (a) SEM and (b) TEM images of β-FeOOH. (c,d) SEM, (e)TEM, (f) elemental mapping images of F-modified β-FeOOH.

Figure 2. XPS spectra: (a) Fe 2p core-level spectra of F-modified β-FeOOH and pure β-FeOOH, (b) O 1s core-level spectra of F-modified β-FeOOH and pure β-FeOOH, (c) Fe 2p core-level spectrum of Fe1.9F4.75·0.95H2O, (d) F 1s core-level spectrum of Fe1.9F4.75·0.95H2O. (e) Chargedensity difference plots (red contour corresponds to charge accumulation and blue contour corresponds to charge depletion; red, white, gray, andbaby blue spheres represent the O, H, Fe, and F atoms, respectively).

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sented in Figure S7a−f. The theoretical calculations clearlyshow the Hirshfeld charge distributions on the Fe and O atomsin these optimized six species (Table 1), which clearly identify

that the Fe atoms carry more positive charge in the F-modifiedβ-FeOOH compared with their charge values in β-FeOOH. Inthese cases, the Fe atoms will become highly oxidized by OH−

and even by C2H5O−, which can be verified by the more

positive charge of the Fe atoms in F-modified β-FeOO-H(110)+OH− and F-modified β-FeOOH(110)+C2H5O

−.DFT calculations were also carried out to study the

adsorption energies of the reactants on the Fe sites under therole of F moderation. The optimal adsorption energies towardthe reactants will play a crucial role for the electrocatalyticactivity.17 This can be visually observed in the charge densitydifference plots after reactant adsorption on the surfaces of β-FeOOH and F-modified β-FeOOH. From Figure 2e, uponOH− adsorption on F-modified β-FeOOH instead of pure β-FeOOH, less charge is accumulated in the OH− or C2H5O

−

molecules, while less charge density is depleted on the Featoms.18 More specifically, the Eabs value of OH− or C2H5O

−

on pure β-FeOOH is high (−3.84 or −4.71 eV, respectively),which indicates the strong adsorption of OH− or C2H5O

− onpure β-FeOOH. Compared with pure β-FeOOH, the Eabs valueof OH− or C2H5O

− on F-modified β-FeOOH is relatively low(−2.33 or −3.42 eV, respectively). It means that F modificationcan weaken adsorption of OH− or C2H5O

− on β-FeOOH thatshould lead to moderate OH− or C2H5O

− adsorption on thesurface of β-FeOOH, similar to the previous methods by theincorporation of Ni/Co cations into FeOOH.11 Therefore, theFe atoms will become highly oxidized by OH− or C2H5O

−, andthe adsorption of OH− or C2H5O

− will be optimized on the Fesites in F-modified β-FeOOH. Accordingly, the Fe sites after Fmoderation are expected to benefit the anodic reactions.To study the above effects, we conducted systematic

electrochemical experiments. First, the cyclic voltammetry(CV) behavior of the F-modified β-FeOOH is different fromthat of pure β-FeOOH, despite the little difference of theelectrochemical surface areas between them (Figure S8).Compared with that of pure β-FeOOH, a negative shift ofthe oxidation wave (by 18 mV) for F-modified β-FeOOH isobserved (Figure 3a), which is close to the oxidation potentialof Fe1.9F4.75·0.95H2O. The oxidation wave shift to lowerpotential suggests that Fe3+ is more easily oxidized to Fe4+,agreeing with the results of the DFT calculations. As waspreviously reported, the presence of Fe4+ in FeOOH plays a keyrole in achieving high catalytic activity.19 Predictably, the lowerFe3+/4+ redox potential for the Fe-site catalysts can lead to alower overpotential for the subsequent anodic reactions. Asshown in the polarization curves (Figure 3b), a much lower

OER potential (1.59 V vs reversible hydrogen electrode(RHE)) at a current density of 10 mA cm−2 were achievedwith F-modified β-FeOOH compared with pure β-FeOOH(1.64 V vs RHE) for OER, which compares favorably with thatof the most of recently reported noble and non-noble metalbased catalysts, i.e., RuO2 (1.58 V vs RHE),20 IrO2 (1.63 V vsRHE),20 CoFeOx (1.59 V vs RHE)21 (Table S3). Additionally,F-modified β-FeOOH shows a Tafel slope of 74.4 mV dec−1

(Figure 3c), which is smaller than that of pure β-FeOOH (81.5mV dec−1). In addition, the reaction kinetics is a crucial qualityof a good electrocatalyst. The Nyquist plot of F-modified β-FeOOH (Figure 3d) shows a much smaller semicircle diameter(corresponding to the charge-transfer resistance, R3) than thatof pure β-FeOOH, demonstrating the faster OER kinetics of F-modified β-FeOOH compared with that of pure β-FeOOH.The above results confirm that β-FeOOH after F modificationcan facilitate Fe4+ formation and accelerate OER kinetics with amoderate adsorption energy for OH−, which enhance theelectrocatalytic OER activity. These effects were also confirmedby the polarization curve, Tafel slope, and Nyquist plot of thecomplete fluoride compound (Fe1.9F4.75·0.95H2O), whichshows a lower onset potential, smaller Tafel slope, and smallercharge-transfer resistance than those of F-modified β-FeOOHand pure β-FeOOH. Moreover, though F-modified β-FeOOHshows inferior stability for OER than pure β-FeOOH, F-modified β-FeOOH still requires a lower potential (<1.63 V vsRHE) to drive 10 mA cm−2 than that of pure β-FeOOH (>1.65V vs RHE) in the whole process of stability test (Figure S9).Recently, a thermodynamically more favorable reaction was

employed to replace OER at the anode for electrolytichydrogen production by integrating the oxidation of biomass.22

Here, we chose ethanol (EtOH) oxidation as an example study;ethanol solvent is expected to stabilize the F-based anodes.According to the polarization curves collected using variousvolume ratios of EtOH to water, a ratio of 15:5 was determinedto be the optimum proportion (Figure 4a). Figure 4b comparesthe electrocatalytic EtOH oxidation performances of pure β-FeOOH, F-modified β-FeOOH, and Fe1.9F4.75·0.95H2O in theoptimum EtOH proportion solution. Pure β-FeOOH showslower activity, requiring a potential of 1.236 V (vs RHE) todrive 10 mA cm−2. Remarkably, F-modified β-FeOOH andFe1.9F4.75·0.95H2O show enhanced activities and demandpotentials of 1.207 and 1.195 V (vs RHE), respectively, to

Table 1. Comparison of the Hirshfeld Charge andAdsorption Energy (Eabs) Values

Hirshfeld charge

components Fe O FEabs(eV)

β-FeOOH (1 1 0) 0.23 −0.20 -- --β-FeOOH (1 1 0)+OH− 0.30 −0.22 -- −3.84β-FeOOH (1 1 0)+C2H5O

− 0.34 −0.23 -- −4.72F-modified β-FeOOH (1 1 0) 0.28 −0.20 −0.17 --F-modified β-FeOOH(1 1 0)+OH− 0.41 −0.22 −0.15 −2.33F-modified β-FeOOH(1 1 0)+C2H5O

− 0.37 −0.23 −0.15 −3.42

Figure 3. (a) CV curves, (b) polarization curves, (c) Tafel plots, and(d) Nyquist plots of pure β-FeOOH, F-modified β-FeOOH andFe1.9F4.75·0.95H2O.

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drive the same current density. Their Tafel plots were alsostudied to compare the EtOH oxidation kinetics (Figure 4c).The Tafel slope of 30.1 mV dec−1 for F-modified β-FeOOH ismuch lower than that of pure β-FeOOH (44.6 mV dec−1),suggesting its favorable catalytic kinetics for EtOH oxidation.This result also corroborates that β-FeOOH after Fmodification with an moderate adsorption energy of C2H5O

−

can facilitate the EtOH oxidation kinetics. Based on the13C{1H} nuclear magnetic resonance (NMR) spectroscopy(Figure S10), acetic acid is confirmed as the main oxidationproduct, and unsaturated fatty acid and ethyl acetate arebyproducts. Further DFT calculations (Figure S11) show thatthe Eabs value of C2H4O (possible intermediate) or CH3COO

−

(product) on F-modified β-FeOOH is lower (−3.72 or −4.52eV, respectively) than that of pure β-FeOOH (−4.02 or −5.69eV, respectively), which indicate F modification can alsoweaken adsorption of intermediate or CH3COO

− on β-FeOOH facilitating products desorption to proceed nextreaction.Durability tests were carried out to access practical

application. Figure 4d reveals the current density dependenceof the electrolysis performance. During the process, cyclingsteps of the current densities were changed successively from10 mA cm−2 to 30 mA cm−2 and then to 10 mA cm−2. F-modified β-FeOOH shows excellent stability with no potentialchange after 12 h of electrolysis, and Fe1.9F4.75·0.95H2O showsinferior stability with the potential increased by 41 mV after 12h of electrolysis. However, the stability of two F-basedelectrodes are better than that of pure β-FeOOH. The

stabilization effect of EtOH and the optimized adsorptionenergy imparted efficient diffusion and transfer of the reactantson the surfaces of the catalysts, which could be responsible fortheir acceptable stabilities. This is confirmed by the SEMimages of the three electrodes (Figures S12, S13a, and S14)after the stability measurements, in which F-modified β-FeOOH and Fe1.9F4.75·0.95H2O show well-retained originalmorphologies, while pure β-FeOOH shows a damagedmorphology. The stabilization effect of EtOH was speciallyverified by the retained structure of Fe1.9F4.75·0.95H2O on thebasis of the XRD and TEM results (Figure S13b−d) after thedurability test. Moreover, the Nyquist plots by EIS experiments(Figures S15−S17) show slightly increased charge-transferresistances on F-modified β-FeOOH and Fe1.9F4.75·0.95H2Othan that of pure β-FeOOH after the 12 h stability tests,indicating the retentive charge-transfer kinetics of the F-basedelectrodes.Electrochemically catalyzed HER on a Pt−C catalyst cathode

was assessed for integrating ethanol oxidation on the F-modified β-FeOOH anode. The impact of ethanol on the HERactivity of the Pt−C catalyst cathode was evaluated undervarious volume ratios of EtOH to water. According to thepolarization curves (Figure 4e) and the optimum proportionfor the F-modified β-FeOOH anode, 15:5 is an acceptableproportion showing limited impact on the Pt−C catalystcathode. Based on the above study, a two-electrodeconfiguration using F-modified β-FeOOH as the anode and aPt−C or Ni2P catalyst cathode were assembled to achieve H2generation in 1.0 M KOH with a 15:5 volume ratio of EtOH towater as an electrolyte. Remarkably, catalytic currents of 10 mAcm−2 were observed when the applied voltages were 1.43 V forPt−C//F-modified β-FeOOH (Figure 4f) and 1.46 V forNi2P//F-modified β-FeOOH (Figure S18), respectively. Thisvalue compares favorably with the behavior for the recentlyreported advanced electrolyzers integrating biomass oxidationbased on porous Ni3S2/Ni foam bifunctional electrocatalyst(1.46 V)23 and 3D Ni2P/Ni foam bifunctional electrocatalyst(1.44 V).24 For comparison, overall water splitting tests werealso conducted in the absence of EtOH. The cells requirevoltages of 1.51 V for Pt−C//F-modified β-FeOOH and 1.55 Vfor Ni2P//F-modified β-FeOOH, respectively, to achieve 10mA cm−2, which are higher than that of the cell systems withEtOH oxidation but lower than or comparable to those ofrecently reported electrolyzers based on nonprecious electro-catalysts, that is, NiFe LDH (1.70 V),25 Ni5P4 (1.70 V),

26 NiSenanowires (1.63 V),27 Ni8P3 (1.61 V),

28 and MoS2/Ni3S2 (1.56V)29 (Table S4). To evaluate the Faradaic efficiencies for bothH2 evolution and O2 evolution/EtOH oxidation, gaschromatography (Figure S19) is used to measure the amountof H2 and O2 and NMR (Figure S20) is used to measure aceticacid. For the system in 1.0 M KOH solution without EtOH, theFaradaic efficiencies were calculated as 93.10% for HER and96.53% for OER (Figure S21a, Tables S5 and S6). For thesystem in 1.0 M KOH solution with ethanol, O2 cannot bedetected in the whole process (Figure S21b). The Faradaicefficiencies were calculated as 91.66% for HER and 72.78% foracetic acid (Table S7 and Figure S22).For further proof, γ-FeOOH and F-modified γ-FeOOH were

also prepared for comparison. The characterization analysis bySEM, TEM, PXRD, XPS, and EDS measurements wereemployed to confirm their morphologies, structures, andchemical compositions (Figures S23−26, Table S8). Accordingto the electrochemical characterizations (Figures S27−34), F-

Figure 4. (a) Polarization curves for EtOH oxidation collected usingvarious volume ratios of EtOH to water as an electrolyte. (b)Polarization curves, (c) Tafel plots, and (d) chronoamperometrymeasurements of pure β-FeOOH, F-modified β-FeOOH, andFe1.9F4.75·0.95H2O collected using a 15:5 volume ratio of EtOH towater as an electrolyte. (e) Polarization curves of Pt−C for HERcatalysis collected using various volume ratios of EtOH to water. (f)Current−potential curves for Pt−C//F-modified β-FeOOH in a two-electrode setup collected using pure water and a 15:5 volume ratio ofEtOH to water as electrolytes.

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modified γ-FeOOH shows superior OER and EtOH oxidationcatalytic capabilities than those of pure γ-FeOOH, similar to thecase of F-modified β-FeOOH and pure β-FeOOH. Notably, theCV curve of F-modified γ-FeOOH displays a negative shift ofthe oxidation wave compared with that of pure γ-FeOOH; thepolarization curves of F-modified γ-FeOOH show a muchlower onset potential and much higher current density thanthose of pure γ-FeOOH for OER and EtOH oxidation catalysis.Therefore, the excellent catalytic performance of F-modified γ-FeOOH originates from the effects of F moderation, whichfacilitates Fe4+ formation and accelerates the reaction kineticswith moderate adsorption energy of OH− or C2H5O

−, whichare favorable for highly efficient OER or EtOH oxidation,respectively. Moreover, it is interesting that both OER andEtOH oxidation performances and stabilities of F-modified γ-FeOOH are inferior to those of F-modified β-FeOOH. Itsuggests that fluorination is more suitable for β-FeOOH toachieve better performance and the different structures of Fe-based catalysts may significantly influence the OER or EtOHoxidation performances of corresponding F-modified materials.In summary, we successfully demonstrated a F-modified

FeOOH anode with enhanced electrocatalytic performance forOER or EtOH oxidation. DFT calculations and electrochemicalcharacterizations verified that F can result in a more optimizedadsorption energy between the Fe sites and OH− or C2H5O

−,and facilitate Fe4+ formation based on more positive chargedensities of the Fe sites, which led to improved electrocatalyticactivity. Moreover, through integrated ethanol oxidation on theF-modified β-FeOOH anode, a simulative two-electrodeelectrolyzer for H2 production only required a low electrolyticvoltage of 1.43 V to achieve 10 mA cm−2. Although this workcannot accurately control the amount of the F dopant, it revealsthat the rational design of classical electrocatalysts through Fmoderation may provide further development for activityoptimization.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acscatal.7b03319.

Detailed experimental procedures; supported physicalcharacterizations by SEM, XRD, TEM, XPS, and ICP-MS; electrochemical characterizations by CV, LSV,current−potential curves, and EIS; additional DFTcalculations results; gas chromatography and NMRresults (PDF)

■ AUTHOR INFORMATION

Corresponding Authors*E-mail for L.X.D.: lxding@scut.edu.cn.*E-mail for H.H.W.: hhwang@scut.edu.cn.

ORCIDHaihui Wang: 0000-0002-2917-4739NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work was financially supported by the National NaturalScience Foundation of China (21776099, 51621001), the PearlRiver and S&T Nova Program of Guangzhou (201610010076)

and Fundamental Research Funds for the Central Universities(2017JQ007).

■ REFERENCES(1) Subbaraman, R.; Tripkovic, D.; Strmcnik, D.; Chang, K. C.;Uchimura, M.; Paulikas, A. P.; Stamenkovic, V.; Markovic, N. M.Science 2011, 334, 1256−1260.(2) Furlong, D.; Yates, D.; Healy, T.; Trasatti, S. Electrodes ofConductive Metallic Oxides, Part B; Elsevier: Amsterdam, 1981; p 367.(3) He, P.; Yu, X.; Lou, X. Angew. Chem. 2017, 129, 3955−3958.(4) Zhu, Y. P.; Ma, T. Y.; Jaroniec, M.; Qiao, S. Z. Angew. Chem., Int.Ed. 2017, 56, 1324−1328.(5) Kuang, P.; Tong, T.; Fan, K.; Yu, J. ACS Catal. 2017, 7, 6179−6187.(6) Zhao, Z.; Zhang, L.; Xia, Z. J. Phys. Chem. C 2016, 120, 2166−2175.(7) Zou, S.; Burke, M. S.; Kast, M. G.; Fan, J.; Danilovic, N.;Boettcher, S. W. Chem. Mater. 2015, 27, 8011−8020.(8) Hansen, H. A.; Viswanathan, V.; Nørskov, J. K. J. Phys. Chem. C2014, 118, 6706−6718.(9) Seh, Z. W.; Fredrickson, K. D.; Anasori, B.; Kibsgaard, J.;Strickler, A. L.; Lukatskaya, M. R.; Gogotsi, Y.; Jaramillo, T. F.;Vojvodic, A. ACS Energy Lett. 2016, 1, 589−594.(10) Bates, M. K.; Jia, Q.; Doan, H.; Liang, W.; Mukerjee, S. ACSCatal. 2016, 6, 155−161.(11) Friebel, D.; Louie, M. W.; Bajdich, M.; Sanwald, K. E.; Cai, Y.;Wise, A. M.; Cheng, M.-J.; Sokaras, D.; Weng, T.-C.; Alonso-Mori, R.;Davis, R. C.; Bargar, J. R.; Nørskov, J. K.; Nilsson, A.; Bell, A. T. J. Am.Chem. Soc. 2015, 137, 1305−1313.(12) Burke, M. S.; Zou, S.; Enman, L. J.; Kellon, J. E.; Gabor, C. A.;Pledger, E.; Boettcher, S. W. J. Phys. Chem. Lett. 2015, 6, 3737−3742.(13) Ahn, H. S.; Bard, A. J. J. Am. Chem. Soc. 2016, 138, 313−318.(14) Trotochaud, L.; Young, S. L.; Ranney, J. K.; Boettcher, S. W. J.Am. Chem. Soc. 2014, 136, 6744−6753.(15) Candelaria, S. L.; Bedford, N. M.; Woehl, T. J.; Rentz, N. S.;Showalter, A. R.; Pylypenko, S.; Bunker, B. A.; Lee, S.; Reinhart, B.;Ren, Y.; Ertem, S. P.; Coughlin, E. B.; Sather, N. A.; Horan, J. L.;Herring, A. M.; Greenlee, L. F. ACS Catal. 2017, 7, 365−379.(16) Li, C.; Yin, C.; Gu, L.; Dinnebier, R. E.; Mu, X.; van Aken, P. A.;Maier, J. J. Am. Chem. Soc. 2013, 135, 11425−11428.(17) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.;Nørskov, J. K.; Jaramillo, T. F. Science 2017, 355, eaad4998.(18) Gan, L.-Y.; Tian, R.-Y.; Yang, X.-B.; Peng, S.-L.; Zhao, Y.-J. Phys.Chem. Chem. Phys. 2011, 13, 14466−14475.(19) Chen, J. Y. C.; Dang, L.; Liang, H.; Bi, W.; Gerken, J. B.; Jin, S.;Alp, E. E.; Stahl, S. S. J. Am. Chem. Soc. 2015, 137, 15090−15093.(20) Panda, C.; Menezes, P. W.; Walter, C.; Yao, S.; Miehlich, M. E.;Gutkin, V.; Meyer, K.; Driess, M. Angew. Chem., Int. Ed. 2017, 56,10506−10510.(21) McCrory, C. C. L.; Jung, S.; Peters, J. C.; Jaramillo, T. F. J. Am.Chem. Soc. 2013, 135, 16977−16987.(22) Chen, Y. X.; Lavacchi, A.; Miller, H. A.; Bevilacqua, M.; Filippi,J.; Innocenti, M.; Marchionni, A.; Oberhauser, W.; Wang, L.; Vizza, F.Nat. Commun. 2014, 5, 4036.(23) You, B.; Liu, X.; Jiang, N.; Sun, Y. J. Am. Chem. Soc. 2016, 138,13639−13646.(24) You, B.; Jiang, N.; Liu, X.; Sun, Y. Angew. Chem., Int. Ed. 2016,55, 9913−9917.(25) Luo, J.; Im, J.-H.; Mayer, M. T.; Schreier, M.; Nazeeruddin, M.K.; Park, N.-G.; Tilley, S. D.; Fan, H. J.; Gratzel, M. Science 2014, 345,1593−1596.(26) Ledendecker, M.; Calderon, S. K.; Papp, C.; Steinruck, H.-P.;Antonietti, M.; Shalom, M. Angew. Chem. 2015, 127, 12538−12542.(27) Tang, C.; Cheng, N.; Pu, Z.; Xing, W.; Sun, X. Angew. Chem.2015, 127, 9483−9487.(28) Chen, G. F.; Ma, T. Y.; Liu, Z. Q.; Li, N.; Su, Y. Z.; Davey, K.;Qiao, S. Z. Adv. Funct. Mater. 2016, 26, 3314−3323.(29) Zhang, J.; Wang, T.; Pohl, D.; Rellinghaus, B.; Dong, R.; Liu, S.;Zhuang, X.; Feng, X. Angew. Chem. 2016, 128, 6814−6819.

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