Benchmarking of oxygen evolution catalysts on porous nickel supports

Abstract

•The oxygen evolution reaction is a key process in electrolysis reactions•Catalysts deposited on Ni foam were benchmarked by using standardized protocol•Activity was enhanced through support modification•Low overpotentials were achieved for high current density operation Storage of intermittent renewable energy can be achieved through the conversion of electricity into chemical energy. The solar- and electricity-driven production of energy vectors such as hydrogen, alcohols, and hydrocarbons can be achieved in electrochemical cells, where proton and CO2 reduction at the cathode is coupled to water oxidation at the anode. To develop these processes for high-rate, energy-efficient experimental demonstrations, highly active and stable oxygen evolution reaction (OER) catalysts are needed. The present benchmarking study, using a standardized characterization protocol, allows for rigorous identification of the current best solid-state OER catalysts based on non-precious metals immobilized on a Ni foam support. Further optimization of anodes will emerge not only through tuning the composition and morphology of the catalysts but also from the design and integration of novel conductive and structured supports. Active and inexpensive oxygen evolution reaction (OER) electrocatalysts are needed for energy-efficient electrolysis applications. Objective comparison between OER catalysts has been blurred by the use of different supports and methods to evaluate performance. Here, we selected nine highly active transition-metal-based catalysts and described their synthesis, using a porous nickel foam and a new Ni-based dendritic material as the supports. We designed a standardized protocol to characterize and compare the catalysts in terms of structure, activity, density of active sites, and stability. NiFeSe- and CoFeSe-derived oxides showed the highest activities on our dendritic support, with low overpotentials of η100 ≈ 247 mV at 100 mA cm–2 in 1 M KOH. Stability evaluation showed no surface leaching for 8 h of electrolysis. This work highlights the most active anode materials and provides an easy way to increase the geometric current density of a catalyst by tuning the porosity of its support. Active and inexpensive oxygen evolution reaction (OER) electrocatalysts are needed for energy-efficient electrolysis applications. Objective comparison between OER catalysts has been blurred by the use of different supports and methods to evaluate performance. Here, we selected nine highly active transition-metal-based catalysts and described their synthesis, using a porous nickel foam and a new Ni-based dendritic material as the supports. We designed a standardized protocol to characterize and compare the catalysts in terms of structure, activity, density of active sites, and stability. NiFeSe- and CoFeSe-derived oxides showed the highest activities on our dendritic support, with low overpotentials of η100 ≈ 247 mV at 100 mA cm–2 in 1 M KOH. Stability evaluation showed no surface leaching for 8 h of electrolysis. This work highlights the most active anode materials and provides an easy way to increase the geometric current density of a catalyst by tuning the porosity of its support. The oxygen evolution reaction (OER) is one of the most relevant anodic reactions within electrochemical cells, where it is coupled to the hydrogen evolution reaction (HER)1Kim J.H. Hansora D. Sharma P. Jang J.W. Lee J.S. Toward practical solar hydrogen production – an artificial photosynthetic leaf-to-farm challenge.Chem. Soc. Rev. 2019; 48: 1908-1971Crossref PubMed Google Scholar, 2Ager J.W. Shaner M.R. Walczak K.A. Sharp I.D. Ardo S. 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Interfacial engineering FeOOH/CoO nanoneedle array for efficient overall water splitting driven by solar energy.Chem. Eur. J. 2019; 26: 4120Crossref Scopus (10) Google Scholar However, sluggish kinetics of the four-electron OER requires a significant anodic overpotential to achieve relevant geometric current densities, reducing the efficiency of the conversion of electrical to chemical energy. Hence, identifying efficient, cheap, and stable OER catalysts comprising earth-abundant elements is of fundamental importance and has been a prominent field of research during the last 20 years.20Suen N.T. Hung S.F. Quan Q. Zhang N. Xu Y.J. Chen H.M. Electrocatalysis for the oxygen evolution reaction: recent development and future perspectives.Chem. Soc. Rev. 2017; 46: 337-365Crossref PubMed Google Scholar, 21Hunter B.M. Gray H.B. Müller A.M. Earth-abundant heterogeneous water oxidation catalysts.Chem. Rev. 2016; 116: 14120-14136Crossref PubMed Scopus (970) Google Scholar, 22Song F. Bai L. 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Pinaud B.A. Gorlin Y. Jaramillo T.F. Substrate selection for fundamental studies of electrocatalysts and photoelectrodes: inert potential windows in acidic, neutral, and basic electrolyte.PLoS One. 2014; 9: e107942Crossref PubMed Scopus (150) Google Scholar,25Chaudhari N.K. Jin H. Kim B. Lee K. Nanostructured materials on 3D nickel foam as electrocatalysts for water splitting.Nanoscale. 2017; 9: 12231-12247Crossref PubMed Google Scholar and variation in characterization methods and experimental setups further adds to disparities between reported performances. As a matter of fact, very few benchmarking studies have been carried out so far. The last significant efforts in providing meaningful benchmarking studies were carried out in 2012,26Trotochaud L. Ranney J.K. Williams K.N. Boettcher S.W. Solution-cast metal oxide thin film electrocatalysts for oxygen evolution.J. Am. Chem. Soc. 2012; 134: 17253-17261Crossref PubMed Scopus (1177) Google Scholar 2013,27McCrory C.C.L. Jung S. Peters J.C. Jaramillo T.F. Benchmarking heterogeneous electrocatalysts for the oxygen evolution reaction.J. Am. Chem. Soc. 2013; 135: 16977-16987Crossref PubMed Scopus (3951) Google Scholar and 2015.28McCrory C.C.L. Jung S. Ferrer I.M. Chatman S.M. Peters J.C. Jaramillo T.F. Benchmarking hydrogen evolving reaction and oxygen evolving reaction electrocatalysts for solar water splitting devices.J. Am. Chem. Soc. 2015; 137: 4347-4357Crossref PubMed Scopus (2416) Google Scholar These studies, conducted on OER electrocatalysts deposited on Au/Ti or glassy carbon electrodes, performed water oxidation at a current density of 10 mA cm−2 with observed overpotentials higher than 300 mV. A relevant figure of merit from these studies is the overpotential (denoted as η10) required to achieve 10 mA cm−2 current density per geometric surface area at ambient temperature and 1 atm O2. The η10 value is indeed the benchmarking criterion generally used in literature.26Trotochaud L. Ranney J.K. Williams K.N. Boettcher S.W. Solution-cast metal oxide thin film electrocatalysts for oxygen evolution.J. Am. Chem. Soc. 2012; 134: 17253-17261Crossref PubMed Scopus (1177) Google Scholar, 27McCrory C.C.L. Jung S. Peters J.C. Jaramillo T.F. Benchmarking heterogeneous electrocatalysts for the oxygen evolution reaction.J. Am. Chem. Soc. 2013; 135: 16977-16987Crossref PubMed Scopus (3951) Google Scholar, 28McCrory C.C.L. Jung S. Ferrer I.M. Chatman S.M. Peters J.C. Jaramillo T.F. Benchmarking hydrogen evolving reaction and oxygen evolving reaction electrocatalysts for solar water splitting devices.J. Am. Chem. Soc. 2015; 137: 4347-4357Crossref PubMed Scopus (2416) Google Scholar Some OER electrocatalysts reported during the last five years exhibit η10 overpotentials much closer to 200 mV, representing a significant advancement in the field. We, thus, found it timely to provide a fair comparison of the performances of the most active catalysts of this new generation. Here, we establish a protocol to benchmark a range of anodes, consisting of various catalysts synthesized on the same nickel foam (NF) support. Because of its conductivity, mechanical strength, relative inertness at alkaline pH, and low cost, nickel is an efficient current collector and a good support for active material deposition. Furthermore, nickel foam shows extended geometric surface areas and fine three-dimensional structures, which make it attractive as a support for heterogeneous catalysts.25Chaudhari N.K. Jin H. Kim B. Lee K. Nanostructured materials on 3D nickel foam as electrocatalysts for water splitting.Nanoscale. 2017; 9: 12231-12247Crossref PubMed Google Scholar,29Grdeń M. Alsabet M. Jerkiewicz G. Surface science and electrochemical analysis of nickel foams.ACS Appl. Mater. Interfaces. 2012; 4: 3012-3021Crossref PubMed Scopus (153) Google Scholar Although various porous metallic foams have been used in the past for water oxidation,30Hall D.E. Alkaline water electrolysis anode materials.J. Electrochem. Soc. 1985; 132: 41C-48CCrossref Scopus (110) Google Scholar, 31Hall D.E. Ni(OH)2 - impregnated anodes for alkaline water electrolysis.J. Electrochem. Soc. 1983; 130: 317-321Crossref Scopus (91) Google Scholar, 32Corrigan D.A. The catalysis of the oxygen evolution reaction by iron impurities in thin film nickel oxide electrodes.J. Electrochem. Soc. 1987; 134: 377-384Crossref Scopus (649) Google Scholar a recent resurgence in the usage of NF as a support material has occurred. This was partly driven by the development of energy-storage electrochemical systems in alkaline conditions, such as solar-driven water splitting and electrocatalytic CO2RR using gas-fed flow cells. We have followed this trend and used NF as the catalyst support exclusively.17Dinh C.T. Burdyny T. Kibria M.G. Seifitokaldani A. Gabardo C.M. García de Arquer F.P. Kiani A. Edwards J.P. De Luna P. Bushuyev O.S. et al.CO2 electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface.Science. 2018; 360: 783-787Crossref PubMed Scopus (946) Google Scholar, 18Xin Y. Kan X. Gan L.Y. Zhang Z. Heterogeneous bimetallic phosphide/sulfide nanocomposite for efficient solar-energy-driven overall water splitting.ACS Nano. 2017; 11: 10303-10312Crossref PubMed Scopus (132) Google Scholar, 19Qiu C. He S. Wang Y. Wang Q. Zhao C. Interfacial engineering FeOOH/CoO nanoneedle array for efficient overall water splitting driven by solar energy.Chem. Eur. J. 2019; 26: 4120Crossref Scopus (10) Google Scholar For this study, we selected nine promising multimetallic, non-precious-metal catalysts reported in the literature (Table 1). Several selection criteria were adopted such as the η10 overpotential in alkaline conditions, the Tafel slope, the maximum current density, and the long-term stability, as these data were available in previous publications.33Huan T.N. Rousse G. Zanna S. Lucas I.T. Xu X. Menguy N. Mougel V. Fontecave M. A dendritic nanostructured copper oxide electrocatalyst for the oxygen evolution reaction.Angew. Chem. Int. Ed. Engl. 2017; 56: 4792-4796Crossref PubMed Scopus (160) Google Scholar, 34Esswein A.J. Surendranath Y. Reece S.Y. Nocera D.G. Highly active cobalt phosphate and borate based oxygen evolving catalysts operating in neutral and natural waters.Energy Environ. Sci. 2011; 4: 499-504Crossref Google Scholar, 35Jayalakshmi M. Kim W.-Y. Jung K.-D. Joo O.-S. Electrochemical characterization of Ni-Mo-Fe composite film in alkali solution.Int. J. Electrochem. Sci. 2008; 3: 908-917Google Scholar, 36Zhou H. Yu F. Zhu Q. Sun J. Qin F. Yu L. Bao J. Yu Y. Chen S. Ren Z. Water splitting by electrolysis at high current densities under 1.6 volts.Energy Environ. Sci. 2018; 11: 2858-2864Crossref Google Scholar, 37Xu X. Song F. Hu X. A nickel iron diselenide-derived efficient oxygen-evolution catalyst.Nat. Commun. 2016; 7: 12324Crossref PubMed Scopus (649) Google Scholar, 38Zhang J.Y. Lv L. Tian Y. Li Z. Ao X. Lan Y. Jiang J. Wang C. Rational design of cobalt–iron selenides for highly efficient electrochemical water oxidation.ACS Appl. Mater. Interfaces. 2017; 9: 33833-33840Crossref PubMed Scopus (105) Google Scholar, 39Zhang B. Zheng X. Voznyy O. Comin R. Bajdich M. García-Melchor M. Han L. Xu J. Liu M. Zheng L. et al.Homogeneously dispersed multimetal oxygen-evolving catalysts.Science. 2016; 352: 333-337Crossref PubMed Scopus (1435) Google Scholar, 40Liu J. Ji Y. Nai J. Niu X. Luo Y. Guo L. Yang S. Ultrathin amorphous cobalt-vanadium hydr(oxy)oxide catalysts for the oxygen evolution reaction.Energy Environ. Sci. 2018; 11: 1736-1741Crossref Google Scholar, 41Liardet L. Hu X. Amorphous cobalt vanadium oxide as a highly active electrocatalyst for oxygen evolution.ACS Catal. 2018; 8: 644-650Crossref PubMed Scopus (159) Google Scholar We also made sure to select materials composed of a variety of transition metals with different morphologies and synthetic procedures in order to broaden the scope of our comparison. These catalysts were preferentially chosen on the basis that they display low η10 overpotentials (below 300 mV). However, we also included the Co-based catalyst developed by Nocera et al. (CoPi),34Esswein A.J. Surendranath Y. Reece S.Y. Nocera D.G. Highly active cobalt phosphate and borate based oxygen evolving catalysts operating in neutral and natural waters.Energy Environ. Sci. 2011; 4: 499-504Crossref Google Scholar despite it exhibiting a η10 value greater than 300 mV in 1 M KOH, given that it is widely used in literature. The NiMoFe-O catalyst was included as it was reported to be the highest performing catalyst at the time of writing.27McCrory C.C.L. Jung S. Peters J.C. Jaramillo T.F. Benchmarking heterogeneous electrocatalysts for the oxygen evolution reaction.J. Am. Chem. Soc. 2013; 135: 16977-16987Crossref PubMed Scopus (3951) Google Scholar,42Ma W. Ma R. Wang C. Liang J. Liu X. Zhou K. Sasaki T. A superlattice of alternately stacked Ni–Fe hydroxide nanosheets and graphene for efficient splitting of water.ACS Nano. 2015; 9: 1977-1984Crossref PubMed Scopus (531) Google Scholar CoV-O and CoV-OOH were chosen to provide a comparison between two catalysts based on cobalt and vanadium that were synthesized by using different methods. In the publications, these catalysts were characterized under very different conditions. In particular, various electrode supports were used (glassy carbon, Cu plate, or Ni foam), illustrating the difficulty of comparing them just on the basis of the literature data. In our study, we exclusively used the same Ni foam as the conductive support. The syntheses followed the reported procedures as strictly as possible with slight adaptations to enable deposition on a 1 cm2 NF support.Table 1Oxygen evolution catalysts with their method for synthesis used in this studyCatalystSynthesis procedureRefCu-OelectrodepositionHuan et al.33Huan T.N. Rousse G. Zanna S. Lucas I.T. Xu X. Menguy N. Mougel V. Fontecave M. A dendritic nanostructured copper oxide electrocatalyst for the oxygen evolution reaction.Angew. Chem. Int. Ed. Engl. 2017; 56: 4792-4796Crossref PubMed Scopus (160) Google ScholarCoPielectrodepositionEsswein et al.34Esswein A.J. Surendranath Y. Reece S.Y. Nocera D.G. Highly active cobalt phosphate and borate based oxygen evolving catalysts operating in neutral and natural waters.Energy Environ. Sci. 2011; 4: 499-504Crossref Google ScholarNiMoFe-OelectrodepositionJayalakshmi et al.35Jayalakshmi M. Kim W.-Y. Jung K.-D. Joo O.-S. Electrochemical characterization of Ni-Mo-Fe composite film in alkali solution.Int. J. Electrochem. Sci. 2008; 3: 908-917Google ScholarNiFe-OOHprecipitationZhou et al.36Zhou H. Yu F. Zhu Q. Sun J. Qin F. Yu L. Bao J. Yu Y. Chen S. Ren Z. Water splitting by electrolysis at high current densities under 1.6 volts.Energy Environ. Sci. 2018; 11: 2858-2864Crossref Google ScholarNiFeSe-dOhydrothermalXu et al.37Xu X. Song F. Hu X. A nickel iron diselenide-derived efficient oxygen-evolution catalyst.Nat. Commun. 2016; 7: 12324Crossref PubMed Scopus (649) Google ScholarCoFeSe-dOhydrothermalZhang et al.38Zhang J.Y. Lv L. Tian Y. Li Z. Ao X. Lan Y. Jiang J. Wang C. Rational design of cobalt–iron selenides for highly efficient electrochemical water oxidation.ACS Appl. Mater. Interfaces. 2017; 9: 33833-33840Crossref PubMed Scopus (105) Google ScholarFeCoWgel formationZhang et al.39Zhang B. Zheng X. Voznyy O. Comin R. Bajdich M. García-Melchor M. Han L. Xu J. Liu M. Zheng L. et al.Homogeneously dispersed multimetal oxygen-evolving catalysts.Science. 2016; 352: 333-337Crossref PubMed Scopus (1435) Google ScholarCoV-OOHcoprecipitationLiu et al.40Liu J. Ji Y. Nai J. Niu X. Luo Y. Guo L. Yang S. Ultrathin amorphous cobalt-vanadium hydr(oxy)oxide catalysts for the oxygen evolution reaction.Energy Environ. Sci. 2018; 11: 1736-1741Crossref Google ScholarCoV-OhydrothermalLiardet and Hu41Liardet L. Hu X. Amorphous cobalt vanadium oxide as a highly active electrocatalyst for oxygen evolution.ACS Catal. 2018; 8: 644-650Crossref PubMed Scopus (159) Google Scholar Open table in a new tab The materials were characterized and their kinetic performances for the OER in alkaline conditions analyzed. Geometric current densities of OER catalysts must attain several hundreds of milliamperes per square centimeter to facilitate CO2 reduction and solar-driven water splitting under lab-scale conditions.1Kim J.H. Hansora D. Sharma P. Jang J.W. Lee J.S. Toward practical solar hydrogen production – an artificial photosynthetic leaf-to-farm challenge.Chem. Soc. Rev. 2019; 48: 1908-1971Crossref PubMed Google Scholar,2Ager J.W. Shaner M.R. Walczak K.A. Sharp I.D. Ardo S. Experimental demonstrations of spontaneous, solar-driven photoelectrochemical water splitting.Energy Environ. Sci. 2015; 8: 2811-2824Crossref Google Scholar,43Jouny M. Luc W. Jiao F. General techno-economic analysis of CO2 electrolysis systems.Ind. Eng. Chem. Res. 2018; 57: 2165-2177Crossref Scopus (514) Google Scholar Therefore, we not only benchmarked the catalysts at the commonly reported value of 10 mA cm2 but also at a more relevant current density of 100 mA cm−2,17Dinh C.T. Burdyny T. Kibria M.G. Seifitokaldani A. Gabardo C.M. García de Arquer F.P. Kiani A. Edwards J.P. De Luna P. 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Finally, we report a novel NF-based support with different morphology and increased structuration, leading to significant improvements in the performance of almost all studied catalysts. By doing so, we report two OER catalysts with overpotential values of 195 and 198 mV required for a current density of 10 mA cm−2, and overpotentials of 247 mV required for 100 mA cm−2, among the lowest values reported so far. We demonstrate the outstanding electrochemical and compositional stability of these two catalysts, evaluated by long-term electrolysis in a flow cell (Figure S1). Furthermore, we stress the importance and necessity of performing galvanostatic tests as well as surface leaching quantification in order to assess the stability of the catalyst. Nine catalysts were selected among the most active materials reported in the literature (Table 1). Cu-O and NiMoFe-O were cathodically electrodeposited on Ni foam (Figure 1A) from aqueous solutions of the metallic precursors. This simple method resulted in the formation of dendritic structures as shown by SEM analysis (Figures 1B, 1C, S2, and S4). NiMoFe-O showed a particularly strong adhesion to the substrate because the presence of nickel in the as-deposited catalyst ensures a continuous interface with NF in terms of composition and, thus, a small lattice mismatch. EDX was used to confirm the elemental composition (Figures S3 and S5). CoPi was anodically electrodeposited, which resulted in the slow formation of large dendrites with a poor adhesion to the NF support (Figures 1D, S6, and S7) . FeCoW (Figures 1E, S8A, S8B, and S9) and CoV-OOH (Figures 1F, S10, and S11) were synthesized from nanomaterial dispersions mixed with a Nafion ink and drop-cast and dried onto the surface of the support to form thick layers, which remained well attached to the support despite large cracks (Figures S8B and S10C). The major limitation of this method is the hydrophobicity of dry Nafion, which promotes the formation of an air film trapped at the catalyst/electrolyte interface and limits their contact area. In order to reach a stable OER activity, these catalysts must be kept under oxidative conditions (10 min at 5 mA cm−2) in an aqueous electrolyte in order for the Nafion’s hydrophilic domains to swell and become predominant in the bulk.48Zhao Q. Majsztrik P. Benziger J. Diffusion and interfacial transport of water in nafion.J. Phys. Chem. B. 2011; 115: 2717-2727Crossref PubMed Scopus (209) Google Scholar A modification in the nanostructure of FeCoW was observed after this process, revealing the formation of a thin layer structure (Figures S8C and S8D). We conclude that once the Nafion network is fully hydrated the metallic sites are exposed to the electrolyte. This allows dissolution/precipitation equilibria at the interface, resulting in the formation of highly nanostructured surfaces. NiFe-OOH (Figures 1G, S12, and S13) was made through galvanic exchange of the Ni-based substrate with a Fe3+ precursor, followed by the deposition of a bimetallic oxy-hydroxide. As the support is the only source of nickel, it is etched during the reaction. This not only ensures a very high adhesion of the catalyst on the support but also slightly modifies the material by etching its Ni backbone. Nevertheless, this method is particularly interesting as it is very simple. NiFeSe- and CoFeSe-derived oxides were synthesized by a more complex three-step procedure involving the hydrothermal formation of layered double hydroxides (LDHs), followed by selenization and subsequent reoxidation. The SEM images and EDX elemental analysis of the resulting NiFeSe-dO (Figures 1H, S14, and S15) and CoFeSe-dO (Figures 1I, S16, and S17) catalysts revealed the formation of very dense, thick, and mechanically stable deposits at the surface of NF with fine nanostructures in the range of 30–100 nm. However, the formation of a selenide involves hazardous synthesis steps and its reoxidation leads to toxic selenite waste products. CoV-O was synthesized by a simple one-step hydrothermal procedure involving the coprecipitation of Co and V in a mixed-phase composed of a fine LDH nanostructure (Figures 1J, S18, and S19)—this method is simple but results in a very low loading on NF. It should be noted that the atomic compositions of the catalysts differed slightly from those reported in some cases. For example, in the case of NiMoFe-O (Figure S5), the use of NF as the support led to small modifications in the chemical composition of the film. The tungste