(Invited) Ir-Based Catalysts for the Electrochemical Oxygen Evolution Reaction in Acidic Environments

Abstract

In order to overcome the inherent intermittent availability of renewable energies and moving forward to a clean and sustainable energy infrastructure, large-scale energy storage and on-demand energy conversion technologies are required to bridge the time gap between energy supply and demand. In this context, hydrogen has great potential as an energy carrier.1,2 The so-called green hydrogen can be produced using excess renewable electricity, stored, transported if necessary, and finally converted into electrical energy. Promising devices to produce green hydrogen are proton exchange membrane water electrolyzers (PEMWEs).3,4 PEMWEs combine high energy efficiency and high gas purity with flexible partial load capabilities and fast dynamic behavior, which are prerequisite for grid balancing.3,4 However, one of the major drawbacks of PEMWEs is the requirement of large amounts of noble metals in the electrocatalysts due to the inherent highly corrosive acidic working environment.3 In PEMWEs, the anodic oxygen evolution reaction (OER) is undoubtedly demanding from a catalyst standpoint and is mostly responsible for the efficiency loss.5 To date, iridium, one of the least abundant elements in Earth’s crust, is required as the electrocatalyst for the OER in PEMWEs.4,6-8 As a result, in order to facilitate the large-scale application of PEMWEs, developing low-noble metal content, stable electrocatalysts for the OER is requisite.In this talk I present collective results of our research on Ir-based electrocatalysts for the electrochemical OER in acidic environments. To lower the Ir loading in OER catalysts while maintaining or improving the activity, we applied different strategies. One strategy was to synthesize binary alloy nanoparticles (NPs) of Ir and an abundant non-noble metal, transforming them into core-shell NPs consisting of Ir-rich shells and low-Ir content cores.9 Another strategy was to use doped mesoporous oxides as support materials to disperse the catalyst NPs. The doped oxides offered improved corrosion resistance combined with large surface area and adequate electronic conductivity. Moreover, they showed synergistic interactions with the catalysts, improving activity and stability.10,11 Considering the synthesis of catalyst-oxide support couples, it is generally challenging to anchor catalyst NPs on oxide supports due to their incompatible surface properties. Hence we developed a new concept to synthesize oxide supported IrOx catalysts by modifying the surface charge of the catalyst or oxide prior to dispersing the catalyst on the oxide.12 These synthesis routes and strategies can be applied to other noble catalyst systems, pointing out a path forward to nanostructured electrodes with substantially reduced noble metal content and improved efficiency.Importantly, deeper insights into the origin of the OER activity and the OER mechanism on Ir-based electrocatalysts are needed in order to aid in rational design of OER electrocatalysts. Combining electrocatalytic investigations with operando materials characterization techniques and DFT calculations, we identified that the p-band holes in the oxygen ligands play a key role in the OER activity of Ir-based catalysts, in addition, we showed that the rate-determining step for the OER on IrOx is driven by the strong oxidizing power of the deprotonated Ir–O sites and not by an electrostatic-potential gradient across the electrodes double layer.13,14 These findings bridge the gap between thermal catalysis and electrocatalysis and emphasize the ultimate importance to investigate and understand heterogeneous electrocatalytic reactions from different perspectives. References Liu, W.et al., Environmental Science and Pollution Research 2020, 27 (25), 31092-31104.Møller, K. T.et al., Progress in Natural Science: Materials International 2017, 27 (1), 34-40.Carmo, M.et al., J. Hydrog. Energy 2013, 38 (12), 4901-4934.Shirvanian, P.; van Berkel, F., Commun. 2020, 114, 106704.Dau, H.et al., ChemCatChem 2010, 2 (7), 724-761.Greenwood, N. N.; Earnshaw, A.,Chemistry of the Elements. Butterworth–Heinemann: Oxford, 1997; p 1600.Lei, Z.et al., Advanced Energy Materials 2020, 10 (23), 2000478.Alaswad, A.et al., Energies 2021, 14 (1), 144.Nong, H.N. et al., Chem.Sci.2014, 5, 8, 2955-2963.Nong, H.N. et al., Chem. Int. Ed.2015, 54, 2975-2979.Oh, H.S. et al., J.Am. Chem. Soc., 2016, 138, 38, 12552–12563.Tran, H.P. et al., Chem. Mater. 2022, 34, 21, 9350–9363.Nong, H.N. et al., Nature Catalysis 2018, 1, 841-851Nong, H. N.et al., Nature 2020, 587 (7834), 408-413.