REVIEW 1806296 (1 of 24) © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advmat.de Recommended Practices and Benchmark Activity for Hydrogen and Oxygen Electrocatalysis in Water Splitting and Fuel Cells Chao Wei, Reshma R. Rao, Jiayu Peng, Botao Huang, Ifan E. L. Stephens, Marcel Risch, Zhichuan J. Xu,* and Yang Shao-Horn* DOI: 10.1002/adma.201806296 1. Introduction One of the grand challenges of this cen- tury is to develop cost-effective energy storage technologies that would allow the use of low-cost electricity from renewa- bles to meet our energy needs at-scale and on-demand. [1–3] Storing electrical energy in chemical bonds such as water splitting [1,4] to generate H 2 as an energy carrier (processes in red, Figure 1a) pro- vides high energy densities relative to other storage technologies such as Li-ion batteries. Splitting water electrochemi- cally involves two half-cell reactions: water reduction to evolve hydrogen at the negative electrode and water oxidation to evolve oxygen at the positive electrode. These processes are reversed to gen- erate electrical energy (processes in blue, Figure 1a) in fuel cells. [1,4] These half-cell reactions in acid and base are defined in Table 1. The standard potential of the hydrogen electrode is 0 V versus revers- ible hydrogen electrode (RHE), and that of oxygen electrocatalysis is 1.23 V versus Electrochemical energy storage by making H 2 an energy carrier from water splitting relies on four elementary reactions, i.e., the hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). Herein, the central objective is to recommend systematic protocols for activity measurements of these four reactions and benchmark activities for comparison, which is critical to facilitate the research and development of catalysts with high activity and stability. Details for the electrochemical cell setup, measure- ments, and data analysis used to quantify the kinetics of the HER, HOR, OER, and ORR in acidic and basic solutions are provided, and examples of state-of-the-art specific and mass activity of catalysts to date are given. First, the experimental setup is discussed to provide common guidelines for these reactions, including the cell design, reference electrode selec- tion, counter electrode concerns, and working electrode preparation. Second, experimental protocols, including data collection and processing such as ohmic- and background-correction and catalyst surface area estimation, and practice for testing and comparing different classes of catalysts are recommended. Lastly, the specific and mass activity activi- ties of some state-of-the-art catalysts are benchmarked to facilitate the comparison of catalyst activity for these four reactions across different laboratories. Benchmark Catalysts Dr. C. Wei, Prof. Z. J. Xu School of Materials Science and Engineering Nanyang Technological University 50 Nanyang Avenue 639798, Singapore E-mail: xuzc@ntu.edu.sg Dr. C. Wei, Prof. Z. J. Xu The Cambridge Centre for Advanced Research and Education in Singapore 1 CREATE way, Singapore 138602, Singapore Dr. C. Wei, Prof. Z. J. Xu Solar Fuels Laboratory Nanyang Technological University 50 Nanyang Avenue 639798, Singapore Dr. C. Wei, Prof. Z. J. Xu Energy Research Institute @ Nanyang Technological University 50 Nanyang Avenue 639798, Singapore R. R. Rao, J. Peng, Dr. B. Huang, Prof. Y. Shao-Horn Electrochemical Energy Laboratory Massachusetts Institute of Technology Cambridge, MA 02139, USA E-mail: shaohorn@mit.edu R. R. Rao, Prof. Y. Shao-Horn Department of Mechanical Engineering Massachusetts Institute of Technology Cambridge, MA 02139, USA J. Peng, Prof. Y. Shao-Horn Department of Materials Science and Engineering Massachusetts Institute of Technology Cambridge, MA 02139, USA Dr. B. Huang, Prof. Y. Shao-Horn Research Laboratory of Electronics Massachusetts Institute of Technology 77 Massachusetts Avenue, Cambridge, MA 02139, USA The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201806296. Adv. Mater. 2019, 1806296