Theoretical and computational approaches play a key role in developing and optimizing new materials and devices for energy storage and conversion applications. Here we use several examples to illustrate how theory and computations can help accelerate the design and development of materials for fuel cell and electrolyzers by addressing the issue of their durability. We emphasize computational studies of different processes that can lead to decreased durability of fuel cells and electrolyzers under operating conditions. These studies, for instance, include catalyst corrosion [1], the change in the catalyst structure at different cell potentials and pH [2], poisoning of the catalyst with a crossover fuel or fragments from the ionomeric binder [3,4], and phosphoric acid leaching in acid-doped high temperature fuel cell polymer electrolytes [5].We first illustrate how combination of experimental and first principles thermochemical data can be used to predict and understand the stability of catalytic materials in aqueous media as a function of pH, cell potential, and temperature. Examples will include transition metal carbides and nitrides [1] and the size effect on the stability diagrams of precious metal catalysts [2]. Secondly, we discuss how first principles calculations can be used in the design of high temperature membrane fuel cells. The specific example will include a study of cluster energetics between phosphoric acid, water, and proton-accepting or hydroxide-donating bases, which are explored in the design of acid-doped polymer electrolytes [5].[1] I. Matanovic, F. H. Garzon, N. J. Henson, J. Phys. Chem. C, 115, 10640–10650 (2011).[2] I. Matanovic, F. H. Garzon, J. Electrochem. Soc. 167, 046518 (2020).[3] D. Sebastián, A. Serov, I. Matanovic, K. Artyushkova, P. Atanassov, A.S. Aricò, V. Baglio, Nano Energy, 34, 195-204 (2017).[4] I. Matanovic, S. Maurya, E. J. Park, J. Y. Jeon, C. Bae, Y. S. Kim, Chem. Mater, 31, 11, 4195-4204 (2019). [5] I. Matanovic, A. S. Lee, Y.-S. Kim, J. Phys. Chem. B, 124, 7725-7734 (2020).

The kinetics and thermodynamics of electrochemical energy storage and conversion technologies such as fuel cells and electrolyzers often times benefit from high temperature operation. For example, the thermodynamic electrical energy requirements for water electrolysis decrease substantially from 1.23 to below 1 volt by increasing the process temperature from ambient conditions to 850 °C. Unfortunately, materials degradation via sintering, crystal structure disproportion to thermodynamically more stable phases, and interfacial reactions also increases exponentially with increasing temperature. Lifetime requirements for energy conversion technologies often times exceed 10 years of usage with no more than 20% degradation.[1] The requirement of high gas/electrode/solid electrolyte interfacial area for large areal current densities repeatedly necessitates the usage of nano to micro structured materials, further acerbating materials degradation. Oftentimes it is much easier to synthesize a promising new electrode or electrolyte material than it is to characterize the long-term stability of a material in the extremely reducing or oxidizing high temperature environments.[2] We have developed experimental methods in conjunction with thermochemical modeling to evaluate the stability of important classes of perovskite and fluorite oxide materials.[3] We have performed thermogravimetric, FT-IR and electrochemical linear sweep voltammetry methods to rapidly determine oxide materials stability under operationally relevant conditions and these results are compared to stability calculations. Generalizations can be made to predict expected materials redox stability based on relatively straightforward chemical bonding principles, known cation coordination environments and more sophisticated computational chemistry methods. Stability determinations of perovskite oxide electrocatalysts for electrochemical oxidative coupling of methane offer an excellent examples of our approach towards evaluation of materials durability under challenging temperature and reducing conditions.[1] A. Hauch, S. D. Ebbesen, S. H. Jensen, and M. Mogensen, “Highly efficient high temperature electrolysis,” J. Mater. Chem., vol. 18, no. 20, pp. 2331–2340, 2008, doi: 10.1039/B718822F.[2] C. Zhu, S. Hou, X. Hu, J. Lu, F. Chen, and K. Xie, “Electrochemical conversion of methane to ethylene in a solid oxide electrolyzer,” Nat. Commun., vol. 10, no. 1, p. 1173, 2019, doi: 10.1038/s41467-019-09083-3.[3] F. H. G. Kannan P. Ramaiyan, Luke H. Denoyer, Angelica Benavidez, “Electrochemical oxidative coupling of methane to produce higher hydrocarbons using Sr2Fe1.5Mo0.5O6-d electrocatalysts,” Submitt. Commun. Chem., 2021.