Discovering Ce-rich oxygen evolution catalysts, from high throughput screening to water electrolysis

Abstract

Discovering improved electrocatalysts for the oxygen evolution reaction (OER) is of great importance for efficient solar fuels generation, electrowinning of metals, regenerative fuel cells, and recharging metal air batteries. The slow kinetics of the 4- electron OER requires large overpotentials to drive water oxidn. at appreciable current densities. Among the numerous compns. investigated, mixed metal oxides in the (Ni-Fe)Ox and (Ni-Co)Ox compn. spaces are among the most active and most studied OER catalysts. Although this technol. important reaction has been studied for more than 50 years, many of the mechanistic details remain under investigation. Lacking a robust fundamental understanding of the basic science and mechanistic details of multi-electron heterogeneous electrocatalysis, an efficient high-throughput synthesis and property screening methodol. is well-suited to discovering the requisite new catalytic materials. We have established high throughput methods to systematically investigate the performance of pseudo-quaternary material libraries as OER electrocatalysts. We report a new Cerich family of active catalysts composed of earth abundant elements, which was discovered using high-throughput methods to produce 5456 discrete compns. in the (Ni-Fe-Co-Ce)Ox compn. space. The activity and stability of this new OER catalyst was verified by re-synthesis and extensive electrochem. testing of samples in a std. format in 1.0 M NaOH, as well as by operation in a photovoltaic-powered electrolyzer for more than 100 h. The most interesting variations in activity lie in a pseudoternary crosssectional plane contg. 665 compns. Our detailed investigation of this psuedoternary cross-section has revealed systematic trends in Tafel slopes and electrochem. signals with compn., which provide a connection between the previously known Ni-Fe and newly discovered Ni-Co-Ce catalysts. Characterization of selected compns. by XRD, XPS, SEM, TEM, EDS, XRF mapping, and EXAFS, both as-synthesized and after electrochem. testing, reveal important differences in nanostructure and stability along with the obsd. differences in electrochem. performance under OER conditions.