Electrolyte-Induced Restructuring of Acid-Stable Oxygen Evolution Catalysts

Crystalline metal oxide catalysts operating under oxygen evolution reaction (OER) conditions invariably restructure, resulting in active sites with hydroxo/oxo species in an amorphous environment. An increase in the population of terminal hydroxo/oxo species (i.e., edge sites) facilitates proton-coupled electron-transfer (PCET) kinetics for oxygen generation and thus improves catalyst competency. While amorphous films benefit from a greater density of active sites, they suffer from diminished charge transport as compared to that of extended crystalline lattices. Managing this amorphous-crystalline dichotomy is essential when designing OER catalysts, which we highlight with the examination of electrodeposited PbOx materials, which historically are very poor OER catalysts. Along these lines, the presence of phosphate during PbOx electrodeposition truncates the growth of an extended lattice owing to its strong bonding to oxide surfaces to afford an amorphous catalyst film (A-PbOx) with significant charge-transfer resistance (138 +/- 42 omega) and poor OER kinetics (420 +/- 105 mV dec-1 Tafel slope). Conversely, electrodeposition of Pb2+ in the presence of less coordinating electrolytes such as nitrate affords crystalline beta-PbO2 with improved charge-transfer resistance (42.6 +/- 1.1 omega), though still poor OER kinetics (134 +/- 36 mV dec-1 Tafel slope). By operating amorphous A-PbOx in less coordinating electrolytes, however, a new partially crystalline material can be generated (mu c-PbOx) with further reduced charge-transfer resistance (33.0 +/- 1.4 omega) and improved OER kinetics (70 +/- 15 mV dec-1 Tafel slope). The enhanced OER activity of mu c-PbOx is the result of coupling the high edge-site population of an amorphous PbOx phase with crystalline-like charge transport properties. The ability to use an electrolyte to induce OER activity in an inactive amorphous form of PbOx highlights the benefits of optimizing the amorphous-crystalline phase compositions in the design of active OER catalysts.

Automated summary

Crystalline metal oxide catalysts restructuring under OER conditions results in active sites with hydroxo/oxo species in an amorphous environment. Increasing the population of terminal hydroxo/oxo species improves catalyst competency. Managing the amorphous-crystalline dichotomy is essential for designing active OER catalysts.