To move to next-generation air-breathing cathodes in alkaline fuel cells and metal–air batteries will require discovering and formulating catalysts with higher activity for the reduction of molecular oxygen (ORR catalysis) and for the oxidation of water/hydroxide back to O2 (OER catalysis). Strategies to enhance production of solar fuels will especially need improved catalysts for oxygen evolution. The design platform around which our team creates high-activity electrocatalysts entails the use of porous nanoarchitectures, which are built with covalently bonded catalytic nanoparticles intermingled with nanoscale void. These structural attributes give the catalyst designer extra dimensions of control when tailoring electrocatalysts to the demands inherent to reducing and evolving oxygen. Even the free volume within the architecture can be adjusted from &gt;85% (aerogel) to 40–70% (ambigel). In the nickel-ferrite (NiyFe1−yOx) ambigel-vs-aerogel series, we find that the emphasis in literature studies on stoichiometry only (the Ni-to-Fe ratio) undercuts the ability to tune catalytic activity. A single variable does not serve as a definitive predictor of catalytic performance, whether that variable is surface area, crystallite size, stoichiometry, or single-phase purity. Significant interplay is at work between all four—not surprising in light of the mechanistic complexity inherent to cycling molecular oxygen electrochemically, but an indication that much remains to be done to design the optimal catalytic nanoscale interphase. We are now testing catalytic formulations in technologically relevant air-breathing electrode structures in order to evaluate the activity and stability of their nanoarchitectured electrocatalysts. Our ultimate goal is to prototype a rechargeable zinc–air battery that pairs NRL’s rechargeable, dendrite-suppressing zinc sponge anode with a “trifunctional” air cathode that reduces oxygen, provides pulse power, and evolves oxygen.

Effect of stoichiometry and architectural expression on the activity of oxygen reduction and evolution electrocatalysts

256th National Meeting (2018)

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