Oxygen reduction (ORR) and evolution reactions (OER) are cornerstone chemistries in fuel cells and electrolyzers. In this talk, density functional theory (DFT) modeling of ORR and OER on electrocatalysts containing dual active sites that synergistically promote these reactions will be discussed. ORR on dual-metal centers coordinated by six pyridinic nitrogen embedded in graphene sheets showed that reactivities at these dual-metal sites can be depicted in a volcano diagram with distinct potential limiting steps (PLSs). Specifically, the traditional OH removal and the protonation of gas-phase O2 remain the two boundaries for the ORR reactivity volcano when too strong or weak OH binding occurs at the active sites. However, due to the strong molecular O2 adsorption at the dual-metal sites, protonation of adsorbed O2 and O species also emerge as likely the PLSs and thus enrich the electrochemistry for ORR. Fe-Co(OH)s is predicted to have the highest limiting potential among all dual-metal sites, with a of 0.73 V. The PGM-free dual-metal sites such as Fe-Fe(OH), Fe-Cu(OH), and Co-Co(OH) also appear to be competitive and could encourage further explorations for Pt-free ORR electrocatalysts. A novel heterostructured CeO2/CoFeCe-LDH (CoFeCe0.5) was developed as the OER electrocatalyst. The Ce doping creates a new active interface between CeO2 and CoFeCe-LDH, enabling the synergistic effects that reduce the ohmic loss and electrode loss in anion exchange membrane (AEM) electrolyzers. The AEM electrolyzer achieves a current density of 2.7 A/cm2 at 1.8 Vcell, outperforming the electrolyzer with an IrO2. The heterostructures responsible for the improved OER performance were elucidated. In particular, DFT was employed to reveal the reactivity origin of OER by focusing on the edge and basal planes of the layered structures.

Utilizations of dual-sites and heterostructures and oxygen electrochemistries in fuel cells and electrolyzers

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