O2 Electroreduction on Silver and Silver-Based Nanostructured Cathodes: A Review

During the recent decade, the search for catalytically active cathodes of oxygen reduction reaction (ORR) for fuel cells and secondary metal–air batteries has been one of the priority directions of electrochemical energy applications. Existing highly efficient electrocatalysts based on very expensive and rare platinum (Pt) and Pt group elements need replacement with affordable and scalable electrode materials. In this article, we review the recent trends and progress in the research of ORR catalysts alternative to Pt group elements, namely, silver (Ag)-based nanostructured cathodes. The following main research directions are presented: (1) the O₂ electroreduction mechanism; (2) nanostructured and nanoporous Ag surfaces; (3) Ag nanoparticles (AgNPs) anchored to a carbon substrate; (4) Ag nanostructures deposited onto M_xO_y−carbon nanocomposites; (5) bimetallic Ag–M nanostructures. The first direction encompasses the analysis of literature data on ORR on Ag surface, which is stable in alkaline electrolytes and provides a four-electron (4e) pathway for the ORR. In the second direction, we demonstrate the dependence of electroreduction efficiency on the size and shape of AgNPs and their nanoscale porosity. The third direction is based on the enhancement of catalytic activity via designing of fine and ultrafine AgNPs attached to highly porous carbon support nanomaterials such as carbon black, mesoporous carbon (MC), reduced graphene oxide, nitrogen-doped graphene, multi-walled carbon nanotubes (MWCNTs) and nitrogen-doped carbon nanotubes (NCNTs). The fourth direction, hybrid AgNPs–M_xO_y/carbon nanocomposites, is one of the most promising for ORR applications. The catalytic activity of these nanocomposites is based on the synergy of the AgNPs–M_xO_y couple, and the nanoscale porosity of carbon materials ensures mass transport of dissolved oxygen to the electrode surface. We describe the characteristic features of electrochemical oxygen reduction for the systems of AgNPs with nanoparticles of MnO₂, ZrO₂, WO₃, Fe₃O₄ and Co₃O₄, which can tune Ag surface states. The fifth separate research direction is the use of bimetallic Ag–M nanostructures for electrocatalytic ORR. The most studied systems are binary, including Ag–Pd, Ag–Cu, Ag–Co and Ag–Ni. This review outlines the trends and prospects of future development of the Ag-based nanostructured catalytically active cathodes for oxygen reduction.