A review of Ag-based catalysts for oxygen reduction reaction

There is great interest in the oxygen reduction reaction (ORR) in basic solution due to the development of alkaline fuel cell and metal air battery. Pt is a kind of highly active catalyst for ORR to go through 4e pathway. However, the price of Pt is expensive, and the annual production is only 181.6 tons, which is far to meet the usage of electric vehicles. On the contrary, Ag is not only much cheaper than Pt, but can also catalyze ORR to pass 4e procedure. In addition, Ag is more active than Pt for 4e process in more concentrated alkaline solution and at higher temperature. Therefore, this paper briefly reviews different types of Ag-based catalysts, which are used in the field of the ORR, such as, pure Ag, carbon-supported Ag, Ag composite catalysts, Ag binary alloys and Ag-transition metal oxides. We introduce merits and drawbacks, possible research directions of the above Ag-based catalysts, respectively, and show a comparative analysis on the update results of these Ag-based catalysts, especially for the carbon-supported Ag, a proper weight proportion of Ag and particle size and morphology of Ag cluster should be found out and researched in the future. Moreover, the state-of-the-art research and catalytic mechanism of Ag binary alloys are also reviewed, interestingly, two different catalytic mechanism explanations of Ag binary, based on the same institute, University of Texas at Austin, are compared and analyzed. Furthermore, the relationship between catalysis performance for ORR and structure of Ag-based catalysts are properly elaborated, and we also introduce some of the main characterization techniques (electrochemical techniques, structure analysis, morphological analysis, density functional theory etc.) Based on the common characteristics of all of Ag-based catalysts, at the end of this review, we propose some promising practical application and research tendency for Ag-based catalysts. The catalytic activity of transition metals in aprotic electrolytes is similar to that of transition metal in strong basic aqueous electrolytes, the excellent Ag based catalysts for alkaline fuel cell and metal air battery may play well in nonaqueous Li air battery, too.