A New Guide for Searching High-Performance Catalysts for Direct Alcohol Fuel Cells

Direct alcohol fuel cells (DAFCs) are important power sources for distributed energy applications, such as automobiles.  They are superior to combustion engines in lower operation temperatures and to hydrogen fuel cells in higher energy density and ease of fuel-handling.  Two of major issues are the elucidation of the multi-electron oxidation mechanism and the development of high-performance (activity, selectivity, stability) electrocatalysts.  For over two decades, cyclic voltammetry has been used as a convenient and powerful tool to assess catalysts’ performance.  The oxidation peak in the backward scan is assigned to the oxidation of intermediate carbonaceous compounds (mainly CO) that are formed in the proceeding forward scan.  Thus, the ratio of peak intensity in the forward and backward scans (I_f/I_b) is believed to indicate the tolerance of CO poisoning; the higher the I_f/I_b, the better the tolerance. This figure-of-merit has been applied ever since for searching high-performance catalysts, including the Pt-based materials for acidic cells and Pd-based materials for alkaline cells.  However, to date, catalysts that are practically superior to commercial counterparts have not been identified yet.

In this presentation, we will provide direct and unambiguous evidence to show that 1) the backward oxidation peak is not due to the oxidation of intermediate carbonaceous compounds that are formed in the proceeding forward scan; 2) the ratio I_f/I_b has nothing to do with the CO oxidation and a higher I_b/I_f value can indicate superior capability in surface regeneration.  These findings are applicable to Pt in acidic cells, Pd in alkaline cells, and various alcohols.

Our discovery corrects a long-term misunderstanding on the reaction mechanism of catalytic electroxidation of alcohols, provides a new guide for searching high-performance catalysts, and has the potential to be generalized to other fields of electrocatalysis.