Review on electrochemical active surface area characterization methods of Pt alloy catalysts

Proton exchange membrane fuel cells (PEMFCs) operating at low temperatures and high-power densities are considered one of the most promising technologies for clean energy. However, some fundamental challenges remains to be addressed before PEMFCs can truly become the top contender, high performance with low Pt usage for example. In order to achieve high-performance and cost-effective in PEMFCs, Pt alloy are currently one of the most promising catalyst candidates. Electrochemical active surface area (ECSA) is a key indicator to gauge the performance of Pt catalysts and an important parameter in studying catalytic kinetic issues in PEMFCs. Therefore, the accurate estimation on the ECSA is exceptionally critical. Known and mature ECSA characterization methods for Pt/C catalysts have been established, which has been widely available for many decades. However, there are challenging issues remaining for ECSA characterization of Pt alloy catalysts due to the significant changes in physical structure (composition, particle size, shape, etc.) and chemical properties (active site, catalytic performance, etc.) compared with Pt catalysts. Therefore, the ECSA characterization approaches for Pt/C catalysts cannot be transplanted to Pt alloy catalysts since the characterization accuracy will no longer meet the requirement. In this review, the present methods of ECSA characterization for Pt alloy catalysts and their inaccuracy sources are described and explained, including physical methods and electrochemical methods. Physical methods mainly cover X-ray diffractometry (XRD) and transmission electron microscopy (TEM). hydrogen underpotential deposition (H-upd), carbon monoxide stripping (CO-stripping) and underpotential deposition of metals are classified as electrochemical methods. In XRD and TEM methods, the geometrical areas calculated based on the average particle size is regard as ECSA. In more cases, it is considered to be theoretical ECSA to evaluate the catalyst utilization. Therefore, the ECSA calculated from XRD and TEM is widely used as a reference. Owing to the changes in surface adsorption properties after alloying, it will be underestimated the ECSA of Pt alloy catalysts by H-upd and CO-stripping, which leads to exaggerate specific activity of when compared with Pt/C catalysts. It is generally accepted that sites from electrochemical methods are catalytically active, but it is still a controversy point. The surface area of different metal components of Pt alloy could be quantified by metal underpotential deposition method. Since the contribution of different metal components to the catalytic activity is different, the actual ECSA cannot be simply calculated by adding the respective metal surface area. The surface active of Pt alloy catalysts is affected by many factors, which there is currently not a universal method to characterize the ECSA accurately. To evaluate the performance of Pt alloy catalysts more objectively, it is necessary to clarify the relationship between the surface structure of different Pt alloys and adsorption energy and coverage of adsorbed species, then modifying the calculated equations of ECSA. In addition, to innovatively develop new electrochemical methods with high accuracy is also essential. Such knowledge is in urgent need in order to develop new high-performance Pt alloy catalysts.