The performance of oxygen reduction reaction (ORR) catalysts, typically evaluated by potential scanning techniques, fails to capture the significant activity decay occurring at longer time scales. To meet the 65% peak energy efficiency target, the continuous decay of Pt/C ORR catalyst, commonly referred to as “transient performance loss”, is studied at 0.8 V by chronoamperometry under membrane electrode assembly (MEA) testing conditions. Based on the results from the time-resolved cyclic voltammetry (CV), surface oxidation was identified as the primary cause of the transient loss. The reduction of surface oxide was observed to occur at 0.6 V, and the recovery of cathode performance can hence be achieved at equal or lower potentials. In addition, the effects of operating temperature and cathode humidity were also studied. The coverage of Pt surface oxides and the extent of the transient loss were both significantly reduced as temperature decreased. However, the benefit of a lower operating temperature came at the cost of slower recovery kinetics. In terms of the impact of humidity, the presence of liquid water was identified as the critical factor leading to a much more severe performance loss over time.
An atomic-scale tungsten (ASW)-armored carbonsupported catalyst is prepared by applying ultrafine tungsten species dispersed onto the surface of a commercially available Pt/C catalyst. Tungsten-chloro-methoxide has proven to be a useful molecular precursor that allows for the uniform deposition of ASW on the carbon support and at the periphery of the Pt nanoparticles. ASW imparts exceptional thermal stability as evidenced by the minimal sintering of nanoparticulate species at an elevated temperature of 700 °C. Moreover, when employed as an oxygen reduction reaction catalyst, the ASW-armored catalyst exhibits improved activity and impressive durability. With 1.6 times higher initial mass activity, the ASW-armored catalyst only showed a 6.5 mV loss of half wave potential compared to 16 mV for pristine Pt/ C after 30,000 cycles of the accelerated stress test (AST). Cyclic voltammetry and post-mortem materials characterizations show the ASW species substantially alleviated the corrosion of carbon during AST, which caused serious oxygen transport loss observed for pristine Pt/C. Density functional theory calculation supports our hypothesis of which ASW can indeed improve the stability of carbon surface defects when a single-atom tungsten is present in close proximity. This work demonstrates the design of atomic-scale metal species for increasing the robustness of functional nanomaterials under harsh thermal and electrochemical working environments.
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