Correlative Electrochemical-TEM Study: Probing the Pt Stability of Mesoporous N-Doped Carbon Supported Pt Nanoparticle ORR Electrocatalyst

Abstract

In recent years, hydrogen proton exchange membrane fuel cells have gained growing attention in the heavy duty vehicle (HDV) sector due to lower cost with increased driving range and lower refueling time compared to Li ion battery systems.[1] However, special requirements apply to HDV fuel cells such as long lifetime (25000 – 30000 h) at low Platinum loadings. Additionally, future Pt cathode loadings should be reduced from ≥ 0.4 mgPt/cm² (i.e. the current benchmark) to 0.25 mgPt/cm² by 2050, while a constant power density of 840 mW/cm² at 0.769 V should be achieved as defined by the US Department of Energy.[2]Despite Pt nanoparticles finely dispersed on carbon supports are considered the benchmark catalysts, the cathode is yet suffering from several degradation processes such as carbon corrosion, Pt nanoparticle detachment, agglomeration, dissolution and Pt redeposition, hindering the achievement of the aforementioned goals.[3, 4] Therefore, in-depth understanding of the electrocatalyst degradation mechanisms by which Pt losses its catalytic activity is required to develop optimized synthesis methods for stable materials. Recently, we developed novel mesoporous N-doped carbon (MPNC) supports with improved stability against carbon corrosion in comparison to a commercial reference carbon support.[5] Utilizing the advantage of such enhanced stability, we designed Pt/MPNC catalysts for oxygen reduction reaction (ORR) with an extended stability reaching over 10000 cycles in comparison to the commercial Pt/C catalyst, as revealed by the accelerated stress tests (AST) in acidic media and within the carbon corrosion potential range between 1 V and 1.5 V vs RHE.In the present study, we synthesized Pt/MPNC-T catalysts under different reduction temperatures to control the Pt nanoparticle size distribution. We then investigated the impact of the various sizes of the Pt nanoparticles on the stability of Pt/MPNC-T after AST cycles. Therefore, Rotating disc electrode measurements with an established Pt AST protocol using a square wave potential between 0.6 V and 0.95 V vs. RHE over 30000 cycles was applied.[1] The ORR activity, the electrochemical active surface area by HUPD and CO stripping experiments were measured at the beginning of the cycle life test, then after 10000 AST cycles and finally after 30000 AST cycles. The electrochemical measurements before and after AST of the Pt/MPNC-T catalysts were correlated with ex-situ TEM and aberration corrected (AC) STEM data. Experimentally, lower reduction temperature synthesis leads to broader Pt size distributions. Thereby, we noticed the presence of Pt nanoparticles with a diameter between 1 and 3 nm, and smaller Pt entities such as Pt atom clusters and single sites of Pt (i.e. Pt single atoms) at the beginning of cycling. By imaging the surface after 30000 AST cycles, the Pt nanoparticle size substantially increased and the amount of smaller Pt entities was drastically reduced. Such findings indicate that the major degradation process follows the Ostwald ripening mechanism.In conclusion such results reveal the importance of determining the structure-function correlations, which contributes to the general understanding of catalyst degradation processes and allows a knowledge-based optimization of these Pt/MPNC materials in future works.