Efficient Exploration of Electrochemical Pre-Treatment on the Performance of Nanocrystal Electrocatalysts By Design of Experiments

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Activation, break-in, or pre-treatment protocols are electrochemical techniques applied in energy-storage or -conversion devices, like fuel cells, before regular operation. Although these protocols have been demonstrated to optimize the utilization of thin-film electrocatalysts, their impact on the intrinsic physical properties and the performance of the practically relevant nanocrystal-based catalysts remains poorly understood. This study employs a design-of-experiments (DoE) approach to investigate how electrochemical pre-treatment affects the performance of electrocatalytic oxygen reduction reaction (ORR) in carbon-supported Pt-nanocrystal catalysts. First, we designed various pre-treatment protocols using a central composite design for five different levels of five electrochemical factors: 1) upper potential (UPL), 2) potential sweep depth, 3) sweep rate, 4) number of cycles, and 5) hold time at potential extrema. These protocols were then tested in a flow cell combined with an inductively-coupled plasma mass spectrometer (online ICP-MS). Utilizing the DoE approach, we achieved comprehensive insights through a limited set of experiments. Moreover, our online ICP-MS setup yielded mass-transfer limited current densities and simultaneously provided real-time data on catalyst dissolution (down to 0.1 ppb). Such an experimental strategy enabled us to efficiently and rapidly explore the effect of a vast parametric space on the electrocatalytic performance. Surprisingly, unlike Pt-based thin films, our results indicate that none of the pre-treatment protocols tested resulted in significant improvements to Pt nanocrystal ORR performance. Instead, high UPLs (~ 2 V RHE ) during pre-treatment were found to lower the onset potential for ORR. Additionally, combining such high UPLs with a low potential sweep depth (~ 0.7 V) in pre-treatment led to increased dissolution during ORR. To understand these findings, we explored five potential degradation mechanisms: 1) carbon corrosion 2) Ostwald ripening of nanocrystals 3) Pt dissolution 4) coalescence of nanocrystals, and 5) surface changes in Pt by employing various in-situ and ex-situ characterization techniques. Our investigations identified surface changes in Pt at high UPLs in pre-treatment as the primary cause of performance deterioration during ORR. Therefore, by combining DoE with in-situ and ex-situ characterization techniques, we demonstrate a powerful approach to gain a mechanistic understanding of pre-treatment protocols on electrocatalytic performance that can be broadly applied to various reaction chemistries and material libraries.