Catalyst layers in proton exchange membrane fuel cells consist of platinum-group-metal nanocatalysts supported on carbon aggregates, forming a porous structure through which an ionomer network percolates. The local structural character of these heterogeneous assemblies is directly linked to the mass-transport resistances and subsequent cell performance losses; its three-dimensional visualization is therefore of interest. Herein we implement deep-learning-aided cryogenic transmission electron tomography for image restoration, and we quantitatively investigate the full morphology of various catalyst layers at the local-reaction-site scale. The analysis enables computation of metrics such as the ionomer morphology, coverage and homogeneity, location of platinum on the carbon supports, and platinum accessibility to the ionomer network, with the results directly compared and validated with experimental measurements. We expect that our findings and methodology for evaluating catalyst layer architectures will contribute towards linking the morphology to transport properties and overall fuel cell performance.

Liquid-phase transmission electron microscopy (LPTEM) is an essential tool for studying the dynamics of materials interactions at the nanoscale, in and/or with their operational environment. Microfabricated SiNx membrane cells further allow the integration of thin-film electrodes which opens the technique to studies of heterogeneous electrocatalysts under relevant electrochemical conditions. However, experiments remain challenging and the specificities of the dedicated electrochemical cells and of the interactions of the electron beam with the electrolyte demand careful interpretation of the results. Herein, we discuss important aspects of the implementation of ec-LPTEM. We first consider the range of information that can be accessible with the technique for electrocatalytic applications and we detail the influence of the thickness and flow of liquid electrolytes on the operation using membrane-based microcells. Further, we provide guidelines for electrochemical configuration of the substrate working, reference, and counter electrodes. We draw all considerations together by experimentally demonstrating the application of ec-LPTEM for the CO2 reduction reaction, the oxygen reduction reaction, and the oxygen evolution reaction. The probed effects in metallic and oxide catalysts are directly related to the applied electrochemical stimuli and corroborate the representativity of the processes under investigation.

Electrochemical liquid cell transmission electron microscopy (TEM) is a unique technique for probing nanocatalyst behavior during operation for a range of different electrocatalytic processes, including hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), or electrochemical CO2 reduction (eCO2R). A major challenge to the technique's applicability to these systems has to do with the choice of substrate, which requires a wide inert potential range for quantitative electrochemistry, and is also responsible for minimizing background gas generation in the confined microscale environment. Here, we report on the feasibility of electrochemical experiments using the standard redox couple Fe(CN)63−/4− and microchips featuring carbon-coated electrodes. We electrochemically assess the in situ performance with respect to flow rate, liquid volume, and scan rate. Equally important with the choice of working substrate is the choice of the reference electrode. We demonstrate that the use of a modified electrode setup allows for potential measurements relatable to bulk studies. Furthermore, we use this setup to demonstrate the inert potential range for carbon-coated electrodes in aqueous electrolytes for HER, OER, ORR, and eCO2R. This work provides a basis for understanding electrochemical measurements in similar microscale systems and for studying gas-generating reactions with liquid electrochemical TEM.

Electrochemical liquid-phase transmission electron microscopy (TEM) is showing excellent promise in fundamental studies of energy-related processes including lithium-ion battery (LIB) cycling. A key requirement to accurately interpret the measurements and acquire quantitative information is the implementation of a reliable reference electrode. Quasi-reference electrodes (QRE) remain commonly used due to microfabrication constraints of the electrochemical cell, however, they typically yield dramatic potential drifts making the electrochemical results inconclusive. Here, we present a method of producing a stable and readily interpretable lithium-gold alloy micro-reference electrode, which exhibits a reference potential of 0.1 V vs Li/Li+. We first examine the feasibility of electrochemically alloying a pristine gold electrode, patterned on a chip for in situ TEM, using a benchtop setup, and investigate various sources to support the lithiation. We confirm the presence of the Li-Au alloy using chronopotentiometry (CP) and open circuit voltage (OCV) measurements, and by scanning electron microscopy (SEM), electron energy loss spectroscopy (EELS) and high-resolution (HR) TEM. Finally, we apply this methodology in situ and use LiFePO4 as a model cathode material to demonstrate the merit of the Li-Au alloy reference electrode for obtaining reproducible cyclic voltammetry (CV) measurements on a liquid cell microelectrode system.

Non-noble metal catalysts (NNMCs) hold the potential to replace the expensive Pt-based materials currently used to speed up the oxygen reduction reaction (ORR) in proton exchange membrane fuel cell (PEMFC) cathodes, but they feature poor durability that inhibits their implementation in commercial PEMFCs. This performance decay is commonly ascribed to the operative demetallation of their ORR-active sites, the electro-oxidation of the carbonaceous matrix that hosts these active centers, and/or the chemical degradation of the ionomer, active sites, and/or carbon support by radicals derived from the H2O2 produced as an ORR by-product. However, little is known regarding the relative contributions of these mechanisms to the overall PEMFC performance loss. With this motivation, in this study, we combined four degradation protocols entailing different cathode gas feeds (i.e., air vs N2), potential hold values, and durations to decouple the relative impact of the above deactivation mechanisms to the overall performance decay. Our results indicate that H2O2-related instability does not depend on the operative voltage but only on the ORR charge. Moreover, the electro-oxidation of the carbon matrix at high potentials (which for the catalyst tested herein triggers at 0.7 V) seems to be more detrimental to the NNMCs’ activity than the demetallation occurring at low potentials.