Polymer electrolyte membrane water electrolyzers (PEMWE) are promising devices for energy conversion from electricity to chemistry. They are able to deal with important and fast changes in energy production. At the cathode compartment, the hydrogen evolution reaction occurs without kinetic limitation and practically reversibly. Nonetheless, the oxygen evolution reaction (OER) which is the anodic reaction remains a scientific challenge due to kinetic limitations and stability issues. Iridium is currently the best compromise between activity and stability [1]. However, it still undergoes degradation in the harsh oxidative and acidic operating conditions of a PEMWE [2]. The development of optimized OER catalysts is an essential step to practically implement PEMWE. A better understanding of structural effects on the catalysts activity and stability is needed. This study focuses on well-defined electrocatalysts at an atomic scale . The Ir(111) surface was characterized before and after electrooxidation and OER thanks to different electrochemical methods. A Pt(111) single-crystal, widely studied in the literature [3-4], was characterized in the same way and used as a reference. Above 1.4 vs. RHE and 1.3V vs. RHE, respectively for Pt(111) and Ir(111), the initial clean surface undergoes irreversible strucutral changes, related to the formation of defects and leading to an improvement of the oxygen evolution activity. Although classical electrochemical impedance spectroscopy (EIS) is a valuable technique when applied to stable and reversible systems, it fails with irreversible ones. Therefore, we used dynamic electrochemical impedance spectroscopy (DEIS). One can see (fig. 1) that the two methods give the same result in the reversible hydrogen adsorption area (fig.1a) while they differ for oxide formation (fig.1b) and OER (fig.1c). By analyzing the whole set of DEIS spectra, we are able to provide details on mechanism parameters for oxide formation and OER. References [1] Aricò, A. S., et al. (2012). "Polymer electrolyte membrane water electrolysis: status of technologies and potential applications in combination with renewable power sources." Journal of Applied Electrochemistry 43 (2): 107-118. [2] Cherevko, S., et al. (2014). "Dissolution of Noble Metals during Oxygen Evolution in Acidic Media." ChemCatChem 6 (8): 2219-2223. [3] CONWAY, B. E. (1995). "ELECTROCHEMICAL OXIDE FILM FORMATION AT NOBLE METALS AS A SURFACE-CHEMICAL PROCESS." Progress in Surface Science 49 (4): 331-452. [4] Drnec, J., et al. (2017). "Initial stages of Pt(111) electrooxidation: dynamic and structural studies by surface X-ray diffraction." Electrochimica Acta 224 : 220-227. Figure 1
The impact of the carbon structure, the aging protocol, and the gas atmosphere on the degradation of Pt/C electrocatalysts were studied by electrochemical and spectroscopic methods. Pt nanocrystallites loaded onto high-surface area carbon (HSAC), Vulcan XC72, or reinforced-graphite (RG) with identical Pt weight fraction (40 wt %) were submitted to two accelerated stress test (AST) protocols from the Fuel Cell Commercialization Conference of Japan (FCCJ) mimicking load-cycling or start-up/shutdown events in a proton-exchange membrane fuel cell (PEMFC). The load-cycling protocol essentially caused dissolution/redeposition and migration/aggregation/coalescence of the Pt nanocrystallites but led to similar electrochemically active surface area (ECSA) losses for the three Pt/C electrocatalysts. This suggests that the nature of the carbon support plays a minor role in the potential range 0.60 < E < 1.0 V versus RHE. In contrast, the carbon support was strongly corroded under the start-up/shutdown protocol (1.0 < E < 1.5 V versus RHE), resulting in pronounced detachment of the Pt nanocrystallites and massive ECSA losses. Raman spectroscopy and differential electrochemical mass spectrometry were used to shed light on the underlying corrosion mechanisms of structurally ordered and disordered carbon supports in this potential region. Although for Pt/HSAC the start-up/shutdown protocol resulted into preferential oxidation of the more disorganized domains of the carbon support, new structural defects were generated at quasi-graphitic crystallites for Pt/RG. Pt/Vulcan represented an intermediate case. Finally, we show that oxygen affects the surface chemistry of the carbon supports but negligibly influences the ECSA losses for both aging protocols.
In this work, we investigated through-the-plane heterogeneities of an ultramicrotomed MEA aged at constant current density j = 0.24 A cm-2 in a 16 cell stack for t = 1341 h. We used structural markers to unveil the physical and chemical changes occurring at both the GDL | CL and at the CL | PEM interfaces. We evidenced that the degradation of the cathode catalytic layer is more pronounced close to the PEM than close to the GDL. Spatially-localized heterogeneities of ageing are discussed in the frame of different chemical/electrochemical environments as well as incomplete utilization of the cathodic electrocatalyst under PEMFC operation.
The integration of promising bimetallic electrocatalytic active materials for oxygen reduction reaction (ORR) into practical and functional proton-exchange membrane fuel cell (PEMFC) electrodes remains largely impeded by the poor performances that these exhibit at high current loads. The early life of PtNi/C catalysts presenting either structurally faceted/ordered or defective/disordered surface morphology is compared to that of both spherical PtNi/C and Pt/C catalysts. Different single-cell operating conditions were studied. At low current density, in the kinetically limited region, a good agreement between liquid electrolyte and single-cell configuration is reported and the kinetic benefit of PtNi/C catalysts compared to Pt/C is (at least partially) maintained. However, PtNi/C catalysts show severe limitations in the O 2 mass transport limited region. Morphological and compositional changes were monitored at each stage showing that Ni atoms are leached at every step from ink formulation to the first PEMFC test. We show that Ni is already redistributed in the membrane in the fresh membrane electrode assembly (MEA) state. Ni 2+ cationic contamination of the ionomer/membrane contributes to the disappointing results obtained in MEA configuration. In addition, for shaped-controlled PtNi/C, the surface faceting loss combined with restructuring via coalescence and crystallite growth further compromise their transfer in technological devices.
