The hydrogen oxidation and evolution reactions (HOR, HER) are of key importance to the development of novel alkaline membrane fuel cells and electrolyzers, which feature a potential cost advantage over their acid-operating counterparts. However, their mechanism remains poorly understood even on the most catalytically-active platinum surfaces, for which such a fundamental parameter as the reactions' order with respect to the hydrogen concentration is still unknown. With this motivation, we have performed rotating disc electrode measurements on polycrystalline Pt in 0.1 M NaOH with different hydrogen partial pressures (between 10 and 100 kPa H 2 ), from which we have derived a reaction order of 1/2. The latter value has important implications in the procedure to follow in order to derive kinetic currents free of diffusion contributions. More precisely, the HOR currents must be corrected for the diffusion overpotential and converted into a mass-transport free kinetic current using a modified version of the Koutecky-Levich equation that takes into consideration this non-unit reaction order, while the HER side only needs to be ohmically compensated due to the impossibility to supersaturate the electrolye with H 2 . Most importantly, our results point at a mechanism consisting of a dissociative adsorption (Tafel) reaction combined with a one-electron transfer (Volmer) rate-determining step, in terms consistent with the well-established view of the hydrogen-bonding strength as the main HOR/HER activity descriptor. The electrochemical oxidation and evolution of molecular hydrogen are the key reactions at play in the anodes and cathodes of fuel cells and electrolyzers, respectively, which are likely energy conversion and storage devices for renewable energy concepts based on the use of H 2 as energy carrier. Beyond this practical interest, the hydrogen oxidation and evolution reactions (HOR and HER, respectively) have also played a pivotal role in the historical development of fundamental electrocatalysis theories. Indeed, the exponential relation between current and overpotential that describes the kinetics of many electrochemical reactions, viz., the Butler-Volmer equation, was originally validated for the HER, 1 which also became the first electrochemical process for which the rate-determining role of the bond strength of adsorbed intermediates (following Sabatier's principle) was verified.2 Thus, the HOR/HER has become one of the most extensively studied electrochemical reactions, particularly at low pH values and on the catalytically most active platinum-based materials relevant to proton exchange membrane fuel cells (PEMFCs) and electrolyzers. In acid electrolytes, the HOR/HER kinetics on platinum electrodes are extremely fast, so that experimental methods which afford very high mass-transport rates are required in order to unambiguously differentiate kinetic-and diffusion-overpotentials (e.g., hydrogen pump experiments in PEMFC 3 or the recently developed floating porous gas diffusion electrode method 4 ). Using t...

This study examines the effect of ink composition on the ionomer distribution in the catalyst layer of membrane electrode assemblies (MEA) prepared by decal transfer. We combine both structural and electrochemical characterization techniques to investigate the influence of the ionomer distribution on MEA performance determined by 50 cm 2 active area single-cell proton exchange membrane (PEM) fuel cell measurements. Cathodic catalyst layers were prepared from inks with different alcohols (1-propanol or 2-propanol) and varying water content (16-65 wt% H 2 O). The H 2 /air performance of cathode catalyst layers prepared from the different inks with 700 EW ionomer differed drastically, particularly under wet operating conditions, whereby the best performance was obtained for an ink based on 16 wt% H 2 O in 1-propanol. This was successfully correlated with the observation of ionomer patches at the cathode electrode surface (i.e., the surface facing the diffusion medium) determined by scanning electron microscopy (SEM), with N 2 adsorption analysis of the electrodes using a QSDFT model, and with dynamic light scattering data of ionomer/solvent mixtures. No correlation could be obtained between H 2 /air performance and the proton conductivity of the cathodes obtained by electrochemical impedance spectroscopy, and a model to rationalize this behavior will be proposed.

In this study, we provide an overview regarding our recent finding on the mechanism, activity and particle size effect for the hydrogen oxidation and evolution reaction (HOR/HER) on Pt-group metal electrodes, under both acid and alkaline conditions. We show that there is an activity decrease of about two orders of magnitude when going from acid to base conditions on electrodes which are able to form a H-UPD layer like Pt, Ir, Pd and Rh. Similarities in the HOR/HER process between acid and base conditions have been found: the rate determining step, which has been identified by means of electrochemical impedance spectroscopy measurements to be the Volmer reaction, remains the same and there is no particle size dependency on the reactivity of Pt electrodes. In the light of these finding, Volcano plots of the HOR/HER activities in acid and base have been proposed.

Alkaline membrane fuel cells (AMFCs) are attracting increasing attention owing to recent developments in anion-exchange membranes and non-noble-metal O 2 -reduction catalysts. Despite these promising improvements, the sluggish kinetics of the H 2 -oxidation reaction (HOR) on Pt imply the need for considerable amounts of this noble metal at the AMFC anode, rendering the system more expensive and ultimately hindering its commercialization. Interestingly, the majority of the works dealing with the kinetics of the HOR on Pt were performed in relatively dilute (0.1 M) alkaline electrolytes with a pH lower than that of the operative AMFC. Wondering whether this difference in OH − -concentration would have a significant effect on the HOR-kinetics, here we report our results on the HOR-activity of polycrystalline Pt in 0.1 M and 1 M KOH. These were derived from rotating disc electrode measurements performed in a temperature-controlled PTFE-cell, which allowed us to estimate reliable values for the exchange current density and activation energy in both alkaline solutions.

Start-up and shut-down (SUSD) events in proton exchange membrane fuel cells (PEMFCs) are a major source of cathode degradation, causing a loss of electrochemical surface area (ECSA) and carbon corrosion. Our study reveals that also the anode suffers significant damage during SUSD, dominated by the loss of ECSA, induced by potential cycling between ≈0 and ≈1 V upon the passage of H 2 /air fronts. Furthermore, we demonstrate the analogy of SUSD-induced anode degradation and that originating from quasi-square wave potential cycling between 0.05 and 1.05 V RHE . The performance penalties arising from a decrease of the kinetics of the hydrogen oxidation reaction (HOR) and growing H 2 mass-transport resistances are measured via H 2 -pump experiments. The thus projected anode voltage losses for low anode Pt loadings (25 μg Pt cm −2 ) predict HOR kinetic losses of ≈40 mV at 80 • C and 3 A cm −2 for aged anode catalyst layers, suggesting that anode degradation by SUSD could be a significant durability issue in future PEMFC systems with ultra-low Pt loadings and with more stable cathode catalyst carbon supports. Moreover, SUSD-induced H 2 mass-transport related overpotentials were identified and attributed to carbon corrosion, indicated by a thinning of the anode catalyst layer upon aging.