Kinetics of the Hydrogen Oxidation/Evolution Reaction on Polycrystalline Platinum in Alkaline Electrolyte Reaction Order with Respect to Hydrogen Pressure

Abstract

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...

Bulk-Palladium and Palladium-on-Gold Electrocatalysts for the Oxidation of Hydrogen in Alkaline Electrolyte

“…[3][4][5] 83 with the only exception of the Tafel-RDS mechanism, for which a much lower Tafel slope of ≈30 mV · dec −1 would be expected. Unfortunately, unlike in previous studies, 47,82 we were unable to fit our results to Eq. 9.…”

“…9 is only valid over a potential region in which changes in θ H are small (i.e., typically between ≈−0.05 and ≈0.05 V RHE ). 16,82 In the case of Figure 12a, however, the HER-and HOR-branches extend between −0.1 and > +0.5 V RHE , concomitant with a large variation in the H-coverage, so that Eq. 9 cannot describe the overall kinetics.…”

“…82 In brief, following the iR-correction of the polarization curves, the HER-currents were considered as being kinetically controlled, while the HOR-branches were additionally corrected for the diffusion overpotential (η diff ) and transformed into kinetic currents (i kin,E ) using Equations 7 and 8, respectively: [7] i kin,E = i meas,E · i lim i lim − i meas,E 0.5 [8] where i lim represents the diffusion-limited HOR-current that, for the conditions applied in our measurements (i.e., in 0.1 M NaOH at 293 K and with ω = 1600 rpm) amounts to ≈2.5 mA · cm −2 geom (see the polarization curve for Pt PC in Figs. 2b and 7b).…”

“…The HOR/HER-mechanism on these monometallic surfaces has been ascribed 82 to a fast Tafel reaction followed by a rate-determining Volmer step (cf. Eq.…”

Pd/Ni Synergestic Activity for Hydrogen Oxidation Reaction in Alkaline Conditions

“…A better knowledge of the reaction mechanisms still needs to be undergone such as the recent literature on Pt. 34 …”