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

Abstract

The commercial feasibility of alkaline-exchange membrane fuel cells and electrolyzers passes by the development of hydrogen oxidation and evolution reaction (HOR/HER) catalysts featuring an activity and/or cost advantage over platinum, which remains the most active metal for these processes. Among these alternatives, Pd appears as a promising candidate, since its price is typically 2-3 fold lower than that of Pt. With this motivation, the first section of this study displays our attempts at quantifying the kinetic parameters of the HOR/HER on bulk Pd in 0.1 M NaOH, which were prevented by the simultaneous absorption of hydrogen into bulk palladium. We succeeded at circumventing this issue by depositing Pd-adlayers on a polycrystalline Au-substrate by galvanic displacement of underpotentially-deposited Cu or by electrochemical plating of Pd 2+ . The resulting surfaces appear to consist of three-dimensional Pd-structures of an unknown thickness that we believe to scale with the palladium coverage, θ Pd/Au . This last parameter is inversely proportional to the HOR/HER-activity of the Pd-on-Au surfaces, in agreement with numerous theoretical and experimental studies in acid media that correlate this effect to the tensile strain induced by the Au-substrate on the Pd-lattice. Proton-exchange membrane fuel cells (PEMFCs) fueled with hydrogen stored at 700 bar can attain energy densities in excess of 200 Wh · kg −1 (on a PEMFC system basis), making them excellent candidates for the propulsion of environmentally-benign, full range (≥ 300 miles) vehicles.1 However, the actual commercialization of PEMFC powered cars has been deterred by the lack of a hydrogen infrastructure and the high cost of the PEMFC. In this last respect, a significant fraction of the system's price is related to the costly platinum-based catalysts needed to catalyze the oxidation of hydrogen and the reduction of oxygen taking place at the FC anode and cathode, respectively.1-3 The kinetics of the oxygen reduction reaction (ORR) on Pt 4 are ≈7 orders of magnitude slower than those of the hydrogen oxidation reaction (HOR), 5 in agreement with the HOR 5 -and ORR 4 -exchange-current density (i 0 ) values of 4 ± 2 · 10 +2 mA · cmPt estimated by Neyerlin and coworkers (at 80• C and 100 kPa H 2 /air partial pressure). As such, the Pt-loading at the PEMFC's cathode (≈0.2-0.4 mg Pt · cm −2 ) is well above the ≤ 0.05 mg Pt · cm −2 required for the anodic HOR, and thus much research is being devoted to the development of more active and/or less expensive ORR catalysts.6-10 These include alloys of Pt with non-noble metals like Ni, Co or Cu (as well as their de-alloyed derivatives), and catalysts consisting of non-noble metal nanoparticles covered by one or a few monolayers of Pt (i.e., the so-called "core-shell" catalysts).Alternatively, operating a fuel cell in alkaline environment allows for the use a wider variety of potentially inexpensive, noblemetal free ORR-catalysts. Among these, certain materials based on Fe-, Co-and/or Cu-N 4 chelates (or equivalent no...

Size-Dependent Hydrogen Oxidation and Evolution Activities on Supported Palladium Nanoparticles in Acid and Base

“…Alternative methods such as H upd adsorption/desorption peaks and CO-stripping can also be used to determine surface area, all of which have advantages and drawbacks. 13 Although the ECSAs derived from PdO reduction might not be the most accurate in absolute terms, the general trend is consistent with values obtained with other methods. Thus, it is still meaningful to compare activities normalized by ECSAs obtained with PdO reduction.…”

“…ECSAs calculated from CV conducted in KOH (S Base CV ) and HClO 4 (S Acid CV ) are comparable (Table I), in agreement with Henning et al's earlier observations. 13 Consistent with the surface area determined by TEM and XRD, ECSA decreases with increasing particle size (Table I), however, ECSAs are roughly a factor of two smaller than S v/a TEM and S v XRD (Table I). One potential cause for this discrepancy is that a fraction of every particle is in contact with the support and thus is not electrochemically accessible.…”

“…Pd and Pd-based catalysts have been studied as HOR catalysts in fuel cells, 9,10 and lower HOR/HER activities were observed on Pd/C than bulk Pd, [11][12][13] suggesting a particle size effect. However, a fundamental understanding of HOR/HER kinetics on Pd and a systematic study of the particle size effect are largely missing.…”

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

“…This low values (<0.1 mA/cm 2 Pd ) seem to be typical of pure Pd in alkaline conditions and has been corroborated by the study of Pd covered Au surface. 7 In the case of Pd/Ni, lower Pd coverage allows for much higher values of exchange current, even above 1 mA/cm 2 Pd (10 times higher than pure Pd and almost the value of Pt 4 )…”

“…6 Detailed investigation of HOR activity of polycrystalline palladium in alkaline electrolyte has been published more recently. 7 Synergistic effect of Pd-based alloyed bimetallic catalysts (PdRu, PdIr) or Pd supported tungsten oxide has also been reported for the promotion of HOR activity. 8 Combining palladium with abundant transition metals can be a further step towards higher power and lower cost AMFC anodes.…”