Pd-Based Bimetallic Electrocatalysts for Hydrogen Oxidation Reaction in 0.1 M KOH Solution

Abstract

A series of carbon black-supported 7.5 wt.% Pd-2.5 wt.% M/C (M: Ag, Ca, Co, Cu, Fe, Ni, Ru, Sn, Zn) electrocatalysts, synthesized via the wet impregnation method, and reduced at 300 °C, were compared in terms of their hydrogen oxidation reaction (HOR) activity in a 0.1 M KOH solution using the thin-film rotating-disk electrode technique. Moreover, 10 wt.% Pd/C and 10 wt.% Pt/C electrocatalysts were prepared in the same manner and used as references. The 7.5 wt.% Pd-2.5 wt.% Ni/C electrocatalyst exhibited the highest HOR activity among the Pd-based electrocatalysts, although it was lower than that of the 10 wt.% Pt/C. Its activity was also found to be higher than that of Pd-Ni electrocatalysts of the same total metal loading (10 wt.%) and reduction temperature (300 °C) but of different Pd to Ni atomic ratio. It was also higher than that of 7.5 wt.% Pd-2.5 wt.% Ni/C electrocatalysts that were reduced at temperatures other than 300 °C. The superior activity of this electrocatalyst was attributed to an optimum value of the hydrogen binding energy of Pd, which was induced by the presence of Ni (electronic effect), as well as to the oxophilic character of Ni, which favors adsorption on the Ni surface of hydroxyl species that readily react with adsorbed hydrogen atoms on neighboring Pd sites in the rate-determining step.

Keywords: Pd-based electrocatalysts; alkaline medium; hydrogen binding energy; hydrogen oxidation reaction; hydroxyl species binding energy; oxophilic effect; rotating-disk electrode; strain effect.

Figure 1

Polarization curves for the (a) 10 Pt, 7.5 Pd-2.5 Cu, 7.5 Pd-2.5 Fe, 7.5 Pd-2.5 Ni, 7.5 Pd-2.5 Zn and (b) 10 Pd, 7.5 Pd-2.5 Ag, 7.5 Pd-2.5 Ca, 7.5 Pd-2.5 Co, 7.5 Pd-2.5 Ru, 7.5 Pd-2.5 Sn electrocatalysts, obtained in 0.1 M KOH solution saturated with ${\mathrm{H}}_2$, at room temperature and 3000 rpm, by scanning the potential of the electrocatalyst at a rate of 5 mV s⁻¹. Mass-transfer corrected Tafel plots for the HOR with the kinetic current I_k normalized with respect to ECSA_CO (c,d) and the Pt or Pd mass of the electrocatalyst (e,f). Potential values (U-IR) are ohmic drop corrected.

Figure 2

Comparison of the specific activity (S.A.) and mass activity (M.A.) of the (a,b) 10 Pt, 7.5 Pd-2.5 Cu, 7.5 Pd-2.5 Fe, 7.5 Pd-2.5 Ni, 7.5 Pd-2.5 Zn and (c,d) 10 Pd, 7.5 Pd-2.5 Ag, 7.5 Pd-2.5 Ca, 7.5 Pd-2.5 Co, 7.5 Pd-2.5 Ru, 7.5 Pd-2.5 Sn electrocatalysts at selected potentials (ohmic drop-free). Conditions: 0.1 M KOH solution saturated with ${\mathrm{H}}_2$ and room temperature.

Figure 3

Correlation between the Pd lattice strain and (a) HOR specific activity and (b) HOR mass activity of the tested Pd-based electrocatalysts (Table 1) in 0.1 M KOH solution saturated with ${\mathrm{H}}_2$ and room temperature.

Figure 4

X-ray diffraction patterns of 7.5 Pd-2.5 Ni electrocatalyst powders reduced at 200, 300, 450, and 600 °C, as well as of the carbon black support (Vulcan XC72R).

Figure 5

(a) CO stripping linear sweep voltammograms obtained in He-purged 0.1 M KOH solution at room temperature with a potential scan rate of 50 mV s⁻¹ after previous adsorption of CO at −0.7 V vs. Ag/AgCl for 15 min; (b) Polarization curves obtained in 0.1 M KOH solution saturated with ${\mathrm{H}}_2$, at room temperature and 3000 rpm, by scanning the potential of the electrocatalyst at a rate of 5 mV s⁻¹; Mass-transfer corrected Tafel plots for the HOR in ${\mathrm{H}}_2$ saturated 0.1 M KOH solution at room temperature with the kinetic current I_k normalized with respect to ECSA_CO (c) and the Pd mass (d) for the 7.5 Pd-2.5 Ni electrocatalysts reduced at 200, 300, 450 and 600 °C (Table 2). U-IR denotes ohmic drop corrected potential.

Figure 6

Dependence of the specific activity (a) and mass activity (b) at −0.8 V vs. Ag/AgCl (0.16 V vs. RHE) on the average Pd crystallite size for the 7.5 Pd-2.5 Ni electrocatalysts that were reduced at 200, 300, 450, and 600 °C.

Figure 7

X-ray diffraction patterns of the monometallic 10 Pd and 10 Ni and of the bimetallic x Pd-y Ni catalytic powders with different Pd:Ni atomic ratios (Table 3), reduced at 300 °C, as well as of the carbon black support (Vulcan XC72R).

Figure 8

(a) CO stripping linear sweep voltammograms obtained in He-purged 0.1 M KOH solution at room temperature with a potential scan rate of 50 mV s⁻¹ after previous adsorption of CO at −0.7 V vs. Ag/AgCl for 15 min; (b) Polarization curves obtained in 0.1 M KOH solution saturated with ${\mathrm{H}}_2$, at room temperature and 3000 rpm, by scanning the potential of the electrocatalyst at a rate of 5 mV s⁻¹; Mass-transfer corrected Tafel plots for the HOR in ${\mathrm{H}}_2$ saturated 0.1 M KOH solution at room temperature, with the kinetic current I_k normalized with respect to ECSA_CO (c) and to the Pd mass (d) for the monometallic 10 Pd and the bimetallic x Pd-y Ni electrocatalysts with different Pd:Ni atomic ratio, reduced at 300 °C (Table 3). U-IR denotes ohmic drop corrected potential.

Figure 9

Dependence of the specific activity (a) and mass activity (b) at −0.8 V vs. Ag/AgCl (0.16 V vs. RHE) on the Pd content for the 10 Pd and the bimetallic x Pd-y Ni electrocatalysts with different Pd:Ni atomic ratio, reduced at 300 °C (Table 3).