Article Summary: “The Effect of Carbonate and pH on Hydrogen Oxidation and Oxygen Reduction on Pt-Based Electrocatalysts in Alkaline Media” (2016)

In the article by Samuel St. John et al. the authors examined the effects of pH and carbonate on the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) with Ru-Pt alloy catalysts and compared this against Pt monometallic catalyst.

Ru-Pt alloys are of interest because they are the best performing HOR catalysts for alkaline fuel cells. Ruthenium shifts the d-band center closer to the Fermi level versus platinum, increasing the electron density in the catalyst conduction band. This decreases the metal-hydrogen binding energy, and increases the oxophilicity of the electrocatalyst surface, resulting in an improvement in the reactivity of H_ads with adsorbed hydroxyl groups, OH_ads.

Catalysts of varying Ru-Pt compositions were synthesized using a chemical-vapor-phase deposition process, from which inks were produced and deposited onto glassy carbon electrodes. The compositions were verified using extended X-ray absorption fine structure (EXAFS) and energy-dispersive spectroscopy (EDS). A standard three-electrode electrochemical cell was used in order to conduct the electrochemical tests.

From the electrochemical charging currents that were collected in N₂-saturated 0.1 M KOH electrolyte, it was found that, as the Ru content increased, there was a shift in the surface oxide reduction peak toward more cathodic potentials. This indicates that the surface oxides are becoming more difficult to reduce with increasing ruthenium content, as a consequence of the increased oxophilicity.

Hydrogen oxidation data were collected using H₂-saturated KOH while rotating the electrode at 1600 rpm with a 10 mV/s sweep rate. These were conducted with several different pH values within a narrow range (12.08-12.95). It was found that the hydrogen oxidation current moves toward the mass-transport-limited current as the pH decreases. However, the Ru-Pt alloy catalysts exhibited much less pH sensitivity than the monometallic Pt.

The authors believe that this change in pH sensitivity is a result of the rate determining step (rds) for HOR changing, as you switch from monometallic Pt to Ru-Pt alloy, from the electron-transfer Heyrovsky/Volmer step:

H₂ + OH^– ↔ H_ads + e^– + H₂O (Heyrovsky Reaction)

H_ads + OH^– ↔ e^– + H₂O (Volmer Reaction)

to the dissociative hydrogen adsorption Tafel step:

H₂ ↔ 2H_ads

Further evidence in favor of this interpretation comes from Tafel analysis of the kinetic currents conducted by the authors. The authors found that the Tafel slope for the monometallic Pt for all pH examined was around 127 mV/dec, which is consistent with an electron transfer rds. On the other hand, for the Ru-Pt catalysts, the Tafel slopes were all around 30 mV/dec, which is consistent with a hydrogen dissociative adsorption rds, such as the Tafel reaction.

To investigate the impact of carbonate on the oxidation of adsorbed hydrogen, the authors measured the HOR polarization response in both carbonate and potassium phosphate electrolytes, for varying pH levels. The shift in the half-wave potential exhibited the same trend in both cases. The authors suggest that this should not be taken as discounting that carbonate is involved in HOR, especially given that carbonate is known to be a good proton acceptor. Rather, they propose that water is also an active participant, whose effects overwhelm and mask those of carbonate due to concentration effects. This is based on the fact that water is also a proton acceptor and can rapidly self-dissociate, and is the major component in the alkaline electrolyte. Thus, they hypothesize that it serves to shuttle protons to the bulk hydroxide during HOR.

It is also hypothesized that the most significant barrier for HOR/HER is the configurational entropy penalty associated with the transport of protons through the double layer, and the composition of the double layer being the primary source of sensitivity to pH. Catalysts with bi-functional effects would then be less sensitive to pH, due to the surface reaction of adsorbed hydrogen and hydroxyl species removing the need to transport protons through the double layer. Looking back at the shifts in the half-wave potentials for the different catalyst compositions, it was found that there was a minimum shift for Ru-Pt catalysts that had 60-80% Pt composition, suggesting an optimum range for HOR catalyst composition for alkaline electrolytes.

Similarly, ORR data was collected as well, testing for the effects of pH and carbonate, using an O₂-saturated electrolyte and a monometallic Pt catalyst. With ORR, however, no change in the half-wave potential was observed, for either variable.

Source: S. St. John et al. J. Electrochem. Soc., 163, F291 (2016).