Pt–Ru catalyzed hydrogen oxidation in alkaline media: oxophilic effect or electronic effect?

Abstract

A current challenge to alkaline polymer electrolyte fuel cells (APEFCs) is the unexpectedly sluggish kinetics of the hydrogen oxidation reaction (HOR). A recently proposed resolution is to enhance the oxophilicity of the catalyst, so as to remove the H ad intermediate through reacting with OH ad , but this approach is questioned by other researchers.Here we report a clear and convincing test on this problem. By using PtRu/C as the HOR catalyst for APEFC, the peak power density is boosted to 1.0 W/cm 2 , in comparison to 0.6 W/cm 2 when using Pt/C in the anode. Such a remarkable improvement, however, can hardly be explained as an oxophilic effect, because, as monitored by CO stripping, reactive hydroxyl species can generate on certain sites of the Pt surface at more negative potentials than on the PtRu surface in KOH solution. Rather, the incorporation of Ru has posed an electronic effect on weakening the Pt-H ad interaction, as revealed by the voltammetric behavior and from density-functional calculations, which thus benefits the oxidative desorption of H ad , the rate determining step of HOR in alkaline media. These findings further our fundamental understanding of the HOR catalysis, and cast a new light on the exploration of better catalysts for APEFC. 5 process to monitor the generation of reactive hydroxyl species, for the anodic current of CO oxidation has to be triggered by reactive hydroxyl species. 31 As demonstrated in Figure 2, in 0.1 M H 2 SO 4 solution, the CO stripping on Pt/C takes on a single sharp peak at 0.85 V, and, upon alloying with Ru, the CO stripping peak is somewhat broadened and moves negatively by 0.3 V, showing that Ru does accelerate the formation of OH ad in acidic environment.However, the CO stripping on Pt/C behaves rather differently in 0.1 M KOH solution: multiple anodic peaks appear and the onset potential shifts to ~0.2 V, a potential even more negative than the onset of CO stripping on PtRu/C (~0.35 V) in either acid or alkaline media.Such a surprising finding indicates that, in alkaline environment, the reactive hydroxyl species, be it OH ad or OH ad − , can generate on certain sites of the Pt surface more favorably than on the PtRu surface; but when alloyed with Ru, the surface reactivity of Pt is suppressed, thereby no reactive hydroxyl species appearing at the potential region negative to 0.35 V.On the basis of the above observations, the promotion effect of Ru on catalyzing the HOR in alkaline media can hardly be explained as an oxophilic effect. The existence of reactive hydroxyl species on either Pt or PtRu surface at potentials negative to 0.2 V also seems unlikely, as revealed by Figure 2. On the other hand, the Ru has posed an obvious effect on weakening the Pt-H ad interaction, as a consequence of the suppressed surface reactivity of Pt. As illustrated in Figure 3a, in KOH solution, the hydrogen underpotential deposition (H-UPD) and subsequent desorption behavior on PtRu/C is clearly different from that on Pt/C: whereas strong H ad peaks are the major signal for Pt/C...

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

“…However, if part of the results tend to corroborate the oxophilic effect in bifunctional bimetallic catalysts, [30][31][32] some recent results on monometallic oxophilic metals like Ir, show the same trend of HOR decreasing activity from acidic pH to alkaline pH as a non-oxophilic catalyst like Pt. 4 On the other hand, examples have been given for the beneficial effects of surface alloying to HOR rate enhancement in alkaline media, providing both explanations, one based on electronic effects modifying the HBE 33 and the other on added oxophilic function. 32 Such a conclusion may be true for a single metal but the extrapolation to bimetallic surfaces like the Pd/Ni reported in the manuscript may not be accurate.…”

Electrocatalysts for hydrogen oxidation and evolution reactions

“…Tuning by the Cu substrate results in the Pt/Cu NWs having an exchange current density 3.5 times as high as that of pure Pt. Zhuang and co-authors [95] used PtRu alloys as HOR catalysts for the alkaline polymer electrolyte fuel cells. A peak power density of 1.0 W cm −2 is obtained, in comparison to 0.6 W cm −2 that is obtained when Pt/C is used as the anode catalyst (Fig.…”

Importance of balancing membrane and electrode water in anion exchange membrane fuel cells

“…Therefore, to improve the water tolerance at the anode, 5% PTFE was incorporated in the GDL, increasing the ability to reject water, which is especially important at higher current densities when water generation and electroosmotic drag are increased. To support the desire of higher cell performance, the anode catalyst was changed from Pt/C to a more "oxophilic" Pt:Ru/C (2:1) [12,14] with the total amount of Pt loading in the electrode kept constant.…”