Enhanced Alkaline Hydrogen Oxidation Reaction by Modifying Pt Nanoparticle Surface with Another Metal

Abstract

Anion exchange membrane fuel cells (AEMFCs) can reduce the cost of fuel cell systems because they allow the use of non-precious metals in catalysts due to their less corrosive internal environments, which gives them an advantage over conventional proton exchange membrane fuel cells (PEMFCs) with highly corrosive environments. Taking this advantage, non-precious-metal catalysts have been developed in particular for cathode, i.e., for the oxygen reduction reaction (ORR). However, the cell performance of AEMFCs is still inferior to that of PEMFCs. One of the reasons is sluggish hydrogen oxidation reaction (HOR) on anode, as represented by the fact that the HOR rate on Pt catalysts in alkaline conditions is two orders of magnitude slower than that in acidic conditions. It would be reasonable to consider that development of catalysts for the HOR enhances the AEMFC performance. In fact, the previous studies have demonstrated that alloying and size-control of metal nanoparticle catalysts improve the HOR activity and the cell performance. Recently, the cell performance has been getting closer to that of PEMFCs by the improvement of HOR catalysts with the help of the state-of-the-art anion exchange membranes having high ion conductivity. However, a lot of precious metals are needed on AEMFC anode to obtain such high cell performance. Therefore, the HOR catalysts in alkaline conditions have to be further developed for widespread use of AMFCs as an alternative of PEMFCs. Previous studies on the alkaline HOR have suggested two mechanisms accounting for enhancement of the HOR activity: hydrogen-binding-energy (HBE) mechanism and bifunctional mechanism. The HBE mechanism can account for the variation of HOR activity with pH of electrolyte as well as with metal species. The bifunctional mechanism has been recently suggested as another mechanism contributing to the improved HOR kinetics. In the bifunctional mechanism, catalyst surface not only adsorbs hydrogen but also supplies reactive oxygen species to the adsorbed hydrogen. These mechanisms have driven us to modify catalyst surface systematically, because tuning of surface property is expected to change the HBE and/or the reactive oxygen species, and consequently improve the HOR activity based on the bifunctional mechanism. In this paper, we show enhanced or lowered HOR activity of Pt/C by surface modification of Pt nanoparticles with various metals on the group 4-11 of the 4th and 5th period in the periodic table except for Tc, i.e., Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Ru, Rh, Pd, Ag (fifteen metals). Commercially available Pt/C (TEK10E50E, Pt 46.7wt%) is modified with the fifteen metals using a conventional impregnation method to load 7.7at% of the metals on Pt/C. The catalysts are denoted as M/Pt/C. The electrochemically active surface area (ECSA) is determined from the electric charge due to HUPD in 0.1 M NaOH aq. saturated with N2. The HOR activity is determined from linear sweep voltammograms obtained using a rotating disk electrode method in 0.1 M NaOH aq saturated with H2 at 25°C. The exchange current (i0) is determined using the approximate Butler-Volmer equation. The mass activity (MA) is determined by dividing i0 by the total amount of Pt and added metals. Interestingly, most M/Pt/C samples presents higher MA values than Pt/C, although the modifying metals are less active than Pt when used individually for the HOR. Thus, modifying Pt/C enhance the HOR on Pt/C. Modification with Co was most effective for enhancing the MA of the metals in the third period, with a MA twice that of Pt/C; Ru in the fourth period exhibited a MA 2.5 times higher than that of Pt/C. It is noteworthy that the MAs of various M/Pt/C samples including Mo, Ru, and Pd/Pt/C are significantly higher than a state-of-the-art Pt-Ru alloyed nanoparticle catalyst (TEC61E54 supplied by Tanaka Kikinzoku Kogyo). The specific activity (SA) of M/Pt/C for the HOR is determined by dividing i0 by the ECSA. The SA varies with a volcano shape on both elemental periods with a peak at Co on the 4th period and at Ru on the 5th period, where the SA value is enhanced two times compared to Pt/C. The mechanism of the enhanced HOR activity is investigated in terms of the hydrogen binding energy (HBE) on catalysts. However, the variation of HOR activity exceeds the effect of HBE, suggesting contribution of bifunctional mechanism. The bifunctional mechanism is supported by volcano-shaped dependence of the SA on the following descriptors for reactivity of surface oxygen species: the standard oxidation potential and the standard formation enthalpy of the modifying metals.