Heteropoly oxometalates as co-electrocatalysts for methanol oxidation in fuel cell applications

Several heteropoly oxometalates have been synthesized and studied to examine their electrocatalytic activity for methanol oxidation in fuel cell applications. The various heteropoly oxometalates investigated include the Keggin anions (XM{dollar}sb{lcub}12{rcub}{dollar}O{dollar}sb{lcub}40{rcub}rbracksp{lcub}rm q-{rcub}{dollar}, where X = P, Si; M = Mo, W; and q = 3 or 4, the vanadium-substituted Keggin anions, (PV{dollar}sb{lcub}rm n{rcub}{dollar}W{dollar}sb{lcub}rm 12-n{rcub}{dollar}O{dollar}sb{lcub}40{rcub}rbracksp{lcub}rm (3+n)-{rcub}{dollar} where n = 1 to 3 and the Anderson anions (H{dollar}sb6{dollar}XMo{dollar}sb6{dollar}O{dollar}sb{lcub}24{rcub}rbracksp{lcub}3-{rcub}{dollar}, where X = Cr, Fe or Co.; Cyclic voltammetry was performed on the Keggin and vanadium-substituted anion complexes in the solution phase with a glassy carbon electrode. These solution phase voltammetry studies are consistent with those reported in the literature but no enhancement for methanol oxidation was observed at room temperature. These results led to polarization studies to determine whether or not heteropoly oxometalates exhibit co-electrocatalytic activity for methanol oxidation at high temperatures (80-200{dollar}spcirc{dollar}C) in the presence of Pt.; Initial half-cell polarization studies performed at 80, 105 and 148{dollar}spcirc{dollar}C show that the Keggin anions, with Pt, display better activity for methanol oxidation than Pt alone. These studies also indicate that the vanadium-substituted tungstates are slightly better electrooxidants for methanol oxidation than the unsubstituted Keggin anions. Furthermore, the half-cell polarization studies show that the Keggin anions with Pt have comparable activity for methanol oxidation as Pt-Ru alloy. However, the polarization studies performed on the Anderson complexes suggest that the 6-heteropoly molybdates with Pt have similar methanol oxidation activity as Pt itself. The oxometalate compound, H{dollar}sb3{dollar}PW{dollar}sb{lcub}12{rcub}{dollar}O{dollar}sb{lcub}40{rcub}{dollar}, was further studied with Pt in a MeOH/O{dollar}sb2{dollar} proton electrolyte membrane microfuel cell between 150 and 200{dollar}spcirc{dollar}C. The heteoropolytungstate complex with Pt was shown to have better activity for methanol oxidation than Pt alone. These studies also showed that H{dollar}sb3{dollar}PW{dollar}sb{lcub}12{rcub}{dollar}O{dollar}sb{lcub}40{rcub}{dollar} with Pt has comparable activity for methanol oxidation as Pt-Ru alloy.; Vanadium K-edge X-ray absorption near edge structure (XANES) studies on K{dollar}sb4{dollar}PVW{dollar}sb{lcub}11{rcub}{dollar}O{dollar}sb{lcub}40{rcub}{dollar} immobilized on a carbon electrode indicates that the complex undergoes two successive one-electron reductions. The added electrons are shown to be localized onto the vanadium atom reducing V(V) {dollar}to{dollar} V(IV) {dollar}to{dollar} V(III). Tungsten L{dollar}sb{lcub}rm III{rcub}{dollar}-edge XANES studies performed on H{dollar}sb3{dollar}PW{dollar}sb{lcub}12{rcub}{dollar}O{dollar}sb{lcub}40{rcub}{dollar} immobilized on a carbon electrode show that complete electron delocalization occurs when the complex undergoes a two electron reduction. Also, tungsten L{dollar}sb{lcub}rm III{rcub}{dollar}-edge extended X-ray adsorption fine structure (EXAFS) experiments conducted on H{dollar}sb3{dollar}PW{dollar}sb{lcub}12{rcub}{dollar}O{dollar}sb{lcub}40{rcub}{dollar} immobilized on a carbon electrode show no evidence for changes in the W--O bond distances or the W--W distances when the complex undergoes a two electron reduction.