Electrochimica Acta, Vol.52, No.5, 2051-2061, 2007
Electrocatalyst materials for fuel cells based on the polyoxometalates-K-7 or H-7[(P2W17O61FeIII(H2O)] and Na(12)or H-12[(P2W15O56)(2)Fe-4(III)(H2O)(2)]
Free acids of the iron substituted heteropoly acids (HPA), H-7[(P2W17O61)Fe-III(H2O)] (HFe1) and H-18[(P2W15O56)(2)Fe-2(III)(H2O)(2)] (HFe2) were prepared from the salts K-7[(P2W17O61)Fe-III(H2O)] (KFe1) and Na-12[(P2W15O56)(2)Fe-4(III)(H2O)(2)] (NaFe4), respectively. The iron-substituted HPA were adsorbed on to XC-72 carbon based GDLs to form HPA doped GDEs after water washing with HPA loadings of ca. 1 mu mol. The HPA was detected throughout the GDL by EDX. Solution electrochemistry of the free acids are reported for the first time in sulfate buffer, pH 1-3. The hydrogen oxidation reaction was catalyzed by KFe1 at 0.33 V, with an exchange current density of 38 mA/cm(2). Moderate activity for the oxygen reduction reaction was observed for the iron substituted HPA, which was dramatically improved by selectively removing oxygen atoms from the HPA by cycling the fuel cell cathode under N-2 followed by reoxidation to give a restructured oxide catalyst. The nanostructured oxide achieved an OCV of 0.7 V with a Tafel slope of 115 mV/decade. Cycling the same catalysts in oxygen resulted in an improved catalyst/ionomer/carbon configuration with a slightly higher Tafel slope, 128 mV/decade but a respectable current density of 100 mA/cm(2) at 0.2 V. (c) 2006 Elsevier Ltd. All rights reserved.