ELECTROCATALYTIC ACTIVITIES OF SOME METALLIZED GRAPHITE-ELECTRODES FOR AQUEOUS FE(2+ 3+) REDOX COUPLES FROM POLARIZATION STUDIES/

Exchange current densities (i0) of the Fe2+/3+ redox couple in aq. H2S O4 solution have been determined at three different temperatures on ba re Pt, graphite C and on electrochemically deposited surfaces of some noble metals (M = Pt, Pd, Rh, Ir, Ru and Au) over cheap C substrates, so as to develop cheaper counter electrodes for a potential aqueous PE C solar cell based on the said redox couple. The apparent electrocatal ytic activities as based on the apparent area of the electrode substra tes, have been found to follow the sequence (C) Ir > (C) Rh > (C) Ru > (C) Pt > (C) Pd > (C) Au > C. As the roughness factor R(f)(= (i0)/(i0 )true) is likely to be independent of temperature, the virtual heats o f activation DELTAH-0, double dagger obtained by means of the relation DELTAH-0, double dagger = -R(deltaln i0/partial derivativeT-1 = - R ( partial derivative In (i0)tr/partial derivativeT-1), have been found t o reflect the intrinsic catalytic activity of the electrode surfaces. Analysis of the relative i0 and DELTAH-0, double dagger values lead to the conclusion that surface geometry/microheterogeneity, electronic w ork function and the coverage by water molecules (theta(w)) on the sur faces play important role resulting in the observed order. The results further suggest that although the cheapest C substrate is not that go od an electrocatalyst, a thinly coated surface on cheap carbon substra te might serve as the desired cheap but efficient counter electrode fo r the reaction concerned.