Catalysis of hydrogen evolution via deposition of palladium onto electrodes modified with an N,N′-dialkyl-4,4′-bipyridinium-based polymer: Dependence of rate on palladium coverage

The H₂ evolution activity of ITO/(PQ^2+/+)_n/Pd surfaces has been characterized in pH 2.8/1 M KCl aqueous solution where (PQ^2+/+)_n is a redox polymer derived from an N,N′-dialkyl-4,4′-bipyridinium monomer and Pd is deposited onto the outer surface of the polymer by reduction of PdCl₄^2-. The coverage of (PQ^2+/+)_n used, (4-5) × 10^-8 mol/cm², can carry a maximum current density, i_E, of 11-15 mA/cm². When the Pd coverage is ∼5 × 10^-8 mol/cm² the maximum current from kinetic limitations only, i_k, for H₂ evolution is ∼20 mA/cm². However, the observed current, i_obsd, is limited by both i_E and i_k. We predict that a polymer coverage of ∼1 × 10^-8 mol/cm² with a Pd coverage of 5 × 10^-8 mol/cm² is sufficient to give an observed H₂ evolution current density of 15-20 mA/cm². These considerations apply at pH 2.8 and an electrode potential of -0.78 V vs. SCE. The Pd deposited onto the polymer is found to be significantly less active than Pd deposited directly onto ITO, but structured ITO/(PQ^2+/+)ⁿ/Pd interfaces have been demonstrated to have good integrity and can provide significant current density, ≥10 mA/cm², at modest, <10^-7 mol/cm², Pd coverages.