Investigation of hydrophobic porous electrodes. II. Pseudocapacitance changes during underpotential copper deposition

Differential capacitance measurements in 96%o-phosphoric acid at 160° C have been used to follow the underpotential deposition of copper on supported platinum in a porous electrode. At a potential in the range 0·2–0·3 V versus RHE the capacitance,C, increased as copper deposition proceeded and a plot ofC versust ^1/2 was linear during a substantial part of the deposition process. The slope dC/dt ^1/2 was proportional to the copper concentration in solution, suggesting that the Cu/Cu²⁺ pseudocapacitance reflected the quantity of copper deposited and that copper deposition was diffusion controlled. The effective diffusion coefficient estimated from the results was 10^–7 cm² s^–1 — some 40 times smaller than the copper ion diffusion coefficient measured separately. At potentials of 0·4 V and above, capacitance measurements could not be used to follow copper deposition because the Cu/Cu²⁺ pseudocapacitance decayed with time.