RECENTLY it has been observed that under certain conditions of anodic polarization small amounts of platinum can dissolve in both acidl-s and alkaline solutions.4 A similar observation was made in the work reported here during an investigation of the cycle life of a zinc-oxygen cell. The effect became apparent when spurious behaviour was observed at the anode during charge after many charge-discharge cycles.
The cell anode consisted of electrodeposited zinc on a smooth nickel sheet, whereas the cathode consisted of platinum black supported on a porous nickel substrate. The catalyst loading of the cathode was about 40 mg/cm2. Oxygen (Matheson dry grade) was supplied to the rear surface of the cathode, which was wetproofed with FEP 120 (E. I. du Pont de Nemours and Company, Inc.). The electrodes were 8.5 x 8.5 cm and the active area was 50 cm2. A mercury/mercuric-oxide reference electrode was connected to the solution compartment of the cell so that single-electrode-potential measurements could be made. The diffusion of dissolved mercuric oxide into the cell electrolyte was minimized by inserting an asbestos plug into the connecting tube. The solution, 21 y0 aqueous potassium hydroxide saturated with zinc oxide (both materials were analytical reagent grade), was contained in a 2-1 reservoir provided with a heater, stirrer, and temperature controller. The solution was circulated through the cell by a Gorman-Rupp pump (model number 12500-13) and the flow rate could be measured on a Brooks flowmeter (model number 3602). A flow rate of about 400 ml/min was generally used. All of the work reported here was carried out at 80Β°C unless the contrary is stated. The materials of construction were polypropylene, Teflon, Lucite, nickel, and stainless steel. I&free potential measurements were made using a 60-c/s current interrupter.s
Initially the cell was given a charge of O-5 Ah at a rate of 60 mA/cm2. The potential of the zinc electrode Z.XY the mercury/mercuric-oxide reference electrode was -1~38 VT, close to the reversible value of -1.37 V under the experimental conditions used.6 Upon discharge the cell delivered 0.4 Ah at the same rate. The potential of the zinc electrode was -1.36 V. After 58 cycles, most of which were carried out at temperatures between 25" and BO"C, the cell was again given a charge of 0.5 Ah at a rate of 60 mA/cm2. This time the potential of the negative electrode decreased from -1.06 V to a steady value of -1.15 V. When the cell was discharged at 60 mA/cm2 the output was zero, and the potential of the negative electrode increased rapidly to 0 V.