The ECSA was measured with the CO stripping method using the cell in half cell mode where the anode electrode acts as a pseudo reference electrode. The cathode voltage was controlled at 0.125 V at 80°C, 100% RH and 50 kPa gauge whilst purging with 1% CO in N2 at 300 ml min -1 for 15 min. Afterwards the cathode was purged with N2 at the same flow rate for 2 h to ensure that CO is removed from the bubblers and the catalyst layer pores. The adsorbed CO is oxidised electrochemically by scanning the cathode voltage from 0.125 V to 0.85 V and back to 0.05 V, at 20 mV s -1 for three cycles. The area under the CO oxidation peak is integrated by subtracting the third scan from the first scan and using a 420 μC cm -2 constant for a CO monolayer on Pt.