Sulphonated poly-ether-ether-ketone porous diaphragms were prepared by immersion precipitation using chemically induced phase separation to obtain a tight diaphragm skin layer and underneath finger-like bulk morphology. Different variables including sulphonation degree, diaphragm thickness, precipitation temperature and the presence of an inorganic filler were analyzed in order to evaluate the performance of the resulting diaphragms in water alkaline electrolysis using a bipolar electrolyzer and a commercial scale (50 kW) electrolyzer stack. Their performance was compared with a commercially available diaphragm in terms of cell voltage and oxygen purity (HTO) under normal operation conditions (10 bar, 80 degrees C) and under transient operation with up to 80 shutdown cycles (a total of 20 days in operation). Long term stability and operation reliability were assured for the SPEEK diaphragms showing lower cell voltage and HTO than the ones obtained with the commercial Zirfon (R) HIP 500 diaphragm. (C) 2013 Elsevier B.V. All rights reserved.