In situ grown carbon nanotubes (CNTs) on carbon paper as an integrated gas diffusion layer (GDL) and catalyst layer (CL) were developed for proton exchange membrane fuel cell (PEMFC) applications. The effect of their structure and morphology on cell performance was investigated under real PEMFC conditions. The in situ grown CNT layers on carbon paper showed a tunable structure under different growth processes. Scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) demonstrated that the CNT layers are able to provide extremely high surface area and porosity to serve as both the GDL and the CL simultaneously. This in situ grown CNT support layer can provide enhanced Pt utilization compared with the carbon black and free-standing CNT support layers. An optimum maximum power density of 670 mW cm(-2) was obtained from the CNT layer grown under 20 cm(3) min(-1) C2H4 flow with 0.04 mg cm(-2) Pt sputter-deposited at the cathode. Furthermore, electrochemical impedance spectroscopy (EIS) results confirmed that the in situ grown CNT layer can provide both enhanced charge transfer and mass transport properties for the Pt/CNT-based electrode as an integrated GDL and CL in comparison with previously reported Pt/CNT-based electrodes with a VXC72R-based GDL and a Pt/CNT-based CL Therefore, this in situ grown CNT layer shows a great potential for the improvement of electrode structure and configuration for PEMFC applications. (C) 2011 Elsevier Ltd. All rights reserved.