WO3/CNTs composite electrodes were fabricated using the following procedure. The composite was mixed with Nafion solution (5 wt%, Dupont, USA) at a 9:1 weight ratio to form slurry, which was sprayed on a PTFE-treated carbon paper (Toray TGH-060) to prepare the composite electrodes with desired WO3 loadings. Cells were assembled using Aquivion® R79-02S as the membrane electrolytes. Pt/C cathodes were prepared by spraying a homogeneous ink, which was prepared from commercial Pt/C (40 wt% Pt, JM) and Nafion solution (5 wt%, Dupont) in ethanol on the membrane to achieve a Pt loading of 0.4 mg cm−2. The MEAs were then made by sandwiching the membrane electrolyte between a WO3 composite electrode and a gas diffusion layer (GDL, prepared by spraying XC-72 carbon black on Toray carbon paper with a loading of 2 mg cm−2). The structure of MEA is also schematically shown in Supplementary Fig. 3 .
Electrochemical measurements were carried out on a Solartron 1860/1287 Electrochemical Interface (Solartron Analytical). The charging process was conducted by supplying the Pt/C cathodes with hydrogen (0.1 L min−1) and the WO3 anode with nitrogen (0.1 L min−1). The cells were charged to 0 V (vs. DHE) using a constant current of 0.2 A g−1. The inlet gas of cathode was then switched to oxygen (0.1 L min−1) (see Supplementary Fig.3 ). The cells were discharged at different rates.
Electrochemical measurements were carried out on a Solartron 1860/1287 Electrochemical Interface (Solartron Analytical). The charging process was conducted by supplying the Pt/C cathodes with hydrogen (0.1 L min−1) and the WO3 anode with nitrogen (0.1 L min−1). The cells were charged to 0 V (vs. DHE) using a constant current of 0.2 A g−1. The inlet gas of cathode was then switched to oxygen (0.1 L min−1) (see Supplementary Fig.