A current challenge to alkaline polymer electrolyte fuel cells (APEFCs) is the unexpectedly sluggish kinetics of the hydrogen oxidation reaction (HOR). A recently proposed resolution is to enhance the oxophilicity of the catalyst, so as to remove the H ad intermediate through reacting with OH ad , but this approach is questioned by other researchers.Here we report a clear and convincing test on this problem. By using PtRu/C as the HOR catalyst for APEFC, the peak power density is boosted to 1.0 W/cm 2 , in comparison to 0.6 W/cm 2 when using Pt/C in the anode. Such a remarkable improvement, however, can hardly be explained as an oxophilic effect, because, as monitored by CO stripping, reactive hydroxyl species can generate on certain sites of the Pt surface at more negative potentials than on the PtRu surface in KOH solution. Rather, the incorporation of Ru has posed an electronic effect on weakening the Pt-H ad interaction, as revealed by the voltammetric behavior and from density-functional calculations, which thus benefits the oxidative desorption of H ad , the rate determining step of HOR in alkaline media. These findings further our fundamental understanding of the HOR catalysis, and cast a new light on the exploration of better catalysts for APEFC. 5 process to monitor the generation of reactive hydroxyl species, for the anodic current of CO oxidation has to be triggered by reactive hydroxyl species. 31 As demonstrated in Figure 2, in 0.1 M H 2 SO 4 solution, the CO stripping on Pt/C takes on a single sharp peak at 0.85 V, and, upon alloying with Ru, the CO stripping peak is somewhat broadened and moves negatively by 0.3 V, showing that Ru does accelerate the formation of OH ad in acidic environment.However, the CO stripping on Pt/C behaves rather differently in 0.1 M KOH solution: multiple anodic peaks appear and the onset potential shifts to ~0.2 V, a potential even more negative than the onset of CO stripping on PtRu/C (~0.35 V) in either acid or alkaline media.Such a surprising finding indicates that, in alkaline environment, the reactive hydroxyl species, be it OH ad or OH ad − , can generate on certain sites of the Pt surface more favorably than on the PtRu surface; but when alloyed with Ru, the surface reactivity of Pt is suppressed, thereby no reactive hydroxyl species appearing at the potential region negative to 0.35 V.On the basis of the above observations, the promotion effect of Ru on catalyzing the HOR in alkaline media can hardly be explained as an oxophilic effect. The existence of reactive hydroxyl species on either Pt or PtRu surface at potentials negative to 0.2 V also seems unlikely, as revealed by Figure 2. On the other hand, the Ru has posed an obvious effect on weakening the Pt-H ad interaction, as a consequence of the suppressed surface reactivity of Pt. As illustrated in Figure 3a, in KOH solution, the hydrogen underpotential deposition (H-UPD) and subsequent desorption behavior on PtRu/C is clearly different from that on Pt/C: whereas strong H ad peaks are the major signal for Pt/C...

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Aromatic ether-based alkaline polymer electrolytes (APEs) are one of the most popular types of APEs being used in fuel cells. However, recent studies have demonstrated that upon being grafted by proximal cations some polar groups in the backbone of such APEs can be attacked by OH(-), leading to backbone degradation in an alkaline environment. To resolve this issue, we performed a systematic study on six APEs. We first replaced the polysulfone (PS) backbone with polyphenylsulfone (PPSU) and polyphenylether (PPO), whose molecular structures contain fewer polar groups. Although improved stability was seen after this change, cation-induced degradation was still obvious. Thus, our second move was to replace the ordinary quaternary ammonia (QA) cation, which had been closely attached to the polymer backbone, with a pendant-type QA (pQA), which was linked to the backbone through a long side chain. After a stability test in a 1 mol/L KOH solution at 80 °C for 30 days, all pQA-type APEs (pQAPS, pQAPPSU, and pQAPPO) exhibited as low as 8 wt % weight loss, which is close to the level of the bare backbone (5 wt %) and remarkably lower than those of the QA-type APEs (QAPS, QAPPSU, and QAPPO), whose weight losses under the same conditions were >30%. The pQA-type APEs also possessed clear microphase segregation morphology, which led to ionic conductivities that were higher, and water uptakes and degrees of membrane swelling that were lower, than those of the QA-type APEs. These observations unambiguously indicate that designing pendant-type cations is an effective approach to increasing the chemical stability of aromatic ether-based APEs.

To minimize the ohmic loss in the cell voltage of fuel cells, the electrolyte should be made as thin as possible, in particular when alkaline polymer electrolytes (APEs) are employed, where both the mobility and the concentration of OH À are relatively low. A practical strategy for fabricating thin APE membranes is to impregnate APE ionomers into an ultrathin, rigid framework (such as a porous PTFE film), so that high ion conduction is achieved by the APE with a high ion-exchange capacity (IEC), while good mechanical stability is provided by the robust host. Our previous study has realized a prototype of an APE fuel cell (APEFC) using this kind of composite membrane but we found later that the APE component, quaternary ammonium polysulfone (QAPS), will leach out gradually under fuel cell operating conditions because of the poor interaction between the QAPS guest and the PTFE host. To address this problem, we demonstrate in the present work a new approach for making ultrathin composite membranes of APEs. The APE ionomer (TQAPS) is impregnated into a porous PTFE film, followed by a self-crosslinking process, so as to form a semi-interpenetrating network. The resulting ultrathin composite membrane (xQAPS@PTFE, 25 mm thick) is highly tolerant to leaching in 80 C water and possesses low area resistance (0.09 U cm 2 ), a low swelling degree (3.1% at 60 C) and high mechanical strength (31 MPa). Making use of such an xQAPS@PTFE membrane, the H 2 -O 2 APEFC exhibits a peak power density of 550 mW cm 2 at 60 C under 0.1 MPa of back pressure.