Alkaline polymer electrolyte fuel cells (APEFCs) are a class of promising energy conversion devices that are attracting ever-growing attention from the academic and industrial energy technology communities. Considerable efforts have been made towards the development of advanced alkaline polymer electrolytes (APEs), and manipulating the balance between high ionic conductivity and low swelling degree is consistently one of the most important trade-offs in APE design. Constructing hydrophilic/hydrophobic microphase-separated morphologies in APEs has long been accepted as an effective way to optimize the ionic conductivity of these materials. However, not all patterns of phase separation lead to high APE ion conductive efficiency. Here we compare two kinds of polysulfone-based APE materials (i.e. self-aggregated quaternary ammonium polysulfone (aQAPSF) and pendant quaternary ammonium polysulfone (pQAPSF)). Experimental and simulation observations unambiguously reveal the existence of distinctly different patterns of microphase separation in aQAPSF and pQAPSF. In aQAPSF, the hydrophobic side chains residing apart from the quaternary ammonium (QA) group help to build broad and percolated pathways, which contribute to boosting the ion conductive efficiency of the material. The aQAPSF membrane with IEC equal to 0.98 mmol g−1 shows ionic conductivity as high as 108.3 mS cm−1 at 80 °C. While in pQAPSF, the introduction of a side chain between the backbone and the cation locates the QA group away from the backbone and helps to build strong hydrophobic networks, which results in limited development of efficient ionic channels. However, when doubling the IEC of pQAPSF to 2.04 mmol g−1, the conductivity can be increased to 75.1 mS cm−1 at 80 °C, and the hydrophobic network restrains the swelling of pQAPSF effectively (swelling degree is 25.0% at 80 °C). These materials with obvious phase separation showed good chemical stabilities, and can be considered competitive candidates for application in fuel cells.

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 Had intermediate through the reaction with OHad, 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 the 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 as the anode catalyst. 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–Had interaction, as revealed by the voltammetric behavior and from density-functional calculations, which thus benefits the oxidative desorption of Had, the rate determining step of HOR in alkaline media. These findings further our fundamental understanding of the HOR catalysis, and cast new light on the exploration of better catalysts for APEFCs.

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.Keywords: alkaline polymer electrolyte; backbone degradation; chemical stability; fuel cell application; pendant-type quaternary ammonia cation; phase separation

•Carbonation effects on the performance of APEFC were studied.•Carbonation anions lead to increase of ionic and overall reaction resistance in APEFC.•Low performance of H2-air APEFC due to low O2 partial pressure and CO2 contamination.The carbonation effect on alkaline polymer electrolyte fuel cells (APEFCs) was investigated in the present work by the study of single-cell performance and electrochemical AC impedance. The carbonate anions in alkaline polymer electrolyte (APE) were found to lead to a decline in cell performance due to the increase in both the ionic resistance, in comparison to OH− conduction, and the overall reaction resistance. When the OH− in APE was partially or completely replaced with carbonate anions, through exposure to air or immersion in KHCO3 solution, respectively, the peak power density of H2–O2 APEFC single cell (operated at 60 °C) dropped from 0.61 W/cm2 to 0.47 and 0.43 W/cm2, respectively. When air was used as the oxidant instead of pure O2, the performance of APEFC single cell was further reduced because of the lower partial pressure of O2; the peak power density decreased to 0.32 W/cm2. If CO2 was removed from the air, the cell performance increased, to some degree, to 0.41 W/cm2. Whereas the influence of deliberate replacement of OH− with carbonate anions in APE is rigid, the carbonation effect caused by air can be alleviated to a large degree by operating the cell under high current density, in which case enormous OH− can be produced to refresh the APE and the electrode surface.

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− 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 μm thick) is highly tolerant to leaching in 80 °C water and possesses low area resistance (0.09 Ω cm2), a low swelling degree (3.1% at 60 °C) and high mechanical strength (31 MPa). Making use of such an xQAPS@PTFE membrane, the H2–O2 APEFC exhibits a peak power density of 550 mW cm2 at 60 °C under 0.1 MPa of back pressure.

Alkaline polymer electrolyte fuel cells (APEFCs) promise the use of nonprecious metal catalysts and thus have attracted much research attention in the recent decade. Among the challenges of developing practical APEFC technology, the chemical stability of alkaline polymer electrolytes (APEs) seems to be rather difficult. Research found that, upon attachment of a cationic functional group, an originally stable polymer backbone, such as polysulfone (PSF), would degrade in an alkaline environment. In the present work, we try to employ poly(ether ether ketone) (PEEK), a very inert engineering plastic, as the backbone of APEs. The PEEK is functionalized with both a sulfonic acid (SA) group and a quaternary ammonia (QA) group, with the latter as the majority amount. Ionic cross-linking between SA and QA has rendered the thus-obtained membrane (xQAPEEK) with high mechanical strength and low swelling degree. More importantly, the xQAPEEK membrane exhibits outstanding stability in a 1 mol/L KOH solution at 80 °C for a test period of 30 days: the total weight loss of xQAPEEK is only 6 wt %, in comparison to a large degradation of quaternary ammonia PSF (more than 40 wt %) under the same conditions. Our findings not only have demonstrated an effective approach to preparing PEEK-based APE but also cast a new light on the development of highly stable APEs for fuel-cell application.Keywords: alkaline polymer electrolyte; fuel cell; high chemical stability; ionic cross-linking; poly(ether ether ketone);

•An APEFC using nonprecious metal catalyst in both the anode and the cathode.•A novel catalyst, W-doped Ni, is fabricated and applied in the anode.•CoPPY/C, an efficient catalyst for 4-electron reduction of O2 in alkaline media.Alkaline polymer electrolyte fuel cells (APEFCs) are a new class of fuel cell that has been expected to combine the advantages of alkaline fuel cells (AFCs) and polymer electrolyte fuel cells (PEFCs). In recent decade, APEFCs have drawn much attention in the fuel cell world. While great efforts have been devoted to the development of high-performance alkaline polymer electrolytes (APEs), prototypes of APEFC using nonprecious metal catalysts in both the anode and the cathode have not been well implemented, except for our previous report where Ni–Cr was used as the anode catalyst and Ag was employed as the cathode catalyst. In the present work, we report our recent progress in this regard. The self-crosslinked quaternary ammonia polysulfone (xQAPS), a high-performance APE that possesses both good ionic conductivity and extremely high dimensional stability, is applied as both the electrolyte membrane and the ionomer impregnated in the electrodes. Carbon-supported Co-polypyrrole (CoPPY/C) is employed as the cathode catalyst and a new Ni-based catalyst, W-doped Ni, is used as the anode catalyst, which features in high oxidation tolerance. H2–O2 and H2-air APEFCs are thus fabricated and show a decent performance with peak power density being 40 and 27.5 mW/cm2 at 60 °C, respectively.

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− 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 μm thick) is highly tolerant to leaching in 80 °C water and possesses low area resistance (0.09 Ω cm2), a low swelling degree (3.1% at 60 °C) and high mechanical strength (31 MPa). Making use of such an xQAPS@PTFE membrane, the H2–O2 APEFC exhibits a peak power density of 550 mW cm2 at 60 °C under 0.1 MPa of back pressure.

Alkaline polymer electrolyte fuel cells (APEFCs) are a class of promising energy conversion devices that are attracting ever-growing attention from the academic and industrial energy technology communities. Considerable efforts have been made towards the development of advanced alkaline polymer electrolytes (APEs), and manipulating the balance between high ionic conductivity and low swelling degree is consistently one of the most important trade-offs in APE design. Constructing hydrophilic/hydrophobic microphase-separated morphologies in APEs has long been accepted as an effective way to optimize the ionic conductivity of these materials. However, not all patterns of phase separation lead to high APE ion conductive efficiency. Here we compare two kinds of polysulfone-based APE materials (i.e. self-aggregated quaternary ammonium polysulfone (aQAPSF) and pendant quaternary ammonium polysulfone (pQAPSF)). Experimental and simulation observations unambiguously reveal the existence of distinctly different patterns of microphase separation in aQAPSF and pQAPSF. In aQAPSF, the hydrophobic side chains residing apart from the quaternary ammonium (QA) group help to build broad and percolated pathways, which contribute to boosting the ion conductive efficiency of the material. The aQAPSF membrane with IEC equal to 0.98 mmol g−1 shows ionic conductivity as high as 108.3 mS cm−1 at 80 °C. While in pQAPSF, the introduction of a side chain between the backbone and the cation locates the QA group away from the backbone and helps to build strong hydrophobic networks, which results in limited development of efficient ionic channels. However, when doubling the IEC of pQAPSF to 2.04 mmol g−1, the conductivity can be increased to 75.1 mS cm−1 at 80 °C, and the hydrophobic network restrains the swelling of pQAPSF effectively (swelling degree is 25.0% at 80 °C). These materials with obvious phase separation showed good chemical stabilities, and can be considered competitive candidates for application in fuel cells.