Yingdan Cui

This manuscript reports a mixed‐spacer architecture for resolving the conductivity–swelling–stability tradeoff in ether‐free anion‐exchange membranes (AEMs). Unlike conventional spacer engineering, which usually relies on a single spacer length, the mixed‐spacer approach provides an additional degree of freedom for regulating membrane properties. Using mixed‐spacer engineering in polycarbazoles with mixed short and long spacer side chains, we show that hydroxide transport and membrane durability can be jointly optimized through control of hydration, ionic connectivity, and local cation solvation, rather than through ion exchange capacity alone. A series of membranes with varied ratios of short ( C4 ) and long ( C6 ) alkyl spacers were synthesized via acid‐catalyzed Friedel–Crafts polymerization followed by quaternization. The optimized membrane achieves a hydroxide conductivity of 184 mS cm ⁻¹ at 80°C, outperforming both short‐spacer and long‐spacer counterparts. Membranes rich in C4 spacers exhibit weaker alkaline stability, whereas those containing dominant C6 spacers show excellent stability. In a fuel cell device, the membrane delivers a peak power density of 2.51 W cm ⁻² , among the highest reported for polycarbazole AEMs. Mechanistic understanding was achieved through both experimental and computational investigations. These results demonstrate that mixed‐spacer molecular engineering is a promising and effective strategy for designing high‐performance AEMs.

This manuscript reports a mixed‐spacer architecture for resolving the conductivity–swelling–stability tradeoff in ether‐free anion‐exchange membranes (AEMs). Unlike conventional spacer engineering, which usually relies on a single spacer length, the mixed‐spacer approach provides an additional degree of freedom for regulating membrane properties. Using mixed‐spacer engineering in polycarbazoles with mixed short and long spacer side chains, we show that hydroxide transport and membrane durability can be jointly optimized through control of hydration, ionic connectivity, and local cation solvation, rather than through ion exchange capacity alone. A series of membranes with varied ratios of short ( C4 ) and long ( C6 ) alkyl spacers were synthesized via acid‐catalyzed Friedel–Crafts polymerization followed by quaternization. The optimized membrane achieves a hydroxide conductivity of 184 mS cm ⁻¹ at 80°C, outperforming both short‐spacer and long‐spacer counterparts. Membranes rich in C4 spacers exhibit weaker alkaline stability, whereas those containing dominant C6 spacers show excellent stability. In a fuel cell device, the membrane delivers a peak power density of 2.51 W cm ⁻² , among the highest reported for polycarbazole AEMs. Mechanistic understanding was achieved through both experimental and computational investigations. These results demonstrate that mixed‐spacer molecular engineering is a promising and effective strategy for designing high‐performance AEMs.

Energy technology development is closely tied to society needs, particularly as global efforts toward carbon emission reduction continue to intensify. Fuel cells can provide efficient electricity generation with low or zero carbon emissions, little noise and fast refueling, becoming an important component of the evolving low-carbon energy systems. Among various fuel cell technologies, proton exchange membrane fuel cells (PEMFCs) have established a strong presence in the commercial market due to their high power density, rapid start-up capability, and compatibility with a wide range of applications including transportation, stationary power, and portable devices. However, the gaseous impurities in the gas feed can easily cause cell performance degradation. Inorganic sulfur compounds are of particular concern due to the strong poisoning effects on Pt catalysts commonly used in PEMFCs [1]. Among the various sulfur compounds, carbon disulfide (CS₂) was overlooked, because its concentration in the mainstream steam methane reforming hydrogen production pathway is much lower than that of hydrogen sulfide [2], and it is also far less familiar to the public than sulfur dioxide, which is widely known for causing acid rain. Nevertheless, CS₂ is present in industrial emissions, incomplete combustion exhausts and especially the warehouses for products containing slowly off-gas sulfur compounds such as vulcanized rubber. It makes CS₂ a real-world airborne contaminant for fuel cells since the cathode is directly fed with the ambient air.

Heat treatment (115 °C, 24 h) of HDPE film (T) led to higher crystallinities ( cf. untreated, UT). HDPE(T) films yield radiation-grafted AEMs with more consistent ion-exchange capacities, and higher bulk water contents and fuel cell performances.

Using fossil fuels, power generation is always accompanied by environmental pollution. However, hydrogen-oxygen fuel cells, using H₂ generated from renewable energy, can provide electricity with zero emissions, meeting the significant demands. Proton exchange membrane fuel cells (PEMFCs), though commercialized, are hindered by the high cost due to relying on expensive Pt. By contrast, as the potential alternative, anion exchange membrane fuel cells (AEMFCs), have attracted more and more attention as they can provide the benefit of using low-cost cathode catalysts while still having high power density. Although the oxygen reduction reaction (ORR) on the cathode can be well achieved by platinum group metal (PGM)-free catalysts in an alkaline environment, the high loading of Pt for the anode is still needed for high power output caused by the sluggish hydrogen oxidation reaction (HOR) kinetics. As the cheapest PGM, Ru with lower HOR activity is often added as an activity promoter to reduce the Pt loading. Recently, more efforts have been put into using exclusively Ru as the main catalysis active site, but the synthesis methods are often complex and involve toxic organics. In this project, boron-activated Ru supported on carbon was developed for HOR catalysis in alkaline media. The B-Ru/C was synthesized by a simple organic-free method, making it possibly attractive for industrial application. This method adopted water as the only solvent and boric acid as the boron source to avoid the toxic problem and the tedious removal step of organics. It produced uniformly dispersed Ru nanoparticles in smaller particle sizes with a boron-rich environment verified by TEM and STEM images. Compared to Ru/C or Ru/BC, B-Ru/C not only has a higher electrochemical active surface area (ECSA) but also reached a much higher ECSA-normalized exchange current density which means boron does promote the activity of Ru. From the XPS spectra, the more significant peak shifts of Ru_3p and B_1s in B-Ru/C relative to Ru/C and BC indicate that the boron-ruthenium interaction is more robust in B-Ru/C than in Ru/BC. A theoretical simulation was conducted further to understand the mechanism of the HOR activity enhancement. From an electrochemical perspective, the B-Ru/C had a very high exchange current density in ex-situ tests and was also studied as an anode in operating AEMFCs. We believe this cost-effective Pt-free HOR catalyst with excellent performance prepared through an organic-free process can contribute to the industrial application of AEMFCs in the future. Acknowledgments This work is supported by the Research Grant Council (HKUST C6011-20G).