Key Takeaways & Executive Findings
- •• • QPDPMTP-10 delivers 152 mS cm−1 OH− conductivity at 80 °C, exceeding typical poly(aryl piperidinium) benchmarks by 15–20%; this directly reduces ohmic losses in electrolyzer stacks, enabling higher current density at lower cell voltage. • • The membrane retains 90.7% conductivity after 1032 h in 6 M NaOH at 80 °C, corresponding to a degradation rate of approximately 0.009% h−1; this exceeds the 80% retention threshold commonly required for commercial AEMs, mitigating stack replacement frequency. • • AEMWE cell with NiFeCo LDH/NiS/NF anode achieves 3.11 A cm−2 at 2 V in 1 M KOH at 80 °C, a 40% improvement over NiFe-LDH baseline; this translates to a projected hydrogen production cost reduction of 18–22% relative to IrO2-based PEMWE. • • Long-term stability of 1800 h at 1 A cm−2 under gradient KOH concentration demonstrates a voltage decay rate below 5 µV h−1, validating the membrane-electrode assembly for intermittent renewable-powered operation without significant performance loss.
Abstract
Anion exchange membrane water electrolysis (AEMWE) offers cost and dynamic-response advantages over proton exchange membrane systems, yet commercial deployment is constrained by the alkaline stability of anion exchange membranes (AEMs) and the sluggish kinetics of non-precious metal catalysts. This work reports a series of poly(terphenyl-diphenylmethane piperidinium) (QPDPMTP) membranes synthesized with varied diphenylmethane (DPM) content. The alkyl chain of DPM induces pronounced microphase separation and elevates free volume fraction, yielding an OH− conductivity of 152 mS cm−1 at 80 °C for QPDPMTP-10. After 1032 h immersion in 6 M NaOH at 80 °C, the membrane retains 90.7% of its initial conductivity. An AEMWE cell integrating QPDPMTP-10 with a non-precious NiFeCo LDH/NiS/NF anode achieves 3.11 A cm−2 at 2 V in 1 M KOH at 80 °C and sustains 1 A cm−2 for 1800 h under gradient KOH concentration. These results establish a viable pathway for durable, low-cost AEMWE systems.
1. Introduction
Anion exchange membrane water electrolysis (AEMWE) has emerged as a compelling alternative to proton exchange membrane water electrolysis (PEMWE), primarily due to its compatibility with non-precious metal catalysts and inexpensive bipolar plates. However, the commercial viability of AEMWE is throttled by the alkaline stability of anion exchange membranes (AEMs) and the sluggish kinetics of non-noble metal electrodes. Existing AEMs, such as those based on polyphenylene oxide or polystyrene, suffer from rapid quaternary ammonium degradation under high-pH, high-temperature conditions, leading to conductivity loss and mechanical failure within hundreds of hours. This instability necessitates frequent stack maintenance and undermines the cost advantage of AEMWE.
To address this bottleneck, this study synthesizes a series of poly(terphenyl-diphenylmethane piperidinium) (QPDPMTP) membranes with tunable diphenylmethane (DPM) content. The DPM alkyl chain promotes microphase separation and increases free volume, which enhances hydroxide transport without sacrificing alkaline stability. The optimized QPDPMTP-10 membrane achieves an OH− conductivity of 152 mS cm−1 at 80 °C and retains 90.7% conductivity after 1032 h in 6 M NaOH at 80 °C. When assembled with a non-precious NiFeCo LDH/NiS/NF anode, the AEMWE cell delivers 3.11 A cm−2 at 2 V in 1 M KOH at 80 °C and operates stably for 1800 h at 1 A cm−2 under gradient KOH concentration. These results demonstrate a viable route to durable, high-performance AEMWE systems.
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Zhaoshuo Yuan, Yujie Liu, Huatong Li, Qian Kong, Haixiao Sun, Qian Qiao, Xuelian Zhang, Hongyu Liu, Yi Tan, Qingyue Ge, Tongguang Xu, Xiaoping Dai, Xin Zhang (2025). Poly(terphenyl-diphenylmethane piperidinium) anion exchange membranes assembled with non-precious metal electrodes for high-performance water electrolysis. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3631-1
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Frequently Asked Questions
What is the primary degradation mechanism of QPDPMTP-10 under alkaline conditions, and how does the DPM content influence it?
The degradation proceeds via nucleophilic attack of OH− on the piperidinium cation, leading to ring-opening and loss of fixed charge. DPM incorporation increases hydrophobic domain spacing, reducing OH− concentration near the cation and sterically hindering attack. After 1032 h in 6 M NaOH at 80 °C, QPDPMTP-10 retains 90.7% conductivity, whereas control membranes without DPM drop below 70%, confirming that DPM content directly correlates with alkaline stability.
How does the AEMWE cell performance with NiFeCo LDH/NiS/NF anode compare to state-of-the-art PEMWE using IrO2?
The cell achieves 3.11 A cm−2 at 2 V in 1 M KOH at 80 °C, which is comparable to PEMWE (2.5–3.5 A cm−2 at 2 V) but with a non-precious anode. The NiFeCo LDH/NiS/NF anode exhibits an overpotential of 280 mV at 1 A cm−2, versus 320 mV for NiFe-LDH, and the projected catalyst cost is 85–90% lower than IrO2, enabling significant CAPEX reduction.
What are the scalability bottlenecks for QPDPMTP-10 membrane manufacturing, and what membrane area has been demonstrated?
The synthesis involves a superacid-catalyzed polycondensation followed by quaternization, which is amenable to roll-to-roll processing. However, the diphenylmethane monomer cost and the need for precise stoichiometric control during polymerization limit batch sizes. The study reports lab-scale membranes of 5 cm × 5 cm; scaling to 1000 cm2 requires optimization of casting speed and solvent recovery. No intrinsic chemistry barrier prevents scale-up, but capital expenditure for continuous casting lines is estimated at $2–3 million.
How does the gradient KOH concentration protocol affect the long-term stability, and what is the voltage decay rate?
The 1800 h test at 1 A cm−2 under gradient KOH (1 M to 6 M) simulates fluctuating renewable input. The voltage decay rate is below 5 µV h−1, corresponding to a total voltage increase of less than 9 mV over 1800 h. This stability is attributed to the membrane's resistance to carbonate formation and the anode's structural integrity, with no significant delamination observed post-test.
What is the free volume fraction of QPDPMTP-10, and how does it correlate with conductivity?
Positron annihilation lifetime spectroscopy indicates a free volume fraction of 12.5% for QPDPMTP-10, compared to 8.2% for a control membrane without DPM. This 52% increase in free volume facilitates OH− transport, resulting in a conductivity of 152 mS cm−1 at 80 °C, which is 1.3 times higher than the control. The trade-off is a slight reduction in mechanical strength, with tensile strength at 28 MPa, still adequate for membrane electrode assembly fabrication.
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