Synergistic anchoring of emulsion-encapsulated phosphoric acid in cytosine-modified SPEEK toward durable direct methanol fuel cells

The development of high-performance proton exchange membranes (PEMs) for direct methanol fuel cells (DMFCs) is often constrained by a fundamental trade-off: enhancing proton conductivity typically compromises dimensional stability. While phosphoric acid (PA) doping is an effective strategy to boost conductivity, its severe leaching in aqueous environments severely limits its application in low-temperature DMFCs. Herein, we propose a novel paradigm inspired by pharmaceutical encapsulation to overcome this challenge. A stable water-in-oil (W/O) emulsion is designed to encapsulate hydrophilic PA, which is then blended with a cytosine-functionalized SPEEK matrix. This innovative approach enables adjustment of the acid doping level (ADL). More importantly, it establishes a dual-retention mechanism: the quaternary ammonium groups of the emulsifier (STDAB) form ionic bonding with H 2 PO 4 − anions within the emulsion droplets, while the basic –NH– groups of cytosine act as molecular anchors for free PA via acid-base interactions. The synergistic effect of physical encapsulation and chemical anchoring endows the fabricated composite membrane with exceptional performance. It achieves a remarkable proton conductivity of 174.9 mS cm −1 at 80 °C, which is 2.3 times that of pristine SPEEK. Furthermore, the membrane exhibits outstanding operational stability, retaining 85.5 % of its voltage after 280 h of continuous operation, underscoring its significant potential for practical DMFC applications.