Synergistic effects of MOF catalysts and a 3D junction enable thin bipolar membranes to reach the thermodynamic minimum potential for water dissociation

Bipolar membrane water electrolysis (BPMWE), which can convert the surplus electricity generated from renewable energy resources into hydrogen, suffers from high overpotential, high ion exchange layer resistance, and delamination, arising from a BPM's limited interfacial area, a large membrane thickness, and the interface's dehydration inevitably occurring during water dissociation. To overcome the limitations, we developed a thin BPM with a catalytic 3D junction by dual electrospinning and air-spraying a metal–organic framework (MOF). A 3D junction could reduce the water dissociation overpotential by enlarging the interfacial area while providing better diffusion pathways for ions and water molecules. In addition, a 3D junction could improve mechanical stability by interlocking the interface of the cationic and anionic exchange layers. To make the most of the 3D junction, we added MOF catalysts to a 3D junction to further promote water dissociation without catalyst leakage owing to the large size of MOF. As a result of the synergistic effects of a 3D junction and MOF catalysts, the best 3D-BPM (3D-0.67, 0.88 V) revealed a remarkably low transmembrane voltage (TMV) at 100 mA cm⁻² (U₁₀₀), which was 4.9 and 3.6 times lower than that of the BPM with a 2D interface (2D-0, 4.32 V) and 3D junction (3D-0, 3.18 V) without catalysts, respectively. Furthermore, the U₁₀₀ of the 3D-0.67 was lower than even a commercial BPM (FBM, 1.017 V) and very close to a thermodynamic minimum potential (0.83 V). In addition, the 3D-0.67 (no TMV changes) demonstrated better long-term and high-current–density stabilities than the FBM (2% and 44% increases in the TMV in the long-term and high-current–density stability tests, respectively). Lastly, the 3D-0.67 was comparable to the FBM in BEMD, despite being prepared without proprietary ionomers.