Beyond superaerophobicity: gradient-porous hydrogels synchronize gas detachment and charge flow for high-current HER Makafui Y. Folikumah 1 , Sehun Seo 1 , Yvonne Pieper 1 , Matthias Heuchel 1 , Axel Neffe 2 , Francesca M. Toma 1,3,4* 1 Institute of Functional Materials for Sustainability, Helmholtz-Zentrum Hereon, Kantstraße 55, 14513 Teltow, Germany, 2 Institute of Materials Chemistry, BTU Cottbus-Senftenberg, Universitätsplatz, 01968 Senftenberg, Germany, 3 Liquid Sunlight Alliance and Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States, 4 Faculty of Mechanical and Civil Engineering, Helmut Schmidt University, Holstenhofweg 85, 22043 Hamburg, Germany * Corresponding author Email: francesca.toma@hereon.de Superaerophobic surfaces created via soft material deposition accelerate bubble detachment 1-3 , yet their effect on hydrogen-evolution catalysis remains ambiguous. We report salt-templated polyethyleneimine (PEI) hydrogels spin-cast on Ti/Pt electrodes as a soft, solution-processable interface that decouples wettability from transport. All hydrogel films display near-perfect bubble contact angles (~180°) in KOH, H 2 SO4 and HNO3. Strikingly, only electrodes carrying a 20 nm Pt layer exhibit ~30 % higher steady-state current density and markedly flatter potential drift at –0.20 V (KOH) or –0.10 V vs RHE (acids); the same hydrogel on 50 nm Pt yields no improvement or slight deterioration. Electrochemical impedance and high-speed imaging confirm equally rapid bubble detachment for both cases, indicating that gas removal aided by the hydrogel layer may be necessary but not sufficient. This is particularly true for excessively hydrophilic hydrogels may suppress bubble nucleation altogether by eliminating the local supersaturation zones necessary for nucleation 4 . Furthermore, swollen hydrogel matrices can physically obstruct ion pathways and alter the structure of the electric double layer (EDL), ultimately reducing catalytic activity 5,6 . Atomistic simulations underway attribute the performance gap to restricted water and ion flux across the hydrogel film and highlight the benefit of introducing vertical porosity gradients to synchronize gas escape with ionic conductivity. Our results advocate architectured, conductive hydrogels as a versatile pathway toward bubble- tolerant, high-rate solar water-splitting electrodes under industrially relevant alkaline conditions. References 1. Bae et al. Adv. Energy Mater . 2022 ,12, 2201452 2. Jeon et al . Sci. Adv. 2020 , 6 , eaaz3944 3. Park et al. J. Mater. Chem. A 2023 , 11 , 1658-1665
4. Pradhan et al. Processes 2022 , 10 , 1073 5. Hill R.J. J . Fluid Mech . 2022, 936, A27 6. Nordness et al. Macromolecules 2023 , 56 , 4669-4680
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