Multiscale Pores in 3D Printed Cobalt Phosphide-Carbon Electrodes Unlock Mass Transfer for Hydrogen Evolution at High Current Densities

The high-current-density hydrogen evolution reaction (HER) is essential for large-scale industrial H₂ production, yet bubble release disrupts mass transport at elevated current densities, significantly impeding reaction kinetics and HER efficiency. We present a self-supported cobalt phosphide-carbon electrode with multiscale pores, specifically engineered to address these challenges. Using a hybrid gel ink composed of chitosan, ZIF-67, and zinc trimesic acid (Zn-BTC), millimeter-scale pores are precisely tailored through direct ink writing with filament spacings of 2, 3, and 4 mm. Hierarchical nanopores are further refined via scaffold pyrolysis and Zn evaporation at controlled temperatures between 700°C and 950°C. The 3-CoP-NC-900 electrode, developed through 3 mm grid printing and 900°C pyrolysis, facilitates efficient mass transport of H₂O, OH^-, and H₂, achieving sustained HER at 900 mA cm^-2 with an overpotential of -287 mV and a Tafel slope of 38.0 mV dec^-1. In situ confocal microscopy fluorescence imaging highlights enhanced OH^- mobilization, optimizing H₂O transport and kinetics. Simulations confirm that the 3 mm grid and 3.7 nm mesopores together optimize bubble release and internal species transport, achieving a peak diffusion flux density of 2.1  µm² s^-1. Integrated with a NiFe LDH/NF anode, the system delivers 0.5 A cm^-2 at a hydrogen cost of $0.88-1.07/GGE. This study utilizes hierarchical pore structures to enhance mass transfer, offering insights for designing advanced electrodes in electrochemical energy systems.