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

ABSTRACT The high‐current‐density hydrogen evolution reaction (HER) is essential for large‐scale industrial H 2 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 H 2 O, OH − , and H 2 , 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 H 2 O 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 µm 2 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.