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.