Nanocomposite Electrodes with CuFe Layered Double Hydroxides and Hydrochar for Alkaline HER and HOR: A Multifunctional Platform for Hydrogen Electrocatalysis

Abstract

The hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) in alkaline media remain limited by slow kinetics and the reliance on noble metal catalysts. Copper-iron layered double hydroxides (CuFe-LDHs), composed of non-rare and low-toxicity elements, offer a sustainable alternative, though their performance is restricted by low conductivity and limited accessibility of active sites. In this work, CuFe-LDHs were synthesized hydrothermally and modified through three complementary strategies: ammonium fluoride (NH₄F) as a structuring agent, poly(2,6-dimethyl-1,4-phenylene oxide) functionalized with trimethylammonium (PPO-LC) as hydroxide-conducting ionomer, and a nitrogen-doped hydrochar derived from pine needles as electron-conducting additive. Structural tuning with 1.5% NH₄F produced highly crystalline LDH-1.5 with increased exposure of catalytic centers, while PPO-LC enhanced OH⁻ transport and electrode integrity. Hydrochar contributed additional conductive domains, facilitating charge transfer and improving electrode architecture. The materials were characterized using BET surface area analysis, scanning electron microscopy with EDS element mapping, X-ray diffraction, and Fourier-transform infrared spectroscopy. All catalytic electrodes demonstrated a high electrochemically active surface area. The LDH-1.5/PPO-LC electrode delivered the best HER activity with an overpotential of -332 mV at -10 mA.cm⁻². For HOR, hydrochar-containing electrodes displayed reduced onset potential values: the LDH-1.5/PPO-LC/HC composite reached the lowest onset potential of 38 mV, though the current densities remained limited. Tafel analysis indicated that the HER is governed by the Volmer step associated with water dissociation, whereas the HOR follows a Heyrovsky-Volmer or Tafel-Volmer pathway depending on the electrode composition. These results highlight the effectiveness of combining structural, ionic, and electronic modifications to enhance the performance of CuFe-LDH-based bifunctional electrodes in alkaline environments, while indicating that further optimization is required to overcome intrinsic kinetic limitations.