From amorphous to crystalline: the impact of crystallisation on iron-nickel boride electrocatalysts.

Efficient electrocatalysts for direct seawater electrolysis are critical for the sustainable production of green hydrogen without straining freshwater resources. However, linking structure-property relationships to catalytic activity and stability remains challenging. Here, FeNiB electrocatalysts with tuneable crystallinity were synthesised via scalable chemical reduction and controlled thermal treatment, producing amorphous, partially crystalline (200 °C), and fully crystalline (400 °C) structures. Crystallinity strongly influenced surface morphology, electrochemically active surface area (ECSA), and charge transfer. The amorphous FeNiB showed surface fractals with the highest ECSA and double-layer capacitance, delivering the best hydrogen evolution reaction (HER) performance with an overpotential of -0.364 V vs. RHE at 100 mA cm-2, compared to -0.416 V vs. RHE and -0.427 V vs. RHE for the 200 °C and 400 °C samples, respectively. In contrast, the partially crystalline FeNiB (200 °C) exhibited optimal oxygen evolution reaction (OER) activity, requiring 0.314 V vs. RHE at 100 mA cm-2, outperforming the amorphous (0.364 V vs. RHE) and fully crystalline (0.402 V vs. RHE) samples. This enhanced OER performance arises from a balance between structural order and accessible active sites. The best-performing samples maintained stable operation over 24 h in alkaline saline and natural seawater with minimal degradation. These results highlight crystallinity as a key parameter for tuning electrocatalyst performance and provide design insights for practical seawater electrolysis systems.