Waste-to-value: Spent coffee grounds derived Ni-decorated porous biochar for efficient plasma-catalytic ammonia synthesis

The shift from centralized Haber–Bosch to decentralized renewable ammonia synthesis demands catalysts with high activity and renewable compatibility. Here we report a Ni-decorated porous biochar catalyst (Ni/HBC-2) derived from spent coffee grounds. The optimized support (HBC-2) features a hierarchical micro-mesoporous structure (2062 m 2 g −1 ), with Ni nanoparticles uniformly dispersed over the carbon support. Complementary XRD and XPS results indicate the coexistence of metallic Ni and surface-oxidized Ni species. In a DBD plasma reactor, Ni/HBC-2 achieves an NH 3 yield of 4473 μmol g −1 h −1 under optimal SEI and 3400 μmol g −1 h −1 under mild conditions (energy yield: 0.58 g-NH 3 ·kWh −1 ). The catalyst retains 90% activity after 50 h and exhibits excellent cyclic stability (25 cycles). Mechanistic studies show that the high permittivity (εᵣ = 30) concentrates the electric field in the gas gap (3.0 → 16.3 kV mm −1 ), enhancing ionization, while moderate acid sites (355 °C, 0.28 mmol g −1 ) promote NH 3 desorption and suppress decomposition. This work demonstrates that waste-derived biochar is a high-performance, low-cost support for plasma-catalytic NH 3 synthesis, enabling distributed green ammonia production.