Carbon dots-boosted active hydrogen for efficient electrocatalytic reduction of nitrate to ammonia

Electrocatalytic nitrate reduction reaction (NO₃RR) has gained great attention as a means of transforming environmental waste into fertilizers. However, the activity and efficiency of NO₃RR is severely hindered by the inevitable self-coupling of active hydrogen (H*_ads) competing with nitrate (NO₃^-) hydrogenation. Here, an efficient electrocatalyst is developed by combining multifunction carbon dots (CDs) with bornite (Cu₅FeS₄) towards the comprehensive manipulation of the adsorption and conversion of H*_ads. As a result, the optimized CDs/Cu₅FeS₄ presents a remarkable NH₃ yield of 215.19 μmol h⁻¹ cm⁻² with a Faradaic efficiency of 86.52 %. Importantly, the introduction of CDs can greatly increase the NH₃ yield and FE compared with pure Cu₅FeS₄, even under a wide range of NO₃^- concentrations. Meanwhile, the catalyst can successfully transform simulated NO₃^- groundwater into NH₃ by using a flow cell. The cyclic voltammetry and electron paramagnetic resonance tests indicate that CDs effectively promote the production of H*_ads and participate in NO₃^- hydrogenation. Kinetic isotope experiments reveal the role of CDs in accelerating water dissociation and proton transfer processes. In-situ Raman spectroscopy demonstrate that CDs effectively boosts the adsorption and conversion of NO₃^- to NO₂^- and NO₂^- to NH₃. And DFT calculations reveal CDs significantly reduce the energy barrier of the key rate-determining step (*NO₃ to *NO₂), which thus promotes the hydrogenation reaction between H*_ads and N-containing intermediates. This work provides an interesting insight for enhancing the catalytic activity of ammonia by utilizing CDs to promote the generation and conversion of H*_ads.