Metal-free boron carbonitride with tunable boron Lewis acid sites for enhanced nitrogen electroreduction to ammonia

From a thermodynamic point of view, electrocatalytic nitrogen reduction reaction (NRR) is possible for the carbon-based metal-free catalysts, which are gradually becoming a class of potential alternatives to metal-based catalysts. However, the clarification of true active sites and corresponding exact catalytic mechanism of metal-free catalysts is urgently required. Taking full advantage of boron “Lewis acid”, a series of boron carbonitride (BCN) materials were designed and synthesized and the “Lewis acid catalysis” sites were tuned easily by adjusting the relative contents of boron and nitrogen atoms. Boron-enriched BCN exhibited outstanding NRR performance with an ammonia yield of −8.39 μg h−1.cm−2cat. (−41.9 μg h−1.mg−1cat.) and Faradaic efficiency of −9.87 %, together with excellent stability. Density functional theory calculations indicate that the boron sites of BCN enable the low energy barrier of the NRR rate-determining steps and the spontaneity of nitrogen adsorption. Our philosophy opens a new possible avenue to explore high-performance metal-free materials for nitrogen activation.