Ammonia is the largest chemical product produced on earth, with an annual production of more than 150 million tons, and has been synthesized by the Haber-Bosch process for more than 100 years. However, the Haber-Bosch process uses hydrogen derived from natural gas reforming to reduce nitrogen under high temperature and pressure, so it is a process with high energy consumption and CO2 emission. However, the electrochemical nitrogen reduction reaction (NRR), which directly uses water and nitrogen as raw materials and reacts at room temperature and pressure, is expected to solve this problem, and has been widely concerned by researchers recently. However, nitrogen has a high bond dissociation energy (940 kJ mol-1), which makes it difficult to activate, and NRR has a similar REDOX potential with hydrogen precipitation reaction (HER), but is often several orders of magnitude lower than HER in terms of kinetics. Therefore, NRR in aqueous solution inevitably faces strong competition from HER. Therefore, most of the existing NRR electrocatalysts have very low Faraday efficiency and low ammonia production rate, which is far from the goal of industrialization. Therefore, it is necessary to further develop NRR catalysts with higher activity and selectivity.
A niobium dioxide (NbO2) nano-particle electrocatalyst has been reported in Small Methods to show high activity in electrochemical nitrogen reduction. According to the theoretical calculation results of electrocatalytic NRR of various oxides reported in the previous literature, NbO2 is not only very close to the peak of the volcano diagram, but also has similar adsorption energy for *NNH and *H. Therefore, it is expected to achieve higher NRR activity while avoiding too serious HER competition. Inspired by these results, NbO2 nanoparticles were synthesized using Nb3O7(OH) nanorods as precursors, and Nb2O5 was synthesized under similar conditions as control. The crystal structure of NbO2 and Nb2O5 is shown in Figure 1. Both oxides are composed of [NbO6] octahedrons, but the connection mode and the valence state of Nb are different. For Nb at +4 valence NbO2, the 4D orbital is occupied by an electron, while the 4D orbital of Nb2O5 is empty.
In homogeneous catalysis, heterogeneous catalysis and photocatalysis, it is generally believed that the transition metal can form σ-π bond with N2 molecules, and transfer electrons to the 1π G * antibonding orbital of N2, thus activating N2. The authors speculate that a similar situation may exist in electrocatalysis. Nb4+ with an electron in the 4D orbital should form a π feedback bond more easily than Nb5+ with an empty 4D orbital. Therefore, NbO2 may have a higher NRR electrocatalytic activity than Nb2O5. XRD, SEM, TEM, Raman spectroscopy and other characterization of NbO2 and Nb2O5 (FIG. 2) show that both oxides conform to the expected structure and have high crystallinity. At the same time, THE XPS map of Nb 3D (FIG. 3) showed that Nb4+ was abundant in NbO2, while Nb5+ was only present in control group Nb2O5. NRR test in 0.05 M H2SO4 solution showed that NbO2 showed better NRR activity than Nb2O5 in the whole potential range, with a Faraday efficiency of 32% at −0.60 V vs. RHE, which is one of the highest Faraday efficiency reported among non-noble metal NRR catalysts. At − 0.65V vs. RHE, the highest ammonia production rate and Faraday efficiency of 11.6 μg h−1 MgCAT.−1 are achieved, and the stability is also good, proving that NbO2 can be used for efficient electrocatalytic nitrogen reduction (FIG. 4). In addition, in order to avoid false positive results in the test, the samples were synthesized and tested without using or contacting any reagent containing N other than N2. A series of contrast and auxiliary experiments, such as Ar contrast experiment, gas chromatography and ion chromatography, further confirmed that the ammonia produced in electrolysis should come from the electric reduction of N2. This work demonstrated an electrochemical nitrogen reduction catalyst of NbO2 with a Faraday efficiency of 32%, providing a valuable idea for the design of Nb4+ based materials and the further improvement of the activity and selectivity of NRR electrocatalysts.
