Simultaneous Boosting Nitrogen Reduction Activity and Suppressing Hydrogen Evolution Reaction Activity of Transition Metal Oxide Based Electrocatalyst

Abstract

Ammonia (NH3) is widely served as fertilizer, safer chemical hydrogen storage, and energy conservation materials with an annual production around 200 million tons per year. Currently, NH3 is mainly synthesized via the Haber-Bosch process, which is an extremely energy- and capital-intensive (high temperature 400–500 °C and high pressure 100–350 atm) process, consuming over 1% of the global annual energy output, emitting massive amount of greenhouse gases (~420 Mt CO2 annually), and causing serious concerns with climate change. Electrochemical nitrogen reduction reaction (eNRR) allows NH3 production from nitrogen (N2) and water (H2O) under ambient conditions. Especially if driven by renewable energies, NRR represents a potentially clean and sustainable strategy for replacing the traditional Haber-Bosch process and alleviating climate change effect. One of the most critical fundamental challenges in production of ammonia via electrochemical nitrogen reduction reaction (eNNR) is developing catalysts which are capable to boost eNRR activity and concurrently suppress the overwhelming competitive hydrogen evolution reaction (HER). This work reports a new strategy to develop such catalysts to alleviate this challenge. This new strategy for the first time takes advantage of the outstanding capability of some redox active transition metal oxides (TMOs) in their super capability in electrochemically controlled proton interstitial doping and the associated remarkable structural changes to boost their NRR activity. Further, inspired by their unique hydrogen intercalation determined HER activity, this new strategy introduced cationic substitutional doping of the TMOs with nonmetal elements, such as phosphorus (P), to modulate their proton intercalation behavior, achieving HER suppression. In this work, tungsten oxide nanosheets (WO3) have been exploited as a typical example of such TMOs to demonstrate the feasibility of this innovative strategy. It is found that cationic P doping also accompanies introduction of large quantity of oxygen vacancies (OVs) in the matrix of the WO3 and structural changes of the WO3 lattice. The synergy of the substitutional doping and interstitial doping in tuning the electronic and the local geometric structures leads to significantly enhanced eNNR performance of WO3 nanosheets. An unprecedented high Faradaic efficiency (FE) of 64.1% with NH3 yield of 24.5 µg h-1 has been reached, representing the highest NRR performance for WO3 based catalysts. It is worth mentioning that the P-doped WO3 catalyst (200 nm square with a thickness of 20-40 nm) is much larger than most of the reported catalyst. The catalytic yield is even higher than that achieved with atomic thin two-dimensional (2D) WO3 nanosheets (OV-rich) catalysts (17 µg h-1 with a FE of 7.0%), which is known for their high catalytic efficiency due to the excellent accessibility of all the catalytic active sites. It is also worthwhile to mention that the FE of 64.1% is comparable to that of the natural nitrogenase can be reached (65%), which is unprecedented high compared to most of catalysts reported so far (<25%). In addition, inclusion of Li ions in the electrolyte further increased the NH3 yield to 60.1 µg h-1 with the high FE largely remained, reached the highest eNNR performance for all the TMO based catalysts. Density functional theory calculations further revealed the synergistic role of interstitial doping of H* and substitutional cationic doping of P, and the associated generation of OVs in increasing the electronic states crossing the Fermi level of the WO3 and thus reducing reaction energy barrier in N2 activation and hydrogenation. An interesting non-PCET (proton coupled electron transfer) mechanism is revealed in the first step and the rate determining step during eNNR, providing a new design principle for TMO-based catalysts. Figure 1