ABSTRACT
Renewable production of ammonia, a building block for most fertilizers, via the electrochemical nitrogen reduction reaction (ENRR) is desirable; however, a selective electrocatalyst is lacking. Here we show that vanadium nitride (VN) nanoparticles are active, selective, and stable ENRR catalysts with an ENRR rate and a Faradaic efficiency (FE) of 3.3 × 10-10 mol s-1 cm-2 and 6.0% at -0.1 V within 1 h, respectively. ENRR with 15N2 as the feed produces both 14NH3 and 15NH3, which indicates that the reaction follows a Mars-van Krevelen mechanism. Ex situ X-ray photoelectron spectroscopy characterization of fresh and spent catalysts reveals that multiple vanadium oxide, oxynitride, and nitride species are present on the surface and identified VN0.7O0.45 as the active phase in the ENRR. Operando X-ray absorption spectroscopy and catalyst durability test results corroborate this hypothesis and indicate that the conversion of VN0.7O0.45 to the VN phase leads to catalyst deactivation. We hypothesize that only the surface N sites adjacent to a surface O are active in the ENRR. An ammonia production rate of 1.1 × 10-10 mol s-1 cm-2 can be maintained for 116 h, with a steady-state turnover number of 431.
ABSTRACT
Much effort has been devoted in the development of efficient catalysts for electrochemical reduction of CO2. Molecular level understanding of electrode-mediated process, particularly the role of bicarbonate in increasing CO2 reduction rates, is still lacking due to the difficulty of directly probing the electrochemical interface. We developed a protocol to observe normally invisible reaction intermediates with a surface enhanced spectroscopy by applying square-wave potential profiles. Further, we demonstrate that bicarbonate, through equilibrium exchange with dissolved CO2, rather than the supplied CO2, is the primary source of carbon in the CO formed at the Au electrode by a combination of in situ spectroscopic, isotopic labeling, and mass spectroscopic investigations. We propose that bicarbonate enhances the rate of CO production on Au by increasing the effective concentration of dissolved CO2 near the electrode surface through rapid equilibrium between bicarbonate and dissolved CO2.
ABSTRACT
Two-dimensional V2O5 and manganese-doped V2O5 sheet network were synthesized by a one-step polymer-assisted chemical solution method and characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, thermal-gravimetric analysis, and galvanostatic discharge-charge analysis. The V2O5 particles were covered with thin carbon layers, which remained after decomposition of the polymer, forming a network-like sheet structure. This V2O5 network exhibits a high capacity of about 300 and 600 mA·h/g at a current density of 100 mA/g when it was used as a cathode and anode, respectively. After doping with 5% molar ratio of manganese, the capacity of the cathode increases from 99 to 165 mA·h/g at a current density of 1 A/g (â¼3 C). This unique network structure provides an interconnected transportation pathway for lithium ions. Improvement of electrochemical performance after doping manganese could be attributed to the enhancement of electronic conductivity.
ABSTRACT
Nanostructured MoO2/graphite oxide (GO) composites are synthesized by a simple solvothermal method. X-ray diffraction and transmission electron microscopy analyses show that with the addition of GO and the increase in GO content in the precursor solutions, MoO3 rods change to MoO2 nanorods and then further to MoO2 nanoparticles, and the nanorods or nanoparticles are uniformly distributed on the surface of the GO sheets in the composites. The MoO2/GO composite with 10 wt % GO exhibits a reversible capacity of 720 mAh/g at a current density of 100 mA/g and 560 mAh/g at a high current density of 800 mA/g after 30 cycles. The improved reversible capacity, rate capacity, and cycling performance of the composites are attributed to synergistic reaction between MoO2 and GO.
