Efficient electrochemical nitrogen fixation at iron phosphide (Fe₂P) catalyst in alkaline medium

Abstract

A catalytic system based on iron phosphide (Fe₂P) has exhibited electrocatalytic activity toward N₂-reduction reaction in alkaline medium (0.5 mol dm⁻³ NaOH). Based on voltammetric stripping-type electroanalytical measurements, Raman spectroscopic and spectrophotometric data, it can be stated that the Fe₂P catalyst facilitates conversion of N₂ to NH₃, and the process is fairly selective with respect to the competing hydrogen evolution. A series of diagnostic electrocatalytic experiments (utilizing platinum nanoparticles and HKUST-1) have been proposed and performed to control purity of nitrogen gas and to probe presence of potential contaminants such as ammonia, nitrogen oxo-species and oxygen. On the whole, the results are consistent with the view that the interfacial reduced-iron (Fe⁰) centers, while existing within the network of P sites, induce activation and reduction of nitrogen, parallel to the water splitting (reduction) to hydrogen. It is apparent from Tafel plots and impedance measurements that mechanism and dynamics of nitrogen reduction depends on the applied electroreduction potential. The catalytic system exhibits certain tolerance with respect to the competitive hydrogen evolution and gives (during electrolysis at -0.4 V vs. RHE) the Faradaic efficiency, namely, the selectivity (molar) efficiency, toward production of NH₃ on the level of 60%. Under such conditions, the NH₃-yield rate has been found to be equal to 7.5 µmol cm⁻² h⁻¹ (21 µmol m⁻² s⁻¹). By referring to classic concepts of electrochemical kinetic analysis, the rate constant in heterogeneous units has been found to be on the moderate level of 1-2*10⁻⁴ cm s⁻¹ (at -0.4 V). The above mentioned iron-phosphorous active sites, which are generated on surfaces of Fe₂P particles, have also been demonstrated to exhibit strong catalytic properties during reductions of other electrochemically inert reactants, such as oxygen, nitrites and nitrates.