title: Synthesis and investigation of low valent transition metal nitrides as electrocatalysts for the hydrogen evolution reaction
Creators: name: Sun, Youyi
Publisher: name: University of Glasgow
Year: 2022

description: With the help of renewable electricity, a device called electrolyser can split water into main components: O2 and H2. Hydrogen is a good fuel and can be used on demand in a range of energy applications as well as feedstock for chemicals. The hydrogen evolution reaction from water is commonly assisted by Pt-group metal electrocatalysts. The high cost of Pt-group metals makes the search for cheaper alternatives important. The presented work aims to assess the suitability of low valent transition metal nitrides as electrocatalysts for the hydrogen evolution reaction (HER) and compare their performance with transition metal alloys. After brief introduction and experimental sections, the synthesis, crystal structure and electronic properties of two common ordered Co-Mo intermetallics (Co3Mo and Co7Mo6) are discussed. These were tested in both acidic and alkaline conditions to have a reliable benchmark for comparison with metal nitrides. Despite relatively good electrocatalytic performance the stability of the intermetallic was found limited which is consistent with previous literature reports. The following chapter is focused on comparison of three isostructural ternary nitrides (Co3Mo3N, Fe3Mo3N and Co6Mo6N) which crystallize in the -carbide structure type. As the focus is given to the comparative investigation a great care was given to isolate them as phase pure materials which was confirmed by Rietveld refinement of PXRD data. The synthesis was carried out by reduction of the relevant metal oxides in H2/N2 gas mixtures while Co6Mo6N was prepared through topotactic reaction from Co3Mo3N in H2/Ar gas mixture. Co3Mo3N delivered a relatively low overpotential of just 108 mV at 10 mA cm-2 while it also showed high current densities when immobilised on Ni-foam substrate. The stability of the nitride was excellent as was evidenced by chronoamperometric studies. Remarkably, Co6Mo6N showed a similar electrocatalytic properties with slightly higher overpotential but poor electrochemical stability. Hence, the initial hypothesis that the introduction of N atoms within the lattice could lead to higher stability was confirmed. Replacing Co for Fe into isostructural Fe3Mo3N on the other hand led to deteriorated performance suggesting that Co sites are catalytically active. This observation was also supported by computational DFT studies presented in this work. Overall, it was shown that low valent nitrides are probably superior catalyst than similar metal alloys. To study the effect of Mo substitution a previously unreported nitride with a composition Co6W6N was isolated. The additional use of urea precursor along with H2/N2 was required with elemental analysis unambiguously pointing out that the resulting compound was pure nitride as no traces of carbon were found. In comparison, isostructural Fe6W6N showed poor electrochemical performance confirming that the Co-sites are probably responsible for catalytic activity in these low valent nitrides. To pursue the hypothesis about crucial role of late transition metals play in catalytic performance Co2Mo3N and the isostructural solid solutions with the compositions Ni2- xFexMo3N (x= 0 – 1.25) were investigated. The substitution of Ni for Fe led to a deterioration in the performance further suggesting that d8 /d9 metals are essential for good electrocatalytic properties. An additional XPS investigation suggested that preserving low oxidation state of the metals is important. As Co2Mo3N showed higher overpotential than Co3Mo3N at a benchmark current density this suggested that higher number of d8 /d9 transition metals per formula unit could be a way forward for improving the catalytic properties. To test the hypothesis that increased number of late transition metal per formula unit would lead to improved catalytic activity several ternary nitrides with anti-perovskite structure were synthesised, including Co3SnN, Fe3SnN, Co3ZnN and Co3InN. These were prepared with the help of melamine as a nitrogen source and were found phase pure according to the Rietveld refinement of the PXRD data. The nitrides showed poor stability in acidic electrolyte but demonstrated a good stability in alkaline conditions. Among these nitrides, despite relatively higher number of Co atoms per formula unit, the lowest overpotential of 128 mV was achieved on Co3InN. However, this was higher than that of 108 mV demonstrated by Co3Mo3N at a benchmark current density of 10 mA cm-2 . From this perspective it was not possible to determine whether increasing the number of late-transition metal sites could lead to an improved catalytic performance. Overall, the results pointed out that ternary low valent transition metal nitrides are promising targets for application in electrocatalytic water splitting. Ni2Mo3N was identified as the most cost-effective material due to a relative simplicity of immobilizing it on a high surface area nickel foam substrate. This creates future opportunities for further research that could lead to an implementation of nitrides in proton exchange electrolysers.