Exploring Transition Metal Molybdates for Electrochemical Water Splitting

The current global energy system strongly depends on non-renewable energy sources such as oil and coal. The main issues with the use of these kinds of energy sources are their finite availability, which makes them unsustainable for the long-term use and the high emission of pollutants associated with their use. The most promising solution is the use of green hydrogen as energy source; this type of gas can be obtained through the electrochemical splitting of water. Currently the main industrial method to carry out this process involves the use of the so-called platinum-group metals (PGMs) such as Pt, Ir, and Ru, which are scarce, expensive, and often unstable under operating conditions. For this reason, it is extremely important end urgent to find an alternative catalyst based on earth-abundant elements. In particular, the most promising materials in this field are, transition-metal based materials, which have been shown to be highly active in catalyzing this process. Moreover, they are also very abundant on the planet and therefore low-cost. In particular, transition-metal oxides show a remarkable high stability under operating conditions. Previous studies have shown that combining 3d transition metals (such as Fe, Ni, or Co) with high-valence metals like Mo or W within the same compound can create strong synergistic interactions. These interactions help stabilize the redox states, which are crucial for enhancing catalytic activity in both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). For these reasons, this work focuses on synthesizing, characterizing, and testing the water splitting performances of different kinds of transition-metal molybdates with the general formula MMoOₓ (M = Mn, Fe, Co, Ni). Accordingly, the results show that in the OER process the material that showed the best performance was Fe2(MoO4)3. Inspired by these outcomes, a mixed Ni–Fe molybdate was prepared to investigate the synergistic effect that could take place when both Fe and Ni are present together with Mo. The synthesis was conducted using different proportions of Fe and Ni precursors to investigate which proportion of the two led to the best performances. From the results it was seen that the samples with the highest proportion of iron are those that show better performances than the initial samples in both the OER and HER processes. This work demonstrates that engineered coexistence of 3d metals with high-valence element, Mo, enables the stabilization of active sites, significantly enhancing both HER and OER activity thereby providing a promising design strategy for next-generation water-splitting electrocatalysts.