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The insulator-to-metal phase transition from VO2(M1) to VO2(R) results in extensive improvements in electrocatalytic activity for both the OER and the HER.

A MnV2O6/graphene nanocomposite was fabricated through hydrothermal synthesis and high energy milling to introduce it as an efficient OER electrocatalyst. The MnV2O6/graphene nanocomposite with 20 wt% graphene exhibited superior electrocatalytic OER performance with a low overpotential and high stability and durability in 1 M KOH aqueous solution, exhibiting even after 1000 CV cycles.

The discovery and deployment of earth-abundant electrocatalysts for hydrogen evolution is central to the use of molecular hydrogen as a viable fuel. The edges of molybdenum sulfide are able to mediate proton adsorption and dihydrogen formation at relatively low overpotentials in acidic media. From a practical perspective, an optimal electrocatalyst must combine electrode level efficiency (reflected by high current densities and low Tafel slopes and overpotentials) with high intrinsic catalytic activity (measured by turnover frequency). Herein, we map both sets of parameters for molybdenum sulfide catalysts as a function of the annealing temperature across their amorphous to crystalline phase transition. Studies of local structure indicate that with increasing annealing temperature, molecular precursors are initially cross-linked to form [Mo3S13]2− clusters characterized by both apical/bridging S22− and unsaturated/terminal S22− moieties, which in turn are consumed to nucleate ultra-thin crystalline MoS2 domains. With increase of the annealing temperature, these nuclei coalesce to form larger nanosheets. Annealing and the resulting amorphous to crystalline transition involves a trade-off between the number of available sites (which is decreased with increasing crystallite size) and the intrinsic activity of the sites (which is improved with increasing crystallinity). Optimal hydrogen evolution reaction (HER) activity is observed for the molybdenum sulfide sample prepared by annealing at 300 °C, which comprises ultra-thin MoS2 nuclei embedded within a matrix of [Mo3S13]2− clusters. This sample is characterized by an overpotential value η10 of 176 mV, a Tafel slope of 49.2 mV dec−1, a turnover frequency of 1.15H2 per s per active site at −0.2 V versus reversible hydrogen electrode (RHE), and furthermore exhibits reasonable stability upon prolonged electrochemical cycling. The amorphous samples are found to be more susceptible to oxidation, which degrades the stability of the catalysts. The mapping of electrode-level parameters and intrinsic activity as a function of crystallite size provides vital design principles for constructing a viable electrocatalyst.

Novel inks and pastes based on a copper(II) formate tetrahydrate precursor were formulated with controllable viscosities in the range 5000–10 000 cP for use in printed electrodes. In particular, the addition of ethyl cellulose increased the adhesion of the printed paste films to the glass substrates. For the facile fabrication of electrodes with improved performance, the formulated pastes were printed on glass substrates under ambient conditions by a doctor-blade method. The printed films were thermally sintered at 170–250 °C in air and subsequently reduced under a formic acid atmosphere. The phase and microstructural evolution of the electrode films were systematically investigated by X-ray diffraction (XRD) and cross-sectional, focused ion beam scanning electron microscopy (FIB-SEM) in each processing step. Highly adhesive, polycrystalline Cu electrode films decorated by ethyl cellulose with a vermicular microstructure and large interconnected pore channels were well formed on the glass substrates. The electrode films sintered for 1 min in air and then reduced for 5 min under formic acid atmosphere at 250 °C showed the lowest electrical resistivity of ∼8 μΩ cm (electrical conductivity of ∼125 000 Ω cm−1, equivalent to ∼22% of bulk Cu), despite their maximum porosity of 27.31%.