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The low quantum yield of photocatalytic hydrogen production in carbon nitride (CN) has been improved upon via the modulation of both the extrinsic and intrinsic properties of the material. Although the modification of extrinsic properties has been widely investigated in the past, recently there has been growing interest in the alteration of intrinsic properties. Refining the intrinsic properties of CN provides flexibility in controlling the charge transport and selectivity in photoredox reactions, and therefore makes available a pathway toward superior photocatalytic performance. An analysis of recent progress in tuning the intrinsic photophysical properties of CN facilitates an assessment of the goals, achievements, and gaps. This article is intended to serve this purpose. Therefore, selected techniques and mechanisms of the tuning of intrinsic properties of CN are critically discussed here. This article concludes with a recommendation of the issues that need to be considered for the further enhancement in the quantum efficiency of CN photocatalysts.

 

Nickel nitride (Ni 3 N) is known as one of the promising precatalysts for the electrochemical oxygen evolution reaction (OER) under alkaline conditions. Due to its relatively low oxidation resistance, Ni 3 N is electrochemically self-oxidized into nickel oxides/oxyhydroxides (electroactive sites) during the OER. However, we lack a full understanding of the effects of Ni 3 N self-oxidation and Fe impurity incorporation into Ni 3 N from electrolyte towards OER activity. Here, we report on our examination of the compositional and structural transformation of Ni 3 N precatalyst layers on Ni foams (Ni 3 N/Ni foam) during extended periods of OER testing in Fe-purified and unpurified KOH media using both a standard three-electrode cell and a flow cell, and discuss their electrocatalytic properties. After the OER tests in both KOH media, the Ni 3 N surfaces were converted into amorphous, nano-porous nickel oxide/(oxy)hydroxide surfaces. In the Fe-purified electrolyte, a decrease in OER activity was confirmed after the OER test because of the formation of pure NiOOH with low OER activity and electrical conductivity. Conversely, in the unpurified electrolyte, a continuous increase in OER activity was observed over the OER testing, which may have resulted from the Fe incorporation into the self-oxidation-formed NiOOH. Our experimental findings revealed that Fe impurities play an essential role in obtaining notable OER activity using the Ni 3 N precatalyst. Additionally, our Ni 3 N/Ni foam electrode exhibited a low OER overpotential of 262 mV to reach a geometric current density of 10 mA cm geo −2 in a flow cell with unpurified electrolyte. 

Electrochemical water splitting is one of the most promising approaches for sustainable energy conversion and storage toward a future hydrogen society. This demands durable and affordable electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). In this study, we report the preparation of uniform Ni–P–O, Ni–S–O, and Ni–S–P–O electrocatalytic films on nickel foam (NF) substrates via flow cell-assisted electrodeposition. Remarkably, electrodeposition onto 12 cm 2 substrates was optimized by strategically varying critical parameters. The high quality and reproducibility of the materials is attributed to the use of a 3D-printed flow cell with a tailored design. Then, the as-fabricated electrodes were tested for overall water splitting in the same flow cell under alkaline conditions. The best-performing sample, NiSP/NF, required relatively low overpotentials of 93 mV for the HER and 259 mV for the OER to produce a current density of 10 mA cm −2 . Importantly, the electrodeposited films underwent oxidation into amorphous nickel (oxy)hydroxides and oxidized S and P species, improving both HER and OER performance. The superior electrocatalytic performance of the Ni–S–P–O films originates from the unique reconstruction process during the HER/OER. Furthermore, the overall water splitting test using the NiSP/NF couple required a low cell voltage of only 1.85 V to deliver a current density of 100 mA cm −2 . Overall, we demonstrate that high-quality electrocatalysts can be obtained using a simple and reproducible electrodeposition method in a robust 3D-printed flow cell. 

This paper reports a highly active and stable nonprecious metal electrocatalyst based on bimetallic nanoscale nickel molybdenum nitride developed for the hydrogen evolution reaction (HER). A composite of 7 nm Ni 2 Mo 3 N nanoparticles grown on nickel foam (Ni 2 Mo 3 N/NF) was prepared through a simple and economical synthetic method involving one-step annealing of Ni foam, MoCl 5 , and urea without a Ni precursor. The Ni 2 Mo 3 N/NF exhibits high activity with low overpotential ( η 10 of 21.3 mV and η 100 of 123.8 mV) and excellent stability for the HER, achieving one of the best performances among state-of-the-art transition metal nitride based catalysts in alkaline media. Supporting density functional theory (DFT) calculations indicate that N sites in Ni 2 Mo 3 N with a N–Mo coordination number of four have a hydrogen adsorption energy close to that of Pt and hence may be responsible for the enhanced HER performance.