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Summary H2 as a storable fuel can be sustainably generated from direct cleavage of water with a catalytic approach. However, H2 generation is severely affected by poor catalytic activities and the instability of catalyst materials. Recently, transition-metal-based nitrides (TMNs) have been widely explored because of their intrinsic abilities to catalyze water splitting, wide pH stability, high corrosion resistance, and potential for structural modulations. Most investigations have focused on the design of advanced heterostructures for improving catalytic activity. However, identification of the active sites and decoding the inherent mechanisms are often neglected. Here, we investigate the fundamental aspects of H2 production to elucidate the cutting-edge progress of TMNs. First, we explore the engineering of the active sites of ordered and disordered structures and the relation with hydrogen evolution reaction activity. Second, we explain the development of advanced oxygen evolution reaction catalysts by focusing on minimizing autoxidation. Third and finally, we discuss complementary strategies for converting unifunctional TMNs to bifunctional catalysts for overall water splitting.

Herein, we report the expedient synthesis of new nanocomposite Sn0.39Ti0.61O2·TiO2 flakes using simple sol–gel and calcination methods. In order to prepare this material, first, we generated a polymeric gel using cost-effective and easily accessible precursors such as SnCl4, titanium isopropoxide, and tetrahydrofuran (THF). A small amount of triflic acid was used to initiate THF polymerization. The calcination of the resulting gel at 500 °C produced a Sn–Ti bimetallic nanocomposite. This newly synthesized Sn0.39Ti0.61O2·TiO2 was characterized by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and UV–visible spectroscopy. The photoelectrochemical (PEC) studies were performed for the first time using Sn0.39Ti0.61O2·TiO2 coated over fluorine-doped tin oxide (FTO) under simulated 1 sun solar radiation. The chronoamperometric study of the Sn0.39Ti0.61O2·TiO2/FTO revealed the repeatable and substantially higher photocurrent for the oxygen evolution reaction (OER) when compared to only TiO2. Moreover, the synthesized material exhibited high stability both in the presence and absence of light. The photocatalytic studies suggested that the sol–gel-synthesized Sn0.39Ti0.61O2·TiO2 can be efficiently used as a photoanode in the water-splitting reaction.

Aerosol-assisted chemical vapor deposition (AACVD) is successfully implemented for the fabrication of nanostructured palladium (Pd) thin films on FTO substrate. The Pd films are produced using a commercially available precursor palladium acetylacetonate (Pd(C5H7O2)2) under the flow of N2 gas without incorporating hydrogen gas or any other reducing agent. This synthetic strategy can effectively confront the previous problems related with conventional CVD approach while depositing palladium thin films such as reduced volatility and thermal instability of precursors. The palladium thin films were characterized by powder X-ray diffraction (pXRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) analysis. Hydrazine is a colorless and flammable chemical routinely used in rocket fuel, agro-chemicals, textile dyes, and as corrosion inhibitors in various industries. However, hydrazine is a toxic and highly carcinogenic chemical. The newly fabricated palladium nanostructured films were tested for electrochemical sensing of hydrazine in this study. A highly stable and reproducible amperometric response for hydrazine oxidation was obtained with a superior limit of detection (LOD) of 10 nM, and limit of quantification (LOQ) of 33 nM while the sensitivity value calculated was 2.94 μA μM−1 cm−2. A linear concentration range of 1–325 μM was achieved by chronoamperometry.

Metal–organic frameworks (MOFs) are an emerging class of materials exhibiting high surface areas, controlled pore sizes, open metal sites and organic linkers. Utilizing MOFs as direct electrode materials for electrochemical sensing can offer inherent advantages such as containing a sensing element and a redox mediator in a single molecule; however, the direct use of MOFs as electrode materials is hindered because of their insulating nature and less stability in an aqueous medium. In this study, a new water stable Zn-MOF was synthesized and used directly as an electrode material. The Zn-MOF possesses good ability to electrocatalyze the hydrazine oxidation reaction. The Zn-MOF's inherent poor conductivity was overcome by including a hydrophobic electrolyte, tetrakis(4-chlorophenyl)borate tetradodecylammonium salt (ETH 500), during the fabrication of the Zn-MOF membrane. After coating a thin film of the nafion-ETH500 supported Zn-MOF over a glassy carbon electrode (GCE), the response for hydrazine oxidation was substantially improved. Linear sweep voltammetry (LSV) demonstrated a wide linear range from 20 to 350 μM (R2 = 0.9922) for hydrazine. A detection limit of 2 μM (n = 3) was observed. The electrochemical behavior of the ZnMOF/ETH500/nafion modified GCE revealed that MOFs have a promising future as electrode materials for direct electrochemical sensing.

In direct methanol fuel cell technology, highly stable electrochemical catalysts are critically important for their practical utilization at the commercial scale. In this study, sub ~10 nm hollow Pt-Ni (1:1 at. ratio) nanoboxes supported on functionalized Vulcan carbon (Pt-Ni/C-R2) were synthesized through a facile method for the efficient electrooxidation of methanol. Two reaction procedures, namely, a simultaneous reduction and a modified sequential reduction method using a reverse microemulsion (RME) method, were adopted to synthesize solid Pt-Ni NPs and hollow nanoboxes, respectively. To correlate the alloy composition and surface structure with the enhanced catalytic activity, the results were compared with the nanocatalyst synthesized using a conventional NaBH4 reduction method. The calculated electroactive surface area for the Pt-Ni/C-R2 nanoboxes was 190.8 m2.g−1, which is significantly higher compared to that of the Pt-Ni nanocatalyst (96.4 m2.g−1) synthesized by a conventional reduction method. Hollow nanoboxes showed 34% and 44% increases in mass activity and rate of methanol oxidation reaction, respectively, compared to solid NPs. These results support the nanoreactor confinement effect of the hollow nanoboxes. The experimental results were supported by Density Functional Theory (DFT) studies, which revealed that the lowest CO poisoning of the Pt1Ni1 catalyst among all Ptm-Nin mixing ratios may account for the enhanced methanol oxidation. The synthesized hollow Pt-Ni/C (R2) nanoboxes may prove to be a valuable and highly efficient catalysts for the electrochemical oxidation of methanol due to their low cost, numerous catalytically active sites, low carbon monoxide poisoning, large electroactive surface area and long-term stability.