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Texturing, i.e., preferentially oriented deposition of a film with a specific crystallographic direction, enables the manipulation of the charge transport properties and surface reactivity of photoelectrodes for solar water-splitting. The advancement of solar water-splitting systems under neutral conditions is a vital strategy to reduce the economic and ecological traits of the prevailing strong acid or alkaline-based solar water-splitting systems. However, the photoelectrodes have to endure cumulative barriers in neutral media to convalesce the performance of the neutral solar water-splitting system. The implication of texturing in materials enforces the synergistic effect that is essential to confine the barriers to improve the performance of the photoelectrodes in eco-friendly neutral pH conditions. Here, we synthesized tungsten trioxide (WO3) films to achieve a columnar-type nanostructure with (0 2 0) texture, through a laser ablation deposition. Specifically, we modulated both deposition temperature and working pressure, enabling the (0 2 0) textured deposition of films, as well as the fine-tuning of the surface morphology. With optimized fabrication conditions, the (0 2 0)-textured WO3 film (thickness: 3.6 µm) showed improved photoelectrochemical water-oxidation performance, and the photocurrent density was ~3 mA/cm2 at 1.23 V versus reversible hydrogen electrode in an economic and ecological neutral condition. The WO3 films were further characterized using various methods, namely a UV–Vis spectroscopy, X-ray photoelectron spectroscopy (XPS), and Hall Effect measurements. Based on the measured film characteristics, we attributed enhanced charge transport and transfer characteristics to the (0 2 0)-texturing, and the formation of the optimal amount of oxygen vacancies.

Carbon-based materials are considered to be promising cadidates for energy storage systems because of their superior conductivity, low price, and high specific surface area. Moreover, heteroatom-doped graphene or graphitic carbons are prepared by various methods for modulating their physicochemical properties, improving their wettabililty, and achieving favorable pseudocapacity for use in electrochemical supercapacitors. Here we report a simple solvent-free scale-up technique to synthesis fluorine-doped graphene oxide (FGO) by direct plasma treatment of graphene oxide (GO) powder at ambient pressure. The FGO enabled fast electrochemical charge transfer and provided a large number of active sites for redox reactions during supercapacitor operation, and the mechanisms were thoroughly studied by various electrochemical analyses. As a result, fabricated symmetric supercapacitor using FGO electrodes exhibited a maximum power density (~3200 W/kg) and energy density (~25.87 Wh/kg) with superior cycle stability (20000 cycles) without capacitance loss. Furthermore, the computational calculation results clarified the roles of semi-ionic C–F bonding of FGO: huge charge accumulation on the electrodes and superior electrical conductivity. Thus, our study demonstrates a facile strategy to develop promising functionalized materials, which can enhance the viability of supercapacitor for the next generation of energy storage systems.

Charge storage in supercapacitors is strongly related to the bond characteristics and electronic structure of electrode materials. Graphene-based materials are widely used in a supercapacitor due to the easily tunable properties and high surface/volume ratios. However, we claim that the typical covalent bond characteristics of 2D carbon materials originating from this 2pπ orbital is not very suitable to the application in supercapacitor. Here, we suggest an efficient way to improve the supercapacitor performance by tuning the covalency of bonding between the graphene-based electrode and potassium ion. We, for the first time, also introduce a simple solvent-free scale-up doping technique to prepare fluorine-doped graphene oxide (FGO) by direct plasma treatment on graphene oxide (GO) powder at ambient pressure. The FGO enabled fast electrochemical charge transfer and provided a large number of active sites for redox reactions during supercapacitor operation, and those mechanisms were thoroughly studied by various electrochemical analyses. As a result, the fabricated symmetric supercapacitor using FGO electrodes exhibited a maximum power density (~3200 W/kg) and energy density (~25.87 Wh/kg) with superior cycle stability (20000 cycles) without capacitance loss. Furthermore, the computational calculation results clarified the roles of semi-ionic C–F bonding of FGO: huge charge accumulation on the electrodes and superior electrical conductivity. Thus, our study demonstrates a facile strategy to develop promising functionalized materials, which can enhance the viability of supercapacitor for the next generation of energy storage systems.

The vacuum deposition method requires high energy and temperature. Hydrophobic reduced graphene oxide (rGO) can be obtained by plasma-enhanced chemical vapor deposition under atmospheric pressure, which shows the hydrophobic surface property. Further, to compare the effect of hydrophobic and the hydrophilic nature of catalysts in the photoelectrochemical cell (PEC), the prepared rGO was additionally treated with plasma that attaches oxygen functional groups effectively to obtain hydrophilic graphene oxide (GO). The hydrogen evolution reaction (HER) electrocatalytic activity of the hydrophobic rGO and hydrophilic GO deposited on the p-type Si wafer was analyzed. Herein, we have proposed a facile way to directly deposit the surface property engineered GO.

Because of a high global energy demand and challenging sustainability requirements, hydrogen has been promoted as a clean and green energy carrier allowing the potential replacement of nonrenewable fossil fuels. Hydrogen is a renewable energy carrier that does not contribute to CO2 emissions, and it holds the highest gravimetric energy density. It can be produced by the electrochemical splitting of water into hydrogen and oxygen molecules. Herein, we investigate the bifunctional activity of bimetallic oxide, sulfide, and selenide nanostructures in water electrolysis. Spinel-type, phase-pure NiCo2O4, NiCo2S4, and NiCo2Se4 with comparable morphological properties were synthesized by a hydrothermal method. The electrocatalytic characterization of NiCo2Se4 was found to demonstrate higher oxygen and hydrogen evolution reaction activities (245 mV and 122 mV @ 10 mA cm−2, respectively) compared to those of NiCo2O4 and NiCo2S4. Furthermore, a lab-scale water-splitting system was fabricated to examine the bifunctional properties of bimetallic nanostructures. A NiCo2Se4-based water-splitting system was found to require a cell voltage of 1.58 V, which is lower than that required by NiCo2S4 (1.64 V)- and NiCo2O4 (1.75 V)-based systems. In summary, this study explores bimetallic NiCo2Se4 as an efficient electroactive material that can be employed in various electrochemical energy systems.

The unique physical and chemical properties of spinels have made them highly suitable electrocatalysts in oxygen evolution reaction and oxygen reduction reaction (OER & ORR). Zinc–air batteries (ZABs), which are safer and more cost-effective power sources than commercial lithium-ion batteries, hinge on ORR and OER. The slow kinetics of the air electrode reduce its high theoretical energy density and specific capacity, which limits its practical applications. Thus, tuning the performance of the electrocatalyst and cathode architecture is vital for improving the performance of ZABs, which calls for exploring spinel, a material that delivers improved performance. However, the structure–activity relationship of spinel is still unclear because there is a lack of extensive information about it. This study was performed to address the promising potential of spinel as the bifunctional electrocatalyst in ZABs based on an in-depth understanding of spinel structure and active sites at the atomic level.

Photoelectrochemical water splitting is a promising field for sustainable energy production using hydrogen. Development of efficient catalysts is essential for resourceful hydrogen production. The most efficient catalysts reported to date have been extremely precious rare-earth metals. One of the biggest hurdles in this research area is the difficulty of developing highly efficient catalysts comparable to the noble metal catalysts. Here, we report that non-noble metal dichalcogenide (Co3Se4) catalysts made using a facile one-pot electrodeposition method, showed highly efficient photoelectrochemical activity on a Si photocathode. To enhance light collection and enlarge its surface area even further, we implemented surface nanostructuring on the Si surface. The nanostructured Si photoelectrode has an effective area greater than that of planar silicon and a wider absorption spectrum. Consequently, this approach exhibits reduced overvoltage as well as increased photo-catalytic activity. Such results show the importance of controlling the optimized interface between the surface structure of the photoelectrode and the electrodeposited co-catalyst on it to improve catalytic activity. This should enable other electrochemical reactions in a variety of energy conversion systems.

The silicon-based photoelectrochemical (PEC) conversion system has recently gained attention owing to its ability to provide cost-efficient and superior photoresponsive behavior with respect to other semiconductor photoelectrodes. Carbon-based co-catalysts have always been the focus of research as alternative metal-free electrocatalysts intended for hydrogen evolution reaction (HER). In particular, reduced graphene oxide (rGO), a representative carbon-derived material, has attracted much attention as a non-metal catalyst for efficient and durable HER. Herein, we deposited rGO on a silicon nanowire (SiNW) structure, which showed the highest reduction in the overpotential for HER up to date. This can be attributed to the synergistic effects of rGO and SiNW with unique anisotropic morphology, facile tuning capabilities, and scalable fabrication methods. Combined with nanostructured photocathode, rGO-deposited SiNW showed better photon to current conversion efficiency of 3.16% (half solar-to-hydrogen conversion efficiency), which is 158 times higher than that of the bare planar Si system. In light of this development, we believe that rGO-SiNW photoelectrodes will pave the way for state-of-the-art highly efficient non-metal catalysts for energy conversion technologies.