Hydrogen (H₂) production is a latent feasibility of renewable clean energy. The industrial H₂ production is obtained from reforming of natural gas, which consumes a large amount of nonrenewable energy and simultaneously produces greenhouse gas carbon dioxide. Electrochemical water splitting is a promising approach for the H₂ production, which is sustainable and pollution-free. Therefore, developing efficient and economic technologies for electrochemical water splitting has been an important goal for researchers around the world. The utilization of green energy systems to reduce overall energy consumption is more important for H₂ production. Harvesting and converting energy from the environment by different green energy systems for water splitting can efficiently decrease the external power consumption. A variety of green energy systems for efficient producing H₂, such as two-electrode electrolysis of water, water splitting driven by photoelectrode devices, solar cells, thermoelectric devices, triboelectric nanogenerator, pyroelectric device or electrochemical water–gas shift device, have been developed recently. In this review, some notable progress made in the different green energy cells for water splitting is discussed in detail. We hoped this review can guide people to pay more attention to the development of green energy system to generate pollution-free H₂ energy, which will realize the whole process of H₂ production with low cost, pollution-free and energy sustainability conversion.

Biomass-derived nitrogen self-doped porous carbon was synthesized by a facile procedure based on simple pyrolysis of water hyacinth (eichhornia crassipes) at controlled temperatures (600–800 °C) with ZnCl₂ as an activation reagent. The obtained porous carbon exhibited a BET surface area up to 950.6 m² g⁻¹, and various forms of nitrogen (pyridinic, pyrrolic and graphitic) were found to be incorporated into the carbon molecular skeleton. Electrochemical measurements showed that the nitrogen self-doped carbons possessed a high electrocatalytic activity for ORR in alkaline media that was highly comparable to that of commercial 20% Pt/C catalysts. Experimentally, the best performance was identified with the sample prepared at 700 °C, with the onset potential at ca. +0.98 V vs. RHE, that possessed the highest concentrations of pyridinic and graphitic nitrogens among the series. Moreover, the porous carbon catalysts showed excellent long-term stability and much enhanced methanol tolerance, as compared to commercial Pt/C. The performance was also markedly better than or at least comparable to the leading results in the literature based on biomass-derived carbon catalysts for ORR. The results suggested a promising route based on economical and sustainable biomass towards the development and engineering of value-added carbon materials as effective metal-free cathode catalysts for alkaline fuel cells.

Advanced materials for electrocatalytic water splitting are central to renewable energy research. In this study, MoO₂ nanobelts@nitrogen self-doped MoS₂ nanosheets are produced by nitridation and sulfuration treatments of MoO₃ nanobelts. The material structures are characterized by a variety of techniques including scanning electron microscopy, transmission electron microscopy, Raman scattering, X-ray photoelectron spectroscopy, and X-ray diffraction spectroscopy. It is found that because of nitrogen doping and the abundance of exposed active edges, the heterostructures exhibit high electronic conductivity, and more importantly, enhanced and stable electrocatalytic activity in hydrogen evolution reaction (HER), as manifested in electrochemical studies. The onset potential is found to be only −156 mV (vs. RHE), which is 105 mV more positive than that of pure MoS₂ under identical experimental conditions. The corresponding Tafel slope is estimated to be 47.5 mV dec⁻¹, even slightly less than that of commercial 10 wt% Pt/C (49.8 mV dec⁻¹), suggesting that the reaction dynamics is largely determined by the electrochemical desorption of hydrogen. This is accounted for by nitrogen doping that leads to an enhanced electronic conductivity of the heterostructures as well as a high density of spinning electron states around the N and Mo atoms in MoS₂ nanosheets that are the active sites for HER, as manifested in density functional theory studies of a N-doped MoS₂ monolayer.