Source Sciencedirect.com Karim Kakaei, … Ali Ehsani, in Interface Science and Technology, 2019
https://www.sciencedirect.com/topics/engineering/hydrogen-h2
7.3.1 Introduction
Hydrogen (H2) is regarded as the future energy carrier to replace the finite fossil fuels, so that all the issues, including environmental emissions, sustainability, and security, are addressed. Jules Verne was the first person who discovered the hydrogen economy in 1874, and after that, hydrogen attracted interest as a novel source of transportation fuel and energy storage medium [123]. Because H2 is the molecule possessing the highest energy density per unit mass, and when it is combusted in an engine or transformed into electricity in a fuel cell, it produces water as the only by-product, its utilization as an energy carrier has become vastly desirable. In contrast, carbon-based fuels produce CO2 as well as water [124]. Hydrogen is usually produced from natural gas, coal gasification, biomass, or water. However, not only the finite resources but also the greenhouse gas emissions of coal gasification have made this strategy inappropriate despite it producing large amounts of H2 [125]. In recent years, although natural gas has been extensively utilized to produce H2, it is still problematic because of the ever-increasing environmental emissions and depletion rate of fossil fuels [10]. Conversely, biomass is not capable of supplying sufficient amounts of H2 despite being a clean and sustainable strategy [126]. Since water is an approximately inexhaustible resource on the earth, it is considered the only sustainable and clean resource among all the aforementioned ones to produce hydrogen in the future (forward direction in Eq. 7.4) [10,124].
(7.4)H2O+energy⇄H2+0.5O2
In this equation, the forward reaction represents splitting of water into hydrogen and oxygen using energy from renewable sources and the reverse direction the production of energy on demand by the combination of H2 and O2.
The HER, involving proton reduction and attendant hydrogen evolution, as depicted in Fig. 7.22, not only is accounted a crucial process in electrocatalysis but also plays a focal part in energy conversion using water splitting. Hence, it has become the center of attention in the production of hydrogen [18,51,127]. Nonetheless, there are several major problems in the use of the HER to produce hydrogen. Not only does the direct thermal splitting of water need significantly high temperatures (≈2000°C), but also H2 and O2 generated during the procedure recombine rapidly [10]. Therefore, catalysts utilized as HER electrocatalysts may exhibit a poor stability [51]. Under small overpotentials (overpotential is considered the difference between the applied and the thermodynamic potentials of a given electrochemical reaction), H2 can be separately produced using the cathodic HER in electrocatalytic water splitting, promoted via efficient electrocatalysts, under ambient conditions [10,124]. Therefore, it is indispensable to make use of efficient electrocatalysts for HER so that not only is the overpotential reduced, but also HER is carried out with high energy efficiency [18,128]. Utilization of nanostructures instead of microstructures and the addition of carbon materials, e.g., graphene, are considered effective and useful solutions to improve the catalyst’s performance [51]. Nevertheless, the investigation of novel HER electrocatalysts possessing high activity and low cost has attracted a great deal of attention because of their crucial role in clean energy generation [18,129].
Figure 7.22. Schematic representation of the hydrogen evolution reaction [130].
Reproduced with permission from Y. Yan, et al., Nano-tungsten carbide decorated graphene as co-catalysts for enhanced hydrogen evolution on molybdenum disulfide, Chem. Commun. 49 (43) (2013)4884–4886. Copyright 2013, Royal Society of Chemistry Group. The combination of graphene with traditional NP catalysts has been demonstrated to possess a great potential in the improvement of catalyst performance for HER. This combination is capable of preventing aggregation and deactivation during the reaction, still major issues lessening the stability and activity of the traditional NP catalysts. Hence, numerous graphene- based catalysts have been developed for water splitting so that the energy conversion efficiency is enhanced from electricity to hydrogen molecules [51].