# Electrospun Nanomaterials for Energy Applications: Recent Advances
## 8 Hydrogen Production by Photo-Assisted Electro-Chemical Water Splitting  
The need for limiting the effects of using fossil fuels on climate changes and the increasing energy demand associated with the growth of the world population and the enhancement of living standards call for developing clean energy sources. Hydrogen is widely recognized to be a carbon-free, environmentally friendly, renewable, and efficient fuel with high energy density. It has a calorific value nearly three times greater than hydrocarbon fuels. Currently, the production of H\({}_{2}\) mainly comes from steam reforming of natural gas [213], consuming natural resources and generating carbon dioxide as an undesired by-product. Solar energy conversion for H\({}_{2}\) production via photo-induced water splitting (WS) represents one of the most promising alternative approaches, but presently still contributes to a very small extent (<4%) [213]. The process is schematically depicted in Figure 28. Essentially, photo-electrochemical (PEC) cells act as \"artificial photosynthetic devices\" that mimic the process of hydrogen production through the light-assisted water splitting occurring in nature [214]. In the cell (Figure 28a), the anode (based on a photoactive \(n\)-type semiconductor) is the oxygen-evolving electrode, whereas hydrogen is generated at the cathode, which acts as the counter-electrode [216]. Figure 27: (**a**) Schematic illustration for the preparation of nitrogen-doped carbon fibers (N-CFs) and graphene-enriched nitrogen-doped carbon fibers (G/N-CFs). (**b**) Electro resorption capacity of the N-CFs and G/N-CFs electrodes at different NaCl concentrations. (**c,d**) Morphology, as resulting from SEM analysis, and (**e**) elemental composition of the G/N-CFs. SEM images refer to (c) N-CFs and (d) G/N-CFs. Insets: fiber diameter distributions (reproduced from reference [55] with permission. Copyright 2017, Elsevier). When a light source illuminates the \(n\)-type semiconductor acting as photo-anode, the electrons (\(e^{-}\)) in the valence band (VB) are excited to the conduction band (CB), and holes (\(h^{+}\)) are generated in the VB (Figure 28b). The photo-generated \(e^{-}\) and \(h^{+}\) that do not undergo recombination act as the reducing and oxidizing agents, respectively. Electrons participate in the reduction reaction and produce H\({}_{2}\), whereas holes carry out oxidation reaction and produce O\({}_{2}\)[213]. To achieve an _overall_ WS in hydrogen and oxygen, both the reduction and oxidation potentials of water should lie within the band gap (\(E_{g}\)) of the photo-catalyst. The bottom of the CB must be more negative than the reduction potential of H\({}^{+}\)/H\({}_{2}\) (0 V vs. normal hydrogen electrode, NHE), while the top of the VB must be more positive than the oxidation potential of O\({}_{2}\)/H\({}_{2}\)O (1.23 V vs. NHE) [216, 217, 218]. Figure 29 displays the energy gap (in eV) and band edge positions of some representative semiconductor materials relative to water redox potentials. Only some of them are suitable for overall WS in hydrogen and oxygen. Moreover, since the intensity of the solar spectrum dramatically falls off below 400 nm, to absorb the visible-light irradiation from sunlight, \(E_{g}\) should be lower than 3.1 eV [216]. Figure 28: Schematic illustration of (**a**) a cell for photo-assisted water splitting employing an \(n\)-type semiconductor as photo-anode (inspired by references [214,215]), and (**b**) photo-generation process of electrons and holes (top), acting as the reducing and oxidizing agents (bottom), respectively. Figure 29: Energy gap (in eV) and band edge positions of some semiconductors relative to normal hydrogen electrode (NHE) (inspired by references [218,219]). The reduction potential of H\({}^{+}\)/H\({}_{2}\) and the oxidation potential of O\({}_{2}\)/H\({}_{2}\)O are also reported. Titanium dioxide (TiO\({}_{2}\)) has been the pioneering semiconductor for the photo-catalysis of water [220]. It is environmentally friendly, easily available, cheap, and stable.