# Electrospun Nanomaterials for Energy Applications: Recent Advances
## 8 Hydrogen Production by Photo-Assisted Electro-Chemical Water Splitting
Nevertheless, absorbing in the near UV region, it can use only an exiguous fraction (5%) of the solar light. In principle, the narrower band gap (2.1-2.3 eV) of hematite enables the absorption of about 40% of the solar spectrum. Thanks to its favorable properties (VB edge lower than the water oxidation potential, chemical stability in a oxidative environment, low cost, and non-toxicity) \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) might represent an attractive alternative to TiO\({}_{2}\) for the application in PEC WS on a large scale [214]. However, poor conductivity (of the order of 10\({}^{-6}\) S cm\({}^{-1}\) for high purity hematite), short hole diffusion length (typically 2-4 nm) and fast electron-hole recombination (10\({}^{-12}\) s) at the grain boundaries mainly limit the PEC activity of polycrystalline hematite films. The design of efficient and stable photo-catalysts for the harvesting of solar energy has become one of the primary challenges in the development of solar H\({}_{2}\) economy [221; 222]. Many efforts have been made to increase the optical absorption of large-\(E_{\mathrm{g}}\) photo-catalysts by varying their chemical composition with metal or non-metal doping or self-impurity [223; 224; 225]. Also, nano-structuring has allowed obtaining even 1000 h stable photo-anodes [226]. 1D-nanostructures, such as nanorods, nanowires, or nanofibers, offer advantages over their bulk counterpart [227]. Besides, minimizing the length scale through which the minority carriers must diffuse and, hence, reducing the probability of recombination losses, 1D-nanostructures may contribute to enhance the photo-conversion efficiency thanks to their higher surface area. In this scenery, the electrospun nanofibres, featured by high porosity, aspect ratio, and surface area, represent very promising candidates for the efficient production of H\({}_{2}\) by PEC WS. Table S7 reports some examples of electrospun photo-catalysts and summarizes their preparation conditions. Figure 30 shows the morphology of some of them. Under UV light irradiation, by the use of electrospun TiO\({}_{2}\) nanofibers, hydrogen evolution increases by a factor of \(\sim\)2.8 with respect to TiO\({}_{2}\) prepared by a hydrothermal method, and PEC activity is higher than for reference commercial TiO\({}_{2}\) nanoparticles [228]. [PERSON] et al. have reported an H\({}_{2}\) evolution rate of 62.7 umol/h and up to 200 umol/h by using TiO\({}_{2}\)/CuO [229] and TiO\({}_{2}\)/SnO\({}_{2}\) (Figure 30a) [230] composite fibers, respectively. After optimization of the preparation conditions, photo-current densities ranging between 150-200 uA/cm\({}^{2}\) and 0.6-1.4 mA/cm\({}^{2}\) have been measured for pure and N-doped electrospun TiO\({}_{2}\) anodes, respectively [231; 232]. Dispersions of the sintered TiO\({}_{2}\) fibers produced via ES have been employed to obtain photo-anodes for WS in the form of homogenous films on FTO/glass substrates by the use of doctor-blade, sol-gel dip coating, or by combining the two techniques. In this case, the highest photocurrent density obtained has been 0.5 mA/cm\({}^{2}\)[233]. [PERSON] et al. [234] reported an improvement in the generation of hydrogen gas through the hydrolysis of ammonia borane complex by the use of carbon fibers decorated with Co-TiC nanoparticles-like superficial protrusions (Figure 30(b) prepared through a one-step electrospinning approach. Interesting results have also been obtained by combining electrospinning with different synthesis techniques [235; 236]. For instance, under optimized conditions, C-coated TiO\({}_{2}\)/WO\({}_{3}\) fibers prepared by electrospinning combined with hydrothermal method have shown remarkable light absorption in the visible region and enhanced H\({}_{2}\)-generation rate thanks to the multichannel-improved charge-carrier photosynthetic hetero-junction system with the C layer on the surface of TiO\({}_{2}\) as an electron collector and WO\({}_{3}\) as a hole collector, leading to effective charge separation on these components [236]. [PERSON] et al.