Solar Photocatalytic Hydrogen Production: Current Status and Future Challenges

Abstract

Due to the increase of the worldwide demand for energy along with the global warming and the increasing level of atmospheric CO 2 , solar hydrogen has been proposed as an optimal fuel as it can be produced from water using solar energy which emerges as the most promising energy source in terms of abundance and sustainability. So far, the main commercial process for producing molecular hydrogen is steam reforming of hydrocarbons which is, however, connected with a CO 2 emission disadvantage. Carbon-free hydrogen production can be achieved by water splitting employing an electrolyzer powered by photovoltaics, but a potentially more cost-effective route is to perform direct photocatalytic water splitting using semiconductor photocatalysts. Herein, the authors present the principles of this process, the maximum solar-to-hydrogen conversion efficiency, the most active photocatalysts reported so far and the challenges for the development of the optimum photocatalyst for the efficient release of hydrogen from water. Since the efficiency of the overall photocatalytic water splitting is still very low and the photocatalytic reforming of biomass compounds can be considered as an intermediate step between the current fossil fuel consumption and the dream for an efficient direct photocatalytic water splitting utilizing solar energy, the photocatalytic hydrogen production employing different so-called sacrificial reagents, i.e., electron donors, is also presented and discussed herein. Commonly, the system employing TiO 2 as the photocatalyst and methanol as the sacrificial reagent is investigated, thus, details concerning the possibility of enhancing the photocatalytic activity of this system as well as the current knowledge of the underlying mechanism are presented at the end of this chapter.

Photocatalytic H2 Evolution from Oxalic Acid: Effect of Cocatalysts and Carbon Dioxide Radical Anion on the Surface Charge Transfer Mechanisms

“…We believe, hence, that noble metals act as a sink for electrons, with platinum being more efficient than gold (work functions of 5.65 eV for Pt and 5.10 eV for Au) . When such a Schottky barrier is created, the photocatalytic activity is enhanced due to a lower probability for the recombination of photogenerated charge carriers …”

TiO2 Photocatalysis for the Transformation of Aromatic Water Pollutants into Fuels

“…Consequently, a volcano-type dependence between HER rates and metal-H • ads bond strength has been proposed [209], in which platinum provides the best activity to drive the HER as shown in Figure 12. In conclusion, Pt/TiO2 has been demonstrated to exhibit the highest photocatalytic activity towards H2 production compared to other metalloaded TiO2 [105,155,210], such as Au/TiO2. This has been explained by the highest work function of Pt that enhances electrons "sinking" properties, the lowest overpotential for H2 formation, and the optimal binding energy adsorbing atomic hydrogen.…”

“…In the photocatalytic reforming process, the photogenerated holes in the valence band can oxidize adsorbed organic substrates (electron donors or sacrificial reagents), whereas the photogenerated electrons in the conduction band can reduce the protons (electron acceptor) to molecular hydrogen As a hybrid field, dual-functional photocatalysis is a combination of different photocatalytic fields for 2-fold purposes achieved in a single step [130]. The coupling of H 2 evolution and photocatalytic degradation of organic pollutants yielding CO 2 can be achieved in the so-called photoreforming process [38,132,[153][154][155][156]. Such a technique has a great advantage as it can benefit from solar light to treat wastewater, meanwhile, the evolved CO 2 can be consumed by natural photosynthesis [157].…”

“…HER on metallic platinum, as an example, induce via the Volmer reaction, in which H • ads atoms are produced when the accumulated electrons in Pt are transferred to the proton adsorbed H + ads and H 2 O ads , respectively, as described in Equations (7a) and (7b) [196]. The reaction proceeds afterward through two possible pathways, either the Heyrovsky reaction (Equation (7c)) or the Tafel reaction (Equation (7d)), in which H • ads react with H + ads or/and the direct recombination of two H • ads with each other, respectively [155]. Figure 10 illustrates the two-electron transfer reaction that occurs on the metal surface in acidic solutions.…”

Unravelling the Mechanisms that Drive the Performance of Photocatalytic Hydrogen Production

“…One of the alternatives to improve the performance of the photocatalytic process consists of using other compounds as sacrificial molecules to act as electron donors, providing electrons for proton reduction. The most common substrates for photocatalytic hydrogen production are methanol, ethanol, triethanolamine (TEOA), and Na 2 S/Na 2 SO 3 [1][2][3][4][5][6]. The photocatalytic reaction involves three sequential processes: (i) absorption of photons with energy equal to or greater than the bandgap of the semiconductor, exciting electrons from the valence band (VB) to the conduction band (CB) and creating electron (e − )/hole (h + ) pairs; (ii) separation of carriers and migration to active sites; and (iii) initiation of redox reactions [7][8][9].…”