Photocatalytic Water Splitting: How Far Away Are We from Being Able to Industrially Produce Solar Hydrogen?

Abstract

Solar water splitting (SWS) has been researched for about five decades, but despite successes there has not been a big breakthrough advancement. While the three fundamental steps, light absorption, charge carrier separation and diffusion, and charge utilization at redox sites are given a great deal of attention either separately or simultaneously, practical considerations that can help to increase efficiency are rarely discussed or put into practice. Nevertheless, it is possible to increase the generation of solar hydrogen by making a few little but important adjustments. In this review, we talk about various methods for photocatalytic water splitting that have been documented in the literature and importance of the thin film approach to move closer to the large-scale photocatalytic hydrogen production. For instance, when comparing the film form of the identical catalyst to the particulate form, it was found that the solar hydrogen production increased by up to two orders of magnitude. The major topic of this review with thin-film forms is, discussion on several methods of increased hydrogen generation under direct solar and one-sun circumstances. The advantages and disadvantages of thin film and particle technologies are extensively discussed. In the current assessment, potential approaches and scalable success factors are also covered. As demonstrated by a film-based approach, the local charge utilization at a zero applied potential is an appealing characteristic for SWS. Furthermore, we compare the PEC-WS and SWS for solar hydrogen generation and discuss how far we are from producing solar hydrogen on an industrial scale. We believe that the currently employed variety of attempts may be condensed to fewer strategies such as film-based evaluation, which will create a path to address the SWS issue and achieve sustainable solar hydrogen generation.

Keywords: hydrogen production; large scale evolution; photocatalytic water splitting; solar energy; thin films.

Figure 1

A digital image of a quartz reactor with a capacity of 70 mL (56 mm in diameter at the center) filled with 40 mL of solution and 25 mg of titania-based catalyst powder while it was spinning (left), and in static circumstances, using thin films produced with 1 mg of the same photocatalyst (right). Replicated from ref. [56].

Figure 2

Dispersion of light in nanostructured and flat films as well as in suspensions of particles. A α⁻¹ is the optical penetration depth, and d is the film or particle thickness. Short arrows denote light that has been reflected or dispersed. Replicated from ref. [13].

Figure 3

Images of panels and thin sheets in various sizes: (I) picture of the large-scale hydrogen generation system for photocatalysis illuminated by the sunshine. Nine stainless steel plates, each measuring 28 × 30 × 0.1 cm³, make up a photocatalyst (Pt@mp–CN) panel. The mesoporous carbon nitride photocatalyst emitting hydrogen bubbles can be seen when magnified. (II) Nine 33 × 33 cm² sheets make up a 1 × 1 m² SrTiO₃:Al photocatalyst panel. (III) Sizes of thin films (a) 1.25 × 3.75 of P25 (b) 1.25 × 3.85, (c) 2.5 × 3.75, and (d) 2.5 × 7.5 cm² of Pd/P25. Replicated from ref. [36].

Figure 4

An illustration showing how photocatalyst particles are transferred across glass substrates to create photocatalyst sheets: (a) preparation for powder suspension, (b) preparation of HEP/OEP plates on carbon tape and without a metal layer, and (c) preparation of HEP/M/OEP plates with sandwiched metal layers on carbon tape. Replicated from ref. [41].

Figure 5

(a) Schematic for screen printing method, and (b) photo image of a 10 × 10 cm² screen printed device. Replicated from ref. [82,29].

Figure 6

Photos taken while being illuminated via hydrophilic and hydrophobic windows. Replicated from ref. [36].

Figure 7

Bulk heterojunctions are demonstrated throughout the thin film employing (a) FESEM, and EDX chemical mapping analysis of (b) Ti, (c) Au, (d) Cd, (e) S, and (f) Pb. EDX result shows the uniform distribution of chalcogenides. Replicated from ref. [60].

Figure 8

Photoluminescence spectra of TiO₂, TiO₂/CdS, and TiO₂/Mn–CdS obtained with a 420 nm excitation source; emission characteristics are normalized. Additionally, photoluminescence is compared to the UV-visible absorption spectra that were obtained for all three materials (shown as a dashed line in the same color). In the picture, a solid triangle area emphasizing the secondary light source accessible for chalcogenide absorption highlights the wavelength range where the absorption and emission spectra coincide. The photoluminescence spectra of TiO₂, TiO₂/CdS, and TiO₂/Mn-CdS, which were obtained at a wavelength of 370 nm, are displayed in the inset. Replicated from ref. [93].