Recent Progress and Approaches on Carbon-Free Energy from Water Splitting

Abstract

Sunlight is the most abundant renewable energy resource, providing the earth with enough power that is capable of taking care of all of humanity's desires-a hundred times over. However, as it is at times diffuse and intermittent, it raises issues concerning how best to reap this energy and store it for times when the Sun is not shining. With increasing population in the world and modern economic development, there will be an additional increase in energy demand. Devices that use daylight to separate water into individual chemical elements may well be the answer to this issue, as water splitting produces an ideal fuel. If such devices that generate fuel were to become widely adopted, they must be low in cost, both for supplying and operation. Therefore, it is essential to research for cheap technologies for water ripping. This review summarizes the progress made toward such development, the open challenges existing, and the approaches undertaken to generate carbon-free energy through water splitting.

Keywords: Hydrogen generation; Nanostructure; Photocatalysis; Renewable energy sources; Water splitting.

Fig. 1

Cost comparison and evaluation of solar–H₂ generators that integrate different catalytic components. Reproduced from Ref. [26] with permission from the Centre National de la Recherche Scientifique (CNRS) and The Royal Society of Chemistry

Fig. 2

Device proposal/design for solar cells (a, b) and analogous designs for photoelectrochemical cells (c, d). Reprinted from Ref. [42] with permission from the Centre National de la Recherche Scientifique (CNRS) and The Royal Society of Chemistry

Fig. 3

a Extinction efficiencies for Ag cubes and Au spheres as functions of particle size. b UV–Vis extinction spectra of samples. c H₂ and O₂ production upon visible illumination of photocatalysts. d Photocurrent responses upon illumination with a broadband visible light source. Reprinted with permission from Ref. [47], Copyright © 2011, American Chemical Society

Fig. 4

a Schematic represent of the MoS₂/Mo₂C; reprinted with permission from Ref. [50], Copyright © 2019, Springer Nature. b Current densities of photoanodes in the dark (broken lines) and under simulated solar are shown as a function of the applied potential, V, with respect to the RHE. An atmospheric pressure chemical vapor deposition sample covered with one ALD cycle of Al₂O₃ has been measured after deposition (red squares), after annealing for 20 min at 300 °C (green triangles) and after annealing for 20 min at 400 °C (blue diamonds), sample before ALD (black circles). Reprinted from Ref. [53] with permission from The Royal Society of Chemistry, c J–V curves of Ta₃N₅ thin film photoanode in the electrolyte with and without H₂O₂ sacrificial agent. Reprinted from Ref. [54] with permission from the Centre National de la Recherche Scientifique (CNRS) and The Royal Society of Chemistry

Fig. 5

Defect engineering using re-growth scheme of hematite. Reprinted with permission from Ref. [69], Copyright © 2015, Springer Nature

Fig. 6

Active water splitting catalyst pair with minimal biological toxicity. a Natural photosynthesis pathway and b reaction diagram and scanning electron microscopy images for Co–P alloy cathode and CoPi anode. Reprinted with permission from Ref. [9], Copyright © 2012, Springer Nature and Ref. [75], Copyright © 2016, American Association for the Advancement of Science

Fig. 7

Schematic representation of energy diagram of a photoelectrochemical cell for the photoelectrolysis of water with various losses. Reprinted with permission from Ref. [86], Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Fig. 8

Schematic representation of the two-step solar thermochemical splitting of H₂O using nonstoichiometric metal oxide redox pairs and concentrated solar energy [100]

Fig. 9

a Energy levels of the 4f one orbital of Ce³⁺. Ce³⁺ splits initially by spin orbit coupling and subsequently by cubic crystal field of the fluorite structure. b Calculated $S_{\text{elec}}^{\text{onsite}}$ for lanthanides ions. Reprinted with permission from Ref. [103], Copyright © 2017, Springer Nature

Fig. 10

a Schematic diagram of the combined device of gradient-doped W/BiVO₄ and a-Si solar cell. b The corresponding band diagram of the hybrid photoelectrode device. ITO tin-doped indium oxide, TCO transparent conducting oxide. c Carrier-separation efficiency (${\eta }_{\text{sep}}$) as a function of applied potential for 1% W-doped BiVO₄ (black triangle), W/BiVO₄ homojunction (red inverted triangle), W/BiVO₄ reverse homojunction (green circle), and gradient-doped W:BiVO₄ (blue square). Reprinted with permission from Ref. [114], Copyright © 2013, Springer Nature

Fig. 11

Schematic illustration of the optical absorption mechanism and electron transport of nanoporous BiVO₄ on the flat substrate and the conductive nanocone substrate [21]

Fig. 12

Classical structure analysis by SEM, BF-TEM, and identification of champion nanostructures. a Cross-sectional SEM images show electrodes from a 45 viewing angle. b The areal densities of nanoparticle aggregates. c A DF-TEM analysis allows each region of a nanoparticle aggregate with a unique crystallographic orientation to be imaged separately. d Analysis by C-AFM examines the charge transport properties of individual nanostructures. Each color represents a different crystal orientation. Reprinted with permission from Ref. [125], Copyright © 2012, American Chemical Society

Fig. 13

a A simplified scheme of the light-driven reactions of photosynthesis. b Construction of an artificial leaf [127]

Fig. 14

Design of an artificial leaf prepared of a single semiconductor having huge band gap (a), a dual photoelectrodes that are assembled with a proton exchange membrane in the back-to-back configuration (b), a dual photoelectrodes wired (c), and a dual photoelectrodes loaded on a single support in the side-by-side configuration (d). The dual photoelectrodes in the three latter cases are made of semiconductors with small or narrow band gaps. Reprinted with permission from Ref. [131], Copyright 2017, Elsevier

Fig. 15

Artificial leafs made of photovoltaics e water electrolyser assemblage: a wired and b wireless. Reprinted with permission from Ref. [131], Copyright 2017, Elsevier