Advanced nanomaterials for highly efficient CO2 photoreduction and photocatalytic hydrogen evolution

Abstract

At present, CO₂ photoreduction to value-added chemicals/fuels and photocatalytic hydrogen generation by water splitting are the most promising reactions to fix two main issues simultaneously, rising CO₂ levels and never-lasting energy demand. CO₂, a major contributor to greenhouse gases (GHGs) with about 65% of the total emission, is known to cause adverse effects like global temperature change, ocean acidification, greenhouse effects, etc. The idea of CO₂ capture and its conversion to hydrocarbons can control the further rise of CO₂ levels and help in producing alternative fuels that have several further applications. On the other hand, hydrogen being a zero-emission fuel is considered as a clean and sustainable form of energy that holds great promise for various industrial applications. The current review focuses on the discussion of the recent progress made in designing efficient photocatalytic materials for CO₂ photoreduction and hydrogen evolution reaction (HER). The scope of the current study is limited to the TiO₂ and non-TiO₂ based advanced nanomaterials (i.e. metal chalcogenides, MOFs, carbon nitrides, single-atom catalysts, and low-dimensional nanomaterials). In detail, the influence of important factors that affect the performance of these photocatalysts towards CO₂ photoreduction and HER is reviewed. Special attention is also given in this review to provide a brief account of CO₂ adsorption modes on the catalyst surface and its subsequent reduction pathways/product selectivity. Finally, the review is concluded with additional outlooks regarding upcoming research on promising nanomaterials and reactor design strategies for increasing the efficiency of the photoreactions.

Keywords: CO2 reduction; Photocatalysis; TiO2; hydrogen generation; nanomaterials; non-TiO2 materials; water splitting.

Graphical abstract

Figure 1.

Schematic representation of and stepwise mechanism in photocatalytic reactions (left) and CO₂ photoreduction to chemical fuels on the surface of metal-doped photocatalyst (right). Source: [18,19].

Figure 2.

Different routes for Hydrogen production using conventional and renewable energy sources. Source: [24].

Figure 3.

Bandgap energies and band edge positions of several photocatalysts for water splitting. (Reprinted with permission from Ref [16]. Copyrights 2018 American Chemical Society).

Figure 4.

Redox potentials and bandgap of various photocatalysts with respect to different chemical species measured at pH 7. (Reprinted with permission from Ref [17]. Copyrights 2019 Elsevier).

Figure 5.

Electronic localization function of a) pure g-C₃N₄ and b) B-doped g-C₃N₄ on the parallel plane. (The red areas represent a high probability of electrons, while the blue areas represent a low probability. The grey, blue, and pink spheres represent C, N, and B atoms, respectively). (Reprinted with permission from Ref [58]. Copyrights 2019 John Wiley and Sons).

Figure 6.

Possible structures of CO₂ anchoring on the surface of the catalyst. (Reprinted with permission from Ref [74]. Copyrights 2006 Elsevier).

Figure 7.

Stepwise CO₂ reduction mechanism with (A) formaldehyde pathway, (B) carbene pathway and (C) glyoxal pathway. source [75].

Figure 8.

Schematic of Volmer-Heyrovsky and Volmer-Tafel pathways for HER process. Source [84]: .

Figure 9.

(a) Scheme highlighting the role of Cu⁺¹ sites and oxygen vacancies in CO₂ photoreduction; (b) Schematic for the photocatalytic and the structural adsorption processes occurring with TiO₂ anatase, Cu-doped TiO₂ (with presence of oxidation state Cu¹⁺) and CuO; (c) Plot for the CH₄ concentration with respect to the irradiation time for a) Cu-1; b) Cu-2; c) Cu-5; d) Cu-10; e) CuO f) TiO₂ nano; (d) Photocatalytic methane production as a function of Cu¹⁺ sites (%) and O-vacancy sites (%) in Cu-doped TiO₂ samples (Reprinted with permission from Ref [69]. Copyrights 2021 the journal of physical chemistry C).

Figure 10.

(a) Time-dependent photocatalytic H₂ production and (b) the corresponding rates of H₂ evolution over N-TC, Ru-NPs/SAs@N-TC, Ru-SAs@N-TC, and 1% Pt/N-TC under 300 W Xe lamp, λ = 320–780 nm (20 mg of catalyst dispersed in 100 mL of a mixed solution of water and methanol with v/v = 4:1). (c) Photostability of H₂ evolution and (d) photo-assisted electrocatalytic LSV polarization curves of Ru-NPs/SAs@N-TC and Ru-SAs@N-TC in 1 m KOH. (Reprinted with permission from Ref [110]. Copyrights 2020 John Wiley and sons).

Figure 11.

(a, b, c) SEM image of NiTe₂, NiSe₂, NiS₂ respectively, (d) XRD pattern of NiTe₂, NiSe₂, NiS₂, (e) LSV curve for HER measurement in 0.5 M H₂SO₄, (f) Tafel slope of materials, (g) Crystal structure of Nickel chalcogenides NiS₂, NiSe₂ and NiTe₂ Source: [125].

Figure 12.

(a, b) SEM image of Co9S8@NS-C-900 composite at 50 and 5, (c) LSV curve of three different Co-MOF derived catalysts, (d) Tafel slope of different Co-MOF derived catalysts. Source: [148].

Figure 13.

Specific energy variation with cluster size. Source [151].

Figure 14.

(a) Nyquist plots of W-SAC, (b) Overpotential for W-SAC compared with WC, WN, and 20% Pt/C, (c) LSV curve of the W-SAC for HER performance in alkaline condition (0.1 M KOH), (d) Tafel slope of materials. (Reprinted with permission from Ref [159]. Copyrights 2018) John Wiley and Sons).

Figure 15.

(a) Scheme of CN synthesized by MW process, (b) Absorbance of Tetracycline at 357 nm as a function of time during photocatalytic oxidation reaction using bulk CN and CN_MW-ins catalyst, (c) Proposed Mechanism of photocatalytic oxidation of Tetracycline. (Reprinted with permission from Ref [181]. Copyrights 2021, springer nature).