Lead-Free Metal Halide Perovskites for Hydrogen Evolution from Aqueous Solutions

Abstract

Metal halide perovskites (MHPs) exploitation represents the next big frontier in photovoltaic technologies. However, the extraordinary optoelectronic properties of these materials also call for alternative utilizations, such as in solar-driven photocatalysis, to better address the big challenges ahead for eco-sustainable human activities. In this contest the recent reports on MHPs structures, especially those stable in aqueous solutions, suggest the exciting possibility for efficient solar-driven perovskite-based hydrogen (H₂) production. In this minireview such works are critically analyzed and classified according to their mechanism and working conditions. We focus on lead-free materials, because of the environmental issue represented by lead containing material, especially if exploited in aqueous medium, thus it is important to avoid its presence from the technology take-off. Particular emphasis is dedicated to the materials composition/structure impacting on this catalytic process. The rationalization of the distinctive traits characterizing MHPs-based H₂ production could assist the future expansion of the field, supporting the path towards a new class of light-driven catalysts working in aqueous environments.

Keywords: aqueous solutions; lead-free; materials properties; metal halide perovskites; photocatalysis; solar-driven hydrogen evolution; water-stable.

Figure 1

Solar-driven perovskite-based H₂ production systems: (a) photoelectrochemical (PEC) cell; (b) particulate photocatalyst system in dynamic equilibrium; (c) particulate water-stable photocatalyst system. (d) Schematic representation of the processes on the perovskite photocatalyst surface under irradiation and possible reactions involved in the different systems (Equations (1)–(5)).

Figure 2

Examples of schematic crystal structures with the related chemical formula of (a) 3D, (b) 2D, (c) 0D and (d) double MHPs used for H₂ evolution from aqueous solutions.

Figure 3

Redox potentials of TiO₂ and Fe₂O₃ taken as example of traditional photocatalysts, Pb-based halide perovskites and Pb-free MHPs already exploited for hydrogen evolution processes.

Figure 4

(a) Schematic representation of DMASnBr₃ crystal structure, (b) valence band (VB) to conduction band (CB) edges of g-C₃N₄ and DMASnBr₃ aligned with the band edges of liquid water and with the H⁺/H₂ and triethanolamine (TEOA)/TEOA⁺ redox level through the vacuum level. Values (eV) are referred to the vacuum level (black) and to the standard H₂ electrode (SHE, grey), (c) H₂ evolution rates for DMASnBr₃@g-C₃N₄ composites at different percentages of MHP loading in 10% v/v TEOA aqueous solution. Reproduced from [17], with permission from Whiley, 2020.

Figure 5

(a) Sn 3d high-resolution X-ray photoelectron spectroscopy (XPS) spectra of the as-prepared PEA₂SnBr₄ and after storage in water; (b) Calculated band edge positions (continuous lines) for g-C₃N₄ and PEA₂SnBr₄ relative to Normal Hydrogen Electrode (NHE) potential; the dashed lines indicate the water redox reaction potentials. Reproduced with permission [16] Copyright 2020, Royal Society of Chemistry.

Figure 6

Schematic illustration of (a) the conversion process in the colloidal HI-H₃PO₂ aqueous solution and (b) the proposed mechanism for the H₂ generation. [(CH₃)₂NH₂]⁺ ions correspond to DMA⁺ ones. Reproduced from [31], with permission from Elsevier, 2018.

Figure 6

Schematic illustration of (a) the conversion process in the colloidal HI-H₃PO₂ aqueous solution and (b) the proposed mechanism for the H₂ generation. [(CH₃)₂NH₂]⁺ ions correspond to DMA⁺ ones. Reproduced from [31], with permission from Elsevier, 2018.

Figure 7

(a) Indirect band gap Tauc plot of as synthesized samples; (b) Energy level diagram of a MA₃Bi₂I₉/DMA₃BiI₆ heterojunction for the photocatalytic HI splitting. Reproduced from [78], with permission from Wiley, 2020.

Figure 8

Schematic mechanism of H₂ evolution in aqueous HBr solution by Cs₂AgBiBr₆ (CABB)/RGO composite under visible-light irradiation. Reproduced from [32], with permission from Elsevier, 2020.