Rationally Designed, Efficient, and Earth-Abundant Ni–Fe Cocatalysts for Solar Hydrogen Generation

Abstract

Developing highly efficient and affordable catalysts for solar hydrogen (H2) generation is crucial, and employing a cocatalyst from earth-abundant elements has a critical role to play. In this context, different compositions of earth-abundant Ni–Fe alloy (1:1, 1:3, and 3:1) have been prepared by hydrothermal method; subsequently, 1 wt % of these Ni–Fe cocatalysts were integrated with TiO2-P25 and thoroughly characterized. The resultant catalysts have been evaluated for solar H2 production, in powder and thin film forms, under one sun condition and in direct sunlight. Interestingly, all the catalysts in the thin film form exhibit superior hydrogen yield (HY), up to 27 times higher activity than its powder counterpart. Among the photocatalysts, Ni–Fe/TiO2 (3:1 = Ni/Fe; NFT31) composition exhibits the best HY in thin film (8.27 mmol·h–1·g–1) and exceeds all other compositions of catalyst. It is also to be reported that HY measured for the powder form with 1 mg shows 3–17 times higher activity than that measured with 25 mg. This is mainly attributed to effective solar light absorption with a smaller amount of photocatalyst either spread over large area in a thin film form or well-dispersed in suspension forms. Furthermore, the enhanced activity obtained with Ni–Fe/TiO2 photocatalysts is also ascribed to strong electronic integration of Ni–Fe cocatalyst with TiO2 and higher performance obtained with a thin film is attributed to increased charge carrier generation and subsequent charge separation and effective utilization. A decrease in work function of TiO2 by 0.6 eV was observed after its integration with cocatalyst in NFT31.

Aqueous Methanol to Formaldehyde and Hydrogen on Pd/TiO2 by Photocatalysis in Direct Sunlight: Structure Dependent Activity of Nano-Pd and Atomic Pt-Coated Counterparts

“…Furthermore, we compared the thin film activity with conventional testing approach with 25 mg of powder and activity values suggest that the thin film form definitely outperforms its powder counterpart. These results are consistent with earlier reports on Pd/TiO 2 , mesoporous Ag/TiO 2 , graphene, and high surface area carbon based PdAu/TiO 2 , NiFe/TiO 2 , and CuNi/TiO 2 catalyst systems. − The reason for the high activity with thin films is ascribed to effective light absorption and possible internal scattering within the film, which induces more charge carrier generation and therefore more hydrogen production compared to powder counterparts, where light scattering, rather than light absorption, is prevalent. − In the case of thin films, local utilization of charge carriers occurs within a few hundred nm 2 distance and leads to better hydrogen generation. This is, in fact, in contrast to solar cells where the charge carriers need to travel several microns to reach the back contact or current collector.…”

“…However, the prevalent light scattering with powder suspension of the catalyst under light irradiation leads to low activities. , To overcome this issue, the thin film form of powder counterparts are employed with much higher activity compared to powder counterparts. Enhanced light absorption and hence a greater number of charge carriers helps for efficient hydrogen generation; this is established with various photocatalyst systems such as Pd/P25, mesoporous Ag/TiO 2 , PdAu/graphene/P25, CuNi/P25, and NiFe/P25. − …”

Transition‐Metal‐Based Cocatalysts for Photocatalytic Water Splitting

“…In addition, compared to single‐component metals, due to the synergistic effect in optimizing the water adsorption energy and improving the electronic conductivity, bimetals with different components are studied as promising cocatalysts for photocatalytic water splitting. For instance, Saikia et al [ 82 ] found that the photocatalytic H 2 evolution performance of TiO 2 with the form of thin film was obviously improved when NiFe alloy nanoparticles were loaded as cocatalyst ( Figure A), which was higher than those induced by pure Ni and Fe nanoparticle cocatalysts. The enhancement of photocatalytic performance over NiFe loaded TiO 2 was due to the increased charge generation as well as effective separation and utilization of photoinduced charge carriers (Figure 10B).…”

“…A,B) Reproduced with permission. [ 82 ] Copyright 2021, American Chemical Society. C) SEM and D–G) HRTEM images of ZnIn 2 S 4 /CoNi.…”

“…The enhanced photocatalytic performance resulted from the cocatalytic function of introduced Cu clusters (Figure 9D), which can tune the electronic structure of TiO 2 to achieve optimized Gibbs free energy (Figure 9E) and enable the efficient transfer of electrons from TiO 2 to Cu (Figure 9F). The work by Gao et al [74] found that Fe, Co, and Ni clusters with [29] CdS Co Photodeposition λ > 420 nm (Xe) (NH 4 ) 2 SO 3 25980 (H 2 ) -18 (2018) [69] g-C 3 N 4 Co Photodeposition AM 1.5G (Xe) TEOA 2295 (H 2 ) 6.2 (400 nm) 48 (2018) [70] Zn 0.5 Cd 0.5 S Ni Impregnation λ > 420 nm (Xe) Na 2 SþNa 2 SO 3 5930 (H 2 ) -13.5 (2021) [71] HNb -12 (2020) [74] CdS Ni Photodeposition λ > 420 nm (Xe) Na 2 SþNa 2 SO 3 630100 (H 2 ) -16 (2020) [57a] g-C 3 N 4 Ni Impregnation-calcination (Xe) TEOA 354.9 (H 2 ) -36 (2020) [75] TiO 2 Cu Hydrothermal-calcination (Xe) Methanol 3700 (H 2 ) -16 (2021) [76] pyridyl-functionalized conjugated microporous polymer (PCMP) [77] ZnIn 2 S 4 Ni Photodeposition λ > 420 nm (Xe) TEOA 22000 (H 2 ) 56.14 (450 nm) 12 (2022) [78] ZnIn 2 S 4 CoNi Impregnation λ > 420 nm (Xe) Ascorbic acid 3336 (H 2 ) 2.15 (420 nm) 16 (2019) [30] TiO 2 Cu-Ni [79] g-C 3 N 4 MoNi-MoO 2 Grinding λ ≥ 400 nm (Xe) TEOA 1359 (H 2 ) 6.9 25 (2020) [80] mpg-C 3 N 4 Fe-Co Impregnation-Calcination λ > 420 nm (Xe) TEOA 1957.8 (H 2 ) 2.26 (405 nm) 16 (2021) [81] TiO 2 Ni-Fe Impregnation AM 1.5G (Xe) Methanol 8270 (H 2 ) -30 (2021) [82] a)…”