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Dependence on fossil fuels in almost every aspect of our lives is believed to be the greatest threat to the environment, but also entails many geopolitical problems. Therefore, turning to other types of fuels and means of energy production, especially such derived from renewable sources, is a necessary step in order to mitigate environmental damage and ensure a greater degree of energy security. Solar fuels, in particular alcohols or hydrogen, are viewed as an alternative source of energy for replacing fossil fuels. They are made from common substances like water and carbon dioxide using the energy of sunlight.

Solar fuels have the potential to diversify our fuel supply and make our overall energy system more sustainable. They can use existing fuel infrastructure for a huge range of applications. These fuels can be stored for a long time and be transported anywhere, making them a valuable and flexible resource for a more reliable electric power grid.

Now (2023), scientists managed to produce carbon-based fuel with the help of a catalyst and a light-absorbing device. They designed artificial leaf devices containing an oxide-derived Cu₉₄Pd₆ electrocatalyst with perovskite–BiVO₄ tandem light absorbers and were thus able to couple CO₂ reduction with water oxidation. The wired Cu₉₄Pd₆|perovskite–BiVO₄ tandem device had a Faradaic efficiency of ~7.5% for multi-carbon alcohols. The study demonstrated the direct production of multi-carbon liquid fuels from CO₂ over an artificial leaf and, therefore, brings us a step closer to using sunlight to generate value-added complex products.

The bimetallic Cu₉₄Pd₆ electro catalyst was used to produce multicarbon alcohols from CO₂ at low over potentials. An additional poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) hole transport layer was added to the perovskite architecture of the device for improved open-circuit voltage (Voc) which would increase the available bias for catalysis. The Cu₉₄Pd₆ catalyst was successfully integrated in a bias-free, wired perovskite-BiVO₄ tandem PEC device as well as a standalone artificial leaf configuration for unassisted multicarbon alcohol production from aqueous CO₂ powered by simulated sunlight.

As copper is the only known metal that can form multicarbon products from CO₂ electro reduction, the scientists tuned its product selectivity by doping a second metal to it. To do so, a template assisted electrodeposition method was employed to form a dendritic macroporous structure of the bimetallic CuxPdy catalyst. The prepared CuxPdy material was further activated by a three-step activation process. First, the material was anodized in a basic ethylene glycol-water mixture to form a rough oxide layer. Second, the anodized material was annealed in air to extend the bulk oxide phase. Third, the thick oxide layer was electrochemically reduced to form oxide-derived bimetallic material.

The formation of multicarbon alcohols on the activated Cu₉₄Pd₆ catalyst took place in the course of a synergistic process including both the Cu-rich and the Pd-rich phases. The multicarbon product formation on the phase separated Cu₉₄Pd₆ catalyst was a *CO and *H spillover from the Pd-rich to the Cu-rich phase, where C-C coupling was followed by hydrogenation.

An artificial leaf device assembly was built for solar multicarbon alcohol generation from aqueous CO₂ using simulated solar irradiation. Cesium formamidinium methylammonium (CsFAMA) triple cation mixed halide perovskite inverse structure devices were coupled with the activated Cu₉₄Pd₆ catalyst with the help of a conductive graphite epoxy (GE) paste to obtain the photocathodes. An additional PTAA hole transport layer was created to improve the open-circuit voltage (Voc) compared to previously reported photoelectrodes. A graphite epoxy encapsulant afforded improved long-term stability in aqueous medium and enabled integrating the catalyst directly in the perovskite layer to fabricate the perovskite׀Cu₉₄Pd₆ photocathode. After 6 h of the PEC experiment, a faradaic efficiency of (6.4±0.9)% multicarbon alcohol was obtained with an almost ~1:1 ethanol and n-propanol ratio.

The scientists designed two models of the artificial leaf: (a) a wired tandem device where both the photocathode and photoanode were connected to Cu wires and the photocurrent transient could be measured with a potentiostat; (b) a standalone artificial leaf where the photoanode and photocathode were directly connected by a conductive copper tape. A CO₂ saturated aqueous 0.5 M KHCO₃ solution (pH 7.2) was used for the bias-free experiments since it can act as an efficient buffer for both CO₂ conversion and O₂ evolution reactions and the BiVO₄ photoanode shows optimal operation and stability at neutral pH.

Scientists have long tried to synthesise fuels from sunlight. In 2019, the synthesis of heterostructures with compatible band positions and a favourable surface area for the efficient photocatalytic production of molecular hydrogen (H₂) was researched. 3-dimensional Nb₂O₅/g-C₃N₄ heterostructures with suitable band positions and high surface area were constructed using a hydrothermal method. When Nb₂O₅ was combined with a low charge carrier recombination rate and a g-C₃N₄ exhibiting high visible light, absorption showed remarkable photocatalytic activity under simulated solar irradiation in the presence of various hole scavengers (triethanolamine (TEOA) and methanol). The scientists also studied the influence of different molar ratios of Nb₂O₅ to g-C₃N₄ on the heterostructure properties, the role of the employed hole scavengers, and how the co-catalyst and the charge carrier densities impacted the band alignment. The separation/transfer efficiency of the photogenerated electron-hole pairs was found to increase significantly. This enhanced photocatalytic activity was ascribed to a sufficient interfacial interaction favouring the fast photogeneration of electron-hole pairs at the Nb₂O₅/g-C₃N₄ interface through a direct Z-scheme.

Image: The proposed reaction mechanism for Nb₂O₅ formation



Source: Faryal Idrees, Ralf Dillert, Detlef Bahnemann, Faheem K. Butt/ In-Situ Synthesis of Nb₂O₅/g-C₃N₄ Heterostructures as Highly Efficient Photocatalysts for Molecular H₂ Evolution under Solar Illumination/ Catalysts 9(2), February 2019/ DOI:10.3390/catal9020169/ Open Access. This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

In 2020, scientists synthesized noble metal-free core-shell nanoparticles of graphene (G)-wrapped CdS and TiO₂ (CdS@G@TiO₂) using a hydrothermal method. The interlayer thickness of G between the CdS core and TiO₂ shell was tuned by varying the amount of graphene quantum dots (GQD) during the synthesis procedure. The most optimized sample, i.e., CdS@50G@TiO₂ generated 1510 µmole g−1 h−1 of H₂ from water under simulated solar light with air mass 1.5 global (AM 1.5G) condition, alongside a stable generation of H₂ during 40 h of continuous operation. The increased photocatalytic activity and stability of the CdS@50G@TiO₂ sample could be ascribed to the enhanced visible light absorption and efficient charge separation and transfer between the CdS and TiO₂ due to incorporation of graphene between the CdS core and TiO₂ shell. This was also confirmed by UV-vis, photoelectrochemical and valence band XPS measurements.

Image: FE-SEM images of pure TiO₂ (a), pure CdS (b), CdS@TiO₂ (c), CdS@50G@TiO₂ (d) and HR-TEM images of CdS@50G@TiO₂ with lattice spacing (e)



Source: Muhammad Zubair, Ingeborg-Helene Svenum, Magnus Rønning, Jia Yang/ Core-Shell Nanostructures of Graphene-Wrapped CdS Nanoparticles and TiO₂ (CdS@G@TiO2): The Role of Graphene in Enhanced Photocatalytic H₂ Generation/ Catalysts 10(4):358, March 2020/ DOI:10.3390/catal10040358/ Open Access. This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0) 

The tandem device has several advantages: It shows a good steady state photocurrent of 280±30 µA cm⁻² and produces 7.5% multicarbon alcohols under 1 sun irradiation. The standalone artificial leaf produced ~1 µmol cm⁻² multicarbon alcohols after 20 h of operation with a production rate of ~50 µmol h−1 gCuPd⁻¹. This proof-of-concept can be applied to form different long chain complex fuels and chemicals by developing novel artificial leaf systems. 

The research will have to be continued and tested in a large-scale application before it can be launched onto the market. If successful, the world will have an efficient and economic means to produce energy without polluting the environment.

By the Editorial Board