The synthesis of fuels using sunlight offers a promising sustainable solution for chemical energy storage, but inefficient utilisation of the solar spectrum limits its commercial viability. Apart from fundamental improvements to (photo)catalyst materials, solar fuel production systems can also be designed to improve solar energy utilisation by integrating complementary technologies that more efficiently utilise the solar spectrum. Here we review recent progress on emerging complementary approaches to better modify, enhance, or distribute solar energy for sunlight-to-fuel conversion, including advanced light management, integrated thermal approaches, and solar concentrators. These strategies can improve the efficiency and production rate of existing photo(electro)chemical systems and, therefore, the overall economics of solar fuel production. More broadly, the approaches highlight the necessary collaboration between materials science and engineering to help drive the adoption of a sustainable energy economy in the near future using existing technologies.3 Solar-driven photo(electro)chemical systems utilise solar energy to split water (H2O) and convert carbon dioxide (CO2) to produce hydrogen gas (H2) and carbon-based fuels in a process usually referred to as 'artificial photosynthesis'. Many artificial photosynthetic technologies exist in various stages of development, and these can be broken down into the following three main categories: photocatalysis, photoelectrochemistry, and photovoltaic-driven electrolysis (PVelectrolysis) (Figure 1a-c). 1 However, techno-economic analyses have shown that none of current laboratory-scale technologies meet the criteria for practical sustainable fuel production due to insufficient solar-to-chemical conversion efficiency (𝜂 !"#$ ), the absence of catalysts that are highly active and selective, and unsatisfactory scalability and stability. 2In general, PV-electrochemical systems benefit from commercially available components and already display high 𝜂 !"#$ . Nonetheless, PV-electrochemical and photoelectrochemical devices still face technical challenges due to high materials and manufacturing costs, mass transport limitations, and substantial resistive losses that must be addressed before such systems can be efficiently scaled up. 3 In contrast, photocatalytic systems are relatively simple, requiring far less auxiliary hardware, and are therefore expected to generate solar fuels more cost-effectively on a large scale, even though the 𝜂 !"#$ of such systems are currently limited to 1-2%. 4,5 Economic viability of solar fuels from artificial photosynthesis may be achieved by reducing the fabrication and operation costs, improving 𝜂 !"#$ without substantial increases to costs, or employing low-cost materials and auxilliary systems at sufficient scale (or with high enough production rates) to meet fuel production cost-thresholds despite relatively low efficiency. This review presents a range of complementary solar technologies, including light management, photon wavelength manipula...

The production of clean fuels and chemicals from waste feedstocks is an appealing approach towards creating a circular economy. However, waste photoreforming commonly employs particulate photocatalysts, which display low product yields, selectivity, and reusability. Here, a perovskite-based photoelectrochemical (PEC) device is reported, which produces H 2 fuel and simultaneously reforms waste substrates. A novel Cu 30 Pd 70 oxidation catalyst is integrated in the PEC device to generate value-added products using simulated solar light, achieving 60-90% product selectivity and ≈70-130 µmol cm −2 h −1 product formation rates, which corresponds to 10 2 -10 4 times higher activity than conventional photoreforming systems. The single-light absorber device offers versatility in terms of substrate scope, sustaining unassisted photocurrents of 4-9 mA cm −2 for plastic, biomass, and glycerol conversion, in either a two-compartment or integrated "artificial leaf " configuration. These configurations enable an effective reforming of non-transparent waste streams and facile device retrieval from the reaction mixture. Accordingly, the presented PEC platform provides a proof-ofconcept alternative towards photoreforming, approaching more closely the performance and versatility required for commercially viable waste utilization.

Photoelectrochemical (PEC) artificial leaves hold the potential to lower the costs of sustainable solar fuel production by integrating light harvesting and catalysis within one compact device. However, current deposition techniques limit their scalability, 1 while fragile and heavy bulk materials can affect their transport and deployment. Here, we demonstrate the fabrication of lightweight artificial leaves by employing thin, flexible substrates and carbonaceous protection layers. Lead halide perovskite photocathodes deposited onto indium tin oxide coated polyethylene terephthalate achieve an activity of 4266 µmol H2 g -1 h -1 using a platinum catalyst, whereas photocathodes with a molecular Co catalyst for CO2 reduction attain a high CO:H2 selectivity of 7.2 under a lower 0.1 sun irradiation. The corresponding lightweight perovskite-BiVO4 PEC devices display unassisted solar-to-fuel efficiencies of 0.58% (H2) and 0.053% (CO), respectively. Their potential for scalability is demonstrated by 100 cm 2 standalone artificial leaves, which sustain a comparable performance and stability of ≈24 h to their 1.7 cm 2 counterparts. Bubbles formed under operation further enable the 30-100 mg cm -2 devices to float, while lightweight reactors facilitate gas collection during outdoor testing on a river. The leaf-like PEC device bridges the gulf in weight between traditional solar fuel approaches, showcasing activities per gram comparable to photocatalytic suspensions and plant leaves. The presented lightweight, floating systems may enable open water applications, while avoiding competition with land use.

Photoelectrochemical (PEC) fuel synthesis depends on the intermittent solar intensity of the diurnal cycle and ceases at night. Here, an integrated device that does not only possess PEC water splitting functionality, but also operates as an electrolyzer in the nocturnal period to improve the overall capacity factor is described. The bifunctional system is based on an “artificial leaf” tandem PEC architecture that contains an inverse‐structure lead halide perovskite protected by a graphite epoxy/parylene‐C coating (conferring 96 h stability of operation in water), and a porous BiVO4 semiconductor. The light‐absorbers are interfaced with a H2 evolution catalyst (Pt) and a Co‐based water oxidation catalyst, respectively, which can also be directly driven by electricity. Thus, the device can operate in PEC mode during irradiation and switch to an electricity‐powered mode in the dark through bypassing of the semiconductor configuration. The bifunctional perovskite‐BiVO4 tandem provides a solar‐to‐hydrogen efficiency of 1.3% under simulated solar irradiation and an onset for water electrolysis at 1.8 V. The compact design and low cost of the proposed device may provide an advantage over other technologies for round‐the‐clock fuel production.

Solar-driven conversion of CO2 and plastics into value-added products provides a potential sustainable route towards a circular economy, but their simultaneous conversion in an integrated process is yet to be accomplished. Here, we introduce a versatile photoelectrochemical (PEC) platform for CO2 conversion which is coupled to the reforming of plastic. The perovskite-based photocathode enables the integration of different CO2 reduction catalysts such as molecular cobalt porphyrin, Cu91In9 alloy, and formate dehydrogenase, which produce CO, syngas, and formic acid, respectively. The Cu27Pd73 alloy anode selectively reforms polyethylene terephthalate (PET) plastics into glycolic acid. The overall single lightabsorber PEC system operates with the help of an internal chemical bias and under zero applied voltage. The system performs similarly to bias-free, dual-light absorber tandems and shows ~10-100 fold higher production rates than photocatalytic suspension processes. This finding demonstrates efficient CO2-to-fuel conversion coupled to plastic-to-chemical PEC conversion as a promising sustainable technology powered by sunlight.