Scaling-up photoelectrochemical (PEC) water splitting requires an end-to-end approach that encompasses photoelectrode fabrication, reactor design, optical coupling and operating strategy, yet few studies address these constraints simultaneously.^[1] Our strategy involves a scale-up-led workflow, from materials optimisation in deployment-relevant conditions to field testing and life cycle assessment (LCA).

We developed up-scalable FTO|WO₃|BiVO₄|NiFeOOH photoanodes, mechanically stable under electrolyte flow, sustaining a photocurrent density of 1.75 mA cm^-2 for 24 hours.^[2] Elevated temperature stability tests revealed that a WO₃ seed layer between the FTO and nanostructured WO₃ improves adhesion and mechanical robustness under flow-induced shear stress.^[3] Next, we developed an up-scaled PV-PEC reactor (30 cm² photoanode + Ni cathode + 30 cm² c-Si PV), characterised outdoors on a dual-axis tracking platform. Sunlight was concentrated (up to 4×) laterally onto the photoanode and PV using linear Fresnel lenses and aluminium lightguides, delivering photocurrents up to 77.5 mA.^[4] Returning to the 1 cm² scale, we integrated wide-angle (± 30°) crossed-compound parabolic concentrators (CCPCs, 3.6× optical concentration) with the PEC reactor. We demonstrated broad-angular light concentration, enhancing photocurrent (up to 3×) and enabling simplified solar tracking or stationary configurations, akin to those deployed for solar photovoltaics.^[5,6] Finally, annual hydrogen production was modelled for a theoretical pilot-scale PEC plant. Cradle-to-gate LCA showed that CCPC integration reduces the global warming potential by 21% (from 50.6 to 40.2 kg CO₂-eq. kg^-1 H₂), with further reductions with single-axis tracking.^[5]

I shall present results from materials development to field testing, highlighting operability challenges and how optics-coupled modules can enhance performance with reduced balance-of-system complexity.

Solar radiation is the most abundant renewable energy source but it is distributed and intermittent, thereby necessitating its storage via conversion to a fuel or chemical commodity for practical use. Solar thermochemical and photoelectro-chemical approaches (and combinations thereof) provide viable, non-biological routes for the direct synthesis of solar fuels and chemical commodities. I will review the state of the art of the technical solar fuel processing approaches [1]. Given the economic and sustainability advantage of utilizing concentrated radiation in photoelectrochemistry, a focus will lie on discussing the challenges associated with the utilization of concentrated solar irradiation and the provided opportunities by thermal integration [2]. Detailed multi-scale and multi-physics models and demonstrations will be used to support the development of general design guidelines on the materials and reactor scale. I will specifically discuss the advantages to integrate high-temperature heat and thermochemistry into photo-electrochemical devices [3]. I will end by highlighting engineering challenges to integrated high-temperatures into photo-electrochemical devices.

Long-term missions to Moon and Mars will require in-situ resource utilization (ISRU) technologies to synthesise materials in harsh environments and reduced gravitation. This opens a path for research on the sustainable fabrication and synthesis of materials potentially utilised in energy conversion technologies in these environments[1,2].

Photoelectrochemical (PEC) energy conversion is currently investigated for space applications, due to its potential in converting water and carbon dioxide using sunlight into oxygen, hydrogen, and useful carbon compounds[1,2]. Further, the monolithic design of PEC devices, which includes integrated semiconductor-electrocatalyst systems, offers significant advantages for long-term space missions, such as a compact and lightweight payload[3]. Particularly important is hereby the choice of electrocatalyst material (metal(s) or metal alloys) for the respective anticipated redox reaction to minimize activation polarization overpotentials of the device[4].

Since microgravity is known to affect the synthesis of nanomaterials by inducing increased crystallinity and increased porosity - which are attractive qualities in a catalyst material - the present study investigates the effect of this environment on the synthesis of Rhodium nanoelectrocatalysts via photoelectrodeposition, conducted in the Bremen Drop Tower (Germany)[5,6].

We present the different morphologies obtained in gravity- versus microgravity-bound synthesis processes, and compare their performance as catalysts for the hydrogen evolution reaction in both environments.

Artificial leaves based on III-V tandem photoabsorbers currently deliver the highest solar-to-hydrogen (STH) efficiencies for direct solar water splitting [1]. These devices typically employ a thin, wide-bandgap AlInP window layer to electronically passivate the photoabsorber surface, suppress surface recombination, and promote selective electron transport, together with a TiO₂ overlayer [2] that protects the III-V photoabsorber from corrosion while enabling electron extraction to the catalyst. However, their unassisted PEC performance is critically limited by the uncontrolled oxidation of the AlInP surface. During storage and upon subsequent wet-chemical removal of the protective GaAs cap layer, the AlInP surface oxidizes, forming a chemically complex mixture of Al-, In-, and P-containing oxides and hydroxides that becomes buried at the AlInP/TiO₂ interface. This interfacial oxide introduces an electronic barrier that hinders selective charge carrier transport, leading to increased interfacial losses and reduced PEC performance.

Here, we present a systematic interface-engineering strategy to optimize the AlInP surface oxides prior to atomic layer deposition of the TiO₂ protection layer. Two complementary surface modification pathways are investigated: selective wet-chemical etching using HCl and NH₄OH, and in situ H₂ plasma treatment immediately before TiO₂ deposition. X-ray photoelectron spectroscopy reveals that wet-chemical treatment selectively removes the electronically unfavorable In(OH)₃ species while largely preserving phosphate/phosphite surface species. The H₂ plasma treatment eliminates residual carbon contamination and creates a chemically activated surface that, as confirmed by in situ spectroscopic ellipsometry, promotes uniform TiO₂ nucleation, and thereby minimizes interfacial charge carrier transfer losses. The resulting interface chemistry, electronic structure, film growth, and PEC performance are correlated using X-ray and ultraviolet photoelectron spectroscopy, atomic force microscopy, and PEC characterization.

While wet-chemical treatment alone provides only a modest improvement in fill factor, H₂ plasma treatment completely restores the fill factor by minimizing interfacial charge carrier transfer losses. Owing to the limited penetration depth of neutral hydrogen radicals, the highest performance is achieved by combining wet-chemical etching with H₂ plasma treatment, thereby simultaneously reducing oxide thickness and optimizing the interfacial electronic structure. Transient photocurrent measurements further reveal efficient selective charge carrier transport with strongly suppressed interfacial charge carrier trapping. The optimized interface enables photocurrent densities exceeding 14 mA/cm², STH efficiencies approaching 18%, and stable unassisted water splitting for more than 5 h in pH 0 electrolyte. To the best of our knowledge, this represents one of the highest reported stability–efficiency combinations for 1 cm² III-V artificial leaves operating under highly acidic conditions. These results demonstrate that precise control of the buried semiconductor/oxide heterointerface is a key design principle for realizing durable, high-efficiency artificial leaves for scalable solar hydrogen production.

The sun represents a vast inexhaustible source of energy. Harnessing and efficiently exploiting this resource would potentially uplift the livelihoods of all mankind and improve the economic prosperity of nations while mitigating climate change. The diurnal nature of sunshine as well as the momentary and seasonal variation of solar irradiance, require both short- and long-term storage solutions for a steady and reliable energy supply. Aside from batteries that directly store the solar generated electricity, fuels enable storage of solar energy using chemical bonds. Hydrogen is the chemically simplest fuel that can be produced by dissociating water using only solar energy. Various solar hydrogen generation technologies such as photocatalysis, photo-electrochemistry (PEC), photovoltaic (PV) coupled electrochemistry (EC), solar thermal catalysis as well as photo-thermo-catalysis (PTC) among others are thus being investigated in the community. While indirectly coupled PV-EC and solar thermal catalysis systems have been deployed at utility scale, direct PV-E coupling, PEC, PC and PTC still require developments for the active materials, device configuration and operation strategies.  Additionally, the conversion efficiency of all approaches is limited by the suboptimal use of the harvested solar energy which is commonly addressed by strategies to enhance broadband absorption and/or utilisation of incident photons. This contribution will present an analysis of examples of our research and from recent developments in the literature with specific focus on the use of heat to enhance solar to fuel conversion. As a result, issues common to all, or unique to each pathway shall be explored. Identification of practically relevant solutions to these problems are expected to advance their development towards large scale deployment.

The practical deployment of photoelectrochemical (PEC) systems for solar fuel and chemical production requires moving beyond conventional laboratory conditions toward device operation under realistic and application-relevant environments. While most PEC studies are performed at atmospheric pressure, many target applications involve gas handling, pressurization, and integrated downstream processes that fundamentally alter device behavior. In this context, elevated-pressure operation emerges as an interesting yet underexplored parameter in PEC system design. In this talk, I will first briefly motivate the importance of coupling PEC hydrogen production with value-added chemical synthesis, illustrated through a solar-driven system where in situ generated hydrogen is directly utilized for homogeneous hydrogenation of biomass-derived feedstocks.^[1] This approach highlights how system-level integration can enhance the overall utility of solar-driven devices beyond hydrogen generation alone. Building on this motivation, the main focus of the talk is on engineering PEC systems under elevated-pressure conditions. Through combined multiphysics modelling, operando diagnostics, and high-pressure device development, we show how pressure modifies key processes governing PEC performance, including gas bubble evolution, optical transmission, interfacial mass transport, and product gas crossover.^[2-4] Our results reveal that moderate pressurization can significantly suppress bubble-induced optical losses and mitigate photocurrent saturation under operating conditions, while introducing only minor thermodynamic penalties. Finally, we also examine coupled transport phenomena such as electrolyte convection and pH gradients, which become increasingly important for scalable device operation.

References

1. K. Obata; M. Schwarze; T. A. Thiel; X. Zhang; B. Radhakrishnan; I. Y. Ahmet; R. van de Krol; R. Schomäcker; F. F. Abdi, Nat. Commun., 2023, 14, 6017.
2. F. Liang; R. van de Krol; F. F. Abdi, Nat. Commun., 2024, 15, 4944.
3. F. Liang; R. van de Krol; F. F. Abdi, Chem. Eng. J., 2025, 512, 162513.
4. F. Liang; H. Kong; D. S. Babu; R. van de Krol; F. F. Abdi, Nat. Commun., 2025, 16, 11139.

Lead halide perovskites have emerged as outstanding alternatives for solar-driven chemistry, enabling bias-free photoelectrocatalytic (PEC) water splitting[1-3] and CO₂ conversion.[4,5] While perovskite semiconductors degrade rapidly in water, recent design principles have led to substantial advances in device stability and performance. Here, we will first discuss the role of charge selective layers in increasing device photocurrent and photovoltage, by fine-tuning the band alignment and enabling efficient charge separation.[2,3] The lifetime of both perovskite photocathodes and photoanodes can be extended to multiple days in aqueous media,[3,4,6] by replacing low melting alloys with graphite epoxy paste as a conductive, hydrophobic and low-cost encapsulant.[3,6,7] These design principles are successfully applied to an underexplored BiOI light absorber, increasing the photocathode stability for H₂ evolution from minutes to months.[8] Next, we will present our recent progress in device manufacturing for scalable solar fuels production. Lightweight substrates can decrease device cost tenfold and expand device functionality, resulting in flexible, floating artificial leaves.[5] Those materials are compatible with large-scale, automated fabrication processes, which present the most potential towards real-world applications.[6,9,10] Square-meter PEC reactors can also take advantage of the modularity of artificial leaves,[11] while thermoelectric generators bolster water splitting by harvesting waste heat to suply an additional Seebeck voltage.[12,13] Finally, I will showcase PEC devices as versatile platforms to produce value-added chemicals, by interfacing the perovskite semiconductor with copper nanoflower catalysts for CO₂ reduction to C₂ hydrocarbons (ethene, ethylene), and silicon nanowire photoanodes for glycerol oxidation.[14]

The increasing levels of CO₂ in the atmosphere have prompted researchers to investigate new technologies for producing carbon-based fuels and value-added chemicals through photocatalytic reduction of CO₂.^[1] Recently, covalent organic frameworks (COFs) have been explored as suitable porous supports for photocatalysts due to their remarkable physical and chemical stability, structural diversity and large surface areas.^[2] Moreover, their light-harvesting capability can be improved by careful selection of building blocks such as perylene, while also tuning the bandgap to extend the lifetime of electron-hole pair separation in a z-scheme, thus establishing a thermodynamically favourable process.^[3]

Light-driven catalytic conversion can be enhanced by forming an electron “donor-acceptor” type photocatalyst by embedding nanoparticles (NPs) into the COF network. NPs have been widely used for catalysis due to their high surface energy and quantum size effects and can accept the electrons excited within the porous network to reduce CO₂. Small Au NPs, in particular, have been shown high efficiency and selectivity towards the production of CO,[4] while RuO₂ NPs, which possess an excellent affinity to O₂ gas with a favourable O₂ binding energy low overpotential, and high water oxidation activity,[5] retain the photogenerated holes in the network to facilitate the oxidation of a sacrificial agent.

In this work, size-controlled Au NPs synthesised in-situ into a thiol-functionalised perylene-based COF and the incorporation of prestabilised RuO₂ NPs will be shown. Well-distributed NPs into a z-scheme-designed photoactive porous network produced a novel robust hybrid material for the purpose of simultaneously reducing CO₂ and oxidising water. A wide range of microscopy, spectroscopic and computational methods utilised to describe the structure and optical properties of this material will be described. In addition, preliminary results of its photocatalytic efficiency for CO₂ reduction under visible light will be presented.