Photoelectrochemical cells (PECs) hold great promise as an environmentally friendly method of converting sunlight into energy-dense chemicals. For example, the PEC approach can find application in the synthesis of multi-carbon-based hydrocarbons (C2+) by solar driven reduction of carbon dioxide (CO2R), and in the generation of hydrogen by solar water splitting. However, the electrochemical environment poses significant challenges to the performance and stability of semiconductor based photoelectrodes. Tackling these challenges requires careful understanding of the behaviour of photoelectrode interfaces at the microscopic scale.

Here we show examples of characterization techniques that can help to quantify the performance and stability of photoelectrode surfaces with nanometer resolution, using, as a model system, thin films of TiO₂ deposited for corrosion protection by atomic layer deposition (ALD). A detailed analysis of Kelvin probe force microscopy (KPFM) measurements under intermittent illumination allows us to analyze the evolution of surface potential over time and extract localized time constants for carrier dynamic processes on the surface. Furthermore, using operando spectroscopic ellipsometry (SE), we can directly quantify the intrinsic stability of these protective overlayers under PEC water splitting conditions, particularly as a function of the degree of crystallinity of the TiO₂ film. In addition, we show how rationally designed catalytic environments can significantly improve the stability and performance of photoelectrodes for PEC CO2R. We demonstrate that by tuning microenvironmental factors such as pH, wettability, and CO₂ mass transport, we can enhance C2+ selectivity on halide perovskite photoelectrodes. Understanding photoelectrode interfaces at the micro and nanoscale allows the systematic improvement of photoelectrode stability and performance, opening avenues for the implementation of PEC technologies towards sustainable energy production and climate change mitigation.

Efficient charge separation is a crucial step in any photoelectrochemical (PEC) or photocatalytic (PC) process. The role of the electric field is, however, often misunderstood in the PEC and PC communities [1]. While it is often stated that the charges are separated by the electric field, this is not the case for a system without externally applied bias; instead, charge separation requires the presence of selective contacts. Another commonly encountered statement is that by increasing the externally applied potential, one can increase the field strength and thereby improve charge separation. Under normal conditions, however, the applied potential only affects the width of the space charge layer and has zero effect on the actual strength of the electric field within the photoelectrode. In this talk I will discuss these misconceptions and offer a tentative explanation of where they originate from. I will discuss the true driving force for charge separation and outline various strategies for making selective contacts, with a focus on metal oxide photoelectrodes. Most of these strategies originate from the field of photovoltaics, in which the concept of selective concepts has been well established. Some approaches are relatively easy to implement for metal oxide absorbers and several examples will be shown. Others strategies may be too complex and costly for scaleup to large areas but can still offer valuable insights and help us to design efficient PEC and PC systems.

The accelerated consumption of fossil fuels and the concomitant rise in greenhouse gas emissions emphasize the need for transitioning towards renewable “green” resources, and environmentally sustainable processes. Photocatalysis has the potential to simultaneously mitigate the energy and environmental concerns.[1] However, the development of economically and environmentally sustainable processes creates the pressing need for new materials of low cost and toxicity, photoabsorbers and catalysts, which are robust and maintain good performances under outdoor conditions.

Carbon dots (CDs) and carbon nitride (CN_x) can efficiently serve as photoabsorbers for this purpose since they fulfil these requirements.[2-6] In particular, they are hydrophilic materials of low toxicity which are chemically and photochemically robust, can be synthesized at low cost, and show good photocatalytic properties upon pre-designed synthesis. In this work, we describe the synthesis of CN_x and CDs from low-cost organics and/or Earth abundant waste (i.e., circular economy), the structure of which bestows the derived photoabsorbers with distinctive photocatalytic performances. These light harvesters, when combined with noble-metal free catalysts in aqueous photocatalytic systems, not only facilitate “green” fuel synthesis but also waste/water pollutant utilization. The use of waste and aqueous pollutants, eliminates the need for additional sacrificial reagents traditionally used in great excess, which add to the overall cost of the process, and result in toxic by-products.[7] We anticipate that this approach could be a breakthrough in the development of robust, scalable, economically, and environmentally sustainable systems, which can efficiently serve energy and environmental applications.

References

[1]   Kamat, P. V.; Bisquert, J., J. Phys. Chem. C 2013, 117, 14873-14875.

[2]   Achilleos, D. S.; Kasap, H.; Reisner, E., Green Chem. 2020, 22, 2831-2839.

[3]   Achilleos, D. S.; Yang, W.; Kasap, H.; Savateev, A.; Markushyna, Y.; Durrant, J. R.; Reisner, E., Angew. Chem. Int. Ed. 2020, 59, 18184-18188.

[4]   Kasap, H.; Achilleos, D. S.; Huang, A.; Reisner, E., J. Am. Chem. Soc. 2018, 140, 11604-11607.

[5]   Kasap, H.; Godin, R.; Jeay-Bizot, C.; Achilleos, D. S.; Fang, X.; Durrant, J. R.; Reisner, E., ACS Catalysis 2018, 8, 6914-6926.

[6]   Ren, J.; Achilleos, D. S.; Golnak, R.; Yuzawa, H.; Xiao, J.; Nagasaka, M.; Reisner,   E.; Petit, T., J. Phys. Chem. Lett 2019, 10, 3843-3848.

[7]   Pellegrin, Y.; Odobel, F., C. R. Chim. 2017, 20, 283-295.

Halide perovskites have gained wide interest for their application in photovoltaics, sensors, photocatalysis, and photoelectrochemistry, due to their excellent optoelectronic properties. However, their development in photoelectrochemistry for solar fuels and chemicals is hampered by their instability in aqueous environments. In this talk, I will present our recent progress in the protection of halide perovskite-based photoanodes with different sheets of carbon allotropes, namely mesoporous carbon, graphite, glassy carbon, and boron-doped diamond, decorated with electrocatalysts. The protection of a CsPbBr₃ photoabsorber film with a mesoporous carbon layer results in photoanodes that last a few hours [1]. Addition of porous graphite sheets increases the stability from hours to days, especially with the addition of water-oxidation NiFeOOH electrocatalyst on the surface of the porous graphite sheet [2]. Longer stabilities of weeks can be achieved by creating porosity gradients with different graphite sheets and by replacing the top graphite sheet upon signs of deterioration. Finally, the best stabilities, projected to be of months, can be realized with more chemically and mechanically stable glassy carbon and boron-doped diamond sheets [3]. These achieve world-record stability: 97% of initial photocurrent close to 8 mA cm^-2 is preserved for 210 h under harsh +1.23 V vs RHE of applied bias (stability projection = 10 months). All these carbon allotropes provide the halide perovskites with durable protection, efficient charge separation and transport, electrocatalyst support, and, importantly, photothermal properties for enhanced thermal management, which are invaluable additions for their future development and application of halide perovskite photoelectrochemical devices.

Photoelectrochemical (PEC) water splitting is one of the few truly renewable pathways toward “green” H₂ production. All PEC water splitting devices demonstrated thus far operate at atmospheric pressure. However, most applications and processes that use H₂ require it to be supplied at elevated pressure. Although PEC-generated H₂ can be pressurized afterward with e.g. mechanical compression, operating the PEC water-splitting device itself at elevated pressure offers an intriguing alternative and several possible advantages. For example, bubble formation is strongly reduced at elevated pressure, which lowers bubble-induced electrode deactivation and bubble-induced product crossover.^[1,2] In addition, the optical reflection and diffraction losses induced by bubbles can be minimized.^[3] Finally, the electrochemical production of hydrogen at higher pressure requires only a small (though not insignificant) increase in the thermodynamic cell voltage (29 mV for a 10-fold increase in pressure).^[4] To quantitatively evaluate these pros and cons, a device that can be used for PEC water splitting at higher pressure is required.

In this work, we demonstrate a membrane-free, laminar flow cell that enables PEC water splitting at elevated pressure (see the TOC graphic for the schematic of the cell). In such a cell, a continuous, laminar liquid flow is essential for adequate product separation. Therefore, the flow velocity profile between the two parallel electrodes was examined using particle image velocimetry (PIV). A nearly parabolic velocity profile was obtained. This observation agrees well with the finite element simulation results. Shadowgraph images also show that O₂ bubbles are restricted to the vicinity of the anode due to the laminar liquid flow. The design of our PEC flow cell also allows operation at moderate pressure elevation, up to 5 barg, without any observed leakage. More importantly, the pressure elevation significantly diminished the bubble curtain, potentially leading to a higher photoelectrochemical performance due to smaller bubble-induced drawbacks. Finally, we will discuss a quantitative comparison of the benefits and drawbacks of performing PEC water splitting at elevated pressure using our laminar flow cell.

Electrochemical reduction of CO₂ into valuable chemicals from renewable electricity is envisioned as a means to avoid fossil resources in many applications. As the field is urged to increase its technology readiness, numerical models are necessary tools to accelerate device development.[1], [2] Nevertheless, fully reliable and accurate models have yet to be developed. The challenges lie in the complexity of the reaction and the convolution of many effects including mass-transport, microkinetics, and kinetics. In this study, we aim at deconvoluting these aspects by studying the reduction of CO₂ into carbon monoxide at a flat silver electrode, one of the most studied catalyst material under controlled mass-transport conditions. We hypothesize that one of the major gap between previous model and experiments is the carbon monoxide affinity for some active sites that prevent other electrochemical reactions to take place. In order to model this effect independently, we conducted a series of measurement in presence of CO and determined equilibrium constants to be fed into a microkinetic model. These experiments confirmed our hypothesis that the produced CO significantly hinders the competitive hydrogen evolution reaction. We then developed a computational model and show that the consideration of the CO effect on hydrogen evolution reaction allows for improved accuracy in predicting experimental observations.[3] The study also provides unexpected insights in the different nature of active sites responsible for CO and H₂ evolution at Ag electrodes.

High conversion efficiencies are key for solar fuel technologies to compete with natural photosynthesis. Here, photoelectrochemical approaches should be in a similar efficiency range as the combination of photovoltaics with electrolysis. So far, the highest photoelectrochemical solar-to-hydrogen efficiencies (STH) are achieved using multi-junction solar absorbers based on the III-V semiconductor material class. Reported areas, however, only lie in the range of up to 0.3 cm2 as surface defects quickly lead to corrosion and failure of the device. I will present our latest results where we demonstrate significantly higher areas at >18% STH using a photoelectrochemical surface functionalisation approach in a Schlenk cell. The higher level of interface quality and reproducibility is a prerequisite for scale-up and optical in situ control with electrochemical reflection anisotropy spectroscopy contributes to this development [1,2]. Furthermore, I will discuss some of the challenges and potential pitfalls related to the benchmarking of multi-junction solar water splitting devices which are still present in the literature.

Mimicking photosynthesis and producing solar fuels is an appealing way to store the huge amount of renewable energy from the sun in a durable and sustainable way. Various technologies exist with different readiness levels [1]. A variety of solar fuels can also be produced, including hydrogen but also syngas or methane. In that context, we will exploit data from our recent contributions regarding the preparation of photoelectrode materials and the design of tandem photoelectrochemical cells [2], but also artificial leaves [3] and integrated PV-EC cells to benchmark these approaches and propose a path forward for solar fuels technologies.                                                                                                                                          

High power density devices are required for economically competitive and sustainable solar fuel devices. I will review our solar hydrogen demonstrations at high current density and based on thermally integrated photo-electrochemical approaches. I will show how the design evolved when moving from lab-scale, to system-scale, and to industrial scale [1]. I will comment on the indoor experimental conditions compared to outdoor experiments. I will show how the thermal-integration principle can be extended for CO₂ reduction devices [2] and for reversibly operated devices (forward mode for hydrogen generation, backward mode for power generation). I will show how indoor experiments with high-flux solar simulators can be used for the performance assessment of such devices, including the assessment of longevity and degradation [3]. I will end with the introduction of a novel design concept for photocatalytic solar fuel generation based on droplets. I will show how such drop-based system con provide a continuously operating system with high throughput and competitive production rates.

While hydrogen production via photoelectrochemical (PEC) water splitting has been demonstrated on a small scale, developing an industrial scale device is a challenge that intrigues and brings together researchers from a range of disciplines. I will present our prototype PV-PEC device, with a photo-absorbing area >100 cm², which operates with a photoanode comprising of a WO₃/BiVO₄ heterojunction on FTO. A key bottleneck in the scalability of PEC devices remains the development of scalable photocatalyst materials for the water splitting reaction. Our photoanodes are produced by conformally coating BiVO₄ onto WO₃ nanoneedles using low-cost and scalable chemical vapour deposition methods. With the band gap of BiVO₄ enabling light absorption up to 517 nm and a theoretical STH of up to 9.2%, the WO₃/BiVO₄ heterojunction system is one of the most promising in terms of performance, cost and durability. Combined with a Ni mesh cathode and externally connected homojunction silicon PVs this creates a cost-effective and scalable photoelectrochemical-photovoltaic (PV-PEC) device with a commercially viable fabrication method. I will discuss how the arrangement of the PVs, either behind (in tandem) or next to (side-by-side) the photoanode, affects the operation and performance of the device. Many photoelectrodes are produced as thin films on transparent conducting oxides, such as fluorine-doped tin oxide (FTO) or indium-doped tin oxide (ITO). However, one difficulty to overcome is the resistivity of FTO glass, which can result in severe resistance losses in scale-up. Without mitigation, this can lead to reductions in photocurrent of over 80%. I will present steps we have taken to mitigate this issue, with minimal performance losses between 1 cm² and 36 cm² electrodes.

Considering heat and mass transfer, as well as fluid dynamics, is not only critical when optimising the efficiency of scaled-up devices, but also to address safety challenges associated with increasingly larger systems. I will therefore discuss our work to optimise the temperature and electrolyte flowrates for long-term, stable operation. To deliver stable operating conditions, the PEC stability of materials must also be addressed. I will present work to improve the PEC stability of BiVO₄ photoanodes, through the use of co-catalysts, doping, different electrolytes and varying fabrication conditions. In preliminary results, WO₃/BiVO₄ electrodes with NiO_x co-catalyst layers have shown stability for 24 hours with an applied potential of 1.23 V_RHE in pH 9 potassium borate buffer.

This research seeks to elucidate challenges of developing up-scaled materials for water splitting, to facilitate the pathway to commercially viable photoelectrochemical hydrogen production.

The Liquid Sunlight Alliance (LiSA) has demonstrated two photochemical architectures that combine a light absorber and a multi-catalyst cascade to achieve conversion of CO₂ and water into liquid fuels using sunlight. Both are complete systems including a light absorber and a multi-catalyst cascade to achieve CO₂ reduction to carbon-based fuels. Realization required careful management of light absorption and conversion and supply of electrons to multiple catalyst sites, while at the same time controlling reactant, intermediate, and product fluxes. I will describe the work of two task forces within LiSA that designed the systems, synthesized, and integrated the components, and evaluated their performance.

The first architecture is a three-terminal tandem (3TT) system which uses a monolithic semiconductor photoelectrode using sunlight to drive chemical reactions in a cascade that produces a liquid fuel. The semiconductor architecture, based on a 3TT solar cell, absorbs light and generates electrons that are used for two separate reduction reactions at two of the terminals. The product of the CO₂ reduction reaction in the first location, CO, moves from where it is formed to the second location where it is reduced further to form methanol. The protons needed for the reduction reaction are generated from water at the third terminal. The photoelectrochemical cascade has two steps: (1) two-electron reduction of CO₂ to CO (driven by the GaInP subcel) and subsequent four-electron reduction of CO to methanol (driven by the GaInP/GaAs tandem subcell).

The second architecture combines photoelectrochemical and solar-driven thermocatalytic environments. A photoelectrochemical (PEC) reactor is used to reduce CO₂ to ethylene (with minimal coproduction of H₂, CO, and CH₄). Its design minimizes losses due to crossover between the electrodes and can achieve efficient conversion of CO₂ as well as high selectivity. The PEC cell can achieve an ethylene Faradaic efficiency (FE) of ~30% and a single pass concentration of > 0.5 vol.% to ethylene. Ethylene is then oligomerized in a second reactor using a supported Ni catalyst. This PEC – solar thermal tandem system has successfully produced butene and hexene. Notably also, the thermocatalytic reactor operating by itself in batch mode can produce C₇ – C₂₄ products from a pure ethylene feed using 1 Sun illumination.

The establishment of so-called artificial photosynthetic reactions that use solar energy to drive the synthesis of organic matter using H₂O and CO₂ as raw materials is significant for reducing CO₂ emissions and realizing carriers of abundant solar energy. ^{1, 2 3} Our primary approach to artificial photosynthetic reactions is the combinatorial technologies that effectively utilize the excellent properties of solid semiconductor photosensitizers and molecular metal complex catalysts. One important progress is the electrochemical and photocatalytic CO₂ reduction reaction (CO₂RR) over metal complex catalysts at a low reaction overpotential approaching the theoretical lower limit in aqueous solution. The essential point of the low overpotential CO₂RR is the operation of the metal complex catalysts with coexisting carbon support and metal cations such as K⁺. ^{3, 4} In addition, we have realized the low overpotential CO₂RR combined the H₂O oxidation reaction (WOR), which is a pair reaction for artificial photosynthesis, in a single aqueous solution at near neutral pH. ⁵ Simultaneous operation of the CO₂RR and the WOR in a single solution is one of the concepts necessary to construct a new simplified form of an artificial photosynthetic system that does not require a setup for separation of sites for CO₂RR and WOR like the thylakoid membrane existing in natural photosynthesis.

In this presentation, I will explain systems that make effective use of semiconductors and metal-complex catalysts driven in a single aqueous solution under simulated sunlight or visible light irradiation: a photocatalytic system functioning by a two-step photoexcitation (Z-scheme) mechanism in a self-organized manner using a combination of particulate (CuGa)_0.3Zn_1.4S₂, BiVO₄, and Co-bipyridine complex ^{6, 7}, a so-called artificial leaf with a thickness of less than 1 mm consisting of Ru-bipyridine complex, IrO_x, and an amorphous Si-Ge triple junction ⁵, and a 1m²-sized cell composed of Ru-bipyridine complex, IrO_x, and a crystalline Si solar cell. ^8-10

The most challenging deal we face today is the need to lower greenhouse gas (GHG) emissions and tackle climate change. Though calls to reduce it are growing louder yearly, emissions remain unsustainably high. CO₂ is the key contributor to global climate change in the atmosphere. Electrochemical CO₂ reduction (EC CO₂R) into chemicals or fuels holds great research interest as a promising approach to mitigate CO₂ emissions and reach a carbon-neutral future.[1] In this regard, an extraordinary effort has been made to discover new efficient and sustainable catalysts at the laboratory level over recent years. High-performance electrocatalysts in aqueous electrolytes often rely on noble metals, which may hinder their industrial applications. Herein, we successfully synthesized core-shell Cu₂O/SnO₂ nanoparticles functionalized with a silane group, using a simple and versatile methodology based on a three-step scalable synthesis method involving wet precipitation followed by salinization and, finally, a rhenium-based complex has been assembled by electro-polymerization. The carbon paper-supported Cu₂O/SnO₂-Re electrocatalyst was characterized at 10 cm² scale, demonstrating a steady-state production of syngas at -20 mA·cm^-2 up to 24 hours, achieving a CO:H₂ ratio from 3 to 9. To translate those developments from the laboratory level to a higher TRL towards the practical application for CO₂ capture and utilization[2], an additional chamber was added to the system for continuous CO₂ capture and electrochemical conversion, increasing the electrode area from 10 cm² to 100 cm². Captured CO² co-electrolysis to syngas (H₂:CO ratio of 5) in one step was demonstrated with a high CO₂ conversion at a current up to -2 A, indicating the scale-up potential of this intensified system. The technology is under validation in a TRL4 reactor composed of an array of 5 modules (i.e., 5 x 4 cells x 100 cm² or 0.2m²) for direct CO₂ conversion from simulated anthropogenic sources. The design ensures a self-bias operation by integrating low-cost perovskite photovoltaic (PV) cells to provide any required additional bias to drive the reaction with Perovskite PV panels with a cost of up to 5 times lower (10 €/m²) than Si PV cells. Besides, to further enhance the performance, ionic liquids (ILs), which have unique properties, have been proposed to perform CO₂ capture and to boost CO₂-derived products. Some of us have identified as role of the anions of imidazolium based ILs the tuning of the CO/H₂ ratio over Ag-based catalyst in aprotic media.[3] In this work, for the first time, a Cu₂O/SnO₂-Re-based electrode has been used within a continuous flow cell and in the presence of ILs-based solutions. However, we have observed different stability issues, such as the blackening of typical carbon-based gas diffusion layers (GDLs) and the degradation/colour changes of ILs-electrolyte. Field Emission Scanning Electron Microscopy (FESEM) and Electrochemical Impedance Spectroscopy (EIS) techniques have been employed to carry out the physicochemical characterization of the electrodes and to assess the electrochemical interfaces within the system, respectively. The observed findings offer openings for large-scale carbon capture and CO₂ reduction technology deployment. The TRL5 demonstration our most stable developed technology is planned in 2024 at the facilities of IIF Spain with real flue gas emissions.