In recent years, significant progress has been accomplished in the development of CSP (Concentrating Solar Power) systems making use of heliostat fields, with central receivers on top of a tower, capable of achieving irradiances well above 2000kW/m². Such high solar radiation fluxes allow the efficient conversion of solar energy at temperatures well beyond 1000ºC, which are needed for the application of two-step thermochemical cycles using metal oxide redox reactions, either for use in thermochemical energy storage or for the production of solar fuels (Romero and Steinfeld, 2012).

In that respect, nonstoichiometric oxides become a well proven material for solar fuels production. Ceria (CeO₂) is currently considered the state-of-the-art redox material because of its rapid redox kinetics and long-term stability, being able to maintain its crystal structure. For typical operating conditions of the reduction step at 1500°C and 0.1 mbar, and the oxidation step at 900°C and 1 bar, thermodynamics predict a δ = 0.04, where δ denotes the non-stoichiometry – the measure of the redox extent. Thermodynamic analyses of solar hydrogen processing cycles with ceria provide peak solar-to-hydrogen efficiencies in the range of 12–20 %, though the associated challenge involves optimized gas-to-gas heat recovery systems and strict oxygen partial pressure control during the reduction step. The best on-sun results as of today are reported with the framework of the European Sun-to-Liquid project in a solar tower plant for the thermochemical production of kerosene from H2O and CO2 (Zoller et al., 2022)

The 50 kW solar reactor, mounted on top of the solar tower, consisted of a cavity-receiver containing a reticulated porous structure made of ceria which was directly exposed to a mean solar flux concentration of up to 2,500 suns (see figure below). Approximately 5,000 standard liters of syngas were produced with full selectivity during 55 hours of on-sun operation, yielding a maximum solar-to-syngas energy efficiency of 4.1% without applying heat recovery. It was observed that a limitation on the energy conversion efficiency in the reactor persists through the inhomogeneous temperature distribution, which is created when the material is heated up during the reduction step. A part of the material reached critical temperatures of 2000 K before the rear part of the reticulated porous ceramic (RPC) structure had achieved high enough temperatures for efficient reduction. Follow up research is oriented to produce 3D-printed structured redox materials with porosity gradient leading to conversion efficiencies of 10% and designing integrated solar reactor+heat recovery system with attached rear chamber for heat exchange for increased efficiency up to 15% at TRL5 (Project Sun-to-Liquid II) and more refined definitions of KPI (Project Suner-C).

Solar hydrogen generation systems’ path towards commercialization relies on their competitiveness in terms of performance, longevity, cost and sustainability. Utilizing solar irradiation concentration and thermal management are two features that can lead to a competitive system [1,2]. I will discuss our journey from our laboratory scale demonstration (output power in the range of 10 W) under controlled irradiation conditions to our demonstrator scale (output power in the range of 1 kW) under realistic solar irradiation conditions [3], and the planned industrial demonstrator (targeted output power in the range of 100 kW). I will provide data that shows and quantifies the operational stability of the system. I will discuss operational strategies that increase the competitiveness and capacity factor of the system, including strategies for 24h operation of the system or for constant hydrogen production rates. I will end with an outlook on how such systems can be further scaled to the MW scale, how they can be integrated in the current energy landscape, and how they can be extended to incorporate C-based inputs to contribute to the production of sustainable liquid fuels.

Solar-assisted water splitting is an environmentally friendly method to obtain storable hydrogen fuel. To reduce the overpotential of the Oxygen Evolution Reaction (OER), which is the bottleneck of the overall process, an efficient photoanode is required. Building, a high-throughput device with low-cost and stable material and using easy and highly reproducible techniques are the main challenges to covering the gap between research and industry.

In this respect, Silicon (Si)  is a promising candidate material due to its excellent optoelectronic properties, cost-effectiveness, and potential for large-scale production. However, its low catalytic properties and instability in alkaline solutions hinder its application as a photoanode. To overcome these limits, a strategy is offered by coupling Silicon with a protective layer of Transition Metal Oxides (TMOs), which are cost-effective, abundant, and highly catalytic toward the OER. In such a system a high charge transfer and low charge recombination at the interface of the two solid materials is mandatory to boost the efficiency of the OER. To this respect, a passivating thin interlayer, i.e. AlO_x/Au or TiO₂, can be introduced or a buried junction can be adopted, though increasing significantly the complexity of the system [1],[2].

As a representative case study, we focused on a system of nanostructures of Co₃O₄/SiO_x/Si prepared by magnetron sputtering deposition of Co and thermal annealing[3].

Here, we engineered the electric field at the solid-solid interface by reducing SiO_x defects through a pre-deposition annealing of Si at high temperatures in a vacuum. In a related way, a significant improvement in the onset of the photocurrent is observed and 500 mV of photovoltage at 10 mA/cm² is achieved. The same results were found in several similar photoanodes, showing a high reproducibility. Furthermore, a stable photocurrent in 1M KOH under working conditions has been measured for more than 70 h[4].

Si/SiO_x interface was investigated by Electron Paramagnetic Resonance (EPR) and X-ray Photoelectron Spectroscopy (XPS). Current-voltage and Impedance spectroscopy in the solid state were used to study the electrical properties of the Co₃O₄/SiO_x/Si heterojunction, elucidating the charge transfer process. A full set of electrochemical measurements (LSV, CV, EIS) were employed to investigate the photo-electrocatalytic properties. Morphological and structural characterization by GIXRD and SEM complete the analyses.

The experimental results show that pre-deposition annealing of Si is mandatory to reduce defects, and the charge transfer at the solid-solid junction is pivotal for an efficient solar-to-energy conversion system. Similar results were also obtained by a system of NiO/SiO_x/Si corroborating that this approach can be used as a general strategy to optimize the solid-solid interface of Si-TMOs photoanodes.

Considering the feasibility of the fabrication process using the versatile and highly scalable technique of sputtering deposition this work paves the way for enhancing the efficiency of Si-TMO photoanodes and potentially accelerates the transition to renewable hydrogen fuel production.

"The development of large-scale, binder-free photo-catalyst panels, resembling solar panels, is highly appealing, which,along with efficient light management and easy recycling, offers great simplicity by configuring them in a flow-reactor. Here, we report a facile and scalable method for binder-free flat and porous melon-type polymeric carbon nitride (CN) panels with tunable structural and photophysical properties. The CN panel exhibits good photoactivity in a homemade bench-scale reactor for several important catalytic reactions: the production of hydrogen peroxide in concentrations that meet industrial requirements; biomass-derived 5-hydroxymethyfurfural (HMF) oxidation to 2,5-diformyl-furan (DFF) under visible light, resulting in a high HMFconversion rate of 1185 μmol h^−1 g^−1, achieved in heterogeneous photocatalysis with a DFF yield of 13% after 24 h under continuous-flow conditions (1 mL min^−1); and hydrogen production. The binder-free deposition method and its good photocatalytic activity broaden avenues for CN photocatalyst-based and other semiconductor panels toward multiple energy-related applications."

Cost-effective and efficient photoelectrochemical (PEC) water splitting stands out as one of the most promising strategies to address sustainable energy supply in the form of green H₂. However, large-area photoelectrodes and electrocatalysts featuring fine chemical and morphological control are key for effective industrial deployment. Consequently, up-scaling of laboratory synthetic strategies to an industrial relevant scale is mandatory. In this presentation, we will discuss two different approaches for the development of large-area (photo)electrodes. On the one hand, the preparation of BiVO₄ nanoparticles with a continuous flow-system method,^[1] and their subsequent deposition onto large-area conductive substrates as water splitting photoanodes will be discussed. BiVO4 photoanodes are the most promising platforms for upscalable photoelectrochemical water splitting.^[2] The proposed system allows to up-scale the synthesis through a microreactor and provides an affordable methodology for the fabrication of cost-effective, large-scale BiVO₄ photoanodes with areas up to 50 cm² and competitive performance. On the other hand, we will discuss a low-temperature, straightforward, and fast synthetic method with an extraordinary ability to in situ generate homogeneous NiFe-based electrocatalytic thin films from a precursor solution compatible with different industrial printing techniques such as inkjet printing and slot-die coating.^[3] Remarkably, this in situ synthesis approach can be easily integrated into scale-up processes like roll-to-roll printing for large-area and cost-effective mass production. The OER activity of the as-synthesized electrocatalysts was promising and the method is fully compatible with deposition onto metal oxide photoanodes.

The electrochemical reduction of CO₂ (eCO₂R) is a promising way to convert detrimental CO₂ emissions into sustainable fuels and chemicals, and thus to achieve a net zero or net negative carbon economy_.¹ Depending on catalyst type and reaction conditions, different gaseous and liquid products can be obtained during eCO₂R, with their production rates significantly varying over time. Thus, for any catalytic performance analysis, it is key to assess the product distribution online during eCO₂R_.^2,3 However, while gaseous products are readily analyzed online by gas chromatography, liquid products are typically only assessed at the end of the reaction due to the lack of suitable automated liquid sampling and analysis methods. In addition, relevant reaction parameters such as CO₂ flow rates, electrolyzer temperatures and flow pressures are seldom recorded, causing the loss of important information on catalysts, electrodes, and electrolyzer behavior.

In this work, we overcome these issues by assembling a comprehensive analytical system coupling online gas and liquid product analysis by gas and liquid chromatography (leveraging a special automated liquid sampling valve) with electrochemical protocols that assess CO₂RR performance, electrolyzer cell resistance and electrode surface area. In addition, we record CO₂ volumetric flow rates, electrolyzer temperatures, as well as gas and liquid flow pressures_.⁴ To rapidly and reproducibly handle the large and heterogeneous data volume obtained we implement a standardized data pipeline based on our own open-source software,⁵ which automatically parses the numerous different raw data files, composes a data set following FAIR practices,⁶ and post-processes and plots the data in a standard way. We validate this analytical system by carrying out eCO₂R at 200 mA/cm² on Cu gas diffusion electrodes, following the changes in selectivity with reaction time for > 12 gaseous and liquid products while recording volumetric flow rates, electrolyzer cell resistance, electrode surface area, electrolyzer temperatures and flow pressures. The modular nature of our analytical system, combined with the standardized data pipeline, allows us to freely increase the number and type of sensors used with minimal impact on the data analysis time, as well as to multiplex our analysis to 8 parallel electrolyzer cells, providing a deep understanding of the function of eCO₂R catalysts, electrodes, and electrolyzers, and paving the way to accelerated discoveries.

References

[1] Senocrate, Battaglia, J. Energy Storage 2021, 36, 102373.

[2] Wang, de Araújo, Ju, Bagger, Schmies, Kühl, Rossmeisl, Strasser, Nat. Nanotechnol. 2019, 14, 1063. [3] Löffler, Khanipour, Kulyk, Mayrhofer, Katsounaros ACS Catal. 2020, 10, 6735.

[4] Senocrate, Bernasconi, Kraus, Plainpan, Trafkowski, Tolle, Weber, Sauter, Battaglia, Nat. Catal. 2024, 7, 742.

[5] https://dgbowl.github.io

[5] Kraus, Vetsch, Battaglia, Journal of Open Source Software 2022, 7, 4166.

(Photo)electrocatalysis has emerged as a promising process to store renewable energy into fuels and high added-value chemicals to decarbonise the energy and fine chemical sectors. As such, the water splitting process to generate H₂ and O₂ is of particular interest, where the oxygen evolution reaction (OER) is the bottleneck of this process, especially when using Earth-abundant metal oxide catalysts. The efficiency of these catalysts, in particular towards scaling them up, does not only depend on the nature of the metal oxide, but also on their physical characteristics such as composition, magnetic susceptibility and doping variations, and even their nanostructure amongst others. Additionally, replacing water for small organic molecules as electron donor to increase the efficiency of H₂ production concomitant to high added-value product synthesis has been attracting increasing attention.

Such variations in efficiency are closely related to the nature and availability of active sites, whose characterisation is not trivial. In this talk, I will present recent advances on the use of in-situ spectroscopic techniques, in particular UV-Vis and XAS spectroelectrochemistry, to shine light into the kinetics and reaction mechanism of OER and glycerol oxidation.  

Solar fuels are expected to play a major role for the decarbonisation of the industrial and energy sectors. The European Green Deal identifies electrolytic green hydrogen powered by renewable energy (zero emissions) as a cornerstone for the energy transition. The latter is technologically feasible, but limited due to high production costs. Optimistic forecasts do not envision competitive green hydrogen prices for the next decades. Alternative strategies towards low-cost solar fuels would be welcome, and many scientific efforts are devoted to this task. It is still unclear if the most obvious electrolyzer+renewable electricity (EL+PV) tandem can be surpassed in technoeconomic terms by more simple, compact, and well-integrated photo-electrocatalytic or photocatalytic devices. However, the role of co-catalysts will be essential and crucial to reach their respective goals. The challenge becomes even more complex when the expected technologies will require high selectivity in, for example, a CO₂ reduction cycle to yield the desired products.

Regardless of the preferred architecture, or the desired final product, the production of renewable fuels will require electrons and protons, which must be extracted from a substrate, via oxidation. Thus, oxidation catalysis will remain a major limiting factor for the realization of solar fuels. This has been well understood in the case of water electrolysis, where the oxidation catalysts have been designed to offer low energy losses and fast kinetics, being adapted to the optimum reaction conditions imposed by the cathode. But the simple hydrogen evolution reaction (HER) cannot compare to C- or N- based fuels in complexity and in limitations imposed by operation conditions. In this talk, we will discuss some oxidation catalysts for the realization of solar fuels developed in our labs, and their application into scalable, and efficient photoelectrocatalytic architectures [1].

The advancement of cost-effective water-splitting systems for hydrogen production has emerged as a strategic focus to mitigate climate change and energy crisis. This prioritization follows the U.S. Department of Energy's (DOE) set cost targets of 2$/kg_H2 in 2026 and 1$/kg_H2 in 2031. Photovoltaic systems integrated with electrolyzers, and photoelectrocatalytic water-splitting techniques, are sustainable approaches for harnessing renewable energy to produce hydrogen. Nevertheless, the projected levelized cost of hydrogen (LCOH) from PEC systems (6.32 $/kg_H2) or PV-EC integrated systems (3.86 $/kg_H2) ^[1] remains uncompetitive compared to the 1.06- 1.64$/kg_H2 via traditional steam methane reforming (SMR) ^[2] and is far away from DOE cost targets.

Supported by the HydroGEN Advanced Water Splitting Materials Consortium, established under the U.S. DOE’s Energy Materials Network through the Office of Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office, we have developed a robust set of device- and system-level performance metrics to effectively compare the level of PEC and PV+EC development between lab-scale experiments and larger demonstration systems. We found that although numerous studies reported promising solar-to-hydrogen (STH) efficiencies (e.g. >10%), none come withing an order of magnitude of projected lifetime metrics. Clearly, durability of photoelectrochemical systems has become the main barrier to meeting the DOE's target for hydrogen production.

Motivated by the durability challenge, we have been operating a long-term outdoor PV-EC system, assembled from commercially available and scalable components, to study the effects of environmental parameters on the performance and the system durability of hydrogen generation. This system consists of a silicon minimodule with illuminated area of 69.3 cm² coupled with a proton-exchange-membrane Pt/C-based electrolyzer with initial current of 491 mA at 1.86 V and active area of 2.9 cm². Initial performance results from 18-day continuous outdoor operation reflecting PV-EC water-splitting activity will be presented. Additionally, an initial diagnosis and modeling analysis of effects of environmental factors, corrosion resistance and electrolyte circulation on hydrogen generation performance, will be discussed.

[1] Energy Fuels 2024, 38, 13, 12058–12077

[2] Ind. Eng. Chem. Res. 2024, 63, 16, 7258–7270

AI-accelerated synthesis is an emerging field that uses machine learning algorithms to improve the efficiency and productivity of chemical and materials synthesis. Modern machine learning models, such as (large) language models, can capture the knowledge hidden in large chemical databases to rapidly design and discover new compounds, predict the outcome of reactions, and help optimize chemical reactions. One of the key advantages of AI-accelerated synthesis is its ability to make vast chemical data accessible and predict promising candidate synthesis paths, potentially leading to breakthrough discoveries. Overall, AI is poised to revolutionise the field of organic synthesis, enabling faster and more efficient drug development, catalysis, and other applications.