Tailoring and elucidating the structure of the electrified interface and the electrocatalytically active sites at the atomic and molecular levels is key to designing advanced materials for sustainable energy conversion and production of renewable fuels and chemicals. This talk will focus on recent strategies to understand and tune the structure-activity and structure-selectivity relationships for different electrocatalytic reactions of interest to produce renewable fuels. These reactions include oxygen evolution for green hydrogen production, electrochemical carbon dioxide conversion, and methane conversion into liquid fuels.

First, I will present our work toward understanding and tuning the structure-activity relations on Ir-based oxides for oxygen evolution in acidic electrolytes. Then, I will show our model studies on well-defined Cu-based surfaces to assess the interfacial properties of the electrochemical CO2 and CO reduction reactions. Finally, I will discuss some strategies for selective oxidation reactions including the electrochemical methane activation and conversion on metal oxides to produce liquid fuels such as methanol.

BiVO4 has emerged as one of the most promising materials to fabricate efficient photoanodes for photoelectrochemical(PEC) solar water splitting. BiVO4 is an n-type semiconductor, with a 2.4 eV bandgap and a theoretical solar to hydrogen(STH) efficiency of 9.2% with a maximum photocurrent of 7.5 mA cm2 under AM 1.5 G illumination, low overpotential andfavourable band-edge positions towards the Oxygen Evolution Reaction (OER).
However, BiVO4 also presents poorelectron transport, high surface recombination and slow water oxidation kinetics. Hence, enormous efforts have been madein the past few years to mitigate these drawbacks through different approaches such as nanostructuring, doping, heterostructuring, the employment of post-synthetic treatments and the use of efficient co-catalysts.

The present study proposes two different strategies for boosting the water oxidation performance of BiVO4 photoanodes: i) a laser treatment and ii) a transition metal doping (Ni, Fe and Co). The origin of this enhanced performance towards Oxygen Evolution Reaction (OER) through these two efficient routes was studied by a combination of a suite of structural, chemical, and mechanistic advanced characterization techniques including Electrochemical Impedance Spectroscopy and Transient Absorption Spectroscopy, among others.

With the introduction of high-entropy oxides (HEO) as a novel class of materials, unexpected and interesting properties have emerged. A HEO consists of five or more ions occupying a single crystallographic site and inducing a high degree of configurational disorder, which increases the entropic contribution to the Gibbs free energy of formation, thus stabilizing their crystallographic structure. Significant efforts have been devoted to the development of new HEO phases; however, the large majority of synthetic approaches are based on solid-state, rather than sol-gel chemistry allowing only the preparation of micrometer-sized, low-surface-area particles. Sol-gel chemistry requires precise control of reaction kinetics in order to form uniform structures, which is most likely the reason why the preparation of ordered mesoporous HEO thin films by the soft-templating and evaporation induced self-assembly (EISA) approach has not been reported yet.

The presentation informs about sol-gel synthesis of (ordered) mesoporous (CrMnFeCoNi)₃O₄^[1] and (CoNiCuZnMg)Fe₂O₄^[2] high-entropy spinel oxides prepared by dip-coating and EISA process. A synthetic route was developed, utilizing the unique copolymer (poly(ethylene-co-butylene)-block-poly(ethylene oxide), known as KLE, in order to obtain periodically ordered and 15−18 nm sized mesopores within the high-entropy ferrite (HEF) framework.^[2] The meso-structured HEF electrodes were found to be crack-free on the nano- and macroscale. Time-over-flight secondary ion mass spectrometry and electron microscopy verified a homogenous distribution of all elements within the structure. The fundamental impact of a nanoscale frame-work on the photoelectrochemical and electrocatalytic properties was investigated: mesoporous HEF applied as both n-type photoanode and oxygen evolution cocatalyst for solar water oxidation, showed near-metallic electric conductivity, which was related to an electron hopping mechanism induced by the interaction of 3d-states of the inserted transition metals, and was found to improve performances. The photoresponse of HEF photoanodes was limited owing to severe surface recombination as evidenced by intensity-modulated photocurrent spectroscopy. The novel high-entropy nanostructures can be considered as interesting candidate for energy conversion applications

The energy crisis and climate change are two of the most critical challenges facing society today. The ongoing reliance on fossil fuels has significantly contributed to global warming, necessitating an urgent transition to clean, renewable energy sources such as solar and wind. In this context, (photo)electrocatalysis has emerged as a transformative technology with the potential to decarbonize key sectors, including energy, industry, and transportation. By enabling the production of sustainable energy vectors and added-value chemicals with minimal environmental impact, (photo)electrocatalysis offers a viable path toward achieving a carbon-neutral future.

In this talk, we will explore some of the key challenges facing (photo)electrocatalytic technologies, providing a broad overview to establish the current state of the field. We will also present research conducted by our group on advanced materials, electrodes, and device architectures designed to improve the efficiency, stability, and sustainability of energy conversion schemes and synthetic processes. A central focus of our work is the fundamental understanding of the processes that govern device operation. To this end, we utilize a comprehensive suite of spectroscopic tools to probe the underlying mechanisms, providing valuable insights into the factors that drive performance and efficiency.[1]

Examples from our research will highlight advancements in material design (specifically metal oxides, organic semiconductors and halide perovskites),[2], [3] catalytic systems (Ni, Fe, Co, Cu based catalysts), [4], [5] and the integration of innovative device configurations (tandem architectures).[6], [7] By addressing both applied and fundamental aspects, this presentation aims to provide a holistic view of the potential of (photo)electrocatalysis to tackle global energy and climate challenges effectively.

Photothermal catalysis may provide highly efficient conversion of CO₂ to value-added products. Here, both the principles of heterogeneous photocatalysis and ‘classic’ thermocatalysis are combined to achieve high reaction rates and to possibly open up new chemical pathways [1-3]. Photothermal catalysis can be realized by depositing specific metal nanoparticles on a support material, typically a semiconductor. Such nanoparticles can absorb a broad part of the solar spectrum, including visible light. As a result, plasmonic or non-plasmonic effects will result in the conversion of the absorbed photon energy in high energetic charge carriers. These charge carriers are either injected into the photocatalytic particle, or they thermalize and thereby release localized heat to the environment. This causes an increase in (photo)catalytic performance of the photothermal system. To enhance the activity even further, external heating can be used as well.

In this study, we demonstrate that we can successfully photothermally convert CO₂ and H­₂ into CH₄ (i.e., the Sabatier reaction) using Ru/TiO₂ catalysts, where Ru nanoparticles are deposited on a TiO­₂ support. By raising the external temperature mildly, we see a significant increase in light sensitivity of the particles, with the effect being strongest in the range of 180 – 200 ^oC. A further increase in temperature yields a shift in selectivity towards CO production. Interestingly, we see photothermal activity not only under UV-vis illumination, but also clearly under visible light illumination. To understand the photophysics, we performed time-resolved photoluminescence (PL) spectroscopy experiments [4]. We studied the behavior and migration of charge carriers within the system when either the TiO­₂ is dominantly photoexcited by 267 nm, or when the Ru is dominantly photoexcited by 532 nm visible light. Additionally, we performed diffuse reflectance infrared Fourier Transform (DRIFT) spectroscopy studies to understand the surface chemistry taking place during the photothermal methanization of CO₂. Based on our findings, we propose a model how these physical and chemical aspects are connected in the photothermal methanization of CO₂ using Ru/TiO₂ nanostructures. We will also discuss strategies how these insights can be used to modify these structures to yield even higher activities and selectivities.

Recent advancements in the molecular design of organic semiconductors have significantly boosted their optoelectronic properties, achieving notable gains in organic photovoltaic device's efficiency and, more recently, serving as photocatalysts for solar-to-fuel production. [1] In my talk, I will discuss the challenge of the solar-driven synthesis of sustainable fuels and the potential role of organic semiconductor photocatalysts for artificial photosynthesis. I will then introduce our recent investigations on the correlation of the charge carrier dynamics and photocatalytic efficiency of polymer nanoparticulate systems by combining advanced optical transient spectroscopies over twelve orders of magnitude in time and using pulsed and continuous light illumination. [1,2] I will focus on single conjugated polymer nanoparticles and donor/acceptor bulk heterojunction nanoparticles photocatalysts, addressing the correlation between charge carrier lifetime and photocatalyst performance for hydrogen production, and discussing the similarities and differences between the function of such organic nanoparticle photocatalysts and thin films fabricated from the same materials.

Solar energy is the largest exploitable source of renewable energy [1], making the development of new photocatalysts for artificial photosynthesis conversion crucial [2]. One of the materials that has attracted much attention for energy applications are the Covalent Organic Frameworks (COFs). They are crystalline polymers with ordered channel structures, relatively large pore apertures and high chemical and thermal stabilities [3].

In this work, we described the first COF based on BOPHY (IEC-2), along with two new COFs based on BODIPY (IEC-3 and IEC-4), ever reported in the literature. IEC-2 was employed as platform to prepare TiO₂ based hybrid photocatalyst (10 wt% of COF loading) for solar driven hydrogen evolution reactions. The performance of this hybrid photocatalyst (IEC-2@T10) was studied at both lab scale and pilot plant scale under natural sunlight. The hybrid heterojunction results in an enhancement of the photonic efficiency, increased by 36% with respect to benchmark TiO₂ [4]. 

In future studies, the process will be replicated using the new BODIPY-based COFs (IEC-3 and IEC-4) to investigate their photocatalytic behavior in the hydrogen evolution reaction.

Photocatalytic hydrogen production from water is a research area of immense interest as hydrogen has been identified as a potential energy carrier of the future. Most of the studied photocatalysts are inorganic and organic materials have been far less studied. Organic polymers have the potential advantage of being easily tunable through the use of a range of chemical building blocks.

Here, I will present our work on the application of conjugated materials as photocatalysts for hydrogen production from water. [1-3] We have used a range of different techniques that helped us to gain understanding of the properties that are important for the materials performance, including transient absorption spectroscopy. [4,5] Sacrificial water oxidation [6] and non-sacrificial overall water splitting [7] were also studied which shows the potential of these materials for the future clean energy generation. A focus of the presentation will be on processing of polymers into films and onto substrates, which will enable the fabrication of scale up devices going forward.[1,2,4]

Electrocatalysis is currently at the heart of the transition to sustainable management of energy and chemical production schemes. Processes such as H2 production, CO2 conversions to carbon fuels, or O2 selective reduction to H2O2 or H2O or water through electrocatalytic approaches are very popular on account of the compliance with new guidelines on sustainable chemistry. Development of high-performance catalysts for the specific reaction is one of the key steps for a realistic implementation of such schemes. Typically, useful levels of activity and selectivity are achieved by including metals in the formulation of the catalytic material. However, a sensible use of metals is one of the new recommendations of European Union on account of recent raw critical materials analysis.[1] Carbon nanostructures (CNSs) can serve as useful conductive supports to boost the activity of metal phases in electrocatalytic reactions. This is possible because of the synergistic electronic interfacial effects between the carbon and the inorganic phases. In particular, the electron collection ability of CNSs can be exploited to increase the electron density of the metal surface states, improving charge transfer kinetics.[2] The promoting effect of CNSs can be so large to allow of reducing the metal loading to single metal atoms, or even to perform metal-free electrocatalysis using the CNS directly as catalyst.[3] By this means, metal loadings can be decreased without compromising electrocatalytic performance.  In this presentation, a few examples will be discussed to illustrates the principle behind the strategy of integrating CNSs in electrocatalyst formuation, focusing on O2 and CO2 reduction and H2 evolution.

Heterogenous photocatalysis is an attractive enabler of the efficient execution of different important environmental (e.g., production of hydrogen and C-based fuels) and organic reactions utilizing solar energy. In a typical batch reactor, a heterogenous photocatalyst is dispersed in a given solution together with reactants, and upon illumination, a chemical reaction occurs. Despite its simplicity, the overall process faces some challenges related to the intrinsic nature of the reaction conditions: light penetration significantly decreases with distance, the photocatalyst should be continuously stirred to avoid sedimentation, separating the products from the photocatalyst is not trivial, the catalyst is hard to recycle, the reaction is dependent on the concentration of starting reactants, and scalability is questionable.

An alternative approach is to use a panel based on a photocatalytic material. This configuration improves light management and enables easy recycling and scalability, similar to that of solar cells. Furthermore, the photocatalyst panel can be easily incorporated into a continuous-flow reactor, facilitating constant reactant feed and product separation. In recent years, polymeric carbon nitride (CN) materials have emerged as a class of photocatalysts for many reactions, from solar fuel production to biomass conversion and complex organic transformations. Nevertheless, most studies have focused on using CN powders as heterogeneous photocatalysts. Some pioneering works showed the utilization of CN panels, mainly for H₂ production. However, all these panels were prepared by drop casting or screen printing a synthesized catalyst with a polymer containing perfluoro groups (i.e., Nafion) or a SiO₂ binder on frosted glass or steel plates. The use of a binder can lead the photocatalyst to detach from the substrate because of the formation of radicals during the reaction (e.g., reactive organic species, ROS); this happens mainly in organic chemical reactions, where the ROS intermediates might react with the binder and give unwanted side products.

This talk will introduce a facile and scalable method for binder-free melon-type flat and porous polymeric carbon nitride (CN) panels with tunable structural and photophysical properties. The CN panel exhibits excellent photoactivity in a homemade bench-scale reactor for four important catalytic reactions: the production of hydrogen peroxide in concentrations that meet industrial requirements, biomass-derived 5-hydroxymethyfurfural (HMF) oxidation to 2,5-diformylfuran (DFF) under visible light, resulting in 75% HMF conversion with a DFF yield of 13% after 24 h under continuous-flow conditions (1 mL min^–1); continuously-monitored water purification via UV–vis spectroscopy; and hydrogen production. The binder-free deposition method and its excellent photocatalytic activity broadens avenues for CN photocatalyst-based and other semiconductor panels towards multiple sustainable energy-related applications.

Photocatalytic water-splitting is a renewable way to store solar energy under chemical energy and, at the same time, produce alternative fuels. The photoconversion of water consists in a reduction half-reaction that produces H₂ and an oxidation half-reaction that produces O₂. O₂ has limited applications (e.g. production of high-purity O₂ for medical purposes), so the performance of the overall process depends only on the H₂ production part. The amount of H₂ produced and the efficiency of the photocatalysts are critical in amortising the environmental and financial costs of the device.

Alternatively, the Marie Skłodowska-Curie OMATSOLFUEL European project explores materials for the photoconversion of industrial effluents rich in sugars (e.g. dairy or brewing effluents). Such an alternative reaction simultaneously produces H₂ and high value-added co-products (e.g. arabinose, erythrose, lactobionic acid), distributing the efforts on both half-reactions. Depending on the quantity, the purity and the cost of the molecules, they can be sold to help achieve a solar H₂ production cost of 1 $/kg [1].

The challenge is therefore not only to design more efficient materials that reduce protons in H₂ and achieve the highest solar to hydrogen value, but also to design materials that are efficient and selective for reduction and oxidation. Proof of concept for photoconversion of sugar-rich reactive mixtures already exists, but more comprehensive studies are needed [2], [3]. Fine structure-activity relationships need to be established between the intrinsic properties of the materials and their photocatalytic activity.

Therefore, we have been working on TiO₂-based model photocatalytic systems; thin films prepared by plasma-based techniques (i.e. magnetron sputtering (MS) and plasma-enhanced chemical vapour deposition (PECVD)). These techniques provide access to a wide range of experimental parameters that can be used to tune the structural, electronic and morphological properties of the photocatalysts.

For example, we studied the impact of the TiO₂ thickness on its catalytic activity for the photoconversion of glucose. The TOC image shows a clear correlation between this thickness and the photocurrent that has been measured for the photoconversion of a Na₂SO₄/glucose mixture. This finding is consistent with a previous work about the degradation of methylene blue with TiO₂ films prepared by PECVD [4].

We are currently investigating two approaches to boost the photocatalytic performance of the thin films. On the one hand, gold nanoparticles have been deposited by MS. The plasmonic effect is expected to increase visible light absorption and the overall photocatalytic activity. By tuning the power of the plasma or the Ar pressure, we can adjust the density and the morphology of the gold nanoparticles. On the other hand, by using a mixture or Ar/O₂/N₂ during the MS procedure we were able to prepare TiO_2-xN_y thin films with different electronic, structural and morphological properties. The experimental parameters were studied in relation to the catalytic activity measured for the photoconversion of sugars.

Simultaneous thermal and photonic activation has provided interesting opportunities for solar upgrading of catalytic processes [1]. Photothermal catalysis works at the interface between photochemical processes, in which photon energy is converted into chemical energy, and thermal catalysis, with the catalyst activated by temperature. This combined catalysis is particularly promising for the activation of small, unreactive molecules at moderate temperatures compared to thermal catalysis and with higher reaction rates than those attained in photocatalysis. CO₂ is an archetype of this kind of molecule: its sustainable conversion into fuels or chemicals is one of the major challenges of modern chemistry. Among the many suggested catalyst formulations, those featuring metals dispersed on semiconductors show promise. These systems offer multiple photonic and thermal pathways for electron transfer, which can be exploited to regulate both the rate and selectivity of the reaction. We report here the photothermal catalytic hydrogenation of CO₂ using systems with selectivities: Cu/ZnO/Al₂O₃ and Ni/TiO₂. Cu/ZnO/Al₂O₃ is a common catalytic system for methanol synthesis that, containing a photocatalytically active phase like ZnO, shows great promise for photothermal activation. In turn, Ni is a low-cost alternative to noble metals for hydrogenation reactions. Ni/TiO₂ catalysts can achieve high performance and, interestingly, it has been found that metal support interactions can strongly modify the selectivity under photothermal conditions, changing the proportion of CO and CH₄ generated [2].

The main products of the thermal reaction over Cu/ZnO/Al₂O₃ are CH₃OH and CO. Cu content modulates the reaction: increasing Cu content reduces CO₂ conversion, while intermediate Cu loadings lead to higher CH₃OH selectivities. Light exerts a positive effect on activity and selectivity, improving CO₂ conversion and favouring CH₃OH production. The observed changes cannot be explained by a thermal effect of light, which suggests it also induces charge transfer processes that favour CH₃OH production. From in-situ XAS during TPR, it is deduced that increasing Cu wt.% leads to a faster reduction and lower amount of Cu+ intermediate species. NAP-XPS under reaction conditions shows light-induced charge transfer between Zn and Cu centres. Analysis of the surface species suggests that CO₂ adsorbs via carbonate/bicarbonate species and that CO₂ hydrogenation proceeds through the formation of CO₂^-  species upon charge transfer from Cu centres.

Good dispersion of Ni over TiO₂ is confirmed by the presence of clusters of Ni of about 5 nm seen by XEDS mapping for the catalyst with the higher metal loading. Ex-situ Ni 2p XPS spectra of the samples afer reaction show in the two samples a major contribution of Ni²⁺ with a small amount of metallic Ni, consistent with metal passivation in air. Similar Ni²⁺ contributions have been related to Ni²⁺ interacting with TiO₂ supports or coordinatively unsaturated Ni²⁺ in NiO. The activity increases with the metal content. Thus, at 350 °C and under UV irradiation the conversion of CO₂ for 10%Ni/TiO₂ is more than 10 times higher than for the catalysts lower loading. At this temperature the enhancement of the activity by UV light is modest, but at 150 °C reaches more than 8%. The Ni content has high impact on selectivity. At 250 °C, the CH₄ selectivity is about 72 % for 10%Ni/TiO₂, while it is lower with lower metal loading, which yields mainly CO [3].

This work demonstrates that the combination of light and heat has a synergistic effect on catalytic CO₂ hydrogenation: light produces both thermal and photonic effects that modify the reaction mechanism. Photothermal activation gives rise to new mechanistic routes and new possibilities for selectivity control.

Ammonia (NH₃) is a cornerstone of modern society, serving as the basis for all nitrogen fertilizers, which sustain nearly half of the world’s population [1]. Furthermore, thanks to its high energy density (4.32 kWh L⁻¹ for liquid NH₃) and ease of liquefaction, ammonia is emerging as a potential renewable energy carrier and fuel for decarbonization efforts. However, current NH₃ production relies heavily on the Haber-Bosch process (HBP), which is highly energy-intensive, consuming 1–2% of global fossil fuel supplies and accounting for approximately 2% of worldwide CO₂ emissions. This underscores the urgent need for sustainable and decentralized NH₃ synthesis technologies. [2]

Electrochemical nitrogen and nitrate reduction reactions (E-NRR and E-NO₃RR) have garnered significant attention as greener alternatives to the HBP [3]. These processes enable the utilization of renewable electricity and the on-site, on-demand production of ammonia. Additionally, nitrate (NO₃⁻), a widespread pollutant in groundwater due to its high solubility, can be converted into valuable NH₃ via E-NO₃RR. However, both E-NRR and E-NO₃RR face challenges, including low production rates, insufficient Faradaic efficiencies, and high overpotentials. These limitations present intriguing opportunities for research and development.

The main components influencing the overall system performance are the catalyst, the electrolyte, and the reactor; thus, a comprehensive understanding of their interplay is crucial to advancing E-NRR and E-NO₃RR technologies. Despite recent advancements, issues related to reproducibility and scalability remain significant obstacles.

Electrification of industrial chemical processes is on its way to being fully integrated into the existing chemical plants. In this context, hydrogen (H₂), one of the main utility gases, can be easily produced through water electrolysis or, even more promisingly, through water photo-electrolysis. According to the International Energy Agency (IEA), the demand for green hydrogen should rapidly increase in the near future, sustained by the large number of projects and investments being conducted worldwide [IEA (2023), Net Zero Roadmap].

The use of a photo-electrochemical (PEC) cell, rather than a conventional water electrolyser, is a valid alternative to meet the increasing green H₂ demand. Basically, a PEC cell is an improvement of a classic electrochemical cell by replacing one of the two electrodes with a semiconductor material capable of absorbing visible light to drive oxidation or reduction reactions. The main advantage of PEC technology lies in its ability to directly convert sunlight into chemical energy (e.g., H₂ or carbon solar fuels), avoiding the step of converting sunlight into electricity by a photovoltaic system, an advantage in terms of process intensification.

Despite its potential, the PEC approach is a novel technology that requires further development before being commercially viable. For CO₂ reduction reaction (CO₂RR), the integration of a photovoltaic system with an electrochemical reactor remains the more efficient route [1]. This contribution discusses the key aspects and current limitations of PEC cells, providing insights into critical parameters that must be considered for a correct evaluation of the process efficiency. The limitations of PEC cells will be assessed and step-by-step strategies will be proposed to mitigate or overcome these challenges, with the ultimate aim to enhance the overall efficiency of the process.

The discussion begins with the selection of the photoactive material, starting from titanium dioxide (TiO₂) being the most widely studied in the literature due to its suitable bandgap (3.2 eV), low toxicity, corrosion resistance, and abundance [2]. However, TiO₂ faces significant drawbacks that strongly limit its efficiency: 1) poor absorption of visible light, as it primarily absorbs only the UV portion of the solar spectrum, and 2) a high charge recombination rate. Through advanced synthesis techniques and electrode engineering, we will show how these limitations can be addressed to improve TiO₂ performance. The selection of different materials than TiO₂ will also be discussed, as well as the importance of electrode and cell design will be remarked [3]. We will discuss the importance of reactor configuration, specifically how to move from a classic H-type cell to a more advanced membrane electrode assembly (MEA) flow device to significantly reduce diffusivity and resistivity, enabling higher current densities that are more suitable for industrial applications. To achieve higher efficiencies, we examine the potential of novel 2D and 3D architecture electrodes and the possibility of replacing TiO₂ with alternative semiconductor materials that offer better light absorption and charge transport characteristics. Additionally, we will discuss the application of PEC devices for solar fuel production through the CO₂RR, focusing on key parameters to investigate in lab-scale systems to facilitate scale-up for commercialization [4].

Electrolysis technologies such as water splitting, CO2 electroreduction, and other emerging reactions, present sustainable alternatives to power large industries such as transport (fuels), manufacturing (chemical feedstock) and agriculture (fertilizers). The viability of these technologies hinges upon achieving sufficient achieving sufficient performance in metrics such as product selectivity, productivity (or current density), energy efficiency, and stability, at scale. Conventionally, improvements in these reactions have been sought by tuning the electronic and physicochemical properties of (pre)catalysts through compositional and structural modifications. Here, I will show approaches to tune electrocatalytic activity in water-based electrolysis by manipulating precatalyst reconstruction and reaction environment, addressing the liquid-side of the reaction. I will discuss the need of tailored activation protocols and situ and operando spectroscopies at relevant working conditions to achieve reliability in catalyst design and operation. To conclude, I will overview sustainability issues in the scale up and path to market of CO2 electrolysis technologies.

In response to the dramatic worldwide energy demand that we are currently suffering, artificial photo-synthesis (AP) has emerged as a sustainable alternative to the use of fossil fuels. AP systems are able to efficiently capture and convert solar energy and then store it in the form of chemical bonds. Therefore, solar energy induces the water splitting to produce hydrogen, and/or to transform carbon dioxide and water into a renewable source of energy rich carbon containing products. For this purpose, TiO₂ based photocatalysts are widely used owing to its availability at low cost. Unfortunately, TiO₂ is active only under UV light (4% of solar energy), while visible light contributes 43%, resulting in a low AP pathways efficiency. To carry out this process more efficiently, we are witnessing the renaissance of organic polymers (OP) and their participation in hybrid systems with inorganic semiconductors (ISs) in last years. The main advantage of OPs versus ISs ones is their tunable pore size, low density, structural periodicity and on demand chemical functionalization, that allows an excellent optoelectronic and surface catalytic properties [1]. Indeed, OPs and hybrids composites containing OPs have been widely applied in areas such as gas stor-age and separation, heterogeneous catalysis, energy storage and optoelectronic devices.

When designing a hybrid photocatalyst, an important aspect to consider is the type of charge transfer mechanism between both semiconductors, being the rate-determining step of the oxidation-reduction photocatalytic processes. At the same time, slightly molecular structure differences of OPs can lead to the increase of the hybrid transient absorption lifetimes during the charge transfer pathway and so, increasing the photocatalytic efficiency of the AP processes in terms of hydrogen production, CO₂ photoreduction or nitrogen fixation to ammonia (Figure 1). Herein, we demonstrate the usefulness of the photophysical techniques by means Steady-state and Time-Resolved Photoluminescence and well as Transient Absorption Spectroscopy (TAS) from the fs-to-s timescale to establish the mechanism governing the photocatalytic efficiency in hybrids photocatalysts under UV-visible light. The unique interfacial interaction between OP and IS results in longer carriers’ lifetimes of hundreds of ns, also taking into account parameters such as the dynamic quenching efficiency. These findings demonstrate a higher driving force for the electron transfer, which directly lead to an enhanced performance of the hybrids based on OPs and ISs compared to the bare materials [2,3]. These results establish the key between performance-structure relationships and highlight the pivotal role of highly tunable OPs and their respective hybrids with ISs for solar fuels production, as well as for a multitude of light-mediated energy applications.

Photocatalysts can convert CO₂ to useful products such as formic acid and carbon monoxide using visible light. However, most photocatalysts are limited by impractical stability and low conversion rates.

Conjugated polymers have emerged as promising visible-light-active photocatalysts for solar fuel production due to their ease of optoelectronic tuneability and extensive building block scope to name but a few. Similarly, molecular photocatalysts based on metal complexes have been well studied for photocatalytic CO₂ reduction to useful products.

By assembling hybrid photocatalysts[1] consisting of conjugated polymers and a binuclear ruthenium(II)-ruthenium(II) complex, very active photocatalysts were discovered for the conversion of CO₂ to formate with augmented activity compared to previously reported literature. The best system produced a turnover number of 349,000 (one-order higher than the previously reported most durable system),[2] a turnover frequency of 6.5 s^-1 (surpassing that of CO₂ fixation by RuBisCO in natural photosynthesis, ~3 s^‑1),[3] and an apparent quantum yield of 11.2% at 440 nm (the highest amongst hybrid photocatalysts reported to date).[4]

Remarkably, quantitative conversion of CO₂ to formate was achieved at standard conditions, thereby enabling the use of low concentration CO₂ feedstocks (especially flue gas streams which are ~3-13% CO₂ by composition) which is very relevant for practical application. After full consumption of CO₂, further replenishment with more CO₂ feedstock produced a very high concentration of formate up to 0.40 M within the timeframe studied and without significant decomposition of the photocatalyst. The use of transient absorption spectroscopy allowed insightful elucidation of the key structure-activity factors which led to the remarkably high photocatalytic activity in the most active system.

This direct light-driven conversion of CO₂ to highly concentrated formate offers a more sustainable alternative than current feedstocks by enabling a carbon neutral pathway mediated by solar energy in the supply chain.

Owing to their highly conjugated structure and central metal ion, which can readily interconvert between different oxidation states to accomplish oxidation and reduction reactions, metalloporphyrins have been selected by Nature to fulfil the two main catalytic phenomena allowing life, i.e. photosynthesis by chlorophylls and respiration by cytochromes. For efficiency and sustainability considerations, it is highly desirable to employ metalloporphyrins in conductive assemblies for heterogeneous catalysis. Nevertheless, due to the lack of synthetic approach, the design and application of conjugated metalloporphyrin assemblies is a largely unexplored topic in view of the plethora of available metalloporphyrin patterns.

Oxidative chemical vapor deposition (oCVD) was recently demonstrated as a convenient method for the simultaneous synthesis and deposition of metalloporphyrin conjugated polymers [1]. In oCVD, the monomer and a suitable oxidant are both supplied from the vapor phase to a surface on which oxidative polymerisation and doping occur in a single step [2]. Metalloporphyrins possessing free meso-positions [1,3,4] and porphyrins bearing thienyl substituents have both been successfully polymerised using oCVD to yield the formation of formation fused metalloporphyrin tapes and thienyl-bridged metalloporphyrins covalent organic frameworks (COFs), respectively. Importantly in the perspective of practical application, including heterogeneous electrocatalysis, the metalloporphyrin conjugated polymers are readily deposited on virtualy any substrate in the form of smooth and thickness-controlled thin films. In addition, metalloporphyrin conjugated polymers are formed almost independently from their substituents [3,4] and central metal cations [3,4] enabling the engineering of their electrocatalytic properties.

Up-to-date, porphyrin conjugated polymer thin films prepared by oCVD have been successfully investigated for the electrochemical hydrogen evolution reaction (HER) [3], nitrate reduction reaction (NRR), oxygen reduction reaction (ORR), oxygen evolution reactions (OER) [4]. Experimental and theoretical data demonstrate the impact of both the central metal cations [3,4] and substituents [3,4] on the catalytic activities [3,4] and stability [5] of the metalloporphyrin conjugated polymer thin films. The approach reported in this work, also suitable for the preparation of heterometallic porphyrin conjugated polymer thin films [6], circumvents many limitations of solution-based approaches and pave the way to the facile engineering and integration of efficient electrocatalysts from metalloporphyrins.

Conjugated polymers (CPs) are a novel class of highly crosslinked materials with extended π- conjugated systems, predominantly composed of low-cost, earth-abundant elements such as C, N, S, O, and H. Their tuneable optoelectronic and photophysical properties make CPs ideal for catalysis in water splitting and related energy conversion applications. The donor-acceptor (D- A) approach is a powerful strategy for tailoring these properties. By carefully selecting donor and acceptor units, D-A conjugated polymers with low band gaps can efficiently harvest a broader spectrum of solar energy, enhance charge transfer, and improve separation efficiency, making them well-suited for applications such as organic photovoltaics (OPVs), photodetectors, and photoelectrochemical (PEC) water splitting.
In this study, we utilized oxidative chemical vapor deposition (oCVD)—a solution-free, versatile, and scalable thin-film fabrication technique—to synthesize homopolymers from 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBTD) and benzo[1,2-b:3,4-b′:5,6-b″]trithiophene (BTT) by varying the oxidant-to-monomer ratios. DTBTD inherently possesses a donor-acceptor structure, where the benzothiadiazole unit serves as the acceptor and the thiophene groups act as the donor. Additionally, we synthesized copolymers of BTT-DTBTD with different monomer-to-monomer ratios to explore their compositional versatility. The objective of copolymerization was to strengthen the donor component by incorporating benzo[1,2-b:3,4-b′:5,6-b″]trithiophene (BTT) and to explore its impact on the overall electronic properties of the copolymer.
The successful polymerization of DTBTD and BTT via oCVD was confirmed through HRMS, which identified oligomers with up to 12 repeating units in the oCVD polymer films of pBTT, pDTBTD, pBTT-DTBTD—absent in sublimed monomer thin films. SEM revealed distinct morphological differences between the monomer and polymer films, showcasing the transformative effect of polymerization on thin-film structure. UV-Vis-NIR spectroscopy provided insights into the electronic structure of the oCVD films, showing a significant red shift in absorption spectra compared to their monomer counterparts. This shift signifies extended π-conjugation within the polymerized films, a critical feature for efficient light harvesting. XPS data further confirmed the chemical composition and bonding environments within the films, solidifying evidence of successful polymerization.
Energy band diagrams from UV-Vis and XPS data demonstrated the tunability of electronic properties via oCVD. The band gap of pDTBTD decreased from 2.34 eV in the sublimed monomer to 1.34 eV after polymerization, while pBTT showed a reduction from 3.91 eV to 2.76 eV. Copolymerization further adjusted the band gaps of pBTT-DTBTD to a range of 1.71–1.77 eV, depending on the monomer-to-monomer ratios. Photoelectrochemical analysis of the oCVD films under simulated sunlight demonstrated their capability for efficient water reduction. The photocurrent densities measured at 0.331 V vs. RHE were 4.23, 11.9, and 20.83± 2 μA·cm⁻² for pBTT, pDTBTD, and pBTT-DTBTD copolymers, respectively. These values highlight the superior performance of the copolymer, driven by combination of donor and acceptor units.
These results emphasize the potential of oCVD for the synthesis, engineering and integration of donor-acceptor homopolymer and copolymer thin films as scalable, efficient, and environmentally friendly materials for renewable energy applications. This work represents a critical advancement in sustainable energy solutions, combining innovative materials with a green fabrication process to enable high-performance photoelectrocatalysis.

Graphdiyne (GDY) is an emerging two-dimensional (2D) carbon material composed of sp² and sp-hybridized carbon atoms, resulting in a regular and porous structure. GDY, rich in π-conjugated electrons, exhibits remarkable properties such as low weight, high mechanical strength, excellent conductivity, and tunable electronic and optical characteristics [1]. These outstanding features make GDY appealing for various applications, including electronic devices, catalysis and photocatalysis, purification membranes, and energy harvesting [2]. Notably, GDY’s intrinsic band gap (0.7–1.4 eV) plays a critical role in charge carrier mobility, making it particularly attractive for solar-to-chemical energy conversion [3]. Despite its exceptional properties, GDY faces challenges such as low processability and a narrow energy band arrangement, which limit its practical application in photocatalysis. To address these limitations, GDY needs structural modification, such as molecular functionalization, hybridization with metals and metal oxides, and heteroatom doping [4].

Here we present strategies for structural modifications of the GDY configuration, aiming to tailor its photophysical properties. In the first approach, pristine GDY was oxidized to generate oxygen defects, followed by size reduction to form quantum dots (QDs). In a complementary strategy, we synthesized nitrogen-doped GDY to tune its band gap and optical characteristics. The structural properties of GDY, both before and after modification, were characterized using techniques such as high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The experimental and theoretical results demonstrated that introducing oxygen defects or heteroatom doping induces significant changes in the electronic and optical properties of GDY. The engineered materials exhibited exceptional photosensitization when combined with commercial TiO₂-P25 for photocatalytic hydrogen generation. Notably, a hybrid material containing 1 wt% defect-rich GDY-QDs achieved a hydrogen evolution rate of 1322 μmol g⁻¹ h⁻¹, five times higher than TiO₂-P25 alone.

Electrocatalytically driven reactions that produce alternative fuels and chemicals are considered as a useful means to store renewable energy in the form of chemical bonds. in recent years there has been a significant increase in research efforts aiming to develop highly efficient electrocatalysts that are able to drive those reactions. Yet, despite having made significant progress in this field, there is still a need for developing new materials that could function both as active and selective electrocatalysts.

In that respect, Metal–Organic Frameworks (MOFs), are an emerging class of hybrid materials with immense potential in electrochemical catalysis. Yet, to reach a further leap in our understanding of electrocatalytic MOF-based systems, one also needs to consider the well-defined structure and chemical modularity of MOFs as another important virtue for efficient electrocatalysis, as it can be used to fine-tune the immediate chemical environment of the active site, and thus affect its overall catalytic performance. Our group utilizes Metal-Organic Frameworks (MOFs) based materials as a platform for imposing molecular approaches to control and manipulate heterogenous electrocatalytic systems. In this talk, I will present our recent study on electrocatalytic schemes involving MOFs, acting as: a) electroactive unit that incorporates molecular electrocatalysts, or b) non-electroactive MOF-based membranes coated on solid heterogenous catalysts.

Carbon dioxide reduction reaction (CO2RR) is a key technology for the chemical industry in a highly electrified energy scenario. To achieve its economic feasibility, it is necessary to reduce the operating voltage of the electrolyser. To this end, replacing the sluggish anodic oxygen evolution reaction (OER) with the glycerol oxidation reaction (GOR) is an interesting approach that also offers the opportunity to upcycle a low-value product. 
In addition to the need to develop efficient and selective catalysts, the production routes should be easily scalable. In this context, we present electrodeposition as an effective and versatile tool to obtain thin film layers of electrocatalysts on metal foams and microporous layers of gas diffusion electrodes (GDE).
This paper presents the recent efforts of the group in the development of coupled systems. On the one hand, AuCu, CuIn alloys have been successfully deposited on GDEs and tested in continuous flow towards the CO2RR using different electrolytes. The results show that the presence of halides in the catholyte can avoid the undesirable hydrogen evolution reaction and favour the formation of carbon monoxide or formic acid, an effect that is enhanced by the use of binary alloys. On the other hand, NiCo layered double hydroxides [1] and AuIn alloys were deposited on nickel foams to carry out the electrooxidation of glycerol, with an electrode potential reduction of 0.2 to 0.5 V with respect to the OER reaction. The results show a strong dependence of the product distribution on the operating conditions, being feasible to obtain C3 products (DHA or lactic acid) under continuous flow of the electrolyte [2].  Finally, both half-reactions were coupled in a continuous flow reactor at operating current densities up to 200 mA cm-2. 

Solar energy conversion via photoelectrochemical (PEC) cells offers a promising solution to current energy challenges, but significant advancements in materials and cell configurations are necessary. Until now, inorganic semiconductors such as metal oxides have been widely studied due to their affordability, non-toxicity and high stability. However, challenges such as charge recombination, low electron mobility and in some cases limited visible absorption limit their ability to obtain high conversion efficiencies. There are several strategies to improve the performance of photoelectrodes, ranging from the modification of materials to the incorporation of more systems that favor the reaction. In our group we are tackling several of them, such as the modification of electrode surfaces for optoelectronic modification, the incorporation of cocatalysts and, as a more innovative part, the use of organic semiconductors to prepare hybrid electrodes. Organic polymers, particularly conjugated polymers (CPs), show potential due to their light-harvesting and conductive properties. In particular, conjugated porous polymers (CPPs), with their 3D structure, provide improved stability and higher surface area. The combination of CPPs with widely studied materials like TiO₂ that suffer from limited visible light absorption due to their high bandgap energy can enhance the general performance.

In this talk, I will present to the audience different strategies to use CPP as multifunctional layers in photoelectrodes, which has been a challenge due to the normally used methodology to obtain these materials, generally composed of micron-sized particles. In this sense, our group has made an effort to design and develop several routes for the use of these materials in photoelectrochemical systems, in particular nanostructuring and electropolymerisation. In fact, these polymers have proven to be efficient as multifunctional layers since they have light absorption properties while they are good conductors of holes in the case of p and electrons in the case of n. We have been able to observe improved photocurrents and photopotentials in these hybrid electrodes. Furthermore, advanced characterizations have been carried out using electrochemical impedance spectroscopy and transient absorption spectroscopy and it has been shown that in most cases, when well designed, these heterojunctions can promote better charge transfer and longer lifetimes of the photogenerated charges. This finding opens the door to the use of these systems not only in photoelectrochemical devices but in any optoelectronic device where the preparation of quality thin films is of vital importance.

An approach to exploring and developing synthetic pathways based on colloidal and plasma deposition processes combined with rapid photonic heating of high entropy oxide (HEO) materials will be presented – to break present limitations and bottlenecks in achieving stable and efficient photo- and electrochemical Fuel Synthesis. HEOs are a new class of single-phase materials comprising near-equimolar compositions of 5 metal cations or more with remarkable thermodynamic and chemical stability, enhanced properties, and tunable multifunctionalities that can present material properties between the constituent metal-oxides, and entirely new properties. Therefore, interest in HEOs is growing exponentially as materials for energy conversion. However, as photo- and electrocatalytic materials, HEOs are still underdeveloped for two significant interrelated challenges: i) material complexity, which often exceeds the robustness of their in-depth characterization and research and development, and ii) synthetic issues of these ceramic compounds, which involve energy-intensive fabrication processes, potentially limiting control of structural and electronic quality and tunable multifunctionalities. Furthermore, electronic quality is critical in the case of light-absorbing semiconductors for solar-energy conversion (note: when typical conventional heating methods are used, using glass-based transparent conductive substrates is limited due to the substrates’ thermal stability). In light of these challenges, there is a pressing need for innovative approaches to close synthesis gaps and identify pathways for achieving high-quality multi-functional HEOs to advance their research and development. In my talk, I will demonstrate the practicality of our approach using the rare earth HEO materials and the model HEO (MgZnCuCoNi)O for photoelectrochemical and electrochemical conversion (respectively), showing to enhance their properties either as free-standing powder form or thin film (photo)electrodes. In summary, our synthesis methods can successfully address a primary need to focus on novel syntheses and design approaches of disruptive and innovative materials and NextGen devices that meet the chemical and physical requirements for reducing global warming through sustainable development. Furthermore, insights and lessons learned would be strongly transferable to other emerging materials for Photo- and Electrochemical Fuel Synthesis.

Halide perovskite (HaP) solar cells, known for their high voltage efficiency (>70%) and a conduction band minimum with low electron affinity, hold significant potential as photocathodes for cost-effective solar fuel generation. However, their instability in aqueous environments, where they readily dissolve, poses a formidable challenge. To mitigate this, ultrathin Al2O3 layers (< 10 nm), applied via atomic layer deposition, serve as protective barriers against water penetration, albeit with the drawback of electronic insulation. To facilitate selective electron transport through these insulating encapsulation layers, linear conjugated organic molecules, termed "molecular relays," are incorporated in the ultrathin Al2O3 layers. The electronic properties of these molecular relays are verified through conductive probe atomic force microscopy, energy level alignment analysis, and photo-electrodeposition of metal particles (Pt and Ag). Furthermore, a feasibility study of utilizing this composite structure for CO2 reduction was conducted, leveraging the unique characteristics of bromide perovskite-based photoelectrodes. Encapsulated HaP photoelectrodes, when immersed in CO2-saturated aqueous electrolytes, demonstrated a photocurrent of approximately 100 µA/cm² at around -0.32 V versus Ag/AgCl. This work presents a robust approach to enhance the stability of HaP materials in polar, protonic electrolytes, paving the way for their application as photoelectrodes in solar fuel production.

There is a misunderstanding in the PEC community about the actual physical cause of charge separation in semiconductors at non-equilibrium conditions. Briefly, the most common explanation for photocurrent generation in photoelectrodes assumes that electron-hole pair separation occurs near the surface due to the internal built-in electric field created by band-bending at the solid/liquid interface in equilibrium (i.e. dark conditions, no bias applied). This interpretation suggests that the built-in electric field independently drives positive and negative charges toward their respective contacts. However, this view leads to the several discrepancies that will be described in the talk. As it will be discussed, it is the gradient of the charge carrier's chemical potential (i.e. their QFLs under non-equilibrium conditions) that actually drives the charge separation, via the establishment of selective contacts for electrons and holes [1, 2]. The usual FTO/semiconducting photoanode interface is likely acting as a "good enough" electron selective contact, thus imposing enough gradient to the QFLs to drive the charge separation under illumination conditions. It will be discussed how the so-far lack of optimized selective contacts for semiconducting oxides (together with defect trap states and polaron-type of transport) could explain why the observed photovoltage for this class of materials is consistently well below their detailed balance (Schottky-Queisser) limit [3].

In the second part of the talk, the influence of various acidic electrolytes (KPi, K₂SO₄, Na₂SO₄, NaClO₄, and NaNO₃; pH = 2) on the PEC glycerol oxidation over BiVO₄ will be discussed. We observed that BiVO₄ exhibited the following GOR performance trend: NaClO₄, NaNO₃ > Na₂SO₄ > K₂SO₄ > KPi, with the photocurrent in NaClO₄ ∼3-fold of that in KPi [4]. Although our BiVO₄ photoanodes exhibited the highest photocurrent in NaClO₄, the low production rate of GOR products, due to the poor stability of BiVO₄ in this electrolyte solution, made it less promising than NaNO₃. NaNO₃ emerged as the preferred electrolyte for PEC glycerol oxidation on BiVO₄, offering superior performance in terms of photocurrent, stability, and selectivity towards value-added GOR products [4]. Glycolaldehyde was identified as the most dominant GOR product in our study, achieving a selectivity of more than 50% in NaNO₃.

Photoelectrochemical (PEC) water splitting shows promising potential for green hydrogen production. However, it currently lacks the technological readiness to allow commercialisation due to limitations in material performance, particularly in developing a scalable, efficient photoanode. 𝛂-Fe₂O₃ (Commonly known as Hematite), BiFeO₃, and BiVO₄ are all semiconductor photoanode materials with great potential due to their desirable bandgap sizes and positionings, yet they face issues to varying degrees with electron-hole recombination rates and slow charge transport¹. Charge carrier dynamics are significantly influenced by a material's phase purity and morphology. Developing a scalable fabrication technique that promotes high phase purity and favourable morphology is therefore essential. Aerosol assisted chemical vapour deposition (AACVD) is a method of depositing semiconductor thin films that has had relatively little reporting in the field of PEC. Unlike other classes of chemical vapour deposition, AACVD does not require volatile precursors, thus allowing precursors to be more tailored towards producing highly efficient, specifically designed photoelectrodes².

Herein, we report some of the first instances of depositing phase pure 𝛂-Fe₂O₃, BiFeO₃, and BiVO₄ via AACVD. A ‘Universal’ precursor approach was taken, in which a common ligand framework based upon an amino-tris alcohol was utilised on various metal centres. By aligning precursors at molecular level, complementary decomposition pathways could be achieved, enabling the development of dual-source precursors to deposit mixed metal oxides of high phase purity. Each precursor was synthesised under inert conditions, with their structure and purity confirmed via single-crystal X-ray diffraction, ¹H NMR, ¹³C NMR, and elemental analysis. The suitability of each compound as an AACVD precursor was assessed using thermogravimetric analysis (TGA). By studying TGA results, our dual-source precursors were effectively matched together based on compatibility/overlap in decomposition pathway. All three materials were deposited on FTO substrate at a temperature <500^oC, with subsequent annealing in air at 500-650^oC. Compositional analysis of the films was carried out using powder X-ray diffraction, Raman spectroscopy, and energy dispersive X-ray spectroscopy. These techniques confirmed that in all cases, phase pure films of 𝛂-Fe₂O₃, BiFeO₃, and BiVO₄ were produced. Scanning electron microscopy revealed distinctive morphologies: 𝛂-Fe₂O₃ formed as nanoflakes, while BiFeO₃ and BiVO₄ grew as high-surface-area nanorods. Photoactivity was assessed by chopped-light linear scanning voltammetry, with the films displaying impressive activity compared to previous reporting’s for the same materials. Most notably, BiVO₄ achieved a photocurrent density of 1.25 mA.cm⁻² at 1.23 V_RHE, the highest photoactivity reported to date for BiVO₄ films synthesized via AACVD. These results suggest that the high phase purity and optimised morphologies of AACVD-derived photoanodes enhances charge carrier dynamics, leading to improved PEC performance.

Our research group, FRONCAT (www.uam.es/jose.aleman), aims to utilize green synthetic methodologies for the synthesis of molecules of relevant interest. In this regard, photo-induced transformations have been at the forefront of chemical research for many years, yet lately they have received enormous interest. The basis for modern photocatalytic methodologies is set on the transmission of photons to a specific molecule - a photosensitizer, which can be parlayed into the population of the molecule’s excited state. This energy can then be transferred to other substrates via energy or electron transfer, wherein the pairing of excited-state energies and of redox potentials, respectively, of the sensitizer and the reactive substrate is crucial for a successful outcome in photochemical reactions. Our research group has mainly focused in recent times on the development of new photocatalytic reactions,^[1-4] emphasizing primarily the search for more sustainable alternatives, among which the use of flow chemistry is included.^[5-7] In addition, we will show the most recent works in the field of electrochemistry.^[8-9]