Hydrogen is now confirmed as a key component of a CO₂-neutral economy, we need to transition towards. The production of large quantities of hydrogen now requires breakthroughs in finding new catalysts that are efficient, stable and cheap, i.e. based on abundant elements. Indeed fuel formation involves multi-electron multi-proton reactions that are inherently kinetically sluggish. Efficient catalysts can be found in living micro-organisms producing or metabolizing hydrogen thanks to hydrogenases. Catalysis in these enzymes only requires Earth-abundant metal centers, the reactivity of which is enhanced thanks to the presence of basic sites acting as proton relays [1] at their vicinity. Such active sites have been used as an inspiration to design new synthetic catalysts for H₂ evolution [2-4] and oxidation [5-6]. Specification, catalytic platforms with installed proton relays display bidirectional [7] and, in rare cases, reversible catalysis [5]. In this presentation we will show how a detailed molecular electrochemistry study can help understanding and quantifying the role of the protons relays related to these remarkable behaviors.

Hydrogenation reactions are fundamental functional group transformations in chemical synthesis. Initially, research on hydrogenation reactions focused on the use of noble metal complexes based on Ru, Rh, Ir, Pd, Pt and remarkable achievements have been made using such complexes. However, recently the focus shifted towards the use of base metal catalysts, as these metals exhibit a higher availability in earth’s crust.[1] In the first instance, the direct reduction of C=O bonds with H₂ seems to be a very atom and redox economic approach. However, the reaction protocols for base-metal catalyzed hydrogenation reactions require often harsh reaction conditions and high H2 pressure to activate H2 and each ton of H2 that is formed via steam reforming produces about 12 tons of CO₂.[2]

I will present an electrochemical approach utilizing electrons and protons for the catalytic hydrogenation of polar double bonds, such as C=O in CO₂, ketones, and aldehydes.[3] In a proof of principle study we demonstrated that in-situ generated Mn hydride species hydrogenate CO₂ forming formic acid. This protocol was later expanded to aromatic, and aliphatic ketones as well as aldehydes. The method is selective for C=O bonds over the thermodynamically favored hydrogenation of C=C bonds. The competing hydrogen evolution reaction was suppressed successfully, and the reactions proceed with high Faraday efficiencies. A large variety of substrates was accessible by this method and in-depth mechanistic studies gave valuable insights for catalyst improvement.

With a two million ton global production hydrogen peroxide is a valuable product.^[1] Yet it is produced via a non-catalytic and energy intensive anthraquinone process rather than via a sustainable electrochemical procedure.

We have shown that Cu-tmpa can produce hydrogen peroxide upon electrochemical reduction of dioxygen at rates of 1.8 x 10⁶ s^–1  and in significant amounts providing that no mass transport limitations in O₂ occur.^[2] When H₂O₂ starts to accumulate and O₂ concentrations drop overreduction of peroxide to water becomes significant.

We have shown that the rate determining step for O₂ reduction is binding of dioxygen to Cu(I), in line with studies by the Karlin group,^[3] while reduction of H₂O₂ appears to proceed via Fenton-like chemistry, showing similarities with lytic polysaccharide monooxygenase chemistry.^[4] Despite that strongly oxidative reactive oxygen species are likely to be involved turnover numbers exceed 1000 without any sign of catalyst deactivation.

Detailed mechanistic studies, structure-correlation studies, comparisons with Cu-monooxygenase chemistry, DFT calculations, and experiments to boost the hydrogen peroxide production employing flow cell chemistry and gas diffusion electrodes are in progress.

In the early studies, on Oxygenic Photosynthesis, the “quantasome hypothesis” led to seminal discoveries correlating the structure of natural photosystems with their complementary photo-redox functions.[1,2] Indeed, and despite the vast bio-diversity footprint, just one protein complex is used by Nature as the H₂O-photolyzer: photosystem II (PSII). Man-made systems are still far from replicating the complexity of PSII, showing the ideal co-localization of Light Harvesting antennas with the functional Reaction Center (LH-RC). Here we report the design of multi-perylenebisimide (PBI) networks shaped to function by interaction with a polyoxometalate water oxidation catalyst (Ru₄POM).[3-5] Our results point to overcome the classical “photo-dyad” model, based on a donor-acceptor binary combination, with integrated artificial “quantasomes” formed both in solution and on photoelectrodes, showing a: (i) red-shifted, light harvesting efficiency (LHE>40%), (ii) favorable exciton accumulation and negligible excimeric loss; (iii) a robust amphiphilic structure; (iv) dynamic aggregation into large 2D-paracrystalline domains.[5] Photoexcitation of the PBI-quantasome triggers one of the highest driving force for photo-induced electron transfer applied so far.[5-7]

[1] Scheuring, S., Sturgis, J. N. Chromatic Adaptation of Photosynthetic Membranes. Science 309, 484–487 (2005);

[2] Sartorel, A., Carraro, M., Toma, F. M., Prato, M., Bonchio, M. Shaping the beating heart of artificial photosynthesis: oxygenic metal oxide nano-clusters. Energy Environ. Sci. 5, 5592 (2012);

[3] Piccinin, S.; Sartorel, A.; Aquilanti, G.; Goldoni, A.; Bonchio, M.; Fabris, S. Water oxidation surface mechanisms replicated by a totally inorganic tetraruthenium-oxo molecular complex. Proc. Natl. Acad. Sci. 110, 4917–4922 (2013)

[4] Toma, F. M.; Prato, M.; Bonchio, M. et al. Efficient water oxidation at carbon nanotube–polyoxometalate electrocatalytic interfaces. Nature Chemistry 2, 826-831 (2010).

[5] Bonchio, M.; Sartorel, A.; Prato, M. et al. Hierarchical organization of perylene bisimides and polyoxometalates for photo-assisted water oxidation. Nature Chemistry 11, 146-153 (2019).

[6] Gobbo, P.; Bonchio, M.; Mann, S. et al. Catalytic processing in ruthenium-based polyoxometalate coacervate protocells Nature Commun 11, 41 (2020).

[7] GobbatoT.; Rigodanza F.; Benazzi, E.; Prato, M.; Bonchio M. et al. J. Am. Chem. Soc.144,14021-14025 (2022).

Tailoring and understanding the structure of the electrochemical interface at the atomic level is key to elucidating the design principles for the discovery of new electrocatalysts for renewable energy conversion. This talk will focus on our recent work on new catalyst (nano)materials and engineered interfaces for electrochemical energy conversion and storage. In particular, I will discuss structure sensitivity and electrolyte effects for oxygen and carbon dioxide/carbon monoxide electrocatalysis.

First, I will present our work toward understanding and tuning the structure-activity-stability relations for oxygen reduction [1] and oxygen evolution [2], for applications in fuel cells and water electrolysers, respectively. Then, I will discuss our model studies on Cu-based surfaces to understand structure-properties relations for CO₂ reduction [3-4]. We have investigated the effect of pH, specific anion adsorption, and potential dependence for CO reduction on Cu single crystals. We show how well-defined studies are essential to design efficient electrocatalysts to produce renewable fuels.

Catalysis is key in the future development of green chemistry and in the renewable production of chemicals and fuels. However, this transition requires a new approach to catalysis, as central chemical reactions cannot be catalyzed with existing materials. The grand challenge is to discover new catalyst materials, which are both stable and active. We identify three energy conversion reactions, which are corner stones for the green transition and in urgent need of new catalysts.

High Entropy Alloys are solid solutions where five or more elements are mixed randomly together. The realization of HEAs has opened for a vast composition space with a practically infinite number of new not yet explored catalyst materials. We can tune their properties by smoothly change their composition. This has led to the statement that: HEA is a shift of paradigm “from using the materials we have, to engineer the materials we need”. The hypothesis is that among the HEAs there are catalysts with superior stability and activity for the important green reactions.

will present the challenges regarding simulations and prediction of the relation between surface structure and catalytic activity on HEA surfaces. Further, I will show studies where the flexibility of the vast chemical space of HEAs is utilized to predict new catalysts.

Hydrogen and other solar fuels have been highlighted as one of the future energy vectors. Having natural photosynthesis as inspiration, we can develop a device capable to split water using sunlight, obtaining oxygen and hydrogen. [1], [2] Different strategies can be used to achieve this: from separated light harvesting and catalytic systems to all integrated devices able to transform directly sunlight into fuels. In both approaches the catalyst is pivotal to improve the system efficiency. Although rapid progress is being made in the field, understanding of the limiting factors of these catalysts has allowed remarkable improvements in their performance.

In this talk I will discuss different approaches to modify the activity of metal/metal oxide nanoparticles towards redox catalytic processes for water splitting. Nanoparticles are excellent catalysts due to their high surface/volume ratio and electronic and chemical surface properties.[3],[4] Consequently, several strategies can be used to modulate the activity of NP towards catalytic processes. In this talk I will discuss how doping, NP size and surface functionalization effects the catalytic activity of Ni/NiOOH NPs for oxygen evolution in the first case and Ru NP for hydrogen in the other two.[5] I will discuss these effects in terms of electrocatalytic activity and thoughtful characterisation of the system: spectroelectrochemical experiments, XPS, TEM, SEM, TOF-Sims, ATP, etc. All this acquired knowledge will help to systematically improve the next generation of catalysts for water splitting.

The construction of carbonyl compounds via carbonylation reaction using a safe CO source remains a long-standing challenge to synthetic chemists.[1] In this work, we explored the electrosynthetic approach using CO₂ as a CO surrogate in the carbonylation of benzyl chlorides, utilizing a combination of two earth-abundant metal catalysts. As proof of concept, the protocols allow the synthesis of symmetric ketones from good to excellent yields in an undivided cell with non-sacrificial electrodes. The mechanistic studies suggested a synergistic effect between the two metal complexes. Based on the mechanistic studies, we propose the coupling of the electrocatalytic CO formed from CO₂ reduction with the benzyl chlorides activated by the Ni-catalyzed. Our approach that has proven more challenging is the use of an undivided cell to perform tandem CO₂ reduction to CO, followed by CO utilization in carbonylation reactions. While this approach has the potential to offer several benefits such as simple experimental setups, straightforward synthetic protocols, direct utilization of the CO formed, decrease of the employed toxic CO, and facilitation of the formation of labeled compounds. This is complex due to the need to balance all required catalytic cycles. Few methods used CO₂ as a CO source because of its inertness but also due to its inherent reactivity that delivers preferably carboxylation products. In this line, Skrydstrup, Daasbjerg and co-workers developed a set of classical Pd-catalyzed carbonylation reactions using the CO evolved from the electrochemical CO₂ reduction in a two-compartment setup.[2] Before that, Perichon and co-workers reported a stepwise electrochemical/chemical method for the stoichiometric synthesis of ketones.[3]

Porphyrins have been actively investigated as oxygen evolution reaction (OER) catalysts, yet only a few studies refer to their integration into electrode devices as heterogeneous catalysts [1-2], mainly due to processability constraints. Moreover, the catalytic activity of porphyrin films can be substantially improved by their modification in the form of conjugated polymers. The enhanced conductivity of metalloporphyrin conjugated polymers, resulting from the extended delocalization of π-electrons in the conjugated system, may promote a faster charge transfer to the active sites, improving the catalytic performance. Recently, our group developed a methodology allowing the straightforward preparation of fused metalloporphyrins conjugated polymers thin films over a wide variety of substrates, based on the oxidative chemical vapour deposition (oCVD) [3,4]. In this way, a new venue for the study of the electrocatalytic activity of fused metalloporphyrin conjugated polymers bearing different metal centres and substituents was opened. Consequently, we have investigated the electrocatalytic activity of Ni(II) and Co(II) fused porphyrins conjugated polymers on FTO substrates towards the OER. We found out that there is a significant role of the direct fusion reaction to form organized electrocatalysts able to operate at lower overpotentials than the monomeric counterparts, resulting in enhanced electrocatalytic performance. Nonetheless, previous reports on Ni and Co complexes, including porphyrins, have demonstrated the decomposition of the molecular matrix to form a material thin film[2,5], possibly a metal oxide specie, acting as the true catalyst for water oxidation at the electrode surface. Therefore, we have conducted a deep-through investigation of the possible conversion of fused metalloporphyrin polymers into a material catalyst, using the combination of structural and compositional analysis with theoretical studies. We pay special attention to the role of polymerization and the presence of substituents of different natures on the transformation kinetics and resulting electrocatalytic activity.

Electrochemical water oxidation (2e^- WOR) to hydrogen peroxide (H₂O₂) is a topic of growing interest. Still material development, mechanistic understanding and process conditions are yet to be defined to allow for efficient and durable hydrogen peroxide production at industrially-relevant current densities [1]. Carbonate ions (CO₃²⁻) are suggested to enable indirect water oxidation to H₂O₂ and alkali metal cations (Li⁺, Na⁺, and K⁺) are known to influence electrochemical activity of various electrochemical reactions including the oxygen evolution reaction [2,3]. Also, in electrocatalysis the electrolyte composition is known to influence electrode stability. Material stability in the context of anodic H₂O₂ production is barely addressed in recent literature. Moreover, the influence of the electrolyte used for selective electrochemical water oxidation is yet to be fully disclosed [4].

Herein, we discuss the effect of different electrolytes (K₂CO₃ and Na₂CO₃) on the H₂O₂ production and electrode stability using commercially available fluorine-doped tin oxide (FTO) anodes frequently used as active material and support. The determined faradaic efficiencies (FE) suggest that the use of K₂CO₃ over Na₂CO₃ electrolyte (FE_H2O2 = 34.8 % in K₂CO₃ and FE_H2O2 = 25.3 % in Na₂CO₃ at 5 mA/cm²) is preferred. We observe a maximum production rates of 0.158 mmol min⁻¹cm⁻² and 0.118 min⁻¹cm⁻² at a current density of 100 mA cm⁻² in K₂CO₃ and Na₂CO₃, respectively highlighting the influence of alkali metal cations on the H₂O₂ formation. Moreover, we will highlight the influence of electrolyte composition on the stability of the FTO electrodes using ICP-MS and SEM analysis. Overall, in this contribution we will disclose our understanding of the role of carbonate on the H₂O₂ formation in the context of the reaction parameter space.

Hydrogen peroxide is known as an essential green chemical oxidant for advanced oxidation processes such as wastewater treatment, sterilization (which includes the pathogenic SARS-CoV-2), decolorization of waste dyes, and as an oxidant in chemical reactions. Besides its oxidizing ability, hydrogen peroxide is considered an emerging green fuel and energy career. Because of these properties, hydrogen peroxide is an ideal green energy carrier comparable to that of compressed hydrogen gas and a medium for energy storage. It is also set to play an important role in energy conversion technologies such as fuel cells and batteries. These emergent energy aspects promise an even higher global demand for this valuable chemical. In this talk, I will discuss our efforts in electrosynthesis of hydrogen peroxide, as well as the potential applications of hydrogen peroxide in battery technologies, and how computational material design can lead to the design and discovery of novel materials for a revolutionary new application.

Transition-metal phosphides are of high interest for electrochemical applications, owing to their chemical stability, composition and phase tunability, and economic potential.  
In this talk, I will discuss our new route for metal phosphides synthesis, utilizing a direct reaction of transition-metal nitrate salts with triphenylphosphine (PPh₃) in molten-state. This method was found straightforward, benign and scalable. We demonstrated this method for nanoscale transition-metal phosphides synthesis, as an attractive alternative to common solvothermal or gas-solid phosphidation synthetic methods, allowing tunable phase composition of the produced nanoparticles, along with significant simplification of procedures and equipment.

We initially demonstrated the new method for nickel-phosphides molten-state synthesis from Ni(NO₃)₂^∙6H₂O and triphenylphosphine (PPh₃)[1]. By combining analytical and computational methods, we elucidated the reaction mechanism, indicating that the reaction propagates dominantly through favored Ni-P bonding and consecutive cleavage of phenyl–P bonds of PPh₃. Interestingly, we found both by DFT and XRD that an intermediate formation of metallic nickel nanoparticles, originating from ligand-to-metal charge transfer at the Ni-P bond, is essential for in-situ production of phosphides.

Furthermore, we exemplified the advantages of this simple and controllable method for the investigation of phase/composition-activity correlation of different Ni/P products for hydrogen evolution reaction (HER) and as anode-materials for Li-ion batteries.
We found a clear composition-activity trend for both applications, providing high performance which is comparable to nickel phosphides produced by other common methods. We found Ni₃P products favorable as HER electrocatalysts, with 145 mV overpotential at 10 mA/cm², and Ni₂P was found favorable as LIBs anode materials, with 206 mAh g^-1 capacity; both with high morphological and electrochemical stability.

This new simple synthetic path offers new possibilities for low-cost synthesis and design of other transition-metal based materials. I will present and discuss our latest results of an improved and generalized synthesis method based on the above detailed molten-state method, allowing the production of a variety of single and multi-component transition-metal phosphides for electrochemical applications.

Perovskite oxides are commonly viewed as excellent electrocatalysts for oxygen evolution reaction (OER). However, OER conditions can substantially damage the perovskite structure and can lead to a wide range of phenomena, including cation leaching, dissolution, oxygen intercalation and modification of electronic structure.

In this talk, using single-crystalline perovskite electrocatalysts, I will demonstrate how operando electrochemical atomic force microscopy (EC-AFM) can be used to record nanoscale changes in the surface morphology during anodic corrosion of the perovskite surface. I will also show that perovskite surfaces can drive a spontaneous ORR in an aqueous environment and, through this reaction, become substantially oxidized, reaching potentials close to the OER, which results in cation leaching, dissolution and oxygen intercalation. Finally, I will discuss the effect of oxygen intercalation and how it can lead to extreme oxidation of a perovskite oxide, which we observe for the first time using resonant inelastic X-ray scattering and rationalize using DFT and molecular dynamics.

Ultimately, our results highlight the complexity of the electrochemical and structural behavior of perovskites and the importance of taking their structural evolution into account when considering elementary reaction steps on such surfaces.

Solar panels can not only produce electricity, but are also in early-stage development for the production of sustainable fuels and chemicals. They can therefore mimic plant leaves in shape and function as demonstrated for overall solar water splitting for green H2 production by the laboratories of Nocera and Domen.[1,2] This presentation will give an overview of our recent progress to construct prototype solar panel devices for the conversion of carbon dioxide and solid waste streams into fuels and higher-value chemicals through molecular surface-engineering of solar panels with suitable electrocatalysts. Specifically, a standalone ‘photoelectrochemical leaf’ based on an integrated lead halide perovskite-BiVO4 tandem light absorber architecture has been built for the solar CO2 reduction to produce syngas using an integrated Co porphyrin catalyst.[3] Recent advances in the manufacturing have enabled the reduction of material requirements to fabricate such devices and make the leaves sufficiently light weight to even float on water, thereby enabling application on open water sources.[4] The tandem design also allows for the integration of biocatalysts as ideal model catalysts and the selective and bias-free conversion of CO2-to-formate has been demonstrated using enzymes.[5] The versatility of the integrated leaf design has been demonstrated by replacing the perovskite light absorber by BiOI for solar water and CO2 splitting.[6] An alternative solar carbon capture and utilization technology is based on co-deposited semiconductor powders on a conducting substrate.[2] Modification of these immobilized powders with a molecular Co bis-terpyridine catalyst provides us with a photocatalyst sheet that can cleanly produce formic acid from aqueous CO2.[7] CO2-fixing bacteria grown on the photocatalyst sheet enable the production of multicarbon products through clean CO2-to-acetate conversion.[8] The deposition of a single semiconductor material on glass gives panels for the sunlight-powered conversion plastic and biomass waste into H2 and organic products, thereby allowing for simultaneous waste remediation and fuel production.[9] The concept and prospect behind these integrated systems for solar energy conversion, related approaches,[10] and their reliance on suitable transition metal catalysts will be discussed.

References
[1] Reece et al., Science, 2011, 334, 645–648.
[2] Wang et al., Nat. Mater., 2016, 15, 611–615.
[3] Andrei et al., Nat. Mater., 2020, 19, 189–194.
[4] Andrei et al., Nature, 2022, 608, 518–522.
[5] Moore et al., Angew. Chem. Int. Ed. 2021, 60, 26303–26307.
[6] Andrei et al., Nat. Mater., 2022, 21, 864–868.
[7] Wang et al., Nat. Energy, 2020, 5, 703–710.
[8] Wang et al., Nat. Catal., 2022, 5, 633–641.
[9] Uekert et al., Nat. Sustain., 2021, 4, 383–391.
[10] Wang et al., Nat. Energy, 2022, 7, 13-24.

Molecular electrocatalysts have received a renewed interest due to their capabilities towards sustainable and energy–efficient redox chemical transformations.¹ Particularly, those involved in new strategies towards energy storage application.^{1a, 2} Along these lines, I will present our first means of molecular cooperativity under electrochemical reduction conditions targeting carbon dioxide reduction.³

First, I will provide direct experimental evidence on the correlation of remote interactions between a synthesized MnI-complex and different alkali cations with redox potential tuning. Additionally, the electrochemical behavior of the Mn-complex towards carbon dioxide will be discuss, including the effects of added alkali salts using cyclic voltammetry.^3a

Later, I will present the formation, characterization and use of a dinuclear cobalt complex bearing a pyrazole-based ligand substituted with terpyridine groups at the 3 and 5 positions, for the electrocatalytic reduction carbon dioxide to carbon monoxide in the presence of Brönsted acids (94 % selectivity towards CO formation, at –1.35 V vs Saturated Calomel Electrode in DMF/TFE mixtures).^3b Chemical, electrochemical and UV-vis spectro-electrochemical studies under inert atmosphere of this complex indicate pairwise reduction processes. And, infrared spectro-electrochemical studies under carbon dioxide and carbon monoxdie atmosphere are consistent with a reduced CO-containing dicobalt complex which results from the electroreduction of carbon dioxide. Additionally, our theoretical results indicate the cooperativity of the ligand platform during the reduction process, delocalizing the electron density at the ligand and reducing the overpotential of the reduction reaction.

References

(1) a) N. W. Kinzel, C. Werlé, W. Leitner, Angew. Chem. Int. Ed. 2021, 60, 11628–11686; b) N. Wolff, O. Rivada‐Wheelaghan, D. Tocqueville, ChemElectroChem 2021, 8, 4019–4027.

(2) a) T. R. Cook, D. K. Dogutan, S. Y. Reece, Y. Surendranath, T. S. Teets, D. G. Nocera, Chem. Rev. 2010, 110, 6474–6502.

(3) a) A. Srinivasan, J. Campos, N. Giraud, M. Robert, O. Rivada–Wheelaghan, Dalton Trans. 2020, 49, 16623–16626; b) A. Bohn, J. J. Moreno, P. Thuéry, M. Robert, O. Rivada-Wheelaghan, 2022, 10.1002/chem.202202361.

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Enzymatic systems have evolved complex strategies to maximize the efficiency and product selectivity in small molecule activation, among which CO2 reduction. Beside unique active sites containing by definition earth-abundant elements, enzyme further control catalytic activity through second sphere interactions and a fine control of electron transfer chains. In this talk, we will introduce a series of bio-inspired strategies for the design of electrocatalytic systems for CO2 reduction. We will highlight a series of earth-abundant metal based molecular catalysts inspired by the active sites of enzymatic systems and will introduce a new strategy for the electrocatalytic metal hydride generation using synthetic Fe4S4 clusters acting as concerted proton electron transfer (CPET) mediators. We will demonstrate that the combination of synthetic Fe4S4 clusters with the CO2 electroreduction catalyst [MnI(bpy)(CO)3Br] (bpy = 2,2’-bipyridine) allows the preparation of a benchmark catalytic system for HCOOH generation. We will finally introduce the preparation of the first complete redox series of Fe4S4 complexes and their use to generate potent strong reducing agents via entatic activation strategies. References: 4. L. Grunwald, M. Clémancey, D. Klose, L. Dubois, S. Gambarelli, G. Jeschke, M. Wörle, G. Blondin, V. Mougel* A complete biomimetic iron-sulfur cubane redox series PNAS, 2022, 119 (31), e2122677119 3. S. Dey, F. Masero, E. Brack, M. Fontecave, V. Mougel* Electrocatalytic metal hydride generation using CPET mediators Nature, 2022, 607 (7919), 499-506 2. S. Dey, T. Todorova, M. Fontecave*, V. Mougel* Electroreduction of CO2 to Formate with low overpotential using Cobalt Pyridine Thiolate Complexes Angew. Chem. Int. Ed., 2020, 59(36), 15726-15733 1. A. Mouchfiq, T. Todorova, S. Dey , M. Fontecave*, V. Mougel* A Bioinspired Molybdenum-Copper Molecular Catalyst for CO2 Electroreduction Chem. Sci., 2020, 11, 5503-5510

A current chemistry challenge is the (photo)electrochemical production of ammonia (NH₃) from atmospheric N₂ without using H₂ or emitting CO₂, as an alternative to the current Haber-Bosch process which consumes 1% of the world energy and emits nearly 1.5% of the CO₂. The nitrogen reduction reaction (N₂ + 6H⁺ + 6e^- -> 2NH₃) implies a cascade of elementary steps that require a delicate balance of reactivity at the metal active site M (cleavage of the N-N triple bond without creating a too strong, poisoning M-N interaction; avoiding competitive reduction of protons to H₂ …) [1,2]. To date, examples of molecular electrocatalysts including abundant metals remain scarce, although recent spectacular progress has been made, notably with Fe and Mo based catalysts [3,4]. In this presentation, we will present and discuss reactivity and mechanistic aspects of a Mn catalyst able to reduce dinitrogen to ammonia in homogeneous as well as in electrochemical conditions [5].

Climate change concerns have spurred a growing interest in developing environmentally friendly technologies for energy generation, including green H₂ production from water splitting. Moreover, concerted attempts are also being made to re-utilize CO₂ by electrocatalytically converting it into value-added chemicals and fuels, offering the possibility to store renewable energy into chemical bonds. Thus, efficient, selective and durable electrocatalysts that can operate under mild reaction conditions (atmospheric pressure and low overpotential) are urgently needed. However, the latter challenging goal can only be achieved when fundamental understanding of the electrocatalyst structure and surface composition under reaction conditions becomes available. It should also be kept in mind that even morphologically and chemically well-defined pre-catalysts are commonly susceptible to drastic modifications under operando conditions, especially when the reaction conditions themselves change dynamically. Here, a synergistic experimental approach taking advantage of a variety of cutting-edge microscopy (EC-AFM, EC-TEM), spectroscopy (NAP-XPS, XAS, Raman Spect., GC/MS) and diffraction (XRD) has been employed to unveil the complexity of energy conversion electrocatalysts.

In particular, I will provide new insights into the electrocatalytic reduction of CO₂ as well as the oxygen evolution reaction using operando characterization methods while targeting model pre-catalyst systems ranging from single crystals to thin films and metal nanoparticles. Some of the aspects that will be discussed include: (i) the design of size- and shape-controlled catalytically active nanoparticle pre-catalysts (Cu, Cu₂O cubes, ZnO@Cu₂O cubic NPs, CoOx NPs), (ii) the understanding of the active state formation and (iii) the correlation between the dynamically evolving structure and composition of these electrocatalysts under operando reaction conditions and their activity, selectivity and durability.

Our results are expected to open up new routes for the reutilization of CO₂ through its direct conversion into industrially valuable chemicals such as ethylene and ethanol and the generation of H₂ through water splitting.

[1] Grosse, P.; Yoon, A.; Rettenmaier, C.; Herzog, A.;. Chee, S.W.; and Roldan Cuenya, B., Nature Comm. 2021, 12, 6736

[2] Timoshenko, J.; Bergmann, A.; Rettenmaier,  C.; Herzog, A.; Aran Ais, R.; Jeon, H.; Haase, F.; Hejral, U.; Grosse, P.; Kühl, S.; Davis, E.; Tian, J.; Magnussen, O.; Roldan Cuenya, B.; Nature Catalysis 2022, 5, 259.

[1] Haase, F.;  Bergmann, A.; Jones, T.; Timoshenko, J.; Herzog, A.; Jeon, H.; Rettenmaier, C.; Roldan Cuenya, B., Nature Energy (2022) 7, 765.

Correlating activity, selectivity and stability with the structure and composition of catalysts is crucial to advancing the knowledge in chemical transformations which are essential to move towards a more sustainable economy. Among these, the electrochemical CO₂ reduction reaction (CO₂RR) holds the promise to close the carbon cycle by storing renewable energies into chemical feedstocks. Yet it still suffers from the lack of efficient, selective and stable catalysts.

In this talk, I will showcase a few examples which highlight how shape-controlled nanocrystals can contribute to address the selectivity challenge in CO₂RR. First of all, I will discuss how size control of Cu nanocubes and Cu octahedra has revealed the importance of facet-ratio to maximize the selectivity towards ethylene and methane, respectively. Second, I will describe our recent efforts towards using shape-controlled NCs beyond pure Cu, in the framework of tandem schemes and bimetallics. Finally, I will present our strategies to enhance the stability of these Cu catalysts during operation.

CO2 reduction to long-chain compounds has the potential to close the carbon cycle and if using excess renewables be a route to sustainability. However, most of the materials we have only produce C2. I will analyze theoretically some of the aspects behind these difficulties in going higher in the C-C bond forming strategies. On one side I will describe why easy one material one orientation do not provide enough flexibility to generate large chains and why the role of the microenvironment and the materials complexity is so intrinsically crucial to the improvement of the electrocatalytic activity. At the same time I will demonstrate how mastering the microenvironment properties improvements can be introduced following different strategies. The work presented will illustrate how much complexity both at the electrocatalytic site but also in the solution and with different operational modes are needed and how the integration of different methodologies is crucial.

The electrocatalytic CO₂ reduction reaction (CO₂RR) powered by renewable energy and promoted by transition metal catalysts, will play an important role in the global decarbonization process of the chemical industry, since represents a sustainable route for the production of value-added chemicals using CO₂ as a feedstock. [1]-[2] However, despite the significant progress made in the field, selectivity, durability and intrinsic activity of the catalysts are still key challenges to achieve an efficient CO₂RR. For organometallic molecular systems, characterized by a well-defined chemical environment of the active site, a rational tuning of the CO₂RR efficiency and selectivity can be precisely controlled through a rational modification of the ligand scaffold. [3]-[5] Moreover, the encapsulation of molecularly defined active units into reticular frameworks was recently shown as an effective strategy to boost the CO₂RR performances of molecular catalysts, but also to alter their redox behavior. [6]

In recent years, the synergy between molecules and nanostructured materials has been proposed as a promising approach to design efficient heterogeneous catalysts for CO₂ electroreduction. For instance, the presence of organic modifiers on metallic surfaces was found to tune the stability of key reaction intermediates or the local surface microenvironment, thus altering the product selectivity. [7]-[8] In this contribution, we will discuss novel strategies and approaches to form hybrid electrocatalysts based on the utilization of reticular or molecular chemistry as tools to steer the CO₂RR selectivity and activity of transition metal-based nanostructured catalysts. The discussion will focus on providing a molecular perspective towards a rational design of heterogeneous catalysts.  

Global warming, climate change and our over-dependence on non-renewable fossil fuels demand long-term solutions to reduce CO₂ emissions and develop sustainable energy technologies. The electrochemical CO₂ reduction has the potential to accomplish a “carbon-neutral energy cycle”, which incorporates CO₂ as the unlimited carbon source for the production of high-density fuels, and renewable energy as the driving force behind the process.[1] However, for an industrial application, there are still challenges to overcome, such as low selectivity, short durability and low current densities along with high overpotentials. New sustainable, modular, robust and efficient catalytic platforms are needed. In this regard, this work entails a fundamental understanding of the CO₂ mechanisms using Single Atom Catalyst (SAC) within Covalent Organic Frameworks (COFs). This work focuses on the investigation of the electrochemical CO₂ reduction (CO₂RR) in water using new Manganese based-Covalent Organic Frameworks with emphasis on understanding the relationship between structure and electrocatalytic activity. The initial center of interest was to accomplish active performance and durability for CO₂RR using highly-organized {Mn(CO)₃} active sites within COFs. The catalytic activity of the materials was benchmarked against other molecular supported catalysts reported in the literature. Compared to equivalent Mn derivates, COFs exhibited higher selectivity and activity towards CO₂ reduction [2]. Additionally, mechanistic studies based on in situ / in operando spectroelectrochemical techniques (ATR-IR, UV-vis, SEIRA, EPR) together with DFT calculations were used to detect key catalytic intermediates and correlate the catalytic activity with the mechanical constraints impose to the {Mn(CO)₃} active sites by reticular framework. Of particular note is the detection of a radical intermediate within a Mn based COFs avoiding the detrimental formation of a dimeric specie determined as a resting state in the catalytic cycle. In addition, the study of the Mn centers within the COF was expanded and focused on the understanding of the mechanism and dynamic processes at the electrode interface. This study show case the richness and complexity of reticular materials and can serve as a guide to investigate the dynamics of these organic frameworks.

Copper (Cu) is an excellent catalyst capable to convert CO₂ beyond CO and produces fuels and hydrocarbons. However, it is low product-selective.(1) One strategy to improve product-selectivity on Cu is tuning the geometry of the surface active sites positions, i.e., the group of atoms in the catalyst surface that catalyse the CO2 conversion. In this communication, I show an easy and feasible strategy to tune and quantify the number and geometry of the surface active sites on Cu nanostructures. (2) At first, we have assessed the reversible adsorption/desorption of lead (Pb), or Pb under potential deposition (UPD), on different well-ordered Cu single facets (Figure 1). The lead adsorption/desorption on Cu, is monitored by a cyclic voltammetry technique (CV), which provides distinguishable peaks on each single facet, i.e., it is highly sensitive to the active site´s geometry (Figure 1). Then, we have nanostructured a Cu electrode using and electroplating method. We have recorded the Pb UPD on the nanostructured Cu surface, which voltammetric shape has several peak-contributions. Using the Pb UPD voltammetric data recorded on Cu single facets, we decouple the different facet contributions on our nanostructured Cu surfaces. This voltammetric analysis can be used to rationally design nanostructured copper catalysts with tailored geometry at the active sites positions, which is relevant to improve the CO₂ reduction reaction.

Figure 1 shows the voltammetric Pb UPD profiles recorded on Cu single facets at pH 3 and from a 0.1 M KClO₄ + 1mM NaCl solution.

In recent years, two dimensional metal organic frameworks (2D-MOF) have attract much interest not only for the ease of their synthesis, but also for their semiconducting properties and catalytic activities. The appeal of these materials is that they are layered and can be easily exfoliated to obtain a mono (or few) layer material with interesting optoelectronic properties. Moreover, they have great potential for CO2 reduction to obtain solar fuels with more than one carbon atom, such as ethylene and ethanol, in addition to methane and methanol. The production of ethylene has been reported in a recent communication, but its mechanism of action is still unknown.

In this talk, I will explore how a particular class of 2D-MOF based on a phthalocyanine core can be the reactive center for the production of ethylene and ethanol, focusing on the mechanism of action by mean of a novel computational method called 'Gran Canonical Potential Kinetic', in which an applied potential is explicitly considered to obtain accurate results which are directly comparable with experimental electrocatalytic measurements.

We observed that not only methane and ethylene can be formed, along with methanol, acetylene, ethanol and other different alcohols, but also that the key reaction step -the insertion of CO-  occur without the presence of a CO-CO dimerization, as commonly observed on metal surfaces, but with a novel method which can still lead to the formation of C2 products, making the 2D-MOF behave like a single atom catalyst.

The electrochemical reduction reaction of CO₂ (CO₂RR) into valuable chemical products is foreseen as a promising lever to move from fossil to renewable energy with the benefit of closing the carbon look. Copper is a unique catalyst for this reaction as it generates products beyond CO, such as CH₄ or C₂H₄. While polycrystalline copper is unselective, faceting copper at the nanoscale has proven to improve its selectivity while retaining attractive activity.^1,2 However, its severe lack of stability during operation dramatically hampers its further use on the industrial scale. Indeed, Cu surfaces suffer from drastic restructuring which often results in a performance loss.^3,4

Herein, we propose the encapsulation of Cu nanocatalysts with a thin amorphous oxide coating as one promising solution against their structural changes during CO₂RR. Specifically, we utilize colloidal atomic layer deposition (c-ALD) to grow an alumina (AlOx) shell with tunable thickness around 7 nm copper spheres to form well-defined Cu@AlOx core@shell  structures. We find that the improved morphological stability of the Cu@AlOx is reached when full encapsulation is achieved. To understand the mechanisms behind the observed behavior, we correlate the shift in product distribution with observations from ex situ electron microscopy and operando X-ray absorption spectroscopy. Overall, this work offers the big opportunity to create stable Cu catalysts during CO₂RR which is currently one of the most important challenges in the field.

Electrochemical CO₂ reduction is an appealing approach to diminish CO₂ emissions, while obtaining valuable chemicals and fuels from renewable electricity. However, efficient electrocatalysts exhibiting high selectivity and low operating potentials are still needed. Herein , we present a preparation method for the obtention of Cu and Fe nanoparticles supported on porous N-doped graphitic carbon matrix as efficient and selective electrocatalysts for CO₂ reduction to CO at low overpotentials. XRD and Raman spectroscopy confirmed independent Cu and Fe metals as the main phases. HRSEM and HRTEM images show the coral-like morphology of the porous N-doped graphitic carbon matrix supporting Cu and Fe metal nanoparticles (about 10 wt. %) homogeneously distributed with an average size of 1.5 nm and narrow size distribution. At the optimum Fe/Cu ratio of 2, this material present high activity for CO₂ reduction to CO at -0.3 V vs RHE with a Faradaic efficiency of 96 %. Moreover, at -0.5 V vs RHE this electrocatalyst produces  27.8 mmol of CO x gcat-1 x h-1, the production rate being stable for 17 h. A synergy between Cu and Fe nanoparticles due to their close proximity in comparison with independent Cu or Fe electrocatalysts is discussed.

Artificial photosynthesis requires a hybrid assembly that integrates light absorption, electron transfer, and catalysis. Covalent anchoring of a molecular catalysts to a light harvesting semiconductor results in a photocathode that is able to perform selective photoelectrocatalytic CO₂ reduction (PEC-CO₂RR). In hybrid materials, the molecular catalyst provides with selective catalytic activity whereas the photoactive semiconductor material is used as support and for light harvesting [1]. In these systems, charge accumulation and kinetics of photoinduced electron transfer from the photocathode to the molecular catalyst and to the CO₂ are crucial for an efficient CO₂ conversion [2, 3]. In this work, a mesoporous anatase TiO₂ layer is used as a solid photosensitizer support to anchor iron porphyrin-based molecular catalysts for CO₂ to CO obtention. Micro to mili second Transient absorption spectroscopy (TAS) is employed to investigate the accumulation and lifetime of photogenerated electrons in TiO₂ by controlling the applied bias and in the presence of CO₂. By probing the transient signal of TiO₂ conduction band electrons, we observe a decrease in the accumulation of charges in non-functionalized TiO₂ under applied bias when CO₂ is present (at -0.5 V vs. Ag/AgCl, by 35%), which proves the transference of electrons from the photocathode to the CO₂. Furthermore, we observe a shorter lifetime of photogenerated electrons when the TiO₂ is functionalized with iron porphyrins (10-fold decrease) derived from a fast electron transfer from the photocathode to the molecular catalyst. Ultimately, we correlate the results to the potentiostatic electrocatalysis performance of TiO₂ and TiO₂-Iron porphyrin hybrid materials under CO₂; with bare TiO₂, CO and CH₄ along with H₂ are produced, whereas TiO₂-Iron porphyrin shows 100% selective CO formation. However, we identify moderate CO₂RR performances caused by interfacial charge recombination between the oxidized iron porphyrin and conduction band electrons of TiO₂ which decrease the charge transfer process to the CO₂ molecule.

Global warming, as a consequence of the dependence on the use of fossil fuels, is one of the major problems that our current society is facing. For this reason, the production of energy by renewable sources has become a great challenge for the scientific community. Thus, researchers have recently focused their efforts on the development of new technologies that guarantee the production of high amounts of energy with low to minimal carbon footprint. Intermittence of renewable energies is a present challenge that requires to develop technologies able to store huge amounts of energy to be easily supplied at periods of low or zero energy production. In this scenario, (photo)electrochemical technologies have become an attractive candidate with the purpose of producing green fuels in order to store energy at the moments of excess of production. Research in the field of (photo) electrocatalysis has mainly been focused on the generation of green H₂, from the oxygen evolution reaction (OER) through the oxidation of H₂O; and the reduction of CO₂ (CO2RR) to obtain green fuels.[1] in these studies, notable advances have been made in recent years, both in terms of understanding the mechanisms of reactivity, as well as the selectivity of the processes activated by sunlight.[2,3]  In both cases, O₂ is obtained as by-product at the anode, with a very low price at the market, thus reducing the economic viability and the interest in this technology.[4] With the aim to store energy at the same time that obtaining compounds with added value, several organic transformations have been pointed out as alternative to the OER^.at the anode and H₂ production or CO2RR at the cathode. [5,6]

One very interesting alternative anodic reaction is based on the oxidation of lignin or compounds from biomass, since these species also allow reducing overpotentials while obtaining valuable products for the chemical industry, with the subsequent biomass revalorization. 5-hydroxymethylfurfural (HMF) is a biomass derived species, which can be easily obtained from C6 natural sugars. The selective oxidation of one of the hydroxyl groups of HMF leads to 2,5-furandicarbox-aldehyde (DFF), which can be used as a monomer in the synthesis of various polymers based in furan; [7,8] while further oxidation of HMF gives rise to 2,5-furandicarboxylic acid (FDCA), a very important component in the pharmaceutical and polymer industry, which can replace the monomers traditionally used in the synthesis of ethylene terephthalate (PET).[9] Besides, the reduction of HMF can lead to 2,5-bi(hydroxymethyl)furan (BHMF), a species that can be used as a precursor for the synthesis of many bio-based polymers, [10] consequently making this process very interesting as alternative to H₂ production or CO2RR in photoelectrochemical processes.

In our project we have performed the transformation of HMF to added value products by means of electrochemistry. The electrochemical oxidation of HMF to 2,5 FDCA has been achieved using easily made and inexpensive NiO-OH electrodes, obtained by Ni electrodeposition on pencil graphite rods (Ni/PGR). We have optimized the reaction conditions to prevent HMF degradation, leading to a nearly complete conversion of HMF into FDCA, with 88% Faradaic Efficiency (FE) and very low degradation in less than 2 hours. The electrochemical reduction of HMF to BHMF has been achieved using Cu foils electrodes decorated with Ag deposition using the galvanic replacement technique. BHMF has been obtained with 89% yield and 87% FE at pH 9 media in less than 3 hours. Impedance Spectroscopy (IS) has been analysed in both electrochemical processes to elucidate the electron transfer mechanism and the adsorption of reactants and intermediate species on the surface of the electrodes.

Photocatalytic liposomes are a promising supramolecular platform for artificial photosynthesis,^[1] but the understanding of reaction mechanisms and surface dynamics between membrane-bound molecular components still remain a challenge. In this presentation, we will report the photocatalytic CO₂ reduction performance of a series of 3d transition metal terpyridine and porphyrin catalysts immobilised on liposomes,^[2-4] and briefly about their use in (photo)electrocatalytic CO₂ reduction.^[5] Time-resolved spectroscopy was utilised to provide new insights into electron transfer processes taking place between the electron donor (sodium ascorbate) and membrane-bound photosensitiser and catalyst molecules. The most active molecular photocatalyst system, containing a cobalt porphyrin, was studied by (spectro)electrochemistry and density functional theory to gain a better understand of its enhanced performance compared to the other studied assemblies, and to propose a possible reaction mechanism for CO₂ reduction.^[4] Furthermore, the same cobalt porphyrin catalyst is successfully utilised in a (photo)electrochemical device that can simultaneously reduce CO₂ and reform plastic waste.^[5] Therefore, our findings demonstrate the great potential of liposomes as versatile photocatalytic scaffolds. We also show how the same molecular catalyst can be utilised successfully in photocatalytic and (photo)electrocatalytic systems, and illustrate the power of combining time-resolved and in situ spectroscopic techniques to understand molecule-based systems.

References:

[1]         Pannwitz, A.; Klein, D. M.; Rodríguez-Jiménez, S.; Casadevall, C.; Song, H.; Reisner, E.; Hammarström, L.; Bonnet, S., Roadmap towards solar fuel synthesis at the water interface of liposome membranes. Chem. Soc. Rev. 2021, 50, 4833-4855.

[2]         Wang, Q.; Warnan, J.; Rodríguez-Jiménez, S.; Leung, J. J.; Kalathil, S.; Andrei, V.; Domen, K.; Reisner, E., Molecularly engineered photocatalyst sheet for scalable solar formate production from carbon dioxide and water. Nat. Energy 2020, 5, 703–710.

[3]         Zhang, X.; Cibian, M.; Call, A.; Yamauchi, K.; Sakai, K., Photochemical CO₂ Reduction Driven by Water-Soluble Copper(I) Photosensitizer with the Catalysis Accelerated by Multi-Electron Chargeable Cobalt Porphyrin. ACS Catal. 2019, 9, 11263-11273.

[4]         Rodríguez-Jiménez, S.; Song, H.; Lam, E.; Wright, D.;Pannwitz, A.; Bonke, S. A.; Baumberg, J. J.; Bonnet, S.; Hammarström, L.; Reisner E., Self-Assembled Liposomes Enhanced Electron Transfer for Efficient Photocatalytic CO₂ Reduction. J. Am. Chem. Soc. 2022, 144, 9399-9412.

[5]         Bhattacharjee, S.; Rahaman, M.; Andrei, V.; Miller, M. Rodríguez-Jiménez, S.; Lam, E.; Pornrungroj C.; Reisner, E., Photoelectrochemical CO₂-to-fuel conversion with simultaneous plastic reforming. Nat. Synth (2023). in print (https://doi.org/10.1038/s44160-022-00196-0).

Electrocatalytic CO2 reduction to renewable fuels has drawn increasing attention due to its great potential in addressing the issues of the greenhouse effect, energy crisis, and industrial transformation thus having the utmost importance in attaining a carbon-neutral society. Nevertheless, there are general practical limitations such as poor selectivity for multi-carbon products and most importantly the electrocatalysts suffer from high overpotentials while directly converting the CO2 to C2+ products. To overcome these challenges and enhance the ECO2R, a cascade mechanistic approach is explored as an alternative to the direct conversion of CO2 to C2+ products. A two-step tandem set-up is employed where CO2 is converted to CO in the first step, enabled with a highly efficient Ni single-atom catalyst on activated carbon black, and sequentially CO is converted to multi-carbon products in the second step. The single-atomic catalytic sites yield a nearly 100 % faradaic efficiency towards CO at an average current density of over -40 mA·cm-2. The as-produced product stream is directly supplied as the reactant stream to a second electrolyzer that is equipped with oxide-derived copper supported on carbon black. Initially, it is noticed that the presence of unreacted CO2 in the second cell has an adverse effect and results in poor performance. When the unreacted CO2 is trapped by using an absorption chamber in between these two electrolyzers, a surge in the faradaic efficiency towards ethylene, ethanol, and n-propanol thus accounting for more than 80% total faradaic efficiency at an average current density of over 120-130 mA·cm-2 and the unwanted yet competitive H2 is limited to just 10-12 % faradaic efficiency. The aforementioned approach is found to be highly efficient not only in terms of faradaic efficiency but also energy efficiency. It is evident that multi-step electrolysis yields a highly promising conversion of CO2 to multi-carbon products. On the flip side, direct or indirect routes are also explored to electrocatalytically reduce the carbonated solvent stream placed in between the electrolyzers. The observed results clearly are imperative by enabling a direct cascade approach and exclusive separation of unreacted feed gas aid in attaining higher electro-conversion of CO2 to alternative fuels.

Electrocatalytic hydrogen production or C-based fuels synthesis via water splitting and  CO2 reduction, respectively, is considered a promising technology to store renewable energy into chemical bonds when using Earth-abundant electrocatalysts. The efficiency of these catalysts does not only depend on the nature of the metal centre, but also on the morphology and crystalline structure. However, chemical and morphological changes in the catalysts take place in operando conditions that are poorly understood.

In this poster, I will present recent advances on the understanding of morphological transformations of Cu-based electrocatalysts for the hydrogen evolution and CO2 oxidation reactions during catalysis and the effect of such transformations in the reactions kinetics. In particular, the effect of morphological transformations on increasing the electrochemically active surface area (ECSA) will be discussed. In this poster, I will show that such increase in the ECSA is found to increase the efficiency of the catalyst noting this work is performed in mild pH conditions.