The electrocatalytic reduction of CO2 is very sensitive to the nature of the electrolyte near the electrode surface. The presence of cations and anions, and how they are indirectly affected by the (local) pH, play a crucial role, both for CO2 reduction and for the competing hydrogen evolution reaction. In my talk, I will demonstrate these effects for CO2 reduction on flat and nanoporous gold electrodes, and show how mass transport regulates them. I will discuss the effect of monovalent cations, multivalent cations, bicarbonate concentration (and how bicarbonate responds to the presence of cations), and varying mass transport rates as regulated by a rotating disc electrode.  

      In the quest for developing a sustainable energy economy, the electrocatalytic reduction of carbon dioxide (CO₂RR) into value-added chemicals and fuels offers the potential to close the anthropogenic carbon cycle and store renewable energy into chemical bonds. It is therefore of particular interest to develop efficient and selective electrocatalysts, which reduce the reaction overpotential and steer the reaction toward hydrocarbons and multicarbon oxygenates. Nonetheless, in order to tailor the chemical reactivity of CO₂RR nanocatalysts at the atomic level, fundamental understanding of their structure and surface composition under reaction conditions must be obtained. The latter requires a synergistic experimental approach taking advantage of a variety of cutting-edge microscopy (EC-AFM, EC-TEM, LEEM) and spectroscopy (XPS, XAS, MS/GC, XPEEM, Raman spect.) methods.

This talk will provide new insights into the CO₂RR and CORR, with special focus on: (i) the evolution of the surface structure of well-oriented single crystal Cu catalysts [Cu(100), Cu(111), Cu(310)]; (ii) the design of size- and shape-controlled catalytically active nanoparticle pre-catalysts (Cu, Cu₂O, Ag-Cu₂O cubes, Au-Cu₂O cubes, ZnO/Cu₂O cubes), and (iii) the correlation between the dynamically evolving structure and composition of the electrocatalysts under operando reaction conditions, including pulse electrolysis treatments, and their activity and selectivity. The results are expected to open up new routes for the reutilization of CO₂ through its direct conversion into valuable chemicals and fuels such as ethylene and ethanol.

Electrochemical reduction of carbon dioxide (CO₂) to carbon monoxide (CO) and other C₁ products under mild conditions opens a way to produce fuels and chemical feedstock. Molecular catalysts made of Earth-abundant metals may be operated in homogeneous conditions or as supported heterogeneous systems.

Co and Fe complexes demonstrate excellent conversion of CO₂ to CO with a selectivity reaching 99 %, in organic solvent as well as in water. By setting proper conditions, we recently discovered that formaldehyde and methanol (Co complex) as well as methane (Fe complex) could be obtained. In other words, CO₂ could be reduced with 2, 4, 6 and 8 electrons with molecular catalysts. Our most recent results will be presented.

In this work, we present a mechanistic study of the electrochemical CO₂ reduction catalyzed by a highly active fac‐[Mn^I(CO)₃(bis‐MeNHC)MeCN]⁺ complex (Mn-MeCN⁺).^[1] By combining in-situ IR-SEC, chemical synthesis of intermediates and DFT calculations, we have identified and fully characterized Mn-CO⁺ (under CO₂) and Mn-H (under Ar) as key intermediates in the CO₂ reduction mechanism. Our observations confirm that, in the presence of low water concentrations, the CO₂ reduction to CO occurs through a protonation-first pathway. However, at higher concentrations of water, Mn-H is formed and easily inserts CO₂ to give the corresponding Mn-O₂CH intermediate. The formate release from the latter intermediate is a slow process which explains the decrease of the catalytic activity in the presence of high concentrations of added acid.

Greenhouse gases have maintained the conditions on Earth habitable. Nonetheless, the
atmospheric level of these gases, in particular carbon dioxide (CO2), has increased considerably in
the latest years. One way to cope with this increase is by electrochemically reduce CO2 into other
building blocks or fuels. However, the kinetic inertness and the thermodynamic stability of this
molecule makes its activation rather difficult, requiring the use of catalysts. Molecular catalysts,
such as iron porphyrins, are among the most efficient, robust, and selective catalysts towards the
reduction of CO2 to CO, by also being optimal catalysts in both organic and aqueous solutions.1,2 In
order to consider the reduction of CO2 by iron porphyrins as an interesting path to re-use and take
advantage of a waste product, fundamental knowledge is a pre-requisite to better understand this
reaction, improve their performance and orient their selectivity.

Even if electrochemical methods have allowed the determination of kinetic parameters,3,4 a
complete picture of the mechanism is still missing due to the lack of spectroscopic evidence. By
coupling electrochemical techniques with X-Ray absorption spectroscopy, we aimed at obtaining
a complete view of the modifications undergone by the catalyst during electroreduction and
catalysis. In this context, we have recently developed a spectroelectrochemical set-up for in situ
CO2RR studies by iron porphyrins in organic media. We present here the spectral changes
observed at the pre-edge and XANES region. This has allowed to give insights on the structural
and electronic changes that occur at the iron center and its interactions with the CO2, proposing
some possible intermediate species which are crucial to describe the CO2 reduction mechanism.

References:
(1) Costentin, C.; Robert, M.; Savéant, J.-M.; Tatin, A. Proc Natl Acad Sci USA 2015, 112 (22), 6882–6886.
(2) Costentin, C.; Drouet, S.; Robert, M.; Savéant, J.-M. Science 2012, 338 (6103), 90–94.
(3) Bhugun, I.; Lexa, D.; Savéant, J.-M. J. Am. Chem. Soc. 1996, 118 (7), 1769–1776.
(4) Costentin, C.; Passard, G.; Robert, M.; Savéant, J.-M. J. Am. Chem. Soc. 2014, 136 (33), 11821–11829.

Effective large-scale CO₂ conversion to fuels or value-added chemicals using renewable energies is critical to reduce our environmental impact [1]. To this end, better understanding of the CO₂ mechanism is needed to develop efficient and selective catalysts that operates in water controlling H₂ evolution. Covalent Organic Frameworks (COFs) are reticular materials, which can be used to combine the advantages of the well-defined molecular catalysts and the heterogeneous ones [2]. In this work, we present the first COF based on tricarbonyl Mn units (COF_bpyMn), that by π-π stacking is attached to MWCNTs form electrocatalytic electrodes active for CO₂ reduction in neutral water. The activity of the catalyst was evaluated by electrochemical techniques with good stability in aqueous solution. The COF_bpyMn shows a low CO2RR onset potential (η = 190 mV) and high current densities (>12 mA·cm^–2, at 550 mV overpotential) in water. TOF_CO and TON_CO values are as high as 1100 h^–1 and 5800 (after 16 h), respectively, which are more than 10-fold higher than those obtained for the equivalent manganese-based molecular catalyst. The encapsulation of  tricarbonyl Mn single-atom centers within the reticular covalent organic structure plays an important role by favouring the electrocatalytic CO₂ reduction over competitive H₂ evolution reaction. The spectroelectrochemical studies evidence the formation of five-coordinate species in the catalytic cycle for CO formation. The COF imposes mechanical constraints on the {fac-Mn(CO)₃S} centers.

Cytochrome c nitrite reductase (CcNiR) is a multi-heme enzyme that catalyzes the six-electron, seven-proton reduction of nitrite to ammonia as part of the global nitrogen cycle. In this talk, the activity of iron and cobalt complexes that perform the multielectron reduction of nitrite, mimicking the activity of CcNiR, will be described. One complex is a small cobalt-peptide complex (CoGGH) that catalyzes the complete reduction of nitrite to ammonium with an onset potential at -0.65 V vs Ag/AgCl (1 M KCl) at pH 7.2. CoGGH also catalyzes the reduction of proposed intermediates nitric oxide and hydroxylamine to ammonium. The second catalyst is an iron complex featuring a macrocyclic redox-active ligand (FeN₅H₂), which catalyzes nitrite reduction with an onset potential of -0.90 V to form hydroxylamine. FeN₅H₂ is unusual for being an iron-based catalyst for multielectron nitrite reduction that functions near neutral pH. CoGGH also catalyzes proton reduction to H₂; the study of this reaction yields insights into proton handling by this catalyst relevant to understanding the nitrite reduction mechanism.

Mediated electrochemistry complements conventional electrocatalysis and introduces flexibility in catalyst design by enabling electrode-driven reactions to take place off-electrode under more conventional thermochemical conditions.[1] This talk will highlight the development and application of electron-proton transfer mediators that support redox processes relevant to fuel cells, emphasizing the oxygen reduction reaction (ORR).[2-4] Off-electrode mediated oxygen reduction proceeds effectively with non-precious-metal M-N-C (M = Fe, Co) catalysts that are similar or identical to those commonly used for direct electrocatalytic ORR. Mechanistic studies have been conducted to probe the mechanism of the thermochemical ORR with a hydroquinone mediator. In principle, this mediated ORR could proceed on the catalyst surface via the coupling of two independent half-reactions (IHR), consisting of hydroquinone oxidation and O₂ reduction, or by direct inner-sphere reaction (ISR) between O₂ and the hydroquinone. Data have been obtained that support the inner-sphere reaction pathway,[5] and these results have important implications for mediated electrochemical processes. Different mechanisms have different linear-free energy relationships (i.e., Tafel slopes) that offer potential advantages in overpotentials and/or rates of (net) electrochemical reactions.

In this famous quote that most surface scientists love and hate in equal proportions, Pauli said: „God made the bulk; surfaces were invented by the devil“.

In this talk,  I will show you how even the most well designed and atomically controlled 2D catalysts, and even the most common metal surfaces suffer from dynamic structural transformations when you look at them at the nanoscale, under electrochemical reaction conditions. The „devil“  face of electrocatalysts can affect the product selectivity trends in detrimental ways, but it can also be tuned to increase their performance in key reactions in artificial photosynthesis schemes. I will show you a few examples on the surface dynamics of Cu electrodes involved in CO₂ electroreduction (CO₂RR), and 2D transition metal-based materials that catalyze oxygen evolution reactions (OER). I will discuss the use of scanning probe microscopy and solid state spectroscopy to track the surface dynamics of electrocatalysts in-situ, and present a few unconventional strategies to reach significant boosts in the catalytic activity of Earth-abundant 2D materials (using spin-selective mechanisms).

I will present the most recent advances on the role of reconstructions and electrolytes and how they affect the modeling of electrocatalytic systems.

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. There are several 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”. I the talk I will present a simulation approach to discover new catalysis materials based on HEAs.

Hydrogen plays a critical role in the energy transformation to a sustainable future, due to its widespread applications as a fuel for transport, feedstock to chemical industries and a heat source for buildings. Low temperature water electrolysis can enable hydrogen production renewably and at scale, but device efficiencies are currently limited by poor kinetics of water oxidation at the anode [1]. The low operating pH and high oxidizing conditions experienced at the anode in proton exchange membrane water electrolyzers limits the choice of materials to catalyse the oxygen evolution reaction to oxides of iridium, which is highly scarce [1],[2]. Mechanistic understanding of the water oxidation reaction on iridium oxides can enable rational development of more active catalysts with lesser precious metal loading.
Iridium oxides exhibit complex redox chemistry, which cannot be understood using electrochemical measurements alone. Particularly, the redox transitions on iridium oxides can exhibit non-Nernstian behavior, resulting in the redox transitions being dependent on pH and cations present in the electrolyte. In this talk, I will demonstrate the power of optical spectroscopy to track potential-dependent electroabsorption changes as a function of electrolyte pH from 1 to 13 on iridium based oxides. These measurements enable a direct comparison of the nature of redox active species in acidic and alkaline solution and also provide insights about the electrolyte environment on the redox potential, and consequently binding energetics of oxygenated species on IrOx. Using complementary stepped potential spectroelectrochemical measurements, the density of these redox active sites can also be computed as a function of potential [3] and the kinetics of water oxidation in acid and base can be linked directly to the density of oxidized species [4], [5]. Furthermore, in addition to quantitative detection of redox states, measurements of the kinetics of charge accumulation in natural and deuterated electrolyte provides insights into the role of ion transport within the oxide and the degree of bulk iridium redox. Therefore, based on our spectroscopic results on iridium-based oxides, we can experimentally probe the effect of interfacial pH and ions on charge accumulation and its impact on water oxidation kinetics. 
 

Iridium oxide is the state-of-the-art electrocatalysts for water oxidation in polymer electrolyte membrane (PEM) electrolysers for green hydrogen production. Hydrous, amorphous iridium oxides (IrO_x) show far higher activity than the rutile crystalline iridium oxides (IrO2), but the reason why they are more active is still under intense debate. Various differences between IrO_x and IrO₂, including the differences in active surface area, chemical state of Ir, fractional coverage of -OH groups and the range of ordered structure, renders it difficult to identify the origin of the differing activity.[1], [2] Thus far, ambiguities in measuring the number of sites participating in the reaction have prevented the accurate measurement of intrinsic catalytic activity, or turnover frequency, for amorphous iridium oxides and rutile iridium oxide.

In this talk, I will present the results from a range of different spectroscopic techniques and electrochemical measurements to identify the redox species present in IrO_x and IrO₂ and establish how these control the reaction kinetics. The techniques include (i) time-resolved UV-vis absorption spectroscopy (ii) on chip electrochemical mass spectrometry (iii) time of flight secondary ion mass spectrometry.

We have identified the same short-lived and long-lived oxidised species for both IrO_x and IrO₂ at potentials from 1.4 V to 1.55 V. We have correlated these species to the O₂ evolution kinetics. We attribute the lower activity of rutile IrO₂ to a lower concentration of the species responsible for driving water oxidation. The electrochemical mass spectrometry allows us to separate capacitive currents from O₂ evolution on the two catalysts. Combining TOF-SIMs with a paraffin wax cover method,[3] the depth of proton penetrating into the bulk catalysts was also revealed.

By comparing the concentration of oxidised species to reaction rates, we can directly measure turnover frequency (TOF) for IrO_x and IrO₂ as a function of applied potential. In summary, this study quantitively measured the intrinsic activity of amorphous and crystalline iridium oxides without any assumption of surface area and therefore shines insights into the origin of activity difference between amorphous and rutile iridium oxides.

A carbon-neutral and sustainable future requires advanced electrocatalysis systems[1]. Magnetism-induced enhancement of electrocatalytic reactions has been gaining increasing attention [2],[3],[4]. A simple and efficient way to achieve an enhancement is to apply an external magnetic field to the reaction[5]. Various mechanisms concerning different aspects of the reactions, e.g., reaction kinetics[5],[6] and mass transfer processes[7], have been introduced. However, the enhancement is likely a result of a synergistic effect of at least two mechanisms. Here, using key electrocatalytic reactions, i.e., oxygen evolution reaction (OER), oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER), and homogeneous magnetic fields provided by an electromagnet, we demonstrate the importance of experimentally decoupling the mechanisms affecting the electrocatalytic reactions at electrodes with different magnetic properties. Our results show that the effect on the reaction kinetics and selectivity is exclusive to ferromagnetic catalysts and electron spin-dependent reactions under sufficient magnetic field strength; however, an enhancement of the mass transfer is universal. We provide a versatile strategy to optimize the magnetic enhancement that can be implemented to boost electrocatalytic reactions.

The conversion of renewable energy into storable, high value fuels is a key outstanding challenge in the transition to a carbon neutral economy. The primary source of electrons for the production chemically reduced fuel compounds is from the electrocatalytic oxidation of water. However, large quantities of energy are currently lost in the form of catalytic overpotential during water oxidation. This creates a challenge in scaling up electrolysis technology. To surmount these challenges, water oxidation activity must be improved using earth abundant catalysts. However, progress towards this goal is stymied by the complex ways in which applied voltage can drive water oxidation. This complexity is a result of the cooperative effects arising from interactions between oxidized species when large fraction of an electrocatalytic surface becomes oxidised. Competing, but not mutually exclusive, hypotheses, point to changes in activation energy¹ and changes in the catalytic mechanism involving the coupling of multiple oxidising equivalents during catalysis² as driving changes in rate constant and Tafel slope as a function of potential.

Cobalt compounds are used as water oxidation electrocatalysts for water oxidation across a wide rage of pH. However, the convolution of overlapping,  non-Nernstian redox processes and changes in catalytic mechanism complicate interpretation of Tafel slopes. In-situ spectroelectrochemistry can be used to separate redox driven changes in surface electronic structure from catalytic effects³. In this talk I demonstrate the construction and capabilities of a new potential and time operando spectroelectrochemical system to help resolve these questions in two model cobalt catalysts. First,  cobalt-iron hexacyanoferrate⁴, in which catalytic Co sites should be physically isolated from one another,^5,6 and secondly, amorphous cobalt phosphate oxide, in which Co centres interact during catalysts by oxo coupling and also via non-nernstian effects.^7,8 Using global analysis of a rich, 3d spectreoelectrochemial data set,  the potential dependence of the density of catalytically active states in both materials can quantified and compared. Time resolved spectral measurements as the catalysts relax from an operating potential to rest under open circuit conditions are used to access the kinetics of water oxidation.  These results show that contrast to cobalt phosphate oxide, catalysis in cobalt hexacyanoferrate shows negligible cooperative effects, both in terms of an enthalpic change in surface reactivity and multi-metallic catalysis, indicating a simple connection between the density of catalytic states and the Tafel slope.

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. We will show how such active sites can be used as an inspiration to design new synthetic catalysts for H₂ evolution [2-4] and oxidation [5-6] and how the introduction of proton relays significantly impacts the catalytic properties of such catalytic platforms, opening in some cases the possibility for bidirectional and even reversible catalysis.

Electrosynthesis strategies have seen a resurgence over the last decade, although most electrocatalysts for electrosynthesis are either traditional metal catalysts (i.e. Pt, Pd, or alloy catalysts) or molecular electrocatalysts. However, biocatalysts can be utilized for improving the selectivity of electrosyntheses. This talk will discuss the use of microbial and enzymatic bioelectrocatalysis for electrosynthesis. It will compare and contrast microbial and enzymatic bioelectrocatalysis. Then, the talk will show the use of enzymes and microbes for mediated and direct bioelectrosynthesis of ammonia and value-added chemical products (i.e. chiral amines). Finally, the talk will discuss the future directions in bioelectrocatalysis for electrosynthesis.

Heterogeneous electrocatalysts for the oxygen evolution reaction (OER) are complicated materials with dynamic structures. They exhibit potential-induced phase transitions, potential-dependent electronic properties, variable oxidation and protonation states, and disordered local/surface phases. These properties make understanding the OER, and ultimately designing higher-efficiency catalysts, challenging. Measurements of intrinsic activity show that, by far, the most-active phases for OER under alkaline conditions are Fe-containing mixed-metal oxyhydroxides, but exactly how they function remains controversial. I will discuss our work to understand the key properties of these catalysts, including morphology, composition, and molecular/electronic structure, and how they evolve and are dynamic under active catalytic conditions. Specifically, I will highlight evidence for the critical role of “surface” Fe-O clusters on CoOOH and NiOOH porous supports as the key active site motif for OER. These concepts inform design strategies for higher-performance catalyst architectures and for their incorporation into practical electrolyzer devices to make clean hydrogen fuel from inexpensive renewable electricity for green long-distance transportation and long-duration energy storage.