Utilizing solar energy and achieving artificial photosynthesis are vital process to building a sustainable society and an effective way to achieve the goal of carbon neutrality. This report presents our research progress in the construction of bias-free water splitting systems and the optimization of CO2 reduction systems. We have proposed a strategy to spatially decouple the light absorption site from the surface reaction site and developed a method to regulate the bending degree of the energy band, thus constructing an efficient charier transport channel to achieve efficient conversion of solar energy to hydrogen energy. In addition, we have combined theoretical calculations and in-situ spectroscopic techniques to clarify the relationship between active site structures and CO2 conversion pathways, leading to the development of a series of site-controlled construction methods. By introducing the basic roles of hydrodynamics into the design of reaction systems, we have elucidated the matching principle between solar energy and electrolyzer power. Through modulating the temperature, velocity and concentration fields in the CO2 reduction system, we have initially achieved the stable production of multi-carbon products under scaling-up conditions.

The electrochemical reduction of CO₂ to value-added products (e.g., CO, HCOO⁻)¹ is increasingly regarded as an appealing strategy to valorize CO₂ while decreasing its emissions. Driven by the sluggish kinetics of the CO₂-reduction reaction (CO₂RR), an ever-increasing body of work is being devoted to the development of new CO₂RR- electrocatalysts.² However, such studies are preponderantly performed in liquid electrolytes that suffer from a limited CO₂-solubility (≈ 30 mM) that restricts the attainable currents to values well below the > 200 mA·cm⁻² relevant for industrial applications. Those high currents can be attained in so-called co-electrolysis cells in which the CO₂ is supplied as a humidified gas, and gas diffusion electrodes and ion-exchange membrane electrolytes are implemented to minimize mass-transport and ohmic losses, respectively.³ However, the development of such co-electrolyzers remains at an early stage in which elementary aspects like the choice of ion exchange membrane remain unclear. In this regard, the proton-exchange membranes used in polymer electrolyte water electrolyzers are not applicable, since they entail an acidic reaction environment (pH ≈ 0) that would favor H₂-evolution over the CO₂RR. Alternatively, recent studies have implemented anion exchange membranes (AEMs) featuring promising performances,^4,5 but in which the (bi)carbonate formed at the cathode is transported to the anode and oxidized back to carbon dioxide, lowering the net CO₂-utilization.^6,7

This contribution will summarize our efforts from catalysts, component and cell development including a techno-economic analysis of technical scale-up feasibility.

[1] J. Durst, A. Rudnev, A. Dutta, Y. Fu, J. Herranz, V. Kaliginedi, A. Kuzume, A. A. Permyakova, Yohan Paratcha, P. Broekmann, and Thomas J. Schmidt, Chimia, 69, 769 (2015).

[2] J. Herranz, J. Durst, E. Fabbri, A. Patru, X. Cheng, A. A. Permyakova, and T. J.Schmidt, Nano Energy, 29, 4 (2016).

[3] J. Herranz, A. Patru, E. Fabbri, and T. J. Schmidt, Curr. Opin. Electrochem., 23, 89 (2020).

[4] K. Wu, E. Birgersson, B. Kim, and P. J. A. Kenis, J. Electrochem. Soc., 162, F23 (2014).

[5] J. J. Kaczur, H. Yang, Z. Liu, S. D. Sajjad, and R. I. Masel, Frontiers Chem., 6, 263 (2018)

Continuous-flow electrolyzers allow CO₂ reduction at industrially relevant rates, but long-term operation is still challenging. In this talk, I am going to present some interesting findings on the role of different ions crossing the anion exchange membrane in zero-gap electrolyzer cells, contributing to unstable operation.

In the first part of my talk, I will show that while precipitate formation in the cathode gas diffusion electrode is detrimental for the long-term stability, the presence of alkali metal cations at the cathode improves performance. To overcome this contradiction, we develop an operando activation and regeneration process, where the cathode of a zero-gap electrolyzer cell is periodically infused with alkali cation-containing solutions. [1] This enables deionized water-fed electrolyzers to operate at a CO₂ reduction rate matching that of those using alkaline electrolytes (CO partial current density of 420 ± 50 mA cm⁻² for over 200 hours). We deconvolute the complex effects of activation and validate the concept with five different electrolytes and three different commercial membranes. Finally, we demonstrate the scalability of this approach on a multi-cell electrolyzer stack, with a 100 cm² / cell active area.

In the second part of the presentation, I will discuss the role of anode catalyst in CO2R cells. The urge to substitute Ir is driven by its high- and steeply rising market price as well as its limited stability in alkaline media. Although Ni is a ~ten thousand times cheaper, active, and stable oxygen evolution reaction (OER) catalyst in alkaline media, I will demonstrate that there are factors, which hinder its application in CO₂ electrolysis. While Ni is a suitable OER catalyst in short experiments, the cell voltage increases, and the measured total Faradaic efficiency decreases continuously during prolonged electrolysis. This is caused by the local acidic pH at the anode surface, the crossing CO₃^2- ions and by the gradual change in the anolyte composition, leading to Ni dissolution. The catalyst loss is only a minor part of the problem; the dissolving metal ions also penetrate into the anion exchange membrane, where precipitate forms due to the high local carbonate ion concentration, inducing cell failure.

Electrochemical CO₂ reduction (eCO₂R) is a promising process to store renewable energy into chemical bonds and close the carbon cycle. However, to date the industrial exploitation of this technology is limited to CO and HCOO^–, whilst production of desirable C₂₊ chemicals is possible only at laboratory scale and on copper-based catalysts. Dynamic phenomena, such as surface reconstruction during operation [1] or specific electrolyte effects,[2] can promote selectivity toward multi-carbon compounds and long-term stability of the device, thus calling for computational modeling to unveil this complexity.

Here, by means of ab initio molecular dynamics (AIMD) simulations, we modeled surface reconstruction on an oxide-derived copper material,[3] and we confirmed experimental observations on the crucial role of metal cations in electrochemical CO₂ reduction.[4]

In the first study, we identified the main ensembles which control the catalytic performance of seven oxygen-depleted oxide-derived copper models. Generally, copper differentiates in three classes: metallic Cu⁰, polarized Cu^ð+, and oxidic Cu⁺, respectively coordinated to 0, 1, and 2 oxygens. These three species form 14 ensembles. Low coordinated Cu adatoms and polarized sites are responsible for binding CO₂ and thus improving eCO₂R activity. Metastable oxygens and metallic fcc-(111) or (100)-like Cu facets promote CO-CO dimerization step via a deprotonated glyoxylate species. In the second study, we rationalized sound experimental evidences on the absence of eCO₂R on gold, silver, and copper without a metal cation in solution. By applying AIMD on a large Au supercell with 72 explicit water molecules and 1 cation, we demonstrated that cations steadily coordinate with adsorbed CO₂, with a coordination rate which increases with the ionic cation radius. Overall, such coordination accounts for a short-range electrostatic interaction which enables eCO₂R by stabilizing CO₂ activation by 0.5 eV and enhancing the first electron transfer from the surface. Specific eCO₂R activity trends are solely due to larger accumulation at the Outer Helmholtz layer for weakly solvated cations. Both studies provide new sets of concepts for modeling dynamic processes driven by high surface polarization and electrolyte species characteristic of CO₂ reduction conditions.

Electrochemical reduction of CO₂ into valuable fuels has entered a pre-industrialization stage with more and more studies oriented toward competitive and scaled designs and processes. This trend is mostly visible in the aim for large current density operation and the use of gas diffusion electrode (GDE) based device approaches. To provide guidelines for optimization, multi-physics modeling of CO₂ electrochemical reduction process, from the molecular to the device scale has become a necessary tool.[1], [2]

To reach a comprehensive and accurate model, each parameters requires to be established and validated independently as a first step. This sub-task is challenging for the determination of electrochemical kinetic parameters as mass transport and kinetics phenomena are always happening concomitantly and are convoluted.

In the present work, we investigate experimental conditions for which the mass transport characteristics can be modeled in detail allowing for the isolated determination of kinetic parameters. Such requirments are meet under certain conditions at rotating disk electrode (RDE). At a silver rotating disk, we investigate the electrochemical reduction of CO₂ into CO and the competitive reduction of water into hydrogen and use the fully modelized convective flow to post determine the exact electrode concentration of reactant and product at steady state. We will show how this strategy enables the determination of finer kinetic models. Such parameters would eventually be implemented into more complex, complete electrode and device models to optimize state of the art silver-based gas diffusion electrode design.[3]

The production of cheap energy from renewable sources, like solar energy, provides the opportunity to use electrochemistry for the synthesis of added-value products in a cost-effective manner. The reduction half-reaction has typically been used for the production of H₂, due to its high-density energy per weight unit. Additionally, the reduction of CO₂, to energy-rich chemicals (CO, formic acid, etc.) is gaining increasing attention these days. In this line, there are other alternative chemical routes, such as the synthesis of products for the chemical industry, which despite being much less developed, may present a good intrinsic economical interest.

The production of aniline by reduction of nitrobenzene is a very useful transformation, as these species are widely employed as building blocks for the production of aniline-based dyes, explosives, pesticides and drugs. This is a 3-steps mechanism, involving  6 electron and 6 proton processes. Each of these steps involves the insertion of 2e^- and 2H⁺ species in the NO₂ group of the molecule. This organic transformation requires the formation (and stabilization) of H^* in the electrode surface to reach an effective reduction and hydrogenation. This requirement highlights the importance of developing materials capable of producing H^* in order to succeed in this electro-transformation.

Electrodes made of Cu and Cu based compounds have efficiently been used for the electro-reduction of nitrobenzene in aqueous media due to their high energy of activation for the competing hydrogen evolution reaction (HER), thus enhancing the reactivity of the hydrogen radical in the organic reduction and increasing the coulombic efficiency for the organic transformation. Compared with copper, palladium shows high activity for the hydrogenation of organic compounds, mainly due to their affinity for the adsorption and storage of H^* species.

In this work, decoration of Cu foil surface with Pd by galvanic replacement technique was used to improve the catalytic properties of this material. Using bare Cu, the reduction of concentrated solutions of nitrobenzene has been tested. The introduction of Pd in the Cu surface enhanced the performance and selectivity of the electrode, achieving a complete reduction of a 30 mM nitrobenzene solution in 12 h at -0.8 V (vs Ag/AgCl), obtaining aniline with 70% yield. Finally, a detailed analysis using Impedance Spectroscopy has revealed the improvement in the catalytic performance of Cu with Pd electrodes by increasing the adsorption of hydrogen in the electrode surface, favoring the selectivity in the reduction of nitrobenzene to aniline.

Photoelectrochemical (PEC) oxidation of alcohol to the value-added aldehyde using semiconductor materials has been one of the promising approaches for green chemistry synthesis. It offers a low-temperature and low-pressure reaction environment to replace high pressure and high energy consumption of the industrial approach using silver catalysis. However, such reaction has been overlooked in the PEC research since alcohol is often used as hole scavengers and the product of this oxidation has not been attracted great attention. In addition, the aldehyde product, as the result of the highly selective oxidation product from alcohol PEC oxidation[1], is still highly reductive. Therefore, the reason that the oxidative photogenerated holes in hematite could not further oxidize aldehyde still remains unclear.

My presentation will primarily focus upon the acetaldehyde formation from PEC ethanol oxidation and its over-oxidation by the highly oxidative photogenerated holes in hematite photoanodes. The kinetic competition of alcohol oxidation and aldehyde oxidation will be primarily elucidated based on operando-spectroelectrochemical measurements[2]. In addition, the Ahhrenius relationship will also be established to elucidate how we can achieve highly selective aldehyde formation by linking thermodynamic and kinetic information learnt from such optical spectroscopies. On the other hand, the challenges in photo-oxidation of organics for the application of water purification will be discussed in detail in terms of the thermodynamic driving force and the reaction kinetics of the photocatalysts.

Hydrogen, when produced by electrochemical water splitting, powered by renewable and clean energy resources such as solar energy, tidal and wind, has emerged as a promising energy vector to respond to this increasing energy demand and to decarbonize transportation, heating and fine chemicals sectors.Traditionally, Pt and Ru based catalysts are considered the most efficient materials for the HER, however, they are expensive and scarce. More recently, electrocatalysts based on earth-abundant and cost-effective materials have been reported with remarkably efficient HER performance.

In this work, we report a (i) cost-effective and (ii) facile to synthetize reproducible Cu₂S electrocatalyst with (iii) current densities close to 400 mA cm^-2 and Faradaic efficiencies near 100% for HER. With this electrocatalyst we achive long stabilities never before reported.

In the search for efficient electrocatalytic materials and devices, parameters such as current density, potential, selectivity and stability have played key roles due to the their links to future scale-up costs. These direct indicators, however, treat both small and large cells as a black box system where spatial variations across a catalyst are averaged into a singular measured output. Within this work we introduce a secondary method to visually compare electrocatalytic activity across a catalyst's surface using time-resolved infrared imaging. Specifically, the spatial temperature of a catalyst's surface is measured with 10 mK accuracy, which we show provides a link to the overpotential and activity of the catalyst at a given location. Such techniques can be applied to multiple material surfaces for combinatorial catalyst testing, as well as using a singular material to observe current density distributions across a gas-diffusion layer.

In general, heat generation within electrochemical systems is poorly studied, and the disambiguation of various heating effects (overpotential, ohmic, gas-diffusion layer resistivity) is useful for improving not only catalyst development, but scale-up of electrochemical systems. As a demonstration of the proposed infrared measurement method, we show the vast temperature difference occurring between a Pt and Ag catalyst for water-splitting, as well as the effects of CO2 dissolution on the temperature rise during CO2 electrolysis in an alkaline environment. Such observations provide a means of detecting premature gas-diffusion layer flooding, spatial variations, temperatures for reaction-diffusion modelling systems and flow imbalances. The thermal imaging method then has broad applications for H2O, CO2, CO and N2 reduction applications which all contain different phenomena and challenges.

Increasing the structural complexity of (photo)electrodes is a possible approach to obtain higher activities and selectivities in – for example – CO₂ reduction or oxygen evolution. Many synthesis methods produce random, porous morphologies in which the chemical and physical conditions may vary significantly with respect to the bulk, affecting activity and selectivity towards the desired product.

Here, we use a combined experimental-numerical technique to obtain, digitalize and quantitatively characterize morphologically complex (photo)electrodes. The material is digitally reconstructed starting from a physical sample investigated by means of the FIB-SEM nanotomography technique, with resolution of up to 4 nm in the three dimensions. The obtained image stacks are processed to obtain a digitalized representation of the morphology, which subsequently can be quantitatively characterized and used in pore-level transport simulations.

We present quantitative morphological characterization of the catalyst layer of five CO₂ reduction gas diffusion electrodes. We calculated and compared roughness factor, specific area, connected volume, volume fraction profile, opening size distribution, chord length distribution, and two points correlation, providing a range of parameters difficult or impossible to obtain through experimental methods giving quantitative insight on electrode morphologies.

The digitalized morphologies can be meshed and used as geometrical domain in simulations for the characterization of transport properties and the extraction of effective physical properties (e.g. effective diffusivity or absorption coefficient) or in direct coupled pore-scale simulations for the characterization of operando physical and chemical conditions within the pores. The presented methodology is widely applicable, and will allow to shine some light on the link between morphology and physical properties of materials.

Electrocatalysts play an important role in the transition towards a sustainable society. Nanomaterials are used as catalysts for a range of electrochemical reactions, and the activity and selectivity of nanocatalysts depend on many factors.1 While extensive efforts have been devoted in optimizing electrode performance with high activity and selectivity, the poor stability of nanomaterials becomes a critical issue. The dynamic nature of nanoparticles under electrochemical conditions results in structural variations that determine the selectivity and activity on the single nanoparticle level. Therefore, thorough understanding of electrocatalysts requires both bulk electrode and single nanoparticle analysis.

TOC. a. Schematic drawing of in situ electrochemical cell for X-ray diffraction studies. b, c. Measured diffraction intensity of Cu nanoparticle in absence (b) and presence (c) of CO2 saturated 0.1 M KHCO3. d. Top isosurface view (top) of the reconstructed modulus. Side isosurface view (bottom) of the reconstructed modulus. The missing electron density corresponding to a part with a different crystallographic orientation (twin) is clearly visible. The tick spacing corresponds to 50 nm. e, f. 2D cross-sections of displacement field of Cu nanoparticle in absence (b) and presence (c) of CO2 saturated 0.1 M KHCO3.

In this work, we demonstrate a versatile in situ X-ray diffraction-based methodology which enables the structure and phase analysis of bulk electrodes and lattice strain analysis of single nanoparticles. A novel in situ electrochemical cell configured in back-illumination geometry has been fabricated to perform X-ray diffraction (XRD) and Bragg Coherent Diffraction Imaging (BCDI) with unfocused and focused beam respectively. In situ XRD has been collected during stepped cyclic voltammetry CO₂ saturated 0.1 M KHCO₃ to trace the phase evolution of Cu electrode, while ex situ BCDI has been performed on single Cu nanoparticle in absence and presence of CO₂ saturated electrolyte. BCDI results demonstrate that that displacement appears mostly at the twin boundary and edges even without the presence of electrolyte. After introducing electrolyte, the displacement field at the twin boundary and the surface change significantly, which can be caused by adsorbed species or surface oxidation. With the strain and displacement field information, we observed the influence of electrolyte on the surface and boundary structure, which can help to identify the active sites and holds great promise for establishing the structure-performance relationship.

The massive quantities of fossil fuels used by our society have led to unprecedented atmospheric CO₂ levels with widespread climate impacts. Carbon capture, utilization, and storage (CCUS) technologies are being developed to mitigate CO₂ emission issues. Large-scale CO₂ capture and sequestration facilities, such as Petra Nova, have been built to store thousands of tons of CO₂ per day. However, the typical capital investment for centralized CCUS facilities is at a billion-dollar scale, making it challenging to finance. Sequestering the captured CO₂ in geological repositories often requires additional investment in CO₂ pipelines and infrastructure, which further increases the financial challenge to the rapid deployment of highly centralized facilities rapidly. More importantly, the carbon capture and sequestration process itself is not profitable without subsidies or a carbon tax. As a critical component in CCUS, carbon utilization holds the key to generating revenues that can offset the capture cost. It enables the captured CO₂ to be converted into valuable materials, such as concrete, building materials, and platform molecules for fuel and chemical productions.

Recently, significant progress has been made in low-temperature CO₂ electrolysis to carbon monoxide, formic acid, ethylene, and ethanol, which raised the demand for systematic techno-economic assessments (TEA) to evaluate its feasibility as a CO₂ utilization process. In this talk, we will present our recent TEA of four major products and prioritizes the technological development with systematic guidelines to facilitate the market deployment of low-temperature CO₂ electrolysis. We will first discuss the present state-of-the-art electrolyzer performance and parameterize figures of merit. Then, we will discuss a detailed roadmap to make C₂ product production economically viable; an improvement in an energetic efficiency to ~50% and a reduction in electricity price to 0.01 USD/kWh. We also propose industrially relevant benchmarks: the 5-year stability of the electrolyzer components and the single-pass conversion of 30% for C₁ and 15% for C₂ products. In addition to the TEA work, we will present our latest experimental efforts to establish a two-step tandem electrolysis process to convert CO₂ into ethylene and acetate with a high selectivity. We will show how operating conditions affect the performance of the tandem system and challenges associated with this system. Finally, we will discuss potential approaches to further improve the efficiency of the overall system.

A sustainable future requires that we harness renewable but intermittent sources of energy and transmit or store it to address real world patterns of use. Renewable energy can be used to sequester CO₂ into a variety of products, such as carbon-neutral fuels and chemical feedstocks, thereby reducing atmospheric CO₂. Reducing atmospheric CO₂ levels requires the substitution of clean power for carbon-intensive fuels as well as CO₂ conversion processes that transform emissions into useful chemical products. To reach cost targets for widespread commercial adoption, materials must enable more effective multiphase flow phenomena than what currently exists. This talk will discuss our latest work on performing in operando imaging of carbon dioxide electrolyzers to understand the role of mass transport losses on overall performance. Designing these materials requires the a priori knowledge of how the heterogeneous properties of the porous materials and their interfacial contacts influence electrochemical performance. I will discuss critical design factors and how they influence the flow and mass transport behaviour in carbon dioxide electrolyzers.

Recently, carbon-based catalyst materials doped with nitrogen and transition metals (MNC) have emerged as a selective and cost-efficient alternative to metal catalysts for the of CO2 electrochemical reduction (CO2RR).  [1] Usually, these solid catalysts are prepared using heat treatment of a carbon precursor, nitrogen containing compounds and inorganic salts to form different nitrogen functionalities, which coordinated to the metal centers resulting catalytically active MNx single sites.These different functionalities tend to be non uniformly distributed making it challenging to establish the correlations between the structure and the catalytic activity

To address these challenges, we have prepared polyanilline derived MNC catalysts and observed that the catalytic performance of MNCs toward the CO2RR is affected by the structure and composition of the catalyst and by the conditions of the catalytic process. In particular, we observe that the metal center plays a crucial role in activity and selectivity of the process. While N functionalities do have some activity, MNx moieties play a predominant role and a higher concentration of FeNx correlates with higher partial current densities towards CO production. [2]

In addition to the catalyst structure and composition the reaction conditions also play a crucial role in determining the reaction’s selectivity. In particular, the pH can be used to tune the ratio of the different products. [3] For a polyaniline derived FeNC catalyst it was observed that hydrogen formation was strongly dependent on pH, while the CO production was not affected, resulting in high selectivities toward CO at high local pH values. Nevertheless, working in the absence of protons (in a non-protic electrolyte) resulted in the suppression of both the HER and the CO2RR, as both reactions require protons. We found that adding small amounts of water enhances the CO2RR while keeping the HER low, showing that there is an optimal proton concentration to ensure a high activity and selectivity. 

CO2 conversion to valuable products has been recently highlighted and made significant progress in terms of current density as well as product selectivity. For instance, CO or formate can be achieved more than 95 % of Faradaic efficiency from electrochemical CO2 reduction reaction (CO2RR). The next technical challenges in the catalyst development is achieving long-term stability for the practical application. During electrochemical CO2 reduction reaction, various factors can affect the stability of the catalyst, and metal impurities (ions) in the electrolyte can be especially detrimental for CO2RR because the transition metal impurities can be depostied on the catalyst surface and activate hydrogen evolution reaction (HER). In this talk, I will compare Ag nanoparticle electrocatalyst with N-doped carbon based electrocatalyst for the electrochemical CO2 reduction reaction to CO production. We found that N-doped carbon catalysts have high resistance to the impurity metal attack in the electrolyte, and long-term durability was achieved^1-2. In addition, for the practical point of view, low concentration of CO2 gas feed has to be considered as well because the flue gas contains diluted CO2 concentration not pure CO2. As decreasing the concentration of CO2 feed, CO2RR selectivity over hydrogen evolution reaction (HER) can be senstively affected. We also find that at low concentration of CO2, Ag nanoparticle showed decreased CO Faradaic efficiency due to increased HER, and modifying the operational condition in the gas-diffusion electrode (GDE) based membrane electrode assembly (MEA) electrolyzer can affect the water supply to the catalyst layer and thus contribution to suppressing HER. We propose that the CO selectivity can be adjustable both of intrinsic catalyst activity as well as extrinsic properties in MEA at low CO2 concentration condition.

I willrevise the modelling advances fro. Our group trying to improve our understanding of CO2 reduction with particular atttention to cations and C3 formation.

In the electrochemical CO2 reduction reaction (CO2RR) selectivity still remains an important issue.

In this talk, I will showcase a few examples which highlight how shape-controlled nanocrystals can contribute to address this challenge. First of all, I will discuss how size control of Cu nanocubes (Cu_cub) and Cu octahedra (Cu_oh) has revealed the importance of facet-ratio to maximize the selectivity towards ethylene and methane, respectively. Second, I will present our recent computational-experimental efforts towards using well-defined NCs to elucidate selectivity rules at the hydrocarbons/alcohols branching nodes in the CO₂RR pathway. The formation of ethanol via *CH_x-*CO coupling or *CO-*CO coupling is an open debate in the literature. As a platform to address this question, we have used CH₄ favoring (i.e. *CH_x populated) Cu_oh and C₂H₄ favoring (i.e. *CO-*CO populated) Cu_cub under enhanced *CO coverage induced by the presence of Ag NCs. The selective promotion of ethanol on the Cu_oh provided evidence for *CH_x-*CO coupling being the preferred pathway. Following theoretical predictions, we have also demonstrated that such pathway is favored on Cu(110) edge sites compared to Cu(100) terraces by studying the size-dependent edge/face ratio of the Cu_cub always in the presence of the CO-generating Ag domains. Indeed, we found that smaller cubes are more selective for ethanol than bigger sizes. Generally, these examples encourage the application of well-defined nano catalysts as a bridge between theory and experiments in electrocatalysis.

The electrochemical reduction of carbon dioxide requires access to ample CO₂ gas and liquid water to fuel reactions at a high rate for industrially relevant applications. The application of gas-diffusion electrodes has notably improved the current densities by over an order of magnitude by positioning the catalyst at gas/liquid interfaces. Such configuration also adds complexity to the electrode that no longer consists of the only catalyst but also a multi-scale network of the gas-liquid-solid interfaces. Therefore, electrode wettability serves a vital role in determining the electrochemical surface area and gas pathways in the three-dimensional electrode structure and thus has a profound impact on the electrode activity, product selectivity, and long-term stability. Based on the content of our recent review paper[1], this presentation will provide an overview of the possible mechanisms underlying the phenomena such as electrowetting and salt precipitation commonly observed in the field of CO₂ electrochemical reduction and discuss the inevitable critical issues in balancing electrochemical surface area and gas pathways. At last, the presentation will conclude with some recent attempts and future outlooks to address these critical issues and redesign the next generation of gas-diffusion electrodes.

In order to address the threat of climate change, the electrochemical conversion of CO2 represents a viable solution[1]. Herein, the electroreduction of CO2 under atmospheric conditions has been performed in a continuous flow cell over gas diffusion electrodes (GDEs). A porous and conductive support has been employed to this end, where a Cu-based[2] catalyst has been manually deposited on the substrate by means of an airbrusher. Several variables of the investigated system have been assessed, with the aim to enhance the production of CO2 reduction liquid products. The most promising conditions have been explored among the cathodic applied potential, catalyst loading, binder content, electrolyte concentration and the presence of metal oxides in the catalytic material, like ZnO or/and Al2O3. In particular, it has been found that the binder content (Nafion) has affected the selectivity toward CO, leading to syngas with a H2/CO ratio of ⁓1 at the lowest Nafion content (15%). On the contrary, the highest Nafion content of 45% has led to a rise in C2+ products formation and a decrease of CO selectivity by 80%. The study undertaken revealed that liquid crossover, linked to electro-wetting, affects the GDE performance by severely compromising the CO2 transport to the active sites of the catalyst, and thus reducing the efficiency of CO2 conversion. A mathematical model[3] confirmed the role of a high local pH in promoting the formation of bi-carbonate species: beside the undesired consumption of CO2 with OH- ions, salts formation may cause the catalyst deactivation and hinder the mechanisms for C2+ liquid products. The ultimate intent of this work is to shift the attention of the scientific community toward other players of the CO2 reduction process, which can impact on both kinetics and mass transport and consequently on carbon efficiency of this systems[4], rather than narrowing the focus on the sole catalytic activity of the materials.

The production of value added compounds within CO₂ electrolyzers has reached sufficient catalytic performance that system and process performance  such as CO₂ utilization have come more into consideration. Efforts to assess the limitations of CO₂ conversion and crossover within electrochemical systems have been performed, providing valuable information to position CO₂ electrolyzers within a larger process. Currently missing, however, is a clear elucidation of the inevitable trade-offs that exist between CO₂ utilization and electrolyzer performance, specifically how the Faradaic Efficiency of a system varies with CO₂ availability. Such information is needed to properly assess the viability of the technology. In this work, we provide a combined experimental and 3D modelling assessment of the trade-offs between CO₂ utilization and selectivity at 200 mA/cm² within a membrane-electrode assembly CO₂ electrolyzer. Using silver gas diffusion electrode and varying inlet flow rates, we demonstrate that the variation in spatial concentration of CO₂ leads to spatial variations in Faradaic Efficiency that cannot be captured using common ‘black box’ measurement procedures. Specifically, losses of Faradaic efficiency are observed to occur even at incomplete CO₂ consumption (80%). Modelling of the gas channel and diffusion layers indicated at least a portion of the H₂ generated is considered as avoidable by proper flow field design and modification. The combined work allows for a spatially resolved interpretation of product selectivity occurring inside the reactor, providing the foundation for design rules in balancing CO₂ utilization and device performance in both lab and scaled applications.

The boost in current density is a major achievement in the past decade for electrochemical CO₂ reduction (ECO2R) process. Nevertheless, low CO₂ utilization coming along with the high performance has been a setback for commercializing CO₂ electrolyzers. The maximum utilization efficiency of CO₂ in a device is plateaued at 50% under common operating conditions due to the parasitic loss of CO₂ in the system. A promising approach is to introduce excess protons near the reaction domain. H⁺ can be provided either directly from an acidic catholyte or via a membrane, both of which will either neutralize the OH^- produced directly, or regenerate CO₂ from (bi)carbonates. Alternatively to acidic catholyte, protons provided by ion exchange membranes can be in a direct relation to the applied current density. Bipolar membranes (BPMs) operated under reversed bias can internally splits water to protons and hydroxide ions, thus protons are sent to the cathode and hydroxide to the anode. In addition, a BPM further allows for the use of an alkaline anolyte and non-noble metal-based anodes. Unfortunately, the use of BPMs reported in membrane electrode assemblies (MEAs) has shown low ECO2R activity, with H₂ remaining a dominant product. In this work, we firstly succeeded to increase the performance of BPMEA by increasing the cation (K⁺) concentration on the catalyst surface, which achieved CO faradaic efficiency of 68%. We then compared the CO₂ conversion and consumption efficiency in above BPMEA with traditional anion exchange membrane in a MEA cell (AEMEA). Results show a 5-fold reduction in lost CO₂, thus 2-3 times higher CO₂ utilization efficiency in a BPMEA than AEMEA at similar current densities. This work addresses the direct importance of alkali cation concentrations when using BPMs in MEA cells, providing an approach for tackling low CO₂ utilization issue in common CO₂ electrolyzers at present.

The electrocatalytic reduction of CO₂ (CO₂R) presents a sustainable pathway for producing fuels and chemicals. Cations have a strong influence on the reaction activity and selectivity, and there are currently three theories regarding their main role: (1) to modify the local electric field, (2) to buffer the interfacial pH and (3) to stabilize reaction intermediates. Here, we tested these theories, and were able to define the main mechanism that explains how cations interact in the CO₂R reaction. Cyclic voltammetry and Scanning Electrochemical Microscopy (SECM) measurements were performed without metal cations and in the presence of Cs⁺. On gold, CO₂R does not take place at all in pure 1 mM H₂SO₄, unless Cs⁺ ions are added to the electrolyte (Fig. a). We used SECM to locally probe if CO is produced on other common CO₂R catalysts, namely copper and silver. Without a cation in solution, CO₂R does not take place at all, regardless the metal surface. These remarkable observations allow us to define the main role of cations in the CO₂R reaction, and to propose a new reaction mechanism in which the cation is actually what enables the reaction to happen, by forming a complex with CO₂ and allowing the first reaction intermediate to be formed (Fig b). We supported this model with Density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations, which confirmed that CO₂R happens only if the CO₂^– intermediate is stabilized via a cation-oxygen chemical bond, which occurs depending on alkali cation solvation shell and, thus, on radius. From the system design point-of-view, it is desired to find charged species that have an even larger stabilizing effect on CO₂^–_(ads) than Cs⁺. Based on that, we have also performed CO₂R in acidic media in electrolytes containing: Li⁺, Cs⁺, Be²⁺, Mg²⁺, Ca²⁺, Ba²⁺, Al³⁺, Nd³⁺ and Ce³⁺. We find via experiments and DFT/AIMD that acidic cations with a moderate hydration radius, promote CO₂R in acidic media/low overpotentials, while the non-acidic weakly hydrated Cs⁺ is the cation that leads to more CO in alkaline media/high overpotentials. These differences come from the promotional effects acidic cations have on water reduction, only allowing these species to favor the selectivity towards CO₂R before the onset of this reaction. In this talk, we elucidate through experiments and simulations the main role of cations on CO₂R, and also which cation properties are important when designing an optimal electrolyte for this system.

Electrochemical CO₂ reduction has the potential to contribute to a closed carbon cycle by recycling CO₂ from various sources and converting it into sustainable chemicals. The reaction environment is crucial to achieve high activity, conversion and selectivity. Hydrogen evolution is typically dominating in acidic environments (due to the sufficient presence of protons) leading to low Faradaic efficiency. Recently, it has been shown that the hydrogen evolution reaction could be suppressed by adding extra non-reactive cations, such as potassium ions in the electrolyte. We developed an electric double layer model, solving for the transport of ionic species in the diffusion layer and charge distribution in Stern layer, to describe the potential and concentration distribution in close vicinity of the electrode. We showed that the potassium cations can migrate to the electrode surface and modulate the electric field strength, and therefore, reduce the migration of protons towards the electrode. The concentration of all ionic species, the potential profile, and the hydrogen evolution current density can be quantitively obtained. This model explained the mechanism of hydrogen evolution suppression by potassium ions which was also observed and convinced in the experiment. The model reveals the possibility of electrochemical CO₂ reduction in acidic environment and proposes a possible way to overcome the depletion of CO₂ in alkaline environments.

Electrocatalytic CO₂ reduction (eCO₂R) to valuable chemicals and fuels is of fundamental scientific and technological interest. However, it remains a huge challenge to control the selectivity of eCO₂R toward certain products with near unity efficiency. Bimetallic electrocatalysts have been regarded as one of the most promising strategies to improve the selectivity of eCO₂R, since the introduction of a second metal tunes the adsorption energy of key intermediates (e.g. CO and hydrogen). Here, we fabricated a bimetallic system of Sn-doped CuO nanoparticles for electrochemical CO₂ conversion. Electrochemical measurements demonstrate that introduction of a trace amount (0.4 %) of Sn enhances the Faradaic Efficiency (FE) of CO up to 98% at –0.75 V vs. RHE cathodic bias, with a stable performance over the course of 15 h. A volcano-shape relationship is found between the CO enhancement and the percentage of Sn (0-0.8%), with maximum FE for 0.4% Sn doping. In situ Raman spectroscopy measurements reveal the subsequent reduction of CuO_x and SnO₂ domains, which tunes the adsorption of CO at the catalyst surface. Our work showcases the potential of bimetallic electrocatalysts to boost the eCO₂R selectivity, and reveals the dynamic restructuring of the electrocatalyst under operating conditions through time-dependent Raman spectroscopy.

Anthropogenic activities have impacted the planet’s carbon cycle by the emissions of large amounts of greenhouse gases (GHGs), shifting the equilibrium of human history since the industrial revolution. The electrocatalytic CO₂ reduction (EC CO₂R) is an interesting technology because renewable energy sources could drive it; as well, it can be used to store both renewable electricity and CO₂ in added-value products such as liquid fuels (ethanol and other high-octane alcohols (>C₂).[1,2] To date, researchers have focused on observing the effects of surface modification (e.g., nano-structuring and surface tailoring) on catalysts selectivity and activity to produce C₂₊ products. Nonetheless, it remains an ongoing challenge due to high C-C coupling barriers. Among these studies, incorporating light heteroatoms such as boron (B) into the Cu catalyst has been reported to induce the formation and stabilization of Cu⁺¹/Cu⁰ interfaces and reduce the *CO dimerization barrier, promoting a high activity for the EC CO₂R towards C₂ products.[3] Herein, we proposed B-doped Cu oxide catalysts for the EC CO₂R process. Briefly, Cu-based powder was prepared by ultrasound-assisted co-precipitation method. Then, the Cu-based powder was impregnated in a solution of B₂O₃ and dried. The obtained material was calcined under air or N₂. The catalyst materials have been characterized by different physico-chemical methods like X-ray diffraction, BET, porosimetry, and filed-emission scanning electron microscopy (FESEM). Incorporating both B and N heteroatoms led to greater selectivity for reducing CO₂ to products of interest: that is, a total FE of 64% to CO₂ reduction products with 49% of liquid products (i.e. 77% selectivity towards formate and alcohols) at a total current density of 23 mA cm^-2(see Figure 1). This behaviour is attributed to the fact that these elements belong to the p-block of the periodic table, which stabilize the carboxyl group by promoting the dimerization of the *CO intermediate. The physical and chemical properties of the synthesized materials can be manipulated to tune the performance of the electrochemical reaction. These interesting results could help find a suitable electrocatalyst to establish this technology at the industrial level.

The electrocatalytic (EC) CO₂ reduction (CO₂R) can be exploited for the energy transition and to store C into valuable products like syngas (H₂/CO mixtures), organic acids (formic acid) and chemicals/fuels (C₁₊ alcohols).¹ A big challenge for the industrialization of this technology is to find low-cost electrocatalyst, efficient reactors and process conditions. Noble metals like Ag and Au are the most used ones for syngas production.² To reduce the catalyst cost, we developed electrodes made of Ag nanoparticles (NPs) on TiO₂ nanotubes (NTs),³ showing a higher electrochemical surface area and electrons transport than a bare Ag. Titania was used as an efficient support, for enhancing the stability of key CO₂^- radical intermediate and decreasing the EC CO₂R overpotential. We are also exploiting the current knowledge of the thermocatalytic CO₂ hydrogenation to develop noble-metal-free CO₂R electrocatalysts.^4–6 For instance, Cu/Zn/Al-based catalysts producing methanol and CO from the CO₂ thermocatalytic (TC) hydrogenation (at H₂ pressure (P) of 30 bar and temperature (T) > 200 ^oC) generates H₂, CO and other C₁ to C₃ liquid products during the electrochemical CO₂R in a gas-diffusion-electrode system; while operating in the liquid phase, the same catalyst produces syngas with a tunable composition (depending on the applied potential) and other liquid C₂₊ products (in both cases at ambient T,P). Our results open a promising path for the prospective implementation of metal-oxides nanostructures for the CO₂ conversion to the chemicals and fuels of the future. From an environmental and technoeconomic analysis applied to the case of study of the scaled-up  methanol production, including downstream purification processes, we determined that optimized EC process conditions could lead to a practical implementation of this technology; that is, recirculation of the not reacted CO₂, achievement of current densities in the range of 100-200 mA/cm² with faradaic efficiencies (> 90%), and use of renewable electricity sources (i.e. 30% of the total energy).⁴  Hence, considering the effective allocation of methanol on a real market scenario, the EC process results to be economically advantageous over the TC one at productivity scales as low as 19.1 kg/h, and could lead to a carbon footprint that is comparable to that of current industrial technologies for methanol production (climate change impact of 2.72 kgCO₂/kgCH₃OH). Moreover, in a scenario with a 100% renewable energy such as photovoltaic, it is possible to reach savings in that carbon footprint of up to 62%.

Disorder in real materials influences their properties and the chemical processes that occur at their interfaces. In order to unravel, and ultimately control processes at these interfaces, it is essential to gain a molecular-level understanding of the underlying physical manifestations caused by disordered materials. To accomplish this, measurement techniques through which disorder can be detected, quantified, and monitored are needed. However, such quantitative measurements are notoriously difficult, as effects often average out in ensemble measurements. In our lab, we have employed a combination of electrochemical and spatially resolved surface spectroscopy measurements to illuminate a molecular-level picture of disorder in materials. We study amorphous carbon which is an intrinsically disordered material. To the amorphous carbon, we covalently attached a monolayer of ferrocene. Interfacial electron transfer across the amorphous carbon–ferrocene interface is highly sensitive to the local microenvironment felt by the ferrocene, and thus to disruptions of order. By systematically varying linker properties and surface loadings, the influence of lateral interactions between nonuniformly distributed ferrocene headgroups on ensemble electrochemical measurements is gleaned.

The electrocatalytic reduction of carbon dioxide is a promising approach for storing (excess) renewable electricity as chemicalenergy in fuels. Here, I will discuss recent advances and challenges in the understanding of electrochemical CO2 reduction. I will summarize existing models for the initial activation of CO2 on the electrocatalyst and their importance for understanding selectivity. Carbon–carbon bond formation is also a key mechanistic step in CO2 electroreduction to high-density and high-value fuels. I will show that both the initial CO2 activation and C–C bond formation are influenced by an intricate interplay between surface structure (both on the nano- and on the mesoscale), electrolyte effects (pH, buffer strength, ion effects) and mass transport conditions. This complex interplay is currently still far from being completely understood.

Many technical challenges remain for implementing CO₂ electrolysis as a practical means for CO₂ utilization. A key challenge for CO₂ electrolysis is developing heterogeneous catalysts that can steer complex reaction networks and selectively convert CO₂ into the desired product. First, I will describe how epitaxially grown Cu thin films can be used to probe CO₂ electrolysis structure-reactivity relationships. We demonstrate that undercoordinated sites are selective motifs for oxygenates and C-C coupling using a combination of electrocatalysis experiments and in situ surface probe microscopy. Next, I will discuss how the same growth methods can be used to controllably deposit noble or base metal atoms onto well-defined Cu electrocatalysts and systematically investigate atomic-scale bimetallic effects. The metal atoms tend to suppress CO reduction to oxygenates and hydrocarbons while promoting competing pathways to CO, formate, and hydrogen, suggesting that the metal atoms segregate to undercoordinated Cu sites during physical vapor deposition. Finally, I will provide some perspectives on how to improve the intrinsic activity and selectivity of Cu through bimetallic effects, showing specific examples of enhancing C-C coupling through tandem catalysis and CO production through alloying with Zn.