Tailoring the structure of the electrochemical interface at the atomic and molecular levels can provide very valuable insight to understand and tune the electrocatalytic activity and/or selectivity of renewable energy conversion reactions. Model studies on well-defined interfaces are pivotal to understand the factors controlling both activity and selectivity in electrocatalysis.

The use of renewable electricity to reduce CO₂ into clean fuels and chemicals is very promising to convert CO₂ into valuable hydrocarbons and alcohols while contributing to close the unbalanced carbon cycle [1,2]. This presentation will summarise some recent strategies aiming to understand the structure-activity-selectivity relations for CO₂ and CO electrocatalysis [1]. We have investigated the interfacial properties of Cu single-crystalline electrodes in contact with different electrolytes. We have studied the effect of pH, specific anion adsorption, and potential dependence of interfacial processes on Cu single-crystalline surfaces for CO electroreduction [3]. In phosphate buffer solutions, the adsorption of phosphate anions depends strongly on both the pH and the geometry of the active site. In the presence of CO, phosphate adsorption affects the potential range at which CO poisons the surface. We show how well-defined studies, under potential control, are essential to understand the structure-function relations and, ultimately, to rationally design highly efficient electrocatalysts for CO₂ reduction.

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 suffers from the lack of efficient, selective and stable catalysts. Furthermore, fundamental catalytic studies should be complemented with investigations under commercially-relevant conditions to assure actual progress in the field.

In this talk, I will present our recent group efforts towards the synthesis of atomically defined nanocrystals (NCs) via colloidal chemistry and their use as CO₂RR catalysts.

First of all, I will illustrate how tailor make copper NCs has helped to reveal synergy between shape and size and the importance of facet ratio in CO2RR. Furthermore, I will show that these relationships hold even when these catalysts are integrated in a gas-fed electrolyzers at technologically relevant conditions with currents up to 300 mA/cm²

I will then provide some examples of hybrid catalysts where copper domains are interfaced with a metallic doman or with a metal oxide doman. Specifically I will focus on Cu-Ag nanodimers, that combine tandem catalysis and electronic effects to promote C-C coupling, and Cu-CeO2 nanodimers, that promote methane formation as a result of ceria reduction and oxygen vacancies formation.

Finally, I will discuss the role of NCs as model systems to study degradation pathways using microscopy tenchiques These insights are essential when moving towards an actual technological implementation.

Recent developments in electrocatalyst design are revealing new trends in carbon dioxide conversion product selectivity. Various design principles -- such as catalysts based on metal oxides, doped metals or metal alloys, and metal atoms in molecular coordination environments -- demonstrate behaviors which differ from their simple metal counterparts, revealing strategies toward enhancing selectivity toward high-value products while suppressing undesired ones. Continued rational development of catalysts demands that we have a detailed understanding of the structure-function relationships which dictate selectivity. However, under the harsh reaction conditions of CO₂ reduction (e.g. highly negative potential, local pH extremes) many of these catalysts are prone to significant structure changes, making it difficult to understand the true catalytically active form of the electrode materials.

X-ray absorption spectroscopy techniques can be uniquely powerful in investigating electrochemical systems under operating conditions. The high energies of X-ray photons enable them to be used under ambient conditions and to pass through liquid electrolyte. Element-specific information can be obtained due to the quantized nature of electronic transitions. A wide range of information can be revealed using X-ray spectroscopy methods, including composition, oxidation states, and local coordination environment. But there are numerous limitations and challenges to successfully investigating an electrochemical system using X-rays, requiring detailed understanding and careful planning. In this talk, I will summarize key concepts of different types of in situ X-ray spectroscopy techniques applied to electrochemical CO₂ conversion, with select examples from the literature as well as a case study on our investigation of a bimetallic catalyst using both hard and soft X-ray absorption methods.

Models based on the computational hydrogen electrode (CHE) [1] have greatly contributed to the discovery and enhancement of electrocatalysts for numerous reactions [2]. Because of its intrinsic complexity, CO2 electroreduction is, so far, a salient exception to the rule [3]. While the shortcomings have traditionally been attributed to the lack of kinetics in CHE-based models, I would also like to point out two thermodynamic factors that limit their accuracy: the presence of systematic errors in the gas-phase molecules simulated with DFT, and the neglecting or insufficient incorporation of solvation effects on the adsorption energies of the reaction intermediates. In my talk, I will show that affordable solutions to those two problems exist that lead to better quantitative agreement with experiments [4]. 

An ever-increasing number of works have assessed the activity of CO2 electrolysis on copper catalysts, specifically targeting high selectivities towards multi-carbon products such as ethylene and ethanol. From a catalytic perspective, many works have examined the impact that catalyst morphology and restructuring have on activity. The conditioning of the pre-catalyst, exposed facets and applied potential have continuously been shown to impact not only C2 product formation, but the stability of the catalyst.

As part of the 'catalyst', the environment around copper is similarly important. The role of electrolyte pH has been relentlessly assessed both experimentally and theoretically, and has shown to be a dominating factor in CO2 reduction; heavily due to hydrogen evolution maintaining a pH-dependent potential as compared to the pH-independent nature of CO2 to *CO. For these reasons the selectivity of CO2 reduction benefits at higher pH, and has shown large performance differences when using KHCO3, KCl and KOH electrolytes.

In this work we describe the impact of changes in the reaction environment at higher current densities for 3 different electrolytes. Specifically, we wanted to understand if the choice of electrolyte was important as it seems when production rate is pushed to the extreme using gas-diffusion electrodes. Through a combined modelling and experimental study we show that 1 M KHCO3, KCl and KOH show identical catalytic performance at 300 mA/cm2. Using experimental data as an input we find that [CO2], [CO], pH and the standard hydrogen potential are similar at higher current densities despite their apparent differences at lower current densities. Such findings help us to consolidate past findings within the field, and begin to better understand the interplay between catalysts, the electrolyte and proximity to a gas-liquid interface.