The electrocatalytic reduction of CO₂ (CO2RR) into valuable base chemicals and fuels is a very complex reaction that depends on the intimate relation between catalyst structure and external reaction conditions. Despite considerable progress over the past few years, it is evident that the precise identification of the active sites of the electrocatalyst under operation remains a challenge, which hinders the rational design and industrial application of advanced electrocatalysts for eCO2RR. For this purpose, in situ characterization techniques are required that probe the catalyst structure, from bulk to surface, with improved time and space resolution.

In this presentation, I will discuss how we deploy in situ time-resolved Raman spectroscopy (TR-SERS), in situ fluorescence and advanced in situ synchrotron-based X-ray scattering and spectroscopy techniques to investigate the electrocatalytic activation of CO₂ and the dynamic chemical structure of the electrode surface at the electrode-electrolyte interface.

Resolving the distribution of ionic species at the electrode-electrolyte interface is of key importance for all electrochemical processes. Specifically, the configuration of cations [1], anions [2], and solvent molecules [3] in the double-layer region governs the binding of surface adsorbates and the accompanying electron transfers at the electrochemical interface. Characterization techniques with such interfacial sensitivity are however limited. While vibrational spectroscopy methods can provide useful insights into this regime under electrochemical conditions, their lack of element specificity (relies on chemical bond vibrations) calls for complementary techniques. In the presented work, we introduce a combination of in situ X-ray spectroscopy techniques applied to the investigation of the electrical double layer (EDL) formed at the metal electrode-aqueous electrolyte interface. We will discuss how the combination of X-ray spectroscopic technique with the dip-and-pull geometry (Fig. 1) can satisfy both conditions necessary to monitor the EDL in situ: (1) an ultrathin electrolyte film and (2) an electrochemically connected system. We will demonstrate how X-ray photoelectron spectroscopy (XPS) with the dip-and-pull and geometry can yield information about the potential dependent distribution of K⁺ cations across the EDL. We present a study of how such potential dependence near a Au surface might be affected by non-adsorbing compared to specifically adsorbing ions. Furthermore, we report how total electron yield X-ray absorption spectroscopy (TEY-XAS) combined with the dip-and-pull geometry can simultaneously indicate changes in the coordination of K⁺ cations at a metal-aqueous electrolyte interface (Fig. 1). Through these case studies, we will highlight the challenges and prospects of the dip-and-pull geometry combined with X-ray spectroscopy for the study of electrocatalytic interfaces. The chemical universality of this approach makes it compatible with various electrolyte compositions and thus, with a broad range of heterogeneous electrocatalytic reactions. In light of the development of the dip-an-pull approach, we will discuss the outlook of applying it to more complex electrocatalytic systems like the alkaline hydrogen evolution reaction (HER) or the CO₂ reduction reaction (CO₂RR).

Proton exchange membrane water electrolysis (PEMWE) demands a highly active and stable bifunctional electrocatalyst for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in acidic electrolytes. In this study, we present a high entropy alloy (HEA) ZnNiCoIrMn as an effective bifunctional electrocatalyst for HER and OER under acidic conditions. A nanoporous structure was obtained by facile dealloying method in a vacuum system using Zn as a sacrificial element. In particular, the electronic structure of Ir was modified by incorporating Mn. The downshift of the d-band center separates the d-band center of Ir away from the Fermi level, weakening the adsorption energy with reaction intermediates. The result is that weaker adsorption energy is beneficial for the catalytic reaction. The electronic structure of Ir can be further tailored by composition control, resulting in optimized adsorption energies that are required not only to improve catalytic activity but also to prevent the dissolution of the metal atoms. The diffusion of elements in ZnNiCoIrMn is limited by high entropy effects, which also contributes to prohibiting the loss of elements during electrolysis. A low overpotential of 29 mV and 242 mV was required for HER and OER to generate the current density of 10 mA cm^-2 despite low Ir content in ZnNiCoIrMn. Furthermore, ZnNiCoIrMn shows high stability for HER and OER over 200 h. It demonstrates that weak adsorption energy of HEA induced by compositional engineering suppress solvation of elements, which is responsible for the enhanced durability of OER and HER under acidic conditions.

For the development of efficient electrochemical interfaces, which are vital for sustainable energy technologies, it is essential to accurately identify the molecular structure of the electric double layer, a critical region where all electrocatalytic reactions occur. While current electrochemical scanning probe microscopy techniques are effectively used to examine structural changes on an electrode surface, even under reaction conditions, they generally lack sensitivity to the solvent structure on the liquid side. Conversely, high-resolution atomic force microscopy (AFM) has demonstrated the ability to visualize the vertical arrangement of solvent molecules perpendicular to the surface, as shown, for example, in Ref. [1]. Nonetheless, there are only a limited number of studies that explore this capability with potential control in an operating electrochemical cell. In this study, we utilize electrochemical AFM equipped with stiff qPlus sensors[2,3] to investigate the potential-dependent solvent layering at clearly defined electrified solid-liquid interfaces with high spatial precision. Our experiments on Au(111) electrodes in various aqueous electrolytes demonstrate pronounced oscillatory shifts in frequency along the z-axis (normal to the surface plane). These shifts, influenced by the electrode's charge, the applied potential, and the specific ions present, are attributed to the layering of water and/or ions near the electrode surface. The observations are supported by corresponding atomistic molecular dynamics simulations.

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

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

Determination of active sites in electrocatalysis is a crucial aspect of understanding and enhancing the performance of electrocatalysts^1,2. Active sites are the specific locations on the catalyst's surface where electrochemical reactions occur, playing a key role in determining the catalyst's efficiency and selectivity. Identifying these sites requires typically a combination of experimental techniques. Common methods include spectroscopic techniques such as X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy (IR), which provide information on the chemical environment and electronic states of the surface atoms. Additionally, advanced microscopy techniques like scanning tunneling microscopy (STM) and transmission electron microscopy (TEM) can visualize the surface structure at the atomic level. Understanding the nature and behavior of active sites allows for the rational design and optimization of electrocatalysts, leading to improved performance in various applications.

In this talk, I show some examples of our research on using in situ X-ray absorption near-edge structure (XANES), synchrotron X-ray diffraction (XRD), and quasi-in-situ X-ray photon spectroscopy (XPS) to understand the nature of the catalytic site two reactions of interest: CO₂ electroreduction,  and oxygen evolution reaction (OER).

This talk will discuss how electrolyte selection affects the thermodynamics and kinetics of water adsorption on metals and metal oxides. Water and ion adsorptions can modify the structure and activities of electrode-electrolyte interfaces; however, mapping their structure-property relationship has been challenging. To address this issue, we measure how bulk experimental variables such as solution pH and ions systematically affect water and ion adsorptions. In the first part, I will discuss the effects of solution pH and ions on the hydroxide and oxygen adsorption on well-defined metal-oxide surfaces [1]. The results will be compared to single-crystal platinum as a model for metals. In the second part, I will discuss the same effects but on kinetics to understand the adsorption mechanism [2]. If time permits, I will also show how pH and solution ions can modify the electrode-electrolyte interface using Second Harmonic Generation (SHG) spectroscopy, where we optically monitor the electrode potentials to infer information on the water orientation [3].

While the performance of an electrocatalyst depends on its structure and composition, there is also a drastic and so far not well-understood influence of the electrolyte composition on the catalyst activity as well as the double layer properties [1-7]. In other words, today’s electrocatalysis operates typically with a paradigm that there are only two major ways of increasing the catalytic reaction rate at a given electrode surface. The first involves modifying the electrode surface structure to maximize the number of active sites. The second one deals with the optimization of the electrode composition. Both approaches often exclude an essential part of the “activity equation”: the electrolytes. Interestingly, when the structure and composition of the electrode surface are fixed, the influence of the “inert” electrolyte species is often drastic. Therefore, the resulting electrocatalyst performance combines at least three major factors: electrode surface structure, electrode composition, and electrolyte composition. The lecture will present and discuss several concepts explaining the “electrolyte
effect” in electrocatalysis.

Electrocatalysis at the electrode-electrolyte interface is fundamentally governed by electron transfer across the electric double layer (EDL), highlighting a crucial mechanistic link between electrocatalytic properties and EDL structure. A central question in this field is the role of alkali metal cations at this interface, commonly known as the “cation effect”. This presentation will outline our ongoing research aimed at uncovering the fundamental principles underlying the cation effect in several important electrochemical reactions in aqueous environments. Using advanced in situ analytical techniques, we have discovered that alkali metal cations are not merely spectators, as traditionally believed, but actively influence the kinetics and mass transport in electrocatalysis. The identity and concentration of the alkali metal cation are critical in modulating electrocatalytic activity and selectivity, as well as affecting electrode stability. We will present a mechanism of cation-coupled electron transfer and its potential role in alkaline hydrogen evolution reactions. By gaining a deeper understanding of how alkali metal cations affect electrocatalysis, we aim to propose a novel conceptual framework for improving electrocatalytic processes.

Electrolyte effects have been the subject of intensive research in the past decade in the field of CO₂ electrocatalysis [1]. In this contribution, I will present some recent results on CO₂ and CO electroreduction on copper and other transition metal electrodes.

First, I will show that the usefulness of CO₂ or CO as feedstocks for electrolysis is determined to a great extent by the morphology of Cu electrodes, as the structural sensitivity of CO₂ and CO in presence of alkaline cations is remarkably different [2].

Moreover, I will show that cation effects on the intermediates of CO₂ electroreduction are systematic for a large number of species involved in CO2 reduction to C₁ products. In view of their systematicity, I will show that the effects are also predictable in simple terms [3].

Finally, I show that a model combining experimental and computational data explains how cationic ammonium surfactants substantially enhance the electroreduction of CO₂ to CO on silver and zinc electrodes but not on gold [4].

Interfacial electric fields play a crucial role in facilitating various electrocatalytic processes. Understanding how these fields emerge from the structure of the electrochemical double layer (EDL), particularly the distribution of ions within the EDL, is essential. In this work, we address this challenge by employing multiple vibrational spectroscopic reporters, which enabled us to simultaneously measure the accumulation of cations in the EDL and the resulting electric field. Specifically, we employed surface-enhanced infrared absorption spectroscopy (SEIRAS) to investigate the accumulation of an infrared-active organic cation, tetramethylammonium, at the aqueous electrolyte/polycrystalline gold (Au) interface as the electrode potential decreases. Additionally, we used vibrational Stark spectroscopy to monitor the resulting interfacial electric field by examining the CO stretch of surface-adsorbed CO. Our findings show a structural change in the EDL at -0.7 V versus the Ag/AgCl electrode. This potential coincides with a change in the Stark tuning slope of the CO stretch, indicating that the structural change in the EDL enhances the interfacial electric field. We will discuss the implications of this finding for electrocatalysis. This study further illustrates how multiple vibrational reporters can be leveraged to gain insights into the connection between interfacial structure and the resulting electric field at catalytic sites.

Overall, the oxygen evolution reaction, written 2 H2O --> O2 + 4 (H+ + e-), involves increasing the formal oxidation state of two oxygen atoms from minus two to zero. Along the way, the electrocatalyst facilitates this overall redox transition through redox movements of its own as metal-oxygen bonds are formed and broken. This interplay is defining both for the electrocatalytic activity but also for the stability of OER catalysts, which invariably metal oxides, as degradation also involves the breaking of metal-oxygen bonds. However, the formal oxidation state of the atoms is rarely communicated clearly in mechanistic proposals. Even if formal oxidation state is as much a bookkeeping method as a description of physical reality, we think that this bookkeeping contains important information, not least as a way of mapping out and comparing different OER mechanistic proposals. In this talk, proposed OER mechanisms will be analyzed according to formal oxidation states and formal movements of electrons and compared to each other on a universal map. The power of this approach will be demonstrated by examining previously proposed (and newly mapped) mechanisms for OER on iridium oxides in acidic electrolyte in the light of our latest in-situ experimental data.

Net-zero initiatives are critical in addressing environmental and climate change issues. Energy transition from fossil fuels to renewable sources is imperative in achieving net-zero initiatives. However, the intermittent nature of renewable energy necessitates the integration of "green hydrogen" into the renewable energy system. Green hydrogen, produced via electrolysis using renewable energy, is applicable in transportation, energy storage and industry, promoting a sustainable and decarbonized energy system. Despite its potential, green hydrogen production remains more expensive than traditional methods due to the low efficiency of water electrolysis. Therefore, the development of efficient electrocatalysts is crucial for sustainable hydrogen production and energy transition. This study presents a novel and comprehensive approach to developing high-performance electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) by focusing on phase engineering, tuning electron configuration, and interfacial engineering. The integrated approach results in developing electrocatalyst with low overpotentials, high current densities, and exceptional stability, making it a promising candidate for sustainable hydrogen production.

Nitrogen reduction to ammonia stands amongst the hardest reactions to decarbonise, with the Haber-Bosch process dominating the market. In that regard, an electrochemical alternative holds potential for sustainable and decentralised production of fertilisers and carbon-neutral fuel. On a solid electrode, the lithium chemistry has long been the only one to split nitrogen selectively to ammonia.^1,2 Huge progress has been made both at fundamental^3,4 and device level.^5,6 However, alkali metals like Li require operation at their plating potential, far beyond the equilibrium potential for ammonia synthesis (>3 V intrinsic overpotential, >70% energy losses to metal plating).^5,7

To address this issue, we provide a theory/experiment informed guide to the discovery of alternative electrochemical systems, considering energetics both from an electrode and electrolyte perspective.⁸ Theoretical analysis pinpoints many candidates energetically relevant to mediate ammonia synthesis, including some alkali-metals. Guided by the flourishing field of beyond-Li batteries, we explore their chemistries through a series of electrolyte and solid-electrolyte interphase characterisations, model experiments and operando gas evolution measurements. This study will show how experimental validation of candidate catalysts requires control over not only the energetics of the electrode but the electrolyte too. When the right balance between the two side of that same coin is found, predicted chemistries eventually work out, with recent examples of Calcium- and Magnesium-mediated nitrogen reduction.^9,10

Learning from such systems, we will conclude on perspectives towards catalytic systems with lower intrinsic overpotentials and how to achieve them experimentally. We hope to stimulate research paths towards breaking the inherent barrier of energy efficiency that nitrogen electroreduction is currently burdened with.