The development of sustainable strategies for the production of added-value chemicals and fuels using renewable resources is particularly attractive to promote a transition towards a more sustainable energetic landscape, overcoming the dependence of fossil fuels at a global scale.[1] One of the most promising alternatives involves the use of renewable electricity (wind, solar, hydropower, etc…) to power electrochemical conversion processes, which convert abundant molecules (e.g., water, carbon dioxide, and nitrogen) into higher-value products (e.g., hydrogen, hydrocarbons, oxygenates, and ammonia). In all these processes, (photo)-electrocatalytic water oxidation stands out as the preferred reaction to provide the protons and electrons needed for the target reduction reactions. In this context, metal oxides of earth-abundant elements (Fe, Ni, Co, etc…) are identified as excellent candidates, since these materials can fulfil most of the needed requirements, although in some cases their performance needs to be improved for a more realistic technological assessment. On the other hand, a clear mechanistic understanding of the physical-chemical processes taking place during operation is essential. In the present contribution, we will describe our efforts to understand the operation mechanisms of different metal oxides (mainly NiO_x)[2-5] and other systems (CoFe prussian blue derivatives) for electrochemical water oxidation, as well as other electrochemical processes, [6-7] with particular emphasis on enhancing their performance. Our studies focus on the correlation of the (photo)electrochemical response of the materials with a detailed structural, optoelectronic and photoelectrochemical characterization carried out by different microscopic and spectroscopic tools.

Iridium oxide is the state-of-the-art electrocatalyst for water oxidation in polymer electrolyte membrane (PEM) electrolysers for green hydrogen production. Amorphous iridium oxides (IrO_x) show far higher activity than the rutile crystalline iridium oxides (IrO₂), but the underlying origin behind the discrepancy in activity for IrO_x and IrO₂ remains poorly understood. Density functional theory (DFT) calculations are usually used to explain the kinetic difference based on the thermodynamic barrier for forming *OH, *O, and *OOH intermediates on a single Ir site of well-defined model catalysts.^{[1, 2]} However, unambiguously correlating the theoretical free energy with experimentally observed kinetic is very challenging, especially for disordered and non-well defined nano-particle systems, such as amorphous IrO_x. In addition, the ambiguities in measuring the number of active states for determining the intrinsic activity per states has further rendered it difficult to understanding the correlation between activity and the observed physical/chemical discrepancies on IrO_x and IrO₂.

In this talk, I will present our results on using time-resolved UV-vis absorption spectroscopy to experimentally probe the energetics of intermediate states and correlating them with water oxidation kinetics on rutile and amorphous iridium oxides.  We have identified the same oxidised species for both IrO_x and IrO₂ and quantified their concentrations as a function of potential. By comparing the concentration of oxidised states to water oxidation reaction rates, we can directly measure turnover frequency (TOF) for IrO_x and IrO₂ and correlated them with the experimentally measured energetics of the states. Therefore, this study shines insights into the origin of activity difference between amorphous and rutile iridium oxides.

The electrocatalysis systems are very sensitive to electrode surface structure as well as the surface composition. A detail investigation of the nature of electrocatalysts is essential not only for the identification of reaction active sites or components, but also for the clarification of mechanism of electrocatalysts deactivation/activation, as the case of oxygen evolution reaction (OER)^1-2. X-ray photoelectron spectroscopy (XPS) coupled with electrochemical methods is a powerful technique for this purpose. However, it is still quite challenging to combine the conventional electrochemical measurement under ambient conditions with XPS measurement under ultra-high vacuum (UHV) conditions. The in situ electrochemical XPS methods can overcome the shortness of sample change during the transfer as it is commonly encountered in ex situ studies³. However, due to presence of both electrolyte and vapor, synchrotron technique is usually required. In addition, the electrochemical signals are usually distorted by poor mass transfer. We have developed a quasi in situ XPS which consist of electrochemical treatment at ambient pressure, air free transfer from ambient pressure to UHV, and detection under UHV conditions. It has great advantages in: 1) acquiring electrochemical signal of good quality that can be directly compared with those in conventional cells; 2) avoiding oxidation of electrocatalysts due to air exposure; 3) collecting high intensity XPS signals by only commercial X-ray source. In this presentation, I will introduction the detailed setup, benchmark practice, and the application of a nearly in situ XPS on obtaining information on oxygen evolution reaction on Ru based catalysts.    

Photoelectrocatalysis has emerged as a promising process to store solar energy into fuels and high added-value chemicals to decarbonise the energy and fine chemical sectors. In this process the generation of H₂, carbon-based chemicals or NH₃ from H⁺, CO₂ and N₂ reduction, is usually limited by the oxidation reaction taking place at the photoanode, in particular when using metal-oxide photoanodes. The photoelectrochemical performance of these photoanodes vary depending on their synthetic route and post-synthesis treatment that can lead to crystal defects such as oxygen vacancies. However, the chemical nature of such oxygen vacancies and their role in photoelectrochemical oxidation of water or organic substrates to produce high added-value chemicals is still in debate.

In this talk, I will present a spectroscopic, microscopic and electrochemical analysis of the chemical nature of light-induced oxygen vacancies in one of the most studied photoanodes such as BiVO₄. Oxygen vacancies in these BiVO₄ photoanodes were produced by light exposure treatments and are associated with the migration of Bi towards the surface forming nanoparticles.^[1] Additionally, I will show the role of oxygen vacancies in the photoelectrochemical behaviour of BiVO₄, WO₃^[2] and a-Fe₂O₃^[3] photoanodes and their role in the water oxidation mechanism as example.

Growing environmental concerns, as a result of fossil fuel utilization, constitute a driving force towards the development of sustainable technologies to produce chemicals. The electroreduction of CO (CORR) or CO₂ (CO2RR) to fuels are part of these technologies as C₁ (methane, methanol) and C₂₊ products (ethylene, propylene, ethanol, propanol etc.) can be obtained [1]. CO is a particularly interesting reactant because it is present in CO-rich waste streams of common industries such as steel manufacture. For instance, it may be valuable to convert CO contained in blast furnaces’ flue gases, considered an irreducible source of COx, into chemical building blocks such as olefins or alcohols. Converting CO to light hydrocarbons and oxygenates by means of renewable electricity could help reducing our dependence on fossil resources. Moreover, CO can also be obtained from the widely studied CO2RR. CO constitutes a key intermediate in C₂₊ products formation in the aforementioned reaction [2]. Alternative to classical CO hydrogenation (Fischer-Tropsch synthesis), we considered here an electrochemical variant to obtain products with C-C bonds (C₂₊ products).

In this study, we focused on the electrosynthesis of C₂₊ products from CO on Cu-based catalysts. We studied the influence of catalyst particle size on the product distribution obtained during CO electroreduction. Particles ranging from smaller than 7 nm up to 45 nm were investigated. Intermediate sizes (20 nm – 30 nm) and a Cu foil have been reported to be the most selective for the formation of C-C bonds (ethanol, ethylene, propanol, etc..) during electroreduction of CO₂ [3]. Our results show a different selectivity trend when using CO as reactant. The chronoamperometry data show a clear dependency of the selectivity on the particle size of the Cu electrocatalyst (Figure 1) with intermediate sizes (11 to 27 nm) showing the highest selectivity towards C₂₊ products while smaller particles favor hydrogen evolution. Among the used characterization methods, in situ XRD was employed to investigate stability of the particles under electrochemical conditions.

Electrolysis of water, CO₂ and nitrogen-based compounds presents the opportunity of generating fossil-free fuels and chemical feedstocks at an industrial scale. These devices are complex in operation, and their activity and efficiency metrics are usually reported as averaged quantities across an electrode. This heat generation, of usually 30-50% of the inputted power, is due to cell inefficiencies. In this work we seek to use these inefficiencies as a measure of spatial electrochemical activity by taking advantage of the link between heat generation and the activity-dependent transport of electrons and ions. To this end we use data from an infrared camera positioned at the backbone of a gas-diffusion electrode in an attempt to develop a 'thermal potentiostat' which provides spatial resolution of electrochemical activity. After a proof-of-concept is displayed for the technique, we present several applications of the technology including catalyst screening, spatial-temporal catalyst measurements, catalyst-defect and ageing detection, and the impact of exothermic CO₂-hydroxide interactions during CO₂ electrolysis. Combined, we find that a spatial-thermal potentiostat has numerous uses for both novel and established electrochemical reactions towards the production of renewable fuels and feedstocks.

Interfaces play an essential role in nearly all aspects of life and are critical for electrochemistry. Electrochemical systems ranging from high-temperature solid oxide fuel cells (SOFC) to batteries to capacitors have a wide range of important interfaces between solids, liquids, and gases which play a pivotal role in how energy is stored, transferred, and converted. This talk will focus on using ambient pressure XPS (APXPS) to directly probe the solid/liquid electrochemical interface of ion exchange membranes and composite water electrolysis electrodes. APXPS is a photon-in/electron-out process that can provide both atomic concentrations, chemical-specific, and local electric potential information at pressures greater than 20 Torr. Using synchrotron X-rays at Lawrence Berkeley Nation Laboratory, the Advanced Light Source has several beamlines dedicated to APXPS endstations to probe a wide range of interfaces ranging from solid/gas to solid/liquid and liquid/liquid. To enable our approach to investigating electrochemical solid/liquid interfaces, we leverage tender x-rays to support investigations at pressures greater than 20 Torr. Electrical leads support applying electrical potentials to enable the ability to collect XPS data of actual electrochemical devices while operating near ambient pressures. This talk will introduce APXPS and share how we can use this technique to directly probe the Donnan potential of an ion exchange membrane at the solid/liquid interface for the first time. In addition, I will share the chemical information we can reveal when probing a composite water electrolysis electrode undergoing electrochemical water splitting, gaining new insight to guide the design and control of future electrochemical interfaces.

Energy storage through the electrocatalytic generation of chemical fuels such as hydrogen is an attractive pathway for storing intermittent renewable energies, and perovskite oxides are among the most attractive candidate materials to catalyze the kinetically limiting half reaction, the oxygen evolution reaction (OER).  OER activity is typically correlated to electronic and atomic structure parameters. But the catalyst surface – i.e. where the reaction happens – changes during the reaction. To design next-generation electrocatalysts, a detailed operando understanding of the relationships between catalytic activity, stability and atomic-level surface properties during the reaction is required.

In my talk I will address two essential ingredients to achieve this next-level understanding: controllable atomic-level surface properties and the development of surface-sensitive operando characterization routes.  

Epitaxial thin films are a direct route for single crystalline model electrocatalysts that can be fabricated with atomically-tailored surface composition. These offer the ideal platform to derive structure-property-function relationships, track the evolution of the surface properties with applied potential and enable direct comparison to the surfaces investigated in density functional theory. I will summarize our recent findings for the role of surface termination, surface contamination, point defects and transformation pathways during the reaction and the role of solid/solid interfaces in proximity of the solid/liquid interface.

Next, I will summarize recent advances in surface-sensitive operando characterization in a liquid medium and with specific attention to studies with atomically-defined thin film model electrode surfaces. A focus will be the current development of meniscus photoelectron spectroscopy, a discussion about utmost surface- or interface-sensitivity using a standing-wave approach^1,2 and a pathway to extract surface-sensitive information from nominally bulk-sensitive techniques such as UV-Vis spectroscopy.^1,3

The key example throughout the talk will be LaNiO₃ thin films, which are atomically flat both before and after application as electrocatalysts for the OER during water electrolysis. We selectively tuned the surface cationic composition in epitaxial growth. The Ni-termination is approximately twice as active for the OER as the La-termination. Using a suite of ex situ, in situ and operando spectroscopy tools, we found that the Ni-rich surface undergoes a surface transformation towards a catalytically active Ni hydroxide-type surface.¹ If LaNiO₃ surfaces are exposed to the ambient, however, surface carbonate groups form, which prohibit the formation of the active phase, leading to an activity decrease compared to the clean surfaces.⁴

Our work thus demonstrates tunability of surface transformation pathways by modifying a single atomic layer at the surface and it shows that active surface phases only develop for select as-synthesized surface terminations, highlighting the instructional value of epitaxial model electrocatalysts. It also highlights the need of and summarizes pathways for the exploration of the three-step relationship between as-prepared surface, transformation under applied potential, and electrocatalytic activity.

The performance of an electrocatalyst (its activity, selectivity, and stability) is strongly dependent on the electrode structure and composition, particularly in the near surface region. A successful approach in trying to understand the impact of structure is the use of well-defined model electrodes such as single crystals, to isolate how various changes in structure and composition impact upon the catalyst behavior. Surface x-ray diffraction gives the average surface structure of an isolated facet, whereas real catalysts often contain multiple facets and edge sites. This contribution will discuss two recent advances in the determination of electrocatalyst structure:

a) The application of high energy surface x-ray scattering to quickly map out large volumes of 3D reciprocal space and then extract crystal truncation rods. These truncation rods can then be used to determine atomic coordinates of surface atoms, in operando. I will present examples during methanol oxidation on both Pt(111) and Au(111) surfaces.

b) I will then discuss how we can complement single crystal studies by using coherent Bragg diffraction imaging (and some of the challenges) to look at single nanoparticle electrocatalysts and ceate strain maps to identify important facets and sites.

The search for commercially viable catalysts for the oxygen evolution reaction (OER), which is typically the bottleneck for electrochemical water splitting, is a key challenge in the worldwide transition to renewables-based electricity generation. Transition-metal oxides in general and cobalt (hydro-)oxides in particular are of great interest in this regard, as they show promising catalytic properties, are stable in alkaline and neutral electrolytes under ambient conditions, and can be easily prepared from earth-abundant materials [1]. A wide variety of different catalysts, based on oxides of Co, Fe, and Ni, have been synthesized and studied, exhibiting rather diverse nanoscale morphologies and usually an unknown or poorly defined surface structure. This makes it difficult to compare their reactivity and correlate it with ab initio theoretical studies, which hinders the development of clear structure-reactivity relationships and unambiguous determin­ation of the OER reaction mechanism.

We here present studies of structurally well-defined Co₃O₄ films of 10-30 nm thickness, prepared by electrodeposition or molecular beam epitaxy (MBE) on Au(111), Au(100), and Ir(100) substrates. In all cases a well ordered epitaxial Co₃O₄(111) arrangement results, but the film morphology differs substantially, as shown by AFM and X-ray diffraction measurements. The oxide structure under reaction conditions was studied by detailed operando surface X-ray diffraction [2,3] and in situ X-ray absorption measurements. These techniques allow characterization of the surface and bulk oxide structure over a wide potential range, including the OER regime, and can be performed simultaneously with electro­chemical measurements, enabling direct correlations of structure and reactivity.

We find that the films prepared by molecular beam epitaxy are perfectly stable whereas all electrodeposited Co₃O₄ films exhibit reversible changes in the oxide structure and oxidation state, in agreement with a previous study of polycrystalline Co₃O₄ catalysts [3]. Specifically, we observe the formation of an ultrathin X-ray-amorphous CoO_x(OH)_y skin layer above 1 V vs. RHE and the gradual buildup of tensile lattice strain with increasing potential. These structural changes occur for all electrodeposited samples, albeit to a different extent, depending on the film morphology. They were found at pH values between 7 and 13 and in the presence of different cations. Potential step experiments show that the structural transformation proceeds within seconds and is highly reversible. Despite the similar structure, differences in the OER overpotential of up to 150 mV are observed for these samples, with the structurally stable MBE-prepared films having a larger overpotential than the electrodeposited films. This indicates a beneficial effect of the CoO_x(OH)_y skin layer on the OER activity of Co oxide catalysts.

[1] C. C. L. McCrory et al., Journal of the American Chemical Society 2015, 137, 4347.

[2] F. Reikowski, et al., ACS Catalysis, 2019, 9, 3811.

[3] T. Wiegmann, et al., ACS Catalysis, 2022, 12, 256.

[4] A. Bergmann, et al., Nature Communications, 2015, 6, 8625.

Platinum dissolution and a resulting decrease in the electrocatalyst active surface area is the main cause of the efficiency decrease of the proton exchange membrane fuel cell (PEMFC) with time. It has been extensively investigated on both the device and simplified model catalyst levels for many years, as summarized in our review [1]. In order of complexity increase, the systems can be ordered as follows: Pt low indexes single crystals; polycrystalline Pt; Pt nanoparticles; supported Pt nanoparticles in thin and thick catalyst layers, where the last one closely reminds the catalyst layer of PEMFC.

Over the last years, all these systems were investigated in our group to understand the complex mechanism of Pt dissolution. In most of these studies, an inductively coupled plasma mass spectrometer (ICP-MS) directly connected to an electrochemical cell was used for time- and potential-resolved Pt dissolution analysis. Scanning flow cell (SFC)- and gas diffusion electrode (GDE)-based setups were developed and successfully applied to study different Pt/electrolyte interfaces [2-4].

In this talk, I will present the most recent results of this research. Two systems: Pt/aqueous electrolyte and Pt/ionomer electrocatalytic interfaces, will be compared and contrasted [4, 5]. The main focus will be the understanding of the influence of system complexity and experimental parameters, simulating cell operational conditions, on Pt dissolution, with a final goal to clarify how the results on Pt dissolution from model catalysts can be extrapolated to PEMFCs. Moreover, mitigating approaches to stabilize platinum electrocatalytic interfaces will be discussed [6].

The quest to design active and stable (electro)catalysts hinges on the ability to identify the reaction mechanism under realistic operating conditions. The past few decades of research have focused on developing a range of spectroscopic and imagining tools to enable the study of the chemical and structural changes at the solid/liquid interface.

This talk will bring the most recent understanding of the electrochemical interface during the energy conversion and storage reactions. The molecular-level understanding of the interfacial electrochemical processes on metal surfaces by operando surface-enhanced infrared absorption spectroscopy (SEIRAS) will be particularly discussed. We will also show that integrating those experimental approaches and density functional theory (DFT) calculation further strengthens the understanding of the reaction mechanism. It will also be demonstrated that the holistic information about the nature of the adsorbed species can provide additional knobs to tune the energetics of key reaction intermediates, leading to further improvements in electrocatalytic activity for energy conversion and storage reactions.

A sustainable future requires highly efficient energy conversion processes. I will discuss two unconventional strategies to reach significant boosts in the electrocatalytic activity of Earth-abundant materials using spin control:

-At the interfacial level,  magnetic fields affect the mass transfer of charged reactants (to) and products (from) the electrolyte. This effect is universal on both magnetic  and non-magnetic electrodes. I will demonstrate this phenomenon operando using Pt and transition metal oxides as catalysts for the oxygen evolution reaction (OER), oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER).

-At the local level,  we use chiral molecules to induce spin selective pathways on the catalyst’s surface. As a case study, I will show the use of functionalized helicenes to boost the oxygen evolution reaction (OER) by ca. 131 % (at the potential of 1.65 V vs. RHE) at state-of-the-art 2D catalysts via a spin-polarization mechanism) [1].

By decoupling the effects involved in interfacial vs local spin control, we provide a versatile strategy that can be easily implemented to boost electrocatalytic reactions on different materials. 

Surface and interface sensitive operando characterization methodologies are required to unlock electrochemical and electrocatalytic processes. Liquid phase electrochemical transmission electron microscopy (TEM) can provide the sensitivity and selectivity at single particle level. Herein, we investigate oxygen-evolving cobalt-based oxides in alkaline solution and we retrieve morphological, structural, chemical information during cyclic voltammetry measurement. We compare the highly active Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) to the spinel Co₃O₄ and rocksalt CoO. The potential-dependent variation of the local contrast is associated to the modification of the wettability from hydrophobic to hydrophilic character at the oxide surfaces through interfacial capacitance phenomena. Molecular oxygen is probed in real-time during its evolution which results to further dewetting of the surfaces. Our work on catalysts exemplifies the crucial role that surface sensitive and electrochemical microcell TEM techniques can play in detailing the mechanism of electrocatalytic processes and can provide a new characterization framework aiding development of novel design routes for targeted catalyst preparation.

For over 50 years, Raman spectroscopy has remained a key tool in the arsenal of the electrochemists investigating structural and ionic (both solid-state and liquid electrolyte) dynamics in battery and electrocatalytic systems. However, imaging of evolving Raman signals has remained challenging due to the small signals (and hence slow acquisition speeds) of spontaneous Raman. Here, we overcome these limitations and develop and apply high-resolution (sub-300 nm) computational Raman imaging for real-time, chemically specific imaging of operating electrocatalysts and batteries.

Firstly, we track the OER from IrO₂ and Li-IrO_x electrocatalysts, demonstrating at low overpotentials that oxygen is evolved by the combination of an electrochemical-chemical mechanism and classical electrocatalytic adsorbate mechanism, whereas at high overpotentials only the latter occurs. We then examine electrochemical degradation in LCO and NMC811  electrodes operating at high voltages (above 4.2 V with LP30 electrolyte). For the LCO system, we observe a build-up of degraded electrolyte through the P-F bond intensity and oxidised Co along the edge of the particles.

Our results demonstrate the power of high-resolution Raman microspectroscopy for non-invasive operando, tracking of electrochemical dynamics and reveal new insights into the operation of state-art-of-the-art battery materials and electrocatalysts.