Palladium hydrides (PdH_x) are a model system for studying phase transitions and hydrogen (H) absorption in materials. Well-studied in the gas phase, they are also relevant in electrochemistry, particularly electrocatalysis, where the H:Pd ratio can be controlled though electrochemical potential. PdH_x hydrides exhibit a slightly expanded lattice at low H content (x < 0.05), known as the α-phase, which transforms into a lattice-expanded β-phase at higher H content. While wide-angle X-ray scattering can be used to monitor the in situ absorption of H into commercial Pd nanoparticles (NPs) 3.6 nm in size [1] as well as related phenomena such as H trapping, crucial aspects of the mechanism and kinetics of PdH_x formation remain elusive. Specifically, it is unclear whether the α- and β-phases coexist, and if the Pd NPs undergo isotropic H insertion, following a core-shell model, or if preferential H absorption pathways exist, as suggested by a spherical cap model. Furthermore, the small size of the facets makes it difficult to determine the distribution of strain fields across single NP.

In this study, employing Bragg Coherent X-ray Diffraction Imaging (BCDI) and focusing on the 111 Bragg reflection, we obtained information on the morphology, projected strain, displacement fields, and d-spacing of single 300 nm Pd NPs at various electrode potentials relevant to H adsorption, H absorption, and H₂ evolution [2]. We examined changes in lattice constants for both α and β phases and reconstructed individual Pd NPs in each individual phase. The reconstructions revealed a continuous increase in the Pd lattice parameter, indicating an isotropic expansion of the NP. Additionally, we observed heterogeneous strain in the reconstructed Pd NPs, with tensile strain accumulating on the {111} and {100} facets, while the lattice in the edges and corners of the atoms appeared compressed. Finally, we will show how BCDI can be used to gain insights into H absorption/desorption mechanism and kinetics.

Figure 1. Diagram showing the in situ cell used to reconstruct a single nanoparticle in 3D under potential control from diffraction images.

The energy transition stands as one of the greatest challenges of today’s society. In this context, electrocatalyst materials at the heart of electrochemical energy conversion devices such as fuel cells and water electrolyzers are expected to play an increasing crucial role in the near future. The urgent bottleneck to be overcome in electrocatalyst materials development to allow the widespread deployment of electrochemical systems is thus reaching combined high activity and long-term stability at low cost. Despite the diversity in electrocatalyst materials, the latter being largely imposed by the various types of electrochemical systems (noble vs. non-noble metals in acidic vs. alkaline media for example) and the prerequisites of the different electrochemical processes (oxidation or reduction reactions of various species at different electrode potential ranges), most activity and stability properties of electrocatalysts directly derive from their (surface) chemistry and structure. Such properties (and their temporal evolution) can thus be directly investigated by means of in situ or operando high energy X-ray scattering (XRD) technique.

In this presentation, the versatility of in situ and operando XRD technique in addressing key bottlenecks in electrocatalyst materials development, notably by probing adsorption and oxidation trends, will be showcased. Finally, the ability of operando XRD to provide device-relevant insights at the macroscale beyond electrocatalysts microstructural properties (such as ionomer hydration in PEMFCs or water distribution in AEMFCs) will be presented [1-4].

Proton exchange water electrolyzer (PEMWE) is rising up as an advanced and effective solution for green hydrogen production [1]. Green hydrogen, offers an alternative to fossil fuel, providing flexible energy storage for extended periods and enabling highly efficient reconversion to electricity through fuel cells. However, its market penetration is still limited by two key challenges: (1) scarcity of iridium (Ir)-based anodes required to catalyze the sluggish oxygen evolution reaction (OER), and (2) the limited understanding of PEMWE performance and durability, which slows technological advancement. Nanostructured and unsupported Ir-based catalysts, which maximize Ir utilization, have demonstrated promising performance [2], however, the nature of their active site during OER and the mechanism driving their deactivation remain unknown.

Advanced X-ray techniques help unlock the complexity of materials and drive innovation in energy applications. In particular, only operando experiments can provide the detailed, real-time understanding necessary to capture these dynamic changes [3].

In this study, operando X-ray total scattering combined with atomic pair distribution function (PDF) analysis is used to probe both the crystalline and amorphous atomic architectures of nanostructured and unsupported Ir-based catalysts in PEMWE [4]. The results give insights on the complex, potential-dependent local structure dynamics occurring during PEMWE operation.

The anodic electrocatalytic reactions are the potential-determining steps in various electrolyzers. Understanding the intrinsic activity and stability of non-noble-metal-based materials is the root to enable future employment of gigawatt-scale system. Here we developed protocols to monitor real-time dissolution of metals, one of the detrimental pathways toward degradation of electrocatalytic activity, using scanning flow cell with inductively coupled plasma mass spectrometry (SFC-ICP-MS) over flame-spray pyrolyzed Co-based nano-oxides under respective oxygen evolution reaction (OER), chlorine evolution reaction (ClER), and glycerol oxidation reaction (GOR) in acidic or alkaline environment. The different geometries and oxidation states of redox-active Co sites among the studied crystal structures present impact on their intrinsic electrocatalytic activity. Furthermore, the difference in electrocatalytic stability of Co-based metal oxides under different applied electrochemical methods, e.g., cyclic voltametric anodic/cathodic scan and/or chronopotentiometry, is observed. In addition, the stability of different Co/M (M = second metal) sites and the shared/independent reaction intermediates for metal dissolution and anodic reaction will be discussed.

With the global push toward sustainable energy technologies, the development of efficient and durable electrocatalysts has become a research priority. Real-time in situ studies are essential to understand the dynamic behavior of catalysts under operational conditions. X-ray absorption spectroscopy (XAS) offers a unique, element-specific probe of electronic and structural changes at the active sites of electrocatalysts during electrochemical reactions.

At BAM, collaborative research efforts leverage the advanced capabilities of the BAMline [1] at the Berlin Synchrotron BESSY-II to study electrocatalytic materials under realistic working conditions. As a dedicated materials research beamline, the BAMline enables in situ and operando XAS across different time and length scales, making it ideally suited for monitoring catalytic transformations in real time.

This presentation highlights the analytical strengths and sample environments developed for electrochemical cells at BAM, showcasing their application to electrocatalysis for energy conversion (e.g., water splitting, CO₂ reduction). Emphasis will be placed on how these insights contribute to the rational design and real-time optimization of functional materials for a sustainable energy future [2].

Electrochemical conversion of small molecules into value-added products is one of the most promising approaches to decarbonize the chemical industry. Improving the efficiency and stability of many of these reactions relies on a molecular-level understanding of the reaction mechanisms. Operando characterizations play a critical role in this regard. Nuclear magnetic resonance (NMR) is an element-specific, quantitative, and non-destructive spectroscopic technique, making it ideal for operando and in situ characterizations.

In this talk, I will showcase how we develop and apply operando NMR to understand chemical and physical processes in the electrolyte and at the electrode in a working electrochemical reactor.[1,2] In the first half of my talk, I will focus on CO₂ reduction, highlighting the coupling of a benchtop NMR system with a gas diffusion electrolyzer, real-time quantification of electrolyte carbonation, water crossover, and their link to device failure.[3] In the second half, focusing on lithium-mediated ammonia synthesis, I will present the development of new in situ NMR techniques for the direct observation of key reaction steps, including the plating of metallic lithium and its concurrent corrosion, nitrogen splitting on lithium metal, and protonolysis of lithium nitride. Informed by these observations, we have developed a new reaction cycle.[4]

By the end of this talk, I hope to demonstrate the versatility and rich informational content that operando NMR can offer in the field of electrochemical conversion, and how it can be used to track reaction stability and guide the design of new reactions.

References

1. Luo R et al. “A parallel line probe for spatially selective electrochemical NMR spectroscopy” J. Magn. Reson. 2024, 361, 107666 (Front cover; Special Issue: New Voices in Magnetic Resonance; Invited).

2. Zhu Z et al. “Operando NMR methods for studying electrocatalysis” Magnetic Resonance Letters, 2024, 4, 100096 (invited).

3. Zhu Z et al. “Operando NMR quantifies liquid product, water crossover and carbonates for electrochemical CO2 reduction” ACS Catalysis, 2025, in press.

4. Luo R et al. “Direct in situ NMR observation of lithium plating, corrosion, nitridation and protonolysis for ammonia synthesis” ChemRxiv, DOI 10.26434/chemrxiv-2024-cpf4.

Solute transport underpins the functionality of many modern electrochemical devices involved in energy generation and storage. In solar fuel generation, mass transport limits the efficiency of product generation, and CO2 reduction intermediaries can cause cell degradation and may further limit efficiency. Despite its immense importance, there are no demonstrations of direct solute imaging of solar fuel generation that could reveal such structure–property relationships and the role of intermediates, limiting rational design. We therefore developed and fabricated a three-electrode microfluidic electrochemical cell that is compatible with a high resolution microscope. By imaging the cell in operando using interference reflection microscopy (IRM), we measure the evolution of voltage-induced spatiotemporal concentration profiles through their changes to the local refractive index in order to extract transport coefficients. 

I will discuss imaging of two partial reactions involved in CO2 reduction. First, to investigate replacing the slow, high-overpotential oxygen evolution reaction at the anode, we image the lower-overpotential oxalic acid oxidation. Second, to overcome the conversion efficiency limitations arising from the low solubility of CO2 in water, we image catalyst-assisted bicarbonate dehydration to increase the supply of CO2 to the cathode for enhanced product generation. Probing these reactions with spatiotemporal imaging has allowed us to measure transport and reaction dynamics, identify limitations on the rate of reaction, separate identify time scales on which different steps of the reactions occur, and measure reactant usage and product generation.

Electrochemical energy conversion technologies such as fuel cells and electrolysers rely on advanced electrocatalysts that must exhibit high catalytic activity, selectivity, and long-term stability, while ideally being composed of abundant and inexpensive materials. However, for most industrially relevant reactions, no catalyst fully meets all these criteria. As a result, significant research efforts have been directed toward the discovery and development of advanced catalysts. In recent years, two fundamentally different approaches have emerged in electrocatalysis research:

• Knowledge-driven design and development of new catalysts, based on mechanistic understanding and guided experimentation;

• Accelerated screening of material libraries, often supported by artificial intelligence (AI), aimed at identifying promising candidates, uncovering hidden structure–performance relationships, or both.

The former approach relies heavily on a wide range of in-situ and operando techniques, which provide insights that are inaccessible through conventional electrochemical testing, also offering real-time information on catalytic behavior and reaction mechanisms. However, the integration of such techniques into high-throughput screening workflows remains limited. This is largely due to practical challenges: in-situ/operando methods can reduce throughput, increase system complexity, and often involve high costs and limited accessibility [1]. Nevertheless, as will be demonstrated in this presentation, the selective application of these techniques can be highly valuable, offering qualitatively different information that is otherwise inaccessible. A representative example is the online inductively coupled plasma mass spectrometry (ICP-MS) for multi-elemental analysis of dissolution products during the screening of catalyst libraries for hydrogen and oxygen reactions – a technique extensively developed and applied in the authors' laboratory [2]. In addition, we will highlight both our work and relevant studies from the literature that elucidate reaction mechanisms and selectivity using advanced analytical techniques. The presentation will conclude with a discussion of the advantages and limitations of integrating real-time characterization into screening workflows, along with an outlook and future directions for the field.

Electrocatalysis plays a central role in the synthesis of valuable chemicals, in fuel cells and water electrolysis. Electrocatalysts enhance reaction rates by lowering the overpotentials of electrode processes. A deep understanding of reaction mechanisms, including the identification of active sites, intermediates, and elementary steps is essential for the rational design of efficient and durable electrocatalytic materials.

To gain such insights and evaluate electrocatalytic activity, a combination of techniques is employed, including electrochemical methods, in situ spectroscopies, and ab initio computational modeling.

In this talk, we will focus on the application of polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) for various electrocatalytic reactions and the progress we made in studying highly dispersed carbon-supported and unsupported nanoparticles (NPs).  PM-IRRAS has emerged as the most sensitive infrared technique for in situ and surface studies [1,2]. PM-IRRAS uses modulated polarization (alternating between p- and s-polarized light) to enhance sensitivity and distinguish surface-specific signals from the bulk. This modulation allows the technique to suppress isotropic background signals, such as those from gases or liquids, and isolate vibrational modes at the surface [1,3]. Furthermore, PM-IRRAS can operate under realistic environmental conditions, such as in liquids or in the presence of gases, making it ideal for studying dynamic processes like adsorption, catalysis, and electrochemical reactions.

PM-IRRAS has established itself as a gold standard for surface-sensitive infrared spectroscopy, enabling researchers to understand molecular-level details of dynamic processes at interfaces. In this presentation, we will demonstrate in situ PM-IRRAS studies for the development of carbon supported and unsupported NPs for ethanol, glycerol, and ammonia electrooxidation, as well as CO₂ electroreduction [4,5].

The molecular structure of the electrical double layer (EDL) fundamentally impacts the chemistry of electrochemical processes. In particular, the orientation and hydrogen-bonding network of interfacial water within the EDL under applied potentials critically influence catalyst performance ^[1-2]. While several mechanistic studies have linked hydration water ordering to reactions such as the hydrogen evolution (HER) ^[3-4], hydrogen oxidation (HOR) ^[5], and oxygen reduction (ORR) reactions ^[6], the specific role of interfacial water structure in CO₂ electroreduction (CO₂RR) remains largely unexplored. In this presentation, I will present our recent works on elucidating the interrelation between surface-adsorbed species, interfacial water structure, and reaction products on Au and Cu catalysts in aqueous bicarbonate electrolytes. Our approach combines on-line differential electrochemcial mass spectrometry (DEMS), in situ attenuated total reflection surface enhanced infrared spectroscopy (ATR-SEIRAS), operando shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), electrochemical atomic force microscopy (EC-AFM) and density functional theory (DFT). We reveal that carbonate induces ordered interfacial hydration networks. On Au, carbonates exist in equilibrium with their radicals (CO₃^•–) that act as proton relays, accelerating HER by facilitating proton delivery. Moreover, these CO₃^•– radicals are found to serve as carbon source for aldehyde formation, alongside the commonly observed CO product in Au-catalyzed CO₂RR. Water is identified as the primary proton donor for both CO₂RR and HER, with bicarbonate predominantly participates in the Heyrovsky step. Our mechanistic insights advance a comprehensive understanding of CO₂RR and the critical role of hydration water at electrochemical interfaces, opening new avenues for future research in energy conversion, photo-electrocatalysis, and surface science.