In situ imaging of electrode processes on solid electrolytes by photoelectron microscopy and microspectroscopy – the role of the three-phase boundary

New Insights into the Hydrogen Evolution Mechanism near the Ni/YSZ Triple Phase Boundary during Steam Electrolysis: A Patterned Model Electrode Study

Solid oxide cell technologies play a crucial role in climate change mitigation by enabling the reversible storage of renewable energy. Understanding the electrochemical high-temperature reaction mechanisms and the catalytic role of the electrode and electrolyte materials is essential for advancing power-to-H2 technologies. Despite its significance, limited in situ spectroscopic research focusing on nickel and yttria-stabilized zirconia (Ni/YSZ) is available. We employ near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to investigate 2D porous Ni/YSZ model electrodes with variable YSZ domain sizes and triple phase boundary (TPB) lengths. Focusing on the hydrogen evolution reaction (HER), we provide a mechanistic explanation for why surface hydroxylation and electrochemical activity are correlated with the YSZ surface area and YSZ domain size and unravel the specific mechanistic role of the YSZ surface. A direct comparison of normalization of the measured total electrolysis current to the purely geometrical length of the TPB vs an electrified "catchment area" next to the TPB, exhibiting strong enough electric fields, is the key to a correct quantitative description of the individual elementary steps of water electrolysis on Ni/YSZ. By combining electrochemical impedance spectroscopy, NAP-XPS, and electric field modeling, the local water reduction process near the TPB can be described, indicating optimized structural parameters for improved HER performance.

Anodization study of epitaxial graphene: insights on the oxygen evolution reaction of graphitic materials

The photoemission electron microscopy and x-ray photoemission spectroscopy were utilized for the study of anodized epitaxial graphene (EG) on silicon carbide as a fundamental aspect of the oxygen evolution reaction on graphitic materials. The high-resolution analysis of surface morphology and composition quantified the material transformation during the anodization. We investigated the surface with lateral resolution <150 nm, revealing significant transformations on the EG and the role of multilayer edges in increasing the film capacitance.

Recent progress on synchrotron-based in-situ soft X-ray spectroscopy for energy materials

Soft X-ray spectroscopy (SXS) techniques such as photoelectron spectroscopy, soft X-ray absorption spectroscopy and X-ray emission spectroscopy are efficient and direct tools to probe electronic structures of materials. Traditionally, these surface sensitive soft X-ray techniques that detect electrons or photons require high vacuum to operate. Many recent in situ instrument developments of these techniques have overcome this vacuum barrier. One can now study many materials and model devices under near ambient, semi-realistic, and operando conditions. Further developments of integrating the realistic sample environments with efficient and high resolution detection methods, particularly at the high brightness synchrotron light sources, are making SXS an important tool for the energy research community. In this progress report, we briefly describe the basic concept of several SXS techniques and discuss recent development of SXS instruments. We then present several recent studies, mostly in situ SXS experiments, on energy materials and devices. Using these studies, we would like to highlight that the integration of SXS and in situ environments can provide in-depth insight of material's functionality and help researchers in new energy material developments. The remaining challenges and critical research directions are discussed at the end.

Investigation of the oxygen exchange mechanism on Pt|yttria stabilized zirconia at intermediate temperatures: Surface path versus bulk path

The oxygen exchange kinetics of platinum on yttria-stabilized zirconia (YSZ) was investigated by means of geometrically well-defined Pt microelectrodes. By variation of electrode size and temperature it was possible to separate two temperature regimes with different geometry dependencies of the polarization resistance. At higher temperatures (550-700 °C) an elementary step located close to the three phase boundary (TPB) with an activation energy of ∼1.6 eV was identified as rate limiting. At lower temperatures (300-400 °C) the rate limiting elementary step is related to the electrode area and exhibited a very low activation energy in the order of 0.2 eV. From these observations two parallel pathways for electrochemical oxygen exchange are concluded.The nature of these two elementary steps is discussed in terms of equivalent circuits. Two combinations of parallel rate limiting reaction steps are found to explain the observed geometry dependencies: (i) Diffusion through an impurity phase at the TPB in parallel to diffusion of oxygen through platinum - most likely along Pt grain boundaries - as area-related process. (ii) Co-limitation of oxygen diffusion along the Pt|YSZ interface and charge transfer at the interface with a short decay length of the corresponding transmission line (as TPB-related process) in parallel to oxygen diffusion through platinum.

Note: Fixture for characterizing electrochemical devices in-operando in traditional vacuum systems

We describe a fixture that allows electrochemical devices to be studied under electrical bias in the type of vacuum systems commonly used in surface science. Three spring-loaded probes provide independent contacts for device operation and the characterization in vacuum or under in situ conditions with reactive gases. We document the robustness of the electrical contacts over large temperature changes and their reliability for conventional electrochemical measurements such as impedance spectroscopy. The optical access provided to the device enables the analysis by many techniques, as we demonstrate using x-ray photoelectron spectroscopy to measure local electrical potentials on a solid-oxide electrolyte device operating at high temperature in near-ambient pressure.

Physical Chemistry of Solids – The Science behind Materials Engineering: Concepts, Models, Methods

The role of Physical Chemistry of Solids as origin of many fundamental concepts for modern materials science as also the bridge between physics, chemistry and materials engineering is emphasized and exemplified. The model of the electrical double layer, the concept of point defects in crystals and transport phenomena and reactions in the solid state are discussed as examples.

Reduction and oxidation of oxide ion conductors with conductive atomic force microscopy

Local accumulation and dissipation of charges on the surface of oxide ion conductors resulting from electric potentials were observed with conductive atomic force microscopy (AFM). After a negative bias was applied at the tip, a sequence of surface potential maps appeared compatible with electron injection onto the electrolyte surface. Applying a positive bias, in contrast, generated a positive surface charge adjacent to the tip contact area. This observation is consistent with the formation of oxide ion vacancies on the oxide surface. In addition, oxide ion conductivity at a low temperature range (100-200 degrees C) was obtained, and the activation energy for diffusion in gadolinia-doped ceria (GDC) was calculated as approximately 0.56 eV, implying that the majority of oxide ion vacancies diffuse on the surface rather than inside the bulk of the electrolyte.