Understanding the evolution of the physicochemical bulk properties during the Li deintercalation process is critical for optimizing battery cathode materials. In this study, we combine X-ray photoelectron spectroscopy (XPS), density functional theory plus dynamical mean-field theory (DFT+DMFT), and charge transfer multiplet (CTM) model to investigate how hybridization between transition metal (TM) 3d and oxygen 2p orbitals evolves upon Li deintercalation. Based on the presented approach combining theoretical calculations and experimental studies of pristine and deintercalated cathodes, two key aspects of ion batteries are examined: i) the detailed electronic structure and involved changes with deintercalation associated with the charge compensation mechanism, and ii) the precise experimental analysis of XPS data which are dominated by charge transfer coupled to final-state effects affecting the satellite structure. As main result for the investigated Li-TM oxides, the results indicate that the electron transfer coupled to the Li+-ion migration does not follow a rigid band model but is influenced by changes in TM 3d and O 2p states hybridization. This integrated approach suggests that 2p XPS satellite peak intensity of TM is sensitive to changes in redox chemistry, providing an indirect experimental descriptor of cathode redox behavior and guiding the design of more efficient battery materials.

ABSTRACT Multi‐absorber III–V semiconductors represent the state of the art in high‐efficiency solar‐to‐electricity and solar‐to‐fuel conversion owing to their tunable bandgaps and favorable optoelectronic properties. GaInP is widely used in monolithic tandem devices as a top absorber or charge‐carrier‐selective contact because of its suitable electronic structure. However, GaInP and related III–V materials are prone to photocorrosion under photoelectrochemical (PEC) operation and require protective layers that ensure chemical and electronic passivation. Atomic layer deposition (ALD), particularly plasma‐enhanced ALD (PE‐ALD), enables conformal deposition of films that enhance interfacial stability while maintaining efficient selective charge carrier transport. Here, the band alignment at the GaInP(100)/ interface is examined to determine whether the initial GaInP surface condition–either an atomically well‐defined, phosphorus‐rich surface or a naturally oxidized surface–affects interface formation during mild, low‐power remote oxygen PE‐ALD. Angle‐dependent X‐ray and ultraviolet photoelectron spectroscopy were used to probe the chemical and electronic structure of the buried interfaces. The results show that interface composition and band alignment are largely insensitive to the initial surface condition, with only minor differences in attenuation and interfacial energetics. These findings demonstrate the robustness of mild PE‐ALD for reproducible interface formation and support the design of protected III–V photoelectrodes for PEC applications.

ABSTRACT Multi‐absorber III–V semiconductors represent the state of the art in high‐efficiency solar‐to‐electricity and solar‐to‐fuel conversion owing to their tunable bandgaps and favorable optoelectronic properties. GaInP is widely used in monolithic tandem devices as a top absorber or charge‐carrier‐selective contact because of its suitable electronic structure. However, GaInP and related III–V materials are prone to photocorrosion under photoelectrochemical (PEC) operation and require protective layers that ensure chemical and electronic passivation. Atomic layer deposition (ALD), particularly plasma‐enhanced ALD (PE‐ALD), enables conformal deposition of TiO2 films that enhance interfacial stability while maintaining efficient selective charge carrier transport. Here, the band alignment at the GaInP(100)/TiO2 interface is examined to determine whether the initial GaInP surface condition–either an atomically well‐defined, phosphorus‐rich surface or a naturally oxidized surface–affects interface formation during mild, low‐power remote oxygen PE‐ALD. Angle‐dependent X‐ray and ultraviolet photoelectron spectroscopy were used to probe the chemical and electronic structure of the buried interfaces. The results show that interface composition and band alignment are largely insensitive to the initial surface condition, with only minor differences in attenuation and interfacial energetics. These findings demonstrate the robustness of mild PE‐ALD for reproducible interface formation and support the design of protected III–V photoelectrodes for PEC applications.

Different combinations of cathode and anode materials have been investigated for alkali(Li,Na)-ion battery materials. The achieved battery voltage depending on the charging state of the battery is given by the change in the Gibbs energy for the exchange of alkali as governed by the exchange of electrons and ions between the electrodes. Systematic experimental studies on the relative influence of both contributions have not been performed yet. We will present in this contributions surface science studies of well-defined electrode surfaces which allows to compare changes in electronic structure and related electronic surface potentials with voltage measurements. In addition, we will present first experiments on the direct measurements of the ionic and electronic work function of the cathodes and of spectral changes in solid-state battery set-ups A number of different cathode materials with different composition of the layered oxide and olivine structural family have been prepared by sputter deposition inside an integrated UHV system. Subsequently, the changes of electronic structure and surface potentials as e. g. the work function induced with charging have been determined by applying photoelectron spectroscopy and X-ray absorption spectroscopy. In addition, surface cleaning procedures by mechanical scratching have been applied to prepare well defined bulk representing surfaces. The experiments allow to relate the energetic changes in the electronic structure and the Fermi level positions to the measured battery voltage and thus to deduce the relative contribution of the electronic electrochemical potential. Based on our data we conclude that a major contribution to the battery voltage is related to the variation of the electronic contribution. Also the tendency to decomposition and side reaction with the electrolyte is governed by changes in the electronic structure of the cathode materials. In addition, we have performed experiments by experimentally determining the electronic and ionic work function of electrode materials. Such experiments allow to determine the chemical potential of the Li atom as given by the electronic and ionic contribution in relation to the vacuum level. With a Born-Haber cycle we can show that the experimentally determined values are in reasonable correspondence to the battery voltage. changes in surface composition and surface potentials can directly be deduced during polarization from in situ quasi operando electron spectra (SXPS and XAS) on complete solid thin film battery structures using NaCoO 2 as cathode. The results indicate that the higher stability of NaCoO 2 with Na loss in contrast to LiCoO 2 is related to a favourable involvement of O2p states to the valence band maximum involved in hole transfer. Summarizing our results we conclude that the performance of Li-ion batteries depend on the relative changes of bonding contributions for electrons and ions within the used electrodes with electrons providing a major role for battery performance.,These are related to deviations of the rigid band behavior of the electrode materials and to possibly induced surface reactions occurring in the given battery set-up.

For the conversion of solar energy to a chemical fuel a lot of different materials as well as device structures have been suggested but only very few provide technological competitive conversion efficiencies. Limitations and loss processes can be deduced from a detailed consideration of the involved photovoltaic and electrochemical elementary steps. Optimized performance can only be reached when the photovoltaic and electrolytic boundary conditions of integrated systems are comparable to those of separated devices with no extra loss due to the coupling process. Besides classical semiconductors transition metal oxides are reported to provide a promising material's class for many components of inorganic artificial leaf devices because of their wide range of available physical properties and expected inherent stability. However, due to their specific electronic properties their applicability and inherent limitations need to be specifically evaluated. For such experiments we have combined thin film synthesis, (photo)electrochemistry and surface science techniques mainly XPS and UPS to investigate the applicability of different oxides for (photo)electrochemical cells. As promising devices buried junctions are needed which must provide surfaces and interfaces free of interfacial barriers with additional losses of potential and current across external or internal double layers. As examples of our approach we will present transition metal oxides used as photovoltaic component as well as for electro catalysis. These oxides are characterized by an electronic structure where the Fermi level is situated between transition metal d n electron states with localized character. As consequence the optical and electronic properties are strongly determined by the electronic structure of the involved d-electron states which may lead to polaron formation or internal redox reactions limiting the photovoltaic performance. This was shown in detailed experiments on thin films of Fe 2 O 3 and BiVO 4 . For efficient electro catalysis it is important that charge transfer rates to the active surface sites are not limited by the electronic coupling. Limitation by internal junctions are demonstrated for MnO x catalysts deposited on different substrates. In contrast transition metal oxide alloys of Fe, Co and Ni show improved properties when they are in-situ formed from precursor Borides compared to their Oxide counterparts. These results demonstrate that for composites the electronic exchange across internal interfaces of the involved semiconducting oxides is an important factor. In summary, it becomes clear that the specific electronic properties of transition metal oxides within the bulk and across interfaces plays a dominant role for obtaining good performance. Therefore, an improved understanding of the correlation of the material’s electronic structure governing the electronic elementary exchange steps in the given device arrangement is a precondition for a target oriented engineering of advanced technological devices.

The wide utilization of Ni-rich oxides as cathode materials in lithium-ion batteries (LIBs) for high-energy applications remains a challenge due to their unstable behaviour at high state-of-charge (SOC) increasing the risk of thermal runaway. To understand this instability, in-depth knowledge of the involved redox processes is needed, which depend on the electronic structure of these materials and their interaction with the electrolyte. X-ray photoemission spectroscopy (XPS) is a promising method to provide information on the electronic structure while inherently limited to a low information depth. The latter is especially problematic for LIB materials, due to the formation of interphase layers on both the anode and the cathode. In our previous studies, we developed an in vacuo scratching method, which allowed the removal of a large fraction of the cathode electrolyte interphase (CEI) and applied the method to the cathode materials LiCoO2 (LCO) and LiNi0.3Mn0.3Co0.3O2 (NMC333) to investigate the charge state dependent transition metal core spectra, exhibiting high spectral quality. In this study, the in vacuo scratching method was applied to the Ni-rich layered oxide compounds LiNi0.6Mn0.2Co0.2O2 (NMC622), LiNi0.8Mn0.1Co0.1O2 (NMC811) and LiNi0.80Co0.15Al0.05O2 (NCA). All three materials showed an equally high oxidation state of Ni in their pristine state. The Ni2+/3+ and Ni3+/4+ redox couples were identified by the analysis of the Ni 2p core spectra as a function of voltage. A shift to lower binding energies of the Co 2p and O 1s photoelectron spectra was observed due to the downward shift of the Fermi level. Charging the materials to high voltages up to 4.8 V vs. Li+/Li resulted in fully deintercalated compounds, as shown by the Li 1s photoelectron spectra, while no further change in the Ni 2p photoelectron spectra suggests deviations from the cationic redox process. The cathode-electrolyte interphase (CEI) composition was also analyzed for non-scratched samples at several voltage steps in the typical operational range of 3.0 to 4.2 V vs. Li+/Li. The results indicate a difference in the quantities of semi-carbonates and lithium alkoxides. At high voltages of 4.5/4.8 V vs. Li+/Li, considerable changes were observed in the CEI due to the onset of decomposition reactions of the CEI components.

Semiconductor/electrolyte interfaces used as photoelectrochemcal junctions strongly depend in their optoelectric performance on the atomic and related electronic structure of the interface. Often a simplified and idealized semiconductor /metal Schottky barrier model is applied to describe the junction properties. However for real devices strong deviations are evident as will be discussed in this contribution due to Fermi level pinning effects and adsorbate related double layer potential shifts of the band edges. This is especially true for photoelectrochemical cells used in water splitting devices as strong interfacial interactions have to be taken into account. For this reason we will present detailed surface science studies on the surface/interface properties of Si and III-V benchmark systems espcially by applying photoelectron spectroscopy. For such studies the effect of source induced surface photovoltages are often not considered but cannot be neglected. Surface band energy diagrams induced by dangling bond related surface states are deduced for differently doped single crystalline Si(111) and (100) after appying different surface preparation and processing procedures. Additionally, the contact to electrolytes and the induced changes in the interfacial electronic structure are investigated by a „frozen electrolyte“ approach adsorbing water at low sample temperature. The results are compared to photoelectrochemical cells based on Si single crystals as well as to thin film multiabsorber cells used as photocathodes. III-V semiconductors in their initial fundamental surface properties are already strongly dependent on the applied wet chemical etching recipy as shown for InP, GaP and GaInP (100) surfaces in comparison to vacuum based processing steps as heating and sputtering. The best surfaces are in their surface compostion very close to samples prepared by MOCVD. Also for the III-V materials the interaction with H 2 O was investigated. In all cases Fermi level pinning close to the valence band maximum was observed and compared to the resulting band diagrams of photoelectrochemical devices used for HER. Our surface science investigations demonstrate that defect levels related to dangling bond states play a dominant role for the properties of the devices. In buried junctions to electronic passivation layers the concentration of remaining defect states can be reduced improving the performance of photoelectrochenical junctions. In conclusion we suggest that surface science studies of semiconductor surfaces and related interfaces before and after electrochemical studies with special emphasis of the valence band regime are essential to obtain improved insights of semiconductor/electrolyte junction properties. References (Photo)electrochemial Reactions on semiconductor surfaces, part A: Si surfaces - atomic and electronic structure D. Moritz, Wolfram Calvet, Mohammad Amin Zare Pour, Agnieszka Paszuk, Thomas Mayer, Thomas Hannappel, Jan P. Hofmann, Wolfram Jaegermann (Photo)electrochemial Reactions on semiconductor surfaces, part B: III-V surfaces - atomic and electronic structure A. Hajduk, Mohammad Amin Zare Pour, Agnieszka Paszuk b , Margot Guidat, Mario Löw, Fabian Ullmann, Dominik C. Moritz, Jan P. Hofmann, Stefan Krischok, Erich Runge, Wolf Gero Schmidt, Wolfram Jaegermann, Matthias M. May, Thomas Hannappel In Encyclopedia of Solid-Liquid Interfaces, Edited by K. Wandelt and G. Bussetti ISBN978-0-323-85670-6 (2024)

The composition and resulting band alignment at the TiO 2 /InP heterointerface are critical for optimizing semiconductor‐based photoelectrochemical and photovoltaic devices. Hence, a systematic investigation of the chemical composition and electronic properties of TiO 2 film grown via atomic layer deposition (ALD) on p‐doped, atomically well‐ordered, phosphorus‐terminated InP(100) surfaces is conducted. A combination of UV and X‐ray photoelectron spectroscopy with ab initio molecular dynamics simulations is applied to provide a comprehensive atomic‐scale understanding of the heterointerface. These results reveal that the P−P dimers in the first monolayer remain intact during the initial ALD cycles, while oxygen preferentially binds between indium in the second monolayer and phosphorus in the first monolayer, leading to the formation of interfacial indium phosphate (InPO x ) species. The presence of chlorine residues from the TiCl 4 precursor persists throughout the deposition process and influences the chemical environment of the interface. Band alignment analysis confirms the formation of a type‐II heterojunction, characterized by a valence band offset of approximately 2.3 eV and a conduction band offset of 0.45 eV, facilitating charge carrier separation essential for high‐efficiency photoelectrochemical applications. These detailed insights into the interfacial chemistry and electronic structure are fundamental to advance the development of efficient semiconductor‐based energy conversion devices.

Voltage-dependent charge compensation mechanisms of nickel-rich cathodes studied by photoemission spectroscopy. Insights into the cathode-electrolyte interphase revealed through in vacuo scratching followed by photoemission spectroscopy.

Understanding the evolution of the physicochemical bulk properties during the Li deintercalation (charging) process is critical for optimizing battery cathode materials. In this study, we combine X-ray photoelectron spectroscopy (XPS), density functional theory plus dynamical mean-field theory (DFT+DMFT) calculations, and charge transfer multiplet (CTM) model simulations to investigate how hybridization between transition metal (TM) 3d and oxygen 2p orbitals evolves with Li deintercalation. Based on the presented approach combining theoretical calculations and experimental studies of pristine and deintercalated cathodes, two important problems of ion batteries can be addressed: i) the detailed electronic structure and involved changes with deintercalation providing information of the charge compensation mechanism, and ii) the precise experimental analysis of XPS data which are dominated by charge transfer coupled to final-state effects affecting the satellite structure. As main result for the investigated Li TM oxides, it can be concluded that the electron transfer coupled to the Li$^{+}$-ion migration does not follow a rigid band model but is modified due to changes in TM 3d and O 2p states hybridization. Furthermore, this integrated approach identifies the 2p XPS satellite peak intensity of TM as an effective indicator of the redox chemistry. With that the redox chemistry of cathodes can be deduced, thus offering a foundation for designing more efficient battery materials.

Nickel-based oxides are among the best performing catalysts for the alkaline O2 evolution reaction (OER). It has long been recognized that iron enhances the catalytic activity of nickel-based catalysts, though only recently has intensive research been done on the interplay between the two transition metals, leading to the excellent performance, surpassing that of either pure metal. It is still not clear how the electronic configuration in these mixed metal compounds changes to enhance their catalytic activity for the OER. We carried out a systematic study of the electronic configuration of thin film mixed metal oxides Ni(1-x)FexOyHz with varying contents x of iron. In this investigation we employed X-ray absorption and resonant valence photoelectron spectroscopy (XAS and resPES) to gain knowledge on the changes induced in the electronic structure by introduction of iron, both before and after electrochemical activation. Based on density functional theory calculations we found iron species to induce a highly oxidizing environment that facilitates generation of oxo species on iron and neighbouring nickel sites. The reduced electron density around Ni-O bonds creates in-gap states near the Fermi level. The magnitude of these in-gap states scales linearly with the OER performance and thus can be used as an activity descriptor. Contrary to literature, we see the in-gap states even before electrochemical activation and conclude that they are a consequence of Ni-O-Fe motifs already present before anodization. Beyond 50% metal content the number of Ni-O-Fe motifs is decreasing again, resulting in an interval of 10-30% iron metal content to be optimal for the OER.

Semiconductor photoelectrochemistry is a dynamic and interdisciplinary field at the forefront of research in solar fuels, energy conversion, and catalysis. This Perspective captures the collective insights from the second Gerischer Electrochemistry Today Symposium, held at Colorado State University in Fort Collins, CO, in August 2024, which convened leading researchers, early-career scientists, and industry partners to define the critical next steps for the field. Through interactive sessions, technical talks, panel discussions, and training initiatives-including a Semiconductor Electrochemistry Bootcamp-the symposium emphasized three pillars of advancement: (i) facilitating the exchange of new ideas in semiconductor electrochemistry and charge separation; (ii) fostering the development of future researchers, research topics, and participation in the semiconductor workforce; and (iii) building community. This Energy Focus distills key themes from the meeting and identifies major knowledge gaps in the following areas: mechanisms of charge separation and recombination, role of defects and disorder, dynamic and operando characterization methods, interfacial chemistry and surface passivation, theoretical and modeling limitations, and standardization and benchmarking. The inclusive and collaborative structure of the symposium enabled the generation of this comprehensive report that will serve as a roadmap for fundamental and applied research in the rapidly evolving field of semiconductor electrochemistry over the next decade.

AlInP (001) is widely utilized as a window layer in optoelectronic devices, including world‐record III‐V multi‐junction solar cells and photoelectrochemical (PEC) cells. The chemical and electronic properties of AlInP (001) depend on its surface reconstruction, which impacts its interaction with electrolytes in PEC applications and passivation layers. This study investigates AlInP (001) surface reconstructions using density functional theory and experimental methods. Phosphorus‐rich (P‐rich) and indium‐rich (In‐rich) AlInP surfaces are prepared with in situ monitoring of the process by reflection anisotropy (RA) spectroscopy and confirmed by low‐energy electron diffraction and photoemission spectroscopy. The experimental RA spectra closely match the theoretical predictions obtained by solving the Bethe–Salpeter equation. It is shown that missing hydrogen on P‐rich surfaces and formation of In–In 1D atomic chains on In‐rich surfaces introduce mid‐gap surface states that pin the Fermi level and induce band bending. Time‐resolved two‐photon photoemission measurements reveal ultrafast near‐surface electron dynamics for both P‐rich and In‐rich surfaces, demonstrating photoexcited electrons reaching the surface conduction band minimum and relaxing to mid‐gap surface states on about hundreds of fs. This work provides the most extensive AlInP surface analysis to date, allowing for more targeted surface and interface engineering, which is crucial for the optimization and design of III‐V heterostructures.

Artificial leaves could be the breakthrough technology to overcome the limitations of storage and mobility through the synthesis of chemical fuels from sunlight, which will be an essential component of a sustainable future energy system. However, the realization of efficient solar‐driven artificial leaf structures requires integrated specialized materials such as semiconductor absorbers, catalysts, interfacial passivation, and contact layers. To date, no competitive system has emerged due to a lack of scientific understanding, knowledge‐based design rules, and scalable engineering strategies. Herein, competitive artificial leaf devices for water splitting, focusing on multiabsorber structures to achieve solar‐to‐hydrogen conversion efficiencies exceeding 15%, are discussed. A key challenge is integrating photovoltaic and electrochemical functionalities in a single device. Additionally, optimal electrocatalysts for intermittent operation at photocurrent densities of 10–20 mA cm −2 must be immobilized on the absorbers with specifically designed interfacial passivation and contact layers, so‐called buried junctions. This minimizes voltage and current losses and prevents corrosive side reactions. Key challenges include understanding elementary steps, identifying suitable materials, and developing synthesis and processing techniques for all integrated components. This is crucial for efficient, robust, and scalable devices. Herein, corresponding research efforts to produce green hydrogen with unassisted solar‐driven (photo‐)electrochemical devices are discussed and reported.

The design of cathode/electrolyte interfaces in high-energy density Li-ion batteries is critical to protect the surface against undesirable oxygen release from the cathodes when batteries are charged to high voltage. However, the involvement of the engineered interface in the cationic and anionic redox reactions associated with (de-)lithiation is often ignored, mostly due to the difficulty to separate these processes from chemical/catalytic reactions at the cathode/electrolyte interface. Here, a new electron energy band diagrams concept is developed that includes the examination of the electrochemical- and ionization- potentials evolution upon batteries cycling. The approach enables to forecast the intrinsic stability of the cathodes and discriminate the reaction pathways associated with interfacial electronic charge-transfer mechanisms. Specifically, light is shed on the evolution of cationic and anionic redox in high-energy density lithium-rich 0.33Li2MnO3·0.67LiNi0.4Co0.2Mn0.4O2 (HE-NCM) cathodes, particularly those that undergo surface modification through SO2 and NH3 double-gas treatment to suppress the structural degradation. The chemical composition and energy distribution of the occupied and unoccupied electronic states at the different charging/discharging states are quantitatively estimated by using advanced spectroscopy techniques, including operando Raman spectroscopy. The concept is successfully demonstrated in designing artificial interfaces for high-voltage olivine structure cathodes enabling stable battery operation up to 5.1 V versus Li+/Li.

Nuclear forward scattering (NFS) is a synchrotron-based technique relying on the recoil-free nuclear resonance effect similar to Mössbauer spectroscopy. In this work, we introduce NFS for in situ and operando measurements during electrocatalytic reactions. The technique enables faster data acquisition and better discrimination of certain iron sites in comparison to Mössbauer spectroscopy. It is directly accessible at various synchrotrons to a broad community of researchers and applicable to multiple metal isotopes. We demonstrate the power of this technique with the hydrogen evolution mechanism of an immobilized iron porphyrin supported on carbon. Such catalysts are often considered as model systems for iron-nitrogen-carbon (FeNC) catalysts. Using in situ and operando NFS in combination with theoretical predictions of spectroscopic data enables the identification of the intermediate that is formed prior to the rate determining step. The conclusions on the reaction mechanism can be used for future optimization of immobilized molecular catalysts and metal-nitrogen-carbon (MNC) catalysts.

Characterizing Li-ion battery (LIB) materials by X-ray photoelectron spectroscopy (XPS) poses challenges for sample preparation. This holds especially true for assessing the electronic structure of both the bulk and interphase of positive electrode materials, which involves sample extraction from a battery test cell, sample preparation, and mounting. Here, we introduce the method of in vacuo scratching as preparation for XPS measurements of LIB positive electrode materials, which can drastically reduce spectral contaminations and enhance spectral quality, enabling higher information gain. The benefits of this effective yet simple method are highlighted in comparison to Ar+-sputtering and air contact using commercially applied powder LiNi1/3Mn1/3Co1/3O2 (NMC333) as showcase material. For assessing interphase formation, the electrode material was soaked in electrolyte, and the effects of rinsing and in vacuo scratching were studied. This treatment results in high signal-to-noise spectra, yielding well-defined photoelectron spectra without the influence of the interphases and spectral artifacts from, e.g., Ar+ sputtering.

Enhancing the high voltage limit of layered LiTMO2 (TM = 3d transition metal) cathode materials has motivated intensive research on their properties at high states of charge. This led to controversies regarding the origin of anionic and cationic redox reaction, the reversibility of oxygen redox reaction, the degree of covalency, and the intrinsic voltage limits of cathode materials. In this study, the commercially used cathode materials LiCoO2 (LCO) and LiNi1/3Mn1/3Co1/3O2 (NMC333) were investigated by core level photoelectron spectroscopy (XPS) after charging to different voltages. A large fraction of the cathode electrolyte interface (CEI) was removed after in vacuo scratching of the surface, enabling insights into the fundamental charge compensation mechanism in the bulk obtained by XPS. To gain insights into the oxidation state using XPS, the origin of the 2p satellite structure seen in the XPS spectra of transition metal oxides is reconsidered in comparison to reference materials. Additionally, both the CEI stability and the electrochemical behaviour are discussed and correlated to the charge compensation mechanism. For LCO, the XPS results confirm the intrinsic voltage limit of 4.2 V vs. Li+/Li with the Co3+/Co4+-redox couple at 4.0 V and subsequent oxygen-redox and CEI instability. For NMC333, the Ni3+/Ni4+ redox couple at 3.7 V, the Co3+/Co4+-redox couple at 4.0 V, and manganese in a stable formal oxidation state of 3.7+ were identified. The intrinsic voltage limit in NMC333 is enhanced to 4.5 V vs. Li+/Li.

Electrocatalysts play an important role in a future renewable energy economy. Therefore, a thorough understanding of their functionality can help to improve and replace existing rare and expensive Pt-group based metal and metal oxide materials for fuel cell applications and also materials used for alkaline electrolysis. Here, we aim to synthesize thin film model systems to study and understand the critical processes taking place at the electrode/electrocatalyst/ electrolyte interface under operating conditions by a surface science approach combined with electrochemical processing and treatment. NiOx(OH)y electrocatalysts are prepared on different substrates by electrochemical and by magnetron sputtering methodologies. The so prepared electrodes are characterized with regard to their activity for the hydrogen evolution reaction (HER) as well as for the oxygen evolution reaction (OER) by standard electrochemical procedures. In our approach we characterize the samples before and after electrochemical treatment mainly by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy with regard to their chemical surface composition. To obtain a deeper insight into the changes taking place under working settings, we also apply Raman spectroscopy during operando conditions for selected electrodes. From the experimental data we conclude, that a mixture of metallic and different oxidic phases leads to the highest overall HER activity. The pure phases on its own show only a strongly reduced activity. In general, the preparation of the samples plays the most important role for the final activity. This is especially observed for the OER, where an easy transformation of the starting phases is required for improved electrical conductivity and electrocatalytic activity. A large number of defects favor the formation of the Ni3+ oxyhydroxide phase, which can then form the O2 evolving site under operating conditions. Even if our model systems are still too complicated to deliver a thorough understanding of all underlying processes, we can show the validity of our approach. Furthermore, we obtained a rich spectral and electrochemical database for NiOx(OH)y based systems as catalysts for electrochemical water splitting.