The preparation of semiconductor surfaces for renewable energy or other optoelectronic applications requires highly precise control of surface structure and composition, rendering vacuum-based techniques typically the method of choice. However, when precise process control is guaranteed, wet-chemical etching processes constitute a resource-efficient and scalable alternative. In this work, we show that reflection anisotropy spectroscopy (RAS)─a nondestructive, optical technique─offers a powerful in situ control of wet-chemical etching processes. Here, we employ RAS to determine the etching depth, rate, and time during the wet-chemical preparation of a photoelectrode for direct solar water splitting. This is achieved by observing and evaluating Fabry-Pérot-type oscillations of reflectance and RAS transients during the etching process. The occurrence of these oscillations confirms the uniform character of the investigated surface in a suitable etching parameter space and allows the determination of the etching depth with a precision of ±8 nm. A variation of the etching procedure demonstrates that RAS control can be used for the development of suitable etching routines and for in situ process control, as validated for two different types of photoelectrodes.

Resource-efficient carbon dioxide removal from the atmosphere will require the development of new materials for processes beyond carbon capture and utilisation. One high-potential pathway for such negative emission technologies is the electrochemical reduction of CO 2 to storable products such as graphitic carbon. Although this can be achieved by employing an electrode made of cerium particles in a liquid gallium-indium-tin matrix, this electrocatalytic pathway currently suffers from comparatively low faradaic efficiencies (FE). In this work, we investigate the temperature dependence of electrochemical CO 2 -reduction to solid carbon on this liquid-metal. An increase in temperature of up to 50 ◦ C leads to significantly higher FEs. A variation of applied potentials leads to a temperature-dependent exfoliation of the product. Moreover, a change in the morphology of the solid carbon product species, from carbon black-like particles, over graphitic flakes to fibers can be induced by varying the operating conditions. From a practical standpoint, the temperature range, we find for optimum FE, lies in the range that can be achieved through thermal coupling with photovoltaic solar cells, suggesting co-benefits of photovoltaics-driven, electrochemical carbon dioxide removal.

Hard carbon is one of the most promising anode materials for post-Li-ion batteries; however, the relationship between the precursor material and performance is still hardly understood. In this work, we present a theoretical investigation of hard carbon anode materials derived from lignin, one of the most prevalent biomass materials. Using density functional theory-based calculations, the formation of several lignin-derived hard carbon structures at different temperatures was investigated, yielding amorphous structures with different morphologies. We observe a significant impact of the preparation routine on the resulting hard carbon structure and the corresponding properties. The created materials show a robust morphology and an increased capacity (AMC(n), n < 6) as compared to pristine graphite. While unsaturated carbon bonds may result in unfavorable insertion potentials, preparation routes that decrease the number of these bonds yield materials that show promising properties for anode applications in Li-, Na-, and K-ion batteries.

Insertion-type metal oxide anode materials for lithium-ion and sodium-ion batteries commonly offer excellent cycling stability. However, their reversible capacity is eventually constrained by the finite number of available active sites within their crystal structures. Herein, the effect of introducing Sb into insertion-type CeO2 (Sb-CeO2) is investigated as an effective strategy to overcome this intrinsic limitation. Compared to neat CeO2, it shows a substantially increased capacity owing to the extended redox activity of the Sb dopant while generally maintaining the insertion-type reaction mechanism of the CeO2 host matrix. Interestingly, Sb-CeO2 demonstrates promising performance in both Li-cells and Na-cells. Although the capacity contribution of Sb is slightly reduced in Na-cells compared to Li-cells, the former benefit from a highly stable solid electrolyte interphase layer and a remarkable rate capability in combination with an ether-based electrolyte. This is demonstrated also in sodium-ion cells comprising an Sb-CeO2 anode and a Na3V2(PO4)3 cathode, showing excellent power performance up to a dis-/charge rate of 50C.

Understanding the structure of semiconductor-electrolyte interfaces under operating conditions is crucial for designing electrodes in photoelectrochemistry and electrocatalysis. However, only few experimental methods exist that give real-time access to the very interface. Here, reflection anisotropy spectroscopy (RAS) is an emerging technique in the field of spectroelectrochemistry. We computationally investigate how the surface structure of clean and oxygenated InP(001) surfaces-in vacuum and in contact with water-and its evolution over time affect the optical response. Depending on the electronic structure of the respective surfaces, different species are adsorbed, resulting in changes of the anisotropy clearly visible in the spectroscopic fingerprint, while the presence of stabilizes certain configurations. Distinct fluctuations of the individual spectra are observed during the molecular dynamics trajectory. However, the resulting time-averaged spectra show a rather good agreement with the respective spectra of the reference structure for most structures. This means that-depending on the surface-the geometry-optimized structures might be suitable for comparison with experiment or not. This behavior differs from the case of metals and can be attributed to the semiconducting nature of the system. Our findings highlight the need to account for the electrochemical environment in computational RAS.

Adsorption of alkali-metal atoms on graphitic surfaces plays a fundamental role in electrochemical energy storage, particularly in carbon-based anodes such as hard carbon. Using first-principles calculations, we systematically investigate alkali-metal adsorption on graphite as a function of surface coverage and demonstrate that the adsorption energetics are governed by long-range dipole–dipole interactions between neighboring atoms. In the context of hard carbon, alkali-metal clustering on graphitic surfaces constitutes a critical initial step in the filling of micro- and nanopores. We reveal pronounced differences in the energetics and growth mechanisms of Li, Na, and K clusters and show that structural defects strongly modulate clustering and stability by providing preferential adsorption sites that alter aggregation behavior. Importantly, neglecting these long-range dipole–dipole interactions, and thus the associated surface-coverage dependence, leads to a systematic overestimation of adsorption energies and an incomplete description of alkali-metal storage in carbon-based anodes. Overall, this work provides a unified physical description of coverage-dependent, long-range dipole–dipole interactions, revealing the atomic-scale mechanisms governing adsorption energetics and high-concentration clustering in graphitic carbon, along with their implications for alkali-metal storage in carbon-based anodes.

The unstable solid-electrolyte interface (SEI) poses a major obstacle to the widespread use of rechargeable magnesium batteries (RMBs) as high-volumetric-capacity next-generation energy storage systems. This issue is effectively mitigated by adding 3 wt.% tris(trimethylsilyl) borate (TMSB, C9H27BO3Si3) to a state-of-the-art Cl-free magnesium tetrakis(hexafluoroisopropyloxy)borate in dimethoxyethane (Mg[B(hfip)4]2/DME) non-aqueous electrolyte. The modified electrolyte enables stable Mg||Mo6S8 (Chevrel phase, CP) full cell operation for up to 1000 cycles at a 1C rate. Tip-enhanced Raman spectroscopy (TERS) reveals that TMSB scavenges degraded electrolyte components and facilitates the formation of a uniform and thin SEI on the magnesium anode. Reflection anisotropy spectroscopy (RAS) further demonstrates that TMSB transforms the interfacial structure, creating a more isotropic and robust SEI during the initial stripping and plating process, thereby extending electrochemical cycling stability. This approach presents a compelling pathway for practical RMB development by stabilizing the SEI and optimizing magnesium electrolyte formulations.

Understanding the structure of semiconductor-electrolyte interfaces under operating conditions is crucial for designing electrodes in photoelectrochemistry and electrocatalysis. However, only few experimental methods exist that give real-time access to the very interface. Here, reflection anisotropy spectroscopy is an emerging technique in the field of spectroelectrochemistry. We computationally investigate how the surface structure of clean and oxygenated InP(001) surfaces - in vacuum and in contact with water - and its evolution over time affect the optical response. Depending on the electronic structure of the respective surfaces, different species are adsorbed, resulting in changes of the anisotropy clearly visible in the spectroscopic fingerprint, while the presence of H2O stabilises certain configurations. Distinct fluctuations of the individual spectra are observed during the molecular dynamics trajectory. However, the resulting time-averaged spectra show a rather good agreement with the respective spectra of the reference structure for most structures. This means that - depending on the surface - the geometry-optimised structures might be suitable for comparison with experiment or not. This behaviour differs from the case of metals and can be attributed to the semiconducting nature of the system. Our findings highlight the need to account for the electrochemical environment in computational reflection anisotropy spectroscopy.

The versatile optoelectronic properties of the material class of III-V semiconductors enable the highest performance in photovoltaic and photoelectrochemical solar cells. While a high level of control and understanding with respect to different surface reconstructions of these compounds in gas-phase ambient has been reached, the situation in an electrochemical environment still poses challenges. Here, we therefore have undertaken a computational study of the InP(100) surface in the presence of hydrogen and chlorine, mimicking the contact with a hydrochloric acid-containing electrolyte, aiming at an understanding of ion adsorption and dominant surface reconstructions with respect to applied potential and electrolyte concentration. For this purpose, the most stable surface terminations for hydrogen and chlorine (co)adsorption from the gas phase as well as the corresponding phase diagrams have been determined with respect to the hydrogen and chlorine chemical potential. In this context, we also introduce a quantitative type of phase diagram to highlight the stability of surface phases with respect to competing structures. Finally, by making use of the computational hydrogen electrode approach, these results were then transferred to the potential domain. We find that hydrogen (chlorine) adsorption is dominating at more (less) cathodic potentials, while coadsorption is limited to small fractions of the phase space. This allows us to determine experimentally accessible phases for which no detrimental effects, such as the creation of in-gap surface states, are to be expected.

Graphite and graphite derivatives, the standard anode materials for Li-ion batteries, are also of great interest for post-Li-ion technologies, such as potassium-ion batteries. However, certain aspects of the intercalation process in these systems, as well as the resulting consequences, still require a deeper understanding. In particular, the first steps of K intercalation in graphitic systems, i.e., at low concentrations, are fundamentally different from the case of Li. Herein, we use density functional theory to elucidate the early-stage intercalation of K in graphitic materials by seeking comparison to the behavior of Li and Na. Our results show the crucial role of the competition between the interlayer van der Waals interaction and the alkali metal-carbon bond formation for the initial stages of intercalation of large alkali metal atoms. As a consequence, and in contrast to the case of Li, K intercalation becomes energetically unfavorable at low concentrations. This is a significant finding, which can explain the origin of the differences observed for Li and K intercalation in graphitic materials. Hence, we identify the first steps of K intercalation as potential reasons for performance loss and battery failure and show that heteroatom doping can open pathways for solving these issues.

This first-principles study provides insights into the stability of oxygenated InP(001) surfaces.

Post-transition liquid metal (LM) alloys are of increasing interest for the electro-reduction of CO2. Besides avoiding coking effects, they often exhibit an electric field-induced accumulation of elements at their interface, which can be used to tune catalytic activity and product selectivity. However, mechanistic insights into these systems remain sparse. In this study, we investigate the ternary GaInSn alloy in DMF-based electrolyte. We demonstrate that even small amounts of water in DMF significantly increase the CO2 reduction rate. Under optimized conditions (2 M water in DMF at -2.7 V versus Ag/Ag+), the overall current density is approximately -3 mA/cm(2), two orders of magnitude higher than in anhydrous electrolysis. The Faradaic efficiency for HCOOH and CO production is around 60% and 30%, respectively, whereas the competing hydrogen evolution remains a minor side product. Analysis of the electrolyte after CO2 electrolysis reveals varying amounts of expelled tin and gallium. This suggest that an enrichment of the interface in tin may be responsible for the high selectivity toward HCOOH. Our study provides a better mechanistic understanding of CO2 reduction on GaInSn and shows that the dynamic elemental composition should always be considered for the interpretation of experimental results.

The adsorption of alkali metal (AM) atoms on graphitic surfaces is one of the processes that determine the performance of carbon-based anode materials. In particular, when graphite derivatives such as hard carbon with increased surface area are considered, adsorption accounts for a significant amount of the AM storage capacity. While it is well known that the adsorption of Li and Na on pristine graphite is energetically unfavorable, this article shows how graphitic surfaces can be modified to tailor their adsorption properties. For this purpose, the adsorption of Li, Na, and K on graphitic model systems, containing defects and impurities as well as combinations thereof, is investigated by means of density functional theory. The results show that particular defects and impurity atoms can modify the adsorption strength of the surface such that Li and Na adsorption become energetically favorable, while at the same time, capacity loss via trapping of AM atoms is minimized.

Due to its negligible capacity with respect to sodium intercalation, graphite is not suited as anode material for sodium ion batteries. Hard carbon materials, on the other hand, provide reasonably high capacities at low insertion potential, making them a promising anode materials for sodium (and potassium) ion batteries. The particular nanostructure of these functionalized carbon-based materials has been found to be crucially linked to the material performance. However, there is still a lack of understanding with respect to the functional role of structural units, such as defects, for intercalation and storage. To overcome these problems, the intercalation of Li, Na, and K in graphitic model structures with distinct defect configurations has been investigated by density functional theory. The calculations confirm that defects are able to stabilize intercalation of larger alkali metal contents. At the same time, it is shown that a combination of phonon and band structure calculations are able to explain characteristic Raman features typically observed for alkali metal intercalation in hard carbon, furthermore allowing for the quantification of the alkali metal intercalation inbetween the layers of hard carbon anodes.

The electrochemical reduction of CO2 (CO2RR) offers a sustainable technology for converting CO2 into valuable products when using electricity generated from renewable sources. Application-oriented research is currently focused mainly on the electrolysis of CO2 to hydrogen-rich fuels or chemical feedstock materials. In contrast our project aims to form solid carbon or carbon rich products (e.g. oxalate), which can finally be disposed in geological repositories. This negative emission technology is developed for a long-term and thereby sustainable CO2 removal from the global CO2 cycle 1. The continuous electrochemical formation of solid carbon from CO2 has so far only been reported on liquid GaInSn-M alloys (M: Ce, V) in water-containing DMF 2,3. Liquid electrodes have the advantage over solid electrodes that solid products do not adhere at the interface and therefore deactivate the catalytic properties of the electrode (coking effect). In first experiments, we studied GaInSn without additional metal alloying in DMF/H2O/ TBAPF6 electrolyte and got differing results to the previously published data. In our experiments, even pure GaInSn shows a significant activity for CO2RR to carbon monoxide and formic acid accompanied by lower quantities of H2. The product distribution (faradaic efficiencies) depends strongly on the water content in the DMF electrolyte. The production rate of CO increases significantly even with small amounts of water. First DFT simulations of the CO2RR provide possible explanations of this effect. Furthermore, it was observed that, depending on the applied cathodic potential, metallic nano particles are released from the GaInSn surface into the organic electrolyte (expulsion effect) which indicates a possible change in the surface composition during CO2 electrolysis 4. In accordance with the literature, our findings confirm the production of carbon flakes from CO2 on cerium modified GaInSn. However, we observe it even independently from the addition of water to the DMF electrolyte. DFT calculations enable us to conclude that the high affinity of cerium to oxygen is responsible for the change in product selectivity, as it can extract oxygen from CO2. 1: May, M. M. and Rehfeld, Earth Syst. Dynam., 10, 1–7. doi.org/10.5194/esd-10-1-2019, 2019. 2: Dorna Esrafilzadeh, NATURE COMMUNICATIONS | (2019) 10:865. doi.org/10.1038/s41467-019-08824-8 3: Mehmood Irfan, J. Mat. Chem. A, Issue 27, 2023 doi.org/10.1039/D3TA01379K 4: Mahroo Baharfar, Chemistry of Materials 2022 34 (23), 10761-10771 doi.org/10.1038/s41467-019-08824-8 Figure 1

In electrochemistry, reactions and charge-transfer are to a large extent determined by the atomistic structure of the solid-liquid interface. Yet due to the presence of the liquid electrolyte, many surface-science methods cannot be applied here. Hence, the exact microscopic structure that is present under operating conditions often remains unknown. Reflection anisotropy spectroscopy (RAS) is one of the few techniques that allow for an in operando investigation of the structure of solid-liquid interfaces. However, an interpretation of RAS data on the atomistic scale can only be obtained by comparison to computational spectroscopy. While the number of computational RAS studies related to electrochemical systems is currently still limited, those studies so far have not taken into account the dynamic nature of the solid-liquid interface. In this work, we investigate the temporal evolution of the spectroscopic response of the Au(110) missing row reconstruction in contact with water by combining ab initio molecular dynamics with computational spectroscopy. Our results show significant changes in the time evolution of the RA spectra, in particular providing an explanation for the typically observed differences in intensity when comparing theory and experiment. Moreover, these findings point to the importance of structural surface/interface variability while at the same time emphasising the potential of RAS for probing these dynamic interfaces.

Zinc-based batteries offer good volumetric energy densities and are compatible with environmentally friendly aqueous electrolytes. Zinc-ion batteries (ZIBs) rely on a lithium-ion-like Zn^2+-shuttle, which enables higher roundtrip efficiencies and better cycle life than zinc-air batteries. Manganese-oxide cathodes in near-neutral zinc sulfate electrolytes are the most prominent candidates for ZIBs. Zn^2+-insertion, H^+-insertion, and Mn^2+-dissolution are proposed to contribute to the charge-storage mechanism. During discharge and charge, two distinct phases are observed. Notably, the pH-driven precipitation of zinc-sulfate-hydroxide is detected during the second discharge phase. However, a complete and consistent understanding of the two-phase mechanism of these ZIBs is still missing. This paper presents a continuum full cell model supported by DFT calculations to investigate the implications of these observations. We integrate the complex-formation reactions of near-neutral aqueous electrolytes into the battery model and, in combination with the DFT calculations, draw a consistent picture of the cycling mechanism. We investigate the interplay between electrolyte pH and reaction mechanisms at the manganese-oxide cathodes and identify the dominant charge-storage mechanism. Our model is validated with electrochemical cycling data, cyclic voltammograms, and in-situ pH measurments. This allows us to analyse the influence of cell design and electrolyte composition on cycling and optimize the battery performance.

The electrochemical reduction of CO2 is a promising realisation of negative emissions to mitigate climate change, aiming at the efficient production and safe longterm storage of carbon-rich sink products. This approach, however, necessitates novel catalyst materials specifically targeting electrochemical carbon dioxide removal. In this work, we investigate synthesis routes for a cerium-incorporated GaInSn-based liquid metal catalyst, focusing on the electrochemical production of graphitic carbon. Preparation and preconditioning of the catalyst are found to be crucial for carbon production, while trace amounts of H2O and OH in the organic electrolyte play a decisive role for the efficiency of the electrocatalytic process. Finally, for a better understanding of the reaction mechanism and the involved active species, experimental findings and density functional theory-based calculations are combined, suggesting a two-step reduction pathway with Ce(OH)x as the catalytically active surface species.