The growing need to store increasing amounts of renewable energy has recently triggered substantial R&D efforts towards efficient and stable water electrolysis technologies. The oxygen evolution reaction (OER) occurring at the electrolyser anode is central to the development of a clean, reliable and emission-free hydrogen economy. The development of robust and highly active anode materials for OER is therefore a great challenge and has been the main focus of research. Among potential candidates, perovskites have emerged as promising OER electrocatalysts. In this study, by combining a scalable cutting-edge synthesis method with time-resolved X-ray absorption spectroscopy measurements, we were able to capture the dynamic local electronic and geometric structure during realistic operando conditions for highly active OER perovskite nanocatalysts. BaSrCoFeO as nano-powder displays unique features that allow a dynamic self-reconstruction of the material's surface during OER, that is, the growth of a self-assembled metal oxy(hydroxide) active layer. Therefore, besides showing outstanding performance at both the laboratory and industrial scale, we provide a fundamental understanding of the operando OER mechanism for highly active perovskite catalysts. This understanding significantly differs from design principles based on ex situ characterization techniques.

In recent years, the oxygen evolution reaction (OER) has attracted increased research interest due to its crucial role in electrochemical energy conversion devices for renewable energy applications. The vast majority of OER catalyst materials investigated are metal oxides of various compositions. The experimental results obtained on such materials strongly suggest the existence of a fundamental and universal correlation between the oxygen evolution activity and the corrosion of metal oxides. This corrosion manifests itself in structural changes and/or dissolution of the material. We prove from basic thermodynamic considerations that any metal oxide must become unstable under oxygen evolution conditions irrespective of the pH value. The reason is the thermodynamic instability of the oxygen anion in the metal oxide lattice. Our findings explain many of the experimentally observed corrosion phenomena on different metal oxide OER catalysts.

Cell designs for the electrochemical reduction of CO 2 from gas phase were developed and investigated, and the critical elements for an efficient process were identified. Various types of polymeric membrane were used to build membrane electrode assembly adapted for CO 2 reduction in gas phase: protonic and anion exchange membrane (AEM), bipolar membrane and a modified bipolar like membrane configuration. Configurations using anion exchange ionomer in the cathodic catalytic layer in contact with an AEM allow for a great enhancement of the cathode reaction selectivity toward CO. However, a severe problem was identified when co-electrolysis is performed using only an AEM: this type of membrane acts as a CO 2 "pump" meaning that for each molecule of CO 2 reduced at the cathode, one or two CO 2 molecules are produced at the anode by oxidation of the carbonate/bicarbonate anion transported in the membrane. A bipolar membrane system was shown to soften this problem, but only a newly developed cell design was able to fully prevent the parasitic CO 2 pumping. Using this new cell configuration, the faradaic efficiency of an alkaline environment is maintained, the parasitic CO 2 pumping to the anode side is completely suppressed, and the overall cell voltage efficiency is highly improved.

The electrochemically active surface area (ECSA) of metal-oxide supported platinum catalysts as obtained from hydrogen underpotential deposition (H upd ) and from carbon monoxide stripping experiments was investigated. It was demonstrated that both methods fail to give meaningful values of the ECSA if they are performed in the conventional way as known for pure Pt and carbon supported Pt catalysts, respectively. For both methods, the reason for this failure is the lack of a correct baseline for the integration of the associated charges. It was found that the cyclic voltammogram recorded in CO saturated electrolyte gives an improved baseline for the H upd analysis. For CO stripping, a novel baseline method was developed by performing a "CO stripping simulation" (COSS) experiment in CO-free electrolyte. The first cycle of this COSS-experiment is an improved baseline for the integration of the CO stripping peak, since possible support reduction/oxidation currents can be accounted for. With these modifications, H upd and CO stripping voltammetry can be used for metal-oxide supported platinum to yield true, reproducible and consistent values for the ECSA. In recent years, an increased attention in the research on oxygen reduction reaction (ORR) catalysts for polymer electrolyte fuel cells (PEFC) has been focused on metal-oxide supported platinum catalysts.1 The reason for this interest is the potentially higher stability of the metal-oxide support in the oxidative electrochemical environment of a PEFC cathode in comparison with the carbon support of standard Pt/C catalysts for ORR. The resistance of the support toward corrosion is especially important under load cycling and start/stop conditions with cathode potentials reaching values up to 1.5 V vs. the adjacent electrolyte.2,3 State-of-the-art cathodes made of Pt nanoparticles supported on high surface area carbon suffer from severe corrosion at potentials above 1.1 V due to the oxidation of the carbon support. [4][5][6] The consequent detachment of Pt catalyst nanoparticles or their dissolution leads to a strong degradation of PEFC performance. 7,8 Replacing the carbon support by suitable metal-oxides in an oxidation state which is thermodynamically stable at PEFC cathode potentials can help to mitigate this degradation mode on the materials side. Furthermore, it is well established in heterogeneous catalysis that metal-oxide supports can possibly influence the intrinsic activity of the supported Pt catalyst toward ORR due to so-called strong metal-support interactions, SMSI.9-12 Thus, metal-oxides may offer a way to kill two birds with one stone: Enhancing the cathode stability and enhancing the cathode catalyst kinetics toward the ORR.In order to experimentally assess both the stability and the ORR activity properties of metal-oxide supported Pt catalyst, the determination of the electrochemically active Pt surface area (ECSA) is inevitable due to its widespread use as an important descriptor of the state of the degraded catalyst and electrode. Measuring the ECS...

An electrochemical three-electrode flow-cell is presented for in situ small-angle X-ray scattering (SAXS) and X-ray absorption spectroscopy (XAS) experiments in transmission mode at synchrotron X-ray sources. The cell also allows for in situ XAS performed in fluorescence mode. Constant experimental conditions, even under moderate gas evolution, are provided by the electrolyte flow with controlled gas saturation. A special configuration of working and counter electrode, respectively, yields low residual ohmic resistance in three-electrode measurements that enables the study of thick porous electrodes of active high surface area materials. The cell proved its functionality and reliability in two studies: First, an in situ anomalous SAXS experiment for the high-potential degradation properties of a Pt/IrO 2 -TiO 2 catalyst for the oxygen reduction reaction at polymer electrolyte fuel cell cathodes; and second, an in situ XAS study of the electronic state of Ir centers inside an IrO 2 -TiO 2 catalyst under oxygen evolution conditions. © The Author Modern research in electrocatalysis makes extensive use of in situ X-ray techniques that provide information about the structure and the electronic state of catalyst materials under electrochemical potential control. The reason for this is the limited, merely indirect information about the state of the catalyst that can be deduced from purely electrochemical testing like cyclic voltammetry (CV) which often does not allow for an unambiguous interpretation of the data. In order to develop an understanding at a more fundamental level, additional information is required about the potential-dependent state of electrocatalyst materials that can be provided by synchrotron-based techniques like X-ray scattering or X-ray absorption spectroscopy.One example is the investigation of polymer electrolyte fuel cell (PEFC) Pt cathode catalyst degradation. Different mechanisms have been proposed for the loss of electrochemically active Pt surface area (ECSA) that occurs most severely at transient high-potential spikes during PEFC start and stop.1,2 Processes like agglomeration of primary Pt particles due to migration or carbon support corrosion, Pt loss due to dissolution, and growth of primary Pt nanoparticles due to dissolution/redeposition cycles have been considered 3,4 and quantified for different operation conditions and electrochemical environments. The most common technique applied for this purpose is transmission electron microscopy (TEM), which has the convenient advantage that changes of the Pt nanoparticle structure can be directly visualized, especially with the use of identical location TEM (IL-TEM).5 Although successfully demonstrated, 6 in situ TEM remains limited to certain electrochemical systems. Whereas the strength of TEM lies in the direct imaging of individual catalyst particles, it is challenging to extract quantitative statistical information about the entire catalyst sample from TEM analysis. Finally, the distinction of Pt nanoparticles from the support material p...

The commercial feasibility of polymer electrolyte fuel cells (PEFCs) passes by the development of Pt-based, O 2 -reduction catalysts with greater activities and/or lower Pt-contents, as well as an improved stability. In an effort to tackle these requirements, unsupported bimetallic Pt-Ni nanoparticles (NPs) interconnected in the shape of nanochain networks (aerogels) were synthesized using a simple one-step reduction and gel formation process in aqueous solution. The products of this novel synthetic route were characterized by X-ray absorption spectroscopy to elucidate the materials' structure. Using electrochemical experiments, we probed the surface composition of the as-synthesized aerogels and of equivalent materials exposed to acid, and concluded that a Ni-(hydr)oxide side phase is present in the aerogel with a larger Ni-concentration. Regardless of this initial surface composition, the Pt-Ni aerogels feature a ≈3-fold increase of surface-specific ORR activity when compared to a commercial platinum-on-carbon catalyst, reaching the mass-specific requirement for application in automotive PEFCs. 1 to account for the large overpotential of the oxygen reduction reaction (ORR). Thus, Pt contributes significantly to the fuel cell system cost, and progress to reduce its loading is crucial to meet the long-term PEFC cost target of 40 $/kW set by the U. S. Department of Energy (DOE).2 One approach to reduce this excessive Pt-loading relies on increasing the catalysts' ORR activity, e.g. by alloying platinum with other metals like Ni, Cu and Co, to form materials which show up to one order of magnitude higher mass-specific activity than commercial Pt/C catalysts.3 On the other hand, these carbon-supported materials suffer from significant carbonand Pt-corrosion during the standard operation of PEFCs, gradually compromising their efficiency and reliability. 4 To partially overcome stability issues, research focuses on unsupported materials (e.g. Ptcoated Ni, Co or Cu nanowires 5-7 ) besides extended metal surfaces (e.g. 3 M nanostructured thin film catalysts 8 ) or alternative supports (e.g. conductive metal oxides 9-12 ). Naturally, those materials should be processable into catalytic layer architectures that provide reactant and product diffusion pathways similar to those in conventional Pt/C electrodes to guarantee high catalyst utilization and PEFC performance. 13To meet the requirements mentioned above, un-supported bimetallic electrocatalysts with high surface area (up to ≈80 m 2 /g metal ) and nanochain network structure, referred to as aerogels, have been synthesized. [14][15][16] The synthetic routes to prepare such materials vary, but generally involve the use of stabilizing surfactants and/or organic solvents [17][18][19] that can poison the catalyst's surface and decrease its activity. In contrast to those approaches, our groups have developed a facile one-step synthesis for mono-and bimetallic aerogels based on the reduction of metal salts by NaBH 4 in aqueous solution without addition of stabilizing surfac...

Superior stability and safety are key promises attributed to all-solid-state batteries (ASSBs) containing solid-state electrolyte (SSE) compared to their conventional counterparts utilizing liquid electrolyte. To unleash the full potential of ASSBs, SSE materials that are stable when in contact with the low and high potential electrodes are required. The electrochemical stability window is conveniently used to assess the SSE-electrode interface stability. In the present work, we review the most important methods to compute the SSE stability window. Our analysis reveals that the stoichiometry stability method represents a bridge between HOMO-LUMO method and phase stability method (grand canonical phase diagram). Moreover, we provide computational implementations of these methods for SSE material screening. We compare 1 arXiv:1901.02251v2 [cond-mat.mtrl-sci] 25 Oct 2019 their results for the relevant Li-and Na-SSE materials LGPS, LIPON, LLZO, LLTO, LATP, LISICON, and NASICON, and we discuss their relation to published experimental stability windows.

In the last two decades, small angle X-ray scattering (SAXS) and X-ray absorption spectroscopy (XAS) have evolved into two well-established techniques capable of providing complementary and operando information about a sample's morphology and composition, respectively. Considering that operation conditions can often lead to simultaneous and related changes in a catalyst's speciation and shape, herein we introduce a setup that combines SAXS and XAS in a configuration that allows optimum acquisition and corresponding data quality for both techniques. To determine the reliability of this setup, the latter was used to study the operando degradation of two carbon-supported Pt-nanoparticle (Pt/C) catalysts customarily used in polymer electrolyte fuel cells. The model used for the fitting of the SAXS curves unveiled the fractal nature of the Pt/C-electrodes and their evolution during the operando tests, and both Xray techniques were complemented with control, ex situ transmission electron microscopy and standard electrochemical measurements. Ultimately, the results obtained with this combined setup quantitatively agree with those reported in previous studies, successfully validating this apparatus and highlighting its potential to study the operando changes undergone by worseunderstood (electro)catalytic systems.