Formation of unexpectedly active Ni–Fe oxygen evolution electrocatalysts by physically mixing Ni and Fe oxyhydroxides

We present an unusual, yet facile, strategy towards formation of physically mixed Ni–Fe(OxHy) oxygen evolution electrocatalysts. We use in situ X-ray absorption and UV-vis spectroscopy, and high-resolution imaging to demonstrate that physical contact between two inferior Ni(OH)2 and Fe(OOH) catalysts self-assemble into atomically intermixed Ni–Fe catalysts with unexpectedly high activity

Influence of Support Material on the Structural Evolution of Copper during Electrochemical CO2 Reduction

The copper-catalyzed electrochemical CO2 reduction reaction represents an elegant pathway to reduce CO2 emissions while producing a wide range of valuable hydrocarbons. The selectivity for these products depends strongly on the structure and morphology of the copper catalyst. However, continued deactivation during catalysis alters the obtained product spectrum. In this work, we report on the stabilizing effect of three different carbon supports with unique pore structures. The influence of pore structure on stability and selectivity was examined by high-angle annular dark field scanning transmission electron microscopy and gas chromatography measurements in a micro-flow cell. Supporting particles into confined space was found to increase the barrier for particle agglomeration during 20 h of chronopotentiometry measurements at 100 mA cm−2 resembling long-term CO2 reduction conditions. We propose a catalyst design preventing coalescence and agglomeration in harsh electrochemical reaction conditions, exemplarily demonstrated for the electrocatalytic CO2 reduction. With this work, we provide important insights into the design of stable CO2 electrocatalysts that can potentially be applied to a wide range of applications

Boosting Photoelectrochemical Water Oxidation of Hematite in Acidic Electrolytes by Surface State Modification

State-of-the-art water-oxidation catalysts (WOCs) in acidic electrolytes usually contain expensive noble metals such as ruthenium and iridium. However, they too expensive to be implemented broadly in semiconductor photoanodes for photoelectrochemical (PEC) water splitting devices. Here, an Earth-abundant CoFe Prussian blue analogue (CoFe-PBA) is incorporated with core-shell FeO/FeTiO type II heterojunction nanowires as composite photoanodes for PEC water splitting. Those deliver a high photocurrent of 1.25 mA cm at 1.23 V versus reversible reference electrode in acidic electrolytes (pH = 1). The enhancement arises from the synergic behavior between the successive decoration of the hematite surface with nanolayers of FeTiO and then, CoFe-PBA. The underlying physical mechanism of performance enhancement through formation of the FeO/FeTiO/CoFe-PBA heterostructure reveals that the surface states' electronic levels of hematite are modified such that an interfacial charge transfer becomes kinetically favorable. These findings open new pathways for the future design of cheap and efficient hematite-based photoanodes in acidic electrolytes

Reduktion von Edelmetallen in der Wasserstoffelektrode bei der Polymerelektrolyt-Wasserelektrolyse

In order to reduce CO emissions, the increased use of renewable energy sources is indispensable. The generation of electricity from renewable energy sources, however, is weather-dependent and intermittent. Chemical energy storage is considered a key technology for the utilization of irregularly generated electricity. Excess electricity can be converted into hydrogen by means of electrolysis of water. If required, a reconversion of the hydrogen can take place. Polymer electrolyte water electrolysis is capable of following the dynamic load profile of renewable energy sources, but large amounts of platinum are needed as a catalyst for the hydrogen production reaction. A main task is to reduce the amount of platinum used without affecting efficiency. Furthermore, the influence of reduced electrode loading on the long-term stability of the electrolyzer is largely unexplored. In this work, the influence of the reduction of the platinum content on the cathode of a polymer electrolyte water electrolyzer on its efficiency and long-term stability is investigated. On the one hand, the reduction of the loading is aimed at by the synthesis of novel, platinum-based catalysts with a high activity, and on the other hand a reduced amount of commercially available catalysts is used. These catalysts are subjected to physicochemical analyses and their electrochemical characteristics are recorded by means of half-cell measurements. For the study of cathode aging, protocols are being developed to simulate accelerated degradation of the cathode. In addition, the characteristics of these catalysts are recorded and analyzed under real electrolysis conditions before and after aging. By means of electron microscopy, the occurring aging mechanisms are examined more closely. A novel, microporous catalyst has been developed, which has a higher exchange current density than the reference material platinum. With the help of platinum-based commercially available catalysts, the loading was reduced by a factor of 80, with a slight deteriorationin performance was found. While at a high cathode loading the accelerated aging did not change the cell characteristics, a small deterioration in cell efficiency was observed at a cathode loading of 0.01 mgPtcm$^{−2}$. Electron microscopic examination of the aging mechanisms revealed platinum particle growth as a result of accelerated aging tests. Furthermore, a migration of the platinum particles was observed, the intensity of which depended on the applied overvoltage

Reduktion von Edelmetallen in der Wasserstoffelektrode bei der Polymerelektrolyt-Wasserelektrolyse

In order to reduce CO2 emissions, the increased use of renewable energy sources is indispensable. The generation of electricity from renewable energy sources, however, is weather-dependent and intermittent. Chemical energy storage is considered a key technology for the utilization of irregularly generated electricity. Excess electricity can be converted into hydrogen by means of electrolysis of water. If required, a reconversion of the hydrogen can take place. Polymer electrolyte water electrolysis is capable of following the dynamic load profile of renewable energy sources, but large amounts of platinum are needed as a catalyst for the hydrogen production reaction. A main task is to reduce the amount of platinum used without affecting efficiency. Furthermore, the influence of reduced electrode loading on the long-term stability of the electrolyzer is largely unexplored. In this work, the influence of the reduction of the platinum content on the cathode of a polymer electrolyte water electrolyzer on its efficiency and long-term stability is investigated. On the one hand, the reduction of the loading is aimed at by the synthesis of novel, platinum-based catalysts with a high activity, and on the other hand a reduced amount of commercially available catalysts is used. These catalysts are subjected to physicochemical analyses and their electrochemical characteristics are recorded by means of half-cell measurements. For the study of cathode aging, protocols are being developed to simulate accelerated degradation of the cathode. In addition, the characteristics of these catalysts are recorded and analyzed under real electrolysis conditions before and after aging. By means of electron microscopy, the occurring aging mechanisms are examined more closely. A novel, microporous catalyst has been developed, which has a higher exchange current density than the reference material platinum. With the help of platinum-based commercially available catalysts, the loading was reduced by a factor of 80, with a slight deterioration in performance was found. While at a high cathode loading the accelerated aging did not change the cell characteristics, a small deterioration in cell efficiency was observed at a cathode loading of 0.01 mgPt cm−2. Electron microscopic examination of the aging mechanisms revealed platinum particle growth as a result of accelerated aging tests. Furthermore, a migration of the platinum particles was observed, the intensity of which depended on the applied overvoltage

High resolution transmission electron microscopy and electronic structure theory investigation of platinum nanoparticles on carbon black

High Resolution Transmission Electron Microscopy (HR TEM) is used to identify the size, shape, and interface structure of platinum nanoparticles and carbon support of a fuel cell catalyst. Using these insights, models accessible to quantum chemical methods are designed in order to rationalize the observed features. Thus, basal plane and prism face models of the carbon black material are considered, interacting with Pt clusters of sizes up to 1 nm. Particular attention is paid to the electronic structure of the carbon support, namely, the radical character of graphene zig-zag edges. The results show that a stronger interaction is found when the nanoparticle is at the zig-zag edge of a basal plane due to the combination of dispersion interaction with the support structure and covalent interaction with carbon atoms at the edge. In this case, a distortion of both the Pt nanoparticle and the carbon support is observed, which corresponds to the observations from the HR TEM investigation. Furthermore, the analysis of the charge transfer upon interaction and the influence of the potential on the charge states and structure is carried out on our model systems. In all cases, a clear charge transfer is observed from the carbon support to the Pt nanoparticle. Finally, we show that changing the potential not only can change the charge state of the system but can also affect the nature of the interaction between Pt nanoparticles and carbon supports

On the mobility of carbon-supported platinum nanoparticles towards unveiling cathode degradation in water electrolysis

This study investigates the influence of the hydrogen evolution reaction (HER) overpotential on the mobility of carbon-supported platinum particles. The migration of the platinum over the carbon support was analyzed by means of identical location transmission electron microscopy (IL-TEM). While at potentials of 0.1 and 0 V vs. reversible hydrogen electrode (RHE), no changes to the Pt/C material were observed. With a decrease of the overpotential to −0.1 V vs. RHE, an increase in the quantity of migrating platinum particles took place. At −0.2 V vs. RHE, a further rise in the particle migration was observed. The effect of the overpotential on the migration was explained by a higher hydrogen generation rate, the formation of a hydrogen monolayer on the platinum and the resulting changes of the platinum support distance. The mechanisms revealed in this study could describe a relevant source of degradation of PEM water electrolyzers