Operando X-ray absorption spectroscopy of Fe–N–C catalysts based on carbon black and biomass-derived support materials for the ORR

Iron nitrogen carbon (Fe–N–C) catalysts are among the most promising non-platinum group metal catalysts for the oxygen reduction reaction (ORR). Their activity and stability are considerably influenced by the structure of the C-support. New biochar materials offer native heteroatom doping, making them a promising precursor for Fe–N–C catalysts. In this study, we apply operando X-ray absorption spectroscopy at the Fe K-edge to characterize the atomic Fe-based active sites of a commercial Fe–N–C catalyst, a carbon black-based catalyst as well as a novel biomass-based Fe–N–C catalyst. We compare the density and the potential-dependent nature of the FeNx-type active sites during operation. Our results demonstrate that the novel biomass-based catalyst exhibits a higher active-site density compared to commercial and carbon black-based Fe–N–C catalysts. Moreover, dynamic detection of the Fe K-edge intensity during potential cycling reveals that their reversible iron redox potential is lower compared to that of conventional catalysts. Evaluation of the Fe K-edge shift as well as of the extended X-ray absorption fine structure (EXAFS) suggests hetero-atom doping and iron under-coordination as potential causes for the observed differences. These insights open the pathway to develop new optimization strategies for Fe–N–C catalysts based on biomass support materials

Photoelectron spectra of alkali metal–ammonia microjets: From blue electrolyte to bronze metal

Experimental studies of the electronic structure of excess electrons in liquids—archetypal quantum solutes—have been largely restricted to very dilute electron concentrations. We overcame this limitation by applying soft x-ray photoelectron spectroscopy to characterize excess electrons originating from steadily increasing amounts of alkali metals dissolved in refrigerated liquid ammonia microjets. As concentration rises, a narrow peak at ~2 electron volts, corresponding to vertical photodetachment of localized solvated electrons and dielectrons, transforms continuously into a band with a sharp Fermi edge accompanied by a plasmon peak, characteristic of delocalized metallic electrons. Through our experimental approach combined with ab initio calculations of localized electrons and dielectrons, we obtain a clear picture of the energetics and density of states of the ammoniated electrons over the gradual transition from dilute blue electrolytes to concentrated bronze metallic solutions

Interfacial Photoelectron and Absorption Spectroscopy Studies of Earth Abundant Catalyst Materials for the Oxygen Evolution and Reduction Reaction

The future use of renewable energy sources is accompanied by the problem of spatially and temporally limited energy supply. To adjust supply and demand fluctuations, especially on the seasonal scale, requires efficient storage solutions. Electrolysis derived fuels, such as hydrogen, are suitable for this purpose, but the efficiency of electrolysis applications has so far been limited by the sluggish kinetics of the oxygen evolution reaction OER and oxygen reduction reaction ORR . The development of suitable catalysts is focused on transition metal based compounds due to their widespread availability and low financial costs. In order to further optimize these materials and design more efficient catalysts, deeper insights into the catalytic processes on the atomic scale are required. In this work, advanced spectroscopic methods are used to characterize the structure of catalyst electrolyte interfaces in different states. For this, I used photoelectron spectroscopy PES and X ray absorption spectroscopy XAS as suitable methods to study the coordination environment and electronic structure. I have exploited the short mean free path of detected electrons in PES to achieve a high sensitivity to interfacial processes. For this purpose I follow different trategies The condensation of a liquid layer at near ambient pressure NAP , the combination of ante and post mortem measurements, and operando PES based on a graphene covered ionomer membrane electrode setup tailored for NAP operation. Along with direct PES, I avail resonant Auger decay channels to gain insights into the occupied as well as unoccupied density of states simultaneously. Meso and nanostructuring of the catalysts facilitates photon detection based XAS also to be interface sensitive. Using spectroelectrochemical cells, I performed operando XAS measurements of transition metal K and L edges. The studies presented in this work focus on the Ni Fe oxy hydroxides material system, which represents one of the most active transition metal based catalysts for OER in the alkaline regime. In addition, iron nitrogen carbon Fe N C compounds are investigated as promising materials for ORR. As a first step, I investigated the interaction between a condensed water film and electro deposited Ni0.75Fe0.25 OH 2 using near ambient pressure PES. My results do not indicate dissociative adsorption of water molecules under these conditions. This suggests, that water dissociation can only be observed with an oxidizing electrode potential applied to the interface. As a step closer to operating conditions, I studied the impact of defect structure on the electrocatalytic properties of reactively sputtered Ni0.75Fe0.25Oy by coupling electrochemical measurements to thorough ante and post mortem investigations based on X ray spectroscopy and complementary methods. I determined changes of the defect structure within a series of samples prepared under systematically varied deposition conditions, which exhibit a constant Fe Ni ratio. Ante and post mortem PE spectra reveal the formation of a hydroxide phase during continuous activation triggered by cyclic voltammetry CV . Moreover, I observed a distinct activity trend with changed deposition conditions, which can be correlated to the observed differences in defect chemistry. My results show, that the defect structure clearly influences the intrinsic activity and redox kinetics, as the amount of active hydroxide phase formed on the surface during potential cycling was similar for different samples and can therefore not explain the trend in activity. Based on the graphene capped ionomer approach, I have measured photoelectron spectra from the Fe Ni oxy hydroxide electrolyte interface, while well defined electrode potentials were applied. Potentialdependent measurements reveal a reversible oxidation of the Ni species, which is accompanied by the formation of metal oxygen hybridized hole

Tailoring the Oxygen Evolution Activity and Stability Using Defect Chemistry

Improving the activity of catalysts for the oxygen evolution reaction (OER) requires a detailed understanding of the surface chemistry and structure to deduce structure-function relationships (descriptors) for fundamental insight. We chose epitaxial (100)-oriented La0.6Sr0.4Mn1−δO3 (LSMO) thin films as a model system with high electrochemical activity comparable to (110)-oriented IrO2 to investigate the effect of Mn off-stoichiometry on both catalytic activity and stability. Extensive structural characterization was performed by microscopic and spectroscopic methods before and after electrochemical characterization using rotating ring-disk studies. Stoichiometric LSMO had the highest activity, while both Mn deficiency and excess reduced the catalytic activity. Furthermore, all samples preserved the crystal structure up to the very surface. Mn excess improved the long-term activity, and we hypothesize that excess Mn stabilizes the surface chemistry during catalysis. Our data show that the defect chemistry should be considered when designing catalysts with enhanced activity and rugged stability

Rotating Ring–Disk Electrode Study of Oxygen Evolution at a Perovskite Surface: Correlating Activity to Manganese Concentration

Transition-metal oxides with the perovskite structure are promising catalysts to promote the kinetics of the oxygen evolution reaction (OER). To improve the activity and stability of these catalysts, a deeper understanding about the active site, the underlying reaction mechanism, and possible side reactions is necessary. We chose smooth epitaxial (100)-oriented La_0.6Sr_0.4MnO₃ (LSMO) films grown on Nb:SrTiO₃ (STNO) as a model electrode to investigate OER activity and stability using the rotating ring−disk electrode (RRDE) method. Careful electrochemical characterization of various films in the thickness range between 10 and 200 nm yields an OER activity of the epitaxial LSMO surface of 100 μA/cm²_ox at 1.65 V vs RHE, which is among the highest reported for LSMO and close to (110)-oriented IrO₂. Detailed post-mortem analysis using XPS, XRD, and AFM revealed the high structural and morphological stability of LSMO after OER. The observed correlation between activity and Mn vacancies on the surface suggested Mn as the active site for the OER in (100)-oriented LSMO, in contrast to similar perovskite manganites, such as Pr_1–<i>x</i>Ca_<i>x</i>MnO₃. The observed Tafel slope of about 60 mV/dec matches the theoretical prediction for a chemical rate-limiting step that follows an electrochemical pre-equilibrium, probably O–O bond formation. Our study established LSMO as an atomically flat oxide with high intrinsic activity and high stability