XANES Research Articles

Unravelling the complex speciation of halozincate ionic liquids using X-ray spectroscopies and calculations.

Using a combination of liquid-phase X-ray spectroscopy experiments and small-scale calculations we have gained new insights into the speciation of halozincate anions in ionic liquids (ILs). Both core and valence X-ray photoelectron spectroscopy (XPS) experiments were performed directly on the liquid-phase ILs, supplemented by Zn 1s X-ray absorption near edge structure (XANES) spectroscopy. Density functional theory (DFT) calculations were carried out on both 1- and 2- halozincate anions, in both a generalised solvation model SMD (Solvation Model based on Density) and the gas phase, to give XP spectra and total energy differences; time-dependent DFT was used to calculate XANES spectra. Speciation judgements were made using a combination of the shape and width of the experimental spectra, and visual matches to the calculated spectra. For 2- halozincate anions, excellent matches were found between the experimental and calculated XP spectra, clearly showing that only 2- halozincate anions were present at all zinc halide mole fractions, x, studied. At specific values of x (0.33, 0.50, 0.60) only one halozincate anion was present; equilibria of different halozincate anions at those values of x were not observed. All findings show that chlorozincate anion and bromozincate anion speciation matched at the same x. Based on the results, predictions are made of the halozincate anion speciation for all values of x up to 0.67. Caution is advised when using differences in calculated total energies obtained from DFT to judge halozincate anion speciation, even when the SMD was employed, as predictions based on total energy differences did not always match the findings from the experimental and calculated spectra. Our findings clearly establish that the combination of high-quality experimental data from multiple spectroscopies and a wide range of calculated structures are essential to have high confidence in halozincate anion speciation identification.

Resolving a structural issue in cerium-nickel-based oxide: a single compound or a two-phase system?

CeNiO3 has been reported in the literature in the last few years as a novel LnNiO3 compound with promising applications in different catalytic fields, but its structure has not been correctly reported so far. In this research, CeNiO3 (RB1), CeO2 and NiO have been synthesized in a nanocrystalline form using a modified citrate aqueous sol-gel route. A direct comparison between the equimolar physical mixture (n(CeO2) : n(NiO) = 1 : 1) and compound RB1 was made. Their structural differences were investigated by laboratory powder X-ray diffraction (PXRD), selected area electron diffraction (SAED), transmission electron microscopy (TEM) with an energy-dispersive X-ray spectroscopy (EDS) detector, and Raman spectroscopy. The surface of the compounds was analyzed by X-ray photoelectron spectroscopy (XPS), while the thermal behaviour was explored by thermogravimetric analysis (TGA). Their magnetic properties were also investigated with the aim of exploring the differences between these two compounds. There were clear differences between the physical mixture of CeO2 + NiO and RB1 presented by all of these employed methods. Synchrotron methods, such as atomic pair distribution function analysis (PDF), X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), were used to explore the structure of RB1 in more detail. Three different models for the structural solution of RB1 were proposed. One structural solution proposes that RB1 is a single-phase pyrochlore compound (Ce2Ni2O7) while the other two solutions suggest that RB1 is a two-phase system of either CeO2 + NiO or Ce1-xNixO2 and NiO.

Inducing molecular orientation in solution-processed thin films of fluorene-bithiophene-based copolymer: thermal annealing vs. solvent additive.

A deep understanding of the factors influencing the morphology of thin films based on conjugated polymers is essential to boost their performance in optoelectronic devices. Herein, we investigated the electronic structure and morphology of thin films of the copolymer poly(9,9-dioctyl-fluorenyl-co-bithiophene) (F8T2) in its pristine form as well as samples processed with the solvent additive 1,8-diiodooctane (DIO) or post-processed through thermal annealing treatment. Measurements were carried out using angle-resolved S K-edge NEXAFS (near-edge X-ray absorption fine structure) in total electron yield (TEY) and fluorescence yield (FY) detection modes. Two main transitions were observed at the S 1s NEXAFS spectra: S 1s → π* and S 1s → σ* (S-C). The observed dichroism pointed to a face-on orientation of the conjugated backbone, which was significantly increased for F8T2 films processed with DIO. Resonant Auger decay spectra were obtained and analyzed using the core-hole clock (CHC) method. An enhancement in the charge transfer process was observed for thermally annealed films, especially for samples processed with DIO, corresponding to an increase in film ordering. Furthermore, the investigated films were characterized using X-ray photoelectron spectroscopy, attesting to the presence of the thiophene unit in the samples and demonstrating that some of its sulfur atoms were positively polarized in the F8T2 films. All these experimental findings were compared with molecular dynamics (MD) simulations of film evaporation with and without DIO. The use of MD, together with mathematical modeling, was able to explain the major effects found in the experiments, including the polarization of sulfur atoms. The simultaneous use of powerful spectroscopic techniques and theoretical methods shed light on key aspects linking film morphology with fabrication procedures.

Phosphorus speciation under long‐term crop rotation management in a tropical soil

AbstractCrop rotation and soil management practices after native tropical forest conversion may impact the dynamics of inorganic and organic phosphorus (Pi and Po) species in the soil. By combining the state‐of‐the‐art spectroscopic methods P K‐edge X‐ray absorption near edge structure (XANES), 31P liquid nuclear magnetic resonance (31P‐NMR) and chemical P fractionation, this study provides important insights on soil P speciation after conversion of native forest to cropland under distinct managements. A field trial was conducted in a weathered tropical soil to assess Pi and Po changes after conversion of native forest to cropland. The crop rotations were managed under no‐tillage (NT) or minimum tillage (MT) since 2003, and repeated annually until 2015, cropped in the fall–winter and spring seasons, followed by soybean in the summer. Soil XANES analysis in the end revealed that P was predominantly associated with ferrihydrite and hematite, suggesting that P speciation was more strongly influenced by the presence of iron(oxyhydr)oxide than by crop rotations and/or soil management. The conversion of native forest to cropland promoted the mineralization of Po species, leading to re‐adsorption in non‐labile forms and consequently worsening the P cycling in the system. To increase the labile P pools and supply crop P demand, inputs of inorganic fertilizers were required owing to the strong soil P sorption. Our results show that even conservation practices such as MT or NT and complex crop rotations are not sufficient to inhibit the impact of cropland conversion on soil P forms.

Zn2+-storage mechanism in V6O13 with nanosheets for high-capacity and long-life aqueous zinc-metal batteries.

V6O13 with a nanosheet structure was employed as a cathode material for aqueous zinc metal batteries. V6O13 delivered a high specific capacity of 425 mA h g-1, outstanding rate performance and durable cycling with high capacity retention of 86% after 3000 cycles. Moreover, in situ X-ray diffractometer (XRD), ex situ X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) were employed to ascertain the reaction mechanism of Zn2+ storage.

Beagle: a near-edge X-ray absorption fine-structure spectroscopy data processing solution for beamline experiments at Pohang Accelerator Laboratory.

Near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy is a powerful tool for identifying chemical bonding states at synchrotron radiation facilities. Advances in new materials require researchers in both academia and industry to measure tens to hundreds of samples during the available beam time on a synchrotron beamline, which is typically allocated to users. Automated measurement methods, along with analysis software, have been developed for beamlines. Automated measurements facilitate high-throughput experiments and accumulate vast amounts of measured spectral data. The analysis software supports various functions for analyzing the experimental data; however, these analysis methods are complicated, and learning them can be time-consuming. To process large amounts of spectral data, a new analysis software, dedicated to NEXAFS spectroscopy, that is easy to use and can provide results in a short time is desired. Herein, the development of Beagle is described, software calculating molecular orientation from NEXAFS spectroscopy data that can report results in a short time comparable with that required to measure one sample at the beamline. It was designed to progress in a single sequence from data loading to the printing of the results with a `click of a button'. The functions of the software include recognizing the dataset, correcting the background, normalizing the plot, calculating the electron yield and determining the molecular orientation. The analysis results can be saved as {\tt{.txt}} files (spectral data), {\tt{.pdf}} files (graphic images) and Origin files (spectral data and graphic images).

First synthesis of RuSn solid-solution alloy nanoparticles and their enhanced hydrogen evolution reaction activity.

Solid-solution alloys based on platinum group metals and p-block metals have attracted much attention due to their promising potential as materials with a continuously fine-tunable electronic structure. Here, we report on the first synthesis of novel solid-solution RuSn alloy nanoparticles (NPs) by electrochemical cyclic voltammetry sweeping of RuSn@SnOx NPs. High-angle annular dark-field scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy maps confirmed the random and homogeneous distribution of Ru and Sn elements in the alloy NPs. Compared with monometallic Ru NPs, the RuSn alloy NPs showed improved hydrogen evolution reaction (HER) performance. The overpotentials of Ru0.94Sn0.06 NPs/C and Ru0.87Sn0.13 NPs/C to achieve a current density of 10 mA cm-2 were 43.41 and 33.19 mV, respectively, which are lower than those of monometallic Ru NPs/C (53.53 mV) and commercial Pt NPs/C (55.77 mV). The valence-band structures of the NPs investigated by hard X-ray photoelectron spectroscopy demonstrated that the d-band centre of RuSn NPs shifted downward compared with that of Ru NPs. X-ray photoelectron spectroscopy and X-ray absorption near-edge structure analyses indicated that in the RuSn alloy NPs, charge transfer occurs from Sn to Ru, which was considered to result in a downward shift of the d-band centre in RuSn NPs and to regulate the adsorption energy of intermediate Hads effectively, and thus enable the RuSn solid-solution alloy NPs to exhibit excellent HER catalytic properties.

Theoretic Study of Sulfur-Doped Graphdiynes by X-ray Spectroscopy.

Sulfur-doped graphdiyne at different sites has a tremendous impact on its electronic structure and properties. Due to the large number of S-doping sites, there is no comprehensive and systematic experimental and theoretical study regarding the identification of S-doped graphdiyne configurations. In this paper, X-ray photoelectron (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectra as well as geometries of 10 sulfur-doped graphdiyne molecules have been simulated at the density functional theory (DFT) level. Different types of carbon spectra were theoretically modeled to analyze the contribution of the spectra. Calculated results show that the NEXAFS spectra exhibit a clear dependence on the local structure. The theoretically simulated XPS spectra are in good agreement with the experimental spectra. The XPS spectra combined with the NEXAFS spectra can provide effective information for identifying the 10 S-doped conformations. Our research results provide further theoretical prediction and guidance for the experimental synthesis of S-doped graphdiyne, which solves the difficult problem of identification of S-doped carbon-based materials.

The migration and transformation mechanism of vanadium in a soil-pore water-maize system

The migration mechanism of vanadium (V) in the soil-pore water-maize system has not been revealed. This study conducted pot experiments under artificial control conditions to reveal V's distribution and transport mechanism under different growth stages and V content gradient stress. The V content in the soil pore water gradually increased by an order of magnitude. The V content of pore water in the no-plant group was higher than that in the plant group, indicating that the maize roots absorbed V. The V exists in the form of pentavalent oxygen anions, in which H2VO4− occupies the most significant proportion. With increasing V content, the root area, root number, root length, and tip number decreased significantly. The malondialdehyde content in maize leaves showed an increasing trend, indicating the degree of lipid peroxidation was gradually enhanced. The V content was in the order of root > leaf > stem > fruit and maturity stage > flowering stage > jointing stage, respectively. The transfer coefficient reached a maximum under natural conditions, and increased gradually with the growth. The results of synchrotron radiation X-ray absorption near edge structure (XANES) analysis showed that Fe in maize roots mainly comprised of Fe2O3 and Fe3O4. The Fe in the soil is primarily existed in lepidocrocite and Fe2O3. The μ-XRF analysis showed that V and Fe enriched in the roots with a positive relationship, indicating the synergistic absorption of V and Fe by roots. Part of the Fe2+ reduced V5+ to V4+ or V3+ in the forms of VO2+, V(OH)2+, or V(OH)3 (s), and fixed V at the root. Soil weak acid-soluble fraction V and soil total V were vital factors to maize extraction. This study provides new insights into V biogeochemical behavior and a scientific basis for correctly evaluating its ecological and human health risks.

Insights into the Complex Oxidation Behaviour of Aqueous H3PO3 through in Situ P K- Edge XANES

High-temperature electrolyte membrane fuel cells (HT-PEMFCs) using H3PO4-doped membranes are an enticing choice for electrochemical energy sources at elevated temperatures [1]. Yet, one of the challenges concerning these systems is the reduction of H3PO4 to lower valency phosphorus compounds, such as H3PO3, during operation [2,3,4]. H3PO3 can strongly adsorb on the Pt catalysts, hence poisoning it and thereby lowering the fuel cell performance [5,6]. Interestingly, the presence of Pt may, under certain conditions, catalyze the chemical (re-)oxidation of aqueous H3PO3 back to H3PO4, illustrating the complexity of the H3POx – Pt interaction. Therefore, to optimize the performance of HT-PEMFCs by rational design of the catalyst/electrolyte interface, further insight into H3PO3 oxidation behaviour and Pt/H3PO3 interaction under conditions relevant to HT-PEMFCs operation is required.In this study, in situ P K- edge XANES (x-ray absorption near edge structure) spectroscopy was conducted to shed light on the oxidation behaviour of aqueous H3PO3 by investigating the impact of (i) different temperatures (25°C vs 75°C), (ii) varying electrode roughness (flat planar Pt vs rough Pt black), (iii) different electrode potentials (open circuit potentials vs more positive potentials [+0.8, +0.9, and +1.0 VRHE]), and (iv) varying molar concentrations of H3PO3 (0.1 , 1 , and 5 mol dm-3). Initially, XANES coupled with electrochemical characterization (e.g. OCP, cyclic voltammetry) of the system was performed under different radiation doses (i.e.: photon fluxes, exposure time) to determine the effect of radiolysis and/or radiation damage during experiments that may lead to misinterpretations of XANES results. Under high radiation dose, we find evidence of H2 presence in the vicinity of the Pt electrode, likely generated by water radiolysis. We have developed experimental procedures to suppress these undesirable effects during the collection of the XANES data, enabling an accurate determination of H3PO3 oxidation behaviour and minimizing the radiolysis contribution to the process. It was found that higher temperature facilitates the oxidation of aqueous H3PO3 to H3PO4, presumably because H3PO3 exists in the thermodynamically preferred “active” pyramidal form at elevated temperatures, which is more prone to react with H2O, forming H3PO4 and H2. In aqueous H3PO3 solutions with higher H2O contents, also more pronounced oxidation of H3PO3 is observed, indicating that oxidation of H3PO3 to H3PO4 in aqueous solutions proceeds via the presence of H2O. The experiments using rough Pt black electrodes additionally hint at the observation of electrochemical oxidation of H3PO3 to H3PO4 during the application of positive potentials. This work provides insights into the underlying chemical processes that occur at conditions relevant for HT-PEMFCs operation and thus it paves the way for possible strategies to mitigate H3PO3 poisoning of the Pt electrode during operation.

Effect of Dissolved Gases on Pt Nanoparticle Catalysts Investigated By in Situ SAXS, XAS Measurements

Understanding of the structural and the chemical properties of Pt catalysts in polymer electrolyte fuel cells (PEFCs) is necessary to improve the activity for the durability of the catalysts. The small-angle X-ray scattering (SAXS) and the X-ray absorption spectroscopy (XAS) measurements are widely used to investigate these properties. We developed a system that combines in situ A-SAXS/XAS measurements in the same field of observation using a channel flow electrode (CFE) cell and characterized the chemical states, average diameter and size distributions of Pt catalysts under controlled electrochemical conditions. In this study, we investigated an effect of dissolved gases on Pt nanoparticle catalysts using the system.All experiments were carried out at SPring-8 BL19B2. A commercial Pt/carbon black catalyst (TEC10E50E, Tanaka Kikinzoku Kogyo) was used as test electrode in this experiment. Prior to SAXS and XAS measurements, the electrochemical potential was cycled between 0.6 and 1.0 V vs. the RHE in O2 or N2-saturated 0.1 M HClO4. For the XAS measurements, the Pt LⅢ edge was used. The ionization chamber and a seven-elements silicon drift detector were used to obtain the XAS spectra. For the SAXS measurements, the X-ray energy was set to 11.5 keV and 11.55 keV, which are below and near to the Pt LIII absorption edge, respectively. The scattered X-ray was detected using an area detector (PILATUS 2M). Figure 1 show the X-ray absorption near edge structure (XANES) spectra of the Pt catalyst after the potential cycles in O2-saturated 0.1 M HClO4. The white-line peak intensity increased and became higher at a higher potential cycle. This indicates the partial electron-transfer from Pt atoms or initial stage of Pt oxidation. We also will discuss the effect of dissolved gases for morphological and chemical states from SAXS and XAS data. Figure 1

Investigation of Iron Based Di-Atomic Catalysts to Identify Active Sites for Coupled Reduction Reactions of CO2 and N2

Urea is a valuable chemical feed stock that is commonly used in fertilizers and other industrial applications, including for the production of plastics and resins. Conventionally, industrial urea is produced through the reaction of ammonia and carbon dioxide under high pressure and temperature, resulting in intensive energy inputs and inefficiencies. In contrast, electrochemical synthesis of urea under mild conditions has been shown to be possible by coupled electrochemical reduction of CO2 (CO2RR) and N2 (NRR)1. While finding active and efficient catalysts remains a major challenge, rational design can be advanced by identification of the favourable atomic arrangements that facilitate C-N bond formation. For this purpose, carbon-supported and nitrogen-coordinated metal single atom catalysts (SACs) are a promising class of electrocatalysts2. Given their well-defined atomic structure, these material systems are valuable model systems for identifying active sites for specific reactions.Cu and Ni are well-known as efficient CO2RR catalysts3,4, whereas Fe-based catalysts are traditionally used for N2 fixation to produce NH3 via the thermochemical Haber-Bosch method5. This background knowledge served as motivation for fabrication of two types of di-atomic catalysts (DACs), containing Cu-Fe and Ni-Fe components anchored in an electrochemically inert carbon framework. A wet-chemical method was used for synthesizing these materials. While no crystalline metallic reflections were observed in X-ray diffraction patterns, X-ray photoelectron spectroscopy confirmed the presence of constituent metals along with lattice N, O, and C. The X-ray absorption near edge structure (XANES) measurements of Fe, Ni and Cu edges show significantly different spectra when compared to corresponding metal foils and oxides. Importantly, a nitrogen-coordinated ‘single atom’ morphology with inter-metal interactions is confirmed by extended X-ray absorption fine structure (EXAFS). Electrochemical reduction reactions in the presence of CO2 and N2 showed formation of 2e- CO2RR products, with CO dominant for the case of Ni-Fe DACs. In contrast, for the case of Cu-Fe DACs, formic acid generation was significantly more pronounced than CO. For the case of isolated NRR, NH3 was observed for both DACs. The production of urea, however, was only observed for Cu-Fe DACs. While mechanistic investigations are underway, the primary difference in these activities is hypothesized to be due to formation of different intermediates and reaction paths followed during CO2RR. Previous studies6 have suggested that the choice of reaction path, CO2→*OCHO→formic acid or CO2→*COOH→CO, could be dependent on oxophilicity of the reaction site. Our observations indicate that C-N coupling is promoted by oxophilic CO2RR catalysts. Since the direct identification of active sites for coupled CO2RR and NRR has rarely been reported using simple ‘single atom’ based catalyst systems, our work opens up a new direction for designing catalysts capable of C-N coupling for the efficient generation of the valuable chemical, urea, under ambient conditions.References Chen, C., Zhu, X., Wen, X., Zhou, Y., Zhou, L., Li, H., Tao, L., Li, Q., Du, S., Liu, T. and Yan, D., Nat. Chem. 12, 717–724 (2020).Chen, Y., Ji, S., Chen, C., Peng, Q., Wang, D. and Li, Y., Joule 2, 1242–1264, (2018).Ferri, M., Delafontaine, L., Guo, S., Asset, T., Cristiani, P., Campisi, S., Gervasini, A. and Atanassov, P., ACS Energy Lett. 7, 2304–2310, (2022).Leverett, J., Yuwono, J.A., Kumar, P., Tran-Phu, T., Qu, J., Cairney, J., Wang, X., Simonov, A.N., Hocking, R.K., Johannessen, B. and Dai, L., ACS Energy Lett. 7, 920–928 (2022).Fuller, J., An, Q., Fortunelli, A. and Goddard III, W.A., Acc. Chem. Res. 55, 1124–1134 (2022).Feaster, J.T., Shi, C., Cave, E.R., Hatsukade, T., Abram, D.N., Kuhl, K.P., Hahn, C., Nørskov, J.K. and Jaramillo, T.F., ACS Catal. 7, 4822–4827 (2017).

Multimodal in Situ synchrotron Analysis Reveals the Effect of the Oxygen Evolution Reaction on Corrosion of a Ni-Cr-Mo Alloy

Nanometer-thin and spontaneously formed oxide films, or passive films, govern the corrosion resistance of many advanced alloys [1]. Our current understanding of the structure and chemistry of the passive film is primarily based on ex situ surface analysis using X-ray Photoelectron Spectroscopy (XPS) [2] and Scanning Tunneling Microscopy (STM) [3]. XPS has historically been limited to Ultra High Vacuum (UHV) conditions, so the results do not represent the true passive film/electrolyte interface. To observe corrosion initiation and progression and obtain a fundamental understanding of corrosion mechanisms, there is a need for techniques that can combine in situ capabilities with detailed chemical and structural information from the surface region. We have previously used Ambient Pressure (AP) XPS to study passive film growth [4] [5] and a combination of X-ray Reflectivity (XRR), Grazing Incidence X-ray Diffraction (GI-XRD), X-ray Fluorescence (XRF) to study the passivity breakdown of duplex stainless steel [6].Ni-based alloys are known for their excellent mechanical properties and corrosion resistance and are used in many demanding industrial environments. Ni is commonly alloyed with Cr and Mo, where Cr is known to improve the corrosion resistance due to the formation of a Cr2O3 oxide film on the surface, while the role of Mo is debated. Commonly used electrochemical techniques for studying passivity breakdown of other alloys may not be applicable to Ni-Cr-Mo alloys because the measured electrochemical current is not only due to corrosion reactions [7]. Ni and Mo are good catalysts for the Oxygen Evolution Reaction (OER) [8, 9], which adds another dimension of complexity to the material system since OER is known to be coupled with dissolution and degradation in other metallic systems [10]. A fundamental holistic understanding is missing regarding how the chemistry and structure of the passive film on Ni-Cr-Mo alloys evolve in realistic aqueous conditions and how that correlates with the onset of dissolution, which determines the breakdown of passivity.Here we present a comprehensive study combining several synchrotron-based techniques to study the surface region of a Ni-Cr-Mo alloy in NaCl solutions in situ during electrochemical polarization, as shown in Figure 1 a). XRR and AP-XPS were used to investigate the thickness and chemistry of the passive film. GI-XRD was used to determine the change in the metal lattice underneath the passive film. XRF was used to quantify the dissolution of alloying elements. X-ray Absorption Near Edge Structure (XANES) was used to study the chemical state of the dissolved species in the electrolyte and the chemical state of corrosion products formed on the surface. The combination of these techniques allowed us to study the corrosion process and detect the passivity breakdown in situ and correlate it to the onset of OER.Growth of the passive film and enrichment of Mo6+ oxide was observed in the passive range below 800 mV vs. Ag/AgCl, followed by a drastic increase in the electrochemical current coupled with the formation of a thick film of MoO3, and Cr(OH)3, as seen from the AP-XPS data in Figure 1 b). The current increase at potentials above 800 mV vs. Ag/AgCl coincided with the dissolution of Ni2+, Cr3+, and Mo6+, as seen in Figure 1 c). Quantitative analysis revealed that a substantial part of the measured current was due to oxygen evolution, as shown in Figure 1 d). The experimental techniques and the unique information they provide will be discussed, as well as the role of OER on the passivity breakdown of the Ni-Cr-Mo alloy. D. D. Macdonald, Pure and Applied Chemistry, 71 (6), 951-978 (1999). H. Strehblow and P. Marcus, CORROSION TECHNOLOGY-NEW YORK AND BASEL-, 22 1 (2006). V. Maurice and P. Marcus, Progress in Materials Science, 95 132-171 (2018). A. Larsson, K. Simonov, J. Eidhagen, A. Grespi, X. Yue, H. Tang, A. Delblanc, M. Scardamaglia, A. Shavorskiy, J. Pan, and E. Lundgren, Appl Surf Sci, 611 155714 (2023). M. Långberg, C. Örnek, J. Evertsson, G. S. Harlow, W. Linpé, L. Rullik, F. Carlà, R. Felici, E. Bettini, U. Kivisäkk, E. Lundgren, and J. Pan, npj Materials Degradation, 3 (1), 22 (2019). E. Bettini, C. Leygraf, and J. Pan, Int J Electrochem Sc, 8 11791-11804 (2013). H. Liao, X. Zhang, S. Niu, P. Tan, K. Chen, Y. Liu, G. Wang, M. Liu, and J. Pan, Applied Catalysis B: Environmental, 307 121150 (2022). A. Lončar, D. Escalera-López, S. Cherevko, and N. Hodnik, Angewandte Chemie International Edition, 61 (14), e202114437 (2022). Figure 1

Single Ru atoms dispersed on MoSe2/MXene nanosheets with multiple interfaces for enhanced acidic hydrogen evolution

Exposing more catalytic active sites is an effective strategy to enhance the catalyst activity for hydrogen evolution reaction (HER). Herein, Ti3C2Tx MXene served as a substrate to load thin and curved 2H–MoSe2 nanosheets by a typical hydrothermal method, forming MoSe2/MXene heterointerface with the fully exposed Mo edge sites. And then, atomically dispersed Ru single atoms (RuSAs) were successfully anchored on the MoSe2/MXene (RuSAs@MoSe2-MXene). The X-ray absorption near edge structure (XANES) demonstrates that Ru is loaded as single atoms on MoSe2 and MXene. The prepared sample exhibits improved catalytic activity with a small overpotential of 49 mV at 10 mA cm−2 and a low Tafel slope of 52 mV dec−1 for HER in 0.5 M H2SO4. It also exhibits excellent stability with a small attenuation voltage of 10 mV for 120 h at 10 mA cm−2 in 0.5 M H2SO4. When assembled into a two-electrode system of RuSAs@MoSe2-MXene||RuO2, it only requires a relatively low potential of 1.626 V at 10 mA cm−2 to drive overall water splitting (OWS). Control experiments disclose the highly improved performance can be primarily attributed to the introduced RuSAs and MoSe2/MXene heterointerfaces exposing more active sites of Ru and Mo. Moreover, the high conductivity of MXene can increase charge transfer in HER process. The interface engineering with multiple interfaces can provide fresh impetus for the development of efficient HER catalysts.

Hydrous Iridium Oxide Thin Films: Revealing the Structure of a Highly Active Model Catalyst By in-Situ Ir L 3-Edge XANES and EXAFS

Currently, only iridium oxide shows favourable activity and stability under the harsh oxygen evolution reaction (OER) conditions for application in proton exchange membrane water electrolysis (PEM-WE) [1-3]. There is a well established inverse relationship between OER activity and stability of iridium oxide-based OER catalysts given its degree of crystallinity and hydration [4-6]. Crystalline, anhydrous iridium oxides are more stable but have diminished OER activity compared to amorphous, hydrous iridium oxides, which are more active. Different OER mechanisms have been theorized to occur on these materials, explaining their difference in performance [6]. Several spectroscopic investigations have been performed to explore the properties of such different iridium oxides, in particular to identify the chemical and electronic state of their active sites. However, a consensus regarding the active site structure and OER mechanism has yet to be reached [7-10].Using scanning electron microscopy, x-ray photoelectron spectroscopy, and combining in-situ Ir L 3-edge x-ray absorption spectroscopy with density functional theory (DFT) calculations and ab initio thermodynamics, we have investigated in-situ electrochemically grown porous hydrous iridium oxide thin films (HIROF) as a model system to examine the chemical and electronic structure of the highly active, hydrous iridium oxide species. In-situ extended x-ray absorption fine structure(EXAFS) results show that HIROF grows preferentially in a form most often associated with the OER catalytically active site of hydrous iridium oxides. Calculations over different possible structures allowed us to identify a unique structural group with enhanced hydrogenation that best fits the EXAFS data. In-situ x-ray absorption near edge structure (XANES) results reveal a lower onset potential for the redox behaviour of HIROF compared to rutile IrO2. Based on this study, we propose a new structural model explaining the high activity and poorer stability of hydrous iridium oxides compared to crystalline IrO2.

The Durability of the Platinum/Polypyrrole Nanotubes' Catalysts in Oxygen Reduction Reaction

Platinum (Pt) catalysts supported on carbon-based materials are effective catalysts for fuel cells. However, the durability of the carbon supports in acidic media needs to be improved. Platinum/polypyrrole (PPy) nanotubes have emerged as promising catalysts for the oxygen reduction reaction (ORR) due to their high surface area and good electrocatalytic activity. Furthermore, the nitrogen atoms within the chemical structure of PPy nanotubes are expected to improve their durability.In this study, we investigated the performance and durability of Pt catalysts during ORR by decorating different carbon-based materials with Pt nanoparticles. The catalytic activity and durability were analyzed using the rotation ring-disk electrode (RRDE) technique. We also examined the influence of the different supports on the electrochemical degradation of the catalysts.Our results showed that Pt/PPy nanotubes exhibited high activity and durability in ORR reactions. We paid particular attention to the chemical changes of the catalysts' surfaces, including both the metal and support components, during the ex-situ electrochemical measurements. The surface of the catalysts was characterized using photoelectron X-ray spectroscopy (XPS), synchrotron radiation photoelectron spectroscopy (SRPES), and near-edge X-ray absorption fine structure (NEXAFS) analysis before and after electrochemical degradation. Based on our findings, we proposed degradation mechanisms of the Pt/PPy nanotubes catalysts.In addition, we tested the Pt/PPy nanotubes catalysts in fuel cells and observed their high performance. These findings suggest that Pt/PPy nanotubes catalysts can serve as durable and effective catalysts for ORR reactions, even under harsh conditions. Further research is needed to optimize the synthesis conditions and operating parameters of these catalysts to improve their performance and durability.

Revealing the Impact of Cell Conditioning on PEMWE Catalyst Layer Morphology

Polymer electrolyte membrane (PEM) water electrolysis is a promising green hydrogen generation technology to mitigate the effects of anthropogenic climate change. To achieve stable cell voltages, a conditioning procedure is typically required, where current or voltage cycling is applied until stable performance is achieved. Previous studies have demonstrated that a decrease in voltage occurs within the first few hours of operation [1], but this conditioning period has not been extensively studied for PEM electrolyzers, despite a variety of standard conditioning protocols available for PEM fuel cells in the literature. Previous works additionally suggest that changes in performance may be induced due to structural changes in the anode catalyst layer (CL) caused by large gas bubble evolution during the first few hours of operation [2]. However, to establish and optimize a conditioning procedure for PEMWEs, the specific structural changes caused by conditioning on the anode CL must be elucidated and correlated with the electrochemical performance parameters.In this work, we applied scanning transmission X-ray microscopy (STXM) to PEMWE catalyst layers to reveal pore and ionomer distributions in pristine and conditioned commercial CL samples. Prior to imaging, CL samples were embedded in epoxy and cut to 50 nm thick slices using an ultramicrotome. Using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, Carbon 1s and Fluorine 1s absorption edges were probed to reveal pore and ionomer distributions, respectively. After image acquisition, the image stacks were processed to obtain the ionomer and epoxy spectra to enable spectral fitting and thresholding. Through this analysis, we quantified various morphological properties of catalyst layers (including CL thickness, porosity, pore size distribution, agglomerate distribution, and ionomer content) and compared these properties to those of pristine CLs to explain changes in cell performance. The insights gained from this work will aid in the development of efficient conditioning protocols and identify optimal post-conditioned anode CL morphology to inform the design of next-generation catalyst materials. A. Weiß et al., J Electrochem Soc, 166, F487–F497 (2019).O. Panchenko et al., Mater Today Energy, 16, 100394 (2020).

In-Situ Scanning Transmission X-Ray Microscopy Studies of MnO2-Based Supercapacitor Electrodes

Supercapacitors (SCs) as energy storage devices provide higher energy density than conventional capacitors and higher power density than batteries1. Understanding how charge is efficiently stored in the electrodes or across the electrolyte/electrode interface is key to developing advanced SC electrodes. Scanning transmission x-ray microscopy (STXM) studies2 have been used to investigate MnO2 based supercapacitor electrodes using a novel three-electrode based in-situ flow electrochemical device (Fig. 1)3, 4. Near-edge X-ray absorption fine structure (NEXAFS) spectra of MnO2 films in-situ deposited and subjected to several different electrochemical processes were measured with high spatial resolution at the Mn L3 and O K edges (Fig. 2) at both working electrode (WE) and counter electrode (CE) regions. In this work, the redox state changes associated with pseudocapacitance during charging/discharging processes in a potential window of -0.5 VAu to +0.9 VAu (+0.1 VRHE to +1.5 VRHE) have been investigated. The spectroscopic data and quantitative chemical mapping by in-situ STXM measurements demonstrated that an as-electrodeposited MnO2 film was reduced to both Mn3+ and Mn2+ oxidation states through a reversable Mn4+ ↔ Mn3+/Mn2+ redox reaction. A significant change from a quasi-uniform MnO2 film to a dendritc MnO2 structure was observed during discharging at +1.5 VRHE (Fig. 3, Fig. 4) corresponding to redeposition of Mn2+ dissolved into electrolyte during the reduction process5. In-situ STXM measurements at the CE showed there is deposition of MnO2 during the reduction reaction (charging process) at +0.1 VRHE. Mn L3 features of Mn2+ appeared in the electrolyte region during the reduction process and disappeared during the oxidation process, confirming the dissolution/redeposition mechanism. We have developed a novel and versatile platform for in situ studies of electrochemical processes, including supercapacitors, batteries, and electro-catalysts3, 4. References A. G. Olabi, Q. Abbas, A. Al Makky, and M. A. Abdelkareem, Energy, 248 123617 (2022). K. V. Kaznatcheev, C. Karunakaran, U. D. Lanke, S. G. Urquhart, M. Obst, and A. P. Hitchcock, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 582 (1), 96-99 (2007). A. P. Hitchcock, C. Zhang, H. Eraky, L. Shahcheraghi, F. Ismail, and D. Higgins, Microscopy and Microanalysis, 27 (S2), 59-60 (2021). C. Zhang, N. Mille, H. Eraky, S. Stanescu, S. Swaraj, R. Belkhou, D. Higgins, and A. Hitchcock, (2023). T.-H. Wu, Y.-Q. Lin, Z. D. Althouse, and N. Liu, ACS Applied Energy Materials, 4 (11), 12267-12274 (2021). Figure 1

Investigating the Molecular Orientation and Thermal Stability of Spiro-OMeTAD and Its Dopants By Near Edge X-Ray Absorption Fine Structure

This work describes the utilization of near edge X-ray absorption fine structure (NEXAFS) to investigate the hole transporting material (HTM) 2,2ʹ,7,7ʹ-tetrakis(N,N-di-p-methoxyphenylamine)- 9,9ʹ-spirobifluorene (Spiro-OMeTAD) and its most common dopants, lithium bis-(trifluoromethylsulfonyl) imide (LiTFSI), 4-tert-butylpiridine (tBP) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ). Utilizing angle resolved NEXAFS, the orientation of the molecules on the surface can be observed. The data suggests it is difficult to determine any orientational preference for Spiro-OMeTAD deposited on a surface due to the 3D propeller-like geometry of this molecule. Both doped and undoped samples show thermal stability beyond the glass transition temperature of the molecules. Significant changes to the Spiro-OMeTAD spectra were observed with the addition of the dopants, in particular the C K-edge. Differences are also observed in the valence band spectra when dopants are added. It is also demonstrated how the doping combination of LiFTSI with tBP and, F4-TCNQ act as p-type dopants by altering the position of the HOMO levels. The F4-TCNQ induces a larger change in the HOMO levels when compared to the LiTFSI and tBP. These results are important to increase our understanding of Spiro-OMeTAD and the effect dopants have on this material for next generation solar cells.

Highly Durable Electrocatalyst with Low-Loading Pt-Co Dispersed over Graphene Nanofiber for Oxygen Reduction Reaction Under PEMFC Condition

Proton exchange membrane fuel cell (PEMFC) has been demonstrated as a highly efficient energy conversion technology. The sluggish oxygen reduction reaction (ORR) at the fuel cell cathode requires 3 to 5 times more catalysts than the hydrogen oxidation reaction at the anode (1). Currently, platinum is still the catalyst material of choice. However, the heavy use of scarce Pt in the electrodes represents a major cost barrier to the commercialization of PEMFCs. Tremendous efforts have been dedicated to reduce or remove Pt usage through the development of Pt-transition metal catalysts, as highlighted recently by dealloyed PtNi (2), Mo-doped PtNi(3), and ordered PtCo (4); or through the development of transition metals-nitrogen-carbon based PGM-free catalysts (5, 6).In the fuel cell, catalysts should be highly dispersed over the electrode surface to be easily accessible by the reactants, especially at the high fuel cell current density where a large influx of O2 must be converted. For the Pt alloy catalyst of large crystallites, there won’t be enough crystallites available to spread over the electrode surface to encounter the incoming O2 if the total loading of platinum has to be maintained ultralow. Furthermore, unprotected Pt nano-alloys may lose their nanostructure and crystallinity from the dissolution and agglomeration during the ORR process. The PGM-free catalysts, on the other hand, have high catalytic site density with uniform distribution when prepared by homogenous precursors such as metal-organic framework or porous organic polymer, rendering them highly efficient in interacting with O2 flux. Their key drawback, however, is the poor stability operated under PEMFC condition.In this presentation, we will describe a method of preparing highly active yet stable electrocatalysts containing ultralow Pt content using polymer fiber containing cobalt zeolitic imidazolate framework as precursor. The new catalyst contains Pt-Co nanoparticels situated over the PGM-free catalystically active support (7,8). The ORR activities of the new catalyst was first tested in O2 saturated acidic electrolyte by rotating ring disk electrode (RRDE) before fabricated into the membrane electrode assembly (MEA) and evaluated under fuel cell operating condition. The synergistic catalysis between PtCo NPs and the PGM-free active substrate led to an unprecedented oRR performance in both RRDE and fuel cell. For example, the fuel cell test demonstrated a mess activity of 2.43 A/mgPt for the new catalysts and > 80% retention of the initial mass activity after 60000 continuous voltage cycles from 0.6 – 1.0 V.The characterizations of the fresh and the post-electrochemical test samples were investigated by x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), x-ray absorption near-edge structure (XANES), extended x-ray absorption fine structure (EXAFS), transmission electron microscopy (TEM), Raman spectroscopy and BET surface analysis. We also performed DFT calculation on the ORR thermodynamic barriers over both Pt-Co NPs and PGM-free site as the descriptors for reaction pathway. The results reveal that the synergistic interaction between Pt-Co and PGM active substrate contribute significantly to both activity and durability enhancements.Acknowledgments:This work was supported by Argonne National Laboratory through Maria Goeppert Mayer Fellowship, US; Overseas Outstanding Youth Fund project, shanghai Jiao Tong university, Shanghai, China, State Key Laboratory of Metal Matrix Composites, shanghai Jiao Tong university, Shanghai, China. The works performed at Argonne National Laboratory’s Center for Nanoscale Materials and Advanced Photo Source, and Hydrogen research center, shanghai Jiao Tong university.Reference: H. A. Gasteiger, N. M. Markovic, Just a Dream-or Future Reality? Science 324, 48-49 (2009).B. Han et al., Record activity and stability of dealloyed bimetallic catalysts for proton exchange membrane fuel cells. Energy & Environmental Science 8, 258-266 (2015).X. Huang et al., High-performance transition metal–doped Pt3Ni octahedra for oxygen reduction reaction. Science 348, 1230-1234 (2015).D. Wang et al., Structurally ordered intermetallic platinum-cobalt core-shell nanoparticles with enhanced activity and stability as oxygen reduction electrocatalysts. Nat Mater 12, 81-87 (2013).L. Chong et al., Investigation of Oxygen Reduction Activity of Catalysts Derived from Co and Co/Zn Methyl-Imidazolate Frameworks in Proton Exchange Membrane Fuel Cells. ChemElectroChem 3, 1541-1545 (2016).J. Shui, C. Chen, L. Grabstanowicz, D. Zhao, D. J. Liu, Highly efficient nonprecious metal catalyst prepared with metal-organic framework in a continuous carbon nanofibrous network. Proc Natl Acad Sci U S A 112, 10629-10634 (2015).L. Chong et al., Ultralow-loading platinum-cobalt fuel cell catalysts derived from imidazolate frameworks. Science, (2018).L. Chong et al., Highly durable fuel cell electrocatalyst with low-loading Pt-Co nanoparticles dispersed over single-atom Pt-Co-N-graphene nanofiber. Chem Catalysis 3, 100541.