Operando X-ray Absorption Spectroscopy Measurements Research Articles

Operando Investigations of Reversible and Irreversible Transformations of Metal Organic Framework Based Catalysts during the Oxygen Evolution Reaction

Intermittency issues due to the growth of renewable energy usage lead to an increasing interest in energy storage systems. Thereby, one can use the excess energy for the production of hydrogen from water by anion exchange membrane (AEM) water electrolysis using earth-abundant, non-noble metal catalysts. However, the kinetics of the oxygen evolution reaction (OER) limit the activity of the catalyst and hence, the development of performance stable and active OER catalysts is of great importance for the commercialization of AEM water electrolyzers. In metal organic frameworks (MOFs), a porous structure is created by linking metal atoms/clusters with other metal centres using organic ligands. The resulting structure with a high surface area and dispersed metal centres is a promising catalyst for OER.MOF catalysts with Ni and Co metal centres show impressive OER activity. Both, Co-MOF-74 and Ni-MOF-74 exhibit higher OER activity than their oxide counterparts produced by flame spray synthesis in form of nanoparticles (Fabbri (2017) and Abbott (2018)).1,2 Thereby, an increasing OER activity of Ni-MOF-74 during rotating disk electrode stability tests, indicates that the catalytic species undergoes favorable electronic and structural transformations.Using an in-house-developed spectro-electrochemical flow cell, these transformations were studied by operando X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD). Operando XAS enabled us to monitor the changes occurring in the Ni metal centers while performing cyclic voltammetry (CV) from 1 VRHE up to potentials above the OER onset potential with a time resolution of 5 sec. The operando XAS measurements show that Ni-MOF-74 develops into a highly OER active and stable catalytic species. However, operando XRD measurements prove that the crystalline MOF structure changes into an amorphous structure during OER. Monitoring the electronic and structural transformations due to potential cycling, three different structures were extracted: an initial state, which disappeared within the first eight CV cycles, and two novel electronic/local structures appearing at low and high potential, respectively. The transformations in the electronic and local structure of the Ni metal centers occurring between low and high potential appear to be reversible, as the Ni metal centers return to their initial state during long-term storage in air. However, XRD measurements indicate that this further transformation does not affect the structure of the catalyst: Even though the Ni centers return to their original local structure and oxidation state after long-term storage in air, the long-range structure stays amorphous.The study focuses on elucidating the structure-performance relations of Ni-MOF-74 based OER catalysts providing the key structural parameters that lead to the formation of the highly OER active species. The mechanism of reversible and irreversible transformations occurring in Ni-MOF-74 catalysts allows the extraction of activity descriptors for the development of highly OER active and stable non-noble metal catalysts for AEM water electrolysis.

High-performance P2-Na0.67Mn0.85Cu0.15O2/Hard carbon full cell Na-ion battery: Pre-Sodiation of anode, p/n ratio optimizations, and Operando XAS studies

Na-ion batteries have gained significant attention as a cost-effective and efficient energy storage option for large scale applications, serving as an alternative to the Li-ion batteries. However, commercialization of these batteries is still many steps away since most cathode materials suffer from significant capacity loss and more full-cell studies are required. In this work, we report the electrochemical properties of half- and full-cells of P2-type Na0.67Mn0.85Cu0.15O2 synthesized by solid state technique. X-ray diffraction, FT-IR, and Raman spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy techniques are used to determine the structural properties. Surface properties are studied by X-ray photoelectron spectroscopy and Brunauer–Emmett–Teller techniques. Half cells and full cells were constructed with Na-metal and hard carbon, respectively. Na-ion diffusion kinetics at 10 °C, room temperature, and 50 °C were determined experimentally. Galvanostatic cycling tests on half-cells show capacity values of 165/124 mAh/g for the 1./100. cycles with 24.8 % capacity fade. Operando x-ray absorption spectroscopy measurements were utilized to study local structural modification around transition metal ions during charge/discharge. In the full-cell studies, electrode mass ratio (p/n) and parameters for presodiation of hard carbon were optimized. Using 30 mA/g current density, the unprocessed and the pre-sodiated full-cells reach capacity values of 48 mAh/g (p/n = 2.5) and 150 mAh/g (p/n = 0.75 and 1.15), respectively.

Ultrafast synthesis of cobalt/carbon nanocomposites by magnetic induction heating for oxygen evolution reaction

Metal/carbon nanocomposites have shown great potential as high-performance, low-cost electrocatalysts owing largely to their unique metal-support interactions. These nanocomposites are typically prepared by conventional pyrolysis that is tedious and energy-intensive. Herein, we report the ultrafast preparation of cobalt/carbon nanocomposites by magnetic induction heating (MIH) of metal organic frameworks within seconds under an inert atmosphere. The resulting samples consist of cobalt nanoparticles encapsulated within defective carbon shells, and effectively catalyze oxygen evolution reaction (OER) in alkaline media. Among the series, the sample prepared at 400 A for 10 s exhibits the best OER performance, needing a low overpotential of +308 mV to reach the current density of 10 mA cm−2, along with excellent stability, and even outperforms commercial RuO2 at high overpotentials. This is ascribed to the charge transfer between the carbon scaffold and metal nanoparticles. Operando X-ray absorption spectroscopy measurements show that the electrochemically produced CoOOH species is responsible for the high electrocatalytic performance. The results highlight the unique potential of MIH in the development of effective nanocomposite catalysts for electrochemical energy technologies.

Operando X-Ray Absorption Spectroscopic Study on Influence of Specific Adsorption of Sulfo Group in Perfluorosulfonic Acid Ionomer Towards ORR Activity of Pt/C Catalyst

Polymer electrolyte fuel cells (PEFCs) are clean energy devices with polymer electrolytes and are expected to play an important role in the development of a global renewable energy system and a pure hydrogen energy society in the future. PEFCs have already been applied for small-scale energy systems such as fuel cell vehicles (FCVs). Pt-based catalysts are basically used due to the relatively inefficient reaction kinetics and catalyst degradation under strong acidic conditions during the oxygen reduction reaction (ORR). However, there are still many difficulties in realizing acceptable cost-effective FCVs. It is known that the ORR activity of Pt-based catalysts is reduced by specific adsorption of anionic species. Perfluorosulfonic acid ionomers such as Nafion® are commonly included in the catalyst layers of PEFCs to provide protonic pathways for promoting the ORR1. However, direct observation of anion adsorption on practical Pt nanoparticle/C catalysts for perfluorosulfonic acid ionomers using X-ray absorption spectroscopy (XAS) still has less report. In this study, to achieve specific adsorption over a higher potential range that is at least equal to the ORR active potential (around 0.90 V vs. RHE), operando XAS measurements were performed; the electronic status and local structural change of Pt atoms induced by the specific adsorption of anions at any potential were observed2.Pt/C (29.1 wt.%; TEC10V30E) catalyst supported was purchased from TKK. Nafion® solution was employed to prepare Pt/C catalyst ink with I/C ranging from 0.0 to 1.0. And the amount of Pt/C was adjusted to form a 20 µgcarbon cm-2 catalyst layer after dropping 10 μL on a glassy carbon RDE. The electrochemical cell was constructed using a model electrode fabricated in the working electrode, a Pt mesh for the counter electrode, a reversible hydrogen electrode for the reference electrode, and 0.1 M HClO4 aq for the electrolyte. operando XAS measurement of Pt L-edge was carried out in SPring-8 (Japan).The specific activity decreased as the I/C ratio increased from 0.0 to 0.20. We utilized many methods to evaluate the adsorption species separately, including the measurements of ECSA and oxygen coverage, CO stripping voltammetry, operando X-ray absorption fine structure, and analysis of 5d orbital vacancy. The ECSA and oxygen coverage did not change with increasing ionomer content, indicating that the Pt/C catalyst activity was affected by other adsorption species. A comparison of the CO displacement charge and 5d orbital vacancies of the Pt/C catalysts with I/C = 0.0 and 1.0 suggests that the ionomer-specific adsorption increases when the I/C ratio of the Pt/C catalyst is 1.0; active sites on the surface of the Pt/C catalyst are occupied, resulting in lower catalyst activity. Acknowledgment This work was supported by the project (JPNP20003) and a NEDO FC-Platform project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Effect of Nitrogen Ligand Type on 3d Orbital Level Rearrangement of Cobalt Single Atom Catalysts

Transition metal single atom catalysts have recently emerged in acid oxygen evolution reactions (OER) due to their maximum atomic efficiency and high durability. Herein, we report a 3d orbital level rearrangement in square planar symmetry according to the ligand type of a cobalt single atom catalyst. We fabricate cobalt single atom catalyst supported on pyrrole type nitrogen doped crumpled graphene (Pyrrolic CoN4-CG) for acidic OER and it shows orbital rearrangement phenomenon because of their longer Co-N bond distance than pyridine type CoN4 sites. When pyrrole type nitrogen is introduced as a ligand of a cobalt single atom catalyst, the degree of oxidation during OER reaction is much greater than when pyridine type nitrogen ligand is introduced, which is confirmed by Operando X-ray absorption spectroscopy measurements. In addition, the reduction of OH- adsorption energy according to the orbital level rearrangement of Pyrrolic CoN4 and the change in the rate determination step are revealed by density functional theory calculations.

Operando Elucidation of Electrocatalytic and Redox Mechanisms on a 2D Metal Organic Framework Catalyst for Efficient Electrosynthesis of Hydrogen Peroxide in Neutral Media.

The practical electrosynthesis of hydrogen peroxide (H2O2) is hindered by the lack of inexpensive and efficient catalysts for the two-electron oxygen reduction reaction (2e- ORR) in neutral electrolytes. Here, we show that Ni3HAB2 (HAB = hexaaminobenzene), a two-dimensional metal organic framework (MOF), is a selective and active 2e- ORR catalyst in buffered neutral electrolytes with a linker-based redox feature that dynamically affects the ORR behaviors. Rotating ring-disk electrode measurements reveal that Ni3HAB2 has high selectivity for 2e- ORR (>80% at 0.6 V vs RHE) but lower Faradaic efficiency due to this linker redox process. Operando X-ray absorption spectroscopy measurements reveal that under argon gas the charging of the organic linkers causes a dynamic Ni oxidation state, but in O2-saturated conditions, the electronic and physical structures of Ni3HAB2 change little and oxygen-containing species strongly adsorb at potentials more cathodic than the reduction potential of the organic linker (Eredox ∼ 0.3 V vs RHE). We hypothesize that a primary 2e- ORR mechanism occurs directly on the organic linkers (rather than the Ni) when E > Eredox, but when E < Eredox, H2O2 production can also occur through Ni-mediated linker discharge. By operating the bulk electrosynthesis at a low overpotential (0.4 V vs RHE), up to 662 ppm of H2O2 can be produced in a buffered neutral solution in an H-cell due to minimized strong adsorption of oxygenates. This work demonstrates the potential of conductive MOF catalysts for 2e- ORR and the importance of understanding catalytic active sites under electrochemical operation.

Continuous-flow reactor setup for operando x-ray absorption spectroscopy of high pressure heterogeneous liquid-solid catalytic processes.

A continuous-flow reactor and a continuous-flow setup compatible with operando x-ray absorption spectroscopy (XAS) were designed for safely studying liquid-phase reactions on solid high atomic number transition metal catalysts (e.g., Au, Pd, and Pt) under pressures up to 100 bars with temperatures up to 100°C. The reactor has a stainless-steel body, 2mm thick polyether ether ketone (PEEK) x-ray windows, and a low internal volume of 0.31 ml. The rectangular chamber (6 × 5 × 1 mm3) between the PEEK x-ray windows allows us to perform XAS studies of packed beds or monoliths in the transmission mode at any position in the cell over a length of 60mm. A 146° wide-angle beam access also allows recording complementary x-ray fluorescence or x-ray diffraction signals. The setup was engineered to continuously feed a single-phase liquid flow saturated with one or more gaseous reactants to the liquid-solid XAS reactor containing no free gas phase for enhanced process safety and sample homogeneity. The proof of concept for the continuous-flow XAS cell and high-pressure setup was provided by operando XAS measurements during the direct synthesis of hydrogen peroxide at room temperature and 40 bars using a 35 ± 5 mg catalyst (1 wt. % Pd/TiO2) and inline near-infrared spectroscopy. The experiments prove that the system is well suited to follow the reaction in the liquid phase while recording high-quality XAS data, paving the way for detailed studies on the catalyst structure and structure-activity relationships.

Role of surface steps in activation of surface oxygen sites on Ir nanocrystals for oxygen evolution reaction in acidic media

Ir and its oxide are the only available oxygen evolution reaction (OER) electrocatalysts with reasonably high activity and stability for commercial proton-exchange membrane electrolyzers. However, the establishment of structure–performance relationships for the design of better Ir-based electrocatalysts is hindered by their uncontrolled surface reconstruction during OER in acidic media. Herein, we monitor the structural evolution of two model Ir nanocrystals (one with a flat surface enclosed by (100) facets and the other with a concave surface containing numerous high-index planes) under acidic OER conditions. Operando X-ray absorption spectroscopy measurements reveal that the promotion of surface IrOx formation during the OER by the concave Ir surface with high-index planes results in a gradual OER activity increase, while a decrease in activity and limited oxide formation are observed for the flat Ir surface. After the activation process, the Ir concave surface exhibits ~ 10 times higher activity than the flat surface. Density functional theory computations reveal that Ir high-index surfaces are thermodynamically preferred for the adsorption of oxygen atoms and the formation of surface oxides under OER conditions. Thus, our work establishes a structure–performance relationship for Ir nanocrystals under operating conditions, providing new principles for the design of nanoscale OER electrocatalysts.

Origin of enhanced water oxidation activity in an iridium single atom anchored on NiFe oxyhydroxide catalyst

The efficiency of the synthesis of renewable fuels and feedstocks from electrical sources is limited, at present, by the sluggish water oxidation reaction. Single-atom catalysts (SACs) with a controllable coordination environment and exceptional atom utilization efficiency open new paradigms toward designing high-performance water oxidation catalysts. Here, using operando X-ray absorption spectroscopy measurements with calculations of spectra and electrochemical activity, we demonstrate that the origin of water oxidation activity of IrNiFe SACs is the presence of highly oxidized Ir single atom (Ir5.3+) in the NiFe oxyhydroxide under operating conditions. We show that the optimal water oxidation catalyst could be achieved by systematically increasing the oxidation state and modulating the coordination environment of the Ir active sites anchored atop the NiFe oxyhydroxide layers. Based on the proposed mechanism, we have successfully anchored Ir single-atom sites on NiFe oxyhydroxides (Ir0.1/Ni9Fe SAC) via a unique in situ cryogenic-photochemical reduction method that delivers an overpotential of 183 mV at 10 mA ⋅ cm-2 and retains its performance following 100 h of operation in 1 M KOH electrolyte, outperforming the reported catalysts and the commercial IrO2 catalysts. These findings open the avenue toward an atomic-level understanding of the oxygen evolution of catalytic centers under in operando conditions.

Key role of chemistry versus bias in electrocatalytic oxygen evolution.

The oxygen evolution reaction has an important role in many alternative-energy schemes because it supplies the protons and electrons required for converting renewable electricity into chemical fuels1-3. Electrocatalysts accelerate the reaction by facilitating the required electron transfer4, as well as the formation and rupture of chemical bonds5. This involvement in fundamentally different processes results in complex electrochemical kinetics that can be challenging to understand and control, and that typically depends exponentially on overpotential1,2,6,7. Such behaviour emerges when the applied bias drives the reaction in line with the phenomenological Butler-Volmer theory, which focuses on electron transfer8, enabling the use of Tafel analysis to gain mechanistic insight under quasi-equilibrium9-11 or steady-state assumptions12. However, the charging of catalyst surfaces under bias also affects bond formation and rupture13-15, the effect of which on the electrocatalytic rate is not accounted for by the phenomenological Tafel analysis8 and is often unknown. Here we report pulse voltammetry and operando X-ray absorption spectroscopy measurements on iridium oxide to show that the applied bias does not act directly on the reaction coordinate, but affects the electrocatalytically generated current through charge accumulation in the catalyst. We find that the activation free energy decreases linearly with the amount of oxidative charge stored, and show that this relationship underlies electrocatalytic performance and can be evaluated using measurement and computation. We anticipate that these findings and our methodology will help to better understand other electrocatalytic materials and design systems with improved performance.

Development of photochemical and electrochemical cells for operando X-ray absorption spectroscopy during photocatalytic and electrocatalytic reactions.

Photochemical and electrochemical reactions are highly relevant processes for (i) transforming chemicals (e.g. photoreduction of isopropanol to acetone, electrochemical hydrogenation of benzaldehyde to benzyl alcohol, etc.), and (ii) sustainable energy production (e.g. photoreduction of CO2 to methanol, electrocatalytic H2 evolution reaction). It is therefore of importance to monitor the structural changes and to understand the properties of active sites under photocatalytic and electrocatalytic reaction conditions. Operando X-ray absorption spectroscopy (XAS) provides the means to investigate the nature of active sites under realistic reaction conditions. In this contribution, we describe the successful development of photochemical and electrochemical cells for operando XAS measurements during photocatalytic and electrocatalytic reactions. We have used the operando photochemical cell to monitor the formation of Pt nanoparticles on graphitic carbon nitride nanosheets (g-C3N4-ns) via photodeposition under visible light illumination and observed the formation of highly dispersed Pt nanoparticles with an estimated size of ∼2.5 nm and >60% dispersion. We have also tested this cell to follow the oxidation state of Pt in Pt/TiO2 and Pt/g-C3N4-ns during H2 evolution reaction (HER). We observed that Pt predominantly existed as metallic (reduced) Pt0 species under HER conditions, and that PtOx species were partially reduced from PtIV to Pt0 upon illumination with UV or visible light. The rates of H2 evolution obtained in the photochemical cell (12.1 mmol g-1 h-1 on Pt/TiO2 and 1.01 mmol g-1 h-1 on Pt/g-C3N4-ns) were comparable to that obtained in a standard top-irradiated photoreactor (16.6 mmol g-1 h-1 on Pt/TiO2 and 1.76 mmol g-1 h-1 on Pt/g-C3N4-ns). The operando electrochemical cell was successfully tested to monitor the changes in the structure and oxidation state of Pd in Pd/C electrocatalyst during electrocatalytic hydrogenation (ECH) of benzaldehyde. It was demonstrated that Pd in Pd/C was present in a partially reduced state (∼80% Pd0 and ∼20% PdII) and Pd nanoparticles did not degrade upon the application of an external potential under ECH reaction conditions.

Highly Active and Selective Electrochemical CO2 Reduction to Formate Enabled By Structural Defects on Converted Bi2O3 Nanotubes

Electrochemical CO2 reduction to fuels and commodity chemicals offers an attractive solution for energy and environmental sustainability. Formic acid (or formate) is suggested to be one of the most economically viable products. However, in order to realize its commercial viability, a drastic improvement of the electrocatalyst materials on both activity and selectivity is required. Here we report that structural defects have a profound positive impact on the electrocatalytic performance of Bi for selective CO2 reduction to formate. Bi2O3 double-walled nanotubes with fragmented outer surface are prepared as the template. They are cathodically converted to highly defective Bi nanotubes as evidenced by operando X-ray absorption spectroscopy measurements. This converted electrocatalyst enables CO2 reduction to formate with excellent activity, selectivity and stability. Importantly, its current density reaches ~288 mA cm-2 at -0.61 V versus reversible hydrogen electrode within a flow cell reactor under ambient conditions, which substantially exceeds the commercialization requirement. Using density functional theory calculations, the excellent activity and selectivity are rationalized as the outcome of abundant defective Bi sites that stabilize the *OCHO intermediate. Furthermore, this electrocatalyst is coupled with Si photocathodes and achieves high-performance photoelectrochemical CO2 reduction.

Co/Fe Oxyhydroxides Supported on Perovskite Oxides as Oxygen Evolution Reaction Catalyst Systems.

Co/Fe oxyhydroxide catalysts have been deposited onto the surface of amorphous carbons or different perovskite oxides. By performing electrochemical characterizations and operando X-ray absorption spectroscopy measurements, novel insights into Co/Fe oxyhydroxide catalysts and their interactions with perovskite oxides have been revealed. The addition of Fe into Co oxyhydroxide catalysts greatly enhances the oxygen evolution reaction (OER) activity by stabilizing the Co cations into a lower oxidation state under operative conditions compared to the case of undoped Co oxyhydroxide. A beneficial Co/Fe electronic interaction for OER can also be achieved by depositing Co oxyhydroxide on Fe-containing oxide supports, such as the LaFeO3 perovskite. Finally, it was found that, despite the lower Fe content in the Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) perovskite structure, Co oxyhydroxides supported on this perovskite exhibit the highest OER activity. Therefore, our findings suggest that perovskite structures presenting a large content of oxygen vacancies and undergoing surface reconstruction, such as BSCF, offer the best interface for Co oxyhydroxides. Finally, profiting from the beneficial Co/Fe electronic interaction and perovskite interface interaction, the highest OER activity has been achieved by depositing Co/Fe oxyhydroxide on the surface of BSCF perovskite.

Thermally-induced arrangement of cobalt and iron in the ZIF-derived Fe,Co,Zn-N/C catalysts for the oxygen reduction reaction

Heteroatom-doped carbon materials (HDCM) are perspective Pt-free alternatives for applications in fuel cells. The Fe,Co,Zn-N/C catalysts were obtained by pyrolysis (at 700 °C in Ar) of sacrificial bimetallic zeolitic imidazolate frameworks (Co,Zn-ZIF), prepared with different Co/Zn ratio by a microwave-assisted solvothermal synthesis (at 140 °C in DMF for 2 h). Co,Zn-ZIF hybrids were impregnated with a FeII-phenanthroline complex before the pyrolysis. The structural properties of prepared materials were assessed primarily by X-ray diffraction (XRD) and transmission electron microscopy (TEM), while X-ray fluorescence (XRF) and the scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy (STEM-EDS) mapping and were used for the elemental content analysis. Because in the obtained HDCM both Fe and Co participate in formation of the bamboo-like structures, synchrotron-based X-ray absorption spectroscopy (XAS) studies were performed at their K-edges. The results of in situ XAS measurements during carbonization of Fe,Co,Zn-ZIF upon heating (up to 500 °C in Ar) as well as operando XAS measurements during the electrochemical cycling of HDCM are reported. The registered changes in the oxidative state of Fe and Co (XANES) and in their coordinative environment (EXAFS) were analyzed. The study is complemented by the electrochemical tests of the synthesized HDCM (in 0.1 M HClO4 solution) towards the oxygen reduction reaction, demonstrating their high efficiency and stability in acidic medium.

Interfacial Behavior of Magnesium Ions at Electrode/Electrolyte Interface during Magnesium Deposition Reaction

Introduction Magnesium has large theoretical volumetric capacity and high negative, also much cheaper than metal lithium on price and simple process for handle. Due to the higher safety compared with lithium batteries. rechargeable magnesium batteries with magnesium as the anode has become a hot spot of the study. Electrolyte for magnesium rechargeable batteries has been developed by Aurbach and co-worker [1,2], in which Grinard reagents are contained. The anodic stability and chemical stability of the Grignard reagents-based electrolyte is not satisfied with practical application. [3]. For the development of rechargeable magnesium batteries, it is necessary to elucidate the reaction mechanism at electrolyte/magnesium metal interface. The key factor that predominates the reaction mechanism is considered to be the coordination structures of magnesium ions in the bulk electrolyte solution and at the negative electrode surface. In this study, we examined the mechanism of magnesium deposition/dissolution reactions in the magnesium bis(trifluoromethanesulfonyl)amide Mg(TFSA)2/triglyme electrolyte, and compared them with those in Mg(AlCl2EtBu)2/tetrahydrofuran (THF) and Mg(TFSA)2/2-methyltetrahydrofuran (2-MeTHF)). The bulk structures of the electrolytes were examined by Raman spectroscopy and the theoretical calculation. And then, we applied operando soft X-ray absorption spectroscopy(XAS) to examine the electronic and local structure of magnesium ions at negative electrode/electrolyte interface during magnesium deposition. Experiments Electrochemical properties of the electrolyte were investigated by cyclic voltammetry with the three-electrode cell. The deposition after electrochemical measurement was characterized by XRD and SEM. The operando XAS spectra for Mg K-edge of the electrolytes were measured by the partial fluorescence yield method at the SPring-8 synchrotron radiation facility (BL27SU) in Hyogo, Japan [4]. Results and discussion In cyclic voltammetry, the currents attributed to magnesium deposition/dissolution reactions occurred in Mg(TFSA)2/triglyme and Mg(AlCl2EtBu)2/THF were observed. The currents were not observed in Mg(TFSA)2/2-MeTHF. operando XAS measurements supported that valence of magnesium ions did not change and local structure distortion around magnesium ions increased in Mg(TFSA)2/triglyme and Mg(TFSA)2/2-MeTHF at the magnesium electrode | electrolyte interface during cathodic polarization. These results indicate that the difference of the behavior of magnesium deposition should be affected by the difference of coordinate structure of [TFSA]- in both electrolytes. Our DFT calculation results indicate that the [TFSA]- strongly bound to magnesium ion in Mg(TFSA)2/2-MeTHF electrolyte undergoes reduction decomposition easily instead deposition of magnesium metal, which makes the electrolyte inactive electrochemically. On the other hand, because the [TFSA]- weakly bound to magnesium ion in Mg(TFSA)2/triglyme, the reduction decomposition occurs relatively slowly, which allows magnesium deposition in the electrolyte. We conclude that controlling the interaction between Mg2+ and [TFSA]- and suppressing reduction decomposition of [TFSA]- is crucial to achieve successful magnesium deposition.

A Combined a-SAXS and XAS Study of the Operando Compositional and Morphological Changes Undergone By High Surface Area IrO2 Oxygen Evolution Electrocatalysts

Sluggish kinetics of the anodic process in polymer electrolyte water electrolyzers (PEWEs) - oxygen evolution reaction (OER) - is the main contributor to the efficiency losses.1 Iridium oxide is the state-of-the-art OER catalyst, however its high price and low abundance2 require minimizing its content in the anode while keeping high OER activity. This can be achieved by maximizing the mass specific surface area (SA) of IrO2, e.g. by synthesizing small IrO2 particles.3 On the other hand, high-SA IrO2 catalysts tend to degrade when operating under the dynamic conditions typical for PEWE’s coupled with intermittent renewable energy source such as wind or solar, and the mechanisms causing this instability remain poorly understood.4 In order to study the degradation mechanisms in detail, we used a modified, chlorine-free Adams fusion method to produce a high-SA IrO2 catalyst3 (referred as ‘IrO2-AS’ in what follows) that displayed a SA of 350 m2∙g-1 based on N2 adsorption / desorption measurements. A fraction of this as-synthetized catalyst was heat treated for 1 h at 400 °C in air to yield a second sample (‘IrO2-HT’) with a SA of 250 m2∙g-1. These two samples were electrochemically tested in 0.1 M HClO4 electrolyte measuring the polarization curve and applying an accelerated stress test (AST) in which the potential is cycled 500 times between 1.0 and 1.6 V vs. the reversible hydrogen electrode (RHE) holding the potential for 10 s at each value, thus simulating the dynamic operation conditions discussed above. The polarization curve measurement indicate an initial activity of 1.490 V and 1.509 V (at i= 10 A∙gox -1) for IrO2-AS and IrO2-HT, respectively. While, the latter AST led to the loss of 50 % vs. 30 % of the initial OER-activity displayed by IrO2-AS vs. IrO2-HT, respectively. To shed light on the reasons for this difference in stability, both catalysts were characterized using a novel setup available at the SuperXAS beamline of the Swiss Light Source and that allows to study the morphological and compositional properties of an (electro)catalyst by combining anomalous small angle X-ray scattering (A-SAXS) and X-ray absorption spectroscopy (XAS), respectively.5 Base on the X-ray absorption fine structure (XANES) and the Extended X-ray absorption fine structure analysis the operando XAS measurements (at 1.0 V vs. RHE) proved that neither of the catalysts suffered from significant changes in their chemical composition in the course of the AST. Complementarily, the A-SAXS curves of the IrO2-AS sample did change during the AST (see Fig. 1a), and were successfully fitted using a model that implies particles with a disk shape assembled in a 2D-agglomerated structure. The model suggests that during the AST the disks’ diameter grew and their thickness became thinner (see Figs. 1c and 1d, respectively), while the 2D-agglomerated structure remained stable. This allowed us determining the variation of SA and corresponding iridium mass, leading to the conclusion that the sample’s loss of OER-performance mainly stems from Ir-dissolution. On the contrary, for IrO2-HT the A-SAXS curves did not change during the AST (cf. Fig. 1b), thus proving that this catalyst displays a stable morphology that is consistent with its enhanced stability in the AST. Chiefly, these A-SAXS-derived conclusions were validated through high-resolution transmission electron microscopy measurements of the initial and post-mortem materials. In conclusion, the combined use of A-SAXS and XAS unveiled the reasons for the different stability behavior of pristine and calcined iridium oxides. Specifically, while XAS showed that the two samples did not change composition in the course of the AST, the A-SAXS results obtained for IrO2-AS indicated the changes in its disk morphology driven by Ir-dissolution. Comparatively, the enhanced stability displayed by IrO2-HT can be attributed to the preliminary agglomeration of its oxide particles caused by the heat-treatment step, which prevents drastic shape variation and limits the loss of Ir during the AST. M. Bernt and H. A. Gasteiger, J. Electrochem. Soc., 163, 3179–3189 (2016).P. C. K. Vesborg and T. F. Jaramillo, RCS Adv., 2, 7933–7947 (2012).D. F. Abbott et al., Chem. Mater., 28, 6591–6604 (2016).C. Rakousky et al., J. Power Sources, 326, 120–128 (2016).M. Povia et al., ACS Catal., 8, 7000–7015 (2018). Figure 1

Effect of Pyrolysis Atmosphere and Electrolyte pH on the Oxygen Reduction Activity, Stability and Spectroscopic Signature of FeNx Moieties in Fe-N-C Catalysts

Two Fe-N-C catalysts comprising only atomically-dispersed FeNx moieties were prepared, differing only in the fact that the second catalyst (Fe0.5-NH3) was obtained by subjecting the first one (Fe0.5-Ar) to a short pyrolysis in ammonia. While the initial ORR activity in acid medium in rotating disk electrode is similar for both catalysts, the activity in alkaline medium is significantly higher for Fe0.5-NH3. Time-resolved Fe dissolution reveals a circa 10 times enhanced Fe leaching rate in acidic electrolyte for Fe0.5-NH3 relative to Fe0.5-Ar. Furthermore, for the former, the leaching rate is strongly enhanced when the electrochemical potential is in the range 0.75–0.3 V vs. RHE. This may explain the reduced stability of ammonia-pyrolyzed Fe-N-C catalysts in operating PEMFCs. In alkaline medium in contrast, Fe0.5-NH3 is more active and more stable, with minimized Fe leaching during electrochemical operation in load-cycling mode. Operando X-ray absorption spectroscopy measurements in alkaline electrolyte reveals similar trends of the XANES and EXAFS spectra as a function of the electrochemical potential for both catalysts, but the magnitude of change is much less for Fe0.5-NH3, as evidenced by a Δμ analysis. This is interpreted as a lower average oxidation state of FeNx moieties in Fe0.5-NH3 at open circuit potential.

Role of Coordination Structure of Magnesium Ions on Charge and Discharge Behavior of Magnesium Alloy Electrode

Mechanism of magnesium ion alloying reaction into bismuth electrode in magnesium bis(trifluoromethanesulfonyl)amide (Mg(TFSA)2)/acetonitrile (AN) and Mg(TFSA)2/2-methyltetrahydrofuran (2-MeTHF) electrolyte was examined by a combination of operando soft X-ray absorption spectroscopy (XAS), Raman spectroscopy, and density functional theory (DFT) calculations. In 0.5 M Mg(TFSA)2/AN, the magnesium ions alloying reaction occurred, whereas the alloying reaction did not occur in 0.5 M Mg(TFSA)2/2-MeTHF. Raman spectroscopy showed that less than 15% of [TFSA]− coordinates with magnesium ions in 0.5 M Mg(TFSA)2/AN, while more than 90% of [TFSA]− coordinates with magnesium ions in Mg(TFSA)2/2-MeTHF. Using operando XAS measurements, we observed that electronic and local structure of magnesium ion changed similarly upon cathodic polarization in both electrolytes. These results indicate that the difference of the behavior of alloy formation should be affected by the difference of coordinate structure of [TFSA]− in both electrolytes. Our DFT calculation results indicates [TFSA]− coordinated to magnesium ions undergoes reduction decomposition more easily than [TFSA]− uncoordinated to magnesium ions. In 0.5 M Mg(TFSA)2/2-MeTHF, the [TFSA]− coordinating to magnesium ions undergoes reduction decomposition, which inhibits the alloying reaction into the bismuth electrode. On the other hand, in 0.5 M Mg(TFSA)2/AN, the [TFSA]− reduction decomposition occurs relatively slowly because of the weak coordination between [TFSA]− and magnesium ions, which allows the magnesium ions alloying into the bismuth electrode in the electrolyte.

Frequency-Resolved Evaluation of Electrode Oxidation State in La0.6Sr0.4CoO3 Thin Films with Operando XAS

Commercial adoption and implementation of renewable, but intermittent, energy sources relies on a new generation of high-performance energy conversion and storage devices, such as solid oxide fuel cells (SOFC) and solid oxide electrolysis cells (SOEC). Performance of the oxygen electrode in SOFC is a limiting factor; however, improvements are hindered by inadequate understanding of electrochemical reaction and ionic diffusion processes. AC impedance and DC polarization techniques are typically used to infer relationships between driving energy and reaction rate, but uniquely understanding reaction mechanisms without direct chemical state measurements of the electrode is challenging. In recent years, workers have developed in operando X-ray absorption spectroscopy (XAS) to study oxygen chemical potentials in SOFC cathodes under operating temperatures and Po2. Determining the oxygen chemical potential is based on shifts in the X-ray absorption edge of 3d transition metals due to valence changes to compensate for oxygen vacancies. So far, operando XAS has only been measured under DC polarization, and thus lacked the ability to probe processes of different timescales [1]. In this work, we extend operando XAS to AC impedance measurements, allowing direct frequency-resolved measurement of electrode oxidation state. This approach can potentially be applied to nonlinear harmonic responses, and the first step in the spatially-resolved mapping of the local steady-periodic electrode response. A dense film of La0.6Sr0.4CoO3 (LSC) was deposited on a mirror polished Gd-doped ceria pellet by pulsed laser deposition as the working electrode with porous Pt counter and reference electrodes. Absorption spectra of the Co K-­edge were collected from fluorescent X-rays to determine the detection energy most sensitive to changes in Co valence state. Operando XAS measurements were conducted at 973 K in 10% O2 at the predetermined X-ray energy, typically around 7.718 keV, under electrical perturbations large enough to excite moderate nonlinearity. The frequency resolved oxidation state response of the electrode is obtained by converting X-ray absorption data to oxygen chemical potentials via sets of calibration XAS spectra at various effective Po2. As a result, the data can be treated by the same analysis methods developed for the voltage response in AC impedance measurements.

Origin of the Normal and Inverse Hysteresis Behavior during CO Oxidation over Pt/Al2O3

The hysteresis in the CO oxidation profile over Pt/Al2O3 catalysts has been intensively studied, but its origin is still controversial. In this work, the influence of the Pt particle size and size distribution was systematically investigated. By the application of conventional and advanced preparation methods, such as flame spray pyrolysis and supercritical fluid reactive deposition, a series of catalysts containing homogeneous distributions of Pt particles were obtained. An optimal Pt particle size of 2–3 nm was identified for the CO oxidation light-off on Pt/Al2O3 catalysts. The CO oxidation results show a clear correlation between the Pt nanoparticle size and the ignition/extinction profile, including the switch of the hysteresis loop. On the basis of high-angle annular dark-field scanning transmission electron microscopy characterization combined with in situ diffuse reflectance infrared Fourier transform spectroscopy and operando X-ray absorption spectroscopy measurements, the appearance of inverse h...