Elucidation of Structure-Activity Correlations in a Nickel Manganese Oxide Oxygen Evolution Reaction Catalyst by Operando Ni L-Edge X-ray Absorption Spectroscopy and 2p3d Resonant Inelastic X-ray Scattering

Electro-Chemo-Mechanical Evolution at the Garnet Solid Electrolyte-Cathode Interface

Solid-state batteries promise higher energy density and improved safety compared with lithium-ion batteries. However, electro-chemomechanical instabilities at the solid electrolyte interface with the cathode and the anode hinder their large scale implementation. Here, we focus on resolving electro-chemo-mechanical instability mechanisms and their onset conditions between a state-of-the-art cathode, LiNi0.6Mn0.2Co0.2O2 (NMC622), and the garnet Li7La3Zr2O12 (LLZO) solid electrolyte. We used thin-film NMC622 on LLZO pellets to place the interfacial region within the detection depth of the X-ray characterization techniques. The experimental probes of the near-interface region included in operando X-ray absorption spectroscopy and ex situ focused ion beam scanning electron microscopy. Electrochemical degradation was not observable during cycling at room temperature with 4.3 V versus Li/Li+ charge voltage cutoff, or with stepwise potentiostatic hold up to 4.1 V versus Li/Li+. In contrast, secondary phases including reduced transition metal species (Ni2+, Co2+) were found after cycling up to 4.3 V versus Li/Li+ at 80 °C and during potentiostatic hold at 4.3 V versus Li/Li+ (Ni2+). Intergranular cracks between NMC622 grains and delamination at the NMC622|LLZO interface occurred readily after the first charge. These interface reaction products and mechanical failure lowered the capacity and cell efficiency due to partial loss of the NMC622 phase, partial loss of contact at the interface, and a higher polarization resistance. Electrochemical instability between delithiated NMC622 and LLZO could be mitigated by using a low charge voltage cutoff or cycling at lower temperature. Ways to engineer the mechanical properties to avoid crack deflection and delamination at the interface are also discussed for enhancing mechanical stability.

Double Hydroxide Nanocatalysts for Urea Electrooxidation Engineered toward Environmentally Benign Products

Recent advancements in the electrochemical urea oxidation reaction (UOR) present promising avenues for wastewater remediation and energy recovery. Despite progress toward optimized efficiency, hurdles persist in steering oxidation products away from environmentally unfriendly products, mostly due to a lack of understanding of structure-selectivity relationships. In this study, the UOR performance of Ni and Cu double hydroxides, which show marked differences in their reactivity and selectivity is evaluated. CuCo hydroxides predominantly produce N2, reaching a current density of 20 mA cmgeo -2 at 1.04 V - 250 mV less than NiCo hydroxides that generate nitrogen oxides. A collection of in-situ spectroscopies and scattering experiments reveal a unique in situ generated Cu(2-x)+-OO-• active sites in CuCo, which initiates nucleophilic substitution of NH2 from the amide, leading to N-N coupling between *NH on Co and Cu. In contrast, the formation of nitrogen oxides on NiCo is primarily attributed to the presence of high-valence Ni3+ and Ni4+, which facilitates N-H activation. This process, in conjunction with the excessive accumulation of OH- ions on Jahn-Teller (JT) distorted Co sites, leads to the generation of NO2 - as the primary product. This work underscores the importance of catalyst composition and structural engineering in tailoring innocuous UOR products.

Unveiling Key Interface Characteristics of Ni/Yttria-Stabilized Zirconia Solid Oxide Cell Electrodes in H2O Electroreduction Using Operando X-ray Photoelectron Spectroscopy

Nickel/yttria-stabilized zirconia (YSZ) composites are the most commonly used fuel electrodes for solid oxide cells. While microstructural changes of Ni/YSZ during operational conditions have been thoroughly investigated, there is limited knowledge regarding Ni/YSZ surface chemistry under working conditions. In this study, we examine the interaction between Ni/YSZ electrodes and water vapor under open circuit and polarization conditions, utilizing near ambient pressure soft and hard X-ray photoelectron spectroscopies. Miniature cells with conventional porous Ni/YSZ composite cermet cathodes were modified to facilitate the direct spectroscopic observation of the functional electrode's areas close to the interface with the YSZ electrolyte. The results highlight dynamic changes in the oxidation state and composition of Ni/YSZ under H2 and H2O atmospheres. We also quantify the accumulation of impurities on the electrode surface. Through adjustments in the pretreatment of the cell, the correlation between the nickel surface oxidation state and the cell's electrochemical performance during H2O electroreduction is established. It is unequivocally shown that nickel surface oxidation in H2O electrolysis favors NiO over Ni(OH)x, providing critical insights into the mechanism of Ni-phase redistribution within the electrode during long-term operation. Depth-dependent photoemission measurements, combined with theoretical quantitative simulations, reveal that NiO and Ni phases are uniformly mixed on the surface during H2O electrolysis. This differs from the conventional expectation of a NiO-shell/Ni-core configuration in gas phase oxidation. These findings provide crucial insights into the surface chemistry of Ni/YSZ electrodes under conditions relevant to H2O electrolysis, elucidating their impact on the electrochemical performance of the cell.

The spin-forbidden transition in iron(IV)-oxo catalysts relevant to two-state reactivity

Quintet oxoiron(IV) intermediates are often invoked in nonheme iron enzymes capable of performing selective oxidation, while most well-characterized synthetic model oxoiron(IV) complexes have a triplet ground state. These differing spin states lead to the proposal of a two-state reactivity model, where the complexes cross from the triplet to an excited quintet state. However, the energy of this quintet state has never been measured experimentally. Here, magnetic circular dichroism is used to assign the singlet and triplet excited states in a series of triplet oxoiron(IV) complexes. These transition energies are used to determine the energies of the quintet state via constrained fitting of 2p3d resonant inelastic x-ray scattering. This allowed for a direct correlation between the quintet energies and substrate C─H oxidation rates.

Excited-State Identification of a Nickel-Bipyridine Photocatalyst by Time-Resolved X-ray Absorption Spectroscopy

Photoassisted catalysis using Ni complexes is an emerging field for cross-coupling reactions in organic synthesis. However, the mechanism by which light enables and enhances the reactivity of these complexes often remains elusive. Although optical techniques have been widely used to study the ground and excited states of photocatalysts, they lack the specificity to interrogate the electronic and structural changes at specific atoms. Herein, we report metal-specific studies using transient Ni L- and K-edge X-ray absorption spectroscopy of a prototypical Ni photocatalyst, (dtbbpy)Ni(o-tol)Cl (dtb = 4,4'-di-tert-butyl, bpy = bipyridine, o-tol = ortho-tolyl), in solution. We unambiguously confirm via direct experimental evidence that the long-lived (∼5 ns) excited state is a tetrahedral metal-centered triplet state. These results demonstrate the power of ultrafast X-ray spectroscopies to unambiguously elucidate the nature of excited states in important transition-metal-based photocatalytic systems.

Strongly-Interacted NiSe2/NiFe2O4 Architectures Built Through Selective Atomic Migration as Catalysts for the Oxygen Evolution Reaction

The interactions between the catalyst and support are widely used in many important catalytic reactions but the construction of strong interaction with definite microenvironments to understand the structure-activity relationship is still challenging. Here, strongly-interacted composites are prepared via selective exsolution of active NiSe2 from the host matrix of NiFe2O4 (S-NiSe2/NiFe2O4) taking advantage of the differences of migration energy, in which the NiSe2 possessed both high dispersion and small size. The characteristics of spatially resolved scanning transmission X-ray microscopy (STXM) coupled with analytical Mössbauer spectra for the surface and bulk electronic structures unveiled that this strongly interacted composite triggered more charge transfers from the NiSe2 to the host of NiFe2O4 while stabilizing the inherent atomic coordination of NiFe2O4. The obtained S-NiSe2/NiFe2O4 exhibits overpotentials of 290 mV at 10 mA cm-2 for oxygen evolution reaction (OER). This strategy is general and can be extended to other supported catalysts, providing a powerful tool for modulating the catalytic performance of strongly-interacted composites.

In Situ X-ray Absorption Spectroscopy of LaFeO3 and LaFeO3/LaNiO3 Thin Films in the Electrocatalytic Oxygen Evolution Reaction

We study the electrocatalytic oxygen evolution reaction using in situ X-ray absorption spectroscopy (XAS) to track the dynamics of the valence state and the covalence of the metal ions of LaFeO3 and LaFeO3/LaNiO3 thin films. The active materials are 8 unit cells grown epitaxially on 100 nm conductive La0.67Sr0.33MnO3 layers using pulsed laser deposition (PLD). The perovskite layers are supported on monolayer Ca2Nb3O10 nanosheet-buffered 100 nm SiNx membranes. The in situ Fe and Ni K-edges XAS spectra were measured from the backside of the SiNx membrane using fluorescence yield detection under electrocatalytic reaction conditions. The XAS spectra show significant spectral changes, which indicate that (1) the metal (co)valencies increase, and (2) the number of 3d electrons remains constant with applied potential. We find that the whole 8 unit cells react to the potential changes, including the buried LaNiO3 film.

Hydrothermally grown Cu doped NiMnO3 perovskite nanostructures suitable for optoelectronic, photoluminescent and electrochemical properties

Transition metal-based perovskites have emerged as highly promising and economically advantageous semiconductor materials due to their exceptional performance in optoelectronics, photovoltaic, photocatalysis, and photoluminescence. In this study, we employed a microwave-assisted hydrothermal process to produce a Cu-doped NiMnO3 nanocomposite electrode material. The appearance of a peak corresponding to the (110) plane with a 2θ value of 36.6° confirmed the growth of the rhombohedral NiMnO3 crystal structure. The presence of metal-oxygen bonds in NiMnO3 was confirmed through FTIR spectra. XPS validates the chemical composition, providing additional support for the results obtained from XRD and FT-IR analyses. FE-SEM affirmed the anisotropic growth of small sphere-like structures that agglomerated to form broccoli-like shapes. Cu doping modified the band gap, reducing it from 2.2 to 1.7 eV and enhancing its photoluminescent (PL) activity by introducing defects. The increase in PL intensity (visible light luminescent intensity) can be attributed to a concurrent rise in complex defects and the rate of recombination of electron-hole pairs. Finally, the electrochemical activity demonstrated the pseudo-capacitor behavior of the synthesized material, with capacitance values increasing as the copper (Cu) content in the parent lattice increased.

Operando studies of Mn oxide based electrocatalysts for the oxygen evolution reaction

Inspired by photosystem II (PS II), Mn oxide based electrocatalysts have been repeatedly investigated as catalysts for the electrochemical oxygen evolution reaction (OER), the anodic reaction in water electrolysis. However, a comparison of the conditions in biological OER catalysed by the water splitting complex CaMn4Ox with the requirements for an electrocatalyst for industrially relevant applications reveals fundamental differences. Thus, a systematic development of artificial Mn-based OER catalysts requires both a fundamental understanding of the catalytic mechanisms as well as an evaluation of the practicality of the system for industrial scale applications. Experimentally, both aspects can be approached using in situ and operando methods including spectroscopy. This paper highlights some of the major challenges common to different operando investigation methods and recent insights gained with them. To this end, vibrational spectroscopy, especially Raman spectroscopy, absorption techniques in the bandgap region and operando X-ray spectroelectrochemistry (SEC), both in the hard and soft X-ray regime are particularly focused on here. Technical challenges specific to each method are discussed first, followed by challenges that are specific to Mn oxide based systems. Finally, recent in situ and operando studies are reviewed. This analysis shows that despite the technical and Mn specific challenges, three specific key features are common to most of the studied systems with significant OER activity: structural disorder, Mn oxidation states between III and IV, and the appearance of layered birnessite phases in the active regime.

Advancements in Liquid Jet Technology and X-ray Spectroscopy for Understanding Energy Conversion Materials during Operation

ConspectusWater splitting is intensively studied for sustainable and effective energy storage in green/alternative energy harvesting-storage-release cycles. In this work, we present our recent developments for combining liquid jet microtechnology with different types of soft X-ray spectroscopy at high-flux X-ray sources, in particular developed for studying the oxygen evolution reaction (OER). We are particularly interested in the development of in situ photon-in/photon-out techniques, such as in situ resonant inelastic X-ray scattering (RIXS) techniques at high-repetition-frequency X-ray sources, pointing toward operando capabilities. The pilot catalytic systems we use are perovskites having the general structure ABO₃ with lanthanides or group II elements at the A sites and transition metals at the B sites. Depending on the chemical substitutions of ABO₃, their catalytic activity for OER can be tuned by varying the composition.In this work, we present our in situ RIXS studies of the manganese L-edge of perovskites during OER. We have developed various X-ray spectroscopy approaches like transmission zone plate-, reflection zone plate-, and grating-based emission spectroscopy techniques. Combined with tunable incident X-ray energies, we yield complementary information about changing (inverse) X-ray absorption features of the perovskites, allowing us to deduce element- and oxidation-state-specific chemical monitoring of the catalyst. Adding liquid jet technology, we monitor element- and oxidation-state-specific interactions of the catalyst with water adsorbate during OER. By comparing the different technical spectroscopy approaches combined with high-repetition-frequency experiments at synchrotrons and free-electron lasers, we conclude that the combination of liquid jet with low-resolution zone-plate-based X-ray spectroscopy is sufficient for element- and oxidation-state-specific chemical monitoring during OER and easy to handle.For an in-depth study of OER mechanisms, however, including the characterization of catalyst-water adsorbate in terms of their charge transfer properties and especially valence intermediates formed during OER, high-resolution spectroscopy tools based on a combination of liquid jets with gratings bear bigger potential since they allow resolution of otherwise-overlapping X-ray spectroscopy transitions. Common for all of these experimental approaches is the conclusion that without the versatile developments of liquid jets and liquid beam technologies, elaborate experiments such as high-repetition experiments at high-flux X-ray sources (like synchrotrons or free-electron lasers) would hardly be possible. Such experiments allow sample refreshment for every single X-ray shot for repetition frequencies of up to 5 MHz, so that it is possible (a) to study X-ray-radiation-sensitive samples and also (b) to utilize novel types of flux-hungry X-ray spectroscopy tools like photon-in/photon-out X-ray spectroscopy to study the OER.

Promoting nickel oxidation state transitions in single-layer NiFeB hydroxide nanosheets for efficient oxygen evolution

Promoting the formation of high-oxidation-state transition metal species in a hydroxide catalyst may improve its catalytic activity in the oxygen evolution reaction, which remains difficult to achieve with current synthetic strategies. Herein, we present a synthesis of single-layer NiFeB hydroxide nanosheets and demonstrate the efficacy of electron-deficient boron in promoting the formation of high-oxidation-state Ni for improved oxygen evolution activity. Raman spectroscopy, X-ray absorption spectroscopy, and electrochemical analyses show that incorporation of B into a NiFe hydroxide causes a cathodic shift of the Ni²⁺(OH)₂ → Ni^3+δOOH transition potential. Density functional theory calculations suggest an elevated oxidation state for Ni and decreased energy barriers for the reaction with the NiFeB hydroxide catalyst. Consequently, a current density of 100 mA cm^–2 was achieved in 1 M KOH at an overpotential of 252 mV, placing it among the best Ni-based catalysts for this reaction. This work opens new opportunities in electronic engineering of metal hydroxides (or oxides) for efficient oxygen evolution in water-splitting applications.

While water-splitting electrolysis offers a potential renewable means to store energy, the oxygen evolution half-reaction’s sluggish kinetics limits performances. Here, authors incorporation boron into nickel-iron hydroxide catalysts to promote electrocatalytic water oxidation activities

Promoting biomass electrooxidation via modulating proton and oxygen anion deintercalation in hydroxide

The redox center of transition metal oxides and hydroxides is generally considered to be the metal site. Interestingly, proton and oxygen in the lattice recently are found to be actively involved in the catalytic reactions, and critically determine the reactivity. Herein, taking glycerol electrooxidation reaction as the model reaction, we reveal systematically the impact of proton and oxygen anion (de)intercalation processes on the elementary steps. Combining density functional theory calculations and advanced spectroscopy techniques, we find that doping Co into Ni-hydroxide promotes the deintercalation of proton and oxygen anion from the catalyst surface. The oxygen vacancies formed in NiCo hydroxide during glycerol electrooxidation reaction increase d-band filling on Co sites, facilitating the charge transfer from catalyst surface to cleaved molecules during the 2^nd C-C bond cleavage. Consequently, NiCo hydroxide exhibits enhanced glycerol electrooxidation activity, with a current density of 100 mA/cm² at 1.35 V and a formate selectivity of 94.3%.

Developing catalysts for biomass electrooxidation are critical in electric refinery. The reaction mechanism, however, is still ambiguous. Here, the authors reveal how proton and oxygen anion deintercalation in hydroxide determine the elementary reaction steps in a model reaction of glycerol oxidation.

Rare earth metal (Sm)-doped NiMnO3 nanostructures for highly competent alkaline oxygen evolution reaction

In the present work, samarium-doped nickel manganese oxide was produced by employing a straightforward co-precipitation method. A peak with a 2θ of 36° corresponded to the (110) plane confirming the formation of the rhombohedral crystal structure of NiMnO₃. The existence of Mn-O and Ni-O stretching vibration modes was confirmed by Raman spectroscopy. FTIR spectra confirmed the existence of the metal-oxygen bond of NiMnO₃. The synthesized ternary Ni-based material was found to be spherical nanoparticles with an average diameter of 0.81 μm. The electrochemical oxygen evolution reaction (OER) performance was explored on 0.02 M samarium (Sm)-doped NiMnO₃ demonstrating outstanding OER action with low 321 mV, a low Tafel slope value (109 mV dec^-1), and low charge-transfer resistance (0.19 Ω). Moreover, the BET results suggest that the 0.02 M Sm-doped NiMnO₃ exhibited elevated surface area (78.78 m² g^-1) with a mesoporous character. Therefore, NiMnO₃ doped with high concentrations of a rare earth metal, Sm, is proposed as a suitable material for next-generation water splitting applications.

First-Row Transition Metal Antimonates for the Oxygen Reduction Reaction

The development of inexpensive and abundant catalysts with high activity, selectivity, and stability for the oxygen reduction reaction (ORR) is imperative for the widespread implementation of fuel cell devices. Herein, we present a combined theoretical-experimental approach to discover and design first-row transition metal antimonates as excellent electrocatalytic materials for the ORR. Theoretically, we identify first-row transition metal antimonates─MSb₂O₆, where M = Mn, Fe, Co, and Ni─as nonprecious metal catalysts with good oxygen binding energetics, conductivity, thermodynamic phase stability, and aqueous stability. Among the considered antimonates, MnSb₂O₆ shows the highest theoretical ORR activity based on the 4e^- ORR kinetic volcano. Experimentally, nanoparticulate transition metal antimonate catalysts are found to have a minimum of a 2.5-fold enhancement in intrinsic mass activity (on transition metal mass basis) relative to the corresponding transition metal oxide at 0.7 V vs RHE in 0.1 M KOH. MnSb₂O₆ is the most active catalyst under these conditions, with a 3.5-fold enhancement on a per Mn mass activity basis and 25-fold enhancement on a surface area basis over its antimony-free counterpart. Electrocatalytic and material stability are demonstrated over a 5 h chronopotentiometry experiment in the stability window identified by theoretical Pourbaix analysis. This study further highlights the stable and electrically conductive antimonate structure as a framework to tune the activity and selectivity of nonprecious metal oxide active sites for ORR catalysis.

Activating lattice oxygen in NiFe-based (oxy)hydroxide for water electrolysis

Transition metal oxides or (oxy)hydroxides have been intensively investigated as promising electrocatalysts for energy and environmental applications. Oxygen in the lattice was reported recently to actively participate in surface reactions. Herein, we report a sacrificial template-directed approach to synthesize Mo-doped NiFe (oxy)hydroxide with modulated oxygen activity as an enhanced electrocatalyst towards oxygen evolution reaction (OER). The obtained MoNiFe (oxy)hydroxide displays a high mass activity of 1910 A/gmetal at the overpotential of 300 mV. The combination of density functional theory calculations and advanced spectroscopy techniques suggests that the Mo dopant upshifts the O 2p band and weakens the metal-oxygen bond of NiFe (oxy)hydroxide, facilitating oxygen vacancy formation and shifting the reaction pathway for OER. Our results provide critical insights into the role of lattice oxygen in determining the activity of (oxy)hydroxides and demonstrate tuning oxygen activity as a promising approach for constructing highly active electrocatalysts.

Elucidating the Mechanism of Excited-State Bond Homolysis in Nickel-Bipyridine Photoredox Catalysts

Ni 2,2'-bipyridine (bpy) complexes are commonly employed photoredox catalysts of bond-forming reactions in organic chemistry. However, the mechanisms by which they operate are still under investigation. One potential mode of catalysis is via entry into Ni(I)/Ni(III) cycles, which can be made possible by light-induced, excited-state Ni(II)-C bond homolysis. Here, we report experimental and computational analyses of a library of Ni(II)-bpy aryl halide complexes, Ni(^Rbpy)(^R'Ph)Cl (R = MeO, t-Bu, H, MeOOC; R' = CH₃, H, OMe, F, CF₃), to illuminate the mechanism of excited-state bond homolysis. At given excitation wavelengths, photochemical homolysis rate constants span 2 orders of magnitude across these structures and correlate linearly with Hammett parameters of both bpy and aryl ligands, reflecting structural control over key metal-to-ligand charge-transfer (MLCT) and ligand-to-metal charge-transfer (LMCT) excited-state potential energy surfaces (PESs). Temperature- and wavelength-dependent investigations reveal moderate excited-state barriers (ΔH^‡ ∼ 4 kcal mol^-1) and a minimum energy excitation threshold (∼55 kcal mol^-1, 525 nm), respectively. Correlations to electronic structure calculations further support a mechanism in which repulsive triplet excited-state PESs featuring a critical aryl-to-Ni LMCT lead to bond rupture. Structural control over excited-state PESs provides a rational approach to utilize photonic energy and leverage excited-state bond homolysis processes in synthetic chemistry.

Phase-Reconfiguration-Induced NiS/NiFe2 O4 Composite for Performance-Enhanced Zinc-Air Batteries

Constructing composite structures is an essential approach for obtaining multiple functionalities in a single entity. Available synthesis methods of the composites need to be urgently exploited; especially in situ construction. Here, a NiS/NiFe₂ O₄ composite through a local metal-S coordination at the interface is reported, which is derived from phase reconstruction in the highly defective matrix. X-ray absorption fine structure confirms that long-range order is broken via the local metal-S coordination and, by using electron energy loss spectroscopy, the introduction of NiS/NiFe₂ O₄ interfaces during the irradiation of plasma energy is identified. Density functional theory (DFT) calculations reveal that in situ phase reconfiguration is crucial for synergistically reducing energetic barriers and accelerating reaction kinetics toward catalyzing the oxygen evolution reaction (OER). As a result; it leads to an overpotential of 230 mV @10 mA cm^-2 for the OER and a half-wave potential of 0.81 V for the oxygen reduction reaction (ORR); as well as an excellent zinc-air battery (ZAB) performance with a power density of 148.5 mW cm^-2 . This work provides a new compositing strategy in terms of fast phase reconstruction of bifunctional catalysts.

Surprisingly Low Reactivity of Layered Manganese Oxide toward Water Oxidation in Fe/Ni-Free Electrolyte under Alkaline Conditions

So far, many studies on the oxygen-evolution reaction (OER) by Mn oxides have been focused on activity; however, the identification of the best performing active site and corresponding catalytic cycles is also of critical importance. Herein, the real intrinsic activity of layered Mn oxide toward OER in Fe/Ni-free KOH is studied for the first time. At pH ≈ 14, the onset of OER for layered Mn oxide in the presence of Fe/Ni-free KOH happens at 1.72 V (vs reversible hydrogen electrode (RHE)). In the presence of Fe ions, a 190 mV decrease in the overpotential of OER was recorded for layered Mn oxide as well as a significant decrease (from 172.8 to 49 mV/decade) in the Tafel slope. Furthermore, we find that both Ni and Fe ions increase OER remarkably in the presence of layered Mn oxide, but that pure layered Mn oxide is not an efficient catalyst for OER without Ni and Fe under alkaline conditions. Thus, pure layered Mn oxide and electrolytes are critical factors in finding the real intrinsic activity of layered Mn oxide for OER. Our results call into question the high efficiency of layered Mn oxides toward OER under alkaline conditions and also elucidate the significant role of Ni and Fe impurities in the electrolyte in the presence of layered Mn oxide toward OER under alkaline conditions. Overall, a computational model supports the conclusions from the experimental structural and electrochemical characterizations. In particular, substitutional doping with Fe decreases the thermodynamic OER overpotential up to 310 mV. Besides, the thermodynamic OER onset potential calculated for the Fe-free structures is higher than 1.7 V (vs RHE) and, thus, not in the stability range of Mn oxides.

In situ fluorescence yield soft X-ray absorption spectroscopy of electrochemical nickel deposition processes with and without ethylene glycol

The electrochemical Ni deposition at a platinum electrode was investigated in a plating nickel bath in the presence and absence of ethylene glycol (EG) using fluorescence yield soft X-ray absorption spectroscopy (FY-XAS) in the Ni L_2,3-edge and O K-edge regions under potential control. At ≤+0.35 V vs. the reversible hydrogen electrode (RHE), the electrochemical Ni deposition was detected by the Ni L_2,3-edge FY-XAS in the presence of EG whereas almost no such event was observed in the absence of EG. A drastic decrease of FY-XAS intensities in the O K-edge region was also observed in the presence of EG at >+0.35 V vs. RHE, suggesting that the nano-/micro-structured Ni deposition initiated by the removal of water molecules occurs on the Pt electrode. The complex formation of Ni²⁺ with EG and the adsorption of EG on the Ni surface could play an important role in the Ni deposition. This study demonstrates that the in situ FY-XAS is a powerful and surface-sensitive technique to understand (electro)chemical reactions including polyol synthesis and electrocatalysis at solid–liquid interfaces.

Schematic drawing of electrochemical reactions of the Pt-coated SiC electrode, which separates the vacuum and the solution containing Ni²⁺ and ethylene glycol, in our spectro-electrochemical setup for the FY-XAS.

In situ activation of Br-confined Ni-based metal-organic framework hollow prisms toward efficient electrochemical oxygen evolution

Fundamental insights into the structural evolution of oxygen electrocatalysts under operating conditions are of substantial importance for designing efficient catalysts. Here, on the basis of operando x-ray absorption fine structure spectroscopy, we probe the in situ activation of Br-confined conductive Ni-based metal-organic framework (Br-Ni-MOF) hollow prisms toward an active oxygen electrocatalyst during the oxygen evolution reaction (OER) process. The successive structural transformations from pristine Br-Ni-MOF to a β-Ni(OH)₂ analog then subsequently to a γ-NiOOH phase during OER are observed. This post-formed γ-NiOOH analog manifests high OER performance with a superior overpotential of 306 mV at 10 mA cm⁻² and a high turnover frequency value of 0.051 s⁻¹ at an overpotential of 300 mV, making Br-Ni-MOF one of the most active oxygen electrocatalysts reported. Density functional theory calculations reveal that the strong electronic coupling between Br and Ni atoms accelerates the generation of the key *O intermediate toward fast OER kinetics.

In Situ Formation of Nano Ni-Co Oxyhydroxide Enables Water Oxidation Electrocatalysts Durable at High Current Densities

The oxygen evolution reaction (OER) limits the energy efficiency of electrocatalytic systems due to the high overpotential symptomatic of poor reaction kinetics; this problem worsens over time if the performance of the OER electrocatalyst diminishes during operation. Here, a novel synthesis of nanocrystalline Ni-Co-Se using ball milling at cryogenic temperature is reported. It is discovered that, by anodizing the Ni-Co-Se structure during OER, Se ions leach out of the original structure, allowing water molecules to hydrate Ni and Co defective sites, and the nanoparticles to evolve into an active Ni-Co oxyhydroxide. This transformation is observed using operando X-ray absorption spectroscopy, with the findings confirmed using density functional theory calculations. The resulting electrocatalyst exhibits an overpotential of 279 mV at 0.5 A cm^-2 and 329 mV at 1 A cm^-2 and sustained performance for 500 h. This is achieved using low mass loadings (0.36 mg cm^-2 ) of cobalt. Incorporating the electrocatalyst in an anion exchange membrane water electrolyzer yields a current density of 1 A cm^-2 at 1.75 V for 95 h without decay in performance. When the electrocatalyst is integrated into a CO₂ -to-ethylene electrolyzer, a record-setting full cell voltage of 3 V at current density 1 A cm^-2 is achieved.

Coordination Geometry Changes in Amorphous Cyanide-Bridged Metal-Organic Frameworks upon Water Adsorption

Amorphous coordination polymers and metal-organic frameworks (MOFs) have attracted much attention owing to their various functionalities. Here, we demonstrate the tunable water adsorption behavior of a series of amorphous cyanide-bridged MOFs with different metals (M[Ni(CN)₄]: MNi; M = Mn, Fe, and Co). All three compounds adsorb up to six water molecules at a certain vapor pressure (P_ads) and undergo conversion to crystalline Hofmann-type MOFs, M(H₂O)₂[Ni(CN)₄]·4H₂O (MNi-H₂O; M = Mn, Fe, and Co). The P_ads of MnNi, FeNi, and CoNi for water adsorption is P/P₀ = 0.4, 0.6, and 0.9, respectively. Although the amorphous nature of these materials prevented structural elucidation using X-ray crystallography techniques, the local-scale structure around the N-coordinated M²⁺ centers was analyzed using L_2,3-, K-edge X-ray absorption fine structure, and magnetic measurements. Upon hydration, the coordination geometry of these metal centers changed from tetrahedral to octahedral, resulting in significant reorganization of the MOF local structure. On the other hand, Ni[Ni(CN)₄] (NiNi) containing square-planar Ni²⁺ centers did not undergo significant structural transformation and therefore abruptly adsorbed H₂O in the low-pressure region. We could thus define how changes in the bond lengths and coordination geometry are related to the adsorption properties of amorphous MOF systems.

Surface site density and utilization of platinum group metal (PGM)-free Fe-NC and FeNi-NC electrocatalysts for the oxygen reduction reaction

Pyrolyzed iron-based platinum group metal (PGM)-free nitrogen-doped single site carbon catalysts (Fe-NC) are possible alternatives to platinum-based carbon catalysts for the oxygen reduction reaction (ORR). Bimetallic PGM-free M1M2-NC catalysts and their active sites, however, have been poorly studied to date. The present study explores the active accessible sites of mono- and bimetallic Fe-NC and FeNi-NC catalysts. Combining CO cryo chemisorption, X-ray absorption and 57Fe Mössbauer spectroscopy, we evaluate the number and chemical state of metal sites at the surface of the catalysts along with an estimate of their dispersion and utilization. Fe L3,2-edge X-ray adsorption spectra, Mössbauer spectra and CO desorption all suggested an essentially identical nature of Fe sites in both monometallic Fe-NC and bimetallic FeNi-NC; however, Ni blocks the formation of active sites during the pyrolysis and thus causes a sharp reduction in the accessible metal site density, while with only a minor direct participation as a catalytic site in the final catalyst. We also use the site density utilization factor, ϕ SDsurface/bulk , as a measure of the metal site dispersion in PGM-free ORR catalysts. ϕ SDsurface/bulk enables a quantitative evaluation and comparison of distinct catalyst synthesis routes in terms of their ratio of accessible metal sites. It gives guidance for further optimization of the accessible site density of M-NC catalysts.

Rare-Earth Metals-Doped Nickel Aluminate Spinels for Photocatalytic Degradation of Organic Pollutants

Visible-light-activated photocatalysts based on samarium-doped, europium-doped, and gadolinium-doped nickel aluminates (SmNA, EuNA, GdNA) were synthesized. The spinel crystalline structures of the doped mixed metal oxides were demonstrated by X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis. The presence of the rare-earth metals (REMs) was confirmed by the energy-dispersive X-ray (EDX) studies. Ultraviolet-visible-near-infrared (UV-Vis-NIR) spectra revealed that the REMs-doped catalysts absorb in the full solar spectrum range covering both visible and near infrared wavelengths. Scanning electron microscopy (SEM) visualized the profound morphological alterations of the doped nickel aluminate samples. Consequently, the pore volume and the Brunauer-Emmett-Teller (BET) surface area decreased, while nanoparticles sizes increased. Fourier-transform infrared spectroscopy (FTIR) exposed that surfaces of REMs-doped nickel aluminates are rich in hydroxyl groups. Finally, the photocatalytic performance was notably increased through doping nickel aluminate (NA) with REMs; the highest activity was observed for EuNA.

In situ X-ray emission and high-resolution X-ray absorption spectroscopy applied to Ni-based bimetallic dry methane reforming catalysts

The promoting effect of cobalt on the catalytic activity of a NiCoO Dry Methane Reforming (DMR) catalyst was studied by a combination of in situ Kβ X-ray Emission Spectroscopy (XES) and Kβ-detected High Energy Resolution Fluorescence Detected X-ray absorption spectroscopy (HERFD XAS). Following the calcination process, Ni XES and Kβ-detected HERFD XAS data revealed that the NiO coordination in the NiCoO catalyst has a higher degree of symmetry and is different than that of pure NiO/γ-Al₂O₃. Following the reductive activation, it was found that the NiCoO/γ-Al₂O₃ catalyst required a relatively higher temperature compared to the monometallic NiO/γ-Al₂O₃ catalyst. This finding suggests that Co is hampering the reduction of Ni in the NiCoO catalyst by modulation of its electronic structure. It has also been previously shown that the addition of Co enhances the DMR activity. Further, the Kβ XES spectrum of the partly reduced catalysts at 450 °C reveals that the Ni sites in the NiCoO catalyst are electronically different from the NiO catalyst. The in situ X-ray spectroscopic study demonstrates that reduced metallic Co and Ni are the primary species present after reduction and are preserved under DMR conditions. However, the NiCo catalyst appears to always be somewhat more oxidized than the Ni-only species, suggesting that the presence of cobalt modulates the Ni electronic structure. The electronic structural modulations resulting from the presence of Co may be the key to the increased activity of the NiCo catalyst relative to the Ni-only catalyst. This study emphasizes the potential of in situ X-ray spectroscopy experiments for probing the electronic structure of catalytic materials during activation and under operating conditions.

Understanding Solid-Gas Reaction Mechanisms by Operando Soft X-Ray Absorption Spectroscopy at Ambient Pressure

Ambient-pressure operando soft X-ray absorption spectroscopy (soft-XAS) was applied to study the reactivity of hydroxylated SnO₂ nanoparticles toward reducing gases. H₂ was first used as a test case, showing that the gas phase and surface states can be simultaneously probed: Soft-XAS at the O K-edge gains sensitivity toward the gas phase, while at the Sn M_4,5-edges, tin surface states are explicitly probed. Results obtained by flowing hydrocarbons (CH₄ and CH₃CHCH₂) unequivocally show that these gases react with surface hydroxyl groups to produce water without producing carbon oxides and release electrons that localize on Sn to eventually form SnO. The partially reduced SnO_2 - x layer at the surface of SnO₂ is readily reoxidized to SnO₂ by treating the sample with O₂ at mild temperatures (>200 °C), revealing the nature of "electron sponge" of tin oxide. The experiments, combined with DFT calculations, allowed devising of a mechanism for dissociative hydrocarbon adsorption on SnO₂, involving direct reduction of Sn sites at the surface via cleavage of C-H bonds and the formation of methoxy- and/or methyl-tin species at the surface.

In Situ X-ray Microscopy Reveals Particle Dynamics in a NiCo Dry Methane Reforming Catalyst under Operating Conditions

Herein, we report the synthesis of a γ-Al₂O₃-supported NiCo catalyst for dry methane reforming (DMR) and study the catalyst using in situ scanning transmission X-ray microscopy (STXM) during the reduction (activation step) and under reaction conditions. During the reduction process, the NiCo alloy particles undergo elemental segregation with Co migrating toward the center of the catalyst particles and Ni migrating to the outer surfaces. Under DMR conditions, the segregated structure is maintained, thus hinting at the importance of this structure to optimal catalytic functions. Finally, the formation of Ni-rich branches on the surface of the particles is observed during DMR, suggesting that the loss of Ni from the outer shell may play a role in the reduced stability and hence catalyst deactivation. These findings provide insights into the morphological and electronic structural changes that occur in a NiCo-based catalyst during DMR. Further, this study emphasizes the need to study catalysts under operating conditions in order to elucidate material dynamics during the reaction.

Stabilization by Configurational Entropy of the Cu(II) Active Site during CO Oxidation on Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O

The mechanisms of CO oxidation on the Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2O high-entropy oxide were studied by means of operando soft X-ray absorption spectroscopy. We found that Cu is the active metal and that Cu(II) can be rapidly reduced to Cu(I) by CO when the temperature is higher than 130 °C. Co and Ni do not have any role in this respect. The Cu(II) oxidation state can be easily but slowly recovered by treatment of the sample with O₂ at ca. 250 °C. However, it should be noted that CuO is readily and irreversibly reduced to Cu(I) when it is treated with CO at T > 100 °C. Thus, the main conclusion of this work is that the high configurational entropy of Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2O stabilizes the rock-salt structure and permits the oxidation/reduction of Cu to be reversible, thus permitting the catalytic cycle to take place.