X-ray Adsorption Spectroscopy Research Articles

Precise modulation of surface lattice to reinforce structural stability of high-nickel layered oxide cathode by hafnium gradient doping

The quest for high-energy lithium-ion batteries has intensified interest in high-nickel layered oxide cathode materials, while the rise in nickel content adversely impacts structural stability and cycling performance. Herein, Hafnium (Hf) anchored at nickel (Ni) site precisely modulates the surface crystal lattice of high-nickel cathode (LiNi0.9Co0.05Mn0.05O2, NCM90) that is confirmed by Rietveld refinement of neutron powder diffraction. Notably, the strengthened structural stability thanks to Hf gradient doping is firmly substantiated by in-situ X-ray diffraction and in-situ X-ray adsorption spectroscopy. This enhancement can be ascribed to the substantial atomic mass of Hf and large radius of Hf4+, facilitating the minimization of potential alterations of interlayer or intralayer. The intensified Hf-O bond significantly increases the formation energy of oxygen vacancies and efficiently restrains the detrimental Li+/Ni2+ disorder, thereby mitigating lattice oxygen loss and enhancing both cycling and thermal stability of high-nickel cathode. Consequently, the 0.3 % Hf-doping cathode exhibits considerable improvement in electrochemical performance, delivering superior capacity retention of 95.0% after 100 cycles at 1C (1 C = 200 mA g−1) and 83.9 % after 500 cycles at 1C/5C in the voltage range of 3.0∼4.3 V. This work highlights the benefits of precise modulation strategy for high-nickel cathodes, offering a viable approach to enhance structural stability and electrochemical performance of high-nickel cathodes.

Developing high-power Li||S batteries via transition metal/carbon nanocomposite electrocatalyst engineering.

The activity of electrocatalysts for the sulfur reduction reaction (SRR) can be represented using volcano plots, which describe specific thermodynamic trends. However, a kinetic trend that describes the SRR at high current rates is not yet available, limiting our understanding of kinetics variations and hindering the development of high-power Li||S batteries. Here, using Le Chatelier's principle as a guideline, we establish an SRR kinetic trend that correlates polysulfide concentrations with kinetic currents. Synchrotron X-ray adsorption spectroscopy measurements and molecular orbital computations reveal the role of orbital occupancy in transition metal-based catalysts in determining polysulfide concentrations and thus SRR kinetic predictions. Using the kinetic trend, we design a nanocomposite electrocatalyst that comprises a carbon material and CoZn clusters. When the electrocatalyst is used in a sulfur-based positive electrode (5 mg cm-2 of S loading), the corresponding Li||S coin cell (with an electrolyte:S mass ratio of 4.8) can be cycled for 1,000 cycles at 8 C (that is, 13.4 A gS-1, based on the mass of sulfur) and 25 °C. This cell demonstrates a discharge capacity retention of about 75% (final discharge capacity of 500 mAh gS-1) corresponding to an initial specific power of 26,120 W kgS-1 and specific energy of 1,306 Wh kgS-1.

Ru Single Atoms Integrated into Cobalt Oxide Spinel Structure with Interstitial Carbon for Enhanced Electrocatalytic Water Oxidation.

Oxygen evolution reaction (OER) plays a critical role in energy conversion technologies. Significant progress has been made in alkaline conditions. In contrast, it remains a challenge to develop stable OER electrocatalysts in acidic conditions. Herein, a new strategy is reported to stabilize single atoms integrated into cobalt oxide spinel structure with interstitial carbon (Ru0.27Co2.73O4), where the optimized Ru0.27Co2.73O4 exhibits a low overpotential of 265, 326, and 367mV to reach a current density of 10, 50, and 100mA cm2, respectively. More importantly, Ru0.27Co2.73O4 has long-term stability of up to 100h, representing one of the most stable OER electrocatalysts. X-ray adsorption spectroscopy (XAS) characterization and density functional theory (DFT) calculations jointly demonstrate that the significant catalytic performance of Ru0.27Co2.73O4 is due to the synergistic effect between the Ru and Co sites and the bridging O ligands, as well as the significant reduction of the OER energy barrier. This work provides a new perspective for designing and constructing efficient non-noble metal-based electrocatalysts for water splitting.

Revealing proton-coupled exchange mechanism in aqueous ion-exchange synthesis of nickel-rich layered cathodes for lithium-ion batteries

Ion exchange is a promising synthetic method for alleviating severe cation mixing in traditional layered oxide materials for lithium-ion batteries, leading to enhanced structural stability. However, the underlying mechanisms of ion exchange are still not fully understood. Such a fundamental study of the ion-exchange mechanism is needed for achieving the controllable synthesis of layered oxides with a stable structure. Herein, we thoroughly unearth the underlying mechanism that triggers the ion exchange of Ni-rich materials in aqueous solutions by examining time-resolved structural evolution combined with theoretical calculations. Our results reveal that the reaction pathway of ion exchange can be divided into two steps: protonation and lithiation. The proton is the key to achieving charge balance in the ion exchange process, as revealed by X-ray adsorption spectroscopy and inductive coupled plasma analysis. In addition, the intermediate product shows high lattice distortion during ion exchange, but it ends up with a most stable product with high lattice energy. Such apparent discrepancies in lattice energy between materials before and after ion exchange emphasize the importance of synthetic design in structural stability. This work provides new insights into the ion-exchange synthesis of Ni-rich oxide materials, which advances the development of cathode materials for high-performance lithium-ion batteries.

Revealing Aqueous-Processing Ni-Rich Cathode Cycling Performance in Term of Cathode Electrolyte Layer Formation and Phase Transition

Traditionally, Ni-rich cathodes for lithium ion batteries are produced by utilizing N-methyl-2-pyrrolidone (NMP) based casting. However, in order to prevent using the harmful solvent NMP, aqueous-processing method becomes one of the options.1 In this study, water-based electrodes have been produced to compare with the NMP-based electrodes. In addition to that, phosphoric acid addition was involved to modify the water-based electrode surface to improve the capacity retention.2 We used X-ray adsorption spectroscopy (XAS) to observe the existence of NiO phase in the aqueous processed electrodes due to Li/Ni disorder. The Galvanostatic cycling results shows that aqueous processed electrodes provide worse capacity retention compared with the NMP processed electrodes because of higher electron transfer resistance, which is caused by the cathode electrolyte layer (CEI) layer formation. Soft X-ray photoelectron spectroscopy (PES) results show that comparing to NMP-based electrodes, water-based electrodes contains more decomposition species from electrolyte in CEI layer after first battery cycle. From Operando X-ray diffraction, we observed the phase transition differences between these three types of electrodes. In this study we aim to answer if and/or how the additive phosphoric acid participates in the electrolyte degradation during battery cycling. A deeper understanding of the roles of acid can provide a new path to develop aqueous-processing electrodes method. Wu, F.; Kuenzel, M.; Diemant, T.; Mullaliu, A.; Fang, S.; Kim, J. K.; Kim, H. W.; Kim, G. T.; Passerini, S., Enabling High‐Stability of Aqueous‐Processed Nickel‐Rich Positive Electrodes in Lithium Metal Batteries. Small 2022, 2203874.Bichon, M.; Sotta, D.; De Vito, E.; Porcher, W.; Lestriez, B., Performance and ageing behavior of water-processed LiNi0.5Mn0.3Co0.2O2/Graphite lithium-ion cells. Journal of Power Sources 2021, 483.

Pyrolysis Free Out-of-Plane Co-Single Atomic Sites in Porous Organic Photopolymer Stimulates Solar-Powered CO2 Fixation.

Herein, a facile strategy is illustrated to develop pyrolysis-free out-of-plane coordinated single atomic sites-based M-POP via a one-pot Friedel Craft acylation route followed by a post-synthetic metalation. The optimized geometry of the Co@BiPy-POP clearly reveals the presence of out-of-plane Co-single atomic sites in the porous backbone. This novel photopolymer Co@BiPy-POP shows extensive π-conjugations followed by impressive light harvesting ability and is utilized for photochemical CO2 fixation to value-added chemicals. A remarkable conversion of styrene epoxide (STE) to styrene carbonate (STC) (≈98%) is obtained under optimized photocatalytic conditions in the existence of promoter tert-butyl ammonium bromide (TBAB). Synchrotron-based X-ray adsorption spectroscopy (XAS) analysis reveals the single atom coordination sites along with the metal (Co) oxidation number of +2.16 in the porous network. Moreover, in situ diffuse reflectance spectroscopy (DRIFTS) and electron paramagnetic resonance (EPR) investigations provide valuable information on the evolution of key reaction intermediates. Comprehensivecomputational analysis also helps to understand the overall mechanistic pathway along with the interaction between the photocatalyst and reactants. Overall, this study presents a new concept of fabricating porous photopolymers based on a pyrolysis-free out-of-plane-coordination strategy and further explores the role of single atomic sites in carrying out feasible CO2 fixation reactions.

Chiolite Na5Ti3F14: A novel sodium titanium fluoride anode for low-cost and high-performance Na-ion batteries

Fluoride-based conversion reaction electrode materials offer exceptional theoretical capacity merit for Na-ion batteries. Nevertheless, it has rarely been considered as potential anode material candidate due to (i) excessive redox potential (> 3 V) and (ii) intrinsically low reaction kinetics related to sluggish structural reorganization process. In this work, we demonstrate that chiolite Na5Ti3F14/carbon nanocomposite can deliver the outstanding electrochemical performances as the promising anode for Na-ion batteries, such as a large specific capacity of ∼425 mAh g−1 at 10 mA g−1 with a low average operating voltage, the capacity retention of ∼78 % compared to the initial capacity after 300 cycle with a high Coulombic efficiency of above 99 %, etc. We demonstrate that the chiolite Na5Ti3F14 phase can store the ∼8.33 mol Na ions through the following conversion reaction; Na5Ti3F14 + 9Na + 9e−⇔ 3Ti + 14NaF, which is clearly confirmed by various ex-situ analyses using X-ray diffraction, synchrotron-based X-ray adsorption spectroscopy, etc. We expect that this research can provide guidance toward the development of a new class of low-cost and high-performance anode materials, not only for Na-ion batteries but also for other rechargeable batteries.

Unravelling the Structure Change and Its Catalytic Consequence of CuO Nanowires Towards CO2 Reduction Reaction

The CO2 reduction reaction (CO2RR) is a complex process with many possible products, and maintaining high selectivity for multi-carbon products remains a challenge. Currently, Cu-based catalysts are the only transition metal catalysts that can produce C2 products, and one of the popular Cu-based catalysts is CuO nanowires (NWs). We studied a series of CuO nanowires ranging from 50 to 200 nm for CO2RR using multiple characterization techniques including transmission electron microscopy(TEM), small angle X-ray scattering (SAXS) and in situ X-ray adsorption spectroscopy. We found that there could be an activation process required for such CuO NWs to serve as efficient CO2RR catalysts. The CuO can go through multiple steps resulting in the severe morphology changes during the activation process, confirmed by TEM and SAXS. In situ XAS results showed that the CuO NWs will be reduced from Cu(II) to Cu(0) during the reaction, consistent with our previous work(1). H-cell testing results also indicated the C2 production can be boosted after specific activation pathway, which highlights the importance of catalyst pre-treatment. These findings not only deepen our understanding of CO2RR mechanism but also provide fundamental insights for CO2RR catalysts synthesis. References (1) Soo Hong Lee, Ian Sullivan, David M. Larson, Guiji Liu, Francesca M. Toma, Chengxiang Xiang, and Walter S. Drisdell. ACS Catalysis, 2020, 10(14), 8000-8011 Acknowledgement The authors acknowledge the funding support from the Laboratory Directed Research and Development Program of the Lawrence Berkeley National Laboratory under the U.S. Department of Energy Contract (No. DE-AC02-05CH11231).

Promotion effects of alkali metals on iron molybdate catalysts for CO2 catalytic hydrogenation

CO2 hydrogenation is an attractive way to store and utilize carbon dioxide generated by industrial processes, as well as to produce valuable chemicals from renewable and abundant resources. Iron catalysts are commonly used for the hydrogenation of carbon oxides to hydrocarbons. Iron-molybdenum catalysts have found numerous applications in catalysis, but have been never evaluated in the CO2 hydrogenation. In this work, the structural properties of iron-molybdenum catalysts without and with a promoting alkali metal (Li, Na, K, Rb, or Cs) were characterized using X-ray diffraction, hydrogen temperature-programmed reduction, CO2 temperature-programmed desorption, in-situ 57Fe Mossbauer spectroscopy and operando X-ray adsorption spectroscopy. Their catalytic performance was evaluated in the CO2 hydrogenation. During the reaction conditions, the catalysts undergo the formation of an iron (II) molybdate structure, accompanied by a partial reduction of molybdenum and carbidization of iron. The rate of CO2 conversion and product selectivity strongly depend on the promoting alkali metals, and electronegativity was identified as an important factor affecting the catalytic performance. Higher CO2 conversion rates were observed with the promoters having higher electronegativity, while low electronegativity of alkali metals favors higher light olefin selectivity.

Modulating the Oxygen Reduction Selectivity in Pt or Pd Chalcogenides via the Ensemble Effect and Electronic Effect.

The atomic ensemble effect will affect the catalysis on catalyst surfaces and determine the selectivity of multielectron reactions, which is an effective potential strategy to modulate the oxygen reduction reaction (ORR) selectivity for hydrogen peroxide (H2O2) production. Here, we reported the investigation of the ensemble effect on Pt/Pd chalcogenides for the two-electron ORR. A series of Pt/Pd chalcogenides were synthesized via introducing chalcogens into Pt/Pd, resulting in catalysts with isolated Pt/Pd active site. X-ray adsorption spectroscopy reveals the change of the electronic structure. Their ORR selectivity transformed from a four-electron process to a two-electron one, which was attributed to the isolated active sites changing the adsorption mode and the tunable electronic property weakening the adsorption energy. Density functional theory calculations revealed that Pt/Pd chalcogenides possessed a lower binding energy of OOH* and inhibited the breakage of the O-O bond, and PtSe2/C with an optimal adsorption energy of OOH* showed 91% selectivity toward H2O2 production. This work provides a design principle for synthesizing highly selective Pt group catalysts for H2O2 production.

Molecular mechanisms of pyrophosphate-mediated selective adsorption of Ni(II) by polyamine-grafted chitosan–wood flour composite from alkaline plating wastewater: EXAFS investigation and theoretical calculations

The efficient removal of complexed heavy metals (HMs) from alkaline electroless plating wastewater is still challenging. A polyamine-grafted chitosan–wood flour-based composite (ACS-AWF) was optimally synthesized. Its adsorption behavior toward Ni(II) in simulated pyrophosphate (PP, the main complextant)–Ni plating wastewater and underlying mechanisms were examined. PP considerably promoted Ni(II) adsorption by up to 108.1%, and the enhancement tolerated excess PP or other substances. Ni(II) adsorption by ACS-AWF was 1.38–∼10 times that of several commercial HM-favored resins in the same complexed system. According to the liquid solute species tracking, solid chemical characterizations (eg. synchrotron radiation X-ray adsorption spectroscopy), and theoretical calculations, it was found that Ni(II), primarily as NiP2O72−, could be captured by charge-assistant coordination with the surface polyamine. Then, the coadsorption of PP altered the interface properties via both electrostatic shielding and bridging effect, enhancing Ni(II)–amine coordination efficiency. A dinuclear multilayer coordination structure [-amine–Ni–PP–Ni–amine-] was proposed as the dominant Ni(II) binding mode in the presence of PP. Ni(II) and PP could be sequentially desorbed from ACS-AWF using gradient elution with ＞95.5% Ni(II) purity. ACS-AWF was then stably reused in five cycles (capacity loss ＜10%). These results may guide adsorbent design and application in HM purification and recycling from complexed wastewater.

Activation of peroxymonosulfate by FeVO3-x for the degradation of carbamazepine: Vanadium mediated electron shuttle and oxygen vacancy modulated interface chemistry

Fast Fe(III)/Fe(II) circulation in heterogeneous peroxymonosulfate (PMS) activation remains as a bottleneck issue that restricts the development of PMS based advanced oxidation processes. Herein, we proposed a facile ammonia reduction strategy and synthesized a novel FeVO3-x catalysts to activate PMS for the degradation of a typical pharmaceutical, carbamazepine (CBZ). Rapid CBZ removal could be achieved within 10 min, which outperforms most of the other iron or vanadium-based catalysts. Electron paramagnetic resonance analysis and chemical probe experiments revealed SO4•−, •OH, O2•− and high valent iron (Fe(IV)) were all generated in this system, but SO4•− and Fe(IV) primarily contributed to the degradation of CBZ. Besides, X-ray photoelectron spectroscopy and X-ray adsorption spectroscopy indicated that both the generated low-valent V provides and oxygen vacancy acted as superior electron donors and accelerated internal electron transfer via the unsaturated V−O−Fe bond. Finally, the proposed system also exhibited satisfactory performance in practical applications. This work provides a promising platform in heterogeneous PMS activation.

Electronic signatures of Lorentzian dynamics and charge fluctuations in lithiated graphite structures

Lithium graphite intercalation compounds (Li-GICs) are essential materials for modern day portable electronics and obtaining insights into their atomic structure and thermodynamics is of fundamental interest. Here we explore the electronic and atomic states of Li-GICs at varying degrees of Lithium loading (i.e., “staging”) by means of ab-initio molecular dynamics simulations and simulated X-ray adsorption spectroscopy (XAS). We analyze the atomic correlation functions and shows that the enhancements of the Li-ion entropy with increased staging result from Lorentzian lithium-ion dynamics and charge fluctuations, which activate low-energy phonon modes. The associated electronic signatures are modulations of the unoccupied π*/σ* orbital energy levels and unambiguous fingerprints in Carbon K-edge XAS spectra. Thus, we extend the canonical view of XAS, establishing that these “static” measurements in fact encode the signature of the thermodynamic response and relaxation dynamics of the system. This causal link between atomic structure, spectroscopy, thermodynamics, and information theory can be generally exploited to better understand stability in solid-state electrochemical systems.

Low-voltage anodizing of copper in sodium bicarbonate solutions

The low-voltage (< 5 V) anodization of copper in aqueous solutions of sodium bicarbonate was studied for the first time. As demonstrated, this method leads to the formation of microstructures on a copper surface, that are composed of malachite (CuCO3·Cu(OH)2). Moreover, by tuning the operating conditions, i.e., applied cell voltage and electrolyte concentration, different surface morphologies can be grown. As shown by electron microscopy investigation, clusters of ribbons corrosion pits or nonuniformly located powdery precipitates are formed when the low anodizing voltage is applied. Anodization at 1.0 V in 0.4 M sodium bicarbonate solution led to the formation of a velvet-like, deep black anodic layer that covered the whole metal surface with ribbon-resembling structures. A thorough investigation of the obtained anodic layers with X-ray diffraction (XRD), X-ray adsorption (XAS), Raman, and X-ray Photoelectron Spectroscopy (XPS) uncovered the mixed crystalline-amorphous nature of the anodic copper species. Besides dominating the crystalline malachite phase, the amorphous cupric oxide was also identified. This composition offers promising features for catalytic applications, hence, low-voltage anodized copper was tested in an electrochemical CO2 reduction reaction to explore one possible application of the presented material. The current density of 4.7 mA cm−2 was registered for the selected sample.

Heterointerface and Tensile Strain Effects Synergistically Enhances Overall Water-Splitting in Ru/RuO2 Aerogels.

Designing robust electrocatalysts for water-splitting is essential for sustainable hydrogen generation, yet difficult to accomplish. In this study, a fast and facile two-step technique to synthesize Ru/RuO2 aerogels for catalyzing overall water-splitting under alkaline conditions is reported. Benefiting from the synergistic combination of high porosity, heterointerface, and tensile strain effects, the Ru/RuO2 aerogel exhibits low overpotential for oxygen evolution reaction (189mV) and hydrogen evolution reaction (34mV) at 10mA cm-2 , surpassing RuO2 (338mV) and Pt/C (53mV), respectively. Notably, when the Ru/RuO2 aerogels are applied at the anode and cathode, the resultant water-splitting cell reflected a low potential of 1.47V at 10mAcm-2 , exceeding the commercial Pt/C||RuO2 standard (1.63V). X-ray adsorption spectroscopy and theoretical studies demonstrate that the heterointerface of Ru/RuO2 optimizes charge redistribution, which reduces the energy barriers for hydrogen and oxygen intermediates, thereby enhancing oxygen and hydrogen evolution reaction kinetics.

Lattice strain and interfacial engineering of a Bi-based electrocatalyst for highly selective CO2 electroreduction to formate

Surface strain tuning in a coupled heterostructure efficiently engineers the catalytic performance of heterogeneous catalysts by altering the electronic structures and boosting electron transport. Generally, Bi-based catalysts are more favorable than ZnO for CO2 electroreduction to formate, but Bi is much more costly than Zn. Herein, a new Bi2O2CO3/ZnO heterojunction catalyst with porous nanoplate morphology is synthesized through a hexadecyl trimethyl ammonium bromide-templated hydrothermal reaction for a highly efficient catalytic CO2 reduction reaction (CO2RR) to produce formate. The Bi2O2CO3/ZnO catalyst shows a maximum Faradaic efficiency of 92% for formate production at −1.0 V vs. reversible hydrogen electrode (RHE) and a large partial current density of −200 mA mgBi−1 at −1.2 V vs. RHE. More importantly, the mass activity of Bi2O2CO3/ZnO normalized by Bi mass is an approximately 3.1-fold enhancement over that of the pristine Bi2O2CO3 at −1.2 V vs. RHE. By coupling X-ray photoelectron spectroscopy and adsorption spectroscopy measurements, the charge transfer from the Zn atom to the Bi atom through a heterogeneous interface results in an electron-enriched Bi2O2CO3 surface, which facilitates CO2 capture and activation. Meanwhile, compressive stress produced on the catalyst surface helps optimize the adsorption energy of the reaction intermediate, synergistically enhancing the catalytic selectivity and activity of Bi2O2CO3/ZnO for electrochemical CO2 reduction to formate.

Atomic-scale interfacial engineering enables high-performance electrochemical glucose detection

Developing noble-metal free electrocatalysts with high sensitivity is critical for the large-scale application of electrochemical glucose sensors. This work reports an atomic-scale interfacial engineering strategy to construct highly-active electrocatalyst for glucose detection. Ni(OH)2 nanosheets are decorated with an ultra-thin layer of Co9S8 using the atomic layer deposition (ALD) technique. After in-situ reconstruction, we obtain Ni@Co heterostructure composed of Ni hydroxide nanosheets and CoOx clusters, which exhibits outstanding electrochemical glucose sensing performance. Combining synchrotron-based X-ray adsorption spectroscopy, in situ Raman Spectroscopy, intermittent electrochemical measurements and density functional theory (DFT) calculations, we find that the presence of surface CoOx not only lowers the valence state of Ni, but also facilitates the deprotonation of Ni(OH)2 to form NiOOH active species for glucose oxidation. The approach used in this work can be adapted to synthesizing high-performance electrocatalysts for other energy and environmental devices.

Rational design of thermally stable polymorphic layered cathode materials for next generation lithium rechargeable batteries

Classical layered transition metal oxides have remained the preferred cathode materials for commercial lithium-ion batteries. Variation in the transition metal composition and local ordering can greatly affect the structure stability. In classical layered cathodes, high concentrations of electrochemically inert Mn elements usually act as a pillar to stabilize the structure. When excess amount of Li and Mn are present in the layered structure, the capacity of the Li-rich layered oxide (molar ratio of lithium over transition metal is larger than one by design) can exceed that expected from transition metal redox. However, the over lithiation in the classical layered structure results in safety issues, which remains challenging for the commercialization of Li-rich layered oxides. To characterize the safety performance of a series of Li-rich layered cathodes, we utilize differential scanning calorimeter and thermal gravimetric analysis; this is coupled with local structural changes using in situ temperature dependent synchrotron X-ray diffraction and X-ray adsorption spectroscopy. These methods demonstrate that the gradual decrease of the Mn–M (M = Ni, Co, Mn and Li) coordination number directly reduces structural stability and accelerates oxygen release. For safety characterization tests in practice, we evaluate the thermal runaway process through accelerating rate calorimeter in 1.0 Ah pouch cells to confirm this trend. Using the insights obtained in this work, we design a polymorphic composition to improve the thermal stability of Li-rich layered cathode material, which outperforms Ni-rich layered oxides in terms of both electrochemical and safety performances.

Understanding the Transition Process of Cu Oxide to Metallic Under the CO2 Reduction Conditions Probed By Operando Quick-XAS

During the last decades, significant efforts have been made to directly convert CO2 as a potential feedstock into hydrocarbons as fuels and/or basic chemicals for industrial applications. The electrochemical CO2 reduction reaction (CO2RR) is a promising alternative for a large-scale production of hydrocarbons. However, there are still some challenges including poor product selectivity and highly complex multiple-step reaction mechanisms.[1] In order to convert CO2 on a Cu electrode, high overpotentials up to 1.0 V are required, which make the energy efficiency still rather poor and lead to competition with the H2 evolution.[2] Additionally, the morphology of the Cu materials i.e. structure and ‘chemical state’ (metallic vs oxidized, high vs low coordinated) strongly influence the performance of the CO2RR. Recently, we have shown the critical potential of oxide-metal transition processes for Cu oxide foam annealed at 300°C probed by operando XAS, XRD and Raman spectroscopy.[3,4] All three operando techniques showed an entire reduction of Cu oxide to metallic before the production of hydrocarbons starts. [3,4]In this work, we have investigated the kinetics of both electrochemical oxide-metal reduction and CO2RR on nanoporous Cu foams as catalyst precursor annealed at four different temperatures (100°C, 200°C, 300°C, 450°C) in air using operando Quick X-ray Adsorption Spectroscopy (Quick-XAS). The Quick-XAS measurements were carried out in a custom-made spectro-electrochemical flow cell using 0.5 M KHCO3 as the electrolyte, while the XAS-Spectra were measured in transmission mode. The Quick-XANES data was analyzed by linear combination fit (LCF) and principle component analysis (PCA) to monitor the potential dependent changes of the chemical state and coordination number of the Cu species. Based on the Cu K-edge XANES and EXAFS data, we show that the annealing temperature strongly influences the chemical state of the Cu species. More precisely, the population of the Cu(II) species within the as prepared foams increases with increasing annealing temperature. Starting from the different ratios of Cu(0):Cu(I): Cu(II), the oxide-metal transition processes are shifted in the cathodic direction by applying potential steps of 100 mV. With an increase in annealing temperature, this oxide-metal transition is more rapid and occurs at lower cathodic overpotentials, but still before the production of hydrocarbons begins. In contrast, the potential jump experiments of several hundreds of mV lead to different kinetics of the oxide-metal reduction of Cu species. These transition processes and the resulting structure of porous Cu foams have a huge impact on the product distribution for CO2RR. Altogether, our results provide deeper insights into the oxide-metal transition processes to form the catalytically active Cu species for hydrocarbon formation during CO2RR.Reference:[1] S. Nitopi et al., Chemical Reviews, 2019, 119, 7610. [2] C. W. Li et al., Journal of the American Chemical Society, 2012, 134, 7231. [3] A. Dutta et al., Chimia, 2021, 75, 733. [4] A. Dutta et al., Journal of Catalysis, 2020, 389, 592.

Interface engineering of Ni/NiO hierarchical heterojunction for oxygen evolution reaction: A combined experimental and theoretical study

NiO is widely considered as one of most forceful noble-metal-free oxygen evolution reaction (OER) catalysts due to its high intrinsic catalytic activity and robust stability. However, the unsatisfactory conductivity has become a stumbling block for further improving its catalytic performance. Here, a Ni/NiO hierarchical heterojunction is synthesized by an interface engineering strategy. The developed strategy endows the electrocatalysts adequately exposed catalytic active sites, optimized kinetics, including rapid charge and mass transfer, bubble release, and reduced intermediate adsorption free energy. Therefore, the obtained Ni/NiO150 catalyst displays an overpotential of 280 mV to reach a current density of 10 mA cm−2 and a small Tafel slope of 67 mV dec−1. In addition, Ni/NiO150 exhibits a negligible current attenuation after 25 h amperometric operation, indicating outstanding stability. Furthermore, the synchrotron X-ray adsorption spectroscopy results demonstrate that, the introduction of metal Ni phase could induce electronic interaction at the heterojunction interface, which plays a crucial role in the catalytic performance enhancement. Further, density functional theory calculations were carried out to reveal the mechanism of optimized conductivity and catalytic activity enhancement caused by interface effect. This work offers a generic interface engineering strategy for developing high-performance and cost-effective electrocatalysts toward OER, water splitting, and beyond.