Search

Electrochemical CO2 reduction is a potential approach to convert CO2 into valuable chemicals using electricity as feedstock. Abundant and affordable catalyst materials are needed to upscale this process in a sustainable manner. Nickel‐nitrogen‐doped carbon (Ni‐N‐C) is an efficient catalyst for CO2 reduction to CO, and the single‐site Ni−Nx motif is believed to be the active site. However, critical metrics for its catalytic activity, such as active site density and intrinsic turnover frequency, so far lack systematic discussion. In this work, we prepared a set of covalent organic framework (COF)‐derived Ni‐N‐C catalysts, for which the Ni−Nx content could be adjusted by the pyrolysis temperature. The combination of high‐angle annular dark‐field scanning transmission electron microscopy and extended X‐ray absorption fine structure evidenced the presence of Ni single‐sites, and quantitative X‐ray photoemission addressed the relation between active site density and turnover frequency. [ABSTRACT FROM AUTHOR]

Sub 10 nm cobalt ferrite CoxFe3−xO4 (x ≤ 1.75) nanoparticles and cobalt-rich wüstite (Cox/3Fe(1−x)/3)O nanoparticles (x ≥ 2) were synthesized in a solvothermal approach and characterized by powder X-ray diffraction (PXRD), selected area electron diffraction (SAED), transmission electron microscopy (TEM) as well as energy dispersive X-ray spectroscopy (EDX), IR, Raman, and 57Fe-Mössbauer spectroscopy. Their electrocatalytic activity in the oxygen evolution reaction (OER) was evaluated and the active state formation was tracked by operando X-ray absorption spectroscopy (XAS). Our studies demonstrate that the cobalt-rich wüstite (Cox/3Fe(1−x)/3)O nanoparticles underwent a phase-transformation into the spinels CoxFe3−xO4 (x ≥ 2) under the applied OER conditions. The overpotential η10 at 10 mA cm−2, serving as a benchmark for the OER activity of the cobalt ferrite nanoparticles in alkaline media, was lower than that of magnetite Fe3O4 even with low cobalt concentrations, reaching a minimum of 350 mV for Co2.25Fe0.75O4 with a Tafel slope of 50 mV dec−1. Finally, we identified that the catalytic activity is linked to the nanoparticle size as well as to the degree of Co redox activity and change in coordination during OER. [ABSTRACT FROM AUTHOR]

This study presents the impact of the side chains in 1-n-alkylimidazolium ionomers with varying side chain lengths (C n H 2n+1 where n = 1, 4, 10, 16) on Cu-catalyzed electrochemical CO 2 reduction reaction (CO 2 RR). Longer side chains suppress the H 2 and CH 4 formation, with the n-hexadecyl ionomer (n = 16) showing the greatest reduction in kinetics by up to 56.5% and 60.0%, respectively. On the other hand, C 2 H 4 production demonstrates optimal Faradaic efficiency with the n-decyl ionomer (n = 10), a substantial increase of 59.9% compared to its methyl analog (n = 1). Through a combination of density functional theory calculations and material characterization, it is revealed that the engineering of the side chains effectively modulates the thermodynamic stability of key intermediates, thus influencing the selectivity of both CO 2 RR and hydrogen evolution reaction. Moreover, ionomer engineering enables industrially relevant partial current density of -209.5 mA cm -2 and a Faradaic efficiency of 52.4% for C 2 H 4 production at 3.95 V, even with a moderately active Cu catalyst, outperforming previous benchmarks and allowing for further improvement through catalyst engineering. This study underscores the critical role of ionomers in CO 2 RR, providing insights into their optimal design for sustainable chemical synthesis., (© 2024 The Author(s). Advanced Science published by Wiley‐VCH GmbH.)

Pulsed CO 2 electroreduction (CO 2 RR) has recently emerged as a facile way to in situ tune the product selectivity, in particular toward ethanol, without re-designing the catalytic system. However, in-depth mechanistic understanding requires comprehensive operando time-resolved studies to identify the kinetics and dynamics of the electrocatalytic interface. Here, we track the adsorbates and the catalyst state of pre-reduced Cu 2 O nanocubes ( ~ 30 nm) during pulsed CO 2 RR using sub-second time-resolved operando Raman spectroscopy. By screening a variety of product-steering pulse length conditions, we unravel the critical role of co-adsorbed OH and CO on the Cu surface next to the oxidative formation of Cu-O ad or CuO x /(OH) y species, impacting the kinetics of CO adsorption and boosting the ethanol selectivity. However, a too low OH ad coverage following the formation of bulk-like Cu 2 O induces a significant increase in the C 1 selectivity, while a too high OH ad coverage poisons the surface for C-C coupling. Thus, we unveil the importance of co-adsorbed OH on the alcohol formation under CO 2 RR conditions and thereby, pave the way for improved catalyst design and operating conditions., (© 2024. The Author(s).)

The electrochemical reduction of nitrate (NO 3 - ) and nitrite (NO 2 - ) enables sustainable, carbon-neutral, and decentralized routes to produce ammonia (NH 3 ). Copper-based materials are promising electrocatalysts for NO x - conversion to NH 3 . However, the underlying reaction mechanisms and the role of different Cu species during the catalytic process are still poorly understood. Herein, by combining quasi in situ X-ray photoelectron spectroscopy (XPS) and operando X-ray absorption spectroscopy (XAS), we unveiled that Cu is mostly in metallic form during the highly selective reduction of NO 3 - /NO 2 - to NH 3 . On the contrary, Cu(I) species are predominant in a potential region where the two-electron reduction of NO 3 - to NO 2 - is the major reaction. Electrokinetic analysis and in situ Raman spectroscopy was also used to propose possible steps and intermediates leading to NO 2 - and NH 3 , respectively. This work establishes a correlation between the catalytic performance and the dynamic changes of the chemical state of Cu, and provides crucial mechanistic insights into the pathways for NO 3 - /NO 2 - electrocatalytic reduction.

A key challenge in electrocatalysis remains controlling a catalyst's structural, chemical, and electrical properties under reaction conditions. While organic coatings showed promise for enhancing the selectivity and stability of catalysts for CO 2 electroreduction (CO 2 RR), their impact on the chemical state of underlying metal electrodes has remained unclear. In this study, we show that organic thin films on polycrystalline copper (Cu) enable retaining Cu + species at reducing potentials down to -1.0 V vs RHE, as evidenced by operando Raman and quasi in situ X-ray photoelectron spectroscopy. In situ electrochemical atomic force microscopy revealed the integrity of the porous organic film and nearly unaltered Cu electrode morphology. While the pristine thin film enhances the CO 2 -to-ethylene conversion, the addition of organic modifiers into electrolytes gives rise to improved CO 2 RR performance stability. Our findings showcase hybrid metal-organic systems as a versatile approach to control, beyond morphology and local environment, the oxidation states of catalysts and energy conversion materials.

The production of green hydrogen through alkaline water electrolysis is the key technology for the future carbon-neutral industry. Nanocrystalline Co 3 O 4 catalysts are highly promising electrocatalysts for the oxygen evolution reaction and their activity strongly benefits from Fe surface decoration. However, limited knowledge of decisive catalyst motifs at the atomic level during oxygen evolution prevents their knowledge-driven optimization. Here, we employ a variety of operando spectroscopic methods to unveil how Fe decoration increases the catalytic activity of Co 3 O 4 nanocatalysts as well as steer the (near-surface) active state formation. Our study shows a link of the termination-dependent Fe decoration to the activity enhancement and a significantly stronger Co 3 O 4 near-surface (structural) adaptation under the reaction conditions. The near-surface Fe- and Co-O species accumulate an oxidative charge and undergo a reversible bond contraction during the catalytic process. Moreover, our work demonstrates the importance of low coordination surface sites on the Co 3 O 4 host to ensure an efficient Fe-induced activity enhancement, providing another puzzle piece to facilitate optimized catalyst design., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Electrochemical CO 2 reduction (CO 2 RR) is a rising technology, aiming to reduce the energy sector dependence on fossil fuels and to produce carbon-neutral raw materials. Metal-nitrogen-doped carbons (M-N-C) are emerging, cost-effective catalysts for this reaction; however, their long-term stability is a major issue. To overcome this, understanding their structural evolution is crucial, requiring systematic in-depth operando studies. Here a series of M-N-C catalysts (M = Fe, Sn, Cu, Co, Ni, Zn) is investigated using operando X-ray absorption spectroscopy. It is found that the Fe-N-C and Sn-N-C are prone to oxide clusters formation even before CO 2 RR. In contrast, the respective metal cations are singly dispersed in the as-prepared Cu-N-C, Co-N-C, Ni-N-C, and (Zn)-N-C. During CO 2 RR, metallic clusters/nanoparticles reversibly formed in all catalysts, except for the Ni-N-C. This phenomenon, previously observed only in Cu-N-C, thus is ubiquitous in M-N-C catalysts. The competition between M-O and M-N interactions is an important factor determining the mobility of metal species in M-N-C. Specifically, the strong interaction between the Ni centers and the N-functional groups of the carbon support results in higher stability of the Ni single-sites, leading to the excellent performance of Ni-N-C in the CO 2 to CO conversion, in comparison to other transition metals., (© 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

Electrochemical reduction of CO 2 (CO 2 RR) is an attractive technology to reintegrate the anthropogenic CO 2 back into the carbon cycle driven by a suitable catalyst. This study employs highly efficient multi-carbon (C 2+ ) producing Cu 2 O nanocubes (NCs) decorated with CO-selective Au nanoparticles (NPs) to investigate the correlation between a high CO surface concentration microenvironment and the catalytic performance. Structure, morphology and near-surface composition are studied via operando X-ray absorption spectroscopy and surface-enhanced Raman spectroscopy, operando high-energy X-ray diffraction as well as quasi in situ X-ray photoelectron spectroscopy. These operando studies show the continuous evolution of the local structure and chemical environment of our catalysts during reaction conditions. Along with its alloy formation, a CO-rich microenvironment as well as weakened average CO binding on the catalyst surface during CO 2 RR is detected. Linking these findings to the catalytic function, a complex compositional interplay between Au and Cu is revealed in which higher Au loadings primarily facilitate CO formation. Nonetheless, the strongest improvement in C 2+ formation appears for the lowest Au loadings, suggesting a beneficial role of the Au-Cu atomic interaction for the catalytic function in CO 2 RR. This study highlights the importance of site engineering and operando investigations to unveil the electrocatalyst's adaptations to the reaction conditions, which is a prerequisite to understand its catalytic behavior., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Spinel-type catalysts are promising anode materials for the alkaline oxygen evolution reaction (OER), exhibiting low overpotentials and providing long-term stability. In this study, we compared two structurally equal Co 2 FeO 4 spinels with nominally identical stoichiometry and substantially different OER activities. In particular, one of the samples, characterized by a metastable precatalyst state, was found to quickly achieve its steady-state optimum operation, while the other, which was initially closer to the ideal crystallographic spinel structure, never reached such a state and required 168 mV higher potential to achieve 1 mA/cm 2 . In addition, the enhanced OER activity was accompanied by a larger resistance to corrosion. More specifically, using various ex situ , quasi in situ , and operando methods, we could identify a correlation between the catalytic activity and compositional inhomogeneities resulting in an X-ray amorphous Co 2+ -rich minority phase linking the crystalline spinel domains in the as-prepared state. Operando X-ray absorption spectroscopy revealed that these Co 2+ -rich domains transform during OER to structurally different Co 3+ -rich domains. These domains appear to be crucial for enhancing OER kinetics while exhibiting distinctly different redox properties. Our work emphasizes the necessity of the operando methodology to gain fundamental insight into the activity-determining properties of OER catalysts and presents a promising catalyst concept in which a stable, crystalline structure hosts the disordered and active catalyst phase.

Electrochemical CO 2 reduction is a potential approach to convert CO 2 into valuable chemicals using electricity as feedstock. Abundant and affordable catalyst materials are needed to upscale this process in a sustainable manner. Nickel-nitrogen-doped carbon (Ni-N-C) is an efficient catalyst for CO 2 reduction to CO, and the single-site Ni-N x motif is believed to be the active site. However, critical metrics for its catalytic activity, such as active site density and intrinsic turnover frequency, so far lack systematic discussion. In this work, we prepared a set of covalent organic framework (COF)-derived Ni-N-C catalysts, for which the Ni-N x content could be adjusted by the pyrolysis temperature. The combination of high-angle annular dark-field scanning transmission electron microscopy and extended X-ray absorption fine structure evidenced the presence of Ni single-sites, and quantitative X-ray photoemission addressed the relation between active site density and turnover frequency., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Copper-based catalysts are established catalytic systems for the electrocatalytic CO 2 reduction reaction (CO 2 RR), where the greenhouse gas CO 2 is converted into valuable industrial chemicals, such as energy-dense C 2+ products, using energy from renewable sources. However, better control over the catalyst selectivity, especially at industrially relevant high current density conditions, is needed to expedite the economic viability of the CO 2 RR. For this purpose, bimetallic materials, where copper is combined with a secondary metal, comprise a promising and a highly tunable catalyst for the CO 2 RR. Nevertheless, the synergy between copper and the selected secondary metal species, the evolution of the bimetallic structural motifs under working conditions and the effect of the secondary metal on the kinetics of the Cu redox behavior require careful investigation. Here, we employ operando quick X-ray absorption fine structure (QXAFS) spectroscopy coupled with machine-learning based data analysis and surface-enhanced Raman spectroscopy (SERS) to investigate the time-dependent chemical and structural changes in catalysts derived from shape-selected ZnO/Cu 2 O nanocubes under CO 2 RR conditions at current densities up to -500 mA cm -2 . We furthermore relate the catalyst transformations observed under working conditions to the catalytic activity and selectivity and correlate potential-dependent surface and subsurface processes. We report that the addition of Zn to a Cu-based catalyst has a crucial impact on the kinetics of subsurface processes, while redox processes of the Cu surface layer remain largely unaffected. Interestingly, the presence of Zn was found to contribute to the stabilization of cationic Cu(i) species, which is of catalytic relevance since Cu(0)/Cu(i) interfaces have been reported to be beneficial for efficient electrocatalytic CO 2 conversion to complex multicarbon products. At the same time, we attribute the increase of the C 2+ product selectivity to the formation of Cu-rich CuZn alloys in samples with low Zn content, while Zn-rich alloy phases result in an increased formation of CO paralleled by an increase of the parasitic hydrogen evolution reaction., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)