Electrocatalytic conversion of nitrogen oxides to value-added chemicals is a promising strategy for mitigating the human-caused unbalance of the global nitrogen-cycle, but controlling product ...selectivity remains a great challenge. Here we show iron-nitrogen-doped carbon as an efficient and durable electrocatalyst for selective nitric oxide reduction into hydroxylamine. Using in operando spectroscopic techniques, the catalytic site is identified as isolated ferrous moieties, at which the rate for hydroxylamine production increases in a super-Nernstian way upon pH decrease. Computational multiscale modelling attributes the origin of unconventional pH dependence to the redox active (non-innocent) property of NO. This makes the rate-limiting NO adsorbate state more sensitive to surface charge which varies with the pH-dependent overpotential. Guided by these fundamental insights, we achieve a Faradaic efficiency of 71% and an unprecedented production rate of 215 μmol cm
h
at a short-circuit mode in a flow-type fuel cell without significant catalytic deactivation over 50 h operation.
Long‐term catalyst stability is essential for the commercialization of hydrogen generation by electrocatalytic water‐splitting. Current research, however, mainly focuses on improving electrode ...activity of the hydrogen evolution reaction (HER) at the cathode and oxygen evolution reaction (OER) at the anode of electrolyzers, although the maintenance of long‐term performance poses a bigger challenge. To shift the focus of research to the issue of catalyst stability, this review describes the mechanism of HER/OER catalyst degradation based on catalyst dissolution and agglomeration, and summarizes representative catalyst designs for achieving stable catalysts in long‐term water electrolysis operation. Additionally, various strategies toward the improvement of HER/OER stability are evaluated, and potential effective guidelines for the design of stable catalysts are suggested.
Long term stability of nanocatalysts is critical to the commercialization of proton/anion exchange membrane water electrolysis. This review discusses various reasons for catalyst degradation such as element dissolution and catalyst agglomeration in conjunction with the relevant catalysis operation conditions. Then, catalyst design strategies are presented to help realize the long‐term operation of water electrolysis in practical environments.
Although metastable crystal structures have received much attention owing to their utilization in various fields, their phase‐transition to a thermodynamic structure has attracted comparably little ...interest. In the case of nanoscale crystals, such an exothermic phase‐transition releases high energy within a confined surface area and reconstructs surface atomic arrangement in a short time. Thus, this high‐energy nanosurface may create novel crystal structures when some elements are supplied. In this work, the creation of a ruthenium carbide (RuCX, X < 1) phase on the surface of the Ru nanocrystal is discovered during phase‐transition from cubic‐close‐packed to hexagonal‐close‐packed structure. When the electrocatalytic hydrogen evolution reaction (HER) is tested in alkaline media, the RuCX exhibits a much lower overpotential and good stability relative to the counterpart Ru‐based catalysts and the state‐of‐the‐art Pt/C catalyst. Density functional theory calculations predict that the local heterogeneity of the outermost RuCX surface promotes the bifunctional HER mechanism by providing catalytic sites for both H adsorption and facile water dissociation.
Crystal phase‐transition of Ru/C from cubic‐close‐packing to hexagonal‐close‐packing creates a ruthenium carbide (RuCX, X < 1) nanosurface on Ru nanocrystal. The as‐created RuCX nanosurface presents a highly active and stable performance for the hydrogen evolution reaction (HER) in alkaline media. Density functional theory calculations predict the RuCX sites as bifunctional configurations for improving alkaline HER kinetics.
Abstract
The voltage reversal of water electrolyzers and fuel cells induces a large positive potential on the hydrogen electrodes, followed by severe system degradation. Applying a reversible ...multifunctional electrocatalyst to the hydrogen electrode is a practical solution. Ir exhibits excellent catalytic activity for hydrogen evolution reactions (HER), and hydrogen oxidation reactions (HOR), yet irreversibly converts to amorphous IrO
x
at potentials > 0.8 V/RHE, which is an excellent catalyst for oxygen evolution reactions (OER), yet a poor HER and HOR catalyst. Harnessing the multifunctional catalytic characteristics of Ir, here we design a unique Ir-based electrocatalyst with high crystallinity for OER, HER, and HOR. Under OER operation, the crystalline nanoparticle generates an atomically-thin IrO
x
layer, which reversibly transforms into a metallic Ir at more cathodic potentials, restoring high activity for HER and HOR. Our analysis reveals that a metallic Ir subsurface under thin IrO
x
layer can act as a catalytic substrate for the reduction of Ir ions, creating reversibility. Our work not only uncovers fundamental, uniquely reversible catalytic properties of nanoparticle catalysts, but also offers insights into nanocatalyst design.
Abstract
The electrosynthesis of formate from CO
2
can mitigate environmental issues while providing an economically valuable product. Although stannic oxide is a good catalytic material for formate ...production, a metallic phase is formed under high reduction overpotentials, reducing its activity. Here, using a fluorine-doped tin oxide catalyst, a high Faradaic efficiency for formate (95% at 100 mA cm
−2
) and a maximum partial current density of 330 mA cm
−2
(at 400 mA cm
−2
) is achieved for the electroreduction of CO
2
. Furthermore, the formate selectivity (≈90%) is nearly constant over 7 days of operation at a current density of 100 mA cm
−2
.
In-situ/operando
spectroscopies reveal that the fluorine dopant plays a critical role in maintaining the high oxidation state of Sn, leading to enhanced durability at high current densities. First-principle calculation also suggests that the fluorine-doped tin oxide surface could provide a thermodynamically stable environment to form HCOO* intermediate than tin oxide surface. These findings suggest a simple and efficient approach for designing active and durable electrocatalysts for the electrosynthesis of formate from CO
2
.
Electrochemical processes coupling carbon dioxide reduction reactions with organic oxidation reactions are promising techniques for producing clean chemicals and utilizing renewable energy. However, ...assessments of the economics of the coupling technology remain questionable due to diverse product combinations and significant process design variability. Here, we report a technoeconomic analysis of electrochemical carbon dioxide reduction reaction-organic oxidation reaction coproduction via conceptual process design and thereby propose potential economic combinations. We first develop a fully automated process synthesis framework to guide process simulations, which are then employed to predict the levelized costs of chemicals. We then identify the global sensitivity of current density, Faraday efficiency, and overpotential across 295 electrochemical coproduction processes to both understand and predict the levelized costs of chemicals at various technology levels. The analysis highlights the promise that coupling the carbon dioxide reduction reaction with the value-added organic oxidation reaction can secure significant economic feasibility.
Oxygen-Cu (O-Cu) combination catalysts have recently achieved highly improved selectivity for ethylene production from the electrochemical CO
reduction reaction (CO
RR). In this study, we developed ...anodized copper (AN-Cu) Cu(OH)
catalysts by a simple electrochemical synthesis method and achieved ∼40% Faradaic efficiency for ethylene production, and high stability over 40 h. Notably, the initial reduction conditions applied to AN-Cu were critical to achieving selective and stable ethylene production activity from the CO
RR, as the initial reduction condition affects the structures and chemical states, crucial for highly selective and stable ethylene production over methane. A highly negative reduction potential produced a catalyst maintaining long-term stability for the selective production of ethylene over methane, and a small amount of Cu(OH)
was still observed on the catalyst surface. Meanwhile, when a mild reduction condition was applied to the AN-Cu, the Cu(OH)
crystal structure and mixed states disappeared on the catalyst, becoming more favorable to methane production after few hours. These results show the selectivity of ethylene to methane in O-Cu combination catalysts is influenced by the electrochemical reduction environment related to the mixed valences. This will provide new strategies to improve durability of O-Cu combination catalysts for C-C coupling products from electrochemical CO
conversion.
Electrochemical carbon dioxide reduction reaction (CO2RR) is a promising approach to mitigate CO2 concentration and generate carbon feedstock. Recently, the (sub‐)nanometer design of catalyst ...structures has been revealed as an efficient means to control the reaction process through the local reaction environment. Herein, the synthesis of a novel tin oxide (SnOx) nanoparticle (NP) catalyst with highly controlled sub‐nanoscale interplanar gaps of widths <1 nm (SnOx NP‐s) is reported via the lithium electrochemical tuning (LiET) method. Transmission electron microscopy (TEM) and 3D‐tomo‐scanning TEM (STEM) analysis confirm the presence of a distinct segmentation pattern and the newly engineered interparticle confined space in the SnOx NP‐s. The catalyst exhibits a significant increase in CO2RR versus hydrogen evolution selectivity by a factor of ≈5 with 20% higher formate selectivity relative to pristine SnO2 NPs at −1.2 VRHE. Density functional theory calculations and cation‐size‐dependent experiments indicate that this is attributable to a gap‐stabilization of the rate‐limiting *OCHO and *COOH intermediates, the formation of which is driven by the interfacial electric field. Moreover, the SnOx NP‐s exhibits stable performance during CO2RR over 50 h. These results highlight the potential of controlled atomic spaces in directing electrochemical reaction selectivity and the design of highly optimized catalytic materials.
The space‐confinement approach to Sn‐based nanoparticles enhances the activity of Sn and its selectivity to formic acid. When the spacing between the active surfaces of Sn reaches <1 nm, an order of magnitude increase in the current density and the product selectivity is observed for the electrochemical reduction of CO2 at a given applied potential.
Single-atom catalysts (SACs) possess the potential to achieve unique catalytic properties and remarkable catalytic mass activity by utilizing low-coordination and unsaturated active sites. However, ...smaller particles tend to aggregate into clusters or particles owing to their high surface energy. In addition, support materials that have strong interactions with isolated metal atoms, extremely large surface areas, and electrochemical stability are required. Therefore, sufficient information about these factors is needed to synthesize and utilize SACs. Herein, we review the recent investigations and advances in SACs for the oxygen evolution reaction (OER). We present not only the structural characterization of SACs, but also
in situ
/
operando
spectroscopic techniques and computational research for SACs to understand the mechanism and reveal the origin of their excellent OER activity. Furthermore, the OER catalytic activity and stability of SACs are summarized to evaluate the current level of SACs. Currently, research on single-atoms as OER catalysts is in the infant stage for synthesis, characterization and mechanism studies. We discuss some challenges for understanding the fundamentals of SACs and enhancing the catalytic performance of SACs for industrial applications.
Issues to be improved in the future for single-atom OER catalysts and related challenges.
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