Showing papers on "Redox published in 2022"

Unraveling of cocatalysts photodeposited selectively on facets of BiVO4 to boost solar water splitting

Abstract: Bismuth vanadate (BiVO4) has been widely investigated as a photocatalyst or photoanode for solar water splitting, but its activity is hindered by inefficient cocatalysts and limited understanding of the underlying mechanism. Here we demonstrate significantly enhanced water oxidation on the particulate BiVO4 photocatalyst via in situ facet-selective photodeposition of dual-cocatalysts that exist separately as metallic Ir nanoparticles and nanocomposite of FeOOH and CoOOH (denoted as FeCoOx), as revealed by advanced techniques. The mechanism of water oxidation promoted by the dual-cocatalysts is experimentally and theoretically unraveled, and mainly ascribed to the synergistic effect of the spatially separated dual-cocatalysts (Ir, FeCoOx) on both interface charge separation and surface catalysis. Combined with the H2-evolving photocatalysts, we finally construct a Z-scheme overall water splitting system using [Fe(CN)6]3-/4- as the redox mediator, whose apparent quantum efficiency at 420 nm and solar-to-hydrogen conversion efficiency are optimized to be 12.3% and 0.6%, respectively.

Metal–organic framework supported Au nanoparticles with organosilicone coating for high-efficiency electrocatalytic N2 reduction to NH3

Abstract: The development of electrocatalysts for nitrogen reduction reaction (NRR) at ambient conditions, with both high NH3 yield and Faradaic efficiency, is currently a great challenge. To this aim, a unique metal–organic framework (MOF) crystalline matrix with disulfide trimeric unit as the building block was in situ synthesized by integration of dynamic covalent chemistry and coordination chemistry. This MOF with high porosity and excellent stability could be used as a host material to encapsulate well-dispersed Au nanoparticles (NPs) with ultrafine size of 1.9 ± 0.4 nm. After surface modification of [email protected] by using organosilicone, the hydrophobic-treated [email protected] (HT [email protected]) composite shows remarkable electrocatalytic performances for NRR, with the highest NH3 yield of 49.5 μg h–1 mgcat.–1 and the state-of-the-art Faradaic efficiency of 60.9% in water medium at ambient conditions. The favorable role of MOFs with functional sulfur groups on modulating the active Au sites and the great effect of hydrophobic coatings on suppressing the competitive hydrogen evolution reaction (HER) have been further demonstrated. This work provides a universal strategy to design composite electrocatalysts for high-efficient and long-term NH3 production.

Metal–organic framework supported Au nanoparticles with organosilicone coating for high-efficiency electrocatalytic N2 reduction to NH3

Abstract: The development of electrocatalysts for nitrogen reduction reaction (NRR) at ambient conditions, with both high NH3 yield and Faradaic efficiency, is currently a great challenge. To this aim, a unique metal–organic framework (MOF) crystalline matrix with disulfide trimeric unit as the building block was in situ synthesized by integration of dynamic covalent chemistry and coordination chemistry. This MOF with high porosity and excellent stability could be used as a host material to encapsulate well-dispersed Au nanoparticles (NPs) with ultrafine size of 1.9 ± 0.4 nm. After surface modification of Au@MOF by using organosilicone, the hydrophobic-treated Au@MOF (HT Au@MOF) composite shows remarkable electrocatalytic performances for NRR, with the highest NH3 yield of 49.5 μg h–1 mgcat.–1 and the state-of-the-art Faradaic efficiency of 60.9% in water medium at ambient conditions. The favorable role of MOFs with functional sulfur groups on modulating the active Au sites and the great effect of hydrophobic coatings on suppressing the competitive hydrogen evolution reaction (HER) have been further demonstrated. This work provides a universal strategy to design composite electrocatalysts for high-efficient and long-term NH3 production.

Tunable metal hydroxide–organic frameworks for catalysing oxygen evolution

Abstract: The oxygen evolution reaction is central to making chemicals and energy carriers using electrons. Combining the great tunability of enzymatic systems with known oxide-based catalysts can create breakthrough opportunities to achieve both high activity and stability. Here we report a series of metal hydroxide–organic frameworks (MHOFs) synthesized by transforming layered hydroxides into two-dimensional sheets crosslinked using aromatic carboxylate linkers. MHOFs act as a tunable catalytic platform for the oxygen evolution reaction, where the π–π interactions between adjacent stacked linkers dictate stability, while the nature of transition metals in the hydroxides modulates catalytic activity. Substituting Ni-based MHOFs with acidic cations or electron-withdrawing linkers enhances oxygen evolution reaction activity by over three orders of magnitude per metal site, with Fe substitution achieving a mass activity of 80 A \({\rm{g}}_{\rm{catalyst}}^{-1}\) at 0.3 V overpotential for 20 h. Density functional theory calculations correlate the enhanced oxygen evolution reaction activity with the MHOF-based modulation of Ni redox and the optimized binding of oxygenated intermediates.

Unraveling of cocatalysts photodeposited selectively on facets of BiVO4 to boost solar water splitting

Abstract: Bismuth vanadate (BiVO4) has been widely investigated as a photocatalyst or photoanode for solar water splitting, but its activity is hindered by inefficient cocatalysts and limited understanding of the underlying mechanism. Here we demonstrate significantly enhanced water oxidation on the particulate BiVO4 photocatalyst via in situ facet-selective photodeposition of dual-cocatalysts that exist separately as metallic Ir nanoparticles and nanocomposite of FeOOH and CoOOH (denoted as FeCoOx), as revealed by advanced techniques. The mechanism of water oxidation promoted by the dual-cocatalysts is experimentally and theoretically unraveled, and mainly ascribed to the synergistic effect of the spatially separated dual-cocatalysts (Ir, FeCoOx) on both interface charge separation and surface catalysis. Combined with the H2-evolving photocatalysts, we finally construct a Z-scheme overall water splitting system using [Fe(CN)6]3-/4- as the redox mediator, whose apparent quantum efficiency at 420 nm and solar-to-hydrogen conversion efficiency are optimized to be 12.3% and 0.6%, respectively.

Realizing Two-Electron Transfer in Ni(OH)2 Nanosheets for Energy Storage.

Abstract: The theoretical capacity of a given electrode material is ultimately determined by the number of electrons transferred in each redox center. The design of multi-electron transfer processes could break through the limitation of one-electron transfer and multiply the total capacity but is difficult to achieve because multiple electron transfer processes are generally thermodynamically and kinetically more complex. Here, we report the discovery of two-electron transfer in monolayer Ni(OH)2 nanosheets, which contrasts with the traditional one-electron transfer found in multilayer materials. First-principles calculations predict that the first oxidation process Ni2+ → Ni3+ occurs easily, whereas the second electron transfer in Ni3+ → Ni4+ is strongly hindered in multilayer materials by both the interlayer hydrogen bonds and the domain H structure induced by the Jahn-Teller distortion of the Ni3+ (t2g6eg1)-centered octahedra. In contrast, the second electron transfer can easily occur in monolayers because all H atoms are fully exposed. Experimentally, the as-prepared monolayer is found to deliver an exceptional redox capacity of ∼576 mA h/g, nearly 2 times the theoretical capacity of one-electron processes. In situ experiments demonstrate that monolayer Ni(OH)2 can transfer two electrons and most Ni ions transform into Ni4+ during the charging process, whereas bulk Ni(OH)2 can only be transformed partially. Our work reveals a new redox reaction mechanism in atomically thin Ni(OH)2 nanosheets and suggests a promising path toward tuning the electron transfer numbers to multiply the capacity of the relevant energy storage materials.

High-Efficiency N2 Electroreduction Enabled by Se-Vacancy-Rich WSe2-x in Water-in-Salt Electrolytes.

Abstract: Electrocatalytic nitrogen reduction reaction (NRR) is a promising approach for renewable NH3 production, while developing the NRR electrocatalysis systems with both high activity and selectivity remains a significant challenge. Herein, we combine catalyst and electrolyte engineering to achieve a high-efficiency NRR enabled by a Se-vacancy-rich WSe2-x catalyst in water-in-salt electrolyte (WISE). Extensive characterizations, theoretical calculations, and in situ X-ray photoelectron/Raman spectroscopy reveal that WISE ensures suppressed H2 evolution, improved N2 affinity on the catalyst surface, as well as an enhanced π-back-donation ability of active sites, thereby promoting both activity and selectivity for the NRR. As a result, an excellent faradaic efficiency of 62.5% and NH3 yield of 181.3 μg h-1 mg-1 is achieved with WSe2-x in 12 m LiClO4, which is among the highest NRR performances reported to date.

A Universal Additive Strategy to Reshape Electrolyte Solvation Structure toward Reversible Zn Storage

Abstract: The benefits of Zn, despite many of its performance advantages (e.g., high theoretical capacity and low redox potential), are compromised by severe side reactions and Zn dendrite growth in aqueous electrolytes, due to the coordinated H2O within the Zn2+‐solvation sheath and reactive free water in the bulk electrolyte. Unlike most efforts focused on costly super‐concentrated electrolytes and single additive species, a universal strategy is proposed to boost Zn reversibility in dilute electrolytes via adding carbonyl‐containing organic solvents. Based on experimental investigations and multiscale simulations, the representative electrolyte with a N‐methyl‐2‐pyrrolidone polar additive is proved to assist in structural reshaping of Zn2+‐solvation and stabilizing the hydrogen bond network of water. This synergy is instrumental in contributing to suppressed water‐induced parasitic reactions and dendrite formation, which enables high average coulombic efficiency of 99.7% over 1000 cycles in an Zn/Cu asymmetric cell, and an ultralong cycling lifespan of 2000 cycles with 99.4% capacity retention in a Zn/VS2@SS full cell. Even with an elevated cathodic mass loading (up to 9.5 mg cm‐2), the cycling stability is still maintained. The proposed strategy provides new insight into electrolyte additive design and sheds light on high‐performance Zn‐ion batteries.

Porous organic polymers for electrocatalysis.

Abstract: Porous organic polymers (POPs) composed of organic building units linked via covalent bonds are a class of lightweight porous network materials with high surface areas, tuneable pores, and designable components and structures. Owing to their well-preserved characteristics in terms of structure and composition, POPs applied as electrocatalysts have shown promising activity and achieved considerable advances in numerous electrocatalytic reactions, including the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, CO2 reduction reaction, N2 reduction reaction, nitrate/nitrite reduction reaction, nitrobenzene reduction reaction, hydrogen oxidation reaction, and benzyl alcohol oxidation reaction. Herein, we present a systematic overview of recent advances in the applications of POPs in these electrocatalytic reactions. The synthesis strategies, specific active sites, and catalytic mechanisms of POPs are summarized in this review. The fundamental principles of some electrocatalytic reactions are also concluded. We further discuss the current challenges of and perspectives on POPs for electrocatalytic applications. Meanwhile, the possible future directions are highlighted to afford guidelines for the development of efficient POP electrocatalysts.

Stable Black Phosphorus Encapsulation in Porous Mesh-like UiO-66 Promoted Charge Transfer for Photocatalytic Oxidation of Toluene and o-Dichlorobenzene: Performance, Degradation Pathway, and Mechanism

Abstract: This work presents a new strategy to enhance the charge transfer from Zr atoms in UiO-66 to black phosphorus (BP) via an atomic-level charge-transfer channel provided by Zr–P bonds for photocatalytic degradation of toluene and o-dichlorobenzene. The formation of Zr–P bonds is the key to covering BP with the UiO-66 encapsulation layer and improving the charge-transfer capability of BP–UiO, which is also verified by a series of characterization and theoretical calculations. The hydrophilic porous mesh-like UiO-66 encapsulation layer endows BP–UiO with enhanced visible-light-harvesting ability, charge transport capability, and photocatalytic activity. Additionally, BP–UiO demonstrates the promoted redox/acidity properties, significantly improving catalytic activity for the degradation of toluene and o-dichlorobenzene even in the presence of water. The degradation pathways of contaminants are investigated via the study of transient variations by in situ infrared (IR) methods, refined by the static analysis of intermates accumulated on the BP–UiO surface by gas chromatography–mass spectrometry (GC–MS). The electron transfer via bond construction and the combination of in situ IR and GC–MS are expected to provide a more complete theoretical basis for environmental catalysis.

Design Rules of a Sulfur Redox Electrocatalyst for Lithium–Sulfur Batteries

Abstract: Seeking an electrochemical catalyst to accelerate the liquid‐to‐solid conversion of soluble lithium polysulfides to insoluble products is crucial to inhibit the shuttle effect in lithium–sulfur (Li–S) batteries and thus increase their practical energy density. Mn‐based mullite (SmMn2O5) is used as a model catalyst for the sulfur redox reaction to show how the design rules involving lattice matching and 3d‐orbital selection improve catalyst performance. Theoretical simulation shows that the positions of Mn and O active sites on the (001) surface are a good match with those of Li and S atoms in polysulfides, resulting in their tight anchoring to each other. Fundamentally, dz2 and dx2−y2 around the Fermi level are found to be crucial for strongly coupling with the p‐orbitals of the polysulfides and thus decreasing the redox overpotential. Following the theoretical calculation, SmMn2O5 catalyst is synthesized and used as an interlayer in a Li–S battery. The resulted battery has a high cycling stability over 1500 cycles at 0.5 C and more promisingly a high areal capacity of 7.5 mAh cm−2 is achieved with a sulfur loading of ≈5.6 mg cm−2 under the condition of a low electrolyte/sulfur (E/S) value ≈4.6 µL mg−1.

Linking oxidative and reductive clusters to prepare crystalline porous catalysts for photocatalytic CO2 reduction with H2O

Abstract: Mimicking natural photosynthesis to convert CO2 with H2O into value-added fuels achieving overall reaction is a promising way to reduce the atmospheric CO2 level. Casting the catalyst of two or more catalytic sites with rapid electron transfer and interaction may be an effective strategy for coupling photocatalytic CO2 reduction and H2O oxidation. Herein, based on the MOF ∪ COF collaboration, we have carefully designed and synthesized a crystalline hetero-metallic cluster catalyst denoted MCOF-Ti6Cu3 with spatial separation and functional cooperation between oxidative and reductive clusters. It utilizes dynamic covalent bonds between clusters to promote photo-induced charge separation and transfer efficiency, to drive both the photocatalytic oxidative and reductive reactions. MCOF-Ti6Cu3 exhibits fine activity in the conversion of CO2 with water into HCOOH (169.8 μmol g-1h-1). Remarkably, experiments and theoretical calculations reveal that photo-excited electrons are transferred from Ti to Cu, indicating that the Cu cluster is the catalytic reduction center.

Selectivity in Electrochemical CO2 Reduction.

Abstract: ConspectusThe electrocatalytic CO2 reduction reaction (CO2RR) to generate fixed forms of carbons that have commercial value is a lucrative avenue to ameliorate the growing concerns about the detrimental effect of CO2 emissions as well as to generate carbon-based feed chemicals, which are generally obtained from the petrochemical industry. The area of electrochemical CO2RR has seen substantial activity in the past decade, and several good catalysts have been reported. While the focus was initially on the rate and overpotential of electrocatalysis, it is gradually shifting toward the more chemically challenging issue of selectivity. CO2 can be partially reduced to produce several C1 products like CO, HCOOH, CH3OH, etc. before its complete 8e-/8H+ reduction to CH4. In addition to that, the low-valent electron-rich metal centers deployed to activate CO2, a Lewis acid, are prone to reduce protons, which are a substrate for CO2RR, leading to competing hydrogen evolution reaction (HER). Similarly, the low-valent metal is prone to oxidation by atmospheric O2 (i.e., it can catalyze the oxygen reduction reaction, ORR), necessitating strictly anaerobic conditions for CO2RR. Not only is the requirement of O2-free reaction conditions impractical, but it also leads to the release of partially reduced O2 species such as O2-, H2O2, etc., which are reactive and result in oxidative degradation of the catalyst.In this Account, mechanistic investigations of CO2RR by detecting and, often, chemically trapping and characterizing reaction intermediates are used to understand the factors that determine the selectivity in CO2RR. The spectroscopic data obtained from different intermediates have been identified in different CO2RR catalysts to develop an electronic structure selectivity relationship that is deemed to be important for deciding the selectivity of 2e-/2H+ CO2RR. The roles played by the spin state, hydrogen bonding, and heterogenization in determining the rate and selectivity of CO2RR (producing only CO, only HCOOH, or only CH4) are discussed using examples of both iron porphyrin and non-heme bioinspired artificial mimics. In addition, strategies are demonstrated where the competition between CO2RR and HER as well as CO2RR and ORR could be skewed overwhelmingly in favor of CO2RR in both cases.

Emerging frontiers of Z-scheme photocatalytic systems

Abstract: Solar-driven fuel production is considered a promising strategy to solve the growing global energy demands and environmental problems. Z-scheme heterojunctions have been reported to exhibit considerably improved photocatalytic fuel production due to enhanced light harvesting, spatially separated reductive and oxidative active sites, and strong redox capability. Understanding the fundamental principles of Z-scheme photocatalytic systems and mastering their improvement strategies will help greatly with the further development of highly efficient Z-scheme photocatalysts for solar-driven fuel production and other photocatalytic applications. Z-scheme photocatalysts have recently received tremendous attention because of their strong light utilization and redox ability. This cutting-edge photocatalytic platform allows photocatalysts to convert light into chemical energy with high activity, selectivity, and stability. In this review, we highlight some of the recent key contributions in the field, including fundamental principles, advanced characterization methods, and a series of photocatalytic applications (e.g., water splitting, CO2 reduction, N2 fixation, H2O2 production). Significant improvement strategies for Z-scheme photocatalysts are also discussed and summarized. With increasing achievements, Z-scheme photocatalytic systems (PSs) will make a historic breakthrough in activity, solar utilization, selectivity, and fabrication cost and move toward practical production in the near future. Z-scheme photocatalysts have recently received tremendous attention because of their strong light utilization and redox ability. This cutting-edge photocatalytic platform allows photocatalysts to convert light into chemical energy with high activity, selectivity, and stability. In this review, we highlight some of the recent key contributions in the field, including fundamental principles, advanced characterization methods, and a series of photocatalytic applications (e.g., water splitting, CO2 reduction, N2 fixation, H2O2 production). Significant improvement strategies for Z-scheme photocatalysts are also discussed and summarized. With increasing achievements, Z-scheme photocatalytic systems (PSs) will make a historic breakthrough in activity, solar utilization, selectivity, and fabrication cost and move toward practical production in the near future. a typical layered 2D material with puckered layers of phosphorus stacked together via van der Waals forces. the energy band formed by free electrons. a unique class of materials that combine extended π-conjugation with a permanently microporous skeleton. one of the most widely used theoretical calculation technologies. a Z-scheme photocatalyst that shows photocatalytic activity under all wavelengths of light. an emerging 2D layered transition-metal carbide/carbonitride/nitride. metal–semiconductor contact that has a negligible contact resistance relative to the bulk or series resistance of the semiconductor. the charge-transfer route in S-scheme mode resembles a macroscopic ‘step’ (from low CB to high VB; Figure 1D). MoS2 with unsaturated S atoms on exposed edges as reactive sites forms three stacked atomic layers. the energy band formed by valence electrons. the minimum energy required to move an electron from the interior of a solid to its surface. the photogenerated carrier transfer route looks like the letter Z.

Steering the structure and selectivity of CO2 electroreduction catalysts by potential pulses

Abstract: Abstract Convoluted selectivity trends and a missing link between reaction product distribution and catalyst properties hinder practical applications of the electrochemical CO 2 reduction reaction (CO 2 RR) for multicarbon product generation. Here we employ operando X-ray absorption and X-ray diffraction methods with subsecond time resolution to unveil the surprising complexity of catalysts exposed to dynamic reaction conditions. We show that by using a pulsed reaction protocol consisting of alternating working and oxidizing potential periods that dynamically perturb catalysts derived from Cu 2 O nanocubes, one can decouple the effect of the ensemble of coexisting copper species on the product distribution. In particular, an optimized dynamic balance between oxidized and reduced copper surface species achieved within a narrow range of cathodic and anodic pulse durations resulted in a twofold increase in ethanol production compared with static CO 2 RR conditions. This work thus prepares the ground for steering catalyst selectivity through dynamically controlled structural and chemical transformations.

Sulfur vacancy engineering of MoS2 via phosphorus incorporation for improved electrocatalytic N2 reduction to NH3

Abstract: Electrocatalytic N2 reduction reaction (NRR) serves as a promising approach for converting N2 to NH3 in a sustainable way to replace the energy-intensive Haber-Bosch process. MoS2-based electrocatalysts hold great potentials in catalyzing N2 reduction due to their similarity with active MoFe-co in biological nitrogenase. In this work, we reported a sulfur vacancy-rich MoS2 as an excellent electrocatalyst for NRR, where the sulfur vacancies (SVs) were easily controlled by regulating the amount of P dopants. MoS2 with abundant SVs (P-M-1) achieved a large NH3 yield rate of 60.27 µg h−1 mg−1cat. and high Faradaic efficiency of 12.22% towards NRR. Further mechanistic study revealed that P dopants not only created SVs as the active centers but also modulated the electronic structure for the enhanced adsorption and activation of N2 molecules, thus immensely promoting the catalytic performance of NRR.