Showing papers on "Redox published in 2021"

A rechargeable zinc-air battery based on zinc peroxide chemistry

Abstract: Rechargeable alkaline zinc-air batteries promise high energy density and safety but suffer from the sluggish 4 electron (e-)/oxygen (O2) chemistry that requires participation of water and from the electrochemical irreversibility originating from parasitic reactions caused by caustic electrolytes and atmospheric carbon dioxide. Here, we report a zinc-O2/zinc peroxide (ZnO2) chemistry that proceeds through a 2e-/O2 process in nonalkaline aqueous electrolytes, which enables highly reversible redox reactions in zinc-air batteries. This ZnO2 chemistry was made possible by a water-poor and zinc ion (Zn2+)-rich inner Helmholtz layer on the air cathode caused by the hydrophobic trifluoromethanesulfonate anions. The nonalkaline zinc-air battery thus constructed not only tolerates stable operations in ambient air but also exhibits substantially better reversibility than its alkaline counterpart.

Chemical vapour deposition of Fe-N-C oxygen reduction catalysts with full utilization of dense Fe-N4 sites.

Abstract: Replacing scarce and expensive platinum (Pt) with metal–nitrogen–carbon (M–N–C) catalysts for the oxygen reduction reaction in proton exchange membrane fuel cells has largely been impeded by the low oxygen reduction reaction activity of M–N–C due to low active site density and site utilization. Herein, we overcome these limits by implementing chemical vapour deposition to synthesize Fe–N–C by flowing iron chloride vapour over a Zn–N–C substrate at 750 °C, leading to high-temperature trans-metalation of Zn–N4 sites into Fe–N4 sites. Characterization by multiple techniques shows that all Fe–N4 sites formed via this approach are gas-phase and electrochemically accessible. As a result, the Fe–N–C catalyst has an active site density of 1.92 × 1020 sites per gram with 100% site utilization. This catalyst delivers an unprecedented oxygen reduction reaction activity of 33 mA cm−2 at 0.90 V (iR-corrected; i, current; R, resistance) in a H2–O2 proton exchange membrane fuel cell at 1.0 bar and 80 °C. Replacing platinum with metal–nitrogen–carbon catalysts for the oxygen reduction reaction in proton exchange membrane fuel cells has been impeded by low activity. These limitations have now been overcome by the trans-metalation of Zn–N4 sites into Fe–N4 sites.

Correlative operando microscopy of oxygen evolution electrocatalysts

Abstract: Transition metal (oxy)hydroxides are promising electrocatalysts for the oxygen evolution reaction1–3. The properties of these materials evolve dynamically and heterogeneously4 with applied voltage through ion insertion redox reactions, converting materials that are inactive under open circuit conditions into active electrocatalysts during operation5. The catalytic state is thus inherently far from equilibrium, which complicates its direct observation. Here, using a suite of correlative operando scanning probe and X-ray microscopy techniques, we establish a link between the oxygen evolution activity and the local operational chemical, physical and electronic nanoscale structure of single-crystalline β-Co(OH)2 platelet particles. At pre-catalytic voltages, the particles swell to form an α-CoO2H1.5·0.5H2O-like structure—produced through hydroxide intercalation—in which the oxidation state of cobalt is +2.5. Upon increasing the voltage to drive oxygen evolution, interlayer water and protons de-intercalate to form contracted β-CoOOH particles that contain Co3+ species. Although these transformations manifest heterogeneously through the bulk of the particles, the electrochemical current is primarily restricted to their edge facets. The observed Tafel behaviour is correlated with the local concentration of Co3+ at these reactive edge sites, demonstrating the link between bulk ion-insertion and surface catalytic activity. Mapping the operational chemical, physical and electronic structure of an oxygen evolution electrocatalyst at the nanoscale links the properties of the material with the observed oxygen evolution activity.

Nickel ferrocyanide as a high-performance urea oxidation electrocatalyst

Abstract: Urea is often present in waste water but can be used in powering fuel cells and as an alternative oxidation substrate to water in an electrolyser. However, an insufficient mechanistic understanding and the lack of efficient catalysts for the urea oxidation reaction have hampered the development of such applications. Here we demonstrate that a nickel ferrocyanide (Ni2Fe(CN)6) catalyst supported on Ni foam can drive the urea oxidation reaction with a higher activity and better stability than those of conventional Ni-based catalysts. Our experimental and computational data suggest a urea oxidation reaction pathway different from most other Ni-based catalysts that comprise NiOOH derivatives as the catalytically active compound. Ni2Fe(CN)6 appears to be able to directly facilitate a two-stage reaction pathway that involves an intermediate ammonia production (on the Ni site) and its decomposition to N2 (on the Fe site). Owing to the different rate-determining steps with more favourable thermal/kinetic energetics, Ni2Fe(CN)6 achieves a 100 mA cm−2 anodic current density at a potential of 1.35 V (equal to an overpotential of 0.98 V). Urea oxidation could be a lower-energy alternative to water oxidation in hydrogen-producing electrolysers, but improved catalysts are required to facilitate the reaction. Geng et al. report nickel ferrocyanide as a promising catalyst and suggest that it operates via a different pathway to that of previous materials.

Modifying redox properties and local bonding of Co3O4 by CeO2 enhances oxygen evolution catalysis in acid

Abstract: Developing efficient and stable earth-abundant electrocatalysts for acidic oxygen evolution reaction is the bottleneck for water splitting using proton exchange membrane electrolyzers. Here, we show that nanocrystalline CeO2 in a Co3O4/CeO2 nanocomposite can modify the redox properties of Co3O4 and enhances its intrinsic oxygen evolution reaction activity, and combine electrochemical and structural characterizations including kinetic isotope effect, pH- and temperature-dependence, in situ Raman and ex situ X-ray absorption spectroscopy analyses to understand the origin. The local bonding environment of Co3O4 can be modified after the introduction of nanocrystalline CeO2, which allows the CoIII species to be easily oxidized into catalytically active CoIV species, bypassing the potential-determining surface reconstruction process. Co3O4/CeO2 displays a comparable stability to Co3O4 thus breaks the activity/stability tradeoff. This work not only establishes an efficient earth-abundant catalysts for acidic oxygen evolution reaction, but also provides strategies for designing more active catalysts for other reactions.

An Organodiselenide Comediator to Facilitate Sulfur Redox Kinetics in Lithium-Sulfur Batteries.

Abstract: Lithium-sulfur (Li-S) batteries are considered as promising next-generation energy storage devices due to their ultrahigh theoretical energy density, where soluble lithium polysulfides are crucial in the Li-S electrochemistry as intrinsic redox mediators. However, the poor mediation capability of the intrinsic polysulfide mediators leads to sluggish redox kinetics, further rendering limited rate performances, low discharge capacity, and rapid capacity decay. Here, an organodiselenide, diphenyl diselenide (DPDSe), is proposed to accelerate the sulfur redox kinetics as a redox comediator. DPDSe spontaneously reacts with lithium polysulfides to generate lithium phenylseleno polysulfides (LiPhSePSs) with improved redox mediation capability. The as-generated LiPhSePSs afford faster sulfur redox kinetics and increase the deposition dimension of lithium sulfide. Consequently, the DPDSe comediator endows Li-S batteries with superb rate performance of 817 mAh g-1 at 2 C and remarkable cycling stability with limited anode excess. Moreover, Li-S pouch cells with the DPDSe comediator achieve an actual initial energy density of 301 Wh kg-1 and 30 stable cycles. This work demonstrates a novel redox comediation strategy with an effective organodiselenide comediator to facilitate the sulfur redox kinetics under pouch cell conditions and inspires further exploration in mediating Li-S kinetics for practical high-energy-density batteries.

Regulating the Local Charge Distribution of Ni Active Sites for the Urea Oxidation Reaction

Abstract: In electrochemical energy storage and conversion systems, the anodic oxygen evolution reaction (OER) accounts for a large proportion of the energy consumption. The electrocatalytic urea oxidation reaction (UOR) is one of the promising alternatives to OER, owing to its low thermodynamic potential. However, owing to the sluggish UOR kinetics, its potential in practical use has not been unlocked. Herein, we developed a tungsten-doped nickel catalyst (Ni-WOx ) with superior activity towards UOR. The Ni-WOx catalyst exhibited record fast reaction kinetics (440 mA cm-2 at 1.6 V versus reversible hydrogen electrode) and a high turnover frequency of 0.11 s-1 , which is 4.8 times higher than that without W dopants. In further experiments, we found that the W dopant regulated the local charge distribution of Ni atoms, leading to the formation of Ni3+ sites with superior activity and thus accelerating the interfacial catalytic reaction. Moreover, when we integrated Ni-WOx into a CO2 flow electrolyzer, the cell voltage is reduced to 2.16 V accompanying with ≈98 % Faradaic efficiency towards carbon monoxide.

Promoting the sulfur redox kinetics by mixed organodiselenides in high-energy-density lithium–sulfur batteries

Abstract: Lithium–sulfur (Li–S) batteries are considered as a highly promising energy storage system due to their ultrahigh theoretical energy density. However, the sluggish kinetics of the complex multi-electron sulfur redox reactions seriously hinders the actual battery performance especially under practical working conditions. Homogeneous redox mediation, through elaborately designing the additive molecules, is an effective approach to promote the sulfur redox kinetics. Herein a promoter of mixed organodiselenides (mixed-Se) is proposed to comprehensively improve the sulfur redox kinetics following the redox comediation principles. Concretely, diphenyl diselenide promotes the liquid–liquid conversion between polysulfides and the solid–liquid conversion regarding lithium sulfide oxidation to polysulfides, while dimethyl diselenide enhances the liquid–solid conversion regarding lithium sulfide deposition. Consequently, the mixed-Se promoter endows a high discharge capacity of 1002 mAh g−1 with high sulfur loading of 4.0 mg cm−2, a high capacity retention of 81.6% after 200 cycles at 0.5 C, and a high actual energy density of 384 Wh kg−1 at 0.025 C in 1.5 Ah-level Li–S pouch cells. This work affords an effective kinetic promoter to construct high-energy-density Li–S batteries and inspires molecular design of kinetic promoters toward targeted energy-related redox reactions.

Mastering the surface strain of platinum catalysts for efficient electrocatalysis

Abstract: Platinum (Pt) has found wide use as an electrocatalyst for sustainable energy conversion systems1–3. The activity of Pt is controlled by its electronic structure (typically, the d-band centre), which depends sensitively on lattice strain4,5. This dependence can be exploited for catalyst design4,6–8, and the use of core–shell structures and elastic substrates has resulted in strain-engineered Pt catalysts with drastically improved electrocatalytic performances7,9–13. However, it is challenging to map in detail the strain–activity correlations in Pt-catalysed conversions, which can involve a number of distinct processes, and to identify the optimal strain modification for specific reactions. Here we show that when ultrathin Pt shells are deposited on palladium-based nanocubes, expansion and shrinkage of the nanocubes through phosphorization and dephosphorization induces strain in the Pt(100) lattice that can be adjusted from −5.1 per cent to 5.9 per cent. We use this strain control to tune the electrocatalytic activity of the Pt shells over a wide range, finding that the strain–activity correlation for the methanol oxidation reaction and hydrogen evolution reaction follows an M-shaped curve and a volcano-shaped curve, respectively. We anticipate that our approach can be used to screen out lattice strain that will optimize the performance of Pt catalysts—and potentially other metal catalysts—for a wide range of reactions. By depositing platinum shells on palladium-based nanocubes, the strain can be controlled by through phosphorization and dephosphorization, making it possible to tune the electrocatalytic activity of the platinum shells.

High-Rate CO2 Electroreduction to C2+ Products over a Copper-Copper Iodide Catalyst.

Abstract: Electrochemical CO2 reduction reaction (CO2 RR) to multicarbon hydrocarbon and oxygenate (C2+ ) products with high energy density and wide availability is of great importance, as it provides a promising way to achieve the renewable energy storage and close the carbon cycle Herein we design a Cu-CuI composite catalyst with abundant Cu0 /Cu+ interfaces by physically mixing Cu nanoparticles and CuI powders The composite catalyst achieves a remarkable C2+ partial current density of 591 mA cm-2 at -10 V vs reversible hydrogen electrode in a flow cell, substantially higher than Cu (329 mA cm-2 ) and CuI (96 mA cm-2 ) counterparts Induced by alkaline electrolyte and applied potential, the Cu-CuI composite catalyst undergoes significant reconstruction under CO2 RR conditions The high-rate C2+ production over Cu-CuI is ascribed to the presence of residual Cu+ and adsorbed iodine species which improve CO adsorption and facilitate C-C coupling

Understanding oxygen-deficient La2CuO4-δperovskite activated peroxymonosulfate for bisphenol A degradation: The role of localized electron within oxygen vacancy

Abstract: An A2BO4-type oxygen-deficient perovskite (La2CuO4-δ, LCO), which owns a better low-temperature reducibility, more abundant oxygen vacancies (OVs) and surface oxygen species than the benchmark CuO oxide, was first applied in peroxymonosulfate (PMS) activation. At 0.7 g/L of LCO and 2 mM of PMS, more than 96 % of bisphenol A (BPA) was removed within 60 min over a wide pH range of 3.1 ‒ 9.1. The cycle of surface Cu(II)/Cu(I) redox couple and lattice oxygen (O2−)/O2 were responsible for PMS activation. Besides, the OVs with localized electrons may directly activate PMS through single electron transfer pathway to generate surface-bound hydroxyl radical ( OH) and sulfate radical (SO4 −), as well as less singlet oxygen (1O2) for BPA degradation. The evolution of OH, SO4 − and 1O2 were revealed by EPR tests. Briefly, this study provides the mechanistic understanding of A2BO4-type perovskite in activation of PMS for water treatment.

The role of O2 in O-redox cathodes for Li-ion batteries

Abstract: The energy density of Li-ion batteries can be improved by storing charge at high voltages through the oxidation of oxide ions in the cathode material. However, oxidation of O2− triggers irreversible structural rearrangements in the bulk and an associated loss of the high voltage plateau, which is replaced by a lower discharge voltage, and a loss of O2 accompanied by densification at the surface. Here we consider various models for oxygen redox that are proposed in the literature and then describe a single unified model involving O2− oxidation to form O2, most of which is trapped in the bulk and the remainder of which evolves from the surface. The model extends the O2 formation and evolution at the surface, which is well known and well characterized, into the electrode particle bulk as caged O2 that can be reversibly reduced and oxidized. This converged understanding enables us to propose practical strategies to avoid oxygen-redox-induced instability and provide potential routes towards more reversible, high energy density Li-ion cathodes. Oxygen redox in Li-rich oxide cathodes is of both fundamental and practical interest in Li-ion battery development. Bruce and team examine the current understanding of oxygen-redox processes, especially those concerning O2 formation, and discuss strategies that can harness oxygen redox with suppressed side effects.

Recent advances in organic electrosynthesis employing transition metal complexes as electrocatalysts

Abstract: Organic electrosynthesis has been widely used as an environmentally conscious alternative to conventional methods for redox reactions because it utilizes electric current as a traceless redox agent instead of chemical redox agents. Indirect electrolysis employing a redox catalyst has received tremendous attention, since it provides various advantages compared to direct electrolysis. With indirect electrolysis, overpotential of electron transfer can be avoided, which is inherently milder, thus wide functional group tolerance can be achieved. Additionally, chemoselectivity, regioselectivity, and stereoselectivity can be tuned by the redox catalysts used in indirect electrolysis. Furthermore, electrode passivation can be avoided by preventing the formation of polymer films on the electrode surface. Common redox catalysts include N-oxyl radicals, hypervalent iodine species, halides, amines, benzoquinones (such as DDQ and tetrachlorobenzoquinone), and transition metals. In recent years, great progress has been made in the field of indirect organic electrosynthesis using transition metals as redox catalysts for reaction classes including C–H functionalization, radical cyclization, and cross-coupling of aryl halides-each owing to the diverse reactivity and accessible oxidation states of transition metals. Although various reviews of organic electrosynthesis are available, there is a lack of articles that focus on recent research progress in the area of indirect electrolysis using transition metals, which is the impetus for this review.

Atomically Dispersed Cobalt Sites on Graphene as Efficient Periodate Activators for Selective Organic Pollutant Degradation.

Abstract: Pollutant degradation via periodate (IO4-)-based advanced oxidation processes (AOPs) provides an economical, energy-efficient way for sustainable pollution control. Although single-atomic metal activation (SMA) can be exploited to optimize the pollution degradation process and understand the associated mechanisms governing IO4--based AOPs, studies on this topic are rare. Herein, we demonstrated the first instance of using SMA for IO4- analysis by employing atomically dispersed Co active sites supported by N-doped graphene (N-rGO-CoSA) activators. N-rGO-CoSA efficiently activated IO4- for organic pollutant degradation over a wide pH range without producing radical species. The IO4- species underwent stoichiometric decomposition to generate the iodate (IO3-) species. Whereas Co2+ and Co3O4 could not drive IO4- activation; the Co-N coordination sites exhibited high activation efficiency. The conductive graphene matrix reduced the contaminants/electron transport distance/resistance for these oxidation reactions and boosted the activation capacity by working in conjunction with metal centers. The N-rGO-CoSA/IO4- system exhibited a substrate-dependent reactivity that was not caused by iodyl (IO3·) radicals. Electrochemical experiments demonstrated that the N-rGO-CoSA/IO4- system decomposed organic pollutants via electron-transfer-mediated nonradical processes, where N-rGO-CoSA/periodate* metastable complexes were the predominant oxidants, thereby opening a new avenue for designing efficient IO4- activators for the selective oxidation of organic pollutants.

Unlocking anionic redox activity in O3-type sodium 3d layered oxides via Li substitution

Abstract: Sodium ion batteries, because of their sustainability attributes, could be an attractive alternative to Li-ion technology for specific applications. However, it remains challenging to design high energy density and moisture stable Na-based positive electrodes. Here, we report an O3-type NaLi1/3Mn2/3O2 phase showing anionic redox activity, obtained through a ceramic process by carefully adjusting synthesis conditions and stoichiometry. This phase shows a sustained reversible capacity of 190 mAh g−1 that is rooted in cumulative oxygen and manganese redox processes as deduced by combined spectroscopy techniques. Unlike many other anionic redox layered oxides so far reported, O3-NaLi1/3Mn2/3O2 electrodes do not show discernible voltage fade on cycling. This finding, rationalized by density functional theory, sheds light on the role of inter- versus intralayer 3d cationic migration in ruling voltage fade in anionic redox electrodes. Another practical asset of this material stems from its moisture stability, hence facilitating its handling and electrode processing. Overall, this work offers future directions towards designing highly performing sodium electrodes for advanced Na-ion batteries. Sodium ion batteries could be an attractive alternative to Li-ion technology but designing high energy density and moisture stable Na-based cathodes is challenging. Adjusting synthesis conditions and stoichiometry, an O3-type NaLi1/3Mn2/3O2 phase with anionic redox activity is reported.

Accelerated Fe2+ Regeneration in an Effective Electro-Fenton Process by Boosting Internal Electron Transfer to a Nitrogen-Conjugated Fe(III) Complex.

Abstract: The regeneration rate of Fe2+ from Fe3+ dictates the performance of the electro-Fenton (EF) process, represented by the amount of produced hydroxyl radicals (·OH). Current strategies for the acceleration of Fe2+ regeneration normally require additional chemical reagents, to vary the redox potential of Fe2+/Fe3+. Here, we report an attempt at using the intrinsic property of the electrode to our advantage, i.e., nitrogen-doped carbon aerogel (NDCA), as a reducing agent for the regeneration of Fe2+ without using foreign reagents. Moreover, the pyrrolic N in NDCA provides unpaired electrons through the carbon framework to reduce Fe3+, while the graphitic and pyridinic N coordinate with Fe3+ to form a C-O-Fe-N2 bond, facilitating electron transfer from both the external circuit and pyrrolic N to Fe3+. Our Fe2+/NDCA-EF system exhibits a 5.8 ± 0.3 times higher performance, in terms of the amount of generated ·OH, than a traditional Fenton system using the same Fe2+ concentration. In the subsequent reaction, the Fe2+/NDCA-EF system demonstrates 100.0% removal of dimethyl phthalate, 3-chlorophenol, bisphenol A, and sulfamethoxazole with a low specific energy consumption of 0.17-0.36 kW·h·g-1. Furthermore, 90.1 ± 0.6% removal of dissolved organic carbon and 83.3 ± 0.9% removal of NH3-N are achieved in the treatment of domestic sewage. The purpose of this work is to present a novel strategy for the regeneration of Fe2+ in the EF process and also to elucidate the role of different N species of the carbonaceous electrode in contributing to the redox cycle of Fe2+/Fe3+.

Sustainable redox processes induced by peroxymonosulfate and metal doping on amorphous manganese dioxide for nonradical degradation of water contaminants

Abstract: Manganese oxides have been applied in advanced oxidation processes (AOPs), however, underlying oxidation regimes are still debatable. In this work, we synthesized various crystalline manganese oxides and single or dual metal-doped amorphous MnO2 (M-AMO, M = Fe, Co, Ni, and Cu) for organic oxidation with peroxymonosulfate (PMS). M-AMO at 1 mol% exhibited much higher activities than crystalline manganese oxides and Cu-AMO displayed the rate constant at 3.5 times as high as that of AMO. Different from conventional radical oxidation, nonradical degradation occurred via two pathways. Cu-doping resulted in relatively higher-crystallized structure, more oxygen vacancies, and a higher ratio of Mn4+/Mn3+ for a faster redox cycle between MnⅣ(s) and MnⅢ(s) to boost PMS activation and direct/indirect (Cu-AMO–PMS complex-based) phenol oxidation. This study contributes to a new insight to the structure-catalysis relationship in manganese-catalyzed PMS oxidation. The outcomes will direct the rational synthesis of reaction-oriented catalysts for nonradical AOPs and novel remediation technologies.

Singlet oxygen triggered by robust bimetallic MoFe/TiO2 nanospheres of highly efficacy in solar-light-driven peroxymonosulfate activation for organic pollutants removal

Abstract: Sulfate-radical (SO4 −) based Advanced Oxidation Process (SR-AOP), which is mainly generated from peroxymonosulfate (PMS) activation, is an excellent route for water treatment. Bimetallic nanoparticles have been widely applied in electronic, chemical, biological, and mechanical fields, etc.; however, few researchers have attempted to adopt bimetallic nanoparticles in environmental remediation. Further, in recent years, element molybdenum (Mo) has addressed much more environmental field attention than ever. Although singlet oxygen (1O2) generated commonly in SR-AOPs, its generation mechanism remains controversial. Hence, in this work, bimetallic MoFe/TiO2 nanospheres were rationally constructed via a facile two-step methodology. Undoubtedly, it exhibited superior performance for the degradation of organic pollutants (e.g., rhodamine, phenol, 4-chlorophenol and sulfadiazine) irradiated by simulated solar light. Both photo-generated electrons and transition metallic redox couples (i.e., Mo6+/Mo4+, Fe3+/Fe2+ and Mo4+/Fe3+) play vital roles in the PMS activation. Distinct from conventional SR-AOPs, sulfate radicals (SO4 −), hydroxyl radicals ( OH) and peroxymonosulfate radicals (SO5 −) indeed participate in the transformation and generation of singlet oxygen (1O2). With the combination of DFT calculation, the Mo sites on the bimetallic MoFe (110) facet are more favorable to adsorb PMS molecules, then followed by the dissociation of PMS progressing on the Mo sites. Electrons transferring from the Mo atoms to the Fe atoms facilitated the adsorption of the negatively charged HSO5- anions, resulting in enhanced PMS activation efficiency. Considering its novelty and generation mechanism, this work highlights the mechanism of 1O2 generation from PMS reduction and oxidation simultaneously and furnishes theoretical support for further relevant studies.

Stable, active CO2 reduction to formate via redox-modulated stabilization of active sites.

Abstract: Electrochemical reduction of CO2 (CO2R) to formic acid upgrades waste CO2; however, up to now, chemical and structural changes to the electrocatalyst have often led to the deterioration of performance over time. Here, we find that alloying p-block elements with differing electronegativities modulates the redox potential of active sites and stabilizes them throughout extended CO2R operation. Active Sn-Bi/SnO2 surfaces formed in situ on homogeneously alloyed Bi0.1Sn crystals stabilize the CO2R-to-formate pathway over 2400 h (100 days) of continuous operation at a current density of 100 mA cm−2. This performance is accompanied by a Faradaic efficiency of 95% and an overpotential of ~ −0.65 V. Operating experimental studies as well as computational investigations show that the stabilized active sites offer near-optimal binding energy to the key formate intermediate *OCHO. Using a cation-exchange membrane electrode assembly device, we demonstrate the stable production of concentrated HCOO– solution (3.4 molar, 15 wt%) over 100 h. Stable electrochemical reduction to formate is still challenging. Here, the authors demonstrate a redox-modulation and active-site stabilization strategy for CO2 to formate conversion over 100 days of continuous operation at 100 mA/cm2 with a cathodic energy efficiency of 70%.

Tailoring Competitive Adsorption Sites by Oxygen-Vacancy on Cobalt Oxides to Enhance the Electrooxidation of Biomass.

Abstract: The electrooxidation of 5-hydroxymethylfurfural (HMF) offers a promising green route to attain high-value chemicals from biomass. The HMF electrooxidation reaction (HMFOR) is a complicated process involving the combined adsorption and coupling of organic molecules and OH- on the electrode surface. An in-depth understanding of these adsorption sites and reaction processes on electrocatalysts is fundamentally important. Herein, the adsorption behavior of HMF and OH- , and the role of oxygen vacancy on Co3 O4 are initially unraveled. Correspondingly, instead of the competitive adsorption of OH- and HMF on the metal sites, it is observed that the OH- can fill into oxygen vacancy (Vo) prior to couple with organic molecules through lattice oxygen oxidation reaction process, which could accelerate the rate-determining step of the dehydrogenation of 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) intermediates. With the modulated adsorption sites, the as-designed Vo-Co3 O4 shows excellent activity for HMFOR with the earlier potential of 90 and 120 mV at 10 mA cm-2 in 1 m KOH and 1 m PBS solution. This work sheds insight on the catalytic mechanism of oxygen vacancy, which benefits designing a novel electrocatalysts to modulate the multi-molecules combined adsorption behaviors.

Complexes of Fe(III)-organic pollutants that directly activate Fenton-like processes under visible light

Abstract: The major challenge of Fenton and Fenton-like technologies is promoting the effective transformation of Fe3+ to Fe2+. Photoinduced ligand-to-metal charge transfer (LMCT) enables charge to transfer effectively from the complex ligand to metal ions for the subsequent redox reactions. This study shows that photoactivated LMCTs relying on internal charge transfers occurred from the pollutant complex to the Fe3+ center and followed the in situ transformation of Fe3+ to Fe2+ without the addition of other ligands or agents. Using the antibiotic pollutant sulfamethoxazole (SMX), a direct Fe-SMX complex is formed and enables visible light to be used to activate peroxydisulfate (PDS) by Fe3+ for the rapid degradation of SMX at a rate 6.5-times higher than that observed by the conventional Fe2+/PDS system. This study outlines a new and cost-effective LMCT activation approach and broadens our knowledge of the ability of Fe3+ to be applied in Fenton-like reactions for environmental remediation.

Activating the I0/I+ redox couple in an aqueous I2–Zn battery to achieve a high voltage plateau

Abstract: Rechargeable iodine conversion batteries possess promising prospects for portable energy storage with complete electron transfer and rich valence supply. However, the reaction is limited to the single I−/I0 redox at a potential of only 0.54 V vs. the standard hydrogen electrode (SHE), leading to a low voltage plateau at 1.30 V when Zn is employed as the anode. Herein, we show how to activate the desired reversible I0/I+ redox behavior at a potential of 0.99 V vs. SHE by electrolyte tailoring via F− and Cl− ion-containing salts. The electronegative F− and Cl− ions can stabilize the I+ during charging. In an aqueous Zn ion battery based on an optimized ZnCl2 + KCl electrolyte with abundant Cl−, the I-terminated halogenated Ti3C2I2 MXene cathode delivered two well-defined discharge plateaus at 1.65 V and 1.30 V, superior to all reported aqueous I2–metal (Zn, Fe, Cu) counterparts. Together with the 108% capacity enhancement, the high voltage output resulted in a significant 231% energy density enhancement. Metallic Ti3C2I2 benefits the redox kinetics and confines the interior I species, leading to exceptional cyclic durability and rate capability. In situ Raman and ex situ multiple spectral characterizations clarify the efficient activation and stabilization effects of Cl− (F−) ions on reversible I0/I+ redox. Our work is believed to provide new insight into designing advanced I2–metal batteries based on the newly discovered I−/I0/I+ chemistry to achieve both high voltage and enhanced capacity.

Catalytic activity of metals in heterogeneous Fenton-like oxidation of wastewater contaminants: a review

Abstract: Innovations in water technology are needed to solve challenges of climate change, resource shortages, emerging contaminants, urbanization, sustainable development and demographic changes. In particular, conventional techniques of wastewater treatment are limited by the presence of poorly biodegradable organic matter. Alternatively, recent Fenton, Fenton-like and hybrid processes appear successful for cleaning of different types of liquid wastewaters. Here, we review the application of metallic catalyst-H2O2 systems in the heterogeneous Fenton process. Each metallic catalyst-H2O2 system has unique redox properties due to metal oxidation state. Solution pH is a major influencing factor. Catalysts made of iron and cerium form stable complexes with oxidation products and H2O2, thus resulting in reduced activities. Copper forms transitory complexes with oxidation products, but copper catalytic activity is restored during the reaction. Silver and manganese do not form complexes. The catalyst performance for degradation and mineralization decreases in the order: manganese, copper, iron, silver, cerium, yet the easiness of practical application decreases in the order: copper, manganese, iron, silver, cerium.

Coupling Glucose-Assisted Cu(I)/Cu(II) Redox with Electrochemical Hydrogen Production

Abstract: Water electrolysis is a sustainable technology for hydrogen production since this process can utilize the intermittent electricity generated by renewable energy such as solar, wind, and hydro. However, the large-scale application of this process is restricted by the high electricity consumption due to the large potential gap (>1.23 V) between the anodic oxygen evolution reaction and the cathodic hydrogen evolution reaction (HER). Herein, a novel and efficient hydrogen production system is developed for coupling glucose-assisted Cu(I)/Cu(II) redox with HER. The onset potential of the electrooxidation of Cu(I) to Cu(II) is as low as 0.7 VRHE (vs reversible hydrogen electrode). In situ Raman spectroscopy, ex situ X-ray photoelectron spectroscopy, and density functional theory calculation demonstrates that glucose in the electrolyte can reduce the Cu(II) into Cu(I) instantaneously via a thermocatalysis process, thus completing the cycle of Cu(I)/Cu(II) redox. The assembled electrolyzer only requires a voltage input of 0.92 V to achieve a current density of 100 mA cm-2 . Consequently, the electricity consumption for per cubic H2 produced in the system is 2.2 kWh, only half of the value for conventional water electrolysis (4.5 kWh). This work provides a promising strategy for the low-cost, efficient production of high-purity H2 .

Understanding the Roles of Electrogenerated Co 3+ and Co 4+ in Selectivity-Tuned 5-Hydroxymethylfurfural Oxidation

Abstract: The Co-based electrocatalyst is among the most promising candidates for electrochemical oxidation of 5-hydroxymethylfurfural (HMF). However, the intrinsic active sites and detailed mechanism of this catalyst remains unclear. We combine experimental evidence and a theoretical study to show that electrogenerated Co3+ and Co4+ species act as chemical oxidants but with distinct roles in selective HMF oxidation. It is found that Co3+ is only capable of oxidizing formyl group to produce carboxylate while Co4+ is required for the initial oxidation of hydroxyl group with significantly faster kinetics. As a result, the product distribution shows explicit dependence on the Co oxidation states and selective production of 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) and 2,5-furandicarboxylic acid (FDCA) are achieved by tuning the applied potential. This work offers essential mechanistic insight on Co-catalyzed organic oxidation reactions and might guide the design of more efficient electrocatalysts.