Correlations among Structure, Electronic Properties, and Photochemical Water Oxidation: A Case Study on Lithium Cobalt Oxides

Formulating the Li sites of Li-CoOx composites for achieving high-efficiency oxidation removal of formaldehyde over the Ag/Li-CoOx catalyst under ambient conditions

Oxide supported noble metals are extensively investigated for ambient formaldehyde oxidation, and the Ag-CoOx complex is one promising combination in terms of cost and activity. Further, we previously observed that cooperating Ag with Li + greatly boosted formaldehyde degradation on CoOx. Yet, there is still room for improvement in removal efficiency, mineralization capacity and resistance to severe conditions. These objectives could be realized via strategically formulating the Li+ sites of Li-CoOx composite in this sister study. Three samples with Li + ---Co3+-O2- connections (L-CO), spinel Li+ (LCO-S) and layered Li+ (LCO-L) were obtained at low (300 °C), moderate (500 °C) and high (700 °C) temperatures, respectively. The specific Li+ positions and componential interaction were demonstrated by Hyperspectral imaging (HSI), XRD, SEM, TEM, HAADF mapping, UV-vis DRS and XPS. Moreover, the effect of reactive oxygen exposure on catalytic oxidation of formaldehyde (330-350 mg/m3) was disclosed through CO-TPR and O2-TPD. Compared with the LCO-S and LCO-L, L-CO exhibited dominant formaldehyde degradation due to the larger content of surface oxygen. After Ag decoration, the Li+---Co3+-O2- connections uniquely caused a strong binding of Ag species with catalyst host, which boosted the amount of reactive oxygen and finally resulted in an even higher elimination of ∼73% (CO2 yield = ∼21%), 47% higher than that of the L-CO (CO2 yield = ∼6%). But in contrast, the Ag@LCO-S only achieved ∼53% removal (CO2 yield = ∼9%) and Ag modification was powerless in altering the inertness of LCO-L, demonstrating that the chemical environment of alkali metal is crucial to effectively tuning the catalyst activity. The advantage of Ag@L-CO in formaldehyde depollution was further reflected from its much better resistance to moisture and aromatic compound omnipresent in indoor air. For the first time, this study extended the understanding of the alkali-metal-promoted formaldehyde oxidation reaction to an in-depth level.

Photoreduction of carbon dioxide and photodegradation of organic pollutants using alkali cobalt oxides MCoO2 (M = Li or Na) as catalysts

The recycling of lithium batteries should be prioritized, and the use of discarded alkali metal battery electrode materials as photocatalysts merits research attention. This study synthesized alkali metal cobalt oxide (MCoO₂, M = Li or Na) as a photocatalyst for the photoreduction of CO₂ and degradation of toxic organic substances. The optimized NaCoO₂ and LiCoO₂ photocatalysts increased the photocatalytic CO₂-CH₄ conversion rate to 21.0 and 13.4 μmol g^-1 h^-1 under ultraviolet light irradiation and to 16.2 and 5.3 μmol g^-1 h^-1 under visible light irradiation, which is 17 times higher than that achieved by TiO₂ P25. The rate constants of the optimized reactions of crystal violet (CV) with LiCoO₂ and NaCoO₂ were 2.29 × 10^-2 and 4.35 × 10^-2 h^-1, respectively. The quenching effect of the scavengers and electron paramagnetic resonance in CV degradation indicated that active O₂^•-, ¹O₂, and h⁺ play the main role, whereas ^•OH plays a minor role for LiCoO₂. The hyperfine splitting of the DMPO-^•OH and DMPO-^•CH₃ adducts was a_N = 1.508 mT, a_Hβ = 1.478 mT and a_N = 1.558 mT, a_Hβ = 2.267 mT, respectively, whereas the hyperfine splitting of DMPO^+• was a_N = 1.475 mT. The quenching effect also indicated that active O₂^•- and h⁺ play the main role and that ^•OH and ¹O₂ play a minor role for NaCoO₂. The hyperfine splitting of the DMPO-^•OH and DMPO^+• adducts was a_N = 1.517 mT, a_Hβ = 1.489 mT and a_N = 1.496 mT, respectively. Discarded alkali metal battery electrode materials can be reused as photocatalysts to address environmental pollution.

High Voltage Electrodes for Li-Ion Batteries and Efficient Water Electrolysis: An Oxymoron?

We demonstrate that key parameters for efficient electrocatalytic oxidation of water are the energetics of the redox complexes associated with their ionization and electrochemical potentials coupled to the change of metal-oxygen band hybridization. We investigate the catalytic activity of the LiCoPO₄-LiCo₂P₃O₁₀ tailored compound, which is a 5 V cathode material for Li-ion batteries. The reason for the weak catalytic activity of the lithiated compound toward the oxygen evolution reaction is a large energy difference between the electronic states involved in the electrochemical reaction. A highly active catalyst is obtained by tuning the relative energetic position of the electronic levels involved in the charge transfer reaction, which in turn are governed by the lithium content. A significant lowering of the overpotential from >550 mV to ∼370 mV at 10 mA cm^-2 is achieved via a decrease of the ionization potential and shifting the electrochemical potential near the electronic states of the molecule, thereby facilitating water oxidation.

Next-Generation Complex Metal Oxide Nanomaterials Negatively Impact Growth and Development in the Benthic Invertebrate Chironomus riparius upon Settling

Most studies of nanomaterial environmental impacts have focused on relatively simple first-generation nanomaterials, including metals or metal oxides (e.g., Ag, ZnO) for which dissolution largely accounts for toxicity. Few studies have considered nanomaterials with more complex compositions, such as complex metal oxides, which represent an emerging class of next-generation nanomaterials used in commercial products at large scales. Importantly, many nanomaterials are not colloidally stable in aqueous environments and will aggregate and settle, yet most studies use pelagic rather than benthic-dwelling organisms. Here we show that exposure of the model benthic species Chironomus riparius to lithium cobalt oxide (Li _xCo_{1- x}O₂, LCO) and lithium nickel manganese cobalt oxide (Li _xNi _yMn _zCo_{1- y- z}O₂, NMC) at 10 and 100 mg·L^-1 caused 30-60% declines in larval growth and a delay of 7-25 d in adult emergence. A correlated 41-48% decline in larval hemoglobin concentration and related gene expression changes suggest a potential adverse outcome pathway. Metal ions released from nanoparticles do not cause equivalent impacts, indicating a nanospecific effect. Nanomaterials settled within 2 days and indicate higher cumulative exposures to sediment organisms than those in the water column, making this a potentially realistic environmental exposure. Differences in toxicity between NMC and LCO indicate compositional tuning may reduce material impact.

Mechanistic Insight into Dioxygen Evolution from Diastereomeric μ-Peroxo Dinuclear Co(III) Complexes Based on Stoichiometric Electron-Transfer Oxidation

Stoichiometric electron-transfer (ET) oxidation of two diastereomeric μ-peroxo-μ-hydroxo dinuclear Co(III) complexes with tris(2-pyridylmethyl)amine (TPA) was examined to scrutinize the reaction mechanism of O₂ evolution from the peroxo complexes, as seen in the final step in water oxidation by a Co(III)-TPA complex. The two isomeric Co(III)-peroxo complexes were synthesized and selectively isolated by recrystallization under different conditions. Although cyclic voltammograms of the two isomers in aqueous solutions showed one reversible wave at 1.1 V vs NHE at pH 2.0, two oxidation waves were observed at 1.0 and 1.4 V at pH 7.0 in the aqueous solutions, the latter of which is responsible for the O₂-releasing process. At pH 7, one diastereomer showed higher reactivity than the other in O₂ evolution, indicating the importance of structures of the μ-peroxo complexes in the reaction. In order to clarify the O₂-evolving mechanism, we performed electron paramagnetic resonance (EPR) and resonance Raman (RR) measurements for characterizing one-electron oxidized species: The observed EPR and RR signals supported the formation of μ-superoxo-μ-hydroxo dinuclear Co(III) complexes; however, no characteristic difference was observed between two isomers in the EPR parameters including g values and superhyperfine coupling constants. ET-oxidation rate constants of the isomers were determined to be much faster than the O₂-evolving rate constants, indicating that the O₂-releasing step is the rate-determining step in the O₂ evolution through the stoichiometric ET oxidation of the dinuclear Co(III)-μ-peroxo complexes. Therefore, the difference of reactivity in the O₂ evolution for the two isomers should be derived from the thermodynamic stability of two-electron oxidized species of the dinuclear Co(III)-μ-peroxo complexes, μ-dioxygen-μ-hydroxo dinuclear Co(III) intermediates.

Three polyoxometalates-based organic–inorganic hybrids decorated with Cu–terpyridine complexes exhibiting dual functional electro-catalytic behaviors

Three new polyoxometalates-based organic–inorganic hybrids decorated with Cu–terpyridine complexes were prepared by using one-pot methods. Compounds 1–3 demonstrate discrepant dual functional electro-catalytic activities toward reduction of nitrite and oxidation of ascorbic acid.

From an Fe2P3 complex to FeP nanoparticles as efficient electrocatalysts for water-splitting

In large-scale, hydrogen production from water-splitting represents the most promising solution for a clean, recyclable, and low-cost energy source. The realization of viable technological solutions requires suitable efficient electrochemical catalysts with low overpotentials and long-term stability for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) based on cheap and nontoxic materials. Herein, we present a unique molecular approach to monodispersed, ultra-small, and superiorly active iron phosphide (FeP) electrocatalysts for bifunctional OER, HER, and overall water-splitting. They result from transformation of a molecular iron phosphide precursor, containing a [Fe2P3] core with mixed-valence FeIIFeIII sites bridged by an asymmetric cyclo-P(2+1) 3- ligand. The as-synthesized FeP nanoparticles act as long-lasting electrocatalysts for OER and HER with low overpotential and high current densities that render them one of the best-performing electrocatalysts hitherto known. The fabricated alkaline electrolyzer delivered low cell voltage with durability over weeks, representing an attractive catalyst for large-scale water-splitting technologies.

Recycled LiCoO2in spent lithium-ion battery as an oxygen evolution electrocatalyst

Lithium cobalt oxide (LCO) is a common cathode material in lithium ion batteries (LIBs).

Coupling carbon dioxide reduction with water oxidation in nanoscale photocatalytic assemblies

Closing the photosynthetic cycle on the nanometer scale under membrane separation of the half reactions for developing scalable artificial photosystems.

A High-Performance Electrocatalyst for Oxygen Evolution Reaction: LiCo0.8 Fe0.2 O2

Layered LiCo0.8 Fe0.2 O2 demonstrates dramatically enhanced oxygen evolution reaction (OER) activity and durability in an alkaline solution over LiCoO2 and other reported state-of-the-art catalysts, including benchmark IrO2 . This superior performance is attributed to Fe-doping-induced synergistic effects.