Metal-organic frameworks as template for synthesis of Mn3+/Mn4+ mixed valence manganese cobaltites electrocatalysts for oxygen evolution reaction

Defect Engineering in Composition and Valence Band Center of Y2(YxRu1-x)2O7-δ Pyrochlore Electrocatalysts for Oxygen Evolution Reaction

Oxygen evolution reaction (OER) takes place in various types of electrochemical devices that are pivotal for the conversion and storage of renewable energy. This paper describes a strategy in the design of solid-state structures of OER electrocatalysts through controlling the cation substitution on the active metal site and consequently valence band center position of site-mixed Y2(YxRu1-x)2O7-δ pyrochlore to achieve high catalytic activity. We found that partially replacing the B-site Ru4+ cation with A-site Y3+ in pyrochlore-structured Y2Ru2O7-δ modifies the oxidation state of B-site Ru from 4+ to 5+, as observed by electron paramagnetic resonance (EPR) spectroscopy but does not continuously increase the oxygen vacancy concentration in these oxygen substoichiometric compositions, as quantified by thermogravimetric analysis (TGA) decomposition studies. We found the increased Ru oxidation state leads to a downshift in valence band center. X-ray photoelectron spectroscopy (XPS) analysis was performed to quantitatively determine the optimal band center to be ∼1.27 eV below the Fermi energy level based on the analysis of the valence band edge of these Ru-based Y2(YxRu1-x)2O7-δ OER electrocatalysts. This work highlights that defect engineering can be a practical, effective approach to the optimization of oxidation state and electronic band center for high OER catalytic performance in a quantitative manner.

Co3O4/activated carbon nanocomposites as electrocatalysts for the oxygen evolution reaction

The Oxygen Evolution Reaction (OER) is crucial in various processes such as hydrogen production via water splitting. Several electrocatalysts, including metal oxides, have been evaluated to enhance the reaction efficiency. Zeolitic Imidazolate Framework-67 (ZIF-67) has been employed as a precursor to produce Co3O4, showing high OER activity. Additionally, the formation of composites with carbon-based materials improves the activity of these materials. Thus, this work focuses on synthesizing ZIF-67 and commercial activated carbon (AC) composites, which were used as precursors to obtain Co3O4/C electrocatalysts by calculating ZIF-67/CX (X = 10, 30, and 50, the mass percentage of AC). The obtained materials were thoroughly characterized by employing X-ray powder diffraction (XRD), confirming the cobalt oxide structure with a sphere-like morphology as observed in the TEM images. The presence of oxygen vacancies was confirmed by infrared spectroscopy and EPR measurements. The electrocatalytic performance in the OER was investigated by linear sweep voltammetry (LSV), which revealed an overpotential of 325 mV at 10 mA cm-2 and a Tafel slope value of 65.32 mV dec-1 for Co3O4/C10, superior in activity to several previously reported studies in the literature and electrochemical stability of up to 8 hours. The reduced value of charge transfer resistance, high double-layer capacitance, and the presence of Co3+ ions justify the superior performance of the Co3O4/C10 electrocatalyst.

Interface engineering of NiS/NiCo2S4 heterostructure with charge redistribution for boosting overall water splitting

Developing highly efficient bifunctional non-noble metal-based electrocatalysts is pivotal to fulfilling practical water electrolysis. In this work, NiS/NiCo2S4 heterostructured electrocatalysts are prepared through a simply controlling sulfurization process by employing a one-pot solvothermal strategy. The alteration of cobalt addition amount can affect the crystalline phase, morphology, and catalytic activity of the resulting heterostructured materials. The successful integration of NiS with NiCo2S4 is realized by deliberately tuning the cobalt addition amount. The resulting Co-Ni-S5:1 delivers high activity with low overpotentials of 198 and 259 mV to attain 10 mA cm-2 when used as electrocatalysts toward hydrogen evolution reaction and oxygen evolution reaction, respectively. Experimental and theoretical calculations evidence the strong interface coupling between NiS and NiCo2S4 leads to increased electronic conductivity, electron migration near lattice-matched interface and interfacial charge redistribution, thereof enhancing the reaction kinetics rate and activity. Moreover, the potential application is demonstrated by employing Co-Ni-S5:1 in a two-electrode electrolyzer which can efficiently catalyze water electrolysis and work stably for 100 h. This work not only provides highly efficient bifunctional heterostructured electrocatalysts by simply regulating the metal components in sulfides but also further broadens the application of interface engineering.

Operando Revealing the Crystal Phase Transformation and Electrocatalytic Activity Correlation of MnO2 toward Glycerol Electrooxidation

In this work, we report for the first time a comprehensive operando investigation of the intricate correlation between dynamic phase evolution and glycerol electrooxidation reaction (GEOR) performance across three primary MnO₂ crystallographic phases (α-, β-, and γ-MnO₂). The results showed that all three electrocatalysts exhibited comparable selectivity toward three-carbon products (∼90%), but γ-MnO₂ exhibited superior performance, with a low onset potential of ∼1.45 V_RHE, the highest current density of ∼1.9 mA cm^-2 at 1.85 V_RHE, and reasonable stability. Operando Raman spectroscopy revealed the potential-induced surface reconstruction of different MnO₂ structures from which a correlation among the applied potential, electrocatalytic activity, and product distribution was identified. The higher the applied potential, the greater conversion from the original structure to δ-MnO₂, resulting in lower C-C cleavage and higher 3C product selectivity. This study not only provides a systematic understanding of structure-controlled electrocatalytic activity for high selectivity toward 3C products of MnO₂ but also contributes to the development of a non-noble and environmentally friendly catalyst for valorizing glycerol.

Design of five two-dimensional Co-metal-organic frameworks for oxygen evolution reaction and dye degradation properties

Two-dimensional (2D) metal-organic frameworks (MOFs) have been extensively investigated as oxygen evolution reaction (OER) materials because of their numerous advantages such as large specific surface areas, ultrathin thicknesses, well-defined active metal centers, and adjustable pore structures. Five Co-metal-organic frameworks, namely, [Co(L) (4.4′-bbidpe)H2O]n [YMUN 1 (YMUN for Youjiang Medical University for Nationalities)], {[Co2(L)2 (4.4′-bbibp)2]·[Co3(L) (4.4′-bbibp)]·DMAC}n (YMUN 2), [Co(L) (3,5-bip)]n (YMUN 3), [Co(L) (1,4-bimb)]n (YMUN 4), and [Co(L) (4.4′-bidpe)H2O]n (YMUN 5), were designed and fabricated from flexible dicarboxylic acid 1,3-bis(4′-carboxylphenoxy)benzene (H2L) and rigid/flexible imidazole ligands. Their frameworks consist of two-dimensional lamellar networks with a number of differences in their details. Their frameworks are discussed and compared, and their oxygen evolution reaction electrochemical activities and photocatalysis dye degradation properties are investigated.

Engineering Oxygen Vacancies on Mixed-Valent Mesoporous α-MnO2 for High-Performance Asymmetric Supercapacitors

Intrinsically poor conductivity and sluggish ion-transfer kinetics limit the further development of electrochemical storage of mesoporous manganese dioxide. In order to overcome the challenge, defect engineering is an effective way to improve electrochemical capability by regulating electronic configuration at the atomic level of manganese dioxide. Herein, we demonstrate effective construction of defects on mesoporous α-MnO₂ through simply controlling the degree of redox reaction process, which could obtain a balance between Mn³⁺/Mn⁴⁺ ratio and oxygen vacancy concentration for efficient supercapacitors. The different structures of α-MnO₂ including the morphology, specific surface area, and composition are successfully constructed by tuning the mole ratio of KMnO₄ to Na₂SO₃. The electrode materials of α-MnO₂-0.25 with an appropriate Mn³⁺/Mn⁴⁺ ratio and abundant oxygen vacancy showed an outstanding specific capacitance of 324 F g^-1 at 0.5 A g^-1, beyond most reported MnO₂-based materials. The asymmetric supercapacitors formed from α-MnO₂-0.25 and activated carbon can present an energy density as high as of 36.33 W h kg^-1 at 200 W kg^-1 and also exhibited good cycle stability over a wide voltage range from 0 to 2.0 voltage (kept at approximately 98% after 10 000 cycles in galvanostatic cycling tests) and nearly 100% Coulombic efficiency. Our strategy lays a foundation for fine regulation of defects to improve charge-transfer kinetics.

Nanoparticles of Mixed-Valence Oxides MnXCO3-XO4 (0 ≤ X ≤ 1) Obtained with Agar-Agar from Red Algae (Rhodophyta) for Oxygen Evolution Reaction

The development of efficient electrocatalysts for the oxygen evolution reaction (OER) is of paramount importance in sustainable water-splitting technology for hydrogen production. In this context, this work reports mixed-valence oxide samples of the Mn_XCo_3-XO₄ type (0 ≤ X ≤ 1) synthesized for the first time by the proteic sol-gel method using Agar-Agar as a polymerizing agent. The powders were calcined at 1173 K, characterized by FESEM, XRD, RAMAN, UV-Vis, FT-IR, VSM, and XPS analyses, and were investigated as electrocatalysts for the oxygen evolution reaction (OER). Through XRD analysis, it was observed that the pure cubic phase was obtained for all samples. The presence of Co³⁺, Co²⁺, Mn²⁺, Mn³⁺, and Mn⁴⁺ was confirmed by X-ray spectroscopy (XPS). Regarding the magnetic measurements, a paramagnetic behavior at 300 K was observed for all samples. As far as OER is concerned, it was investigated in an alkaline medium, where the best overpotential of 299 mV vs. RHE was observed for the sample (MnCo₂O₄), which is a lower value than those of noble metal electrocatalysts in the literature, together with a Tafel slope of 52 mV dec^-1, and excellent electrochemical stability for 15 h. Therefore, the green synthesis method presented in this work showed great potential for obtaining electrocatalysts used in the oxygen evolution reaction for water splitting.

Synthesis of amorphous FeNiCo trimetallic hybrid electrode from ZIF precursors for efficient oxygen evolution reaction

Development of non-noble multi-metallic electrocatalyst with high oxygen evolution reaction (OER) activity via a simple and low-cost method is of great importance for improving the efficiency of water electro-chemical splitting. Herein, a solution impregnation strategy was proposed to synthesize novel FeNi-doped Co-ZIF-L trimetallic hybrid electrocatalyst using Co-ZIF-L as sacrificial templates and Fe and Ni ions as etchants and dopants. This synthetic strategy could be realized via the etching-coprecipitation mechanism to obtain an amorphous hybrid containing multi-metal hydroxides. The as-prepared electrocatalyst loaded on Ni foam displays a low overpotential of 245 mV at 10 mA·cm^-2, a small Tafel slope of 54.9 mV·dec^-1, and excellent stability at least 12 h in the OER process. The facile and efficient synthetic strategy presents a new entry for the fabrication of ZIFs-derived multi-metallic electrocatalysts for OER electrocatalysis.

Research on the quantum confinement effect and enhanced luminescence of red-emitting P5+-doped CaAl12O19:Mn4+,Mg2+ phosphors

Mn⁴⁺-activated oxide red phosphors are always a hot topic in the luminescent material field to solve the lack of red light components in white-light-emitting diodes (WLEDs). Herein, a series of novel deep red-emitting CaAl_12-mP_mO_19+m:0.01Mn⁴⁺,0.2Mg²⁺ (m = 0-0.15) phosphors were synthesized and their crystal structure, luminescence properties and thermal stability were investigated in detail. The high-valence P⁵⁺ is used to replace low-valence Al³⁺ in the luminescent host CaAl₁₂O₁₉ to improve the photoluminescence quantum yield (PLQY) of phosphors. The doping of P⁵⁺ does not change the crystal phase structure of phosphors, and the luminescence intensity and PLQY are significantly enhanced. The analysis of the photocurrent and fluorescence lifetime shows that an electron trap with a quantum-confinement structure is formed in the phosphor host, which plays a key role in buffering photogenerated electrons. Therefore, the PLQY of the P⁵⁺-doped CaAl_11.90P_0.1O_19.10:0.01Mn⁴⁺,0.2Mg²⁺ phosphor increased from 9.8% (P⁵⁺-undoped) to 70.2%, and the mechanism of PLQY enhancement is proposed based on the analysis of the crystal structure. Furthermore, the phosphor has superior thermal stability and color purity (96.8%). Overall, this work provides new insights and ideas on quantum confinement effects for improving the quantum yield of Mn⁴⁺-activated luminescent materials.

Iron Doped in the Subsurface of CuS Nanosheets by Interionic Redox: Highly Efficient Electrocatalysts toward the Oxygen Evolution Reaction

Modifying the electronic structure of electrocatalysts by metal doping is favorable to their electrocatalytic activity. Herein, by a facile one-pot redox process of Fe(III) and Cu(I), Fe(II) was successfully doped into the subsurface of CuS nanosheets (NSs) for the first time to obtain a novel electrocatalyst (Fe_sub-CuS NSs) that possesses not only subtle lattice defects but also an atomic-level coupled nanointerface, greatly enhancing the oxygen evolution reaction (OER) performances. Meanwhile, Fe(II) and Fe(III) coexisting in Fe_sub-CuS nanosheets are favorable to OER through valence regulation. As expected, by simultaneously controlling the abovementioned three factors to optimize Fe_sub-CuS nanosheets, they display a lower overpotential of 252 mV at a current density of 20 mA cm^-2 for OER, better than 389 mV for pristine CuS nanosheets. This discovery furnishes low-cost and efficient Cu-based electrocatalysts by metal doping. Density functional theory (DFT) calculations further verify that Fe_sub-CuS(100) is thermodynamically stable and is more active for OER. This research provides a strategy for the atomic-scale engineering of nanocatalysts and also sheds light on the design of novel and efficient electrocatalysts.

Over-Reduction-Controlled Mixed-Valent Manganese Oxide with Tunable Mn2+/Mn3+ Ratio for High-Performance Asymmetric Supercapacitor with Enhanced Cycling Stability

Manganese oxides composed of various valence states Mn^x+ (x = 2, 3, and 4) have attracted wide attention as promising electrode materials for asymmetric supercapacitor. However, the poor electrical conductivity limited their performance and application. Appropriate regulation content of Mn^x+ in mixed-valent manganese oxide can tune the electronic structure and further improve their conductivity and performance. Herein, we prepared manganese oxides with different Mn²⁺/Mn³⁺ ratios through an over-reduction (OR) strategy for tuning the internal electron structure of mixed-valent manganese, which could make these material oxides a good platform for researching the structure-property relationships. The Mn²⁺/Mn³⁺ ratio of manganese oxide could be precisely tuned from 0.6 to 1.7 by controlling the amount of reducing agent for manipulating the redox processes, where the manganese oxide electrode with the most appropriate Mn²⁺/Mn³⁺ ratio, as 1.65 (OR4) exhibits large capacitance (274 F g^-1) and the assembling asymmetric supercapacitors by combining OR4 (positive) and the commercial activated carbon (as negative) achieved large 2.0 V voltage window and high energy density of 27.7 Wh kg^-1 (power density of 500 W kg^-1). The cycle lifespan of the OR4//AC could keep about 92.9% after 10 000-cycle tests owing to the Jahn-Teller distortion of the Mn(III)O₆ octahedron, which is more competitive compared to other work. Moreover, a red-light-emitting diode (LED) can easily be lit for 15 min by two all-solid supercapacitor devices in a series.