Mn doping of cobalt oxynitride coupled with N-rGO nanosheets hybrid as a highly efficient electrocatalyst for oxygen reduction and oxygen evolution reaction

Boosting oxygen reduction via MnP nanoparticles encapsulated by N, P-doped carbon to Mn single atoms sites for Zn-air batteries

An electrocatalyst of single-atomic Mn sites with MnP nanoparticles (NPs) on N, P co-doped carbon substrate was constructed to enhance the catalytic activity of oxygen reduction reaction (ORR) through one-pot in situ doping-phosphatization strategy. The optimized MnSA-MnP-980℃ catalyst exhibits an excellent ORR activity in KOH electrolyte with a half-wave potential (E1/2) of 0.88 V (vs. RHE), and the ORR current density of MnSA-MnP-980℃ maintained 97.9 % for over 25000 s chronoamperometric i-t measurement. When using as the cathode, the MnSA-MnP-980℃ displays a peak power density of 51 mW cm-2 in Zinc-Air batteries, which observably outperformed commercial Pt/C (20 wt%). The X-ray photoelectron spectroscopy reveal that the doped P atoms with a strong electron-donating effectively enhances electron cloud density of Mn SAs sites, facilitating the adsorption of O2 molecules. Meanwhile, the introduction of MnP NPs can regulate the electronic structure of Mn SAs sites, making Mn SAs active sites exist in a low oxidation state and are less positively charged, which can supply electrons for ORR process to narrow the adsorption energy barrier of ORR intermediates. This work constructs novel active sites with excellent ORR properties and provides valuable reference for the development of practical application.

Catalytic advancements in carbonaceous materials for bio-energy generation in microbial fuel cells: a review

Microbial fuel cells (MFCs) are a sustainable alternative for wastewater treatment and clean energy generation. The efficiency of the technology is dependent on the cathodic oxygen reduction reaction, where the sluggish reaction kinetics hampers its propensity. Carbonaceous materials with high electrical conductivity have been widely explored for oxygen reduction reaction (ORR) catalysts. Here, incorporating transition metal (TM) and heteroatom into carbon could further enhance the ORR activity and power generation in MFCs. Nitrogen (N)-doped carbons have also been a popular research hotspot due to abundant active sites formed, resulting in superior conductivity, stability, and catalytic activity over carbons. This review summarizes the progress in the carbon-based materials (primary focus on the cathode) for ORR and their utilization in MFCs. Furthermore, we discussed the conceptualization of MFCs and carbonaceous materials to instigate the ORR kinetics and power generation in MFC. Furthermore, prospects of carbon-based materials for actual application in bio-energy generation have been discussed. Carbonaceous catalysts and biomass-derived carbons exhibit good potential to replace precious Pt catalysts for ORR. M-N-C catalysts were found to be the most suitable catalysts. Electrocatalysts with MN_x sites are able to achieve excellent activity and high-power output by taking advantage of the active site exposure and rapid mass transfer rate. Moreover, the use of biomass-derived carbons/self-doped carbons could further reduce the overall cost of catalysts. It is anticipated that the research gaps and future perspectives discussed will show new avenues to develop excellent electrocatalysts for better performance and transformation of technology to industrial applications.

Modulating the electrocatalytic activity of N-doped carbon frameworks via coupling with dual metals for Zn-air batteries

N-Doped carbon electrocatalysts are a promising alternative to precious metal catalysts to promote oxygen reduction reaction (ORR). However, it remains a challenge to design the desired active sites on carbon skeletons in a controllable manner for ORR. Herein, we developed a facile approach based on oxygen-mediated solvothermal radical reaction (OSRR) for preparation of N-doped carbon electrocatalysts with a pre-designed active site and modulated catalytic activity for ORR. In the OSRR, 2-methylimidazole reacted with Co and Mn salts to form an active site precursor (MnCo-MIm) in N-methyl-2-pyrrolidone (NMP) at room temperature. Then, the reaction temperature increased to 140 °C under an oxygen atmosphere to generate NMP radicals, followed by their polymerization with the pre-formed MnCo-MIm to produce Mn-coupled Co nanoparticle-embedded N-doped carbon framework (MnCo-NCF). The MnCo-NCF showed uniform dispersion of nitrogen atoms and Mn-doped Co nanoparticles on the carbon skeleton with micropores and mesopores. The MnCo-NCF exhibited higher electrocatalytic activity for ORR than did a Co nanoparticle only-incorporated carbon framework due to the improved charge transfer from the Mn-doped Co nanoparticles to the carbon skeleton. In addition, the Zn-air battery assembled with MnCo-NCF had superior performance and durability to the battery using commercial Pt/C. This facile approach can be extended for designing carbon electrocatalysts with desired active sites to promote specific reactions.

Nickel decorated manganese oxynitride over graphene nanosheets as highly efficient visible light driven photocatalysts for acetylsalicylic acid degradation

In this work, we prepared nanocomposites of nickel-decorated manganese oxynitride on graphene nanosheets and demonstrated them as photocatalysts for degradation of acetylsalicylic acid (ASA). The catalyst exhibited a high degradation efficiency over ASA under visible light irradiation and an excellent structural stability after multiple uses. Compared to manganese oxide (MnO) and manganese oxynitride (MnON) nanoparticles, larger specific surface area and smaller band gap were observed for the nanocomposite accounting for the enhanced photocatalytic efficiency. Besides the compositional effect of the catalyst, we also examined the influence of various experimental parameters on the degradation of ASA such as initial concentration, catalyst dose, initial pH and additives. The best performance was obtained for the nanocomposite when the catalyst dose was 10 mg/mL and the initial pH 3. Detection of intermediates during photocatalysis showed that ASA undergoes hydroxylation, demethylation, aromatization, ring opening, and finally complete mineralization into CO₂ and H₂O by reactive species. For practical applications as a photocatalyst, cytotoxicity of the nanocomposite was also evaluated, which revealed its insignificant impact on the cell viability. These results suggest the nanocomposite of nickel-decorated manganese oxynitride on graphene nanosheets as a promising photocatalyst for the remediation of ASA-contaminated water.

Transition-Metal Oxides/Carbides@Carbon Nanotube Composites as Multifunctional Electrocatalysts for Challenging Oxidations and Reductions

The rapid development of renewable-energy technologies such as water splitting, rechargeable metal-air batteries, and fuel cells requires highly efficient electrocatalysts capable of the oxygen-reduction reaction (ORR) and the oxygen-evolution reaction (OER). Herein, we report a facile sonication-driven synthesis to deposit the molecular manganese vanadium oxide precursor [Mn₄ V₄ O₁₇ (OAc)₃ ]^3- on multiwalled carbon nanotubes (MWCNTs). Thermal conversion of this composite at 900 °C gives nanostructured manganese vanadium oxides/carbides, which are stably linked to the MWCNTs. The resulting composites show excellent electrochemical reactivity for ORR and OER, and significant reactivity enhancements compared with the precursors and a Pt/C reference are reported. Notably, even under harsh acidic conditions, long-term OER activity at low overpotential is reported. In addition, we report exceptional activity of the composites for the industrially important Cl₂ evolution from an aqueous HCl electrolyte. The new composite material shows how molecular deposition routes leading to highly active and stable multifunctional electrocatalysts can be developed. The facile design could in principle be extended to multiple catalyst classes by tuning of the molecular metal oxide precursor employed.

Regulating Phase Conversion from Ni3 Se2 into NiSe in a Bifunctional Electrocatalyst for Overall Water-Splitting Enhancement

Phase engineering has been demonstrated as an efficient method for the enhancement of catalytic activity. This study concerns the phase and morphology modulation of Ni₃ Se₂ /NiSe nanorod arrays through a hydrothermal process. Partial phase conversion can effectively enhance the electrical conductivity and yield more active sites through atom rearrangement during phase transformation. Quite low optimal overpotentials of 166 mV for the hydrogen evolution reaction (HER) and 370 mV for the oxygen evolution reaction (OER) are obtained in a sample containing 32.4 % of NiSe phase and 67.6 % of Ni₃ Se₂ phase. The performance is superior to the samples with only one phase. Furthermore, a water electrolyzer was assembled by using two symmetrical NiSe/Ni foam electrodes as the anode and cathode, which can deliver 10 mA cm^-2 at a low voltage of 1.61 V. More significantly, the water electrolyzer can be operated at 10 mA cm^-2 over 10 h without noticeable degradation, showing extraordinary operational stability. This phase conversion control strategy provides a new way to improve the catalytic activity of NiSe and may have potential use in the design of other selenide electrocatalysts.

A Hybrid Material Combined Copper Oxide with Graphene for an Oxygen Reduction Reaction in an Alkaline Medium

In this work, an electrode material based on CuO nanoparticles (NPs)/graphene (G) is developed for ORR in alkaline medium. According to the characterization of scanning electron microscope and transmission electron microscope, CuO NPs are uniformly distributed on the wrinkled G sheets. The X-ray diffraction test reveals that the phase of CuO is monoclinic. The CuO/G hybrid electrode exhibits a positive onset potential (0.8 V), high cathodic current density (3.79 × 10^-5 mA/cm²) and high electron transfer number (four-electron from O₂ to H₂O) for ORR in alkaline media. Compared with commercial Pt/C electrocatalyst, the CuO/G electrode also shows superior fuel durability. The high electrocatalytic activity and durability are attribute to the strong coupling between CuO NPs and G nanosheets.

Sn(OH)x-assisted synthesis of mesoporous Mn-porphyrinic frameworks and their carbon derivatives for electrocatalysis

5,10,15,20-Tetrakis (4-aminophenyl) Mn-porphyrin and 2,4,6-trihydroxy-1,3,5-benzenetricarbaldehyde were combined into a new mesoporous organic framework by a Schiff-base-type reaction.

Synthesis and Characterizations of Zinc Oxide on Reduced Graphene Oxide for High Performance Electrocatalytic Reduction of Oxygen

Electrocatalysts for the oxygen reduction (ORR) reaction play an important role in renewable energy technologies, including fuel cells and metal-air batteries. However, development of cost effective catalyst with high activity remains a great challenge. In this feature article, a hybrid material combining ZnO nanoparticles (NPs) with reduced graphene oxide (rGO) is applied as an efficient oxygen reduction electrocatalyst. It is fabricated through a facile one-step hydrothermal method, in which the formation of ZnO NPs and the reduction of graphene oxide are accomplished simultaneously. Transmission electron microscopy and scanning electron microscopy profiles reveal the uniform distribution of ZnO NPs on rGO sheets. Cyclic voltammograms, rotating disk electrode and rotating ring disk electrode measurements demonstrate that the hierarchical ZnO/rGO hybrid nanomaterial exhibits excellent electrocatalytic activity for ORR in alkaline medium, due to the high cathodic current density (9.21 × 10⁻⁵ mA/cm²), positive onset potential (−0.22 V), low H₂O₂ yield (less than 3%), and high electron transfer numbers (4e from O₂ to H₂O). The proposed catalyst is also compared with commercial Pt/C catalyst, comparable catalytic performance and better stability are obtained. It is expected that the ZnO/rGO hybrid could be used as promising non-precious metal cathode in alkaline fuel cells.

Template-Directed Growth of Bimetallic Prussian Blue-Analogue Nanosheet Arrays and Their Derived Porous Metal Oxides for Oxygen Evolution Reaction

Coordination polymers (CPs) are ideal precursors for synthesizing porous catalysts. However, the direct thermolysis of CPs is prone to generate agglomerates, greatly reducing the electrical conductivity and active sites of their derived catalysts. The construction of well-ordered CP nanostructures is a promising strategy for alleviating the above issue, but it remains challenging. Here, a facile chemical etching approach is developed for the fabrication of well-aligned three-dimensional (3D) bimetallic Prussian blue-analogue nanosheet arrays. Impressively, the derived porous metal oxide (Fe-NiO) acts as a remarkable oxygen evolution reaction (OER) catalyst, which merely requires overpotentials as low as 218 and 270 mV to achieve 10 and 100 mA cm^-2 in 1.0 m KOH aqueous solution, respectively. The excellent electrocatalytic performance of Fe-NiO is ascribed to the 3D porous nanosheet array architecture, which endows the bimetallic catalyst with abundant electrocatalytic active sites, enhanced surface permeability, and high electronic conductivity. It is expected that the proposed strategy can pave a new way for fabricating highly efficient electrocatalysts for energy storage and conversion.