Robust Covalent Organic Framework Photocatalysts for H2O2 Production: Linkage Position Matters.

Covalent organic frameworks (COFs) are promising photocatalysts for hydrogen peroxide (H2O2) synthesis. However, the nature of organic polymers makes the balance between high activity and stability challenging. We demonstrate that the linkage position matters in the design of robust COF photocatalysts with durable high activity without sacrificial reagents. COFs with ortho- and para-linkages (o-COFs and p-COFs) were constructed by 1,3,5-triformylphloroglucinol with benzene-, pyridine-, pyrazine-orthodiamines and paradiamines. The pyrzaine-containing o-COFs with two pyridinic nitrogen atoms exhibited a H2O2 production rate of 4396 µmol g-1 h-1 together with long-time continuous H2O2 photosynthesis performance in pure water (48 h), superior to the corresponding p-COFs. A four-step reaction mechanism is proposed by density function calculations. Moreover, the active sites and origin of stability enhancement for o-COFs are clarified. This work provides a simple and effective molecular design strategy in the design of robust COF photocatalysts for artificial H2O2 photosynthesis.

Weakly Hydrophilic Imine-Linked Covalent Benzene-Acetylene Frameworks for Photocatalytic H2O2 Production in the Two-Phase System.

Conversing oxygen (O2) to hydrogen peroxide (H2O2) driven by solar energy is a promising H2O2 onsite production route but often short of efficient and durable photocatalysts. Herein, strong π-π conjugate polycyclic aromatic benzene and acetylene units have been constructed into new covalent organic frameworks (COFs) linked by imine C═N bonding. These COFs demonstrated two-dimensional hexagonal crystalline frameworks with higher crystallinity and larger surface area (>600 m2 g-1). Covalent benzene-acetylene frameworks possessed appropriate visible light-responsive band structure and the suppressed charge recombination rate. The -OH groups on their frameworks enable them to be weakly hydrophilic. As a result, it served as high-performance but durable photocatalysts for H2O2 production in the water-benzyl alcohol (BA) two-phase system. It delivered a H2O2 production rate of 1240 µmol h-1 gcat-1 and durable catalytic efficiency within 60 h, comparable to the best COF-based catalysts. This study provides an efficient two-phase photocatalytic system for H2O2 production based on weakly hydrophilic imine-linked benzene-acetylene organic photocatalysts.

Activated biochar derived from peanut shells as the electrode materials with excellent performance in Zinc-air battery and supercapacitance.

The use of activated biochar-based electrode derived from waste biomass in energy technologies, such as metal-air batteries and supercapacitors, is an important strategy for realizing energy and environmental sustainability in the future. Herein, peanut shells (waste biomass) were employed to prepare activated biochar materials by pyrolysis in molten KCl and heat-treatment. The effective dispersion and corrosion effects of molten salt for the pyrolysis products during pyrolysis obviously increase defects and specific surface area of the activated biochar materials. The prepared activated biochar material (PS-800-1000) by pyrolysis in molten KCl at 800 °C and heat-treatment at 1000 °C exhibits excellent catalytic activity with half-wave potential of 0.84 V vs. RHE, comparable to commercial Pt/C for oxygen reduction reaction (ORR) in 0.1 M KOH and outstanding supercapacitance performance in 6 M KOH with high specific capacitance (355 F g-1 at 0.5 A g-1), which exceeds all reported biochar derived from peanut shells. The PS-800-1000-based zinc-air battery (ZAB) displays higher peak power density (141 mW cm-2), specific capacity (767 mAh gZn-1) and cycling stability than Pt/C-based ZAB. The activated biochar prepared by pyrolysis in molten KCl and heat-treatment method from peanut shells can be a promising candidate for replacing precious metals in energy conversion/storage devices.

3D Graphene-Carbon Nanotube Hybrid Supported Coupled Co-MnO Nanoparticles as Highly Efficient Bifunctional Electrocatalyst for Rechargeable Zn-Air Batteries.

The development of high-efficiency and low-cost catalysts is one of the core and important issues to improve the efficiency of electrochemical reactions on electrodes, and it is also the goal we ultimately pursue in the commercialization of large-scale clean energy technologies, such as metal-air batteries. Herein, a nitrogen-doped graphene oxide (GO)-carbon nanotube (CNT) hybrid network supported coupled Co/MnO nanoparticles (Co/MnO@N-C) catalyst was prepared with a hydrothermal-pyrolysis method. The unique three-dimensional network structure of substrate allowed for the uniform dispersion of Co-MnO nanoparticles in the carbon skeleton. These characters enable the Co/MnO@N-C to possess the excellent bifunctional electrocatalysts. In alkaline electrolyte, the Co/MnO@N-C presents the outstanding oxygen evolution reaction (OER) performance comparable to the commercial RuO2 catalyst and the exceedingly good oxygen reduction reaction (ORR) activity with positive half-wave potential of 0.90 V vs. RHE outperforming commercial Pt/C (0.84 V vs. RHE) and the recently reported analogous electrocatalysts. When it is applied to homemade Zn-air batteries, such a non-noble metal electrocatalyst can deliver a better power density, specific capacity and cycling stability than mixed Pt/C and RuO2 catalyst, and exhibits a wide application prospect and great practical value.

Covalent pendulous anthraquinone polymers coupled on graphenes for efficient capacitance storage in both alkaline and acidic media.

Novel crystalline covalent organic polymers (COPs) were constructed by reacting 1,4-diaminoanthraquinone with 1,3,5-triformylphloroglucinol or tris(4-formylphenyl)amine (TPDA or TADA). After they were covalently bonded to amine-functionalized graphene oxides, the resulting mesoporous COPs@graphene composites demonstrated efficient capacitance storage performance in both alkaline and acidic media. In particular, the as-synthesized TPDA@graphene displayed a reversible specific capacitance of 522 F g-1 in a 6.0 mol L-1 aqueous KOH electrolyte, superior to the previously reported COPs with inconspicuous capacitance storage properties in alkaline media. Its specific capacitance also reached 390 F g-1 in 2.0 mol L-1 H2SO4. The impressive capacitance storage properties of this composite can be ascribed to its unique structure with abundant pendulous anthraquinone redox groups and better electrical conductivity enhanced by the coupled graphenes.

Efficient Nitrate Reduction over Novel Covalent Ag-Salophen Polymer-Derived "Vein-Leaf-Apple"-like Ag@Carbon Structures.

Efficient electrocatalysts for nitrate reduction reaction (NO3--RR) that could selectively transfer nitrate into harmless nitrogen are required for water-denitrification treatment. The most widely used electrodes for NO3--RR including noble metals, transition metals, and their alloys still face many challenges such as lower selectivity and efficiency, high cost, and easy corrosion properties. Metallic Ag with acceptable cost possesses strong corrosion resistance in electrolysis, but its activity is often incompetent for NO3--RR. In this work, Ag nanoparticles with a lower loading content (1.99 wt %) on a nitrogen-doped carbon support was successfully used as the robust electrocatalyst for NO3--RR in a Cl--free neutral solution. This Ag@carbon catalyst exhibited an impressive electrochemical performance for NO3--RR, with a NO3--N conversion yield of 53% and a N2-N selectivity of 97% at a low electrolysis overpotential (-0.29 V vs RHE). In particular, the prepared Ag@carbon showed better stability and no secondary Ag ion pollution in electrolysis. Its impressive electrocatalytic performance was attributed to the unique "vein-leaf-apple"-like Ag@carbon structures, prepared by thermal conversion of Ag-salophen polymers@CNTs. CNTs served as veins to enhance the electron transportation in electrocatalysts. Salophen polymer-derived mesoporous N-doped carbon plates acted as leaves to concentrate NO3- from the electrolyte. Like apples on trees, Ag nanoparticles of about 10-20 nm highly dispersed on carbons selectively converted NO3--N into N2-N. It opens up a cost-acceptable and corrosion-resistant Ag-less electrocatalytic pathway for NO3--RR, and the special "vein-leaf-apple"-like Ag@carbon structure could enhance the electrolytic efficiency and N2-N selectivity for NO3--RR.

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. Sn(OH)x helped in improving the yield of this Mn-COF. Further, S-containing dimethyl sulfoxide solvent molecules were tripped in the pores of Mn-COFs. Such heteroatoms-enriched Mn-COFs could be used to fabricate Mn-S-N-doped carbons (Mn-S-N-Cs) with abundant mesopores. In particular, Mn-Nx sites could be partly maintained and highly dispersed on the surface of Mn-S-N-Cs; impressively, Mn-N-S-C-800 could be catalyzed for oxygen electroreduction in both alkaline and acidic media. Its half-wave potential reached 0.86 V in 0.1 M KOH, with a very low yield of HO2- (4.02%) and better durability. The thermal conversion of the synthesized mesoporous porphyrinic Mn-COFs provided an efficient strategy for fabricating high-dispersion Mn-Nx sites on mesoporous S and N codoped carbons.

Pyrolytic Carbon-coated Cu-Fe Alloy Nanoparticles with High Catalytic Performance for Oxygen Electroreduction.

Well-dispersed carbon-coated or nitrogen-doped carbon-coated copper-iron alloy nanoparticles (FeCu@C or FeCu@C-N) in carbon-based supports are obtained using a bimetallic metal-organic framework (Cu/Fe-MOF-74) or a mixture of Cu/Fe-MOF-74 and melamine as sacrificial templates and an active-component precursor by using a pyrolysis method. The investigation results attest formation of Cu-Fe alloy nanoparticles. The obtained FeCu@C catalyst exhibits a catalytic activity with a half-wave potential of 0.83 V for oxygen reduction reaction (ORR) in alkaline medium, comparable to that on commercial Pt/C catalyst (0.84 V). The catalytic activity of FeCu@C-N for ORR (Ehalf-wave =0.87 V) outshines all reported analogues. The excellent performance of FeCu@C-N should be attributed to a change in the energy of the d-band center of Cu resulting from the formation of the copper-iron alloy, the interaction between alloy nanoparticles and supports and N-doping in the carbon matrix. Moreover, FeCu@C and FeCu@C-N show better electrochemical stability and methanol tolerance than commercial Pt/C and are expected to be widely used in practical applications.

Bio-inspired chitosan-heme-vitamin B12-derived Fe-Co bimetallic-doped mesoporous carbons for efficiently electro-activating oxygen.

A bio-inspired bimetallic Fe- and Co-doped nitrogen-carbon material (Fe-Co-N-C) was synthesized using chitosan and porphyrin-like heme and vitamin B12 precursors. Anionic heme with Fe-N4 moieties and vitamin B12 with Co-N4 moieties were grafted onto the surface of acidic chitosan polymers through interactions with cationic -NH3+, and these polymers were coated onto nano-sized silica spheres to form the composite precursors. The optimised thermolysis of these composites yielded mesoporous carbons with abundant surface Fe-Nx and Co-Nx sites. The as-prepared Fe-Co-N-Cs catalyzed the oxygen reduction reaction (ORR) in both alkaline and acidic conditions. It not only provided a low-cost bio-inspired synthesis for bimetal-doped carbons using biologics with the definitive molecular structure but also the as-prepared mesoporous Fe-Co-N-Cs represented as the possible NPMC candidates for replacing Pt-based electrocatalysts in fuel cells.

Electroactive Cu-P-Coupled Moieties Doped in Hierarchically Porous Carbon as Efficient Catalysts for the Oxygen-Reduction Reaction.

A porous carbon material that was co-doped with copper and phosphorus (Cu-P-C) was synthesized by the direct thermal conversion of [(Ph3 P)2 CuCl2 ] in the channels of an SBA-15 template and found to be an impressive Cu-based electrocatalyst. The prefabricated Cu-Px moieties in the starting [(Ph3 P)2 CuCl2 ] were retained during the preparation process of the catalyst. These Cu-Px active sites effectively catalyzed the oxygen-reduction reaction (ORR). Moreover, the hierarchically porous morphology of the Cu-P-C material, which demonstrated a large specific surface area, allowed for a higher density of the Cu-Px active sites, thereby facilitating mass transfer and further boosting the electrocatalytic activity of the Cu-P-C catalyst. The as-obtained catalyst exhibited surprising catalytic activity, with a halfwave potential of 0.833 V in alkaline medium, which was comparable to that of the commercial Pt/C-JM catalyst, and possessed the highest activity among the reported M-P-C catalysts for the ORR.

Covalent Porphyrin Framework-Derived Fe2P@Fe4N-Coupled Nanoparticles Embedded in N-Doped Carbons as Efficient Trifunctional Electrocatalysts.

A new porous covalent porphyrin framework (CPF) filled with triphenylphosphine was designed and synthesized using the rigid tetrakis(p-bromophenyl)porphyrin (TBPP) and 1,3,5-benzenetriboronic acid trivalent alcohol ester as building blocks. The carbonization of this special CPF has afforded coupled Fe2P and Fe4N nanoparticles embedded in N-doped carbons (Fe2P/Fe4N@N-doped carbons). This CPF serves as an "all in one" precursor of Fe, N, P, and C. The porous property and solid skeleton of the CPF endow Fe2P/Fe4N@N-doped carbons with porous structure and a high degree of graphitization. As a result, Fe2P/Fe4N@N-doped carbons exhibited highly efficient multifunctional electrocatalytic performance for water splitting and oxygen electroreduction. Typically, Fe2P/Fe4N@C-800, obtained at a heat-treatment temperature of 800 °C, showed an ORR half-wave potential of 0.80 V in alkaline media and 0.68 V in acidic media, close to that of commercial Pt/C catalysts. Fe2P/Fe4N@C-800 also displayed efficient OER and HER activities, comparable to other phosphide and nitride electrocatalysts. The coupled Fe4N and Fe2P nanoparticles embedded in carbons exert unique catalytic efficiency for water splitting and fuel cells.

Space-confined synthesis of multilayer Cu-N-doped graphene nanosheets for efficient oxygen electroreduction.

Cu-based carbon electrocatalysts for the oxygen reduction reaction are difficult to compare with the corresponding Fe- or Co-based electrocatalytic materials, owing to their insufficient catalytic activity and stability. Herein, as an impressive Cu-based electrocatalyst, a multilayer Cu-N-doped graphene sheet (Cu-N-GR) is directly synthesized by the thermal conversion of copper(ii) 2,2'-bipyridine in the confined space of lamellar montmorillonites. The open layered morphology of Cu-N-GR materials facilitated the exposure of more active centers and enhanced the flexibility and mobility of charge carriers. Combining the unique electronic properties of layered morphology and the synergistic effect of Cu and N, the obtained Cu-N-GR exhibits surprising results in terms of ORR catalytic activity, particularly in catalytic stability and methanol-tolerant properties in alkaline media.

Nanoporous carbon derived from a functionalized metal-organic framework as a highly efficient oxygen reduction electrocatalyst.

High levels of iron-nitrogen doped porous carbon materials are obtained from MOF-253 using a step-by-step post-synthetic modification strategy. MOF-253 possessing open 2,2'-bipyridine nitrogen sites not only serves as a precursor but also provides chelate bonding sites for Fe2+. Followed by further impregnation of 1,10-phenanthroline, high surface area porous carbon materials are obtained. For comparison, when iron-1,10-phenanthroline species as a whole are incorporated into MOF-253, carbon materials with less active sites and low surface area are obtained. The porous carbon materials derived from MOF-253 by using a step-by-step post-synthetic modification strategy demonstrate excellent ORR activity, high selectivity (direct 4e- reduction of oxygen to water) and stability under both alkaline and acidic conditions. The onset potential of the porous carbon material under alkaline conditions (980 mV) is the same as that of Pt/C (20 wt%) (980 mV) and the half-wave potential (E1/2) is 840 mV, which is 20 mV more than that of Pt/C (20 wt%). Under acidic conditions, the onset potential and the half-wave potential are only 20 mV and 30 mV less than those of Pt/C (20 wt%). The developed step-by-step post-synthetic modification route of MOFs has expanded the ways to prepare functionalized porous carbon for energy related applications.

Origin of the Ability of α-Fe2 O3 Mesopores to Activate C-H Bonds in Methane.

Methane is a most abundant and inexpensive hydrocarbon feedstock for the production of chemicals and fuels. However, it is extremely difficult to directly convert methane to higher hydrocarbons because the C-H bonds in methane are the most stable C-H bonds of all hydrocarbons. The activation of the C-H bonds in methane by using an efficient and mild route remains a daunting challenge. Here, we show that the inner surface structures of the pore walls in mesoporous α-Fe2 O3 possess excellent catalytic performance for methane activation and convert C-H bonds into the C-O bonds in an O2 atmosphere at 140 °C. We found that such unusual structures are mainly comprised of turbostratic ribbons and K crystal faces and have higher catalytic activity than the (110) plane. These results are without precedent in the history of catalysis chemistry and will provide a new pathway for designing and preparing highly efficient catalytic materials.

MIL-100 derived nitrogen-embodied carbon shells embedded with iron nanoparticles.

The use of metal-organic frameworks (MOFs) as templates and precursors to synthesize new carbon materials with controllable morphology and pre-selected heteroatom doping holds promise for applications as efficient non-precious metal catalysts. Here, we report a facile pyrolysis pathway to convert MIL-100 into nitrogen-doped carbon shells encapsulating Fe nanoparticles in a comparative study involving multiple selected nitrogen sources. The hierarchical porous architecture, embedded Fe nanoparticles, and nitrogen decoration endow this composite with a superior oxygen reduction activity. Furthermore, the excellent durability and high methanol tolerance even outperform the commercial Pt-C catalyst.

New heterometallic zirconium metalloporphyrin frameworks and their heteroatom-activated high-surface-area carbon derivatives.

Four cubic zirconium-porphyrin frameworks, CPM-99(H2, Zn, Co, Fe), were synthesized by a molecular-configuration-guided strategy. Augmentation of meso-substituted side arms (with double-torsional biphenyl rings) of tetratopic porphyrin linkers leads to a successful implementation of zirconium-carboxylate frameworks with cubic 2.5 nm cage. The hard-templating effect of Zr6-polyoxo-cluster and uniformly embedded (metallo)porphyrin centers endow CPM-99 with highly desirable properties as precursors for oxygen reduction reaction (ORR) catalysts. The pyrolytic products not only retain the microcubic morphology of the parent CPM-99 but also possess porphyrinic active sites, hierarchical porosity, and highly conducting networks. CPM-99Fe-derived material, denoted CPM-99Fe/C, exhibits the best ORR activity, comparable to benchmark 20% Pt/C in alkaline and acidic media, but CPM-99Fe/C is more durable and methanol-tolerant. This work demonstrates a new route for the development of nonprecious metal ORR catalysts from stable metalloporphyrinic MOFs.

From cage-in-cage MOF to N-doped and Co-nanoparticle-embedded carbon for oxygen reduction reaction.

By one-step pyrolysis of a unique "cage-in-cage" cobalt metal-organic framework, nitrogen-doped carbon cubes embedded with numerous metallic Co nanoparticles were obtained. A considerable amount of Co particles was encapsulated in thin carbon shells and formed the core-shell-like Co@C structure. With about 60 wt% Co particles in the prepared sample, the nanocomposites of Co nanoparticles and nitrogen-doped carbon show electrocatalytic activity for the oxygen reduction reaction (ORR) with an efficiency comparable to the commercial Pt/C catalyst, but with better durability and methanol-tolerance performance. The metallic Co nanoparticles were found to play an important role in enhancing the ORR activity of the nanocomposites. This simple one-step pyrolysis method provides a novel synthetic route for the synthesis of core-shell-like Co@C nano-composites. The synthesized material represents a highly active non-precious metal catalyst for ORR.

Efficient oxygen reduction by nanocomposites of heterometallic carbide and nitrogen-enriched carbon derived from the cobalt-encapsulated indium-MOF.

By one-step pyrolysis of an indium-MOF with entrapped cobalt dimers in the presence of melamine, heterometallic carbide nanoparticles (Co3InC0.75) embedded in nitrogen-enriched carbon have been prepared and found to exhibit efficient electrocatalytic activity for oxygen reduction reaction with high durability and methanol-tolerance properties.

Ordered mesoporous Fe-porphyrin-like architectures as excellent cathode materials for the oxygen reduction reaction in both alkaline and acidic media.

Iron ORR: An ordered, mesoporous, Fe-porphyrin-like material was created through the nanocasting and pyrolysis of traditional Fe-N4 porphyrins. The resulting nonprecious metal electrocatalyst was used for the oxygen reduction reaction in both alkaline and acidic media.