Metal organic frameworks as a catalyst for oxygen reduction: an unexpected outcome of a highly active Mn-MOF-based catalyst incorporated in activated carbon

Co(II)-Chelated Polyimines as Oxygen Reduction Reaction Catalysts in Anion Exchange Membrane Fuel Cells

In this paper, a cobalt (Co)-chelated polynaphthalene imine (Co-PNIM) was calcined to become an oxygen reduction reaction (ORR) electrocatalyst (Co-N-C) as the cathode catalyst (CC) of an anion exchange membrane fuel cell (AEMFC). The X-ray diffraction pattern of CoNC-1000A900 illustrated that the carbon matrix develops clear C(002) and Co(111) planes after calcination, which was confirmed using high-resolution TEM pictures. Co-N-Cs also demonstrated a significant ORR peak at 0.8 V in a C-V (current vs. voltage) curve and produced an extremely limited reduction current density (5.46 mA cm-2) comparable to commercial Pt/C catalysts (5.26 mA cm-2). The measured halfway potential of Co-N-C (0.82 V) was even higher than that of Pt/C (0.81 V). The maximum power density (Pmax) of the AEM single cell upon applying Co-N-C as the CC was 243 mW cm-2, only slightly lower than that of Pt/C (280 mW cm-2). The Tafel slope of CoNC-1000A900 (33.3 mV dec-1) was lower than that of Pt/C (43.3 mV dec-1). The limited reduction current density only decayed by 7.9% for CoNC-1000A900, compared to 22.7% for Pt/C, after 10,000 redox cycles.

Electrocatalytic Behavior of an Amide Functionalized Mn(II) Coordination Polymer on ORR, OER and HER

The new 3D coordination polymer (CP) [Mn(L)(HCOO)]n (Mn-CP) [L = 4-(pyridin-4-ylcarbamoyl)benzoate] was synthesised via a hydrothermal reaction using the pyridyl amide functionalized benzoic acid HL. It was characterized by elemental, FT-IR spectroscopy, single-crystal and powder X-ray diffraction (PXRD) analyses. Its structural features were disclosed by single-crystal X-ray diffraction analysis, which revealed a 3D structure with the monoclinic space group P21/c. Its performance as an electrocatalyst for oxygen reduction (ORR), oxygen evolution (OER), and hydrogen evolution (HER) reactions was tested in both acidic (0.5 M H2SO4) and alkaline (0.1 M KOH) media. A distinct reduction peak was observed at 0.53 V vs. RHE in 0.1 M KOH, which corresponds to the oxygen reduction, thus clearly demonstrating the material's activity for the ORR. Tafel analysis revealed a Tafel slope of 101 mV dec-1 with mixed kinetics of 2e- and 4e- pathways indicated by the Koutecky-Levich analysis. Conversely, the ORR peak was not present in 0.5 M H2SO4 indicating no activity of Mn-CP for this reaction in acidic media. In addition, Mn-CP demonstrated a noteworthy activity toward OER and HER in acidic media, in contrast to what was observed in 0.1 M KOH.

3D TM-N-C Electrocatalysts with Dense Active Sites for the Membraneless Direct Methanol Fuel Cell and Zn-Air Batteries

Electrocatalysts with high cost-effectiveness for the oxygen reduction reaction (ORR) are essential for fuel cells (FC) and Zn-Air batteries (ZAB), which need highly active sites and suitable carbon substrates to accelerate the charge transfer kinetics. Herein, a simple and extensible method using ball milling and space-confinement pyrolysis is reported to prepare a series of transition metals and N-C catalysts (M-NLPC), which possess three-dimensional porous carbon substrates and dense active sites for efficient ORR. M-NLPC catalysts (especially Fe-NLPC) exhibit outstanding ORR activity with a half-wave potential (E_1/2, 0.88 V) in an alkaline medium, high stability, and strong methanol resistance. The M-N₄ sites are proven to be the active centers in M-NLPC by theoretical calculation, and methanol molecules are more likely to desorb than react on the Fe-N₄ sites, which is the origin of the inactivity for the methanol oxidation reaction (MOR). Furthermore, Fe-NLPC was applied to membraneless alkaline direct methanol FC (DMFC) in practice, exhibiting outstanding performance. Meanwhile, the Fe-NLPC-based ZAB also shows excellent electrochemical performance.

Waste Face Surgical Mask Transformation into Crude Oil and Nanostructured Electrocatalysts for Fuel Cells and Electrolyzers

A novel route for the valorization of waste into valuable products was developed. Surgical masks commonly used for COVID 19 protection by stopping aerosol and droplets have been widely used, and their disposal is critical and often not properly pursued. This work intended to transform surgical masks into platinum group metal-free electrocatalysts for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) as well as into crude oil. Surgical masks were subjected to controlled-temperature and -atmosphere pyrolysis, and the produced char was then converted into electrocatalysts by functionalizing it with metal phthalocyanine of interest. The electrocatalytic performance characterization towards ORR and HER was carried out highlighting promising activity. At different temperatures, condensable oil fractions were acquired and thoroughly analyzed. Transformation of waste surgical masks into electrocatalysts and crude oil can open new routes for the conversion of waste into valuable products within the core of the circular economy.

Metal-Organic Frameworks and Metal-Organic Gels for Oxygen Electrocatalysis: Structural and Compositional Considerations

Increasing demand for sustainable and clean energy is calling for the next-generation energy conversion and storage technologies such as fuel cells, water electrolyzers, CO₂ /N₂ reduction electrolyzers, metal-air batteries, etc. All these electrochemical processes involve oxygen electrocatalysis. Boosting the intrinsic activity and the active-site density through rational design of metal-organic frameworks (MOFs) and metal-organic gels (MOGs) as precursors represents a new approach toward improving oxygen electrocatalysis efficiency. MOFs/MOGs afford a broad selection of combinations between metal nodes and organic linkers and are known to produce electrocatalysts with high surface areas, variable porosity, and excellent activity after pyrolysis. Some recent studies on MOFs/MOGs for oxygen electrocatalysis and their new perspectives in synthesis, characterization, and performance are discussed. New insights on the structural and compositional design in MOF/MOG-derived oxygen electrocatalysts are summarized. Critical challenges and future research directions are also outlined.

Bifunctional PGM-free metal organic framework-based electrocatalysts for alkaline electrolyzers: trends in the activity with different metal centers

In order to solely rely on renewable and efficient energy sources, reliable energy storage and production systems are required. Hydrogen is considered an ideal solution as it can be produced electrochemically by water electrolysis and renewably while no pollutants are released when consumed. The most common catalysts in electrolyzers are composed of rare and expensive precious group metals. Replacing these materials with Earth-abundant materials is important to make these devices economically viable. Metal organic frameworks are one possible solution. Herein we demonstrate the synthesis and characterization studies of metal benzene-tri-carboxylic acid-based metal-organic frameworks embedded in activated carbon. The conductive composite material was found to be electrocatalytically active for both the oxygen evolution reaction and the hydrogen evolution reaction. Furthermore, several metal organic frameworks sharing the same ligand but with different first-row transition metals (M = Co, Cu, Fe, Mn) were compared, and the trend of their activity is discussed. Cobalt was found to have the highest activity among the studied metal centers, and therefore has the best potential to serve as a bifunctional catalyst for alkaline electrolyzers.

Durable Tungsten Carbide Support for Pt-Based Fuel Cells Cathodes

In an effort to develop durable, corrosion-resistant catalyst support materials for polymer electrolyte fuel cells, modified polymer-assisted deposition method was used to synthesize tungsten carbide (WC, WC_1-x), which was later used as a support material for Pt-based oxygen reduction reaction catalyst, as an alternative for the corrosion-susceptible, carbon supports. The Pt-deposited tungsten carbide's corrosion-resistance, oxygen reduction reaction electrocatalysis, and durability were studied and compared to that of Pt/C. Bulk free carbon was found to be absent from the ceramic matrix which had particle size in the range of 2-25 nm. Tungsten carbide support appears to enhance the oxygen reduction activity on Pt, showing an increase in mass activity (nearly 2-fold at 0.85 V vs RHE) and specific activity (more than 7 times higher), alongside decrease in overpotential, in comparison to Pt/C. A significant increase in durability was also observed with the tungsten carbide-based system.

Performance evaluation of CuBTC composites for room temperature oxygen storage

Oxygen is commonly separated from air using cryogenic liquefaction. The inherent energy penalties of phase change inspire the search for energy-efficient separation processes. Here, an alternative approach is presented, where we determine whether it is possible to utilise simpler, stable materials in the right process to achieve overall energy efficiency. Adsorption and release by Metal-Organic Frameworks (MOFs) are an attractive alternative due to their high adsorption and storage capacity at ambient conditions. Cu-BTC/MgFe2O4 composites were prepared, and magnetic induction swing adsorption (MISA) used to release adsorbed oxygen quickly and efficiently. The 3 wt% MgFe2O4 composites exhibited an oxygen uptake capacity of 0.34 mmol g-1 at 298 K and when exposed to a magnetic field of 31 mT, attained a temperature rise of 86 °C and released 100% of adsorbed oxygen. This water vapor stable pelletized system, can be filled and emptied within 10 minutes requiring around 5.6 MJ kg-1 of energy.

Synthesis and Performance of MOF-Based Non-Noble Metal Catalysts for the Oxygen Reduction Reaction in Proton-Exchange Membrane Fuel Cells: A Review

Hydrogen is widely regarded as a prime energy carrier for bridging the gap between renewable energy supply and demand. As the energy-generating component of the hydrogen cycle, affordable and reliable fuel cells are of key importance to the growth of the hydrogen economy. However, the use of scarce and costly Pt as an electrocatalyst for the oxygen reduction reaction (ORR) remains an issue to be addressed, and in this regard, metal-organic frameworks (MOFs) are viewed as promising non-noble alternatives because of their self-assembly capability and tunable properties. Herein, recent (2018-2020) works on MOF-based electrocatalysts containing N-doped C, Mn, Fe, Co, multiple metals, and multiple sites are reviewed and summarized with a focus on ORR activity, and the principal physicochemical properties and electrochemical performance of these catalysts realized using rotating electrodes are compared.

Recent Developments for Aluminum–Air Batteries

Abstract

Environmental concerns such as climate change due to rapid population growth are becoming increasingly serious and require amelioration. One solution is to create large capacity batteries that can be applied in electricity-based applications to lessen dependence on petroleum. Here, aluminum–air batteries are considered to be promising for next-generation energy storage applications due to a high theoretical energy density of 8.1 kWh kg−1 that is significantly larger than that of the current lithium-ion batteries. Based on this, this review will present the fundamentals and challenges involved in the fabrication of aluminum–air batteries in terms of individual components, including aluminum anodes, electrolytes and air cathodes. In addition, this review will discuss the possibility of creating rechargeable aluminum–air batteries.

Graphic Abstract

Pristine S,N-containing Mn-based metal organic framework nanorods enable efficient oxygen reduction electrocatalysis

Uniform Mn^II[(Tdc)(4,4′-Bpy)]_n nanorods have been synthesized by a hydrothermal method and they show high performance for the oxygen reduction reaction in alkaline medium.

Metal–Organic Frameworks Toward Electrocatalytic Applications

Metal–organic frameworks (MOFs) are a class of porous materials constructed from metal-rich inorganic nodes and organic linkers. Because of their regular porosity in microporous or mesoporous scale and periodic intra-framework functionality, three-dimensional array of high-density and well-separated active sites can be built in various MOFs; such characteristics render MOFs attractive porous supports for a range of catalytic applications. Furthermore, the electrochemically addressable thin films of such MOF materials are reasonably considered as attractive candidates for electrocatalysis and relevant applications. Although it still constitutes an emerging subfield, the use of MOFs and relevant materials for electrocatalytic applications has attracted much attention in recent years. In this review, we aim to focus on the limitations and commonly seen issues for utilizing MOFs in electrocatalysis and the strategies to overcome these challenges. The research efforts on utilizing MOFs in a range of electrocatalytic applications are also highlighted.

Nonpyrolyzed Fe-N Coordination-Based Iron Triazolate Framework: An Efficient and Stable Electrocatalyst for Oxygen Reduction Reaction

Pyrolyzed base-metal-based metal-organic frameworks (MOFs) with FeN_x coordination are emerging as nonprecious metal catalysts for electrochemical oxygen reduction reaction (ORR). However, surprisingly, nonpyrolyzed MOFs involving Fe-N coordination have not been explored for the ORR. This study concerns the catalytic performance of a semiconducting nonpyrolyzed iron triazolate framework (FeTa₂ ) for ORR in alkaline electrolyte. The FeTa₂ catalyst is studied as composites with different amounts of conductive Ketjenblack carbon (KB). The performance of these FeTa₂ -x KB (x denotes the KB/FeTa₂ weight ratio) composites by onset and half-wave potentials of ORR appears to be superior to most previously documented nonpyrolyzed MOFs. Characterization by elemental analysis, FTIR spectroscopy, XPS, and cyclic voltammetry suggest that N-Fe^III -OH^- sites at the surface of FeTa₂ function as the catalytic active sites. This FeTa₂ also shows very stable activity during ORR, as supported by accelerated durability test of the FeTa₂ -x KB sample (20 000 cycles, ca. 90 h). The framework structure of FeTa₂ remains intact during the durability test, which would help to explain its excellent catalytic durability. This would be the first study demonstrating efficient and stable ORR catalysis by a nonpyrolyzed Fe-N coordination-based MOF material.