Enhanced Cathodic Oxygen Reduction and Power Production of Microbial Fuel Cell Based on Noble‐Metal‐Free Electrocatalyst Derived from Metal‐Organic Frameworks

Abstract

several limiting factors can weigh heavily against the overall performance before the MFC technology can gain a signifi cant share of energy market. In particular, the power generated from MFC is commonly limited by the cathode poor performance owing to kinetic sluggishness of cathodic oxygen reduction reaction (ORR) in pHneutral aqueous electrolyte. [ 2 ] The ORR takes place through twoelectron (H 2 O 2 ) or four-electron (H 2 O) pathway depending on the electrocatalyst and electrolyte used. To maximize power density and energy conversion of MFC, the four-electron ORR process is necessary. This gives priority to explore highly active electrocatalyst toward four-electron ORR under pH-neutral condition. [ 2b , 3 ] Much effort has been devoted to enhance the cathode performance by either exploring available electrocatalyst, such as noble metals, nitrogen-doped carbonaceous materials and transition metal or metal oxides, or optimizing the property of three-phase boundary where oxygen reacts with electron and proton to form water. [ 4 ] However, there still exist several challenges for the carbonsupported particle-structured electrocatalysts in MFC, due to the occurrence of migration and agglomeration of catalyst particles at "water-fl ooding" three-phase boundary. [ 2b ] Inevitably, this will cause the loss of electrochemically active surface area for mass transfer of protons and oxygen, and thus the decrease in the activity, selectivity, and stability of the electrocatalyst. [ 5 ] Meanwhile, despite the best performance of platinum catalyst observed hitherto, the high cost and scarcity of platinum constitute the obstacle in the drive for scale-up applications. [ 2b , 6 ] Therefore, developing an inexpensive electrocatalyst with nonparticulate defi ned 3D porous structure, large surface area, and abundant active sites for effi cient cathodic oxygen reduction in MFC is still highly desirable.Metal-organic frameworks (MOFs), which consist of metal ions (clusters) and organic ligands, have a highly defi ned 3D network structure at molecular level. [ 7 ] Given their unique properties of high surface area, tunable porous structures, and uniform open cavities, [ 8 ] MOFs have attracted growing interest in potential applications in many areas, such as gas storage and Microbial fuel cell (MFC) can generate electricity from organic substances based on anodic electrochemically active microorganisms and cathodic oxygen reduction reaction (ORR), thus exhibiting promising potential for harvesting electric energy from organic wastewater. The ORR performance is crucial to both power production effi ciency and overall cost of MFC. A new type of metal-organic-framework-derived electrocatalysts containing cobalt and nitrogen-doped carbon (CoNC) is developed, which is effective to enhance activity, selectivity, and stability toward four-electron ORR in pH-neutral electrolyte. When glucose is used as the substrate, the maximum power density of 1665 mW m −2 is achieved for the optimized CoNC pyrolyzed at 900 °C, which is 39.8...

Design Strategies toward Advanced MOF‐Derived Electrocatalysts for Energy‐Conversion Reactions

“…[38,39,65] For precursor MOFs, You et al indicated that pyrolysis of ZIF-67 at 900 °C for 3 h under Ar generated carbonized ZIF-67 with a N content of only 1.1 wt%. [38] By contrast, carbonized ZIF-8 obtained by using the same pyrolysis conditions contained 8.6 wt% N, in which the percentage of pyrrolic and pyridinic N was 78.3%.…”

“…[58,60] For instance, in order to fabricate N-doped MOF-derived materials, zeolitic imiazolate frameworks (ZIFs), a subclass of MOFs composed of tetrahedrally coordinated transition metal ions (e.g., Zn, Fe, Co, Cu) and imidazolate (IM) ligands, are frequently utilized as the precursor and/or template because of their high N content and other preferred properties, such as large surface area and uniform particle size. [24,33,34,36,[38][39][40][41]44,53,[64][65][66][67][68][69] Recently, You et al reported the conversion of ZIF-67 (Co(MIM) 2 , MIM = methylimidazole) to Co-and N-doped carbon (CoNC) via direct pyrolysis, producing a N-doped material with high N content. [39] Similarly, Chen and co-workers studied porous carbon materials derived from a series of bimetallic ZIFs (BMZIFs), which are based on ZIF-8 and ZIF-67 with varied Zn/Co ratio.…”

“…[24,33,34,36,[38][39][40][41]44,53,[64][65][66][67][68][69] Recently, You et al reported the conversion of ZIF-67 (Co(MIM) 2 , MIM = methylimidazole) to Co-and N-doped carbon (CoNC) via direct pyrolysis, producing a N-doped material with high N content. [39] Similarly, Chen and co-workers studied porous carbon materials derived from a series of bimetallic ZIFs (BMZIFs), which are based on ZIF-8 and ZIF-67 with varied Zn/Co ratio. [53] Uniform N doping was readily carried out within the skeleton of the derived porous carbons.…”

“…[53] Specifically, pyridinic N is able to improve the capacity for accepting protons owing to its charge neutralization effect; [39] pyrrolic and pyridinic N can reduce the energy barrier of O 2 adsorption, thus facilitating the first rate-limiting step in the ORR by increasing electron transfer; [38] and graphitic N has shown to provide more valence electrons for increasing the conductivity and catalytic activity. [39] Metal-coordinated N usually withdraws electrons from the metal center and thus lower its electron density, which is beneficial for reducing the redox potential of the metal center by optimizing the bond strength between the metal center and intermediate. [38] In effect, doping N by pyrolysis always results in a mixture of different N species, whereas theoretical calculations often point out that one N configuration will dominate in a specific catalytic reaction.…”

“…When used as electrode materials in energy-conversion reactions, MOF-derived materials have displayed appealing performance in electrochemical systems, and comprehensive studies indicate that MOF-derived materials have several advantageous characteristics for electrochemical energy generation and storage. These are: 1) the carbon matrix obtained from the organic framework is able to act as a highly conductive network, promoting fast electron transfer; [31][32][33][34][35][36] 2) various heteroatoms (e.g., N, O, S, and P) contained in the organic ligands can be directly doped into the carbon framework, which can not only provide more active sites, but also create a favourable microenvironment for the growth of functional nanoparticles (NPs, e.g., transition metal carbides, nitrides, and oxides); [24,[36][37][38][39][40][41] 3) the metal atoms in MOFs can form corresponding metals and/or compounds and homogeneously distribute in the carbon network, leading to the in situ synthesis of a carbon framework decorated with transition metals and/or transition metal compounds; [24,[41][42][43] 4) a well-defined morphology, size, and structure can be obtained by adjusting synthesis parameters; [42][43][44][45] 5) many of the features (e.g., large surface area and high porosity) inherited from the MOF precursors can be preserved in the resultant materials after post-treatments. [46][47][48] Unsurprisingly, owing to the advantages discussed above, these MOF-derived materials have stimulated much interest in energy fields, [49][50][51] and have been studied as electrode materials in batteries and supercapacitors.…”