Metal‐Organic Frameworks Help Conducting Polymers Optimize the Efficiency of the Oxygen Reduction Reaction in Neutral Solutions

Abstract

respect, the theoretical work of Barzukov and co-workers [ 7 ] suggests that the catalysis of the ORR on CPs can be ascribed to the weakening of molecular O O bonds during the chemisorption of oxygen on the polymer surface. For example, in the case of carbon black/PPy composites, the onset potential for ORR (−0.34 V) is more anodic than that observed in carbon blackcoated electrodes (−0.60 V). However, the ORR current densities are highly dependent on the loading of the PPy catalyst because of the low oxygen diffusion in the electrode surface. [ 13 ] In this regard, one of the major challenges for the optimization of electrochemical energy-conversion devices (e.g., metal-air batteries), is to increase the O 2 reduction and evolution effi ciencies, thus requiring the development of new effective electrocatalytic interfacial architectures compatible with the operation in aqueous electrolytes using air as the oxygen source.This point is where MOFs come into the picture as a valuable tool to engineer the oxygen uptake properties of the electrochemical interface in order to increase its concentration on the electrode surface. MOFs represent an emergent and versatile class of microporous materials constituted by an infi nite arrangement of metal (or metal cluster) centers coordinated via noncovalent interactions with organic linkers. [ 14,15 ] A great deal of effort has been devoted also to test strategies directed to control MOFs' growth and to synthesize mixed composites and supported fi lms, [ 16 ] whose distinctive properties have given rise to an increasing number of interesting applications (e.g., mixed matrix membranes for pervaporation, [ 17 ] or thin fi lms for sensor and separation technologies [ 18 ] ).Very recently, Díaz et al. [ 19 ] reported a comprehensive work describing the selective gas adsorption of fi lms constituted by ZIF-8 MOF (a member of a subclass of MOFs known as Zeolitic Imidazole Frameworks, constituted of Co 2+ or Zn 2+ metal ions tetrahedrally coordinated by imidazole-based linkers). It was shown that selectivity of O 2 adsorption over N 2 is close to 6, thus confi rming that ZIF-8 would undergo preferential uptake of O 2 even in the case of atmospheric O 2 :N 2 composition.Taking into account these concepts and being aware of the outstanding features of gas storage exhibited by metal-organic frameworks, we took this new paradigm one step further by integrating MOFs onto CPs and using them as oxygen reservoirs sitting atop the electrocatalytic active material. However, it is important to consider that such interfacial architectures require the tailored production and organization of complex matter displaying spatially addressed chemistry using different wet chemical processes. Therefore, to achieve this goal it is important to test strategies for the controlled preparation of multicomponent nanostructures on surfaces. Research efforts The oxygen reduction reaction (ORR), where molecular oxygen is electrocatalytically reduced on the electrode surface, plays a decisive role in energy conver...

Metal–organic frameworks meet polymer brushes: enhanced crystalline film growth induced by macromolecular primers

“…36 More recently, Caro and coworkers further extended these notions by using polydopamine layers as platforms to grow ZIF-8 thin films showing that macromolecular surfaces can promote the heterogeneous nucleation of MOF crystals. 37,38 Inspired by the above described attempts directed to the creation of hybrid polymer-MOFs materials, 39 which would certainly help broadening the range of possible applications by circumventing common limitations (e.g., organic phase immiscibility or sensitivity towards hydrolysis); 40,41 we have explored the use of polymer brushes as macromolecular primers to promote the heterogeneous nucleation and growth of ZIF-8 thin films. Polymer brushes refer to assemblies of macromolecules that are tethered by one end to a surface or interface in which chemical groups all along the polymer backbone can be placed in pseudo-3D spatial arrangements.…”

Layer-by-layer integration of conducting polymers and metal organic frameworks onto electrode surfaces: enhancement of the oxygen reduction reaction through electrocatalytic nanoarchitectonics

“…[13][14][15][16][17] MOFs can endow films with ideal characteristics for electrochemical reactions (e.g., water splitting, CO 2 electroreduction), allowing for the coexistence of both, hydrophilic channels for the diffusion of charged species, and hydrophobic porosity needed for gas adsorption. [18][19][20][21][22][23][24] In the last decade the concept of including MOFs into composites, or its use as precursors for the synthesis of electroactive materials for the Oxygen Reduction Reaction (ORR) was extensively explored, and many different strategies were followed. [25][26][27][28][29] Although it suffers from poor control over the morphologies obtained, calcination remains a popular approach; and the use of MOFs as templates for heteroatom-doped micro/mesoporous carbon features electrochemical promotion.…”

“…In order to achieve this end, we first synthesized colloidal suspensions of both, Pani-PSS negatively charged electroactive polymer, and PAH-stabilized (Polyallylamine hydrochloride) positively charged colloidal suspensions of ZIF-8 MOF (a Zn-based archetypal Zeolitic Imidazolate microporous material, which was already shown to act as ORR enhancer). 20,52 ZIF-8 can be described as an infinite network of Zn 2+ ions tetrahedrally coordinated with 2-methylimidazolate N-bidentate organic linkers, featuring high BET surface area (≈ 1500-1800 m 2 g -1 ), and good resistance towards hydrolysis in moderate pH aqueous environments and ionic strengths. 53,54 We show for the first time that a LbL architecture is achievable using these presynthesized nanostructures; and moreover, that these films feature electrochemical activity and can act as enhancers of the neutral pH electrochemical ORR due to a synergetic effect between high surfacearea porous material and electroactive conductive polymer, offering thus a cost-effective alternative to precious metal catalysts usually employed in energy conversion devices.…”

Enhancing the Catalytic Activity of Zeolitic Imidazolate Framework‐8‐Derived N‐Doped Carbon with Incorporated CeO2 Nanoparticles in the Oxygen Reduction Reaction

“…However, the possible structure/morphology damage during carbonization and thereby the loss of active sites inhibit their development to some degree. To enhance the ORR catalytic activity of ZIF‐derived N‐doped carbon, many strategies have been adopted, including 1) template‐guided synthesis, for example, tellurium nanowires, ZnO multiarms, and cetyltrimethylammonium bromide; 2) codoping with other elements, such as Cu, Fe, S, and P; 3) combination with other electroactive materials, such as graphene oxides and conducting polymers . In the above cases, hierarchical porous structures, enhanced conductivity, increased active sites, or certain synergistic effects can be achieved, and better ORR catalytic performance is obtained thereby.…”