Porphyrin Aerogel Catalysts for Oxygen Reduction Reaction in Anion‐Exchange Membrane Fuel Cells

While supported metal nanoparticles cannot achieve full electrochemical utilisation of metal atoms, catalysts featuring single metal atom sites may offer this possibility, along with advantages in selectivity. However, the passage from nanometric to atomic dimension is not without consequences. It first raises the question of efficient and robust synthesis methods, and underlines the need of cutting-edge characterization techniques that can target single metal atoms. These analytical tools are also pivotal to gain insights into the structure of the active sites, and establish atomic structure-catalytic activity-selectivity-stability relationships. Herein, we illustrate these topics for electrocatalysis, with a particular focus on metal-nitrogen-carbon single metal atom catalysts, for which a fantastic leap forward has been achieved in the last 15 years, triggered by the growing interest in sustainable energy storage and conversion systems.C SMAC prepared via pyrolysis in the following decades. Hence, we focus mainly on the synthesis methods of Fe-and Co-N-C SMAC in this section. While in those early days, the use of Metal-N4 macrocycles without pyrolysis secured the well-defined Metal-N4 active centres, the different approaches used to interface them with a conductive support impacted the local environment of metal cations and their accessibility. Phthalocyanines and porphyrins are poorly soluble in water, and different solvents or suspensions in concentrated sulphuric acid were used, removing the solvent by evaporation or precipitating the macrocycle on the support. The dispersion quality of metal macrocycles on supports were shown to depend strongly on the morphology, specific surface area (SSA) but also on acido-basic properties of carbon supports [17,18]. For example, the characterization by 57 Fe Mössbauer spectroscopy, a technique which allows identifying the different coordinations and spin states of Fe (discussed in more detail in 13.3), of iron phthalocyanine (Fe-Pc) precipitated on a carbon powder revealed multiple coordinations and spin states for Fe, whereas a single coordination and electronic state was seen for the Fe-Pc monomer [17]. Controlled deposition or solvent removal however resulted in Fe-Pc/C composites with a single (or vast majority of one) 57 Fe Mössbauer spectroscopic signature [19]. For example, removal of the pyridine solvent for Fe-Pc by boiling it off at 420 °C resulted in a Fe-Pc/C composite with 95 % of the spectral signal assigned to one specific doublet [19]. The latter can nowadays be assigned to an O2-Fe(III)-N4 coordination [20], implying high dispersion and accessibility to O2 of the Fe-N4 sites.While some heterogeneities of metal coordination and site accessibility were exemplified above for SMAC prepared via interfacing Metal-N4 macrocycles without high-temperature pyrolysis, the extent of heterogeneities was progressively increased, and the true nature of the active site blurred for several decades, with the introduction of high-temperature treatments of macrocycles in a...

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“…combining metal porphyrins with the silica hard-templating method [41] or forming aerogels of porphyrins [42] before pyrolysis.…”

Section: Synthesis Methodsmentioning

confidence: 99%

Electrocatalysis with Single‐Metal Atom Sites in Doped Carbon Matrices

Asset¹,

Maillard²,

Jaouen³

2022

Supported Metal Single Atom Catalysis

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“…1F), which reflects the processability of our DCPAs. Notably, most aerogels prepared by the sol-gel method are brittle and unmachinable (26,27). A piece of DCPA-3 (50 mg, 23 (L)  7.0 (W)  2.5 mm (H)) can sustain a load of 100 g without cracking, showing good tensile strength (Fig.…”

Section: Design Fabrication Structural Characterization and Basic Properties Of The Dcpasmentioning

confidence: 99%

Weldable and Closed-loop Recyclable Monolithic Dynamic Covalent Polymer Aerogels

Zhang

Zhao

et al. 2021

Preprint

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Owing to their low density, high porosity and unique micro-nanostructures, aerogels are attractive for applications in various aspects, but at the same time, suffer from shrinkage and/or cracking during preparation, mechanical brittleness, low production efficiency, and non-degradation. Herein, we introduce the concept of dynamic covalent polymer chemistry to produce a new class of aerogelsreferred to as DCPAs. The resulting light DCPAs display large-scale preparation prospect and feature high porosity (90.7-91.3%), large degrees of compression (80% strain) and bending (diametral deflection of 30 mm) without any cracks, as well as considerable tensile property (elongation at break of 32.7%). In addition, our DCPAs showcase emergent characteristics of weldability, repairability, and closed-loop recyclability that are highly desirable for affording versatile materials platforms but hardly achieved by traditional aerogels. Benefiting from the robust porous structures, we demonstrate the potential of the DCPAs for applications in thermal insulation and emulsion separation.

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“…[58][59][60] In recent years, researchers have been continuously attempting to explore available NMF ORR electrocatalysts with superefficiency, high-selectivity, and extremely stability, such as macrocyclic molecule-, metal compound-, covalent organic framework (COF)-, metal organic framework (MOF)-, and carbon-based materials. [61][62][63][64][65][66][67][68][69] Among NMF catalysts, singleatom catalysts (SACs) with atomically separated active sites anchored on carbon matrix saved as one of the most attractive candidates to substitute Pt-based catalysts. [70][71][72][73][74][75][76] SACs featuring maximum atom-utilization efficiency are endowed unique electronic nature compared with corresponding bulk metal, and thus contribute impressed ORR property in both alkaline and acidic media.…”

Section: Introductionmentioning

confidence: 99%

Recent Developments of Microenvironment Engineering of Single‐Atom Catalysts for Oxygen Reduction toward Desired Activity and Selectivity

Huang

Tang

et al. 2021

Adv Funct Materials

107

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Oxygen reduction reaction (ORR) is an essential process for sustainable energy supply and sufficient chemical production in modern society. Singleatom catalysts (SACs) exhibit great potential on maximum atomic efficiency, high ORR activity, and stability, making them attractive candidates for pursuing next-generation catalysts. Despite substantial efforts being made on building diversiform single-atom active sites (SAASs), the performance of the obtained catalysts is still unsatisfactory. Fortunately, microenvironment regulation of SACs provides opportunities to improve activity and selectivity for ORR. In this review, first, ORR mechanism pathways on N-coordinated SAAS, electrochemical evaluation, and characterization of SAAS are displayed. In addition, recent developments in tuning microenvironment of SACs are systematically summarized, especially, strategies for microenvironment modulation are introduced in detail for boosting the intrinsic 4e − /2e − ORR activity and selectivity. Theoretical calculations and cutting-edge characterization techniques are united and discussed for fundamental understanding of the synthesis-construction-performance correlations. Furthermore, the techniques for building SAAS and tuning their microenvironment are comprehensively overviewed to acquire outstanding SACs. Lastly, by proposing perspectives for the remaining challenges of SACs and infant microenvironment engineering, the future directions of ORR SACs and other analogous procedures are pointed out.

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Porphyrin Aerogel Catalysts for Oxygen Reduction Reaction in Anion‐Exchange Membrane Fuel Cells

Cited by 85 publications

References 52 publications

Electrocatalysis with Single‐Metal Atom Sites in Doped Carbon Matrices

Electrocatalysis with Single‐Metal Atom Sites in Doped Carbon Matrices

Weldable and Closed-loop Recyclable Monolithic Dynamic Covalent Polymer Aerogels

Recent Developments of Microenvironment Engineering of Single‐Atom Catalysts for Oxygen Reduction toward Desired Activity and Selectivity

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