Composite Electrocatalyst Derived from Hybrid Nitrogen‐Containing Metal Organic Frameworks and g‐C₃N₄ Encapsulated In Situ into Porous Carbon Aerogels

Abstract

A novel composite Fe‐bpdc‐C3N4‐CA catalyst (where bpdc stands for 2,2′‐bipyridine‐3,3′‐dicarboxylic acid and CA stands for carbon aerogel) with a high nitrogen content was synthesized through the in situ encapsulation of nitrogen‐containing metal organic frameworks along with g‐C3N4 into porous CAs. The characteristics of the catalysts calcined at different temperatures were determined by means of TEM, XRD, Raman spectroscopy, XPS, and BET measurements. The results demonstrate the successful doping of N heteroatoms and the effective formation of Fe‐NX active sites. The electrocatalytic properties were investigated by using rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) measurements. The Fe‐bpdc‐C3N4‐CA catalyst calcined at 800 °C exhibits the highest oxygen reduction reaction activity with the initial reduction potential and half‐wave potential reaching 1.09 and 0.96 V, respectively, versus RHE in 0.1 M KOH. It also possesses high stability and follows approximately a complete four‐electron (4e−) reaction pathway, as commercial Pt/C catalysts do.

Effect of Experimental Operations on the Limiting Current Density of Oxygen Reduction Reaction Evaluated by Rotating‐Disk Electrode

“…The steady‐state limiting current density I lim , which indicates the maximum diffusion current in the ORR, is easily influenced by experimental conditions. The theoretical limiting current density I lim can be calculated by the following equation: , where ω is the rotating speed (rad ⋅ s −1 ), υ is the kinematic viscosity of the electrolyte (m 2 ⋅ s −1 ), C is the bulk concentration of O 2 (mol L −1 ), D is the diffusion coefficient (m 2 ⋅ s −1 ) of the electroactive species, F is the Faraday constant (96485.34 C mol −1 ), and n is the total number of electrons transferred during the electrochemical reaction . The limiting current density I lim is independent on the kinetics of the reaction.…”

“…where ω is the rotating speed (rad · s À 1 ), υ is the kinematic viscosity of the electrolyte (m 2 · s À 1 ), C is the bulk concentration of O 2 (mol L À 1 ), D is the diffusion coefficient (m 2 · s À 1 ) of the electroactive species, F is the Faraday constant (96485.34 C mol À 1 ), and n is the total number of electrons transferred during the electrochemical reaction. [6,[14][15][16][17] The limiting current density I lim is independent on the kinetics of the reaction. For a 4e ORR, I lim should be a fixed value in a particular concentration solution at 1600 rpm on a 5 mm working electrode, such as 5.277 mA cm À 2 for 0.5 M H 2 SO 4 , 6.332 mA cm À 2 for 0.1 M HClO 4 , and 5.968 mA cm À 2 for 0.1 M KOH (The parameters were listed in support information).…”

An Emerging Family of Hybrid Nanomaterials: Metal–Organic Framework/Aerogel Composites

“…Zhu et al 64 also used the direct mixing method and first synthesized the MOF iron 2,2′-bipyridine-3,3′-dicarboxylate (Fe-bpdc). Then together with g-C 3 N 4 , the MOF was encapsulated in a resorcinol−formaldehyde resin during the precursor polymerization.…”

“…Zhu et al 64 used carbon-shell-encapsulated Fe 3 C particles derived from a Fe-bpdc-C 3 N 4 /carbon aerogel composite as a catalyst for the ORR, and the highest ORR activity was obtained using the composite calcined at 800 °C with an initial reduction potential of 1.09 V and a half-wave potential reaching 0.96 V, which were better than those of the commercial Pt/C catalyst. Xia et al 67 also prepared Co-MOF/NGA-derived monodisperse cobalt oxide (CoOx) hollow nanoparticles dispersed in NGA as a Pt-free electro- catalyst and tested its performance.…”

N‐Doped Carbon Aerogel Derived from a Metal–Organic Framework Foam as an Efficient Electrocatalyst for Oxygen Reduction

“…This is because carbon aerogels possess hierarchically porous structures with micro, meso, and macro scales, which could allow the electrolyte and substrates to diffuse rapidly to the accessible active sites. To date, carbon aerogel examples from MOF‐based composites have rarely been reported …”