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

Abstract

Owing to their unique chemistry and physical properties, metal-organic frameworks (MOFs) are an interesting class of materials which can be utilized for a wide array of applications. MOFs have been proposed to be used as catalysts for fuel cells, but their low intrinsic electronic conductivity hampered their utilization as is. In this work, we present the synthesis and application of MOF-based precious-metal-group-free (PGM-free) catalysts for oxygen reduction based on a unique metal-organic framework-carbon composite material. Benzene tricarboxylic acid-based MOFs were synthesized inside activated carbon (AC) with four different, first row transition metals: Mn, Fe, Co, and Cu. The MOFs@AC were analyzed electrochemically to measure their catalytic activity. Further physical and chemical characterization studies are performed to measure the material properties. The MOFs@AC are found to be conductive and active catalysts for the oxygen reduction reaction in an alkaline environment. Surprisingly, the Mn-MOF-based@AC exhibits the best performance with an onset potential of 0.9 V vs. RHE and the almost four-electron mechanism, as opposed to most other known PGM-free catalysts, which show Fe and Co as the most active metals.

Electrocatalytically Active Silver Organic Framework: Ag(I)‐Complex Incorporated in Activated Carbon

“…Ag‐BTC and Ag‐BTC@AC were synthesized according to the procedure described in the experimental section. Two‐probes bulk conductivity measurements showed that the AgBTC is practically an insulator, as many other metal‐BTC, unlike the high electronic conductivity of metallic silver. Therefore, AgBTC cannot be utilized as a stand‐alone electrocatalyst/electrode in its pristine form.…”

“…), metal organic frameworks (MOFs) of metal ions coordinated by polydentate ligands, or heat‐treated materials (M−N−C) . In many cases, macrocyclic compounds and MOFs have been used as the precursor to the M−N−C catalysts rather than being the catalyst itself, since the heat‐treatment raises their overall performance, and due to the lack of conductivity and stability in many cases . The most common metals used in PGM‐free catalysts are first‐row transition metals, usually iron or cobalt …”

“…In a recent work it was shown that other first‐row transition metals can achieve high ORR activity with the suitable ligand, even without going through a heat‐treatment stage . Since it was shown that PGM‐free catalysts do not need to be limited to a specific metal, or even to nitrogen‐based ligands, it was decided to expend this concept and study second‐row transition metal‐based complexes as ORR catalysts.…”

“…In this system, the Ag is atomically dispersed as Ag ions coordinated by the BTC ligand. As many pristine MOFs, including first‐row transition‐metals‐BTCs, Ag‐BTC is not considered an electrically conductive material. In order to overcome this shortfall, it was synthesized inside porous activated carbon (AC).…”

Mn3+ Active Surface Site Enriched Manganese Phosphate Nano‐polyhedrons for Enhanced Bifunctional Oxygen Electrocatalyst

“…It has also been confirmed in Mn−Co spinel oxide compounds that those having largest ratio of Mn 4+ /Mn 3+ has the highest ORR activity . Also, manganese‐based bifunctional ORR and OER catalytic activity have been reported for α‐MnO 2 , layered manganese oxides, cobalt manganese oxides, CoMn 2 O 4 embedded MnOOH nanorods and even in multiple transition metal‐based composites like FeCoNiMn‐nanocarbon, manganese doped multi‐metallic oxysulfide nanoparticles, Mn plays the most important role in bifunctional activity and stability . Additionally, the Jahn‐teller distortion and back bonding with a change in bond length have been observed both in manganese and phosphate‐based materials and hence might boost the kinetics of ORR activity .…”