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.

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Durability of catalyst supports is a technical barrier for both stationary and transportation applications of polymer-electrolyte-membrane fuel cells. New classes of non-carbon-based materials were developed in order to overcome the current limitations of the state-of-the-art carbon supports. Some of these materials are designed and tested to exceed the US DOE lifetime goals of 5000 or 40,000 hrs for transportation and stationary applications, respectively. In addition to their increased durability, the interactions between some new support materials and metal catalysts such as Pt result in increased catalyst activity. In this review, we will cover the latest studies conducted with ceramic supports based on carbides, oxides, nitrides, borides, and some composite materials.

Aerogels offer a great platform for heterogeneous electrocatalysis owing to their high surface area and porosity. Atomically dispersed transition metal ions can be imbedded in these platforms at ultra‐high site density to make them catalytically active for various reactions. Herein, the synthesis of a new class of conjugated microporous organic aerogels that are used as covalent 3D frameworks for the electrocatalysis of oxygen reduction reaction (ORR) is reported. Modified aerogels functionalized with bipyridine ligands enable copper ion complexation in a single‐step synthesis. The aerogels’ structures are fully characterized using a wide array of spectroscopic and microscopic methods, and heat‐treated in order to make them electronically conductive. After heat treatment at 600 °C, the aerogels maintained their macrostructure and became active ORR catalysts in alkaline environment, showing high mass activity and ultra‐high site density.

Nano-crystallites of molybdenum carbide, Mo 2 C, were synthesized via modified polymer-assisted deposition (mPAD) and utilized as platinum support material for polymer electrolyte fuel cells, as alternative for the commonly used, corrosion-prone, carbon supports. The Mo 2 C durability, corrosion-resistance and effect on the oxygen reduction reaction with deposited Pt Nano-particles was studied. The synthesized ceramic compound was found to be devoid of free carbon, as opposed to other carbides synthesized in the past, making it potentially compatible for fuel cell electrode material. The molybdenum carbide appeared to improve the electro-catalytic activity of Pt catalyst, showing an increase in both, the mass activity (three times higher at 0.85 V vs. RHE), and half-wave potential (by 70 mV), when compared to commercial Pt/C catalyst. As anticipated, the durability was also increased, showing 40% more resistance to chemical and physical corrosion than standard commercial Pt/C catalyst/support system.

Cost of polymer electrolyte membrane fuel cells (PEMFC) is one of the most acute setbacks delaying this technology's worldwide deployment and commercialization. Platinum, the most common and effective catalyst for the oxygen reduction reaction (ORR), is a scarce element which accounts for more than 45 % of PEMFC stack cost. One of the preferred proposed paths to substantially reduce stack costs is by dramatically lowering precious metal loading in a membrane electrode assembly (MEA) to account for up to 11.3 gPt per midsize vehicle (US‐DOE 2020 target). This Minireview surveys the latest synthetical and methodical advances in making and utilizing ultra‐low loading precious metal ORR catalyst in PEM fuel cells.

We use first-principles calculations to study the formation of Pt nanorafts and their oxygen reduction reaction (ORR) catalytic activity on MoC. Due to the high Pt binding energy on C atoms, Pt forms sheet-like structures on the MoC surface instead of agglomerating into particles. We find that the disordered MoC surface carbon arrangement limits the Pt sheet growth, leading to the formation of 4-6 atom Pt nanorafts. The O-O repulsion between the O atoms on the MoC and O adsorbate enhances the ORR activity by weakening the O adsorption energy. We find a significant change from the usual scaling of the energies of the intermediates in the ORR pathway and a strong interaction between the nanoraft and water that lead to a high activity of the Pt nanorafts. Fundamentally, our work demonstrates that the activity of metal catalysts can be strongly affected by manipulation of the atomic arrangement of the supporting carbide surface.

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