Advances in platinum-based and platinum-free oxygen reduction reaction catalysts for cathodes in direct methanol fuel cells

Direct methanol fuel cells (DMFCs) have been the focus of future research because of their simple structure, abundant fuel sources, high energy conversion efficiency and low cost. Among the components in DMFC, the activity and stability of the cathode catalyst is the key to the performance and lifetime of the DMFCs. Oxygen reduction reaction (ORR) is an important electrode reaction on DMFC cathode. It is known that Pt is widely used in the fabrication of ORR catalysts, but the limited earth storage of Pt and its high price limit the use of Pt-based commercial catalysts in DMFCs. To overcome these problems, advances have been made on new low Pt-based catalysts and Pt-free catalysts in recent years. In this article, the development of novel ORR catalysts and the carbon supports is reviewed and discussed.

Drop-casted Platinum Nanocube Catalysts for Hydrogen Evolution Reaction with Ultrahigh Mass Activity

Platinum hydrogen evolution reaction (HER) electrocatalysts in the form of nanocubes (NCs) were synthesized at 50 °C by aqueous-based colloidal synthesis and were applied to electrochemical (EC) and photoelectrochemical (PEC) systems by a fast and simple drop-casting method. A remarkable Pt mass activity of 1.77 A mg^-1 at -100 mV was achieved in EC systems (fluorine-doped tin oxide/Pt NC cathode) with neutral electrolyte while maintaining low overpotential and Tafel slope. In the Cu(In,Ga)(S,Se)₂ (CIGS)-based PEC system, a carefully chosen amount of Pt NC loading to achieve a compromise between the catalytic activity (more Pt NCs) and better light transmittance (fewer Pt NCs) led to a maximum onset potential of 0.678 V against the reference hydrogen electrode. The photoelectrodes with Pt NCs also exhibited good long-term operational stability over 9.5 h with negligible degradation of the photocurrent. This study presents an effective strategy to greatly reduce the use of expensive Pt without compromising the catalytic performance because the drop-casting of Pt NC solutions to form electrocatalysts is expected to waste less raw material than vacuum deposition.

A Surface-Oxide-Rich Activation Layer (SOAL) on Ni2 Mo3 N for a Rapid and Durable Oxygen Evolution Reaction

The oxygen evolution reaction (OER) is key to renewable energy technologies such as water electrolysis and metal-air batteries. However, the multiple steps associated with proton-coupled electron transfer result in sluggish OER kinetics and catalysts are required. Here we demonstrate that a novel nitride, Ni₂ Mo₃ N, is a highly active OER catalyst that outperforms the benchmark material RuO₂ . Ni₂ Mo₃ N exhibits a current density of 10 mA cm^-2 at a nominal overpotential of 270 mV in 0.1 m KOH with outstanding catalytic cyclability and durability. Structural characterization and computational studies reveal that the excellent activity stems from the formation of a surface-oxide-rich activation layer (SOAL). Secondary Mo atoms on the surface act as electron pumps that stabilize oxygen-containing species and facilitate the continuity of the reactions. This discovery will stimulate the further development of ternary nitrides with oxide surface layers as efficient OER catalysts for electrochemical energy devices.

Enhancing Electrocatalytic Performance of Bifunctional Cobalt-Manganese-Oxynitride Nanocatalysts on Graphene

We report the synthesis and characterization of graphenesupported cobalt-manganese-oxynitride nanocatalysts (CoMnON/G) as bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). A nitriding treatment of spinel compound CoMnO increased the ORR activity considerably, and the most active material catalyzed the ORR with only a 30 mV half-wave potential difference from the commercial carbon-supported platinum (Pt/C) in alkaline media. In addition to high activity, the catalyst also exhibited an intrinsic stability that outperformed Pt/C. An appropriately designed nitridation thus facilitates new directions for developing active and durable non-precious-metal oxynitride electocatalysts.

A review of metal oxynitrides for photocatalysis

This review highlights the recent progress in the production of hydrogen using a metal oxynitride photocatalyst under visible light irradiation.