Cobalt-Doped Ba2In2O5 Brownmillerites: An Efficient Electrocatalyst for Oxygen Reduction in Alkaline Medium.

A series of compounds with cobalt doping in the indium site of Ba2In2O5 brownmillerites exhibited excellent oxygen reduction activity under alkaline conditions. Doping (25%) retains the brownmillerite structure with disorder in the O3 site in the two-dimensional alternate layer along the ab plane. Further substitution of cobalt in the indium site leads to the loss of a brownmillerite structure, and the compound attains a perovskite structure. Cobalt-doped samples exhibited far better oxygen reduction reaction (ORR) activity when compared to the parent Ba2In2O5 brownmillerite. Among the series of compounds, BaIn0.25Co0.75O3-δ with the highest Co doping and oxygen vacancies randomly distributed in the lattice exhibited the best ORR activity. BaIn0.25Co0.75O3-δ showed a 40 mV positive shift in the onset potential with better limiting current density and a nearly four-electron-transfer reduction pathway when compared to the parent Ba2In2O5 brownmillerite.

Carbon Nanotube/Boron Nitride Nanocomposite as a Significant Bifunctional Electrocatalyst for Oxygen Reduction and Oxygen Evolution Reactions.

It is an immense challenge to develop bifunctional electrocatalysts for oxygen reduction reactions (ORR) and oxygen evolution reactions (OER) in low temperature fuel cells and rechargeable metal-air batteries. Herein, a simple and cost-effective approach is developed to prepare novel materials based on carbon nanotubes (CNTs) and a hexagonal boron nitride (h-BN) nanocomposite (CNT/BN) through a one-step hydrothermal method. The structural analysis and morphology study confirms the formation of a homogeneous composite and merging of few exfoliated graphene layers of CNTs on the graphitic planes of h-BN, respectively. Moreover, the electrochemical study implies that CNT/BN nanocomposite shows a significantly higher ORR activity with a single step 4-electron transfer pathway and an improved onset potential of +0.86 V versus RHE and a current density of 5.78 mA cm-2 in alkaline conditions. Interestingly, it exhibits appreciably better catalytic activity towards OER at low overpotential (η=0.38 V) under similar conditions. Moreover, this bifunctional catalyst shows substantially higher stability than a commercial Pt/C catalyst even after 5000 cycles. Additionally, this composite catalyst does not show any methanol oxidation reactions that nullify the issues due to fuel cross-over effects in direct methanol fuel cell applications.

Synergistically Enhanced Electrocatalytic Performance of an N-Doped Graphene Quantum Dot-Decorated 3D MoS2-Graphene Nanohybrid for Oxygen Reduction Reaction.

Nitrogen-doped graphene quantum dots (N-GQDs) were decorated on a three-dimensional (3D) MoS2-reduced graphene oxide (rGO) framework via a facile hydrothermal method. The distribution of N-GQDs on the 3D MoS2-rGO framework was confirmed using X-ray photoelectron spectroscopy, energy dispersive X-ray elemental mapping, and high-resolution transmission electron microscopy techniques. The resultant 3D nanohybrid was successfully demonstrated as an efficient electrocatalyst toward the oxygen reduction reaction (ORR) under alkaline conditions. The chemical interaction between the electroactive N-GQDs and MoS2-rGO and the increased surface area and pore size of the N-GQDs/MoS2-rGO nanohybrid synergistically improved the ORR onset potential to +0.81 V vs reversible hydrogen electrode (RHE). Moreover, the N-GQDs/MoS2-rGO nanohybrid showed better ORR stability for up to 3000 cycles with negligible deviation in the half-wave potential (E 1/2). Most importantly, the N-GQDs/MoS2-rGO nanohybrid exhibited a superior methanol tolerance ability even under a high concentration of methanol (3.0 M) in alkaline medium. Hence, the development of a low-cost metal-free graphene quantum dot-based 3D nanohybrid with high methanol tolerance may open up a novel strategy to design selective cathode electrocatalysts for direct methanol fuel cell applications.