One-step ultrasonic synthesis of Co/Ni-catecholates for improved performance in oxygen reduction reaction

The inherent periodically arranged M-N_X, M-S_X and M-O_X units (M are usually Fe, Co, Ni, etc.) in metal-organic frameworks (MOFs) can be promising active centers in electrocatalysis. In previous studies, MOFs were usually constructed by energy-consuming hydro- or solvo-thermal reactions. Ultrasonic synthesis is a rapid and environment-friendly technique when envisaging MOFs' industrial applications. In addition, different synthetic pathways for MOFs may lead to difference in their microstructure, resulting in different electrocatalytic performance. Nevertheless, only a handful of MOFs were successfully prepared by ultrasonic synthesis and few were applied in electrochemical catalysis. Herein, we constructed Ni/Co-catecholates (Ni/Co-CATs) synthesized by one-step ultrasonic method (250 W, 40 KHz, 25 W/L, Ultrasonic clearing machine) and compared their performance in oxygen reduction reaction (ORR) with that of Ni/Co-CATs synthesized by hydrothermal method. Ni-CAT and Co-CAT prepared by ultrasonic showed the half-wave potential of -0.196 V and -0.116 V (vs. Ag/AgCl), respectively. The potentials were more positive than those prepared by hydro-thermal method. And they showed excellent electrochemical stability in neutral solution. The latter was only 32 mV lower than that of commercial Pt/C. The improved performance in ORR was attributed to higher specific surface area and mesopore volume as well as more structural defects generated in the ultrasonic synthesis process, which could facilitate their exposure of electrocatalytic active sites and their mass transport. This work gives some perspective into cost-effective synthetic strategies of efficient MOFs-based electrocatalysts.

Unveiling the Axial Hydroxyl Ligand on Fe-N4-C Electrocatalysts and Its Impact on the pH-Dependent Oxygen Reduction Activities and Poisoning Kinetics

Fe-N-C materials have shown a promising nonprecious oxygen reduction reaction (ORR) electrocatalyst yet their active site structure remains elusive. Several previous works suggest the existence of a mysterious axial ligand on the Fe center, which, however, is still unclarified. In this study, the mysterious axial ligand is identified as a hydroxyl ligand on the Fe centers and selectively promotes the ORR activities depending on different Fe-N4-C configurations, on which the adsorption free energy of the hydroxyl ligand also differs greatly. The selective formation of hydroxyl ligand on specific Fe-N-C configurations can resolve contradictories between previous theoretical and experimental results regarding the ORR activities and associated active configurations of Fe-N-C catalysts. It also explains the pH-dependent ORR activities and, moreover, a previously unreported pH-dependent poisoning kinetics of the Fe-N-C catalysts.

Alkaline Anion-Exchange Membrane Fuel Cells: Challenges in Electrocatalysis and Interfacial Charge Transfer

Alkaline anion-exchange membrane (AAEM) fuel cells have attracted significant interest in the past decade, thanks to the recent developments in hydroxide-anion conductive membranes. In this article, we compare the performance of current state of the art AAEM fuel cells to proton-exchange membrane (PEM) fuel cells and elucidate the sources of various overpotentials. While the continued development of highly conductive and thermally stable anion-exchange membranes is unambiguously a principal requirement, we attempt to put the focus on the challenges in electrocatalysis and interfacial charge transfer at an alkaline electrode/electrolyte interface. Specifically, a critical analysis presented here details the (i) fundamental causes for higher overpotential in hydrogen oxidation reaction, (ii) mechanistic aspects of oxygen reduction reaction, (iii) carbonate anion poisoning, (iv) unique challenges arising from the specific adsorption of alkaline ionomer cation-exchange head groups on electrocatalysts surfaces, and (v) the potential of alternative small molecule fuel oxidation. This review and analysis encompasses both the precious and nonprecious group metal based electrocatalysts from the perspective of various interfacial charge-transfer phenomena and reaction mechanisms. Finally, a research roadmap for further improvement in AAEM fuel cell performance is delineated here within the purview of electrocatalysis and interfacial charge transfer.

Advanced Biomass-Derived Electrocatalysts for the Oxygen Reduction Reaction

Recent progress in advanced nanostructures synthesized from biomass resources for the oxygen reduction reaction (ORR) is reviewed. The ORR plays a significant role in the performance of numerous energy-conversion devices, including low-temperature hydrogen and alcohol fuel cells, microbial fuel cells, as well as metal-air batteries. The viability of such fuel cells is strongly related to the cost of the electrodes, especially the cathodic ORR electrocatalyst. Hence, inexpensive and abundant plant and animal biomass have become attractive options to obtain electrocatalysts upon conversion into active carbon. Bioresource selection and processing criteria are discussed in light of their influence on the physicochemical properties of the ORR nanostructures. The resulting electrocatalytic activity and durability are introduced and compared to those from conventional Pt/C-based electrocatalysts. These ORR catalysts are also active for oxygen or hydrogen evolution reactions.

A direct four-electron process on Fe–N3 doped graphene for the oxygen reduction reaction: a theoretical perspective

The reaction mechanism for the ORR on Fe–N₃-Gra is investigated theoretically. Our results indicate that the ORR is a direct four-electron process, and the kinetically most favorable pathway is O₂ hydrogenation.

CoN3 embedded graphene, a potential catalyst for the oxygen reduction reaction from a theoretical perspective

Non-precious metal catalysts have attracted particular interest in recent years due to their promising ORR (oxygen reduction reaction) activity in fuel cells.

Defective-Activated-Carbon-Supported Mn-Co Nanoparticles as a Highly Efficient Electrocatalyst for Oxygen Reduction

A highly active and durable cathodic oxygen reduction reaction (ORR) catalyst is synthesized by introducing a small amount of Mn-Co spinel into a kind of defective activated-carbon (D-AC) support. It is assumed that the synergetic coupling effects between the unique defects in the D-AC and the loaded Mn-Co spinel facilitate the ORR and enhance its durability.