Polystyrene Microspheres-Templated Nitrogen-Doped Graphene Hollow Spheres as Metal-Free Catalyst for Oxygen Reduction Reaction

The CO2 electrolysing mechanism in single-phase mixed-conducting cathode of solid oxide cell

In the field of solid oxide cells (SOC), unveiling the electrochemical reaction and transfer mechanisms in mixed ionic and electronic conducting (MIEC) electrodes is of great importance. Due to the chemical capacitance effects of MIEC materials, SOC often shows large capacitance current during electrochemical tests, which might interfere with the polarization behaviors. This work presents a numerical multiphysical model based on the transport of oxygen species, which accurately and concisely replicates the current-voltage curves of a solid oxide electrolysis cell (SOEC) with MIEC electrodes under various scanning rates. The scanning IV and electrochemical impedance spectra measurement under different SOEC working conditions are combined to enable the separation of Faradic and charging currents. Thus, both the bulk diffusion and surface gaseous diffusion of the oxygen species are encompassed, which explains how the current being generated due to intertwined chemical capacitance effects and chemical reactions in the MIEC electrodes.

Mesoporous Single Atom-Cluster Fe-N/C Oxygen Evolution Electrocatalysts Synthesized with Bottlebrush Block Copolymer-Templated Rapid Thermal Annealing

Current electrocatalysts for oxygen evolution reaction (OER) are either expensive (such as IrO2, RuO2) or/and exhibit high overpotential as well as sluggish kinetics. This article reports mesoporous earth-abundant iron (Fe)-nitrogen (N) doped carbon electrocatalysts with iron clusters and closely surrounding Fe-N4 active sites. Unique to this work is that the mechanically stable mesoporous carbon-matrix structure (79 nm in pore size) with well-dispersed nitrogen-coordinated Fe single atom-cluster is synthesized via rapid thermal annealing (RTA) within only minutes using a self-assembled bottlebrush block copolymer (BBCP) melamine-formaldehyde resin composite template. The resulting porous structure and domain size can be tuned with the degree of polymerization of the BBCP backbone, which increases the electrochemically active surface area and improves electron transfer and mass transport for an effective OER process. The optimized electrocatalyst shows a required potential of 1.48 V (versus RHE) to obtain the current density of 10 mA/cm2 in 1 M KOH aqueous electrolyte and a small Tafel slope of 55 mV/decade at a given overpotential of 250 mV, which is significantly lower than recently reported earth-abundant electrocatalysts. Importantly, the Fe single-atom nitrogen coordination environment facilitates the surface reconstruction into a highly active oxyhydroxide under OER conditions, as revealed by X-ray photoelectron spectroscopy and in situ Raman spectroscopy, while the atomic clusters boost the single atoms reactive sites to prevent demetalation during the OER process. Density functional theory (DFT) calculations support that the iron nitrogen environment and reconstructed oxyhydroxides are electrocatalytically active sites as the kinetics barrier is largely reduced. This work has opened a new avenue for simple, rapid synthesis of inexpensive, earth-abundant, tailorable, mechanically stable, mesoporous carbon-coordinated single-atom electrocatalysts that can be used for renewable energy production.

Progress of Nonmetallic Electrocatalysts for Oxygen Reduction Reactions

As a key role in hindering the large-scale application of fuel cells, oxygen reduction reaction has always been a hot issue and nodus. Aiming to explore state-of-art electrocatalysts, this paper reviews the latest development of nonmetallic catalysts in oxygen reduction reactions, including single atoms doped with carbon materials such as N, B, P or S and multi-doped carbon materials. Afterward, the remaining challenges and research directions of carbon-based nonmetallic catalysts are prospected.

Highly active Fe-N-doped porous hollow carbon nanospheres as oxygen reduction electrocatalysts in both acidic and alkaline media

Hierarchical iron-nitrogen-codoped porous hollow carbon spheres have been synthesized by using melamine-formaldehyde (MF) resin spheres as templates, nitrogen sources and pore-forming agents. FeCl3, 1,10-phenanthroline and carbon black were used as iron, nitrogen and carbon sources. The as-obtained porous hollow carbon spheres possess a high specific surface area of 807 m2 g-1, as well as exhibited excellent electrocatalytic activity for the oxygen reduction reaction (ORR) in both acidic and alkaline media. In 0.1 M HClO4 solution, the onset potential was 0.857 V (vs. RHE) and the half-wave potential was 0.715 V, which are only 78 and 80 mV less than those of the 20% Pt/C catalyst, respectively. In addition, in 0.1 M KOH solution, the onset potential was 1.017 V and the half-wave potential was 0.871 V for the ORR, which are 22 and 28 mV more positive than those of the Pt/C catalyst, respectively. Meanwhile, the catalyst also exhibited excellent methanol tolerance and long-term durability with a more effective four-electron pathway compared to the 20% Pt/C catalyst in both acidic and alkaline media. When used as an air-cathode catalyst for a Zn-air battery, the maximum power density of a Zn-air battery with the MF-C-Fe-Phen-800 cathode was 235 mW cm-2 at a high current density of 371 mA cm-2, and a high open-circuit potential of 1.654 V, superior to that of Pt/C (199 mW cm-2, 300 mA cm-2, 1.457 V). A series of designed experiments suggested that the remarkable performance was attributed to the high specific area, hollow carbon spheres, unique hierarchical micro-mesoporous structures, high contents of pyridinic-N and graphitic-N. The superiority of the as-prepared catalyst makes it promising for use in practical applications.

The self-template synthesis of highly efficient hollow structure Fe/N/C electrocatalysts with Fe-N coordination for the oxygen reduction reaction

The exploration of highly efficient catalysts to replace noble metal platinum for the oxygen reduction reaction, on which M/N/C catalysts have shed brilliant light, is greatly significant but challenging. This paper presents a strategy for synthesizing highly efficient and stabilized hollow structure Fe/N/C catalysts with iron and nitrogen doped into the carbon layer by the self-template method. The prepared Fe/N/C catalysts with NaCl protection during pyrolysis are characterized by a unique hollow structure, porous morphology and Fe-N coordination as the active sites, all of which significantly endow the materials with excellent properties towards the ORR, including high electrical conductivity, long-term durability and outstanding capacity for methanol tolerance. We employed X-ray absorption fine structure spectrometry to investigate the chemical state and coordination environment of the central iron atoms of the Fe/N/C catalysts, which also clarified the promoting effect of the NaCl protection for Fe-N coordination during pyrolysis. In particular, the Fe/N/C catalysts exhibit positive half-wave potentials (0.84 V vs. RHE) and Tafel slope comparable to 20% commercial Pt/C, possessing four-electron transfer pathway as well as excellent long-term stability and methanol tolerance in alkaline medium.

Dealloyed PdAg core Pt monolayer shell electrocatalyst for oxygen reduction reaction

A core–shell nanostructure with dealloyed PdAg nanoparticles as the core and a Pt monolayer as the shell shows much higher catalytic properties for oxygen reduction reaction as compared with its counterpart with alloyed PdAg as the core.

Nanostructured platinum-free electrocatalysts in alkaline direct alcohol fuel cells: catalyst design, principles and applications

A review of the fundamental principles that allow for the intelligent design and synthesis of non-precious metal nanostructured electrocatalysts for ADAFCs.