Electrochemical probing into the active sites of graphitic-layer encapsulated iron oxygen reduction reaction electrocatalysts

Combining Multivariate Electrospinning with Surface MOF Functionalization to Construct Tunable Active Sites toward Trifunctional Electrocatalysis

The development of high-performance multifunctional electrocatalysts operating in the same electrolyte is key to reduce the material and process costs of renewable energy conversion and storage devices. Herein, the fabrication of freestanding integral electrodes by combining multivariate electrospinning with surface metal organic framework functionalization to arrest pyrolytic emissions from fiber interior is reported, resulting in the expression of rich active sites with controlled composition, for example, the tunable Co-P coordination. The as-fabricated electrode of CoP@CF-900, when used as both the cathode and anode for overall water splitting, is able to deliver 200 mA cm^-2 at a cell voltage of 1.89 V, significantly outshining the Pt/C‖RuO₂ couple; when used as the air cathode for a zinc-air battery, is able to operate more than 150 h at 10 mA cm^-2 with a nearly constant round-trip energy efficiency of ≈60%, also outperforming the Pt/C+RuO₂ benchmark. The activity and kinetics origin of the superb multi-functionality is further elucidated through extensive electroanalytical, post-mortem, and operando characterizations, which underscore the construction of robust integral electrodes through synergistic structure and composition engineering.

Crystal Phase Transition Creates a Highly Active and Stable RuCX Nanosurface for Hydrogen Evolution Reaction in Alkaline Media

Although metastable crystal structures have received much attention owing to their utilization in various fields, their phase-transition to a thermodynamic structure has attracted comparably little interest. In the case of nanoscale crystals, such an exothermic phase-transition releases high energy within a confined surface area and reconstructs surface atomic arrangement in a short time. Thus, this high-energy nanosurface may create novel crystal structures when some elements are supplied. In this work, the creation of a ruthenium carbide (RuC_X , X < 1) phase on the surface of the Ru nanocrystal is discovered during phase-transition from cubic-close-packed to hexagonal-close-packed structure. When the electrocatalytic hydrogen evolution reaction (HER) is tested in alkaline media, the RuC_X exhibits a much lower overpotential and good stability relative to the counterpart Ru-based catalysts and the state-of-the-art Pt/C catalyst. Density functional theory calculations predict that the local heterogeneity of the outermost RuC_X surface promotes the bifunctional HER mechanism by providing catalytic sites for both H adsorption and facile water dissociation.

Relevant Properties of Carbon Support Materials in Successful Fe-N-C Synthesis for the Oxygen Reduction Reaction: Study of Carbon Blacks and Biomass-Based Carbons

Fe-N-C materials are promising non-precious metal catalysts for the oxygen reduction reaction in fuel cells and batteries. However, during the synthesis of these materials less active Fe-containing nanoparticles are formed in many cases which lead to a decrease in electrochemical activity and stability. In this study, we reveal the significant properties of the carbon support required for the successful incorporation of Fe-N-related active sites. The impact of two carbon blacks and two activated biomass-based carbons on the Fe-N-C synthesis is investigated and crucial support properties are identified. Carbon supports having low portions of amorphous carbon, moderate surface areas (>800 m2/g) and mesopores result in the successful incorporation of Fe and N on an atomic level and improved oxygen reduction reaction (ORR) activity. A low surface area and especially amorphous parts of the carbon promote the formation of metallic iron species covered by a graphitic layer. In contrast, highly microporous systems with amorphous carbon provoke the formation of less active iron carbides and carbon nanotubes. Overall, a phosphoric acid activated biomass is revealed as novel and sustainable carbon support for the formation of Fe-Nx sites. Overall, this study provides valuable and significant information for the future development of novel and sustainable carbon supports for Fe-N-C catalysts.

Rational design of the carbon doping of hexagonal boron nitride for oxygen activation and oxidative desulfurization

The doping of hexagonal boron nitride (h-BN) materials has a great influence on their catalytic oxidation performance, but the mechanism of doping has still not been studied in depth to date. Herein, carbon-doped h-BN materials were systematically investigated. Three different doping modes were established, and their performance for O₂ activation and oxidative desulfurization (ODS) were explored. DFT calculation showed that not all carbon-doped forms of the h-BN surface could activate O₂. Specifically, two of the dispersed doping forms could activate O₂, whereas the π-doping form could not activate O₂, and thus the ODS reaction could not be carried out. For the two dispersed doping forms, the O₂ adsorption on the C_B-doped h-BN surface (C-doped in B position) was too strong, which hampered its ODS performance; whereas the O₂ adsorption on the C_N-doped h-BN surface (C-doped in the N position) was moderate, resulting in good catalytic activity for ODS. Therefore, to design effective BN-based catalysts by C doping, it is suggested that the C dopant should be dispersed to substitute the N atom of h-BN, and C_N-doped h-BN will play an important role in ODS with moderate O₂ activation. This study can be used as a reference for the catalytic oxidation of boron nitride.

Engineering the electronic and strained interface for high activity of PdMcore@Ptmonolayer electrocatalysts for oxygen reduction reaction

Alloyed nanoparticles with core-shell structures provide a favorable model to modulate interfacial interaction and surface structures at the atomic level, which is important for designing electrocatalysts with high activity and durability. Herein, core-shell structured Pd₃M@Pt/C nanoparticles with binary PdM alloy cores (M = Fe, Ni, and Co) and a monolayer Pt shell were successfully synthesized with diverse interfaces. Among these, Pd₃Fe@Pt/C exhibited the best oxygen reduction reaction catalytic performance, roughly 5.4 times more than that of the commercial Pt/C catalyst used as reference. The significantly enhanced activity is attributed to the combined effects of strain engineering, interfacial electron transfer, and improved Pt utilization. Density functional theory simulations and extended X-ray absorption fine structure analysis revealed that engineering the alloy core with moderate lattice mismatch and alloy composition (Pd₃Fe) optimizes the surface oxygen adsorption energy, thereby rendering excellent electrocatalytic activity. Future researches may use this study as a guide on the construction of highly effective core-shell electrocatalysts for various energy conversions and other applications.

Controlled synthesis of single cobalt atom catalysts via a facile one-pot pyrolysis for efficient oxygen reduction and hydrogen evolution reactions

Metal-nitrogen doped carbon catalysts (M-N/C) with abundantly accessible M-N_x sites, particularly single metal atom M-N/C (SAM-N/C), have been developed as a substitute for expensive Pt-based catalysts. These catalysts are used to increase the efficiency of otherwise sluggish oxygen reduction reactions (ORR) and hydrogen evolution reactions (HER). However, although the agglomerated metal nanoparticles are usually easy to form, they are very difficult to remove due to the protective surface-coating carbon layers, a factor that significantly hampers SAM-N/C fabrication. Herein, we report a one-step pyrolysis approach to successfully fabricate single cobalt atom Co-N/C (SACo-N/C) by using a Co²⁺-SCN^- coordination compound as the metal precursor. Thanks to the decomposition of Co²⁺-SCN^- compound at lower temperature than that of carbon layer deposition, Co-rich particles grow up to larger ones before carbon layers formation. Even though encapsulated by the carbon layers, it is difficult for the large Co-rich particle to be completely sealed. And thus, it makes the Co atoms possible to escape from incomplete carbon layer, to coordinate with nitrogen atoms, and to form SACo-N/C catalysts. This SACo-N/C exhibits excellent performances for both ORR (half-wave potential of 0.878 V) and HER (overpotential at 10 mA/cm² of 178 mV), and is thus a potential replacement for Pt-based catalysts. When SACo-N/C is integrated into a Zn-O₂ battery, battery with high open-circuit voltage (1.536 V) has high peak power density (266 mW/cm²) and large gravimetric energy density (755 mA h/g_Zn) at current densities of 100 mA/cm². Thus, we believe that this strategy may offer a new direction for the effective generation of SAM-N/C catalysts.

Two-Step Synthesis of Cobalt Iron Alloy Nanoparticles Embedded in Nitrogen-Doped Carbon Nanosheets/Carbon Nanotubes for the Oxygen Evolution Reaction

There is a vital need to explore highly efficient and stable non-precious-metal catalysts for the oxygen evolution reaction (OER) to reduce the overpotential and further improve the energy-conversion efficiency. Herein, we report a unique and cost-effective lyophilization and thermal treatment two-step procedure to synthesize a high-performance hybrid consisting of CoFe alloy nanoparticles embedded in N-doped carbon nanosheets interspersed with carbon nanotubes (CoFe-N-CN/CNTs) hybrid. The lyophilization step during the catalyst preparation leads to a uniform dispersion of carbon-like precursors and avoids the agglomeration of metal particles. In addition, the inserted CNTs and doped N in this hybrid provide a good electrical conductivity, an abundance of chemically active sites, good mass transport capability, and effective gas adsorption/release channels. All these lead to a high specific surface area of 240.67 m²  g^-1 , favorable stability, and remarkable OER activities with an overpotential of only 285 mV at a current density of 10 mA cm^-2 and a Tafel slope of 51.09 mV dec^-1 in 1.0 m KOH electrolyte, which is even superior to commercial IrO₂ catalysts. The CoFe-N-CN/CNTs hybrid thus exhibits great potential as a highly efficient and earth-abundant anode OER electrocatalyst.