Calcium Single Atom Confined in Nitrogen-Doped Carbon-Coupled Polyvinylidene Fluoride Membrane for High-Performance Piezocatalysis

A piezoelectric polymer membrane based on single metal atoms was demonstrated to be effective by anchoring isolated calcium (Ca) atoms on a composite of nitrogen-doped carbon and polyvinylidene fluoride (PVDF). The addition of Ca-atom-anchored carbon nanoparticles not only promotes the formation of the β phase (from 29.8 to 56.3%), the most piezoelectrically active phase, in PVDF, but also introduces much higher porosity and hydrophilicity. Under ultrasonic excitation, the fabricated catalyst membrane demonstrates a record-high and stable dye decomposing rate of 0.11 min-1 and antibacterial efficiencies of 99.8%. Density functional theory calculations reveal that the primary contribution to catalytic activity arises from single-atom Ca doping and that a possible synergistic effect between PVDF and Ca atoms can improve the catalytic performance. It is shown that O2 molecules can be easily hydrogenated to produce ·OH on Ca-PVDF, and the local electric field provided by the β-phase-PVDF might enhance the production of ·O2-. The proposed polymer membrane is expected to inspire the rational design of piezocatalysts and pave the way for the application of piezocatalysis technology for practical environmental remediation.

Tuning the coordination environment of Fe atoms enables 3D porous Fe/N-doped carbons as bifunctional electrocatalyst for rechargeable zinc-air battery

As one of the most promising candidates for power sources, the rechargeable Zn-air batteries have attracted much attention due to their high energy density. However, Zn-air batteries suffer from sluggish kinetics of oxygen reduction (ORR) and oxygen evolution reaction (OER) during the discharge and charge process. Herein, a FeN₂-doped carbon with a unique three-dimensional (3D) porous structure (CeO₂-FeNC-5) was synthesized as an electrocatalyst for Zn-air batteries by one-step pyrolysis and introducing CeO₂ to tune the coordination environment of Fe atoms. Extended X-ray absorption fine structure (EXAFS) results indicate that the introduction of CeO₂ can convert FeN₃ moieties into FeN₂ moieties. The CeO₂-FeNC-5 exhibits a more positive half-wave potential of 0.902 V for ORR, and a low overpotential of 0.327 V at 10 mA cm^-2 for OER. Furthermore, the Zn-air battery with CeO₂-FeNC-5 achieve a maximum power density (169 mW cm^-2), a high open voltage platform (1.47 V) and superior cycling stability (200 h). The unique 3D porous structure provides channels for mass transport and exposes sufficient active sites to facilitate the ORR and OER processes. Calculations prove that FeN₂ moieties are beneficial to O₂ adsorption on Fe/N-doped carbon surface. This work provides an effective strategy for designing and synthesizing FeN_x-doped carbon matrix electrocatalysts for sustainable metal-air batteries.

Rational design of M-N4-Gr/V2C heterostructures as highly active ORR catalysts: a density functional theory study

Inspired by the composites of N-doped graphene and transition metal-based materials as well as MXene-based materials, heterostructures (M-N₄-Gr/V₂C) of eight different transition metals (M = Ti, Cr, Mn, Fe, Co, Ni, Cu, and Zn) doped with nitrogen-coordinated graphene and V₂C as potential catalysts for the oxygen reduction reaction (ORR) using density functional theory (DFT) were designed and are described herein. The calculations showed that the heterostructure catalysts (except for Zn-N₄-Gr/V₂C) were thermodynamically stable. Ni-N₄-Gr/V₂C and Co-N₄-Gr/V₂C showed higher activities towards the ORR, with overpotentials as low as 0.32 and 0.45 V, respectively. Excellent catalytic performance results were observed from the change in electronic structure caused by the strong interaction between V₂C and the graphene layers as well as the synergistic effect between the MN₄ groups and the graphene layers. This study further provides insights into the practical application of ORR catalysts for MXene systems through the modulation of the electronic structure of two-dimensional materials.

Non-PGM Electrocatalysts for PEM Fuel Cells: A DFT Study on the Effects of Fluorination of FeNx-Doped and N-Doped Carbon Catalysts

Fluorination is considered as a means of reducing the degradation of Fe/N/C, a highly active FeN_x-doped disorganized carbon catalyst for the oxygen reduction reaction (ORR) in PEM fuel cells. Our recent experiments have, however, revealed that fluorination poisons the FeN_x moiety of the Fe/N/C catalytic site, considerably reducing the activity of the resulting catalyst to that of carbon only doped with nitrogen. Using the density functional theory (DFT), we clarify in this work the mechanisms by which fluorine interacts with the catalyst. We studied 10 possible FeN_x site configurations as well as 2 metal-free sites in the absence or presence of fluorine molecules and atoms. When the FeN_x moiety is located on a single graphene layer accessible on both sides, we found that fluorine binds strongly to Fe but that two F atoms, one on each side of the FeN_x plane, are necessary to completely inhibit the catalytic activity of the FeN_x sites. When considering the more realistic model of a stack of graphene layers, only one F atom is needed to poison the FeN_x moiety on the top layer since ORR hardly takes place between carbon layers. We also found that metal-free catalytic N-sites are immune to poisoning by fluorination, in accordance with our experiments. Finally, we explain how most of the catalytic activity can be recovered by heating to 900 °C after fluorination. This research helps to clarify the role of metallic sites compared to non-metallic ones upon the fluorination of FeN_x-doped disorganized carbon catalysts.

Density functional study on the CO oxidation reaction mechanism on MnN2-doped graphene

The CO oxidation mechanisms over three different MnN2-doped graphene (MnN2C2: MnN2C2-hex, MnN2C2-opp, MnN2C2-pen) structures were investigated through first-principles calculations. The vacancy in graphene can strongly stabilize Mn atoms and make them positively charged, which promotes O2 activation and weakens CO adsorption. Hence, CO oxidation activity is enhanced and the catalyst is prevented from being poisoned. CO oxidation reaction (COOR) on MnN2C2 along the Eley-Rideal (ER) mechanism and the Langmuir-Hinshelwood (LH) mechanism will leave one O atom on the Mn atom, which is difficult to react with isolated CO. COOR on MnN2C2-opp along the ER mechanism and termolecular Eley-Rideal (TER) mechanism need overcome low energy barriers in the rate limiting step (RLS), which are 0.544 and 0.342 eV, respectively. The oxidation of CO along TER mechanism on MnN2C2-opp is the best reaction pathway with smallest energy barrier. Therefore, the MnN2C2-opp is an efficient catalysis and this study has a guiding role in designing effective catalyst for CO oxidation.

A theoretical study of atomically dispersed MN4/C (M = Fe or Mn) as a high-activity catalyst for the oxygen reduction reaction

Carbon-based, non-noble metal catalysts for the oxygen reduction reaction (ORR) are crucial for the large-scale application of metal-air batteries and fuel cells. Density functional theory calculations were performed to explore the potential of atomically dispersed MN4/C (M = Fe or Mn) as an ORR catalyst in an acidic electrolyte and the ORR mechanism on MN4/C was systematically studied. The results indicated MN4 as the active site of MN4/C and a four-electron OOH transformation pathway as the preferred ORR mechanism on the MN4/C surface. The Gibbs free energy diagram showed that the rate-determining step of the FeN4/C and MnN4/C catalysts is the formation of the second H2O molecule and OOH*, respectively. FeN4/C exhibited higher thermodynamic limiting potential (0.79 V) and, thus, higher ORR activity than MnN4/C (0.52 V) in an acidic environment; its excellent catalytic performance is due to the nice electron structure and adsorption properties of the FeN4 site. Therefore, this work demonstrates that atomically dispersed MN4/C is a promising catalyst for the ORR.