Oxygen Reduction Pathway Research Articles

The inhibitory mechanism of humic acids on photocatalytic generation of reactive oxygen species by TiO2 depends on the crystalline phase

To enable the practical use of TiO2 for photocatalytic water treatment, it is important to understand how dissolved natural organic matter affects its photocatalytic activity and the mechanisms involved in generating reactive oxygen species (ROS). In this study, we systematically investigated the inhibitory effects of humic acids (HA) on 4-chlorophenol (4-CP) degradation using two common TiO2 crystalline phases (anatase and rutile) individually. HA strongly hindered the photocatalytic activity of anatase, with an Ra (the ratio of the first-order rate constant for 4-CP degradation in the absence vs. presence of 30 mg/L HA) of 2.30 (±0.13). This ratio was significantly higher than that of rutile (Rr = 1.21 ± 0.08), which was less susceptible to this inhibitory effect and even exhibited improved photocatalytic activity at HA concentrations below 20 mg/L. Similar trends were observed for various HA sources, corroborating the crystalline-phase-dependent effect of HA. Adsorption experiments, Fourier transform infrared spectroscopy, and photoelectrochemical analyses suggested that the adsorption mechanisms and hole-scavenging effect of HA on the two TiO2 polymorphs did not differ. Importantly, scavenger, probe, and electron spin resonance experiments revealed that the difference in inhibitory effects of HA originate from the distinct ROS generation mechanisms for the two polymorphs. The oxygen reduction pathway for OH generation over anatase was hindered by surface-adsorbed HA, while the water oxidation pathway for OH generation over rutile was less affected. Furthermore, surface-adsorbed HA boosted O2− generation on rutile, increasing 4-CP degradation efficiency. This mechanistic insight into NOM-TiO2 interactions informs materials selection and strategies for higher TiO2 photocatalytic performance in different water matrices.

Clean Production of Hydrogen Peroxide: A Heterogeneous Solar‐Driven Redox Process

AbstractHydrogen peroxide is an essential chemical that is attracting strong attention for energy and environmental applications. However, the struggle between the growing market demand and the unsustainability of the conventional anthraquinone method motivates the exploration of alternative H2O2 production processes. Although several new production processes have been proposed, the environmental‐friendly solar‐driven H2O2 production attracts most attention because of the only inputs of water, oxygen, and light energy. The rational design of functional photo‐responsive catalysts promotes H2O2 production in the photocatalytic and photoelectrocatalytic approaches. These are, in general, achieved by facilitating the preferential adsorption of key intermediates of OOH*/OH*/O*, enhancing the light absorption, promoting the charge separation, and accelerating the surface charge transfer with selective number of involved charges. This review systematically summarizes strategies for photo(electro)catalysts toward H2O2 production via both the water oxidation and oxygen reduction pathways. Though the oxygen reduction route is perceived as more popular in the community, selective water oxidation is emerging as a convincing alternative. Furthermore, prevailing hypotheses, state‐of‐the‐art catalysts, critical challenges, and perspectives are discussed in depth. This review aims to enhance the comprehension of this research field and promote interest in sustainable H2O2 production.

Kirigami Photopiezo Catalysts for Self-Sustainable Environment Remediation.

Photo(electro)-piezo catalysis has emerged as one of the most effective strategies for sustainable environmental remediation. While various (nano)materials have been investigated for enhancing the intrinsic properties related to the interfacial band structure, increasing the efficiency by integration of materials with rational design for stress-strain applications has not yet been considered. Herein, we introduce kirigami strain engineering to photopiezo catalysts for enhancing efficiency by increasing the magnitude of applied strain and density of bends. Macroscale stretching motion is converted into localized bending by a pliable kirigami structure using similar or even lower input energy, which can be easily modulated by natural waves. The kirigami structure leads to a significant enhancement (∼250%) in the degradation of dyes, and we discovered the significant contribution of the oxygen reduction pathway in the charge-transfer mechanism, which corresponds to the observed enhancement. The photopiezo catalytic effects of kirigami were further highlighted by the small water reservoir test, showing its feasibility in nature for self-sustainable environmental remediation that can be modulated using motions of winds, waves, and life vibrations.

Insights into the trigger effect of Pt-N bond on lattice distortions of novel PtCu alloys enabling an unusual pathway in oxygen reduction

The precise manipulation of the carbon matrix into a unique hierarchical structure by heteroatoms (N) doping, further applying to load Pt-transition metal (M) nanoparticles (NPs) can enable PtM/NC catalyst with excellent oxygen reduction reaction (ORR) activity. However, the electronic and geometric structure changes of PtM NPs after introducing N, and the corresponding catalytic mechanism during ORR are often neglected. Herein, the N-doped metal organic framework (MOF)-derived carbon loaded PtCu NPs catalyst (PtCu/NC) is prepared to reveal the surface behavior of Pt-N and its modulation mechanism. The Cu and carbon separation property of Cu-MOF pyrolysis supports the preferential N-doping of carbon, followed by PtCu alloying. This lays the structural basis for the formation of Pt-N bond. The density function theory (DFT) proves that N preferentially bonds with Pt to form Pt-N bond to trigger lattice distortions of the subsequently alloyed PtCu NPs. In situ Raman analysis shows the dynamic evolution of OOH* intermediate on distorted PtCu NPs, further proving that there is indeed the electron transfer from N to Pt superior to that of Cu promoting associative pathway for oxygen reduction. These boost PtCu/NC catalyst exhibiting excellent ORR activity with a mass activity (MA) of 0.87 AmgPt−1 (@0.80 V), greater than that of 20 wt% Pt/C. Additionally, a high MA is also obtained in the methanol oxidation reaction (MOR) test, also higher than that of Pt/C.

Phosphorus-Induced Dual-Sites and High-Index PtCu (3 1 1) crystal planes for modifying oxygen reduction pathway

The oxygen reduction reaction (ORR) on Pt-based catalysts is largely retarded by the stronger adsorption of the intermediates on Pt (111) than ideal catalysts. Synergistic catalysis offers a promising approach to reshape the electronic structure of the designed catalyst for optimizing the adsorption energy of the intermediates on its surface. Herein, we certificate that the intermediates adsorption on the C-supported PtCu alloy catalyst can be modulated synergistically by distorted PtCu (111) crystal planes and P-C sites caused by P-doping, leading to a substantial increase in ORR. In situ spectral evidence of two *OOH intermediates on the above dual sites identifies the role of introduced metalloid P elements in promoting the ORR association pathway. Notably, the high-index PtCu (311) crystal planes of the P-doped catalysts have shown a distinct enhancement with abundant terraces and step densities, which contributes to the highest ORR activity of PtCu/PC-3 with the mass activity (MA) of 0.82 A mgPt-1 (0.8 V). Analysis of the *OH real-time adsorption behaviors on this crystal plane suggests that another parallel dissociation pathway is also in operation. Benefiting from the increased active site species and the enhanced intrinsic activity, the optimal PtCu/PC-3 also exhibits the impressive performance in methanol oxidation reaction (MOR).

Insight into oxygen reduction activity and pathway on pure titanium using scanning electrochemical microscopy and theoretical calculations

HypothesisUnlike noble metals, the oxygen reduction reaction (ORR) behavior on Ti is more complicated due to its spontaneously formed oxide film. This film results in sluggish ORR kinetics and tends to be reduced within ORR potential region, causing the weak and multi-reaction coupled current. Though Ti is being used in chemical and biological fields, its ORR research is still underexplored. ExperimentsWe innovatively employed the modified reactive tip generation-substrate collection (RTG/SC) mode of scanning electrochemical microscopy (SECM) with high efficiency of 97.2 % to quantitatively study the effects of film characteristics, solution environment (pH, anion, dissolved oxygen), and applied potential on the ORR activity and selectivity of Ti. Then, density functional theory (DFT) and molecular dynamics (MD) analyses were employed to elucidate its ORR behavior. FindingsOn highly reduced Ti, film properties dominate ORR behavior with promoted 4e– selectivity. Rapid film regeneration in alkaline/O2-saturated conditions inhibits ORR activity. Besides, ORR is sensitive to anion species in neutral solutions while showing enhanced 4e– reduction in alkaline media. All the improved 4e– selectivities originate from the hydrogen bond/electrostatic stabilization effect, while the decayed ORR activity by Cl– arises from the suppressed O2 adsorption. This work provides theoretical support and possible guidance for ORR research on oxide-covered metals.

Molecular Level Modulation of Anthraquinone-containing Resorcinol-formaldehyde Resin Photocatalysts for H2 O2 Production with Exceeding 1.2 % Efficiency.

Designing polymeric photocatalysts at the molecular level to modulate the photogenerated charge behavior is a promising and challenging strategy for efficient hydrogen peroxide (H2 O2 ) photosynthesis. Here, we introduce electron-deficient 1,4-dihydroxyanthraquinone (DHAQ) into the framework of resorcinol-formaldehyde (RF) resin, which modulates the donor/acceptor ratio from the perspective of molecular design for promoting the charge separation. Interestingly, H2 O2 can be produced via oxygen reduction and water oxidation pathways, verified by isotopic labeling and in situ characterization techniques. Density functional theory (DFT) calculations elucidate that DHAQ can reduce the energy barrier for H2 O2 production. RF-DHAQ exhibits excellent overall photosynthesis of H2 O2 with a solar-to-chemical conversion (SCC) efficiency exceeding 1.2 %. This work opens a new avenue to design polymeric photocatalysts at the molecular level for high-efficiency artificial photosynthesis.

Molecular Level Modulation of Anthraquinone‐containing Resorcinol‐formaldehyde Resin Photocatalysts for H2O2 Production with Exceeding 1.2 % Efficiency

AbstractDesigning polymeric photocatalysts at the molecular level to modulate the photogenerated charge behavior is a promising and challenging strategy for efficient hydrogen peroxide (H2O2) photosynthesis. Here, we introduce electron‐deficient 1,4‐dihydroxyanthraquinone (DHAQ) into the framework of resorcinol‐formaldehyde (RF) resin, which modulates the donor/acceptor ratio from the perspective of molecular design for promoting the charge separation. Interestingly, H2O2 can be produced via oxygen reduction and water oxidation pathways, verified by isotopic labeling and in situ characterization techniques. Density functional theory (DFT) calculations elucidate that DHAQ can reduce the energy barrier for H2O2 production. RF‐DHAQ exhibits excellent overall photosynthesis of H2O2 with a solar‐to‐chemical conversion (SCC) efficiency exceeding 1.2 %. This work opens a new avenue to design polymeric photocatalysts at the molecular level for high‐efficiency artificial photosynthesis.

Ultrafast charge transfer in metal-free H2O2 photoproduction by anhydride modified g-C3N4.

As a low-cost, low toxicity and metal-free catalyst with strong light absorption, graphitic carbon nitride (g-C3N4)-based materials have gained wide attention for efficient H2O2 photocatalysis. However, further investigation regarding the charge transfer process and reaction mechanism of H2O2 photoproduction remains to be completed. In this work, bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BTDA) modified g-C3N4 is synthesized through a facile one-step dehydration process, and the H2O2 photoproduction could reach 22.5 μmol within 8 hours. The proposed structure of g-BTDA is confirmed by FTIR, XPS and SEM studies. The transient absorption reveals a 20.88 ps charge transfer process caused by the electron withdrawing ability of the CO group, and a 2-electron oxygen reduction pathway is proposed. Our work represents a new strategy for efficient H2O2 photoproduction using easily acquired materials with future application potential.

Metal-Free Triazine-Based Porous Organic Polymer-Derived N-Doped Porous Carbons as Effective Electrocatalysts for Oxygen Reduction Reaction

In recent years, porous heteroatom-doped carbon materials have been very promising for energy conversion. A newly designed porous organic polymer (POPQ) has been synthesized using two organic monomers, i.e., 2,6-diaminoanthraquinone and cyanuric chloride, under reflux conditions for 72 h in an inert atmosphere. The triazine-containing porous organic polymers undergo pyrolysis, which produces two nitrogen-doped porous carbon materials, N/POPQ600 and N/POPQ800, at 600 and 800 °C temperatures, respectively. Since the resultant N-doped porous materials have a higher surface area than the parent porous organic polymer and the materials have a synergistic effect due to the enriched nitrogen content throughout the matrix, the metal-free N/POPQ600 and N/POPQ800 materials exhibit good electrocatalytic activity toward oxygen reduction reaction (ORR). Among these, the N/POPQ800 material shows excellent ORR activity with a nearly four-electron oxygen reduction pathway where the half-wave potential is estimated to be 0.728 V vs reversible hydrogen electrode (RHE), comparable with the commercially available Pt/C catalyst. Most interestingly, the N/POPQ800 catalyst displays outstanding long-lasting stability. It shows a better methanol tolerance capability than Pt/C, which can be attributed to the high specific surface area and N-doped well-defined crystalline porous structure. Also, the homogeneously distributed active sites throughout the carbon framework are the most precious for the electrochemical oxygen reduction reaction.

Two‐electron Oxygen Reduction Reaction on Nanocarbon Materials for Onsite H2O2 Electrochemical Synthesis: Selectivity Enhancement via Oxygen and Nitrogen co‐Doping

AbstractProduction of hydrogen peroxide (H2O2) through the two‐electron pathway of electrocatalytic oxygen reduction reaction is a green and convenient route, which has attracted considerable attention in the field of energy and catalysis. However, the high cost of noble Pt‐based catalyst and H2O2 selectivity and stability need to be further refined for large‐scale production. Herein, we reported a O, N co‐doped mesoporous nanocarbon catalyst. The introduction of nitrogen atoms increases the electrochemically active surface area, promotes the mass transfer capacity, and reduces the impedance of electron transfer. The absolute contents of each oxygen and nitrogen functional group in the catalyst were calculated according to XPS, which reveals that pyridine nitrogen and pyrrole nitrogen might contribute most to the two‐electron electrocatalytic oxygen reduction reaction. In addition, O, N co‐doping significantly promoted the two‐electron pathway of electrocatalytic oxygen reduction, and the selectivity of hydrogen peroxide reached 93 %. The present work provides a reference for the rational design of functional groups of electrocatalysts for highly selective electrosynthesis of H2O2.

The synergistic effect of the adjacent dual NiNx sites to boost the oxygen reduction reaction activity for proton exchange membrane fuel cell

The investigation on the application of dual metallic sites in electro-catalysis for oxygen reduction reaction (ORR) has been given a lot of consideration. In this study, the reaction mechanisms and corresponding thermodynamic energy evolution in the dual Ni saturated coordinate with N atoms embedded in graphene have been investigated using first-principles density functional theory (DFT) calculations. The results show that the dual NiNx active sites are more stable than that of the NiN4 in view of the comparison of their formation energies. The reduction reaction of O2∗ on NiN4/G obviously promotes the two-electron reduction process due to its loosely bonded with O2. While the oxygen reduction reaction process on these dual NiNx active sites is thermally downhill, all of which promote a single site with four electron ORR process. The dual Ni atoms directly bonded catalysts (Ni2N6-ST2/G and Ni2N6-ST3/G) are in favor of the ORR mechanism through the reaction pathway of O2→O2∗→OOH∗→O∗→OH∗→H2O. However, the dual NiNx catalysts of others (Ni2N6-ST1/G and Ni2N7-ST4/G) prefer to undergo an unconventional 2OH∗ mechanism (O2→O2∗→OOH∗→2HO∗→OH∗→H2O). Moreover, it was found that the first reduction step (O2∗→OOH∗) is the thermodynamic rate-determining step (RDS) for the ORR on these catalysts. Both of Ni2N6-ST2/G and Ni2N6-ST3/G exhibit outstanding ORR activity with the on-set potentials of 0.65 V and 0.68 V, respectively. In particular, the two catalysts are able to adsorb the H2O2 and further reduce it into H2O through a two-electron pathway. Furthermore, it was found that the most stable intermediates in the oxygen reduction pathway depend on their lowest free energy at a given set of pH and applied potential. The free energy for these O-contained intermediates is linearly decreasing with the increment of applied potential. The critical potential for these O-contained intermediates is also linearly decreased with the increment of pH value.

Polythiophene, Polypyrrole and Carbon Nanotube-Based Catalyst for Alkaline Membrane Fuel Cell Cathode

The transformation from hydrocarbons to a cleaner and sustainable H2 economy is considered a favorable approach to meet the increasing energy demand. Among different H2 technologies, fuel cell technology is a viable option for environmentally clean energy conversion (1). Anion-exchange membrane fuel cell (AEMFC) is one of the suitable candidates to meet this demand as they could be more cost-effective than their widely used acidic counterpart, proton-exchange membrane fuel cell. For providing an affordable AEMFC, the non-precious metal catalyst (NPMC) for oxygen reduction reaction (ORR) at the cathode is needed (2).Co-doping (e.g. with N, S and Fe) of the nanocarbon material has been found to be more beneficial for the preparation of highly active ORR electrocatalysts compared to the application of just one dopant atom (e.g. N) (3). Therefore, we have prepared the polypyrrole (PPy), polythiophene (TPh), and multi-walled carbon nanotubes (MWCNT) based composite material that has been further pyrolysed and subjected to the acid treatment procedure to prepare the NPMC for the AEMFC cathode. Versatile optimization procedures were performed to finally obtain the nanocarbon material with the high ORR activity (A-PPy/PTh/MWCNT) (4).According to the scanning electron microscopy images, the composite catalyst exhibited mainly tubular morphology with PPy and PTh wrapped around the MWCNT. The physical characterization of A-PPy/PTh/MWCNT revealed the BET surface area of 379 m2 g−1 and micro-mesoporous tubular structure. The surface composition of the catalyst was found to contain S, C, N, O and Fe, while the latter originates from the FeCl3 polymerization catalyst. The ORR studies with A-PPy/PTh/MWCNT electrocatalyst in 0.1 M KOH showed the ORR half-wave potential of 0.85 V vs. RHE. According to the Koutecky-Levich analysis, the NPMC catalyzed a preferred 4-electron oxygen reduction pathway (4).A single-cell AEMFC experiment with A-PPy/PTh/MWCNT cathode catalyst showed the maximum power density (P max) of 284 mW cm−2, while the P max value of 328 mW cm−2 was recorded for commercial Pt/C in similar AEMFC conditions (Figure 1). The obtained data show that the three-component (PPy, PTh and MWCNT) composite prepared herein is a promising route for the development of AEMFC cathode catalyst (4).References A. Serov, I. V. Zenyuk, C. G. Arges and M. Chatenet, Journal of Power Sources, 375, 149 (2018). T.-W. Chen, P. Kalimuthu, P. Veerakumar, K.-C. Lin, S.-M. Chen, R. Ramachandran, V. Mariyappan and S. Chitra, Molecules, 27, 761 (2022). A. Sarapuu, E. Kibena-Põldsepp, M. Borghei and K. Tammeveski, Journal of Materials Chemistry A, 6, 776 (2018). A. Sokka, M. Mooste, M. Marandi, M. Käärik, J. Kozlova, A. Kikas, V. Kisand, A. Treshchalov, A. Tamm, J. Leis, S. Holdcroft and K. Tammeveski, ChemElectroChem, 9, e202200161 (2022). Figure 1

Highly Active Rutile TiO2 for Photocatalysis under Violet Light Irradiation at 405 nm

Anatase TiO2 is a widely investigated photocatalyst; however, it can only work under ultraviolet (UV) light with wavelengths less than 390 nm (band gap 3.2 eV). Rutile TiO2 can absorb visible light at wavelengths less than 410 nm (band gap 3.0 eV); however, its photocatalytic activity is not high. Herein, we activated rutile TiO2, which was prepared from Evonik TiO2 P 25 through calcination at 800 °C using hydrogen reduction treatment at 700 °C. The photocatalytic activity of the hydrogen-treated TiO2 was as high as P 25 under UV irradiation at 380 nm, which was significantly higher than P 25 under violet light irradiation at 405 nm for the oxidative decomposition of acetic acid in water. Electron spin resonance studies indicate that charge separation is enhanced in reduced TiO2, and their oxygen reduction pathways differ between anatase and rutile. The formation of H2O2 was observed on rutile TiO2; however, it was consumed during photocatalysis to accelerate acetic acid decomposition.

Dyeing sludge carbons activated by alkali-etching for outperformed 4-chlorophenol degradation in electro-Fenton process

Developing outperformed and cost-effective heterogenous electro-Fenton catalysts will be necessary for halogen-containing compounds removal. In this contribution, dyeing based sludge carbons are activated via alkali etching to achieve extremely high removal of 4-Chlorophenols (4-CPs) in electro-Fenton process, fabricating robust and inexpensive catalysts via solid waste resources. The sludge carbons are initially activated by KOH etching and the corresponding structural and morphological features are characterized by pXRD, Raman, FT-IR, BET, and SEM. Further mechanism investigation is conducted via Rotating Ring-disc Electrode (RRDE) and EPR by confirming in-situ oxygen reduction pathway and the type of free radicals. The outcomes of this contribution suggest that the increment of alkali activation level facilitates oxygen reduction towards two-electrons transfer pathway with extremely high H2O2 yield (81.32 %). Meanwhile, the removal rate and mineralization efficiency of 4-CP achieves 100 % and 62.14 %, respectively after reaction time with 150 min. Additionally, the high activated sludge carbons showcase extraordinary stability: H2O2 yield and 4-chlorophenol removal present no obvious decay even enduring five cycles. Thus, alkali-etched dyeing sludge carbons not only provide outperformed, economic, and robust heterogenous electro-Fenton catalysts to remove halogen-containing compounds, but also benefit for solid waste reutilization.

Solution processed 2D SnSe nanosheets catalysts: Temperature dependent oxygen reduction reaction performance in alkaline media

• Single phase SnSe synthesis with non-toxic Se precursor, the monetary synthetic route is employed. • The SnSe catalysts 4-electron pathway for oxygen reduction. • The current density and charge transfer number of SnSe-500 catalysts are 4.50 mA/cm 2 and 3.62. One of the most effective synthesis routes is the simple solution process, which is an inexpensive way to make single phase 2-Dimensional (2D) Tin Selenide (SnSe) nanosheets. X-ray diffraction (XRD), X-ray Photoelectron spectroscopy (XPS), and Field Emission Scanning Electron Microscope (FESEM) were used to confirm the structural and topographical investigations. 2D SnSe served for Oxygen Reduction Reaction (ORR) electrochemical performance in KOH solution. Later, 2D SnSe is annealed at 300, 400 and 500 °C later termed as SnSe-300, SnSe-400 and SnSe-500 respectively to investigate the electrochemical behavior of catalyst. As the temperature increases ORR performance improved. For the first time, 2- and 4-electron transfer pathway mechanism of SnSe catalysts is discussed. As enhancement in the performance of as prepared 2D SnSe catalysts ensured with various parameters like current density, electron charge transfer number and half-wave potential. SnSe-500 has a half-wave potential of −0.75 V, which is lower than other catalysts in this case. In addition, a current density of 4.50 mA/cm 2 was recorded, which is superior to other catalysts in this case.

Competitive Oxygen Reduction Pathways to Superoxide and Peroxide during Sodium‐Oxygen Battery Discharge

AbstractThe sodium‐air battery offers a sustainable, high‐energy alternative to lithium‐ion batteries. Discharge in the cell containing glyme‐based electrolytes can lead to formation of large cubic NaO2 particles via a solution‐precipitation mechanism. While promising, high rates result in sudden death. The exact nature of the discharge product has been a matter of contention, and Na2O2 has never been directly detected in a dry glyme Na−O2 cell. If Na2O2 were to form during discharge in the Na−O2 cell it would have a detrimental impact on cell performance. Here we show that Na2O2 forms during discharge at high overpotential in the glyme‐based Na−O2 batteries. Na2O2 formation is confirmed by spectroscopic and electrochemical analysis and electron microscopy demonstrates that it results in thin insulating films at the electrode surface. The formation of these thin films results in rapid cell death during discharge, introducing an inherent chemical limitation to the Na−O2 battery.

Charge state modulation on boron site by carbon and nitrogen localized bonding microenvironment for two-electron electrocatalytic H2O2 production

Design of electrochemical active boron (B) site at solid materials to understand the relationships between the localized structure, charge state at the B site and electrocatalytic activity plays a crucial role in boosting the green electrochemical synthesis of hydrogen peroxide (H2O2) via two-electron oxygen reduction (2eORR) pathway. Herein, we demonstrate a carbon (C) and nitrogen (N) localized bonding microenvironment to modulate the charge state of B site at the boron-carbon nitride solid (BCNs) to realize the efficient selective electrocatalytic H2O2 production. The localized chemical structure of N-B-N, N-B-C and C-B-C bonds at B site can be regulated through solid-state reaction between boron nitride (BN) and porous carbon (C) at variable temperatures. The optimized BCN-1100 achieves an outstanding H2O2 selectivity of 89% and electron transfer number of 2.2 (at 0.55 V vs. RHE), with the production of 10.55 mmol/L during 2.5 h and the catalytic stability duration for 15000 cycles. Further first-principles calculations identified the dependency of localized bonding microenvironment on the OOH* adsorption energies and relevant charge states at the boron site. The localized structure of B site with BNC2-Gr configuration is predicted to be the highest 2eORR activity.

Manipulating the oxygen reduction reaction pathway on Pt-coordinated motifs

Electrochemical oxygen reduction could proceed via either 4e−-pathway toward maximum chemical-to-electric energy conversion or 2e−-pathway toward onsite H2O2 production. Bulk Pt catalysts are known as the best monometallic materials catalyzing O2-to-H2O conversion, however, controversies on the reduction product selectivity are noted for atomic dispersed Pt catalysts. Here, we prepare a series of carbon supported Pt single atom catalyst with varied neighboring dopants and Pt site densities to investigate the local coordination environment effect on branching oxygen reduction pathway. Manipulation of 2e− or 4e− reduction pathways is demonstrated through modification of the Pt coordination environment from Pt-C to Pt-N-C and Pt-S-C, giving rise to a controlled H2O2 selectivity from 23.3% to 81.4% and a turnover frequency ratio of H2O2/H2O from 0.30 to 2.67 at 0.4 V versus reversible hydrogen electrode. Energetic analysis suggests both 2e− and 4e− pathways share a common intermediate of *OOH, Pt-C motif favors its dissociative reduction while Pt-S and Pt-N motifs prefer its direct protonation into H2O2. By taking the Pt-N-C catalyst as a stereotype, we further demonstrate that the maximum H2O2 selectivity can be manipulated from 70 to 20% with increasing Pt site density, providing hints for regulating the stepwise oxygen reduction in different application scenarios.

Nitrogen‐doped Graphene Loaded with Cobalt Nanoparticles as Efficient Electrocatalysts for Oxygen Reduction Reaction

AbstractDeveloping non‐noble‐metal electrocatalysts used for efficient oxygen reduction reactions (ORR) is greatly significant in the advancement of fuel cells. Herein, a novel electrocatalyst Co/N−G has been synthesized via simply pyrolyzing the mixture of amino phenanthroline (NH2‐Phen), graphene oxide and cobalt salt. The microstructure of the catalyst has been characterized by using HADDF‐STEM, XRD, Raman and XPS, the results indicate that the precursor complex has formed Co nanoparticle during the high‐temperature pyrolysis process. The obtained catalyst of Co/N−G shows relatively good ORR catalytic activity (E1/2=0.80 V) and almost 4e− oxygen reduction pathway in alkaline. In addition, the Co/N−G catalyst shows better methanol‐tolerant ability and longer‐term stability than Pt/C catalyst.