Alkaline Oxygen Reduction Reaction Research Articles

Ultrafine Fe2C in porous N-doped carbon by Polydopamine−Silane Co-deposition for efficient oxygen reduction reaction and zinc-air battery

Pt-based electrocatalysts in oxygen reduction reaction (ORR) have notably frustrated the large-scale application in the relevant energy technologies. Alternatively, polydopamine (PDA)-based materials have displayed great potential due to its adhesive uniqueness, componential and structural tailorability, but the inferior surface area due to the intrinsically dense coating of PDA is an imperfection. Herein, with 3-aminopropyltriethoxysilane (APTS), one mutual deposition method with organic PDA and inorganic APTS was proposed for ultrafine Fe2C embedded into porous N-doped carbon (Fe/PNC), benefitting from synergistic effect of the PDA-derived carbon and APTS-derived SiO2 components produced from the pyrolysis of PDA-APTS hybrid, which can confine metal species more robustly than single PDA, together with the internal pore-making capacity. Accordingly, the as-obtained sample exhibits superior alkaline ORR activity with a half-wave potential of 0.87 V as well as robust long-time durability and strong poison tolerance. Additionally, the constructed zinc–air battery using Fe/PNC as cathode presents higher peak power density and much stronger long-time charge-discharge stability than Pt/C. Such strategy would be generally applicable for the rational design of highly-quality metal-based functional composites.

碱性和酸性条件下均稳定的高效氧电催化剂: 具有多活性中心的多孔纳米片复合材料

The reversible oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are proton-coupled/decoupled four-electron reactions requiring multi-type active centers for catalysis. Herein, porous nanosheets codoped with a carbon-encapsulated FeNi alloy, metal-N species, and N defects (FeNi-N/PCN) were synthesized through a facile and environmentally friendly molten NaCl template method. The obtained FeNi-N/PCN achieved impressive performances as an oxygen electrocatalyst for the ORR and OER in alkaline (E1/2 of 0.88 VRHE, Ej=10 of 1.57 VRHE) and acidic electrolytes (E1/2 of 0.78 VRHE, Ej=10 of 1.76 VRHE). Furthermore, this FeNi-N/PCN catalyst achieved an impressive power density of 224 mW cm−2 and a specific capacity of 691 mA h g−1 in a Zn-air battery, which are 1.34- and 1.20-fold higher than that of the Pt and RuO2 catalysts, respectively. These outstanding performances are due to the layered carbon nanosheet with multi-type active sites, including a carbon-encapsulated FeNi alloy, N defects, and metal-N sites (M-Nx).

Oxygen vacancies endow atomic cobalt-palladium oxide clusters with outstanding oxygen reduction reaction activity

Considering the technological importance of fuel cells, developing highly efficacious, durable, and Platinum (Pt)-free catalysts are crucial. In this work, we propose a novel nanocatalyst (NC) comprising oxygen vacancies (OV) enriched atomic CoPdOx clusters (CoPdOxV) anchored Pd nanoparticles (NP)s on cobalt-oxide support (denoted as CPCo). As-prepared CPCo NC with an additional 3 wt% of Co decoration (denoted as CPCo-3) delivers an exceptionally high mass activity (MA) of 4394 mAmgCo−1 at 0.85 V vs RHE and 426 mAmgCo−1 at 0.90 V vs RHE in alkaline oxygen reduction reaction (ORR) (0.1 M KOH), which surpasses the commercial J.M.-Pt/C (20 wt%) catalyst by 65-times. More importantly, the CPCo-3 NC exhibits outstanding durability in an accelerated durability test (ADT) with a progressively increased MA by 40 % (6,140 mAmgCo−1) as that of the initial condition after 20 k cycles. Through in-depth physical characterization, electrochemical analysis, and in-situ X-ray absorption spectroscopy (XAS), we demonstrated the conceptual framework of potential synergism between the CoPdOxV and neighbouring metallic Pd-sites. In this event, the surface-anchored CoPdOxV species coupling with OV promotes the O2 splitting, while the neighbouring Pd-sites simultaneously trigger the Oads relocation (i.e. OH− desorption) step. In addition, the cobalt oxide support underneath assists the electron injection to surface Pd-sites. This work not only marks a step ahead for designing high-performance transition metal oxide catalysts for fuel cells but also uncovers the material’s aspects of cobalt that shall spark motivation for the other catalytic applications.

Hierarchically porous N-doped carbon nanosheet networks with ultrafine encapsulated Fe3C and Fe-Nx for oxygen reduction reaction in alkaline and acidic media

The electrocatalytic oxygen reduction reaction (ORR) plays an important role in renewable energy storage and conversion systems such as polymer electrolyte membrane fuel cells. However, design of high-performance cathode electrocatalysts is still required to speed up the sluggish kinetics of ORR. In this work, we develop a facile strategy to synthesize hierarchically porous N-doped carbon nanosheet networks with ultrafine encapsulated Fe3C and Fe-Nx for ORR by consideration of thermodynamic (intrinsic and support effect) and kinetic (diffusion effect) parameters. The synthetic process is straightforward and sustainable and the hierarchically porous materials can be synthesized directly starting from raw materials and without any additional template. The as-designed catalyst exhibits excellent ORR catalytic performance in both acidic and basic media with much positive half-wave potential and good stability, which is comparable to commercial Pt/C. The excellent ORR performance may result from highly active Fe-Nx sites and ultrafine encapsulated Fe3C, and the hierarchically porous nanosheet structure for accelerating mass diffusion and increasing exposure of the active site. This work provides a facile strategy for rational design and synthesis of highly active, hierarchically porous non-precious metal electrocatalysts.

Facet Impact of CuMn2O4 Spinel Nanocatalysts on Enhancement of the Oxygen Reduction Reaction in Alkaline Media

Surface-structure engineering represents an attractive strategy to optimize the energy conversion performance of nanocatalysts using their deliberately controlled exposed facets. To further exploit the potential of non-Pt-group metal-based spinel catalysts for the alkaline oxygen reduction reaction (ORR), a cathodic fuel cell reaction, we hereby report a strategy of ORR improvement by controlling the crystallographic facets of ultra-small CuMn2O4 spinel nanocatalysts through a developed colloidal synthesis approach. The synthesis of CuMn2O4 nanocrystals with morphological control relies on the design and selection of the Cu/Mn precursors with striking discrepancies in reaction kinetics. Following carbon loading and an annealing post-treatment of the as-synthesized nanocatalysts, the exclusively {101} facet-exposed CuMn2O4 spinel nano-octahedra exhibit improved electrocatalytic activity toward the ORR in 1 M KOH, when compared to their spherical counterparts, exhibiting a mass activity (MA) of 37.6 A/g at 0.85 V. After 10,000 cycles of the ORR durability test, the nano-octahedra still retain an MA of 24.5 A/g, which is twice that of the CuMn2O4 spinel nanospheres. Structural characterizations after durability testing indicate that the MA decay is likely associated with a decrease in the Mn3+ fraction and the emergence of Cu+ on CuMn2O4 nano-octahedral surfaces. As a paradigm, this synthesis approach could be extended to other Mn-based spinel nanocatalysts with precise shape control, enabling us to understand and establish the relationship between the surface lattice/valence state and electrocatalytic properties.

Using 2D-Phthalocyanine Metal Organic Framework-Based Catalysts for Oxygen Reduction Reaction in Alkaline Media

Developing high performance electrocatalysts to improve the sluggish kinetics of oxygen reduction reaction (ORR) at the cathode is essential in expanding fuel cell applications. Metal organic frameworks (MOFs), given their high surface area, tunable pore size, and diverse metal choices, are promising candidates to catalyze such a reaction. Herein, we report a family of nine 2D-phthalocyanine MOF-based catalysts for alkaline ORR, where M = Co, Ni, Cu. Structurally, these bimetallic materials have two distinct metal sites, where M1 is a phthalocyanine moiety and M2 is a node, existing as M-N4 moiety. X-ray Diffraction (XRD) shows that all as-prepared MOFs have similar structures and inter-layer spacing. X-ray Photoelectron Spectroscopy (XPS) shows slight shifts in binding energy when the same element is placed in a M1 or M2 site, which indicates that the M1 and M2 sites are chemically distinct.Through cyclic voltammetry we evaluate the role of the metal sites for ORR activity and selectivity in 0.1 M KOH. All materials are active for alkaline ORR, while Ni (M1) – Co (M2) has the best overall kinetic activity performance of - 5 mA cm-2 at 0.7 V vs. RHE. Cu – Cu, on the other hand, is the least active combination under the same testing conditions. Within composition series where M2 = Co, the activity trend for M1 is Ni > Cu ~ Co. From a 2-electron selectivity perspective, M1 = Ni > Cu > Co. From these general trends we hypothesize that Co M2 and Ni M1 sites are essential in promoting 4- and 2-electron selectively, respectively. On average we find a nominal ORR performance trend where M2 = Co > Cu > Ni. Interestingly, we note that the M1 identity also plays a non-linear role in activity and selectivity modulation. Though we identify one of the metal sites (M1 or M2) as the dominating site for activity and/or selectivity, the other site also contributes to the total performance. The density functional theory (DFT) calculation supports our experimental results and finds out that Ni as M1 is highly selective for H2O2 formation whereas Co as M2 shows excellent activity for ORR. Our predicted volcano plot shows that MOF with Ni – Co and Cu – Co combinations at Co active site offering lowest ORR overpotential among all other combinations, which is also in good agreement with experimental findings.By evaluating different metal sites performance through electrochemical testing, we can better understand the active species in the MOF catalytic system. By pairing theory with experiment, we gain active site insight to design new specific enhanced active motifs. We believe that these performance trends can assist in better designing efficient MOF-based ORR catalysts that can lead to advances in clean energy technology for sustainable development.

MOF-Derived Bimetallic Pd-Co Alkaline ORR Electrocatalysts.

The development of highly active, durable, and low-cost electrocatalysts for the oxygen reduction reaction (ORR) has been of paramount importance for advancing and commercializing fuel cell technologies. Here, we report on a novel family of Pd-Co binary alloys (PdxCo, x = 1-6) embedded in bimetallic organic framework (BMOF)-derived polyhedral carbon supports. BMOF-derived Pd3Co, annealed at 300-400 °C, exhibited the most promising ORR activity among the family of materials studied, with a half-wave potential (E1/2) of 0.977 V vs RHE and a mass activity of 0.86 mA/μgPd in 1 M KOH, both values being superior to those of commercial Pd/C electrocatalysts. Moreover, it maintained robust durability after 20,000 potential cycles with a minimal degradation in E1/2 of 10 mV. The enhanced performance and stability are ascribed to the uniform elemental distribution of Pd and Co and the Co-containing N-doped carbon (Co-N-C) structures. In anion exchange membrane fuel cell (AEMFC) tests, the peak power density of the cell employing a BMOF-derived Pd3Co cathode reached 1.1 W/cm2 at an ultralow Pd loading of 0.04 mgPd/cm2. Strategies developed herein provide promising insights into the rational design and synthesis of highly active and durable ORR electrocatalysts for alkaline fuel cells.

Improving Electroactivity of N-Doped Graphene Derivatives with Electrical Induction Heating.

Graphene derivatives doped with nitrogen have already been identified as active non-noble metal materials for oxygen reduction reaction (ORR) in PEM and alkaline fuel cells. However, an efficient and scalable method to prepare active, stable, and high-surface-area non-noble metal catalysts remains a challenge. Therefore, an efficient, potentially scalable strategy to improve the specific surface area of N-doped graphene derivatives needs to be developed. Here, we report a novel, rapid, and scalable electrical induction heating method for the preparation of N-doped heat-treated graphene oxide derivatives (N-htGOD) with a high specific surface area. The application of the induction heating method has been shown to shorten the reaction time and improve the energy efficiency of the process. The materials synthesized by induction heating exhibited very high specific surface area and showed improved ORR activity compared to the conventional synthesis method. Moreover, we demonstrated that the temperature program of induction heating could fine-tune the concentration of nitrogen functionalities. In particular, the graphitic-N configuration increases with increasing final temperature, in parallel with the increasing ORR activity. The presented results will contribute to the understanding and development of nonmetal N-htGOD for energy storage and conversion applications.

Strain-Stabilized 1T Phase Ruthenium Sulfoselenide Monolayer Nanotubes for Oxygen Reduction Reaction in Alkaline Medium

Backgrounds: In pursuit of sustainable development, a fuel cell become a key device that can convert chemical energy in hydrogen and oxygen into electrical energy. In this field, the sluggish reaction kinetics of the oxygen reduction reaction (ORR) on cathode side of fuel cell has been a key issue hindering large–scale applications. Although many efforts have been devoted to the development of efficient electrocatalysts for ORR, platinum (Pt)–based catalysts remain the most efficient electrocatalysts owing to their high intrinsic activity. However, their reserve–scarcity and low stability limit their widespread use. Strategy: Therefore, it is highly desirable to develop electrocatalysts with high catalytic activities and stability as well. Among various strategies, metal chalcogenides have led to remarkable electrochemical performances beyond those of pure metals. Also, they have unique characteristics such as multiple crystal phases (e.g., 1H, 2H, 1T, and 1T’); the phase of 1H and 2H correspond to semiconductor, whereas the 1T and distorted 1T’ are metallic phases. Generally, the metallic phase has more active sites and higher electrical conductivity, making it favorable for electrochemical reactions compared to other phases. However, the metastability of 1T phase makes it challenging to directly prepare 1T metal chalcogenides; even, the prepared 1T material easily convert back to stable 2H (or cubic) phase under ambient conditions. Results: Herein, we prepared 1T phase Ru(SxSe1-x)2 (x = 0, 0.2, 0.5, 0.8, and 1) with a shape of monolayer nanotubes (NTs), and they showed remarkable ORR activity and stability as well. The Ru(SxSe1-x)2 nanotube containing abundant anion vacancies was generated from Ru seed particle, which is confirmed by ex-situ TEM, EDS, and EPR. Typically, most nanotubes (e.g., TiO2, and MoS2) are generated via a roll-up mechanism, where the large size of nanosheet is necessary to roll up a sheet into a nanotube; however, the process is not suitable for ultra-small (ø <3 nm) nanotubes. The Ru(SxSe1-x)2 nanotube with a diameter of ø = 1.5 nm takes advantages of larger surface area and more catalytic active sites as well. Moreover, a strain induced by curvature of the nanotube and/or anion vacancies can lead to new bond identities (e.g., bond length and bond angle), resulting in phase stabilization of 1T. In effect, the 1T Ru(S0.8Se0.2)2 nanotubes not only exhibited remarkable ORR activities (E1/2 = 0.864) comparable to commercial Pt/C, but also showed superior durability under alkaline ORR conditions. The electrochemical property was maintained after an accelerated durability test (ADT) for 10K cycles or a long-term air exposure for 6 months. Our results pave the way for the synthesis of stable 1T phase metal chalcogenides, particularly for efficient alkaline oxygen reduction reaction. Figure 1

Controlled introduction of defects into single-walled carbon nanotubes via a fluorination–defluorination strategy using xenon difluoride and their alkaline oxygen reduction reaction catalytic activity

The defect edges in carbon nanomaterials have attracted attention as catalytic active sites for the oxygen reduction reaction (ORR) of the cathode in electrolyte fuel cells, and the defect control in carbon nanomaterials is becoming increasingly important. This study evaluates a fluorination–defluorination strategy for the controlled introduction of defects into single-walled carbon nanotubes (SWCNTs) involving the fluorination of SWCNTs using xenon difluoride (XeF2) and their subsequent defluorination through thermal annealing. We synthesized fluorinated SWCNTs with different fluorine contents using gaseous XeF2 and annealed the fluorinated SWCNTs at 1000 °C for 3 h under nitrogen gas flow. Structural analyses revealed that SWCNTs derived from fluorinated SWCNTs with low fluorine contents primarily had single point defects. In contrast, SWCNTs derived from fluorinated SWCNTs with high fluorine contents had vacancy defects with edges. According to the ORR catalyst evaluation in alkaline aqueous solution, SWCNTs with edge defects, rather than point defects, can improve the efficiency of ORR catalytic activity. The proposed fluorination–defluorination strategy using gaseous XeF2 is expected to enable the controlled introduction of defects in different types of carbon materials.

ZrO2-embedded N-enriched carbon catalyst for alkaline oxygen reduction reaction

The use of transition metal oxides is a promising strategy for accelerating the sluggish kinetics of the oxygen reduction reaction (ORR) in zinc–air batteries. Herein, we propose a facile and cost-effective strategy to synthesize a ZrO2/doped-N carbon (ZrO2/NC) catalyst with high N and Zr contents (8.78 and 4.4 atom%, respectively). The ZrO2/NC catalyst exhibits comparable electrocatalytic activity with a half-wave potential of 0.815 V and better durability in the ORR compared to the commercial Pt/C catalyst. The ORR proceeds via a 4e− transfer pathway under alkaline conditions in the presence of this catalyst. The excellent catalytic performance is attributed to the high densities of the active sites, namely, pyridinic-N, graphitic-N, and Zr. The high Brunner–Emmett–Teller surface area (787.4 m2/g) and an amorphous-crystal morphology of the ZrO2/NC catalyst favored the rapid mass transfer and exposed the active sites of the electrolyte and reactants. The strategy presented herein can be used for the large-scale production of metal–air batteries.

In Situ Mechanistic Insights for the Oxygen Reduction Reaction in Chemically Modulated Ordered Intermetallic Catalyst Promoting Complete Electron Transfer.

The well-known limitation of alkaline fuel cells is the slack kinetics of the cathodic half-cell reaction, the oxygen reduction reaction (ORR). Platinum, being the most active ORR catalyst, is still facing challenges due to its corrosive nature and sluggish kinetics. Many novel approaches for substituting Pt have been reported, which suffer from stability issues even after mighty modifications. Designing an extremely stable, but unexplored ordered intermetallic structure, Pd2Ge, and tuning the electronic environment of the active sites by site-selective Pt substitution to overcome the hurdle of alkaline ORR is the main motive of this paper. The substitution of platinum atoms at a specific Pd position leads to Pt0.2Pd1.8Ge demonstrating a half-wave potential (E1/2) of 0.95 V vs RHE, which outperforms the state-of-the-art catalyst 20% Pt/C. The mass activity (MA) of Pt0.2Pd1.8Ge is 320 mA/mgPt, which is almost 3.2 times better than that of Pt/C. E1/2 and MA remained unaltered even after 50,000 accelerated degradation test (ADT) cycles, which makes it a promising stable catalyst with its activity better than that of the state-of-the-art Pt/C. The undesired 2e- transfer ORR forming hydrogen peroxide (H2O2) is diminished in Pt0.2Pd1.8Ge as visible from the rotating ring-disk electrode (RRDE) experiment, spectroscopically visualized by in situ Fourier transform infrared (FTIR) spectroscopy and supported by computational studies. The effect of Pt substitution on Pd has been properly manifested by X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). The swinging of the oxidation state of atomic sites of Pt0.2Pd1.8Ge during the reaction is probed by in situ XAS, which efficiently enhances 4e- transfer, producing an extremely low percentage of H2O2.

Mesopore-Rich Fe-N-C Catalyst with FeN4 -O-NC Single-Atom Sites Delivers Remarkable Oxygen Reduction Reaction Performance in Alkaline Media.

Fe-N-C catalysts offer excellent performance for the oxygen reduction reaction (ORR) in alkaline media. With a view toward boosting the intrinsic ORR activity of Fe single-atom sites in Fe-N-C catalysts, fine-tuning the local coordination of the Fe sites to optimize the binding energies of ORR intermediates is imperative. Herein, a porous FeN4 -O-NCR electrocatalyst rich in catalytically accessible FeN4 -O sites (wherein the Fe single atoms are coordinated to four in-plane nitrogen atoms and one subsurface axial oxygen atom) supported on N-doped carbon nanorods (NCR) is reported. Fe K-edge X-ray absorption spectroscopy (XAS) verifies the presence of FeN4 -O active sites in FeN4 -O-NCR, while density functional theory calculations reveal that the FeN4 -O coordination offers a lower energy and more selective 4-electron/4-proton ORR pathway compared to traditional FeN4 sites. Electrochemical tests validate the outstanding intrinsic activity of FeN4 -O-NCR for alkaline ORR, outperforming Pt/C and almost all other M-N-C catalysts reported to date. A primary zinc-air battery constructed using FeN4 -O-NCR delivers a peak power density of 214.2mWcm-2 at a current density of 334.1mAcm-2 , highlighting the benefits of optimizing the local coordination of iron single atoms.

Co‐N as a Promoter towards Modulation Surface Chemistry of PtCo Alloy on 2D Thin Layer Hierarchical Porous Nitrogen‐Carbon for the Efficient Oxygen Reduction Reaction

AbstractTo clarify the mechanism of active site conversion and utilization of the Pt−M alloy complexed with nitrogen‐doped carbon (NC) catalysts (Pt−M/NC) for oxygen reduction reaction (ORR), we synthesized the two‐dimensions thin‐layer hierarchical porous nitrogen‐carbon for supporting PtCo nanoparticles (2D PtCo/TNC) and applied to alkaline ORR. The self‐assembled thin layer of carbon nitride (TL‐C3N4) was thermally fused with commercial carbon black (C) to form a 2D‐TNC, which can conduct the optimal mass transfer. Therewith, PtCo nanoparticles were anchored on this carrier through a thermal injection strategy, promoting the formation of Co−N and achieving low Pt loading (2 wt%). As a promoter, the Co−N favours the lattice distortion of PtCo alloys and facilitates the in‐situ adsorption‐diffusion process of oxygen‐containing species. Thus, the obtained catalyst demonstrates an excellent ORR activity in alkaline electrolytes, with a half‐wave potential (E1/2) of 0.885 V and electrochemical active surface area (ECSA) of 111 m2/g, lower Tafel slope of 66 mV/dec.

Surface engineering of ZIF-L renders multidoped leaf-like porous carbon nanosheets for highly efficient oxygen reduction reaction in both alkaline and acidic media

Despite significant progress in developing zeolitic imidazolate frameworks (ZIFs)-derived carbon materials, the crafting of 2D porous carbon catalyst derived from ZIFs with outstanding oxygen reduction reaction (ORR) performance still remains challenge. Herein, we propose on a facile and robust strategy to create leaf-like porous carbon nanosheets (denoted Co,Zn/N,S-LPCS) derived from the surface engineering of ZIF-L with trithiocyanuric acid (TCA) for highly efficient ORR electrocatalyst. By optimizing Co/Zn ratio and TCA amount, the core-shelled ZIF-LCo/Zn@TCA composite is first synthesized. Sequentially, the pyrolysis of ZIF-LCo/Zn@TCA yields Co,Zn/N,S-LPCS with hierarchical porosity and large specific surface area. The resulting Co,Zn/N,S-LPCS electrocatalyst exhibits higher ORR performance than the counterparts of Co,Zn/NC (no TCA) and Co/N,S-LPCS (no Zn). Most importantly, the half-wave potential (E1/2) of Co,Zn/N,S-LPCS is 30 mV larger than Pt/C in alkaline electrolyte, and close to Pt/C in acidic electrolyte. Notably, the long-time stability and anti-methanol property of the Co,Zn/N,S-LPCS largely outperform Pt/C. The Co,Zn/N,S-LPCS-based primary Zn-air battery displays a specific capacity of 808.9 mA h g−1Zn and a power density of 106 mW cm−2. The remarkable performance of the Co,Zn/N,S-LPCS electrocatalyst stems from the synergetic effect of multi-doping, hierarchical porosity, large specific surface area and 2D leaf-like architecture.

Accelerated intermediate conversion through nickel doping into mesoporous Co-N/C nanopolyhedron for efficient ORR

Engineering non-precious metals into nitrogen-doped carbon is employed to improve electrocatalyst activity towards oxygen reduction reaction (ORR). A nickel-doped Co-N/C mesoporous nanopolyhedron is successfully evoluted from a Ni-doped ZIF-67 precursor. The Ni & Co synergistic N/C catalyst exhibits a half-wave potential of 0.895 V (vs. reversible hydrogen electrode (RHE)) with a diffusion-limiting current density of 6.1 mA cm−2 for alkaline ORR at 1600 r min−1, which is competitive to commercial Pt/C in terms of cost, methanol tolerance, and long-term stability. In situ surface-enhanced Raman scattering (SERS) study reveals the formation and fast conversion of superoxide ion (O2-) intermediate on the catalyst surface. Density functional theory (DFT) calculations demonstrate the decrease of energy barrier for potential-determining step (O* protonation) by Co-Ni synergy as well as the reduction of adsorption energy on catalyst surface upon nickel doping. The joint results of in situ SERS study and DFT calculations suggest a favourable ORR process on nickel-doped Co-N/C.

Si–CN for the oxygen reduction reaction in alkaline media, the effect of synthesis temperature

The electrocatalytic properties of silicon-doped carbon nanotubes (Si–CN) for oxygen reduction reaction (ORR) in alkaline media are reported. The Si-CNs were obtained by a modified chemical vapor deposition method. The Si–CN showed good activity towards ORR since most of the process was carried out efficiently by the four-electron transfer pathway, and it exhibited a low Tafel slope of 84.28 mV dec−1 indicating that the material has a good kinetic activation response for ORR. This favorable response was attributed to 0.29 at. % silicon intercalated in the carbon lattice enhancing electrons transport at neighboring silicon sites. In addition, its mechanical robustness and thermodynamic stability influenced by the synthesis temperature contributed favorably to the energetic activation of Si–CN to the adsorption and reduction of oxygen. Therefore, Si–CN could be considered a potential metal-free and low-cost material for the ORR in alkaline fuel cells.

Boron doping induced electronic reconfiguration of Fe-Nx sites in N-doped carbon matrix for efficient oxygen reduction reaction in both alkaline and acidic media

Developing non-precious metal-based catalysts as the substitution of precious catalysts (Pt/C) in oxygen reduction reaction (ORR) is crucial for energy devices. Herein, a template and organic solvent-free method was adopted to synthesize Fe, B, and N doped nanoflake-like carbon materials (Fe/B/N–C) by pyrolysis of monoclinic ZIF-8 coated with iron precursors and boric acid. Benefiting from introducing B into Fe–N–C, the regulated electron cloud density of Fe-Nx sites enhance the charge transfer and promotes the ORR process. The as-synthesized Fe/B/N–C electrocatalyst shows excellent ORR activity of a half-wave potential (0.90 V vs 0.87 V of Pt/C), together with superior long-term stability (95.5% current density retention after 27 h) in alkaline media and is even comparable to the commercial Pt/C catalyst (with a half-wave potential of 0.74 V vs 0.82 V of Pt/C) in an acidic electrolyte. A Zn-air battery assembled with Fe/B/N–C as ORR catalyst delivers a higher open-circuit potential (1.47 V), specific capacity (759.9 mA h g−1Zn at 10 mA cm−2), peak power density (62 mW cm−2), as well as excellent durability (5 mA cm−2 for more than 160 h) compared to those with commercial Pt/C. This work provides an effective strategy to construct B doped Fe–N–C materials as nonprecious ORR catalyst. Theoretical calculations indicate that introduction of B could induce Fe-Nx species electronic configuration and is favorable for activation of OH∗ intermediates to promote ORR process.

Atomically-dispersed Mn-(N-C2)2(O-C2)2 sites on carbon for efficient oxygen reduction reaction

Owing to their ultimate mass-catalytic activity, simple active site configuration and readily tunable electronic structures, transition-metal single atoms (SAs) on carbon support have emerged as a new category of electrocatalysts for oxygen reduction reaction (ORR). Here, we exemplify the use of atomically dispersed Mn with N, O heteroatoms-coordinated Mn sites to enhance alkaline ORR performance. A combined sol-gel/carbonization approach is developed to controllably anchor Mn-SAs onto graphitic carbon nanosheets via a hybridized N, O coordination configuration of Mn-(N-C2)2(O-C2)2 (denoted as Mn-SA@CNSs). The obtained Mn-SA@CNSs exhibits superior alkaline ORR electrocatalytic activity with a half-wave potential of 0.88 V vs. RHE in 0.1 M KOH. The rechargeable Zn–air battery assembled using Mn-SA@CNSs air cathode can readily attain a high power density of 177 mW cm−2 with a narrow voltage gap of 0.76 V at 5 mA cm−2. Density functional theory calculations unveil that altering the coordination environment of Mn-SAs from N-coordinated Mn-(N-C2)4 to N-/O-coordinated Mn-(N-C2)2(O-C2)2 alters the d-band electronic structures and regulates the binding strength of ORR intermediates on Mn-SA sites to dramatically reduce the energy barrier and enhance ORR activity. The exemplified hybridization coordination approach in this work would be applicable to alter the electronic structures of other transition-metal SAs for ORR and other reactions.

Water-regulated and bioinspired one-step pyrolysis of iron-cobalt nanoparticles-capped carbon nanotubes/porous honeycombed nitrogen-doped carbon composite for highly efficient oxygen reduction

To achieve commercial applications of green fuel cells and rechargeable metal-air batteries, rational design and synthesis of low-cost, high-efficiency and ultra-stable transition metal-based electrocatalysts are of significant importance for alkaline oxygen reduction reaction (ORR). In this work, iron-cobalt (FeCo) nanoparticles-capped carbon nano-tubes/-porous nitrogen-doped honeycombed carbon composite (FeCo-CNTs/NHC-800) is synthesized by a water-regulated and bioinspired one-step pyrolysis method at 800 °C, where l-histidine behaves as the C and N sources combined by working as a chelating agent of Fe/Co. The formation mechanism is discussed by adjusting the pyrolysis temperature and water amount. The resultant FeCo-CNTs/NHC-800 exhibits a positive onset potential (Eonset = 1.091 V) and half-wave potential (E1/2 = 0.88 V), showing promising ORR activity, outperforming home-made controls and many lately reported catalysts. The hierarchically porous honeycombed structures have fascinating open porous spaces for fast diffusion of active species, large specific surface area, high conductivity, and stable sites for anchoring FeCo nanoparticles (NPs). Moreover, the N-doped carbon nanotubes coupling with homogeneous FeCo NPs greatly improve the catalytic activity and stability of ORR. This work provides some valuable insights to prepare hierarchical, reliable, and high-efficiency carbon-based ORR catalysts for new energy-correlated devices.