Intrinsic Oxygen Reduction Reaction Activity Research Articles

Efficient Proton-exchange Membrane Fuel Cell Performance of Atomic Fe Sites via p-d Hybridization with Al Dopants.

Orbital hybridization to regulate the electronic structures and surface chemisorption properties of transition metals is of great importance for boosting the oxygen reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs). Herein, we developed a core-shell rambutan-like nanocarbon catalyst (FeAl-RNC) with atomically dispersed Fe-Al atom pairs from metal-organic framework (MOF) material. Experimental and theoretical results demonstrate that the strong p-d orbital hybridization between Al and Fe results in an asymmetric electron distribution with moderate adsorption strength of oxygen intermediates, rendering enhanced intrinsic ORR activity. Additionally, the core-shell rambutan-like structure of FeAl-RNC with abundant micropores and macropores can enhance the density of active sites, stability, and transport pathways in PEMFC. The FeAl-RNC-based PEMFC achieves excellent activity (68.4 mA cm-2 at 0.9 V), high peak power (1.05 W cm-2), and good stability with only 7% current loss after 100 h at 0.7 V under H2-O2 condition.

Intrinsic Defect‐Rich Carbon‐Supported Iron Phthalocyanine as Beyond‐Pt Oxygen Reduction Catalysts for Zinc–Air Batteries

Molecular iron phthalocyanine (FePc) bearing a single‐atom Fe–N4 moiety is a high‐profile non‐platinum catalyst toward the oxygen reduction reaction (ORR), but its unsatisfactory activity and inadequate stability hinder the practical application. Herein, a novel strategy by hybridizing FePc with fullerene‐derived intrinsic defect‐rich carbon to improve its ORR activity and stability is reported. Via alkali‐assisted thermal pyrolysis, C60 molecules are disintegrated into tiny fragments that then restructure into pentagon‐ and edge‐rich carbon (FC). Utilizing FC as a support of FePc leads to a significantly improved ORR activity compared to other supports including reduced graphene oxide and carbon nanotube that hold distinct structural features. The optimized FePc/FC with a Fe loading of 3 wt% presents a half‐wave potential of 0.917 V, far beyond that of Pt/C (0.846 V), via selective four‐electron ORR pathway and excellent stability under accelerated stress test. The practical applicability of FePc/FC is also demonstrated as a high‐performing cathode catalyst of aqueous zinc–air batteries. It is, for the first time, explicitly disclosed that strong interactions between FePc molecules and intrinsic carbon defects (e.g., pentagons and edges) not only strengthen the anchoring effect of supported FePc but also enhance the intrinsic ORR activity of single‐atom Fe–N4 sites.

Boosting Oxygen Reduction at Pt(111)|Proton Exchange Ionomer Interfaces through Tuning the Microenvironment Water Activity.

A proton exchange ionomer is one of the most important components in membrane electrode assemblies (MEAs) of polymer electrolyte membrane fuel cells (PEMFCs). It acts as both a proton conductor and a binder for nanocatalysts and carbon supports. The structure and the wetting conditions of the MEAs have a great impact on the microenvironment at the three-phase interphases in the MEAs, which can significantly influence the electrode kinetics such as the oxygen reduction reaction (ORR) at the cathode. Herein, by using the Pt(111)|X ionomer interface as a model system (X = Nafion, Aciplex, D72), we find that higher drying temperature lowers the onset potential for sulfonate adsorption and reduces apparent ORR current, while the current wave for OHad formation drops and shifts positively. Surprisingly, the intrinsic ORR activity is higher after properly correcting the blocking effect of Pt active sites by sulfonate adsorption and the poly(tetrafluoroethylene) (PTFE) skeleton. These results are well explained by the reduced water activity at the interfaces induced by the ionomer/PTFE, according to the mixed potential effect. Implications for how to prepare MEAs with improved ORR activity are provided.

Enhanced Oxygen Reduction Activity of Platinum Micropore

AbstractThe high overpotential of oxygen reduction reaction (ORR) prevents the wide commercialization of fuel cells and metal‐air batteries. To accelerate the reaction rate, porous electrocatalysts draw attention to obtain a higher specific surface area. However, the effect of micropores on ORR kinetics has not been understood because of the non‐uniform pore sizes, length, and tortuosity of practical electrocatalysts. In this study, we evaluate ORR activity in a micropore using platinum model electrodes with arrays of cylindrical pores with a uniform pore diameter of 1.8 nm. The model electrodes having pore lengths of 45, 100, and 380 nm are fabricated, and their ORR performances are examined by electrochemical measurements and numerical simulations in 0.1 mol dm−3 KOH aqueous solution. The intrinsic ORR activity of the micropore is successfully obtained with the electrode having a pore length of 45 nm, where oxygen transport resistance in the micropore has little effect on ORR. It is found that the intrinsic ORR activity of the micropore is higher than that of a planar Pt surface. X‐ray photoelectron spectroscopy and CO stripping voltammetry indicate a downshift of the d‐band center, which can be the origin of the high intrinsic ORR activity.

Lamellar structured Ce-S-NC heterogeneous electrocatalyst as the highly durable cathode for passive direct liquid fuel cells

Passive direct liquid fuel cells are promising power generation devices with the advantages of convenient transportation, storage for liquid fuel and high power density. This paper focuses on improving the intrinsic electrocatalytic performance and durability of nitrogen-doped carbon (N-C) cathodic electrocatalysts. Then, lamellar structured Ce-S-NC heterogeneous electrocatalyst was synthesized and used as a highly durable cathode for two types of passive direct liquid fuel cells, i.e., direct methanol fuel cells (DMFCs) and direct borohydride fuel cells (DBFCs). In this work, the effect of the S/C molar ratio on the microstructure and electrocatalytic performance of the Ce-S-NC electrocatalysts was investigated, and excellent intrinsic oxygen reduction reaction activity was obtained at an S/C molar ratio of 0.15. The peak power densities of Ce-S-NC based DMFCs reached up to 12.5, 23.6 and 41.9 mW·cm−2 at 20, 40 and 60 °C, respectively. The peak power densities of 59.3, 84.6 and 132.6 mW·cm−2 were obtained in Ce-S-NC based DBFCs at 20, 40 and 60 °C, respectively. In the durability tests, both the Ce-S-NC based air-breathing DBFCs and passive DBFCs showed the stable operation than that of N-C and Ce-NC. When the current density increased from 50 to 150 mA·cm−2, the Ce-S-NC based DBFC can be operated for 882 h, and only 34 and 91 h for that of N-C and Ce-NC. The electrochemical impedance spectroscopy tests proved Ce-S-NC based DBFC showed the faster mass transfer efficiency. The outstanding durability of Ce-S-NC is mainly attributed to the improved defect density and mass transfer.

Local Single Co Sites at the Second Shell of Fe‐N4 Active Sites to Boost Oxygen Reduction Reaction

AbstractFe single‐atom catalysts (SACs) are a promising catalyst for oxygen reduction reaction (ORR) in both Zn–air batteries (ZABs) but have a certain distance to compete with Pt‐based catalysts. Rational modulation of the coordination environment in the second coordination shell of SACs offers an opportunity to improve the intrinsic ORR activity, yet a challenge. Here, a novel strategy is reported to construct a dual‐metal catalyst by introducing a single Co atom in the second coordination shell of the Fe center. The dual‐metal N3─Fe─N─Co site with a certain Fe─Co distance of 0.312 nm is constructed. It allows for manipulation of the positive shift of the Fe center charge state, driving the d‐band center up‐shift by 0.63 eV, thereby offering the optimal adsorption of intermediates. Benefiting from this structure, the as‐prepared Fe(Co2nd)‐NC enables an excellent ORR activity with a half‐wave potential of 0.948 V in 0.1 M KOH. As a cathode in ZABs, it delivered an outstanding peak power density of 218 mW cm−2 and a specific capacity of 915 mAh gZn−1 at 5 mA cm−2, respectively, with superior long‐term durability over 680 h. The second shell layer alignment regulation strategy shows great potential for energy storage applications.

Local Geometric Distortion to Stimulate Oxygen Reduction Activity of Atomically Dispersed Zn‐Nx Sites for Zn–Air Batteries

AbstractLocal geometric strain engineering is useful for modulating the performance of nitrogen‐coordinated transition metal‐carbon catalysts. However, realizing the nano‐level strain is technically challenging. Additionally, the structure‐property relationship between strain degree and performance remains poorly understood. Herein, it is conceptually predict that geometric bending induces more electron transfer from Zn to the coordinated N in Zn─N─C, leading to a positive shift of the d‐band center of the Zn atom, which promotes the adsorption reduction process of the O2 molecule and thus increases the intrinsic oxygen reduction reaction (ORR) activity. Moreover, a low‐temperature non‐saturated coordination strategy is proposed to prepare spherical porous carbon catalysts with surface‐enriched geometrically bent (20‐50°) Zn─N─C sites. Benefiting from the highly active Zn─N─C sites, large specific surface area and abundant pore structure, the optimized catalyst (S─Zn─N─C‐950) exhibited excellent intrinsic alkaline ORR activity (half‐wave potential E1/2 = 0.89 V) and high zinc‐air battery performance (peak power density of 229.2 mW cm−2), exceeding that of commercial Pt/C catalysts. Density functional theory (DFT) calculations show that when the geometrical bending angle is 30–45°, Zn centers with suitable charge transfer to the surrounding N can produce a moderate adsorption strength to the oxygen intermediate state, resulting in optimal ORR activity.

Electronic structure and spin state regulation of vanadium nitride via a sulfur doping strategy toward flexible zinc-air batteries

Owing to the distinctive structural characteristics, vanadium nitride (VN) is highly regarded as a catalyst for oxygen reduction reaction (ORR) in zinc-air batteries (ZABs). However, VN exhibits limited intrinsic ORR activity due to the weak adsorption ability to O-containing species. Here, the S-doped VN anchored on N, S-doped multi-dimensional carbon (S-VN/Co/NS-MC) was constructed using the solvothermal and in-situ doping methods. Incorporating sulfur atoms into VN species alters the electron spin state of vanadium in the S-VN/Co/NS-MC for regulating the adsorption energy of vanadium sites to oxygen molecules. The introduced sulfur atoms polarize the V 3dz2 electrons, shifting spin-down electrons closer to the Fermi level in the S-VN/Co/NS-MC. Consequently, the introduction of sulfur atoms into VN species enhances the adsorption energy of vanadium sites for oxygen molecules. The *OOH dissociation transitions from being unspontaneous on the VN surface to a spontaneous state on the S-doped VN surface. Then, the ORR barrier on the S-VN/Co/NS-MC surface is reduced. The S-VN/Co/NS-MC demonstrates a higher half-wave potential and limiting current density compared to the VN/Co/N-MC. The S-VN/Co/NS-MC-based liquid ZABs display a power density of 195.7 mW cm−2, a specific capacity of 815.7 mA h g−1, and a cycling stability exceeding 250 h. The S-VN/Co/NS-MC-based flexible ZABs are successfully employed to charge both a smart watch and a mobile phone. This approach holds promise for advancing the commercial utilization of VN-based catalysts in ZABs.

Hierarchically Porous, N, P‐Co‐Doped Carbon Structures with Embedded Pt Particles as Effective Electrocatalysts for the Oxygen Reduction Reaction

AbstractThe proton exchange membrane fuel cell now ranks as one of the best alternatives to fossil fuels. Two optimization techniques that have stirred up a lot of controversy include reducing the amount of precious metal Pt catalysts and boosting the kinetics of the cathodic oxygen reduction reaction (ORR). Herein, “template‐etching” method is used to create N−P co‐doped low‐Pt catalysts with hierarchical porous carbon nanospheres (Pt/NPC). The P atoms′ valence electrons, which serve to attach the Pt to the equally disseminated N atoms, further disperse the Pt inside the carbon nanospheres. As a result, the Pt is able to modify the electronic structure and alter the d‐band center, which raises the intrinsic ORR activity. The hierarchical porous structure of the carbon nanospheres, created by the phytic acid activation of the pores, can expose more active sites and efficiently enhance the transfer of oxygen and electrons, increasing the catalytic efficiency. The catalysts synthesized using this method had great stability, 9.87 wt % Pt concentration, and half‐wave potential of 0.832 V for acidic intermediates and 0.855 V for alkaline intermediates. Because of its entire pH ORR catalytic capabilities, this catalyst exhibits tremendous application potential for replacing conventional Pt/C catalysts.

Experimental Determination of the Intrinsic Oxygen Reduction Kinetics on Polycrystalline Platinum and the Role of the Adsorbed Oxygen

The Oxygen Reduction Reaction (ORR) is a fundamental reaction in electrochemical devices such as fuel cells and sensors. The ORR only occurs on few catalytically active surfaces, mainly platinum and its alloys and understanding its mechanism is critically important to develop new catalysts and applications. In contrast to the related Hydrogen evolution/oxidation reaction, the intrinsic ORR activity is rarely determined due to simultaneous formation of Pt-O species. Therefore, the ORR activity is measured at a benchmark potential making its susceptible to the measurement characteristics, such as "activation", scan rate, and scan direction.We found that by using a specially designed experimental procedure one can separate the effect of Pt-O on the ORR kinetics without extensive modelling or fitting. As a results an exemplary Pt-O corrected Tafel plot on polycrystalline platinum is displayed in figure 1.In this presentation we will discuss the underlying method, its application on a range of different ORR environments, the nature of the Pt-O species, and the implications for ORR mechanism itself. Figure 1

Dual-MOFs-Derived Fe and Mn Species Anchored on Bamboo-like Carbon Nanotubes for Efficient Oxygen Reduction as Electrocatalysts

The development of efficient non-precious metal electrocatalysts for oxygen reduction reaction (ORR) to replace Pt-based methods is crucial for the applications of fuel cells and metal–air batteries. In this study, a bimetallic M-N-C catalyst with highly dispersed dual-atom Fe/Mn-Nx sites immobilized on N-doped bamboo-like carbon nanotubes is prepared by the ball-milling and calcination of dual-MOFs as precursors. The rich N-doping and abundant M–Nx species contribute to the excellent intrinsic ORR activity of the catalyst, and the unique bamboo-like nanotubes morphology is beneficial for facilitating electron transfer and mass transport while simultaneously enabling the exposure of active sites. As expected, the optimized Z-Fe1Mn1-NC catalyst exhibits efficient ORR activity with a half-wave potential (E1/2) of 0.80 V in acid and 0.82 V in alkaline, and a higher electrochemical stability with the current density maintained at 91% (in 0.1 M KOH) and 86% (0.1 M HClO4) of its initial current density after 15 h of a chronoamperometric test at a high potential of 0.7 V. When further applied to Zn–air batteries, the catalyst also delivers a high open-circuit voltage, large power density, and outstanding rate performance. This work provides a novel means of designing dual metal M–Nx site-based M-N-C catalysts for ORR sustainable energy applications.

Axial Chlorine Induced Electron Delocalization in Atomically Dispersed FeN4 Electrocatalyst for Oxygen Reduction Reaction with Improved Hydrogen Peroxide Tolerance.

Atomically dispersed iron sites on nitrogen-doped carbon (Fe-NC) are the most active Pt-group-metal-free catalysts for oxygen reduction reaction (ORR). However, due to oxidative corrosion and the Fenton reaction, Fe-NC catalysts are insufficiently active and stable. Herein, w e demonstrated that the axial Cl-modified Fe-NC (Cl-Fe-NC) electrocatalyst is active and stable for the ORR in acidic conditions with high H2 O2 tolerance. The Cl-Fe-NC exhibits excellent ORR activity, with a high half-wave potential (E1/2 ) of 0.82V versus a reversible hydrogen electrode (RHE), comparable to Pt/C (E1/2 = 0.85V versus RHE) and better than Fe-NC (E1/2 = 0.79V versus RHE). X-ray absorption spectroscopy analysis confirms that chlorine is axially integrated into the FeN4. More interestingly, compared to Fe-NC, the Fenton reaction is markedly suppressed in Cl-Fe-NC. In situ electrochemical impedance spectroscopy reveals that Cl-Fe-NC provides efficient electron transfer and faster reaction kinetics than Fe-NC. Density functional theory calculations reveal that incorporating Cl into FeN4 can drive the electron density delocalization of the FeN4 site, leading to a moderate adsorption free energy of OH* (∆GOH* ), d-band center, and a high onset potential, and promotes the direct four-electron-transfer ORR with weak H2 O2 binding ability compared to Cl-free FeN4, indicating superior intrinsic ORR activity.

Boosting oxygen reduction of single atomic iron sites by charge redistribution

Atomically dispersed porphyrin-type iron catalysts have demonstrated great potential to substitute platinum group metals for the oxygen reduction reaction (ORR), yet being subjected to limited active sites and sluggish O2 activation kinetics over the symmetrical planar four-coordinated FeN4 structure. Herein, the charge regulation on FeN4 site is achieved via non-bonding interaction of adjacent nitrogen over high-loading single atomic Fe catalysts to boost their intrinsic ORR activity. The single atomic Fe catalysts exhibit excellent alkaline ORR activity with a half-wave potential of 0.914 V versus reversible hydrogen electrode and tenfold higher kinetic current density than that of Pt/C catalysts. Both experimental and theoretical studies confirm that the secondary nitrogen compensation can anchor greater Fe atoms up to 5.44 wt% with sufficient O2 activation sites and simultaneously break the symmetrical charge distribution of FeN4 site to optimize the adsorption/desorption of oxygen intermediate for accelerating ORR kinetics. Moreover, the single atomic Fe catalysts demonstrate great practical application in liquid and solid zinc-air batteries with long-term cycling stability for 600 and 150 h, respectively, as well as superior flexibility in solid batteries.

Molecular Design of Porous Organic Polymer-Derived Carbonaceous Electrocatalysts for Pinpointing Active Sites in Oxygen Reduction Reaction.

The widespread application of fuel cells is hampered by the sluggish kinetics of the oxygen reduction reaction (ORR), which traditionally necessitates the use of high-cost platinum group metal catalysts. The indispensability of these metal catalysts stems from their ability to overcome kinetic barriers, but their high cost and scarcity necessitate alternative strategies. In this context, porous organic polymers (POPs), which are built up from the molecular level, are emerging as promising precursors to produce carbonaceous catalysts owning to their cost-effectiveness, high electrical conductivity, abundant active sites and extensive surface area accessibility. To enhance the intrinsic ORR activity and optimize the performance of these electrocatalysts, recognizing, designing, and increasing the density of active sites are identified as three crucial steps. These steps, which form the core of our review, serve to elucidate the link between the material structure design and ORR performance evaluation, thereby providing valuable insights for ongoing research in the field. Leveraging the precision of polymer skeletons based on molecular units, POP-derived carbonaceous catalysts provide an excellent platform for in-depth exploration of the role and working mechanism for the specific active site during the ORR process. In this review, the recent advances pertaining to the synthesis techniques and electrochemical functions of various types of active sites, pinpointed from POPs, are systematically summarized, including heteroatoms, surficial substituents and edge/defects. Notably, the structure-property relationship, between these active sites and ORR performance, are discussed and emphasized, which creates guidelines to shed light on the design of high-performance ORR electrocatalysts.

Sustainable chitosan hydrogen derived platinum/N-doped carbon aerogel for efficient oxygen reduction and hydrogen evolution reactions

Platinum (Pt)-based catalysts play a key role in electrocatalytic oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER), and reducing Pt content and maintaining high electrocatalytic activity and durability are essential to its promoted application. In this work, low content of Pt (10.5 wt%) loaded defective N-doped carbon aerogel (Pt/D-NCA) was synthesized using natural protonated chitosan complexed chloride platinate ion (chitosan-PtCl62-) as precursor. Pt/D-NCA exhibits better ORR (half-wave potential of 0.89 V vs RHE) and HER (57 mV vs RHE overpotential at 100 mA/cm2) performance than commercial 20 wt% Pt/C, and can be used as efficient cathode for Zn-air battery and water spitting. The defective N-doped carbon support can effectively reduce energy barrier of Pt sites through interface effect to inducing high intrinsic ORR and HER catalytic activity. This work provides a sustainable route to synthesize low Pt consumption electrocatalyst through a sustainable and environmentally friendly pathway.

Core-shell Fe/Fe3C heterostructure@carbon layers anchored on N-doped porous carbon for boosting oxygen reduction reaction

It is still challenging but crucial to plumb non-noble metal-based oxygen reduction reaction (ORR) catalysts with comparable or better performance than benchmark Pt/C catalysts. Herein, we proffer an unconventional core-shell-structured iron-based heterostructure electrocatalyst constructed by a Fe/Fe3C heterostructure nanoparticle core wrapped by a graphitic shell embedded on a conductive N-doped mesoporous graphite skeleton (Fe/Fe3C@NC) via facile pyrolysis of MIL-53(Fe) (MIL = Matérial Institute Lavoisier) and melamine. The Fe/Fe3C@NC catalyst delivers more positive half-wave potential (E1/2) of 0.86 V and a low Tafel slope of 56 mV dec−1 for ORR, outpacing the benchmark Pt/C catalyst, and it also shows remarkable long-term durability and excellent methanol tolerance. The marvelous ORR behaviors can be attributed to its distinctive morphologic structure: the heterostructure can enhance intrinsic ORR activity; the core-shell nanostructure can enhance ORR activities through unique electron transfer between iron species and their conductive carbon shells, which can also act as protective barriers to ensure long-term stability; the high-content pyridinic-N doping can be used as additional active centers; and the abundant mesopores facilitate the diffusion of electrolyte, which can further upgrade ORR activities. This study proffers a strategy to design and fabricate non-noble catalysts with well-designed nanoarchitecture and high catalytic activity.

FeP nanoparticles highly dispersed on N,P-doped petaloid carbon nanosheet: Interface engineering and boosted intrinsic ORR activity

Modulating the surface structure and intrinsic activity of iron phosphides (FeP) is of great importance to improve their oxygen reduction reaction (ORR) activity. In this work, aminotrimethylene phosphonic acid (ATMP) and melamine (MA) were introduced as acid and base ligand respectively to fabricate hydrogen-bonded organic frameworks (HOF)-like precursor (ATMP-MA). After high-temperature treatment, the obtained FeP supported on 3D nitrogen (N) and phosphorus (P)-doped petaloid carbon nanosheet (FeP/NPC) displayed the desired improved ORR performance. An improved onset potential (0.922 V vs. RHE) and diffusion limiting current (5.480 mA cm−2) could be realized, superior to those of Pt/C (0.911 V, 5.380 mA cm−2). Moreover, ultraviolet photoemission spectroscopycharacterization consistently revealed that the proper P and N doping could effectively regulate the electronic structure, promote electron transport, and lower the d-band center position of Fe. This work may streamline the strategy of transition metal phosphides with enhanced reactive activity for ORR.

Scrutinizing Intrinsic Oxygen Reduction Reaction Activity of a Fe−N−C Catalyst via Scanning Electrochemical Cell Microscopy

AbstractCarbon‐based nanomaterials are renowned for their exceptional properties, making them propitious candidates for oxygen reduction reaction (ORR) electrocatalysis. However, their intrinsic activity is often challenging to investigate unambiguously with conventional methodologies due to the inherent complexities of such systems and the material itself. Zooming into the material and gaining electrochemical information with high resolution is a way to get rid of many experimental factors that influence the catalytic activity in macro‐scale measurements. Herein, we employ nano‐scale scanning electrochemical cell microscopy (SECCM) to investigate individual catalyst agglomerates with and without Nafion content. The intrinsic ORR activity of the catalyst was unravelled by using a unique approach of normalizing the data of all measured points by their distinctive electrochemical surface area (ECSA). When coupling with scanning electron microscopy (SEM), the structure and morphology of the catalytically active agglomerates were visualized.

Revealing the bifunction mechanism of LaCoO3 as electrocatalyst: Oxygen vacancies effect and synergistic reaction process

Bifunctional perovskite catalysts with both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) activity have potential applications in metal-air batteries and regenerative fuel cells etc. Elucidating the microscopic mechanism of the perovskite bifunctionality is of great scientific significance. Here, we studied the effects of oxygen vacancy (VO) on OER and ORR reaction processes on the LaCoO3 surface based on the first-principles calculations. The results reveal the linear relationship between OER overpotential and the d‐band center of Co active site at the surface, which relates to the covalent component of the bond between the active sites and the oxygen atom in the intermediates. The O2 desorption is linearly related to the distance between the d‐band center of Co active site and the p-band center of the oxygen atom in the O2 intermediate. When VO is located at the subsurface, d‐band center of the Co active site and the covalent component of the bond between Co and O increase, and the OER activity of the adsorbate evolution mechanism is significantly enhanced with an overpotential of about 0.50 V. However, the desorption of OH intermediate is too difficult without VO or the adsorption for O2 with VO is too weak for the reverse ORR reaction to occur. Therefore, the synergistic reaction mechanism involving Co and the adjacent surface VO is proposed for the ORR process with an overpotential of 0.31 V. The reverse OER reaction pathway with the lattice oxygen participation mechanism has an overpotential of 0.25 V, whereas the kinetics are limited because of the difficult desorption of O2. Therefore, OER and ORR take a non-reverse reaction pathway, and the results can well explain the microscopic mechanism by which LaCoO3 has excellent intrinsic ORR activity and VO enhances the LaCoO3 bifunctionality, and provide theoretical basis for understanding the microscopic mechanism of bifunctional materials.

Low-Loading Sub-3 nm PtCo Nanoparticles Supported on Co-N-C with Dual Effect for Oxygen Reduction Reaction in Proton Exchange Membrane Fuel Cells.

Developing low-loading Pt-based catalysts possessing glorious catalytic performance can accelerate oxygen reduction reaction (ORR) and hence significantly advance the commercialization of proton exchange membrane fuel cells. In this report, we propose a hybrid catalyst that consists of low-loading sub-3 nm PtCo intermetallic nanoparticles carried on Co-N-C (PtCo/Co-N-C) via the microwave-assisted polyol procedure and subsequent heat treatment. Atomically dispersed Co atoms embedded in the Co-N-C carriers diffuse into the lattice of Pt, thus forming ultrasmall PtCo intermetallic nanoparticles. Owing to the dual effect of the enhanced metal-support interaction and alloy effect, as-fabricated PtCo/Co-N-C catalysts deliver an extraordinary performance, achieving a half-wave potential of 0.921 V, a mass activity of 0.700 A mgPt-1@0.9 V, and brilliant durability in the acidic medium. The fuel cell employing PtCo/Co-N-C as the cathode catalyst with an ultralow Pt loading of 0.05 mg cm-2 exhibits an impressive peak power density of 0.700 W cm-2, higher than that of commercial Pt/C under the same condition. Furthermore, the enhanced intrinsic ORR activity and stability are imputed to the downshifted d-band center and the strengthened metal-support interaction, as revealed by density functional theory calculations. This report affords a facile tactic to fabricate Pt-based alloy composite catalysts, which is also applicable to other alloy catalysts.