Kinetic Current Density Research Articles

Atomically dispersed antimony on N-doped carbon for highly efficient oxygen reduction reaction

• A series of atomically dispersed Sb-N-C catalysts are prepared via a facile adsorption-pyrolysis strategy. • Atomically dispersed Sb are stabilized by N atoms in terms of Sb-N 5 configuration. • Self-evaporation of Sb during synthesis could create abundant micro/meso-pores. • Sb-N 5 sites enable Sb-N-C catalysts outstanding oxygen reduction reaction activity. • Abundant micro/ meso -pores enable Sb-N-C catalysts enhanced mass transport. Main-group metal based single-atom catalysts are attracting increasing research attention in electrochemical catalysis owing to their partially occupied valence p-orbitals. Herein, we report a series of atomically dispersed Sb-N-C catalysts (including Sb-N-C NP , Sb-N-C NL and Sb-N-C NT ) synthesized by a facile adsorption-pyrolysis strategy for oxygen reduction reaction (ORR). Apart from generating atomically dispersed Sb-N 5 sites as active centers, self-evaporation of Sb during pyrolysis process has created abundant micro/meso-pores in Sb-N-C catalysts. Benefitting from these advantageous features, Sb-N-C presents outstanding ORR activity (Sb-N 5 sites) and efficient mass transport (micro/meso-pores). By rotating disk electrode (RDE) test in alkaline media, Sb-N-C exhibits a most positive half-wave potential of 0.90 V vs. RHE and a highest kinetic current density up to 43.8 mA cm −2 at 0.85 V vs. RHE. As gas diffusion electrode (GDE), Sb-N-C NP0.2 demonstrates fast O 2 diffusion and transport that enables smaller mass transport overpotential at high current densities up to 800 mA cm −2 . Finally, Zn-air battery that uses the Sb-N-C NP catalyst as air electrode achieves a maximum power density of 180 mW cm −2 and more than 1000 hs of continuous operation. This work further demonstrates the excellent performance of main-group Sb single-atom catalyst toward ORR and applications in practical energy conversion devices.

Tailoring the stability of Fe-N-C via pyridinic nitrogen for acid oxygen reduction reaction

Developing advanced non-precious metal catalysts towards acidic oxygen reduction reaction (ORR) is critical for electrochemical energy conversion devices. Fe-N-C catalysts are demonstrated to be the most promising alternatives to platinum-based catalysts for ORR. Herein, Fe single atoms (SAs) coordinated by pyridinic nitrogen catalysts (denoted as Fe-pyridinic N-C) are synthesized through pyrolysis of ZIF-8 encapsulating ferrocene. Owing to the synergistic effects between Fe SAs and pyridinic N, Fe-pyridinic N-C exhibits remarkable ORR activity and outstanding stability in acid media, evidenced by golden kinetic current density of 9.71 mA cm−2 at 0.8 V, along with only 21 mV decrease in half-wave potential after 20,000 cycles. Theoretical calculations demonstrate that pyridine-type N possesses stronger binding energy with Fe SAs compared with pyrrole-type N, in other words, high pyridinic N content will help stabilize the catalyst. This study will be of great significance for the development of non-noble metal catalysts towards ORR with enhanced stability in acidic media.

Mesoporous carbon promoting the efficiency and stability of single atomic electrocatalysts for oxygen reduction reaction

Single-atom Fe–N–C electrocatalysts are attracting more attentions as one most promising transition metal based single-atom catalyst towards the oxygen reduction reaction (ORR). However, the inaccessibility of internal Fe-N x active sites and insufficient stability hinder their large-scale application. Herein, a facile benzoate-assisted self-template strategy which could dramatically enhance the oxygen reduction reaction catalytic performance and stability of ZIF-8 derived atomically dispersed Fe–N–C catalysts (Fe-SA/Meso-C) is developed. The sodium benzoate used in the present work is effective for promoting the formation of Fe–N–C catalysts with denser accessible active sites as well as mesopores with diameters ranging from 2.5 to 5 nm. These structural advantages make the synthesized Fe-SA/Meso-C catalysts afford excellent electrocatalytic performance for the ORR in 0.1 M KOH with a positive half-wave potential (E 1/2 ) of 0.926 V vs. RHE and exceptionally high kinetic current density (J k ) of 92.5 mA cm −2 at 0.85 V. Beyond that, Fe-SA/Meso-C shows outstanding long-term stability with over 90% activity retain after 90 h chronoamperometry i-t test as well as superior tolerance to methanol crossover. More importantly, the assembled zinc-air battery with Fe-SA/Meso-C as the cathode material achieves a high peak power density of 166.2 mW cm −2 and a high specific capacity of 776.1 mA h g −1 . Our results reveal that small mesopores in atomically dispersed Fe–N–C electrocatalysts could facilitate mass transport, increase the accessibility of active sites and optimize the interface between electrolyte and carbon matrix, thus optimizing their electrocatalytic performance for the ORR. This work paves a way to the design and synthesis of stable single-atom electrocatalysts with optimized structure, electrochemical performance and promising applications. Ultrastable and highly catalytically active single-atom iron electrocatalysts (Fe-SA/Meso-C) with exclusively Fe–N 4 sites dispersed on ZIF-8 derived mesoporous N-doped carbon frameworks have been synthesized by a novel benzoate-assisted self-template strategy without acid leaching assistance.

Effect of Lithium Sulfate on the Catalytic Activity of Pt for Hydrogen Oxidation Reaction

The effect of Lithium sulfate on the hydrogen oxidation reaction (HOR) in 0.1 M H2SO4 electrolyte was investigated on flat Pt electrode. The Li+ concentration solutions of 0, 10, 25 and 32 g l−1 were studied using cyclic voltammetry and rotating disk electrode (RDE) techniques. The obtained results demonstrate a good repeatability and confidence in analysis method, to understand the influence of lithium sulfate on HOR for an electrocatalysis system. The electrochemical surface area, limiting current and kinetic parameters were measured and analysed using Koutecky-Levich and Tafel representations to investigates the different types of lithium sulfate interactions on the catalytic properties of Pt. In presence of Li2SO4, the H+ adsorption/desorption process, species mass-transport and kinetic current density are reduced. Furthermore, the Tafel’s slope analyse show a change of the rate-determining steps for HOR mechanism. More detailed results of the kinetic analysis and lithium impact on the studied systems are discussed in this work.

Nanoparticle size effect of Pt and TiO2 anatase/rutile phases “volcano-type” curve for HOR electrocatalytic activity at Pt/TiO2-CNx nanocatalysts

• According to the results, we can draw the following conclusions: • Pt nanoparticles are homogenously dispersed on TiO 2 -CNx supports with different content anatase/rutile phases. • TiO 2 anatase/rutile phases show a “volcano-type” curve for the HOR electrocatalytic activity over Pt/TiO 2 -CNx nanocatalysts and Pt/TiO 2 (25 %R)-CNx exhibits the best HOR electrocatalytic activity and stability. • There is a strong metal-support interaction (SMSI) between Pt nanoparticles and TiO 2 (25 %R)-CNx support with 25% rutile. • In the range of 1.8–2.8 nm, the HOR electrocatalytic activity increases with decreasing of Pt nanoparticles size. • Pt/TiO 2 (25 %R)-CNx catalyst is a promising anode catalyst for hydrogen fuel cell. The multicomponent complex Pt/TiO 2 -CNx catalysts with different TiO 2 anatase/rutile phases synthesized by ethylene glycol reduction were investigated. Herein, FESEM, TEM, BET, XANES, EXAFS, XPS, CV and RDE techniques are adopted to systematically investigate the relationship between composition, structure and performance of catalysts with ultrafine Pt nanoparticles, CNx nanowires and TiO 2 co-catalyst. Firstly, we discussed the correlation between HOR electrocatalytic activity and ultrafine Pt nanoparticles in the range of 1.8–2.8 nm. The HOR electrocatalytic activity increases with decreasing of Pt nanoparticles size, showing a “positive” nanoparticle size effect on HOR. Furthermore, the HOR activity of Pt/TiO 2 -CNx are affected by the content of TiO 2 anatase/rutile phases, showing a “volcano-type” curve, and Pt/TiO 2 (25 %R)-CNx containing 25% rutile has the highest HOR electrocatalytic activity with kinetic current density of 16.801 mA cm −2 and exchange current density of 0.962 mA cm −2 . It also revealed a strong metal-support interaction (SMSI) between Pt nanoparticles and TiO 2 -CNx support resulting from the electrons drift from TiO 2 -CNx to Pt promotes the HOR electrocatalytic activity of the Pt/TiO 2 -CNx catalyst. Our results explain to some extent the dependence among Pt nanoparticles, CNx nanowires and TiO 2 co-catalyst, as well as the scientific problem between the size of Pt nanoparticles and HOR electrocatalytic activity.

Uniform zinc electrodeposition directed by interfacial cation reservoir for stable Zn–I2 battery

Rough interfacial Zn electrodeposition and severe parasitic side reactions in aqueous electrolyte, resulting in increased internal resistance and limited reversibility, have significantly hindered the commercial deployment of Zn metal anode. Interfacial engineering offers a versatile platform for manipulating the zinc-ion flux and suppressing side reactions, via pre-formation of an artificial interlayer. Herein, we demonstrate a zincophile interphase based on metal-organic framework cation reservoir. The zinc hexacyanoferrate nanocubes with interconnected open cation transfer channels uniformize the interfacial electric field and render the protected electrode with faster zinc redox kinetic and higher exchange current density. The cations within the reservoir served as an ion-transfer mediator could regulate interfacial Zn2+ even distribution and lower zinc-ion nucleation energy barrier, consequently promoting the homogeneous Zn electrodeposition. Additionally, the in situ pre-formed film also enhances the thermodynamic stability of metal anode and avoids the water-induced side interfacial reaction. The protected anode exhibits an extremely low voltage hysteresis and achieves a high average Coulombic efficiency (∼99.6%) for 400 h with dendrite-free behaviors. When coupled with an iodine cathode, the favorable electrochemical performance of full Zn–I2 cells is realized, representing an advanced practical step toward stable Zn metal anode for real applications.

Superior electrocatalytic ORR performance of Melaleuca Leucadendron L barks derived hierarchical porous carbon with abundant atom-scale vacancies and multiheteroatoms

Biomass feedstocks from agriculture and forestry usually possess unique isometric microstructures that are used as potential renewable precursors to synthesize hierarchically porous carbon materials for energy-conversion applications. Herein, the powders of Melaleuca leucadendron L. barks (MLBs) impregnated with a hydrogen bonded complex (HBC) were carbonized to prepare CoBN-doped porous carbon (CBNC) as oxygen reduction reaction (ORR) electrocatalysts. The microstructure, porous texture and compositions of the as-obtained CBNC samples and their intermediates were characterized by small-angle neutron scattering (SANS) and other techniques. The lamellar structure of MLBs and the HBC strategy endowed the as-obtained CBNC with a unique porous texture, large specific surface area (1041 m 2 g −1 ) and uniformly distributed heteroatoms of Co, B and N. The introduction of Co species in the precures is beneficial to improving the formation of atomic defects. The CBNC sample obtained at the optimized conditions exhibited a half-wave potential ( E 1/2 ) of 0.83 V and a kinetic current density ( j k ) of 9.73 mA cm −2 as well as a robust durability and methanol tolerance in the alkaline electrolyte; its overall ORR properties were highly superior to Pt/C ( j k = 8.2 mA cm −2 , E 1/2 = 0.82 V). This novel approach offers a feasible way towards producing high-performance ORR electrocatalysts from biomass wastes.

Reakcija redukcije kiseonika na elektrohemijski istaloženim tankim slojevima platine na (Nb-Ti)2AlC nosaču

Catalytic activity towards the oxygen reduction reaction (ORR) in 0.5 M H2SO4 was investigated at sub-monolayers and ultra-thin layers (corresponding to 10, 30 and 100 monolayers, (MLs)) of Pt electrochemically deposited on (Nb-Ti)2AlC substrate. Electrochemical deposition of Pt layers on (Nb-Ti)2AlC substrate was achieved from the solution containing 3 mM K2PtCl4 + 0.5 M NaCl (pH 4) under the conditions of convective diffusion (RPM = 400) using linear sweep voltammetry (LSV) at a sweep rate of 2 mV s-1 , by determining limiting potential for deposition of each Pt sample from the QPt vs. E curves. The Pt samples were characterized X-ray photoelectron spectroscopy (XPS). XPS analysis showed that practically the whole surface of (Nb-Ti)2AlC substrate is covered with homogeneous layer of Pt, while Pt ion reduction was complete to metallic form - Pt(0) valence state. Then oxygen reduction was studied at rotating disc electrode by cyclic voltammetry and linear sweep voltammetry. Two different Tafel slopes were observed, one close to 60 mV dec-1 in low current densities region and second one ~ 120 mV dec-1 in high current densities region. This novel catalyst exhibited higher activity in comparison to carbon supported one, in terms of mass activity - kinetic current density normalized to Pt loading.

Electrocatalytic oxygen reduction of COF-derived porous Fe-Nx nanoclusters/carbon catalyst and application for high performance Zn-air battery

The development of non-previous-metal electrocatalyst with low cost, high activity and stability towards oxygen reduction reaction (ORR) remains a challenge. Covalent organic frameworks (COFs) have emerged as promising candidates for the preparation of porous metal/nitrogen co-doped carbon catalysts. In the present work, porous Fe-Nx nanoclusters/carbon catalysts are prepared by pyrolyzing iron-containing covalent-triazine organic frameworks. It is found that the synthesis conditions including pyrolysis temperature and pressure have great impacts on the surface area of micropores, mesopores and macropores, which in turn influences the ORR activity of catalysts by varying the mass diffusion and the contact of active centers with electrolytes. The optimized catalyst (denoted as Fe1.2NC-0.02-800), having the highest both BET and external surface areas, shows much higher kinetic current density and catalytic stability than commercial Pt/C catalyst in ORR process. When being assembled into Zn-air battery, it also exhibits excellent performances in terms of open circuit voltage (1.45 V), power density (210 mW cm−2 at 330 mA cm−2) and specific capacity (713.1 mA h gZn−1), which outperform the corresponding values of Pt/C-based Zn-air battery. Moreover, 15 light-emitting diodes can be powered by two series-connected Zn-air batteries with Fe1.2NC-0.02-800 as the air-cathode catalyst, implying its promising application in clean energy conversion field.

Oxygen adsorption in pores promotes its reduction on metal-free carbon catalysts: A case of carbon blacks

Four as-received carbon blacks from a Black Pearl family with surface areas between 30 and 1600 m 2 g −1 were chosen to be tested as oxygen reduction catalysts in an alkaline environment. As main descriptors of their performance an onset potential, electron tranfer number and kinetic current density have been chosen. Even though the onset potential was smaller than that on Pt/C, the electron transfer number and kinetic current density reached the values measured on Pt/C. To find surface features governing the good performance, porosity and surface chemistry of the samples were evaluated in detail using nitrogen adsorption, thermal analysis, potentiometric titration, and XPS. The results indicated the linear dependence of the onset potential and electron transfer number on the surface area/porosity. The most porous sample exhibited an excellent catalytic behavior due to its developed ultramicropores. They attracted O 2 from an electrolyte and, through strong adsorption forces, contributed to the brake of oxygen-oxygen bonds and thus to its further reduction and protonation. The importance of the surface affinity to attract oxygen for ORR was demonstrated by linear dependences of both the electron transfer number and onset potential on the extent of oxygen adsorption from its saturated solution.

Boosting the ORR performance of Fe-N/C catalyst via increasing the density and modifying the electronic structure of Fe-NX active sites

• Fe-N/C‑meso-evap with unique porous structure is prepared by doping sulfate. • Fe-N/C‑meso-evap achieves high Fe-N X density by a second FeCl 2 evaporation method. • Fe-N/C‑meso-evap shows superior ORR activity with the E 1/2 of 0.835 v in acid media. • Rich mesopores in Fe-N/C‑meso-evap increase the number of accessible active sites. • Doped s facilitates the absorption of O 2 and the breakage of O O bond. Due to the existence of highly active Fe-N X coordination moieties, iron-nitrogen/carbon (Fe-N/C) materials have been considered as potential electrocatalysts toward oxygen reduction reaction (ORR). Increasing the density of Fe-N X sites is supposed to be a useful strategy to enhance the ORR performance of Fe-N/C catalysts. Herein, we report a stepwise synthesis strategy to construct Fe-N/C catalyst possessing a high ratio of mesopores and a high concentration of accessible Fe-N X active sites. Integrated with the properties of composition and morphology, the synthesized catalyst, Fe-N/C‑meso-evap, achieves remarkable ORR activity with a half potential of 0.835 V and a kinetic current density of 31.02 mA cm −2 at 0.8 V in 0.1 M HClO 4 . This work provides new insight for designing single-atom catalysts towards ORR.

Low‐Coordinated CoNC on Oxygenated Graphene for Efficient Electrocatalytic H2O2 Production

AbstractElectrochemical H2O2 production through the 2‐electron oxygen reduction reaction (ORR) is a promising alternative to the energy‐intensive anthraquinone process. Herein, by simultaneously regulating the coordination number of the atomically dispersed cobalt sites and the nearby oxygen functional groups via a one‐step microwave thermal shock, a highly selective and active CoNC electrocatalyst for H2O2 electrosynthesis that exhibits a high H2O2 selectivity (91.3%), outstanding mass activity (44.4 A g−1 at 0.65 V), and large kinetic current density (11.3 mA cm−2 at 0.65 V) in 0.1 m KOH is obtained. In strong contrast to the typical CoN4 moieties for the 4‐electron ORR, the present CoNC catalyst possesses a low‐coordinated CoN2 configuration and abundant epoxide groups, which work in synergy for promoting the 2‐electron ORR, as demonstrated by a series of control experiments and theoretical simulations. This study may provide an effective avenue to modulating the composition and structure of electrocatalysts at the atomic scale, leading to the development of new electrocatalysts with unprecedented reactivity.

Developed Nanomaterials with a Pdcore-Fe-Pdskin Structure for Efficient Electrocatalytic Performance in Oxygen Reduction and Glycerol Oxidation Reactions in Alkaline Electrolytes

One of the unsolved important fundamental and applied questions in heterogeneous electrocatalysis for low-temperature fuel cells and electrolyzers is about our ability to engineer a unique electrocatalyst that can simultaneously operate for both anode and cathode without changing the composition/structure or the synthesis method. Our present contribution is about a solution in the case of iron-diluted Pd nanoparticles. ORR is the common bottleneck of the pollution-free electricity production with alkaline technologies of rechargeable metal-air batteries and fuel cells. As alternative to scarcely Pt, Pd-based materials have attracted much attention for different applications in heterogeneous catalysis and not only for ORR, but also for glycerol (biomass-based byproduct) oxidation that is also common reaction (anode) for fuel cells and low-energy input electrolyzers to produce green H2. So, being able to prepare a nanocatalyst that is simultaneously active and durable for both ORR and glycerol oxidation reaction (GOR) is highly desired. We observed that the support plays an important role and the presence of Fe (20%) dramatically improves the performance. The developed bimetallic PdFe performed efficiently and selectively ORR in alkaline media at the maximum faradaic yield of four-electron transfer (100%) and high kinetic current density j k of 2 mA cm−2 Pd (1 A mg−1 Pd) at 0.9 VRHE, which largely exceeds the tested commercial Pd/C and the literature. For glycerol electrooxidation, the obtained peak current density of j p = 2.3 mA cm−2 Pd (1.1 A mg−1 Pd) and faradaic efficiency of FE = 83-99% outperformed the commercial Pd/C (j p = 0.47 mA cm−2 Pd (0.39 A mg−1 Pd, FE = 78%) and the majority of the reported catalysts.Our conducted DFT calculations reveal that the high performance of the bimetallic PdFe arises from a unique structure with Pd in the core plus skin and Fe in the second layer. This particular configuration of skin surfaces has been reported for PtNi to trigger high electrocatalytic kinetics for ORR. Those findings open new possibilities in the development of multifunctional nanocatalysts for the sustainable electrochemical energy conversion and storage Figure 1

Integration of Morphology and Electronic Structure Modulation on Atomic Iron‐Nitrogen‐Carbon Catalysts for Highly Efficient Oxygen Reduction

AbstractAtomic transition‐metal‐nitrogen‐carbon catalysts (M‐N‐Cs) hold great promise as Pt‐group‐metal‐free candidates for electrochemical reactions, yet their rational design and controllable synthesis remain fundamental challenges. Here, the molten‐salts mediated pyrolysis is demonstrated to be an effective and facile strategy for simultaneous morphology and electronic structure modulation of prototypical Fe‐N‐C materials, which functions as efficient oxygen reduction electrocatalysts. Taking advantage of the strong polarity and salt templating effects, the as‐obtained Fe‐N/C‐single atom catalyst (SAC) possesses hierarchical porous nanosheet morphology with an impressive specific surface area of 2237 m2 g−1 and unique FeN4Cl moieties as isolated active centers. The Fe‐N/C‐SAC delivers remarkable alkaline oxygen reduction reaction (ORR) activity with a half‐wave potential of 0.91 V and record kinetic current density up to 55 mA cm−2, outperforming the benchmark Pt/C. By virtue of dechlorination treatment, it is experimentally identified that the enhanced ORR activities are essentially governed by the axially bound Cl. Theoretical calculations rationalize this finding and demonstrate that the well‐defined fivefold‐coordinated configuration accelerates 4e− pathway kinetics through near‐optimal adsorption of the *OH intermediates and tunes the potential determining step from *OH reduction to *OOH formation. This study provides fundamental insights into the coordination‐engineered strategy in single‐atom catalysis.

Catalyzing Bond-Dissociation in Graphene via Alkali-Iodide Molecules.

Atomic design of a 2D-material such as graphene can be substantially influenced by etching, deliberately induced in a transmission electron microscope. It is achieved primarily by overcoming the threshold energy for defect formation by controlling the kinetic energy and current density of the fast electrons. Recent studies have demonstrated that the presence of certain species of atoms can catalyze atomic bond dissociation processes under the electron beam by reducing their threshold energy. Most of the reported catalytic atom species are single atoms, which have strong interaction with single-layer graphene (SLG). Yet, no such behavior has been reported for molecular species. This work shows by experimentally comparing the interaction of alkali and halide species separately and conjointly with SLG, that in the presence of electron irradiation, etching of SLG is drastically enhanced by the simultaneous presence of alkali and iodine atoms. Density functional theory and first principles molecular dynamics calculations reveal that due to charge-transfer phenomena the CC bonds weaken close to the alkali-iodide species, which increases the carbon displacement cross-section. This study ascribes pronounced etching activity observed in SLG to the catalytic behavior of the alkali-iodide species in the presence of electron irradiation.

Compressive Strain Modulation of Single Iron Sites on Helical Carbon Support Boosts Electrocatalytic Oxygen Reduction

AbstractDesigning and modulating the local structure of metal sites is the key to gain the unique selectivity and high activity of single metal site catalysts. Herein, we report strain engineering of curved single atomic iron‐nitrogen sites to boost electrocatalytic activity. First, a helical carbon structure with abundant high‐curvature surface is realized by carbonization of helical polypyrrole that is templated from self‐assembled chiral surfactants. The high‐curvature surface introduces compressive strain on the supported Fe−N4 sites. Consequently, the curved Fe−N4 sites with 1.5 % compressed Fe−N bonds exhibit downshifted d‐band center than the planar sites. Such a change can weaken the bonding strength between the oxygenated intermediates and metal sites, resulting a much smaller energy barrier for oxygen reduction. Catalytic tests further demonstrate that a kinetic current density of 7.922 mA cm−2 at 0.9 V vs. RHE is obtained in alkaline media for curved Fe−N4 sites, which is 31 times higher than that for planar ones. Our findings shed light on modulating the local three‐dimensional structure of single metal sites and boosting the catalytic activity via strain engineering.

Compressive Strain Modulation of Single Iron Sites on Helical Carbon Support Boosts Electrocatalytic Oxygen Reduction.

Designing and modulating the local structure of metal sites is the key to gain the unique selectivity and high activity of single metal site catalysts. Herein, we report strain engineering of curved single atomic iron-nitrogen sites to boost electrocatalytic activity. First, a helical carbon structure with abundant high-curvature surface is realized by carbonization of helical polypyrrole that is templated from self-assembled chiral surfactants. The high-curvature surface introduces compressive strain on the supported Fe-N4 sites. Consequently, the curved Fe-N4 sites with 1.5 % compressed Fe-N bonds exhibit downshifted d-band center than the planar sites. Such a change can weaken the bonding strength between the oxygenated intermediates and metal sites, resulting a much smaller energy barrier for oxygen reduction. Catalytic tests further demonstrate that a kinetic current density of 7.922 mA cm-2 at 0.9 V vs. RHE is obtained in alkaline media for curved Fe-N4 sites, which is 31 times higher than that for planar ones. Our findings shed light on modulating the local three-dimensional structure of single metal sites and boosting the catalytic activity via strain engineering.

CoN4 active sites in locally distorted carbon structure for efficient oxygen reduction reaction via regulating coordination environment

• The locally distorted CoN 4 active sites in single atom Co catalyst were synthesized via coordination-intercalation assisted strategy. • It exhibits eminent ORR property with high E 1/2 of 0.89 V and J k of 15.4 mA cm −2 at 0.85 V. • It can offer high ORR performance by decreasing reaction energy barrier of the rate-determining step. • The fabricated Al-air battery shows brilliant energy density of 2466.7 Wh kg Al −1 and power density of 453.8 mW cm −2 . Adjusting the coordination environment of active sites for oxygen reduction reaction (ORR) is a desired method to realize efficient energy conversion. Herein, we synthesized a single atom Co catalyst (Co-SA/NC) with CoN 4 active sites in locally distorted carbon configuration via a coordination-intercalation assisted strategy. The distinctive existing form of CoN 4 structure has been confirmed according to X-ray absorption fine spectroscopy. The Co-SA/NC displays satisfactory ORR performance with higher half-wave potential of 0.89 V and kinetic current density ( J k ) of 15.4 mA cm −2 at 0.85 V in alkaline solution, than those of Pt/C (0.87 V and 8.7 mA cm −2 ). Density functional theory calculations demonstrate that locally distorted CoN 4 sites are instrumental in adsorption/desorption of oxygen species, promoting the reaction kinetics of ORR. Furthermore, the fast ORR kinetics is proved by small Tafel slope of 57.9 mV/dec. Applying Co-SA/NC as air electrode in Al-air battery, it presents high specific capacity (1992.7 mAh g Al −1 ) and energy density (2466.7 Wh kg Al −1 ) at 200 mA cm −2 . The electrocatalyst with unique coordination configuration for M-N 4 active sites at atomic level will have wide applications in regulating its intrinsic activity.

Identification of the Evolving Dynamics of Coordination-Unsaturated Iron Atomic Active Sites under Reaction Conditions

Understanding the nature of the catalytic active center and its evolving dynamics under operating conditions is critical for the development of efficient and highly selective catalysts. By combining synchrotron-based operando X-ray absorption and infrared spectroscopies, here we uncover at an atomic level a hydroxyl was coupled on the dynamically released coordination-unsaturated Fe-N2 moieties to form a highly active OH-Fe-N2 structure and then promotes the adsorption of O2 during the catalytic oxygen reduction reaction (ORR), which greatly facilitates the fabrication of the key *OOH intermediate and simplifies the fracture of the O–O bond to accelerate the multielectron reaction kinetics. The resulting covalent organic framework-derived Fe single-site catalyst could efficiently deliver an excellent ORR catalytic activity with an extremely large kinetic current density (Jk) of 81.3 mA cm–2 and an extra high turnover frequency of 5804 h–1, 20 times that of the Pt-C catalyst (288 h–1, 7.7 mA cm–2).

Revealing the Effect of Nickel Nanoparticles for Li Plating and Stripping Processes on Ni−Nx Doped Hollow Carbon Sphere

AbstractIn this work, the effects of the coexistence of Ni−Nx sites and Ni metal nanoparticles of Ni−N−C (nickel‐nitrogen‐carbon) materials on the thermodynamic Li nucleation overpotential (η) and kinetic exchange current density (j0) are systematically studied. Ni‐Nx sites guarantee reduced nucleation resistance on the hollow carbon sphere (HCS) host, while the low amount residual of Ni nanoparticles plays an adverse role. The Li nucleation overpotential was down to 10.6 mV and the exchange current density increased from 1.139 to 2.325 mA cm−2 when the residual Ni nanoparticles in the prepared Ni−N−C composite were removed. As a result, pure Ni‐Nx sites displayed an average CE of 98.4 % over 300 cycles and a stable Li plating/stripping behavior for over 800 h.