Nanostructured Nonprecious Metal Catalysts for Oxygen Reduction Reaction

With increasing demand for energy, the development of the energy storage and conversion technologies has become the focus of an intensive research effort in recent years. Among various technologies, fuel cells, batteries, supercapacitors, and water electrolyzers have been recognized as potentially feasible and efficient devices for portable, stationary, and transportation applications. For both the fuel cells and metal-air batteries, the oxygen reduction reaction (ORR) cathode catalysts play a major role in the device performance characteristics, as determined based on the power output, open circuit voltage, charge-discharge rate, energy efficiency, cycling life (for batteries), etc. Currently, Pt-nanoparticle catalysts, supported on high surface-area carbons, represent the state-of-the-art electrocatalysts for hydrogen oxidation reaction (HOR) and ORR at the polymer electrolyte fuel cell (PEFC) anode and cathode, respectively. However, the prohibitive price and scarcity of Pt have limited its widespread implementation in the PEFC, especially at the cathode, which accounts for approximately 80% of the Pt loading in fuel cells. As a result, non-precious metal catalysts (NPMCs) for the ORR have received more attention in recent years as a possible replacement of precious-metal catalysts.A successful ORR catalyst must combine high activity with good long-term stability – a major challenge in the strongly acidic environment of the PEFC cathode. Since the early work of Jasinski in the 1960s (Nature 201, 1212, 1964), recent breakthroughs in the synthesis of high-performance non-precious metal catalysts (NPMCs) (e.g., Lefèvre et al., Science 324, 71, 2009; Wu et al., Science 332, 443, 2011) make replacement of Pt in ORR electrocatalysts with earth-abundant elements, such as Fe, Co, N, and C, a realistic possibility, though the activity and especially durability of the resulting catalysts need to be improved before the technology can become viable.The NPMC performance depends on the selection of precursors, the synthesis chemistry; and especially the catalyst nanostructure. In addition to those, the CNx structures likely play a major role in the performance of ORR active site(s). Apart from possible direct participation in the active site, the transition metal is crucial to in-situ formation of carbon nanostructures (nanotubes, onion-like structures, graphene) by catalyzing the decomposition of the nitrogen/carbon precursor(s) at a high temperatures (800-1000°C). The formation of different carbon and nitrogen-doped carbon nanostructures can be controlled during the synthesis of such NPMCs. The highly-graphitized carbon nanostructures likely serve as a matrix for the formation of the ORR-active groups with improved catalytic activity and durability, containing nitrogen and possibly also metal atoms.Future NPMC synthesis approaches are certain to focus on the precise control of interactions between precursors of the metal and carbon/nitrogen during the heat treatment, with the main purpose being the maximizations of the population of active sites, optimization of nitrogen doping levels, and generation of carbon morphologies capable of hosting active and stable ORR sites. This is evident not only in catalysts developed for PEFCs but also in materials specifically designed for alkaline fuel cells (Chung et al., Nat. Commun. 4:1922 doi:10.1038/ncomms2944, 2013). In the end, however, the much needed progress in ORR electrocatalysis at NPMCs, especially in acid media, will be decided by better understanding of the origin of the NPMC activity and the nature of the active site. That key part of NPMC development will be addressed in this presentation along with the summary of the progress achieved to date and challenges still awaiting non-precious metal electrocatalysis in polymer electrolyte fuel cells. Acknowledgment Financial support from DOE-EERE Fuel Cell Technologies Office and Los Alamos National Laboratory Laboratory-Directed Research and Development (LDRD) Program and is gratefully acknowledged.

Open AccessCCS ChemistryRESEARCH ARTICLE1 Jan 2022Theory-Driven Design of Electrocatalysts for the Two-Electron Oxygen Reduction Reaction Based on Dispersed Metal Phthalocyanines Yang Wang†, Zisheng Zhang†, Xiao Zhang, Yubo Yuan, Zhan Jiang, Hongzhi Zheng, Yang-Gang Wang, Hua Zhou and Yongye Liang Yang Wang† Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055 , Zisheng Zhang† Department of Chemistry, Guangdong Provincial Key Laboratory of Catalysis, Southern University of Science and Technology, Shenzhen 518055 , Xiao Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027 , Yubo Yuan Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055 , Zhan Jiang Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055 , Hongzhi Zheng Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055 , Yang-Gang Wang Department of Chemistry, Guangdong Provincial Key Laboratory of Catalysis, Southern University of Science and Technology, Shenzhen 518055 , Hua Zhou X-Ray Science Division, Advanced Photon Source, Argonne National Laboratory, Lemont, IL 60439 and Yongye Liang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055 Guangdong-Hong Kong-Macao Joint Laboratory for Photonic-Thermal-Electrical Energy Materials and Devices, Southern University of Science and Technology, Shenzhen 518055 https://doi.org/10.31635/ccschem.021.202000590 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The two-electron electrochemical reduction of oxygen is an appealing approach to produce hydrogen peroxide. Metal and heteroatom-doped carbon (M–X/C) materials have recently been recognized as compelling catalysts for this process, but their performance improvement is generally hindered by the ill-defined structures of active sites. Herein, we demonstrate a theory-driven design of catalysts for oxygen reduction reactions based on molecularly dispersed electrocatalysts (MDEs) with metal phthalocyanines on carbon nanotubes. Density functional theory calculations suggest that nickel phthalocyanine (NiPc) favors the formation of *H2O2 over *O, thus acting as a selective catalyst for peroxide production. NiPc MDE shows high peroxide yields of ∼83%, superior to the aggregated NiPc and pyrolyzed Ni–N/C catalysts. The performance is further enhanced by the introduction of the cyano group (CN). NiPc–CN MDE exhibits ∼92% peroxide yields and good stability. Our studies provide a new perspective for the development of heterogeneous electrocatalysts for hydrogen peroxide production from metal macrocyclic complexes. Download figure Download PowerPoint Introduction Driving economically important chemical reactions with renewable electricity offers an intriguing opportunity to replace current energy-intensive processes.1–3 For example, the electrochemical oxygen reduction reaction (ORR) through the two-electron (2e−) pathway is considered an environmentally benign alternative to the industrial anthraquinone method to produce hydrogen peroxide (H2O2), which is widely used as a green oxidizer in bleaching, waste water treatment, and the chemical industry.4–6 An ideal electrocatalyst should possess high activity toward the 2e− pathway to the peroxide product and suppress the competing 4e− process to water. Platinum (Pt) or palladium (Pd) alloys with mercury (Hg) have been demonstrated as selective electrocatalysts for the 2e− pathway in ORR.7,8 However, due to the toxicity of Hg and the limited reserve of noble metals, these elements are not preferred for practical applications. Carbon-based materials doped with earth-abundant elements, however, are compelling candidates as efficient and affordable electrocatalysts.9–14 Carbon nanotube (CNT), graphene, and activated carbon with oxygen-containing functional groups were reported to be selective in 2e− ORR.15–18 Embedding coupled boron–nitrogen (BN) domains into graphitic carbon showed enhanced selectivity and activity for reducing O2 to HO2− compared with the catalysts with individual B or N doping.19 In addition to metal-free catalysts, metal and heteroatom-doped carbon (M–X/C) catalysts with isolated heteroatom-coordinated metal moieties, one type of single-atom catalysts (SACs), have also been exploited for H2O2 production.20–23 A series of M–N/C (M = Mn, Fe, Co, Ni, and Cu) catalysts with proposed M–N4 active sites were synthesized to investigate their performance in ORR. The Co–N/C catalyst showed preference for the 2e− pathway in acidic condition.24 Moreover, SACs with transition-metal centers coordinated by different heteroatoms, such as O and S, were also reported to show high selectivities for the 2e− reduction pathway in ORR.25–28 However, the lack of well-defined structures and the copresence of various types of active sites prevent understanding the structure–performance relationships and catalyst design principles in these SAC catalysts. Metal macrocyclic complexes, such as metal phthalocyanines (MPcs) and porphyrins with well-defined M–N4 moieties, have been attractive electrocatalysts since the report of cobalt phthalocyanine as an active ORR catalyst.29–31 For instance, iron phthalocyanine (FePc) has been reported to be efficient in catalyzing ORR through the 4e− pathway to water.32,33 However, the performances of metal complexes in heterogeneous form are often limited by their low electric conductivity.34–36 Hybridizing metal macrocyclic complexes with nanocarbon materials were found to promote their catalytic performances.35,37,38 In the carbon dioxide reduction reaction, achieving molecular dispersion on conducting supports is beneficial to reveal the intrinsic performance of molecular catalysts and establish catalyst design principles.39,40 In addition, previous reports of heterogeneous molecular ORR catalysts mainly focused on optimizing the performance toward the 4e− pathway with little exploration of the 2e− pathway to peroxide production.30,41 In this work, we present a theory-driven design of electrocatalysts based on an molecularly dispersed electrocatalyst (MDE) consisting of dispersed MPcs on CNTs for electrochemical production of peroxide. From density functional theory (DFT) calculations, we identify nickel phthalocyanine (NiPc) as a selective catalyst for 2e− ORR with experimental peroxide yields of ∼83% in the form of MDE, in contrast to FePc MDE that is selective for 4e− ORR. Achieving molecular dispersion of NiPc with well-defined Ni–N4 sites is important to the high peroxide selectivity as proven by the lower peroxide yields of the physically mixed NiPc and CNT (containing aggregated NiPc) and a pyrolyzed Ni–N/C SAC. Moreover, molecular engineering of NiPc MDE with the introduction of cyano groups (CNs) to the Pc ligand (NiPc–CN MDE) further enlarges the free-energy preference to the 2e− pathway and enhances the selectivity for the electrochemical production of peroxide. NiPc–CN MDE exhibits a high peroxide yield of ∼92% in the potential range of 0.70–0.20 V versus a reversible hydrogen electrode (RHE). Experimental Methods Preparation of MPc MDEs The preparation of MPc MDEs was based on a reported procedure with the control of the ratio between MPcs and CNTs.40 NiPc and FePc were obtained from commercial sources, and NiPc–CN was synthesized according to a reported method.40 Briefly, 30 mg purified CNTs were dispersed in 25 mL of N,N-dimethylformamide (DMF) with the assistance of sonication, in which a calculated amount of MPcs in 5 mL of DMF was added to obtain a well-mixed suspension. The mixture was further sonicated for 30 min and then stirred at room temperature for 20 h. Subsequently, the precipitate was collected by centrifuge and washed with DMF (three times) and ethanol (twice). Finally, the collected precipitate was lyophilized to yield the final product. Electrochemical measurements About 4 mg of MPc MDEs and 10 μL of 5 wt % Nafion solution were dispersed in 990 μL ethanol under ultrasonication to form a homogeneous ink. About 13 μL catalyst ink was loaded onto the glassy carbon (GC) disk electrode (5.5 mm in diameter) of a rotating ring-disk electrode (RRDE) to achieve a catalyst loading of ∼0.2 mg cm−2. The ink of NiPc + CNT was prepared by dispersing 2.8 mg of NiPc, 1.2 mg of CNT, and 10 μL of 5 wt % Nafion solution in 990 μL ethanol under ultrasonication, then loaded onto the GC electrode. The RRDE experiments were conducted with a four-electrode system using a saturated calomel electrode (SCE) as the reference electrode (calibrated with a homemade RHE), a graphite rod as the counter electrode, and the catalyst-modified GC disk electrode as the working electrode. Meanwhile, the Pt ring electrode was kept at 1.5 V (vs RHE, the same for following potentials unless otherwise stated) for all experiments. The disk and ring electrodes were rotated at a speed of 1600 rpm (Pine research). Electrolytes (0.1 M KOH) were saturated with O2 by bubbling for 30 min prior to each experiment, and a flow of O2 was maintained over the electrolyte during the reaction. Linear sweep voltammetry (LSV) was conducted by scanning the disk electrode potential with a scan rate of 5 mV/s. For the stability test, the disk electrode potential was kept at 0.5 V. Experiments were also performed under an argon environment to record the background currents of the disk and ring electrodes, which were subtracted from the currents under O2. The peroxide yield and electron transfer number (n) were determined by the following equations: Peroxide yield ( HO 2 − ) = 200 × ( I r / N ) I d + ( I r / N ) % n = 4 × I d I d + ( I r / N ) where Ir is ring current, Id is disk current, and N is current collection efficiency of the Pt ring electrode (0.28, calibrated with K3[Fe(CN)6]). Computational Methods DFT calculations of gas-phase MPc molecules catalyzing ORR were conducted using the Gaussian 09 program.42 B3LYP functional43 with D3 correction (Becke–Johnson damping)44 was adopted for calculation.45 The all-electron 6-31G* basis set (for H, C, N, and O)46–48 and the Stuttgart–Dresden (SDD) basis set containing all double-ξ valence with effective core potentials (ECPs)49 (for Ni and Fe) were used. The geometric structures were all optimized at 298.15 K and under 1 atm. The harmonic vibrational frequencies were computed with no imaginary frequency found for all reaction intermediates. The Gibbs free energies of high- and low-spin forms of all intermediates were calculated with the harmonic potential approximation to determine the ground states. The electrocatalytic mechanisms were investigated with the computational hydrogen electrode (CHE) model.50 Additional details of computational methods are available in the Supporting Information. Results and Discussion Theoretical calculations of MPcs catalyzing ORR To understand how the central metals in MPc molecules affect the product selectivity in ORR, DFT calculations of the free-energy changes of ORR through the 2e− and 4e− pathways were conducted on FePc and NiPc at 1.23 V versus RHE. The calculated free-energy diagrams suggest distinctly different ORR behaviors of FePc and NiPc (Figure 1a). On FePc, O2 is first adsorbed on the Fe center, followed by a proton-coupled electron transfer (PCET) process to form *OOH with an uphill free-energy change. The divergence of the 2e− and 4e− pathways came from the preference of *OOH reduction with a protonation mechanism to *H2O2 or an O–O cleavage to *O. The downhill free-energy change to form *O and the large free-energy increase required for the generation of *H2O2 indicate a high preference for the 4e− reduction pathway on FePc, consistent with high selectivities of O2 reduction to water/hydroxide of Fe macrocyclic complexes and Fe-based SACs in previous reports.34,51 By contrast, the *OOH intermediate (generated from O2 through an *O2 intermediate with two uphill free-energy changes) on NiPc shows a slight downhill free-energy change to generate *H2O2, while the formation of *O in the 4e− reduction pathway is energetically uphill (Figure 1a). In contrast to FePc, the reversed trend in free-energy changes to form *H2O2 and *O on NiPc suggests the preference for the 2e− reduction pathway. Therefore, NiPc molecules are predicted to be selective electrocatalysts for 2e− ORR to peroxide product (Figure 1b). Figure 1 | Theoretical calculations of ORR catalyzed by MPcs. (a) Calculated free-energy diagrams of ORR through the 2e− and 4e− reduction pathways on NiPc and FePc at 1.23 V. (b) Schematic presentation of ORR selectivity on NiPc and FePc based on DFT calculations. Download figure Download PowerPoint ORR performance of MPc MDEs and aggregated MPcs Dispersed NiPc and FePc molecules were supported on the CNTs via π–π interactions to fabricate MPc MDEs according to our previous method40 to examine the calculated trends in ORR. The metal contents in MDEs were controlled to be ∼0.7 wt % ( Supporting Information Table S1), which were measured by inductively coupled plasma mass spectrometry (ICP-MS). The electrochemical behaviors of NiPc MDE and FePc MDE were first investigated in O2-saturated 0.1 M KOH electrolytes with the RRDE setup (0.2 mg cm−2 catalyst loading). The MDEs were drop-coated on the disk electrode as the working electrode to reduce O2, while the ring electrode (Pt) was maintained at 1.5 V to detect the produced peroxide. FePc MDE shows more positive onset potential (0.94 V at −0.025 mA, corresponding to a current density of ∼−0.1 mA cm−2) than that of NiPc MDE (0.79 V) (Figure 2a). The current of FePc MDE is saturated to −1.41 mA at ∼0.58 V. The saturation current for NiPc MDE is about −0.63 mA at the same potential. The peroxide yield and n of MPc MDEs calculated from the disk and ring currents are depicted in Figure 2b and Supporting Information Figure S1, respectively. NiPc MDE shows good peroxide yields of ∼83% in the potential range of 0.70–0.53 V, which decline at more negative potentials. Correspondingly, n of NiPc MDE is below 2.34 in the potential range of 0.70–0.53 V, which gradually increases to 2.83 from 0.53 to 0.20 V ( Supporting Information Figure S1). In contrast, low peroxide yields of ∼1% together with n above 3.97 in the potential range of 0.70–0.20 V are observed with FePc MDE (Figure 2b and Supporting Information Figure S1), confirming its strong preference toward the 4e− reduction pathway. These results indicate that the preferred ORR pathways of NiPc MDE and FePc MDE are the 2e− and 4e− reduction pathways, respectively, which are consistent with the DFT calculations (Figure 1a). Figure 2 | ORR performance of MPc-based electrocatalysts on RRDE. (a) Disk and ring currents of NiPc, NiPc + CNT, NiPc MDE, and FePc MDE in O2-saturated 0.1 M KOH electrolytes on RRDE test rotating at 1600 rpm. (b) Calculated peroxide yields of NiPc MDE, NiPc + CNT, and FePc MDE. Download figure Download PowerPoint The effects of aggregation state of NiPc were further investigated. The NiPc molecules directly deposited on substrates easily formed aggregates due to their strong intermolecular interactions ( Supporting Information Figure S2). Due to the poor electric conductivity and limited exposure of active sites of aggregated NiPc, the neat NiPc electrode shows minimal activity in ORR (Figure 2a). Therefore, we physically mixed NiPc with CNTs (denoted as NiPc + CNT) to enhance the conductivity. LSV shows that the physically mixed NiPc + CNT possesses higher activity than that of neat NiPc (Figure 2a). Although NiPc + CNT exhibits even more positive onset potential than NiPc MDE, the low peroxide yields of NiPc + CNT (under 60% in the potential range of 0.70–0.20 V) suggests much less preference toward the 2e− pathway compared with NiPc MDE (Figure 2b). Topological defects and N-dopants have been considered as active sites for ORR.11 However, Pc MDE (prepared by anchoring Pc molecules on CNTs) without metal centers shows inferior ORR activity and selectivity for the 2e− pathway compared with NiPc MDE ( Supporting Information Figure S3). These results suggest the critical role of dispersed Ni centers rather than the N-dopants or topological defects in the selective electrocatalysis of 2e− ORR. To investigate the origin of the different ORR behaviors of neat NiPc, NiPc + CNT, and NiPc MDE, their structures were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The TEM ( Supporting Information Figure S4a) and SEM ( Supporting Information Figure S4b) images of NiPc MDE show the bundles of multiwalled CNTs and excluded the formation of nano- or microsized NiPc aggregates. The isolated bright spots in the high-angle annular dark-field (HAADF) image of NiPc MDE obtained with a Cs-corrected scanning TEM (STEM) indicate the existence of single-Ni sites, suggesting the molecular dispersion of NiPc on CNTs ( Supporting Information Figure S5). No appreciable signature peaks of NiPc molecules are observed in the Raman spectrum of NiPc MDE ( Supporting Information Figure S6), which could be due to the low content of NiPc in NiPc MDE. On the contrary, physically mixed NiPc + CNT contains microsized NiPc aggregates, as revealed by SEM and confirmed by energy-dispersive spectrometry (EDS) mapping of the Ni signals ( Supporting Information Figures S4c and S7). Electrochemical impedance spectroscopy (EIS) was further conducted to gain insights into the ORR kinetics of the NiPc catalysts in aggregated and dispersed states. The Nyquist plots in Supporting Information Figure S4d show that the charge transfer of NiPc MDE for ORR is more favorable than that of NiPc and NiPc + CNT. The neat NiPc exhibited the largest charge-transfer resistance, in agreement with its low activity from LSV (Figure 2a). It should be noted that DFT calculations with individual NiPc molecule-catalyzing ORR suggest high selectivity toward the 2e− transfer pathway, which is only observed in dispersed NiPc as in NiPc MDE but not in aggregated NiPc as in NiPc + CNT. These results emphasize the importance of correlating free-energy diagrams calculated with individual catalyst molecule with the electrocatalytic performance of dispersed molecular catalysts as opposed to aggregated molecules. Comparison with the pyrolyzed Ni–N/C SAC SACs have gained extensive attention recently due to their superior electrocatalytic properties. In electrocatalytic applications, SACs are generally fabricated by pyrolyzing metal salts and N-containing organic precursors at high temperatures. However, these pyrolyzed SACs often contain parasitic active sites due to the insufficient structural control during the high-temperature synthesis.52 For comparison, a nickel SAC with Ni–Nx structures (denoted as Ni–N/C) was synthesized by pyrolyzing a Ni-containing zeolitic imidazolate framework (ZIF) precursor according to the reported method with minor modifications.53 The Ni content of the Ni–N/C catalyst was also controlled to be ∼0.7 wt % (measured by ICP-MS) to compare with NiPc MDE. TEM images of Ni–N/C suggest the absence of metallic Ni particles in the catalyst ( Supporting Information Figure S8). The X-ray diffraction (XRD) pattern of Ni–N/C only shows broad features attributable to graphitic carbon ( Supporting Information Figure S9),53 further indicating the absence of metallic Ni or crystalline Ni-containing compounds. The Fourier-transformed extended X-ray adsorption fine structure (FT-EXAFS) curve of Ni–N/C exhibits a peak at ∼1.4 Å (without phase correction) corresponding to Ni–N coordination, but little signal at ∼2.1 Å corresponding to Ni–Ni coordination ( Supporting Information Figure S10), confirming the presence of single Ni sites in Ni–N/C. Although no Ni particles are observed, parasitic active sites such as N-doped carbon sites could still be present in the Ni–N/C catalyst,54,55 which are known to be active for 4e− ORR (Figure 3a). The ORR performance of the Ni–N/C catalyst was further characterized in the RRDE setup with identical conditions as NiPc MDE. Given the LSV curves (Figure 3b), Ni–N/C possesses more positive onset potential (0.83 V) than that of NiPc MDE (0.79 V). The peroxide yields of Ni–N/C are under 43% with n larger than 3.1 in the potential range of 0.70–0.20 V (Figure 3c and Supporting Information Figure S11). The much worse selectivity of the Ni–N/C catalyst toward the 2e− reduction pathway than that of NiPc MDE is attributed to the structural heterogeneity in the pyrolyzed Ni–N/C catalyst. Additionally, the stability tests were carried out at the constant potentials of 0.50 V for the disk electrode to conduct ORR and at 1.50 V for the ring electrode to detect generated peroxide. As shown in Figure 3d, Ni–N/C shows obvious decay of both the disk and ring currents in the first suggesting the of the pyrolyzed By contrast, NiPc MDE exhibits much stability without appreciable decay of the disk and ring currents during the Therefore, the molecularly dispersed and well-defined Ni–N4 sites in NiPc MDE a ORR catalyst for the 2e− reduction pathway than the pyrolyzed Ni–N/C. NiPc MDE be a catalyst system to establish the between the active structure and electrocatalytic Figure | Comparison of electrocatalytic ORR performance between NiPc MDE and Ni–N/C. (a) Schematic presentation of ORR with NiPc MDE and Ni–N/C. (b) Disk and ring currents of NiPc MDE and Ni–N/C in O2-saturated 0.1 M KOH Calculated peroxide yields of NiPc MDE and Ni–N/C. tests of NiPc MDE and Ni–N/C under the constant potentials of 0.50 V for the disk electrode and 1.50 V for the ring electrode. Download figure Download PowerPoint engineering of NiPc MDEs for ORR A of the MDE system is the of the of the active sites and the electrocatalytic performance through molecular we to further the performance of NiPc MDE for 2e− ORR for peroxide production with the introduction of groups to the Pc with Figure for molecular as an ORR catalyst by DFT calculations. The free-energy diagrams suggest to NiPc, NiPc–CN shows strong preference toward the 2e− reduction pathway, and the of *H2O2 over *O is from with NiPc to with NiPc–CN (Figure indicating that NiPc–CN be a more selective catalyst for 2e− ORR. Figure 4 | ORR electrocatalysis with NiPc–CN MDE. (a) Calculated free-energy diagrams of ORR through the 2e− and 4e− reduction pathways on NiPc and NiPc–CN at 1.23 V. shows the molecular structure of (b) Disk and ring currents of NiPc and NiPc–CN MDEs in O2-saturated 0.1 M KOH Calculated peroxide yields and n of NiPc and NiPc–CN test of NiPc–CN MDE under the constant potentials of 0.50 V for the disk electrode and 1.50 V for the ring electrode. Download figure Download PowerPoint NiPc–CN MDE was synthesized ( Supporting Information Figure and characterized with the RRDE The LSV curve shows more positive onset potential of NiPc–CN MDE V) than that of NiPc MDE (0.79 V) (Figure Moreover, the saturation current for NiPc–CN MDE in the potential range of V together with constant ring The peroxide yields of NiPc–CN MDE were calculated to be ∼92% from to 0.20 V, superior to NiPc MDE with peroxide yields below V (Figure The enhanced peroxide yields of NiPc–CN MDE are consistent with the preference toward the 2e− reduction pathway from DFT calculations with the introduction of (Figure NiPc–CN MDE also shows good stability in ORR with little decay in the disk and ring currents and the peroxide yields during the (Figure The performance of NiPc–CN MDE for oxygen reduction to peroxide is the for the reported and metal catalysts, high peroxide selectivity of ∼92% at a potential in conditions ( Supporting Information Table S2). by catalyst design with DFT calculations, we identify NiPc MDE as a good ORR catalyst for the 2e− reduction pathway and further enhance its performance through molecular The enhanced NiPc–CN MDE with shows superior selectivity with peroxide yield of ∼92% in the potential range of 0.70–0.20 V. The molecularly dispersed and well-defined Ni–N4 sites on CNTs NiPc MDEs with higher 2e− selectivities than the aggregated NiPc and pyrolyzed Ni–N/C catalysts. These results also that the MPc MDE system as electrocatalysts for the of the between active structures and electrocatalytic performances of molecular catalysts and Supporting Information Supporting Information is available and Figures and and of is no of to Information was supported by Guangdong-Hong Kong-Macao Joint Laboratory for Photonic-Thermal-Electrical Energy Materials and the from Zhejiang University, and Guangdong Provincial Key Laboratory of TEM and were measured with maintained by The were obtained with the of the Advanced Photon a Department of Energy of Science for the of Science by Argonne National Laboratory under The computational is supported from the for Computational Science and the in Energy Materials for It for to Zheng Wang Zhang Wang Pt on for O2 Reduction to H2O2 in Wang of Peroxide from and Jiang Wang Design for Electrochemical Oxygen Reduction toward H2O2 through in the Electrochemical of and by Zheng Zhou with

The effective utilization of clean energy and finding alternatives to fossil resources are highly important to ensure the sustainability of human society and are always among the major goals of both chemistry and material science research. Advanced electrochemical devices, such as fuel cells, water electrolysers and metal-air batteries, represent the most promising strategies for clean-energy utilization. In an electrochemical device, the redox reactions are spatially separated by a membrane, allowing direct extraction/transfer of electrons at an electrode-electrolyte interface, which leads to higher intrinsic energy conversion efficiencies, milder process conditions, easy product separation and excellent design features for coupling to renewable energy infrastructure. The performance of such electrochemical processes is fundamentally determined by the physicochemical properties of the electrochemical interfaces, encompassing both the electrocatalyst and the structure of the adjacent electrochemical double layer. Specifically, electrocatalysts play key roles in electrochemical reactions and often limit the performance of entire systems due to their insufficient activity, low durability or high cost. Ideally, the rate, efficiency, and selectivity of the above electrochemical reactions can be substantially improved by developing high-performance electrocatalyst. One of the central tasks for chemists and material scientists is to design and fabricate the high-efficient efficiency but low-cost electrocatalysts systems. The current promising electrochemical reactions mainly focus on the realization of the reversible conversion between chemical and electricity energy, e.g., the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen oxidation reaction (HOR), and hydrogen evolution reaction (HER). Coupling of the above electrochemical reactions provide a solid foundation for various essential electrochemical devices, such as direct hydrogen fuel cells (HOR + ORR); electrolysers (OER + HER); rechargeable zinc (Zn)-air battery (ORR + OER). Therefore, this thesis aims to design and synthesize high-performance electrocatalysts for HER, ORR and OER based on earth-abundant materials with proper hierarchical 2D or 3D nanostructures. Combined with the advanced characterization techniques and density functional theory (DFT) calculations, the relationship between the electrochemical activity and active sites of these earth-abundant electrocatalysts were detailedly explored and confirmed. Furthermore, to emphasize the hierarchical 2D or 3D nanostructures, the actual performance of these electrocatalysts was all evaluated in practical devices including Zn-air battery and proton exchange membrane fuel cell (PEMFC), specifically as follows: (1) The vast majority of the reported HER electrocatalysts performs poorly under alkaline conditions due to the sluggish water dissociation kinetics. In the first work, a hybridization catalyst construction concept is presented to dramatically enhance the alkaline HER activities of catalysts based on 2D transition metal dichalcogenides (TMDs) (MoS2 and WS2). A series of ultrathin 2D-hybrids are synthesized via facile controllable growth of 3d metal (Ni, Co, Fe, Mn) hydroxides on the monolayer 2D-TMD nanosheets. The resultant Ni(OH)2 and Co(OH)2 hybridized ultrathin MoS2 and WS2 nanosheet catalysts exhibit significantly enhanced alkaline HER activity and stability compared to their bare counterparts. The combined theoretical and experimental studies confirm that the formation of the heterostructured boundaries by suitable hybridization of the TMD and 3d metal hydroxides is responsible for the improved alkaline HER activities because of the enhanced water dissociation step and lowers the corresponding kinetic energy barrier by the hybridized 3d metal hydroxides. (2) Nitrogen-coordinated iron atoms on carbon matrix (Fe-N-C) materials are the most active Pt-group-metal-free ORR catalysts but still suffering their low stability and relatively lower activity compared to platinum-based materials. In the second work, Fe and Ni dual sites atomically dispersed in hierarchically ordered macroporous carbon support (Fe-Ni/N-HOMC) was designed and successfully prepared. Isolated atomic Fe- N4 and Ni-N4 active sites were confirmed via various characterizations. The ORR activity and stability of Fe-Ni/N-HOMC in both acid and alkaline electrolyte were much higher than commercial Pt/C and the mono-Fe doping counterpart, which was among the state-of-the-art ORR electrocatalysts. In addition, this 3D ordered interconnected macroporous structure with abundant mesopores and micropores could greatly increase the accessible ORR active site and also enhance the mass transport during the ORR process. When employed as cathodes for PEMFC, we found the excellent ORR activity of Fe-Ni/N-HOMC was completely translated to the cathode in the fuel cell. (3) High-performance bifunctional electrocatalysts with ORR and OER activity is the key to developing efficient rechargeable Zn-air batteries. In the third work, a high-performance bifunctional electrocatalysts for both OER and ORR were synthesized via further hybridizing as-prepared Fe-Ni/N-HOMC with NiFe layer double hydroxides (LDHs). Layered double hydroxides (LDHs) have been reported to be promising OER electrocatalysts with ultrahigh OER performances. The as-synthesized new composites exhibited almost the same ORR activity as Fe-Ni/N-HOMC, revealing that hybridization of NiFe-LDHs would not deteriorate the initial ORR activity. Moreover, the remarkable enhancement of OER activity was observed after the hybridization, which was attributed to the strong coupling of uniformly dispersed small NiFe-LDH nanoparticles with the carbon substrate. The prototype Zn-air battery was assembled using these new composites, which displayed the ultralow voltage gap and long-term stability. (4) Compared with Fe-N-C or Co-N-C based ORR electrocatalysts, the Cu-nitrogen-carbon composites were attracted little attention. However, the natural multicopper oxidases (MCOs) enzymes, such as laccase, can serve as efficient ORR catalyst with almost no overpotential. Inspired by their tris-copper centers in MCO, one novel Cu-nitrogen-carbon composite (Cu SAs/N-CS) with atomic Cu coordination sites were synthesized via the pyrolysis of the Cu-involved metal-organic-framework. The copper contents in Cu SAs/N-CS reaches as high as 3.17 wt.%, and the average distances of adjacent copper sites was around only 3.1 Å. Due to the synergetic effect of abundant single atomic copper active sites with closer distance and ultrathin carbon nanosheet structure, Cu SAs/N-CS exhibited superior ORR activity exceeding commercial Pt/C catalyst, methanol tolerance, and long-term stability in both alkaline and neutral electrolyte. In summary, four kinds of new composites were successfully designed and prepared as high-performance electrocatalysts for HER, ORR and OER. Multi-dimensional heterostructures, atomic metal coordination sites and 3D hierarchically porous structure were designed and observed, which contributed greatly to improve activities of these composites. This thesis suggests several new viewpoints in the design of electrocatalysts based on earth-abundant materials: (i) offering new strategies for the preparation of novel 2D and 3D heterostructures as electrocatalysts; (ii) expanding methods for the synthesis of atomic metal coordination sites and evaluating their activities for ORR; (iii) evaluating the practical performances of achieved electrocatalysts in proton exchange membrane fuel cell and Zn-air battery; (iv) attempting to explain reaction mechanisms of some electrocatalysts by DFT calculation.

Pyrolyzed Fe/N/C catalyst is a promising non-precious-metal catalyst (NPMCs) for oxygen reduction reaction (ORR) in fuel cells, and substantial progresses have been made recently in this field. However, to meet the requirement of practical applications, it still needs to improve significantly the ORR activity and stability of Fe/N/C catalyst, and to reveal active site structure for rational design of NPMCs. Herein, we report our recent progresses in preparation of high performance Fe/N/C catalyst and investigation of the active sites. The Fe/N/C catalyst was prepared by using precursor of poly-m-phenylenediamine (PmPDA), and exhibits an ORR performance of 12.8 A/g@0.80V in acid solution with H2O2 yield less than 1%.[1] When it was applied in proton exchange membrane cell (PEMFC), a peak power density could reach 0.81 W cm-2. We further improved the catalytic activity of PmPDA-Fe/N/C by co-doping of S element. The peak power density has exceed 1.0 W cm- 2. [2] The active sites of Fe/N/C catalyst are mostly located in micropores, so they are susceptible to water flooding. To this point, surface fluorination was applied to the Fe/N/C catalysts (such as the modification of Ar-CF3). The fluorinated Fe/N/C could perform stably over 120 h at 0.5 V with a current density of 0.56 A cm-2 in a H2-O2 PEMFC.[3] Alternatively, hydrophobic dimethyl silicon oil (DMS) was introduced into Fe/N/C catalyst layer, which leads to form triple-phase interface in micropores. As a result, Fe/N/C-based direct methanol fuel cell (DMFC) could yield a performance close to that of Pt-based DMFC.[4] We developed a molecule/ion probe method to study the active sites. It has revealed that the ORR activity of PmPDA-Fe/N/C is not sensitive to CO and NOx, but can be suppressed significantly by halide ions (e.g., Cl−, F−, and Br−) and low valence state sulfur-containing species (e.g., SCN−, SO2 , and H2S).[1] This indicates that the active sites of the Fe/N/C catalyst contains Fe element in acid medium. We further designed a single-atomic-layer Fe/N/C model catalyst based on monolayer graphene (FeNMLG), and systematically investigated the effects of defect density, the number of graphene layers, and the doped nitrogen species on the ORR activity.[5] It has demonstrated also that such Fe/N/C model catalyst is also fit for in situ vibration spectroscopic studies. Acknowledgement: This study was supported by grants from National Key Research and Development Program of China (2016YFB0101202), and NSFC (91645121 and 21621091).

Two-Dimensional Metal–Organic Frameworks with Unique Oriented Layers for Oxygen Reduction Reaction: Tailoring the Activity through Exposed Crystal Facets

Fe nanoparticles encapsulated in doped graphitic shells as high-performance and stable catalysts for oxygen reduction reaction in an acid medium

Nitrogen-doped non-precious metal catalysts show high oxygen reduction reaction (ORR) activity, which are considered as possible alternatives for fuel cell catalysts. It has been reported that the transition metals and nitrogen doped carbon complex shows outstanding performance of ORR activity, suggesting that metal impurities in carbon nanotube play a very important role to the ORR activity. [1] On the other hand, recently the ORR active sites in N—doped carbon materials are carbon atoms with Lewis basicity next to pyridinic N was proposed. [2] However, the mechanism and the ORR active site are still in controversy. In our previous work, we reported that annealing multi-walled carbon nanotubes (MWCNT) with nano-drilled defective structure in Argon atmosphere could reach high ORR activity. [3] We suggest that the edge of defects is essential for the formation of active site. In order to clarify the active site and further enhance the ORR activity, here, by making defective edges on the MWCNT structure accompanied with nitrogen doping, we showed that the ORR activity has been improved with onset potential reached up to 1.1V vs RHE in 0.1M KOH and 0.88V vs RHE in 0.1M HClO4. The MWCNT used were provided by Showa Denko KK Japan (VGFX-XA, diameter: 15nm, approximate length: 1μm; containing Fe impurities <1 wt%). As a precursor, functionalized and purified MWCNT was obtained by heat and acid treatment in the mixture of H2SO4/HNO3. [3] Defective MWCNTs were then prepared following our previous report by nano-drilling purified MWCNT using CoOx as oxidation catalyst. [3,4] Then defective MWCNTs were nitrogen doped in 10% NH3/ Argon at 900℃. The ORR activity of nitrogen doped defective MWCNT in acid was shown in Fig.1 and in alkaline was shown in Fig.2. We found that nitrogen doped defective MWCNTs show very high ORR activity and efficiently doping nitrogen to the defective edge is essential for the enhancement of ORR activity. The optimization of nitrogen doping process and the nitrogen contain along with the role of defect structure on the ORR activity will be discussed further through detailed characterizations with X-ray photoelectron spectroscopy, Temperature Programmed Desorption and Raman Spectroscopy. Acknowledgement This work partially was supported by COI STREAM from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) and Shin-Etsu Chemical Co. ,Ltd. Japan. Their contribution is greatly appreciated. Reference [1] Li, Yanguang, et al. "An oxygen reduction electrocatalyst based on carbon nanotube-graphene complexes." Nature nanotechnology 7.6 (2012): 394-400. [2] Guo, Donghui, et al. "Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts." Science 351.6271 (2016): 361-365. [3] K. Waki, R. A. Wong, H. S. Oktaviano, T. Fujio, T. Nagai, K. Kimoto and K. Yamada, Energy Environ. Sci., 2014, 7, 1950-1958. [4] Oktaviano, Haryo S., Koichi Yamada, and Keiko Waki. "Nano-drilled multiwalled carbon nanotubes: characterizations and application for LIB anode materials." Journal of Materials Chemistry 22.48 (2012): 25167-25173. Figure 1

Pyrolyzed platinum group metal free (PGM-free) oxygen reduction reaction (ORR) electrocatalysts are a promising class of earth-abundant materials for low-temperature polymer electrolyte fuel cell (PEFC) cathodes. Understanding of the atomic scale structure of PGM-free ORR active sites remains a key focus of research efforts with the aim of improving performance of these materials. The pyrolysis process leads to highly heterogeneous catalyst systems which complicates direct active site study. In particular, (i) understanding how these active sites give rise to ORR activity, (ii) how they interact with the environment during material degradation, (iii) their interactions with probe molecules for the purposes of active site quantification, and (iv) their spectroscopic signatures are all important aspects governed by atomic scale structure. Through the use of density functional theory (DFT), we investigate these four structure-function relationships. ORR activity, one of the greatest challenges faced by PEFCs, is explored via several binding energy parameterized descriptor models. These models include the computational hydrogen electrode (CHE) model which yields thermodynamic limiting potentials, and the linearized Gibbs energy relationship (LGER) model which yields reversible potentials for reaction steps. Additionally, kinetics of OOH bond dissociation via nudged elastic band (NEB) DFT calculations are also considered as this has been proposed by some groups to be rate determining for pathways that include binding to local C sites. Combined, these models give insight into how local arrangement of atoms affects reaction pathways and enables exploration of varied reaction pathways on and local to the active site. Durability of PGM-free electrocatalysts remains a key challenge in these materials. While a variety of degradation mechanisms have been proposed, the relation between environment and degradation mechanism has not been firmly established, complicating any mitigation strategies that might be applied. Through the use of an automated ab initio molecular dynamics (AIMD)-based model, the kinetics of bond breaking local to the active site is explored. Resulting degraded structures can then be studied via the activity models previously mentioned to show how such degradation affects calculated ORR activity descriptors. Additionally, the use of reaction rate models applied to activity loss curves can also enable some discrimination of degradation mechanism indicating either a single autocatalytic process or two mechanisms with different temporal behavior. Another issue faced by PGM-free electrocatalysts is the lack of a method for quantifying the number of ORR active sites. Unlike Pt-based systems where integration of charge transferred in the H adsorption/desorption region can give information about density of active sites, no such method has been fully established for determining the number of PGM-free active sites, a value required to deconvolute turn over frequency from ORR current densities. Promising approaches include use of probe molecules that, if bound to ORR active sites (and only ORR active sites) could provide insight into how many active sites are in a given sample. This is highly dependent on the specificity of binding for these molecules which can be explored via binding energy calculations with DFT. Finally, DFT can also be a valuable tool for understanding spectroscopic signatures of highly ORR active materials. The main issue faced in such studies is focusing on just the active sites which generally requires considering changes in the Fe states, particularly for so called atomically dispersed catalysts that exhibit the highest ORR activities to date. X-ray adsorption spectroscopy (XAS, in particular XANES and EXAFS) can give information about local structure and the state of Fe with and without probe molecules. DFT and related theoretical approaches can simulate these spectra as a function of local environment. Vibrational calculations with and without probe molecules give insight into Fe-specific nuclear resonance vibrational spectroscopy (NRVS). Calculation of energy density at the Fe nucleus and electric field gradient gives input regarding isomer shift and quadrupole splitting, giving much needed interpretation of Mössbauer spectroscopy, especially in instances where no standard exists or changes in measured parameters with ligands/probes are required. Combined, these coupled experimental/theoretical approaches give insight into the nature of active sites in PGM-free systems. In particular, in this contribution we will focus on the presence of spontaneously evolved ligands that, using DFT, are shown to be likely contributors to electrocatalyst activity in-situ and experimental signatures thereof.

The activity of oxygen reduction reaction (ORR) catalysts often determine performance of polymer electrolyte fuel cells (PEFCs) as measured by their power output, open circuit voltage, and fuel conversion efficiency. Currently, Pt-nanoparticle catalysts, either supported on high surface-area carbons or prepared in a form of contiguous thin layer on conductive or non-conductive supports, represent the state of the art in ORR electrocatalysis at the PEFC cathode. However, the high and variable price and scarceness of Pt have limited its widespread implementation in the -temperature fuel cells to date, especially in automotive transportation. Under these circumstances, platinum group metal-free (PGM-free) ORR catalysts have received growing attention in recent years as a possible replacement for Pt-based formulations. The progress achieved since the development of the first nature-inspired electrocatalysts of oxygen reduction in the seminal work by Jasinski in the 1960s (Nature 201, 1212, 1964), which mostly happened through the broad implementation of the high-temperature synthesis approach, makes replacement of Pt in ORR electrocatalysts with earth-abundant elements, such as Fe, Co, N, and C, a realistic possibility. In this this presentation, we will summarize recent progress in research targeting development of high-performance PGM-free catalysts for oxygen reduction reaction (ORR) at Los Alamos National Laboratory. Two approaches will be discussed in a greater detail: (i) the approach involving fine-tuning of the catalyst porosity and surface area using pore-forming compounds and (ii) the method specifically focusing on the development of atomically dispersed transition metal moieties and avoiding the formation of transition metal-rich nanoparticles during the heat treatment of catalyst precursors. We will demonstrate the impact that porosity/surface area optimization, through the use of either pore formers (cyanamide, ZnCl2) precursor templating (metal organic frameworks) or both, can have on the fuel cell performance of PGM-free catalysts. We will also show how modifications to the electrode structure through the change in the ionomer content and ionomer equivalent weight can lead to substantial improvements in the fuel cell performance at both low- and high current densities (aerial power density of more than 0.50 W/cm2 in H2-air testing at 80°C). Finally, we will recapitulate the challenges still facing PGM-free research that, in spite of all the progress achieved in recent years, is yet to produce materials capable of competing with the incumbent Pt-based catalysts in terms of oxygen reduction activity, performance durability, and cost (specifically, when extended to the overall cost of a fuel cell stack). Acknowledgement Financial support for this research by DOE-EERE through Fuel Cell Technologies Office is gratefully acknowledged.

Non‐Precious‐Metal Oxygen Reduction Reaction Electrocatalysis

Non-precious metal catalysts (NPMCs) synthesized from earth-abundant elements (Fe, Co, N, C) have the potential to efficiently generate clean electrical energy by harnessing the chemical energy stored in fuels via direct electrochemical conversion. Recent breakthroughs in the synthesis of high-performance NPMCs for the oxygen reduction reaction (ORR), demonstrating high volumetric activity and low H2O2 yield [1,2], suggest that NPMCs can potentially replace scarce and expensive Pt-based cathode catalysts and help the odds for successful commercialization of polymer electrolyte fuel cells (PEFCs) by significantly reducing the cost.State-of-the-art NPMCs are typically synthesized via high-temperature treatment of highly heterogeneous precursors [1,3,4], which makes the understanding of the ORR active site formation a difficult task. Knowledge about the origin of the NPMC activity is imperative for the successful design of improved catalysts that could eventually replace Pt-based formulations. In this project, controlled functionality of NPMCs for ORR was attempted through an experimental investigation of graphene and graphene-oxide-based model systems, combined with advanced theory, modeling and simulation.Model systems based on 2D structures, such as graphene and graphene-oxide (GO), are more homogeneous than typical NPMCs obtained using the high-temperature approach. In addition to well-defined morphology, such systems possess additional attractive properties, such as enhanced chemical stability and excellent conductivity. In the presented approach, successful incorporation of nitrogen heteroatoms into the graphitic structures of the starting precursors was obtained via ammonia treatment at varied temperatures (500°C-900°C). Efficient oxygen reduction with low H2O2 yield was achieved from model systems with low concentrations of Fe and Mn (less than 0.5%).Density functional theory (DFT) and ab initio molecular dynamics were used to characterize the structural properties and activity of possible active sites in NPMCs [5]. The work focused on the active-site molecular configuration, surface accessibility, sensitivity to N and Fe chemical potentials, response to an aqueous environment, and adsorption of ORR intermediates. Structurally, particular attention was paid to the clustering tendencies of different N-coordinated Fe structures in order to illuminate the configuration of the FexNy centers thought to occur at graphene edges. It was found that, depending on the synthesis conditions dictating Fe and N chemical potentials, either 4N or 3N coordinated Fe structures should be the most stable Fe-containing defects studied thus far. The inter-edge Fe structures are considerably higher in energy than these defects and their existence is postulated to be thermodynamically prohibited. Clustered FeN3 structures (Fe2N5) seem to excel at cleaving the O2 bond with zero-barrier and thus are likely to follow a dissociative pathway for ORR. This pathway is possibly more selective, avoiding evolution of H2O2 but at the cost of overbinding ORR intermediates. Ligands attached to such sites lead to tuning of intermediate binding energies and can significantly improve calculated site ORR activity. Ab initio molecular dynamics simulations indicate that such ligands may form spontaneously on these under-coordinated sites through water dissociation and binding of *OH (Figure 1). Solvent does not appear to affect the stability of these edge defects otherwise. Acknowledgment Financial support from the Los Alamos National Laboratory Laboratory-Directed Research and Development (LDRD) Program and DOE-EERE Fuel Cell Technologies Office is gratefully acknowledged. References Wu, G., More, K. L., Johnston, C. M., Zelenay, P., Science, 332, 443-447 (2011).Bashyam, R., Zelenay, P., Nature, 443, 63-66 (2006).Chung, H. T., Johnston, C. M., Artyushkova, K., Ferrandon, M., Myers, D. J., Zelenay, P., Electrochem. Commun., 12, 1792-1795 (2010).Wu, G., Nelson, M., Ma, S. G., Meng, H., Cui, G. F., Shen, P. K., Carbon, 49, 3972-3982 (2011).Holby, E. F., Wu, G., Zelenay, P., Taylor, C. D., J. Phys. Chem. C, to be submitted (2014).

The great challenge of boosting the oxygen reduction reaction (ORR) activity of non-noble-metal electrocatalysts is how to achieve effective exposure and full utilization of nitrogen-rich active sites. To realize the goals of high utilization of active sites and fast electron transport, here we report a new strategy for synthesis of an iron and nitrogen co-doped carbon nanolayers-wrapped multi-walled carbon nanotubes as ORR electrocatalyst (N-C@CNT-Fe) via using partially carbonized hemoglobin as a single-source precursor. The onset and half-wave potentials for ORR of N-C@CNT-Fe are only 45 and 54 mV lower than those on a commercial Pt/C (20 wt.% Pt) catalyst, respectively. Besides, this catalyst prepared in this work has been confirmed to follow a four-electron reaction mechanism in ORR process, and also displays ultra-high electrochemical cycling stability in both acidic and alkaline electrolytes. The enhancement of ORR activity can be not only attributed to full exposure and utilization of active site structures, but also can be resulted from the improvement of electrical conductivity owing to the introduction of CNT support. The analysis of X-ray photoelectric spectroscopy shows that both Fe–N and graphitic-N species may be the ORR active site structures of the prepared catalyst. Our study can provide a valuable idea for effective improvement of the electrocatalytic activity of non-noble-metal ORR catalysts.

Graphdiyne Electrocatalyst

Introduction Fe/N/C catalyst is one of the most studied non-precious metal catalysts for oxygen reduction reaction (ORR). It usually prepared by heat treatment of precursor, such as Fe-N containing polymers or chelates, with or without carbon support. The relation of heat treatment time and ORR activity in Fe/N/C catalyst has been studied from some aspects of view. For example, Kamiya et al. reported existence of an optimum heat treatment time, 45s, investigating catalyst prepared by heat treatment of Fe–pentaethylenehexamine complex and graphene oxide under Ar [1]. It is proposed that Fe-N active sites are initially formed on graphene oxide and then decomposed during heat treatment resulting in the existence of an optimum heat treatment time. In this study, investigating catalyst prepared by heat treatment of hemin, a Fe-porphyrin derivative, and carbon black support under Ar, we unveil a dynamic fluctuation of activity and Fe-N active site content with more than one local maximum with the increase in heat treatment time. These behaviors can be explained by a reaction model considering multiple types of Fe-N active site with individual formation and decomposition rate constant. It could be a support for existence of multiple types of Fe-N active site responsible for ORR activity and gives us a comprehensive understanding how net ORR activity is achieved by constituent active site formed and decomposed during heat treatment. Experimental For preparing catalyst, 200 mg of hemin was mixed with 200 mg of carbon black (Ketchen Black 600JD, Lion Corp.) in dimethylformamide. After ultrasonicating the solution for 30 mins, the dried mixture was obtained by a rotary evaporator and heat treated under Ar by different patterns of heat treatment time: 45s, 100s, 5m, 30m, 2hrs, 5hrs or 10hrs at temperature of 750°C. Finally, the mixture was acid washed in 2M H2SO4(aq). Catalyst's ORR activity was evaluated in 0.5M H2SO4 (aq) by rotating disk electrode (RDE) experiments. Catalyst was coated on a glassy carbon working electrode with the area of 0.2826 cm2 to be 0.57mg/cm2 catalyst loading. Pt and reversible hydrogen electrode (RHE) generated using the same electrolyte were used as the counter and reference electrode respectively. The rotating speed of the working electrode was set to 1500 rpm. Kinetic current density, ik, is compared, where ik = iL*i/(iL -i). Here, i is observed current density at 0.8V and iL is diffusion limit current density defined as current density at 0.4 V vs. RHE. X-ray photoelectron spectroscopy (XPS, PHI 5000 Versaprobe) measurements were performed. Fe 2p3/2 spectrum is observed for estimation of Fe-N active site content in catalyst, and C1s spectrum was collected to calibrate Fe 2p3/2 peak intensity. Results and Discussion Figure.1a shows the Fe 2p spectrum and its fitting result for the sample with heat treatment time of 5 hrs.The spectrum was well fitted and separated into two peaks originated from Fe in hemin-like structure and Fe-N active site formed on carbon support. Peak position and FWHM of Fe in hemin-like structure are set to 711.25 eV and 5.1 eV, respectively. Fe-N active site peak position and FWHM were set free to move through peak separation fitting to collectively represent several possible types of Fe-N active site structures, FeN2, FeN4 or FeN3, as one peak. Its peak position and FWHM exist in the range of 709.0-710.2eV and 2.15-3.06 eV, respectively. Fig. 1b shows the behavior of activity and content of Fe-N active site normalized by that of C. We can see more than one local maximum in activity with the increase in heat treatment time. One local maximum is located around 5m and another exists around 5hrs. Only by investigating over such a wide range of heat treatment time, we can realize existence of multiple local maximums. The discrepancy for the position of the first local maximum with that of Kamiya et al. [1] may come from difference in choice of precursor. Fe-N active site content shows similar behavior to that of activity, which can be fitted by two curves with the expression of A[exp(-kf*t)-exp(-kd*t)]. We attribute them to formation and decomposition of two different types of Fe-N active site, (Fe-N)α and (Fe-N)β with individual formation rate, kf, and decomposition rate, kd. It could be a support for existence of more than one type of Fe-N active site responsible for ORR activity. Detailed reaction model and structure of two active sites will be discussed in session.

To overcome the barrier of high cost caused by the exclusive use of Pt-based catalysts, the development of non-precious metal catalysts (NPMCs) to replace Pt in polymer electrolyte membrane fuel cells has become the goal of intensive research in recent years.1,2 Some of these NPMCs have shown remarkable catalytic activity towards ORR. Among these NPMCs, carbon-supported transition metal/nitrogen ( M/N/C, M = Fe, Co, Mn, etc.) materials have gained increased attention due to their promising catalytic activity and high durability.3-5 However, the role of transition metal playing in the catalysts’ active sites is still a subject of controversy. In order to further clarify the nature of the active sites of NPMCs, in this, with Co(SO4)7H2O as the metal precursor and N,N-bis (salicylidene) ethylenediamine (Salen) as the nitrogen precursor, carbon-supported non-precious metal catalysts, Co-Salen/C, were synthesized using a facile thermal annealing approach. The catalysts were heat-treated at different temperatures (from 600- 1000oC) to optimize oxygen reduction reaction (ORR) activity. To clarify the significance of metal (Co) content for the ORR enhancement, the catalyst was further processed by acid leaching. The electrocatalytic activity and electron transfer mechanism were demonstrated in oxygen-saturated alkaline electrolyte by cyclic voltammetry (CV), linear sweep voltammetry (LSV) as well as rotating disk electrode (RDE) techniques. Scanning electron microscope-energy dispersive spectrometer (SEM-EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) measurements were used to identify the structure and composition of the catalysts.