Active Site Structures in Nitrogen-Doped Carbon-Supported Cobalt Catalysts for the Oxygen Reduction Reaction.

The formation of active sites in the molybdenum-zeolite catalyst for methane dehydroaromatization was studied by the density functional theory method. The interaction of MoO2(OH)2 particle with the Bronsted site, anionic site, and electron hole of the zeolite was studied. The mechanism governing the formation of mononuclear active sites was proposed. It was shown that the formation of the MoO2 mononuclear active site with participation of electron hole of the zeolite is thermodynamically possible and is accompanied by electron density transfer from zeolite oxygen atom to molybdenum atom.

We examine the influence of precursor functional groups on the formation and electrocatalytic performance of iron ion-chelating ordered mesoporous carbon (Fe-OMC) fuel cell catalysts. First, we study whether the active sites in these catalysts consist of Fe–Nx or Fe–Ox chelates. To verify this, catalysts were prepared from two different molecular precursors (furfurylamine and furfuryl alcohol) and the functional groups’ (−NH2 vs −OH) role in the formation of iron ion-chelating active sites in the catalysts was established. From electrochemical tests and EPR spectroscopy, conspicuously different behaviors were obtained for the catalyst prepared from furfurylamine compared to that prepared from furfuryl alcohol. It was unambiguously established that the amine group is central to the formation of electrocatalytically active sites in Fe-OMC catalysts and that these are of the Fe–Nx-OMC type. Additional Fe-OMC catalysts were prepared with the purpose to determine the influence of the two precursors on the formation of the carbon matrix. By complementing the furfurylamine with the more readily polymerizing furfuryl alcohol and using a mixture of the two as precursor solution in the synthesis, an overall improvement over the pure furfurylamine was achieved. The mixture gave a catalyst with a larger pore volume and surface area, a higher conductivity, and a higher oxygen conversion rate.

Host-guest interactions promoted formation of Fe-N4 active site toward efficient oxygen reduction reaction catalysis

The high cost platinum (Pt) catalyst used for the electrocatalysis of oxygen reduction reaction (ORR) is one of the bottlenecks for the widespread deployment of polymer membrane electrolyte fuel cells (PEMFCs). Fe-N-C catalysts, produced by the pyrolysis of iron, nitrogen and carbon precursors together, have been found as one of the promising alternatives to replace Pt with abundant elements and low cost.1-3 The Fe-N coordination structure bonded with carbon, generated during high-temperature pyrolysis, has been identified as the active site for the high ORR activity of Fe-N-C catalysts. However, it remains a grand challenge to prepare Fe-N-C catalysts with abundant Fe-N active sites to achieve high ORR activity, since undesired inactive species (e.g., Fe/Fe3C) are easily obtained during high-temperature pyrolysis due to the improper design and the poor chemistry control in the integration of all precursors, lowering the active sites density of catalysts. We have developed an approach to synthesize Fe-N-C catalysts with exclusively atomic Fe-N active sites instead of inactive Fe agglomeration by using well-defined Fe-containing metal-organic frameworks (MOFs) precursors.4 The morphology of MOF precursors can be directly transferred to Fe-N-C catalysts with the retained and homogenous morphology after pyrolysis. Such well-defined precursors and resulting homogenous catalysts allow us to precisely control and tune the composition and morphology of final Fe-N-C catalysts to understand how the property change of catalysts impact their ORR activity of catalysts. In this presentation, we will discuss the effect of particle size of catalysts on their ORR activity and the critical role of pyrolysis temperature on the formation of Fe-N active sites. The Fe-N-C catalysts with size from 20 nm to 1000 nm are prepared by adjusting the size of MOF crystals in the precursor synthesis. Similar to Pt nanoparticles, the unique size control of the Fe-N-C catalysts enables us to increase the accessible number of Fe-N active sites for ORR. The 50 nm catalyst shows the best ORR activity with a half-wave potential of 0.85 V vs. RHE, only leaving 30 mV gap with Pt/C (60 µgPt/cm2) in 0.5 M H2SO4 along with the excellent stability. When the particle size of catalyst is reduced to 20 nm, significant agglomeration of particles are found in the catalyst, resulting in ORR activity decrease of the catalyst. Using our homogenous model catalysts, the formation of active sites during pyrolysis is investigated by correlated the measured-ORR activity with the bonds change of precursors at various pyrolysis temperature. 800 oC is found to be the critical temperature to form the Fe-N active sites with notable ORR activity, which is related to the generation of new Fe species likely bonded with pyridinic N in the carbon structure. These high performance Fe-N-C catalysts exhibit a promising potential to replace Pt for ORR in future PEMFCs. (1) Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P. Science 2011, 332, 443. (2) Zhang, H.; Osgood, H.; Xie, X.; Shao, Y.; Wu, G., Nano Energy, 2017, 31, 331-350. (3) Wu, G.; Santandreu, A.; Kellogg, W.; Gupta, S.; Ogoke, O.; Zhang, H.; Wang, H.-L.; Dai, L., Nano Energy, 2016, 29, 83–110. (4) Zhang, H.; Hwang, S.; Wang, M.; Feng, Z.; Karakalos, S.; Luo, L.; Qiao, Z.; Xie, X.; Wang, C.; Su, D.; Shao, Y.; Wu, G. Journal of the American Chemical Society, 2017, 139, 14143–14149.

We have been engaged in development of group 4 and 5 oxide-based compounds as non-PGM cathodes for polymer electrolyte fuel cells because of their high stability in acidic and oxidative atmosphere. Recently, we focused on group 4 oxide-based compounds such as titanium and zirconium oxides. From theoretical consideration, these oxides might have different active sites. Theoretical consideration revealed that the active sites of zirconium and titanium oxide-based cathodes are stabilized oxygen vacancies and complex site (doped metal and lattice oxygen), respectively. Thus, we need to perform the experiments to clarify the active sites.With respect to zirconium oxide-based compounds, we prepared zirconium oxide-based electrocatalysts from oxy-zirconium phthalocyanine with multi-walled carbon nanotubes (MWCNTs) as electroconductive supports via heat-treatment under a low oxygen partial pressure. We evaluated the density of the active sites, i.e., oxygen vacancies, using X-ray photon spectroscopy, the average particle size of the oxide particles using transmission electron microscope (TEM), and the surface area utilization of the oxide particles using TEM image analysis. We successfully determined that the increase in the oxygen reduction reaction (ORR) activity was due to the increase in the density of oxygen vacancies and the effective surface area, and that the decrease in the effective surface area was responsible for the decrease in the ORR activity. According to the experimental results, the active sites of the zirconium oxide-based catalysts might be oxygen vacancies stabilized by nitrogen doping or nano-sizing.With respect to titanium oxide-based compounds, we investigated factors affecting the use of niobium-doped titanium oxides for the ORR. MWCNTs were used as support to maintain sufficient electrical conductivity. Nb-doped titanium oxide/MWCNTs prepared by hydrolysis was heat-treated at the desired temperature for a certain time under argon containing 4% hydrogen to investigate the relationship between ORR activity and physicochemical properties such as crystalline structure and electronic state. We confirmed that the density of the low-valence state of titanium ions, Ti3+, affected ORR activity. However, theoretical consideration indicated that Ti3+ located at the top surface of the oxides must be adsorbed by oxygen molecule strongly to form TiO2. This means that the Ti3+ could not be active sites. On the other hand, according to a good relationship between the cell volume of anatase phase in the catalyst and the ORR current at 0.7 V, we discovered that crystalline distortion of the anatase phase might produce Ti3+ on the surface and lead to ORR activity. This means that the active sites of the titanium oxide-based catalysts might be crystalline distortion by niobium doping or phase transition. Both theoretical consideration and experimental results revealed that the active sites of zirconium and titanium oxide-based cathodes are stabilized oxygen vacancies and complex site (doped metal and lattice oxygen), respectively. This difference affected the strategy of the catalyst design for titanium and zirconium oxide-based cathodes in order to enhance their ORR activities.

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.

To reduce greenhouse gases (GHG) emission, we need to move away from fossil-based energy sources. Electrochemical energy conversion systems (EECSs), like fuel cells, are broadly considered to be alternative systems to the currently dominant internal combustion energy-generation systems (ICESs). Among several types of fuel cells, polymer electrolyte fuel cells (PEFCs) are the most suitable one for vehicle applications. Nanoparticle platinum (Pt) catalysts are now used as state-of-the-art catalyst for the PEFCs. However high price, scarcity, and monopolized global distribution of Pt place significant limitations on these energy conversion systems. Particularly oxygen reduction reaction (ORR) at the cathode is inherently slower by six orders of magnitude than hydrogen oxidation reaction at the anode, thus requiring higher Pt loading. Platinum group metal free (PGM-free) ORR catalyst development thus has been a continuous research theme for several decades by many research groups. Metal-nitrogen-carbon (M-N-C) type catalysts have demonstrated the highest activity and durability among several types of PGM-free catalysts. Recently there has been a significant improvement in ORR activity, however further improvement in activity is still needed to compete with Pt catalyst. Furthermore good durability of these catalysts has not been demonstrated. Understanding of active site that has not been clarified yet is a core for solving these issues. Recently we directly observed FeN4 moiety in (CM+PANI)-Fe-C catalysts [1]. If FeN4 is an active site for ORR, increasing the number of this moiety, i.e., making atomically dispersed (AD) Fe, will be a pathway for improving the ORR activity. Thus to achieve high density of FeN4, we synthesized fiber-type zeolitic imidazolate framework (ZIF-F) as a precursor for (AD)-Fe-N-C catalysts (Fig. 1 (a)), in which FeN4 structure already exist. Heat-treatment converts this ZIF-F into fibrous N-doped carbons (Fig. 1(b). Fe atoms are dispersed atomically without aggregation in the fibrous N-doped carbons (Fig. 1(c)). Importantly, electron energy loss spectroscopy (EELS) demonstrates that N is cordinated to the Fe atoms (Fig. 1(d)). Thus we could construct FeNx moieties within the (AD)-Fe-N-C catalysts. In this presentation, we will present the activity and durability of this (AD)-Fe-N-C catalyst in rotating disk electrode (RDE) and fuel cells in conjunction with diverse analysis tools. This will give some insights for the nature of activity/durability for M-N-C type ORR catalysts. Hoon T, Chung, David A. Cullen, Drew Higgins, Brian T. Sneed, Edward F. Holby, Karren L. More, Piotr Zelenay, “Direct atomic-level insignt into the active sites of a high-performance PGM-free ORR catalyst”, Science, 357, 479 (2017). Acknowledgments This research is supported by DOE Fuel Cell Technologies Office, through the Electrocatalysis Consortium (ElectroCat). Figure 1

The objective of this project is to develop novel non-precious metal electrocatalysts for oxygen reduction reaction (ORR), and demonstrate the potential of the catalysts to perform at least as good as conventional Pt catalysts currently in use in polymer electrolyte membrane fuel cell (PEMFC) with a cost at least 50 % less than a target of 0.2 g (Pt loading)/peak kW and with durability > 2,000 h operation with less than 10 % power degradation. A novel nitrogen-modified carbon-based catalyst was obtained by modifying carbon black with nitrogen-containing organic precursor in the absence of transition metal precursor. The catalyst shows the onset potential of approximately 0.76 V (NHE) for ORR and the amount of H2O2 of approximately 3% at 0.5 V (NHE). Furthermore, a carbon composite catalyst was achieved through the high-temperature pyrolysis of the precursors of transition metal (Co and Fe) and nitrogen supported on the nitrogen-modified carbon-based catalyst, followed by chemical post-treatment. This catalyst showed an onset potential for ORR as high as 0.87 V (NHE), and generated less than 1 % of H2O2. The PEM fuel cell exhibited a current density of 2.3 A cm-2 at 0.2 V for a catalyst loading of 6.0 mg cm-2. No significant performance degradation was observed for 480 h continuous operation. The characterization studies indicated that the metal-nitrogen chelate complexes decompose at the temperatures above 800 oC. During the pyrolysis, the transition metals facilitate the incorporation of pyridinic and graphitic nitrogen groups into the carbon matrix, and the carbon surface modified with nitrogen is active for ORR. In order to elucidate the role of transition metal precursor played in the formation of active sites in the non-precious metal catalysts, a novel ruthenium-based chelate (RuNx) catalyst was synthesized by using RuCl3 and propylene diammine as the Ru and N precursors, respectively, followed by high-temperature pyrolysis. This catalyst exhibited comparable catalytic activity and selectivity for ORR as the Pt catalyst. A theoretical analysis is made of the four-electron reduction reaction of oxygen to water over the mixed anion and cation (202) surface of pentlandite structure Co9Se8, one of several selenide phases. Reversible potentials for forming adsorbed reaction intermediates in acid are predicted using adsorption energies calculated with the Vienna ab initio simulation program (VASP) and the known bulk solution values together in a linear Gibbs energy relationship. The effect of hydrophobic and structural properties of a single/dual-layer cathode gas diffusion layer on mass transport in PEM fuel cells was studied using an analytical expression. The simulations indicated that liquid water transport at the cathode is controlled by the fraction of hydrophilic surface and the average pore diameter in the cathode gas diffusion layer. The optimized hydrophobicity and pore geometry in a dual-layer cathode GDL leads to an effective water management, and enhances the oxygen diffusion kinetics.

The objective of this project is to develop novel non-precious metal electrocatalysts for oxygen reduction reaction (ORR), and demonstrate the potential of the catalysts to perform at least as good as conventional Pt catalysts currently in use in polymer electrolyte membrane fuel cell (PEMFC) with a cost at least 50 % less than a target of 0.2 g (Pt loading)/peak kW and with durability > 2,000 h operation with less than 10 % power degradation. A novel nitrogen-modified carbon-based catalyst was obtained by modifying carbon black with nitrogen-containing organic precursor in the absence of transition metal precursor. The catalyst shows the onset potential of approximately 0.76 V (NHE) for ORR and the amount of H2O2 of approximately 3% at 0.5 V (NHE). Furthermore, a carbon composite catalyst was achieved through the high-temperature pyrolysis of the precursors of transition metal (Co and Fe) and nitrogen supported on the nitrogen-modified carbon-based catalyst, followed by chemical post-treatment. This catalyst showed an onset potential for ORR as high as 0.87 V (NHE), and generated less than 1 % of H2O2. The PEM fuel cell exhibited a current density of 2.3 A cm-2 at 0.2 V for a catalyst loading of 6.0 mg cm-2. No significant performance degradation was observed for 480 h continuous operation. The characterization studies indicated that the metal-nitrogen chelate complexes decompose at the temperatures above 800 oC. During the pyrolysis, the transition metals facilitate the incorporation of pyridinic and graphitic nitrogen groups into the carbon matrix, and the carbon surface modified with nitrogen is active for ORR. In order to elucidate the role of transition metal precursor played in the formation of active sites in the non-precious metal catalysts, a novel ruthenium-based chelate (RuNx) catalyst was synthesized by using RuCl3 and propylene diammine as the Ru and N precursors, respectively, followed by high-temperature pyrolysis. This catalyst exhibited comparable catalytic activity and selectivity for ORR as the Pt catalyst. A theoretical analysis is made of the four-electron reduction reaction of oxygen to water over the mixed anion and cation (202) surface of pentlandite structure Co9Se8, one of several selenide phases. Reversible potentials for forming adsorbed reaction intermediates in acid are predicted using adsorption energies calculated with the Vienna ab initio simulation program (VASP) and the known bulk solution values together in a linear Gibbs energy relationship. The effect of hydrophobic and structural properties of a single/dual-layer cathode gas diffusion layer on mass transport in PEM fuel cells was studied using an analytical expression. The simulations indicated that liquid water transport at the cathode is controlled by the fraction of hydrophilic surface and the average pore diameter in the cathode gas diffusion layer. The optimized hydrophobicity and pore geometry in a dual-layer cathode GDL leads to an effective water management, and enhances the oxygen diffusion kinetics.

The nature of the active site(s) of platinum group metal-free (PGM-free) electrocatalysts for the oxygen reduction reaction (ORR) remains a topic of intense debate. This is mainly due to the complex heterogeneous character produced during the high-temperature synthesis process required for the generation of electrocatalytically active materials. Quantum chemical modeling of possible active site structures has proposed heme-like moieties in which pyridinic nitrogen groups are coordinated to transition-metal center forming a Fe-Nx-type of structure (1-3). Nevertheless, unlike heme structures that easily poison with strongly bound molecules such as CO, PGM-free electrocatalysts have shown extreme resilience to a number of commonly known poisoning molecules, e.g., CO, H2S, SO2 (4, 5). Thus, the difficulty of developing a characterization technique based on molecular probes will not only rely on the identification of non-traditional probes but also on the successful recovery of electrocatalytic activity required to estimate active site density based on a proposed electrochemical reaction mechanism of poisoning and stripping. A molecular probe approach in which a particular molecule interacts specifically with the active site by forming a stable chemical adduct causing a decrease in electrocatalytic activity, and when removed fully recovers its initial activity, can be a powerful tool for the characterization and quantification of the active site(s). Development of such technique will not only provide experimental evidence about the nature of the active site(s), vital for the synthesis of next-generation PGM-free catalyst with more durable and increased concentration of active sites, but also will become a valuable tool for the further understanding of more complex processes such as reaction pathways and degradation mechanisms. Due to the extraordinary tolerance of PGM-free electrocatalysts to a number of commonly used poisons, nitrogen-containing molecules: nitrite (NO2 -) and nitric oxide (NO), are proposed as molecular probes as they have been reported to interact with Fe-heme structures in electrochemical environments (6). Furthermore, recent ex-situ studies reported by our group and collaborators showed that the NO probe molecule directly interacts with FexNy structures (7, 8). In this work, the poisoning extent of ORR activity in acidic electrolytes caused by the formed chemical adducts is investigated using electrochemical half-cell experiments. The specificity of the probe molecules was studied via X-ray spectroscopic techniques: XPS, EXAFS, and XANES, to better understand their interaction with Fe-centered FexNy moieties. Quantum chemical models of proposed active site interactions with probe molecules are used to determine possible binding motifs and relative binding energies of different sites/molecules. The combination of experimental and computational approaches in this work will yield a powerful suite of tools for interpreting the nature and density of active sites in PGM-free electrocatalysts. Acknowledgments This research is supported by DOE Fuel Cell Technologies Office, through the Electrocatalysis Consortium (ElectroCat). References E. F. Holby, G. Wu, P. Zelenay and C. D. Taylor, The Journal of Physical Chemistry C, 118, 14388 (2014).Q. Y. Jia, N. Ramaswamy, U. Tylus, K. Strickland, J. K. Li, A. Serov, K. Artyushkova, P. Atanassov, J. Anibal, C. Gumeci, S. C. Barton, M. T. Sougrati, F. Jaouen, B. Halevi and S. Mukerjee, Nano Energy, 29, 65 (2016).E. F. Holby and C. D. Taylor, Sci Rep, 5, 9286 (2015).D. Malko, T. Lopes, E. Symianakis and A. R. Kucernak, J Mater Chem A, 4, 142 (2016).T. Reshetenko, A. Serov, K. Artyushkova, I. Matanovic, S. Stariha and P. Atanassov, Journal of Power Sources, 324, 556 (2016).V. Rosca, M. Duca, M. T. de Groot and M. T. M. Koper, Chemical Reviews, 109, 2209 (2009).P. Zelenay and D. Myers, 2017 Annual Merit Review and Peer Evaluation Meeting (2017).J. L. Kneebone, S. L. Daifuku, J. A. Kehl, G. Wu, H. T. Chung, M. Hu, E. E. Alp, K. L. More, P. Zelenay, E. F. Holby and M. L. Neidig, The Journal of Physical Chemistry C, acs.jpcc.7b03779 (2017).

Non-platinum group metal catalysts for the oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) under acidic conditions were developed using a CoN4 complex with a 14-membered-ring hexaazamacrocyclic ligand (Co-14MR). The carbon-supported Co-14MR catalyst (Co-14MR/C) showed higher ORR and HER activities than a conventional carbon-supported 16-membered-ring Co phthalocyanine (CoPc/C) catalyst. Heat treatment of Co-14MR/C at 600 °C further enhanced its ORR and HER activity through structural modification of the Co active center via deprotonation of ligand amine groups. Density functional theory calculations indicated that the structural modifications of Co-14MR induced by heat treatment adjusted the adsorption energies of important intermediates in the ORR and HER toward optimal values, resulting in enhanced catalytic activity. The Co-14MR/C catalysts also exhibited higher durability in the ORR and HER than CoPc/C and Fe-14MR/C catalysts. Structural analysis suggested that the short Co-N bond lengths and small distortion of the CoN4 active site of the Co-14MR catalysts are the reasons for their high durability. These findings suggest that the Co-14MR structure is a promising design for non-platinum group metal catalysts for proton-exchange membrane fuel cells and water splitting.

State-of-the-art polymer electrolyte fuel cells (PEFCs) utilize platinum (Pt) or Pt-alloy nanoparticles supported on high surface-area carbon as catalysts for oxygen reduction reaction (ORR) at the PEFC cathode.1 Platinum group metal-free (PGM-free) catalysts have the potential to reduce the high PEFC cost by replacing Pt with far less expensive materials. Over the past two decades, the growing interest in PGM-free catalysts has led to significant improvements in their performance, which is now approaching that of Pt-based catalysts.2–5 Several insights into the critical factors, such as the role of transition metal, catalyst precursors and pore-formers have been studied.3–5However, for further development of PGM-free catalyst understanding the nature of active site(s) in PGM-free catalysts, as well as the determination of the active site density and turnover frequency (TOF) are vital. PGM-free catalysts have little affinity to “standard” molecular probes, such as carbon monoxide and hydrogen sulfide, that effectively poison ORR active sites in PGM catalysts.6,7 Only recently, an ex situ and in situ method for reversibly probing the active sites and estimating the active site density has been proposed.8,9 Malko et al. showed that nitrogen-based molecular probes can poison the active site and then be reductively stripped off in a voltammetric scan.9 In this work, we have studied the effect of the transition metal and synthesis approach (type of precursor and catalyst fabrication method) on the active site. Active site density in catalysts derived from different transition metals was estimated using sodium nitrite (NaNO­2) as probe.9Figure 1a shows the reductive stripping curves for Los Alamos-developed Fe- and Co-derived CM-PANI-C catalysts, with Co exhibiting much higher affinity to the probe than Fe. The effect of different synthesis methodology and precursor with same transition metal was also studied. Reductive stripping of the probe from CM-PANI-Fe-C catalysts with and without Zn added as a pore former occurs at the same peak potentials suggesting the presence of identical active site in both cases (though occurring at a higher concentration in the catalyst synthesized using Zn). On the other hand, a catalyst with atomically dispersed Fe appears to bind the probe more strongly than CM-PANI-Fe-C catalysts (Figure 2b). Active site density estimation provides a quantitative assessment for the effect of the transition metal and synthesis methodology for further development of PGM-free catalyst. Acknowledgments This research is supported by DOE Fuel Cell Technologies Office, through the Electrocatalysis Consortium (ElectroCat). References S. Litster and G. McLean, J. Power Sources, 130, 61 (2004).H. T. Chung, J. H. Won, and P. Zelenay, Nat. Commun., 4, 1922 (2013).H. T. Chung et al., Meet. Abstr. MA2016-02, 2825 (2016).L. Lin, H. T. Chung, X. Yin, U. Martinez, and P. Zelenay, Meet. Abstr. MA2016-02, 2826 (2016).U. Martinez, E. F. Holby, J. H. Dumont, and P. Zelenay, Meet. Abstr. MA2016-02, 2827 (2016).Q. Wang, Z. Zhou, Y. Lai, Y. You, J. Liu, X. Wu, E. Terefe, C. Chen, L. Song, M. Rauf, N. Tian and S. Sun, J. Am. Chem. Soc., 136, 10882 (2014).7. D. Malko, T. Lopes, E. Symianakis, and A. R. Kucernak, J. Mater. Chem. A, 4, 142 (2016).N. R. Sahraie, U. I. Kramm, J. Steinberg, Y. Zhang, A. Thomas, T. Reier, J. Paraknowitsch and P. Strasser, Nat. Commun., 6, 8618 (2015).D. Malko, A. Kucernak, and T. Lopes, Nat. Commun., 7, 13285 (2016). Figure 1

Developing high activity, low-cost and long durability catalysts for oxygen reduction reaction is of great significance for the practical application of microbial fuel cells. The full exposure of active sites in catalysts can enhance catalytic activity dramatically. Here, novel Fe-N-doped graphene is successfully synthesized via a one-step in situ ball milling method. Pristine graphite, ball milling graphene, N-doped graphene and Fe-N-doped graphene are applied in air cathodes, and enhanced performance is observed in microbial fuel cells with graphene-based catalysts. Particularly, Fe-N-doped graphene achieves the highest oxygen reduction reaction activity, with a maximum power density of 1380±20 mW/m2 in microbial fuel cells and a current density of 23.8 A/m2 at −0.16 V in electrochemical tests, which are comparable to commercial Pt and 390% and 640% of those of pristine graphite. An investigation of the material characteristics reveals that the superior performance of Fe-N-doped graphene results from the full exposure of Fe2O3 nanoparticles, pyrrolic N, pyridinic N and excellent Fe-N-G active sites on the graphene matrix. This work not only suggests the strategy of maximally exposing active sites to optimize the potential of catalysts but also provides promising catalysts for the use of microbial fuel cells in sustainable energy generation.

Significant progress has been made in recent years in developing platinum group metal-free catalysts, particularly so-called Fe-N-C materials, for the oxygen reduction reaction (ORR) in polymer electrolyte fuel cells.1 However, further improvement in both activity and, especially, durability are still necessary for practical applications. One of the major factors that hinder the further enhancement of the catalysts is the lack of clear understanding of the nature of the active sites in the Fe-N-C catalysts due to the complexity of the Fe-N-C catalysts’ composition and structure. Gaseous nitric oxide was reported to be a suitable probe molecule that can bond to Fe and impede the ORR of the Fe-N-C catalysts.2,3 In this work, the effect of nitric oxide gas-phase adsorption on Fe-N-C catalyst redox feature and ORR activity for a variety of Fe-N-C catalysts from different origin will be investigated. The implication of these results on the nature of the ORR active sites of the Fe-N-C catalysts will be discussed.AcknowledgementsThis work was supported by the U.S. Department of Energy (DOE), Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office (HFTO) under the auspices of the Electrocatalysis Consortium (ElectroCat 2.0). This work was partially authored by Argonne, a U.S. Department of Energy (DOE) Office of Science laboratory operated for DOE by UChicago Argonne, LLC under contract no. DE-AC02-06CH11357.References X.X. Wang, M.T. Swihart, and G. Wu, "Achievements, challenges, and perspectives on cathode catalysts in proton exchange membrane fuel cells for transportation", Nature Catalysis, 2 (2019) 578-589.J.L. Kneebone, et al., "A Combined Probe-Molecule, Mossbauer, Nuclear Resonance Vibrational Spectroscopy, and Density Functional Theory Approach for Evaluation of Potential Iron Active Sites in an Oxygen Reduction Reaction Catalyst", J. Phys. Chem. C 121 (2017) 16283-16290.P. Boldrin, D. Malko, A. Mehmood, U.I. Kramm, S. Wagner, S. Paul, N. Weidler, A. Kucernak, “Deactivation, reactivation and super-activation of Fe-N/C oxygen reduction electrocatalysts: Gas sorption, physical and electrochemical investigation using NO and O2”, Applied Catalysis B: Environmental 292 (2021) 120169.