Non-precious Metal Oxygen Reduction Catalysts Research Articles

Preparation of Fe-BN-C catalysts derived from ZIF-8 and their performance in the oxygen reduction reaction.

Enhancing the oxygen reduction reaction (ORR) activity and stability of the fuel cell cathode electrocatalysts and reducing their costs are critical. In response to this need, Fe, B, and N co-doped hollow mesoporous carbon materials were prepared by a simple chemical doping one-step pyrolysis method using ZIF-8 as a precursor. The results showed that the optimized catalyst displayed a higher limiting current density (6.154 mA cm-2) and half-wave potential (0.859 V), which showed significant enhancement compared with the Pt/C catalyst (5.487 mA cm-2 and 0.853 V). Moreover, the optimized catalyst had outstanding long-term stability with a current density retention higher than 91% after 36 000 s of stability testing. This work provides a facile strategy for the design of outstanding ORR performance of non-precious metal oxygen reduction catalysts.

Heat-Treatment of a Fourteen Membered Macrocyclic Fe Complex for Electrocatalytic Oxygen Reduction

For the broad application of polymer electrolyte fuel cells (PEFCs), the development of non-precious-metal (NPM) catalysts for oxygen reduction is extremely important. To date, many NPM catalyst have been synthesized by pyrolyzing Fe-, N-, and C-containing precursors, but they suffer from the low density and uncertain chemical structure of their active sites. On the contrary, our research group has been studying a fourteen-membered macrocyclic Fe complex, which was inspired by the FeNx sites embed in graphene sheets (Fig. 1).In this study, a fourteen-membered macrocyclic Fe complex was loaded on a carbon support and then heat-treated, and found to be quite active as a non-precious metal oxygen reduction catalyst. The oxygen reduction current at 0.8 V (RHE) was 13.3 A gcat -1. The improvement is attributed to a high turnover frequency rather than the number of active sites. X-ray absorption spectroscopy and temperature programmed decomposition for the Fe complex suggested that the counter anions and the protons on the nitrogen atom in the ligand were eliminated by the heat-treatment. This change probably influenced the adsorption energy of the oxygen species during the oxygen reduction and resulted in the improved catalytic activity (Fig. 2). Simulation based-on the density functional theory well support this hypothesis. Figure 1

Preparation of Fe, N co-doped oxygen reduction catalysts from sacrificial templates and their application to Zn-air batteries

Fe, N co-doped non-precious metal oxygen reduction catalysts has been prepared with nitrogen-rich precursor material (g-C3N4) as a self-sacrificing template and N source. The utilization of g-C3N4 as a nitrogen-rich material could anchor the Fe source sufficiently on its surface, and form abundant active sites. The g-C3N4-derived carbon has a high specific surface area, which facilitates the more active sites to be exposed, and the suitable pore structure provides a site for the oxygen reduction reaction. The half-wave potential of the catalyst is optimized to 0.85 V by doping with different Fe-content sources, exceeding that of commercial 20% Pt/C catalysts. The prepared catalysts also exhibit excellent electron transfer numbers along with superior methanol resistance. The prepared catalysts is loaded into Zn-Air battery devices and presenting high open-circuit voltage, power density, and specific capacitance performance. This work provides an economical and feasible preparation strategy to explore nitrogen-rich precursor materials as Fe-N-C type efficient oxygen reduction catalysts.

(Keynote) Identification of Active Species and Mechanisms in Non-Precious Metal Oxygen Reduction Catalysts By Reductive Treatments and Magnetic Measurements

This talk addresses the Oxygen Reduction Reaction (ORR) utilizing Fe- and Co-based non precious metal (NPM) catalysts. There has long been uncertainty as to the active site for ORR in these materials due to the heterogeneity of the as-prepared catalyst. We report on two innovations directed at understanding the active sites in these materials. In the first, we interrogate the ORR activity in acid of a Fe porphyrin on different supports. While the activity is high when the Fe porphyrin is adsorbed on XC72 (a graphitic carbon), this activity is much lower when the porphyrin is adsorbed on either MoS2 or g-C3N4. Numerous physical characterization techniques show the electronic structure around the Fe center is the same for all three supports. Only the Fe porphyrin supported on XC72 exhibits a pH dependence in its ORR activity. This observation, coupled with the increased hydrophilicity of XC72 relative to the other supports, suggests that the support-electrolyte interaction controls the ORR activity. Modification of MoS2 to increase its hydrophilicity results in a more active ORR catalyst. In the second area, we use magnetometry and other techniques to examine the effect of two different reductive gas treatments on pyrolyzed ORR catalysts. We find that reductive gas treatments yield more superparamagnetic/ferromagnetic Fe species, with specific preparations yielding materials that consist almost entirely of reduced Fe (particularly Fe4N). Interestingly, catalysts exhibiting large percentages of superparamagnetic/ferromagnetic materials exhibit high activities for ORR catalysis as high as those reported in the literature.

(Keynote) Nanostructured Materials for CO2 and Oxygen Reduction

This talk addresses the reactivity associated with CO2 and O2 reduction. For CO2, electrodeposition of metals from plating baths containing 3 an inhibitor yields highly porous materials exhibiting enhanced activity for electrochemical reactions. Electrodeposition of CuAg or CuSn alloy films from such plating baths yields high surface area catalysts for the active and selective electroreduction of CO2 to multi-carbon hydrocarbons and oxygenates. Alloy films containing Sn exhibit greater efficiency for CO production relative to either Cu along or CuAg at low overpotentials. In-situ Raman and electroanalysis studies suggest the origin of the high selectivity towards C2 products to be a combined effect of the enhanced stabilization of the Cu2O overlayer and the optimal availability of the CO intermediate due to the Ag or Sn incorporated in the alloy. Sn-containing films exhibit less Cu2O relative to either the Ag-containing or neat Cu films, likely due to the increased oxophilicity of the admixed Sn. Additionally, modification of the Cu electrode with certain polymers yields substantially enhanced reactivity, due in part to control of the Cu2O layer.For O2 reduction, we report the results of detailed magnetometry studies on non-precious metal oxygen reduction catalysts. The magnetometry clarifies uncertainties about the state of the metal in active catalysts and reports on how the catalysts change following alteration in preparation procedures, such as exposure to NH3.

ZIF-derived Co–N–C ORR catalyst with high performance in proton exchange membrane fuel cells

Metal and nitrogen-doped carbon (M-N-C) materials have been considered as the most promising non-precious metal oxygen reduction (ORR) catalysts to replace expensive Pt catalysts. Due to high Fenton catalytic activity of Fe element and the resulting instability, Co-based N–C (Co–N–C) catalysts without Fenton catalytic activity should be a worthier ORR catalyst being explored. Although the high ORR activity of Co–N–C catalyst has been demonstrated in aqueous half-cell tests, their performance under PEMFC working condition is still far away from that of state-of-the-art Fe–N–C catalysts. In this study, a high-performance Co–N–C catalyst was synthesized by one-step pyrolyzing Co-doped ZIF-8 (zeolitic imidazolate framework-8) particles in-situ grown on the high-surface-area KJ600 carbon black with high electronic conductivity. The resulting Co–N–C catalyst exhibited high intrinsic ORR activity, fast mass transfer rate and high electronic conductivity, and thus yielded a remarkable peak power density of 0.92 W cm-2 in H2–O2 PEMFC, which is comparable to state-of-the-art Fe–N–C catalyst. This strategy is helpful to synthesize highly active M-N-C ORR catalysts with improved mass transfer and electric conductivity.

FeWO4/nitrogen-doped multi-dimensional porous carbon for the highly efficient and stable oxygen reduction reaction

Non-precious metal oxygen reduction catalysts still present considerable challenges in ensuring activity and stability in acidic/alkaline media. In this paper, the tungstate with excellent stability is selected as the electrocatalyst for oxygen reduction reaction (ORR). FeWO4/nitrogen-doped multi-dimensional porous carbon (FeWO4/N-MPC) catalyst was successfully synthesized by freeze-drying and rapid calcination methods. FeWO4/N-MPC exhibited a multi-dimensional porous carbon structure with uniformly distributing FeWO4 nanoparticles. The nitrogen-doped multi-dimensional porous carbon structure composed of nanofibers and nanosheets can significantly improve the transport efficiency of charge and O2 and effectively protect the FeWO4 nanoparticles, further leading to enhance catalytic activity and stability of FeWO4/N-MPC. Thus, FeWO4/N-MPC exhibited excellent catalytic performance for ORR in acidic/alkaline media. The long-term durability of FeWO4/N-MPC was also remarkably more outstanding than that of 20 wt% Pt/C catalyst in acidic/alkaline media. The long-term durability of FeWO4/N-MPC had also made it possible to apply in metal-air batteries and fuel cells.

Identification of Active Species and Mechanisms in Non-Precious Metal Oxygen Reduction Catalysts By Poisoning and Magnetic Measurements

This talk addresses the Oxygen Reduction Reaction (ORR) utilizing Fe- and Co-based non precious metal (NPM) catalysts. There has long been uncertainty as to the active site for ORR in these materials due to the heterogeneity of the as-prepared catalyst. We have developed several methods with which to interrogate these materials. First, by using a purification scheme which removes and then remetallates the active site we substantially reduce catalyst heterogeneity. Second, we have developed a method by which to poison some of these active sites. Third, we use magnetic measurements to probe those sites affected by poisoning. We show as well that there is a minimum metal requirement for high ORR activity and that the metal must be arranged in certain critical configurations.

Molecular-Scale Mechanistic Investigation of Oxygen Dissociation and Adsorption on Metal Surface-Supported Cobalt Phthalocyanine.

Ultrahigh vacuum scanning tunneling microscopy and density functional theory are used to investigate adsorption of oxygen on cobalt phthalocyanine (CoPc), a promising nonprecious metal oxygen reduction catalyst, supported on Ag(111), Cu(111), and Au(111) surfaces at the molecular scale. Four distinct molecular and atomic oxygen adsorption configurations are observed for CoPc supported on Ag(111) surfaces, which are assigned as O2/CoPc/Ag(111), O/CoPc/Ag(111), CoPc/(O)2/Ag(111), and (O)2/CoPc/Ag(111). In contrast, no oxygen adsorption is observed for CoPc supported on Cu(111) and Au(111) surfaces. The results show that for Ag(111), atomic O that is predominantly catalytically produced from the dissociation of molecular O2 at metal surface step edges is responsible for the observed adsorption configurations. However, Cu(111) binds atomic O too strongly, and Au(111) does not produce atomic O. These results show the active role of the supporting metal surface in facilitating oxygen adsorption on CoPc.

Determining Electro-Osmotic Drag of Water in Anion Exchange Membrane Fuel Cells

In an effort to reduce the cost of proton exchange membrane fuel cells (PEMFCs), a significant amount of research into non-precious metal (NPM) oxygen reduction catalysts has been conducted in recent decades. Despite this effort, platinum remains the only material with suitable stability and activity for use in the harsh environment of PEMFCs. Anion exchange membrane fuel cells (AEMFCs) have been gaining increased attention in recent years, providing advantages of higher stability and activity of NPM catalysts in alkaline environments. Unfortunately, the relevant properties, e.g. conductivity and mechanical & chemical stability, of anion exchange membranes (AEMs) are inferior to their proton conducting counterparts. Many studies have been conducted which focus on improving membrane properties such as swelling ratio, conductivity, and stability. In comparison, relatively little attention has been paid to studying ion and water transport in anion exchange membranes. Previous work conducted in our group has indicated that the majority of AEMFC polarization arises from the anode. The performance is relatively insensitive to catalyst loading and initial results indicate that electrode flooding is the primary cause of poor performance. To improve the performance of AEMFCs and better design AEMs, it is therefore important to study the transport of water and ions within AEMs. One key parameter which has received very little attention in the literature is the electro-osmotic transport of water due to the conduction of anions from the cathode to the anode. In this work, we study the electro-osmotic drag (EOD) in anion exchange membranes. The cell design and concept for electro-osmotic drag coefficient (ε) measurements in this work follow from the work of Fuller and Newman as adapted by others1–3. Analysis is conducted on the membranes in pure hydroxide form and carbonate form. The conductivity, water uptake and water mobility of the membranes is also studied over a wide range of relative humidities and temperatures. Better understanding of the transport of water and ions within anion exchange membranes will facilitate the development of models for AEMFC behavior and offer insight for future AEM production. Figure 1: Effect of water activity on the EOD coefficient (ε) and water content (λ) in Tokuyama A201. T. F. Fuller and J. Newman, J. Electrochem. Soc., 139, 1332–1337 (1992).T. A. Zawodzinski, J. Davey, J. A. Valerio, and S. Gottesfeld, Electrochim. Acta, 40, 297–302 (1995).X. Wang, J. P. Mcclure, and P. S. Fedkiw, Electrochim. Acta, 79, 126–132 (2012) http://dx.doi.org/10.1016/j.electacta.2012.06.098. Figure 1

Identification of carbon-encapsulated iron nanoparticles as active species in non-precious metal oxygen reduction catalysts.

The widespread use of fuel cells is currently limited by the lack of efficient and cost-effective catalysts for the oxygen reduction reaction. Iron-based non-precious metal catalysts exhibit promising activity and stability, as an alternative to state-of-the-art platinum catalysts. However, the identity of the active species in non-precious metal catalysts remains elusive, impeding the development of new catalysts. Here we demonstrate the reversible deactivation and reactivation of an iron-based non-precious metal oxygen reduction catalyst achieved using high-temperature gas-phase chlorine and hydrogen treatments. In addition, we observe a decrease in catalyst heterogeneity following treatment with chlorine and hydrogen, using Mössbauer and X-ray absorption spectroscopy. Our study reveals that protected sites adjacent to iron nanoparticles are responsible for the observed activity and stability of the catalyst. These findings may allow for the design and synthesis of enhanced non-precious metal oxygen reduction catalysts with a higher density of active sites.

(Invited) Purification of a Non-Precious Metal Oxygen Reduction Catalyst to Reveal the Active Species

Despite substantial efforts to understand the structure of the active site in Fe-based non-precious metal (NPM) catalysts for the oxygen reduction reaction (ORR) in fuel cells, the identification of the active species remains elusive, impeding progress towards the development of highly active and cost efficient catalysts. We report the isolation and identification of the active Fe species in an NPM catalyst using Cl2 and H2 treatments which decrease the heterogeneity of the catalyst. The Cl2 treatment decreases catalyst activity as shown by a negative shift in the ORR onset of ca. 170 mV. Subsequent treatment with H2 restores the initial activity of the catalyst. Mössbauer spectroscopy and X-ray absorption spectroscopy show many Fe species in the as-prepared catalyst while the catalyst after the Cl2 and H2 treatments contains only reduced Fe species encapsulated by C which are highly active for the ORR.

Is reduced graphene oxide favorable for nonprecious metal oxygen-reduction catalysts?

Reduced graphene oxide (rGO), as a newly emerged carbon material, has attracted great attention concerning its applications for electrocatalysts. Presently, there are mixed opinions regarding the advantages to using rGO as a support for preparing nonprecious metal catalysts for the oxygen reduction reaction (ORR). The primary goal of this work is to determine whether rGO would be favorable for non-precious metal catalysis of oxygen reduction or not. In the case of Fe-free catalysts, when polyaniline (PANI) was used as nitrogen/carbon precursor, the PANI-rGO catalyst is superior to the PANI-Ketjenblack (KJ) carbon black catalyst in terms of ORR activity and H2O2 yield. When comparing the ORR activity of PANI-Fe-rGO to the traditional PANI-Fe-KJ, in more challenging acidic electrolyte, PANI-Fe-rGO performed no better than PANI-Fe-KJ. However, rGO does indeed enhance stability of the Fe–N–C catalyst in acidic media. In addition, in an alkaline electrolyte, ORR activity was significantly improved when using rGO in comparison to the KJ-supported Fe–N–C catalysts. Based on detailed comparisons of structures, morphologies, and reaction kinetics, the traditional KJ support with dominant microporous is able to accommodate more FeNx moieties that are crucial for the ORR in acid. Oppositely, the richness of nitrogen-doped graphene edge sites provided by rGO facilitates the ORR in the alkaline electrolyte.

High pressure pyrolyzed non-precious metal oxygen reduction catalysts for alkaline polymer electrolyte membrane fuel cells.

Non-precious metal catalysts, such as metal-coordinated to nitrogen doped-carbon, have shown reasonable oxygen reduction reaction (ORR) performances in alkaline fuel cells. In this report, we present the development of a highly active, stable and low-cost non-precious metal ORR catalyst by direct synthesis under autogenic-pressure conditions. Transmission electron microscopy studies show highly porous Fe-N-C and Co-N-C structures, which were further confirmed by Brunauer-Emmett-Teller surface area measurements. The surface areas of the Fe-N-C and Co-N-C catalysts were found to be 377.5 and 369.3 m(2) g(-1), respectively. XPS results show the possible existence of N-C and M-Nx structures, which are generally proposed to be the active sites in non-precious metal catalysts. The Fe-N-C electrocatalyst exhibits an ORR half-wave potential 20 mV higher than the reference Pt/C catalyst. The cycling durability test for Fe-N-C over 5000 cycles shows that the half-wave potential lost only 4 mV, whereas the half-wave potential of the Pt/C catalyst lost about 50 mV. The Fe-N-C catalyst exhibited an improved activity and stability compared to the reference Pt/C catalyst and it possesses a direct 4-electron transfer pathway for the ORR process. Further, the Fe-N-C catalyst produces extremely low HO2(-) content, as confirmed by the rotating ring-disk electrode measurements. In the alkaline fuel single cell tests, maximum power densities of 75 and 80 mW cm(-2) were observed for the Fe-N-C and Pt/C cathodes, respectively. Durability studies (100 h) showed that decay of the fuel cell current was more prominent for the Pt/C cathode catalyst compared to the Fe-N-C cathode catalyst. Therefore, the Fe-N-C catalyst appears to be a promising new class of non-precious metal catalysts prepared by an autogenic synthetic method.

Varying N content and N/C ratio of the nitrogen precursor to synthesize highly active Co-Nx/C non-precious metal catalyst

The most promising non-precious metal oxygen reduction catalysts are M-Nx/C (M = Fe and/or Co) materials. Moreover, N-containing precursor is one of the important factors that affect oxygen reduction reaction (ORR) activity of M-Nx/C materials. In this paper, we want to study nitrogen precursor effects on ORR activity of Co-Nx/C catalysts by varying N content and N/C ratio of the nitrogen precursor. In this regard, three Co-Nx/C catalysts were synthesized using a solvent-milling method followed by high temperature treatment. The results showed that the increase in N content and N/C ratio did not necessarily cause the improvement of ORR activity of the Co-Nx/C catalyst. The most active catalyst, Co-HQ/C-800 (heat treatment of carbon supported Co-HQ complex at 800 °C for 2 h), was obtained using 8-hydroxyquinoline (8-HQ) as the nitrogen precursor. XPS analysis demonstrated that more graphitic N and Co-Nx active sites were responsible for better ORR activity of the Co-HQ/C-800 catalyst. The electrochemical property of the three Co-Nx/C catalysts was evaluated by linear sweep voltammetry (LSV), chronoamperometric measurements, accelerated durability tests (ADT) and H2/O2 alkaline fuel cell (AFC) tests.

Model Systems and Modeling of Non-Precious Metal Oxygen Reduction Catalysts

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).

Effect of Post-Treatment to the Co-Based Non-Precious Metal Oxygen Reduction Catalysts

not Available.

Pyrolyzing cobalt diethylenetriamine chelate on carbon (CoDETA/C) as a family of non-precious metal oxygen reduction catalyst

Using diethylenetriamine ligand, a family of non-precious metal oxygen reduction catalyst is synthesized by pyrolysis of cobalt-diethylenetriamine chelate on carbon at elevated temperature from 600 to 900 °C. Cyclic voltammetry results show that pyrolysis temperature plays an important impact on improving catalytic activity and the maximum activity is obtained at 800 °C with its peak potential of 719 mV (SHE). For the best catalyst HT800, rotating-ring disk electrode measurement indicates that the number of electrons transferred is 3.80–3.85 at potential of 0.5 V with rotating rates from 100 to 1600 rpm and the catalyst loading of 648 μg cm−2. XRD indicates that the cobalt-nitrogen chelate decomposes above 600 °C and nanometallic α-Co with different sizes is synthesized. Raman indicates that there are more defective sites on the carbon surfaces induced by N doping. Combined XPS data with electrochemical results, it indicates that a higher total N content does not lead to a higher ORR activity.

Graphene Oxide-Derived Non-Precious Metal Oxygen Reduction Catalysts

not Available.

Metropolis Monte Carlo Search for Non-Precious Metal Catalyst Active Site Candidates

Non-precious metal oxygen-reduction catalysts have the potential to out-perform the rare and expensive platinum group metals used for oxygen reduction reaction at the cathode of low temperature polymer electrolyte fuel cells. Understanding and optimizing non-precious metal catalyst activity is dependent upon an atomistic understanding of active site structures. Unfortunately, there exists a tremendously large configurational active site search space comprised of C, N, Fe (and/or Co), and a variety of edge termination and defect structures. We here present a stochastic search methodology that we have developed and applied to identification of candidate structure attributes using graphene as a template matrix material.