Platinum Group Metal-free Catalysts Research Articles

Platinum Group Metal-Free Catalysts for Oxygen Reduction Reaction: Applications in Microbial Fuel Cells

Scientific and technological innovation is increasingly playing a role for promoting the transition towards a circular economy and sustainable development. Thanks to its dual function of harvesting energy from waste and cleaning up waste from organic pollutants, microbial fuel cells (MFCs) provide a revolutionary answer to the global environmental challenges. Yet, one key factor that limits the implementation of larger scale MFCs is the high cost and low durability of current electrode materials, owing to the use of platinum at the cathode side. To address this issue, the scientific community has devoted its research efforts for identifying innovative and low cost materials and components to assemble lab-scale MFC prototypes, fed with wastewaters of different nature. This review work summarizes the state-of the-art of developing platinum group metal-free (PGM-free) catalysts for applications at the cathode side of MFCs. We address how different catalyst families boost oxygen reduction reaction (ORR) in neutral pH, as result of an interplay between surface chemistry and morphology on the efficiency of ORR active sites. We particularly review the properties, performance, and applicability of metal-free carbon-based materials, molecular catalysts based on metal macrocycles supported on carbon nanostructures, M-N-C catalysts activated via pyrolysis, metal oxide-based catalysts, and enzyme catalysts. We finally discuss recent progress on MFC cathode design, providing a guidance for improving cathode activity and stability under MFC operating conditions.

Platinum and Platinum Group Metal-Free Catalysts for Anion Exchange Membrane Fuel Cells

The development of active hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) catalysts for use in anion exchange membrane fuel cells (AEMFCs), which are free from platinum group metals (PGMs), is expected to bring this technology one step closer to commercial applications. This paper reports our recent progress developing HOR Pt-free and PGM-free catalysts (Pd/CeO2 and NiCo/C, respectively), and ORR PGM-free Co3O4 for AEMFCs. The catalysts were prepared by different synthesis techniques and characterized by both physical-chemical and electrochemical methods. A hydrothermally synthesized Co3O4 + C composite ORR catalyst used in combination with Pt/C as HOR catalyst shows good H2/O2 AEMFC performance (peak power density of ~388 mW cm−2), while the same catalyst coupled with our flame spray pyrolysis synthesised Pd/CeO2 anode catalysts reaches peak power densities of ~309 mW cm−2. Changing the anode to nanostructured NiCo/C catalyst, the performance is significantly reduced. This study confirms previous conclusions, that is indeed possible to develop high performing AEMFCs free from Pt; however, the challenge to achieve completely PGM-free AEMFCs still remains.

Communication—On the Lack of Correlation between the Voltammetric Redox Couple and ORR Activity of Fe-N-C Catalysts

Platinum group metal-free catalysts for polymer electrolyte fuel cells are analyzed in different electrolytes (H2SO4 and HClO4) and at different pH values to gain insight into the oxygen reduction reaction (ORR) mechanism. Two Fe-N-C type catalysts showing a reversible voltammetric redox couple around 0.77 V vs RHE in HClO4 electrolyte are investigated. A notable cathodic shift of the redox couple to 0.62 V is observed in H2SO4 and is assigned to bisulfate adsorption. Concurrently, the ORR activity of the catalysts is unaffected, suggesting an independent nature of the reductive mechanism with the redox peak species.

Elucidating the Role of Ionomer in the Performance of Platinum Group Metal-free Catalyst Layer via in situ Electrochemical Diagnostics

The ionomer content in platinum group metal (PGM)-free polymer electrolyte fuel cell (PEFC) cathode catalyst layer (CCL) plays an important role in the electrode gas transport properties, proton conductivity, and hence, membrane electrode assembly (MEA) performance. In this work, the ionomer content in the CCL is varied, influencing electrode microstructure by altering porosity, tortuosity, as well as ionomer distribution and coverage of the catalyst particles. A novel technique consisting of a H2 pump, combined with a Pt black sensor layer, is used to measure the bulk mass transport resistance of a series of PGM-free CCL prepared with different ionomer contents. The values for bulk electrode mass transport resistance are contrasted with electrode proton transport resistance in the cathode catalyst layer, establishing a clearly defined trade-off between two key performance limiting phenomena and identifying a need for novel PGM-free electrode fabrication strategies.

Polymorphic cobalt diselenide as extremely stable electrocatalyst in acidic media via a phase-mixing strategy

Many platinum group metal-free inorganic catalysts have demonstrated high intrinsic activity for diverse important electrode reactions, but their practical use often suffers from undesirable structural degradation and hence poor stability, especially in acidic media. We report here an alkali-heating synthesis to achieve phase-mixed cobalt diselenide material with nearly homogeneous distribution of cubic and orthorhombic phases. Using water electroreduction as a model reaction, we observe that the phase-mixed cobalt diselenide reaches the current density of 10 milliamperes per square centimeter at overpotential of mere 124 millivolts in acidic electrolyte. The catalyst shows no sign of deactivation after more than 400 h of continuous operation and the polarization curve is well retained after 50,000 potential cycles. Experimental and computational investigations uncover a boosted covalency between Co and Se atoms resulting from the phase mixture, which substantially enhances the lattice robustness and thereby the material stability. The findings provide promising design strategy for long-lived catalysts in acid through crystal phase engineering.

Multicomponent Spinel Oxide Solid Solutions: A Possible Alternative to Platinum Group Metal Three-Way Catalysts

Single-phase quaternary spinel solid solutions, Cu0.05Ni0.95AlyCr2–yO4 (0 ≤ y ≤ 2.0), were prepared over the whole range of y by a polymerized complex method to study as platinum group metal-free three-way catalysts (TWC). Most conventional binary and/or ternary spinel oxides lose their NO reduction activity in the presence of water vapor and/or after high-temperature aging. In contrast, the present quaternary system with y = 1.8, which was aged at 900 °C for 25 h, preserved high activity even under a wet gas stream (5% H2O) simulating real TWC conditions. Comprehensive structural analyses via X-ray absorption fine structure and X-ray Rietveld analysis showed that, in the quaternary system, Cu and Cr prefer to occupy the tetrahedral site and the octahedral site, respectively, whereas Ni and Al are distributed across both sites. The partial replacement of Cr by Al increased the specific surface area from 7 m2 g–1 (y = 0) to 36 m2 g–1 (y = 1.8), which is a common feature of the NiAl2O4-based spinel platform...

Designing the 3D Architecture of PGM-Free Cathodes for H2/Air Proton Exchange Membrane Fuel Cells

Metal–nitrogen–carbon catalysts have emerged as the most promising platinum group metal-free catalysts toward oxygen reduction reaction for proton exchange membrane fuel cell (PEMFC) applications. ...

Developing Platinum Group Metal Free Catalysts with Multiple Metal Centers for the Oxygen Reduction Reaction in Acid

Remarkable advancement of Platinum group metal-free (PGM-free) catalysts for the oxygen reduction reaction (ORR) in acidic media makes it potentially feasible as an alternate to Pt-based catalysts in polymer electrolyte fuel cells (PEMFCs). The current leading PGM-free catalysts are composed of single transition metal atoms embedded in microporous nitrogen-doped carbon matrix (M-N-C). The commercialization of M-N-C catalysts in PEMFCs has been impeded by the insufficient activity and particularly durability. The limited ORR performance is likely owing to the single metal center configuration that causes low active site density, undesired O2 binding configuration and energy toward the metal center, etc. These inherent limitations of M-N-C catalysts motivate us to move toward Mx-N-C with multiple metal center (MMC) sites. On account of this, herein a multidentate ligand platform is utilized to prepare a series of bimetallic coordination complexes. These complexes were then used as the metal precursors to produce Mx-N-C with MMC sites by incorporating them into microporous nitrogen-doped carbon matrix via both pyrolysis and surface deposition approaches without breaking the pre-existing multiple nuclear structure. The formation of MMC sites is supported by multiple characterizations. The high ORR activity achieved is tentatively related to MMC sites that promotes the desorption of oxygen adsorbates and suppress the site-blocking effect at elevated potentials. Further improvement, however, is limited by the low concentration of MMC sites. densification of MMC sites is undergoing. Acknowledgement This work was supported by the US Department of Energy under award number DE-EE0008416. The authors declare no competing financial interests. MRCAT operations are supported by the Department of Energy and the MRCAT member institution. This research used resources of the Advanced Photon Source, a DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

(Invited) Electrocat: Expediting PGM-Free Fuel Cell Catalyst and Electrode Development

The development of platinum group metal-free (PGM)-free catalysts with comparable performance and durability to low- PGM catalysts would hasten the cost competitiveness of fuel cells with alternative technologies. Therefore, addressing electrocatalyst development represents the most pressing technical barrier in fuel cell deployment and in meeting the ultimate U.S. Department of Energy (DOE) automotive fuel cell system cost target of $30/kW. ElectroCat (The Electrocatalysis Consortium), like other DOE Energy Materials Network (EMN) consortia, aligns research and development (R&D) activities at national laboratories and, through DOE-funded projects, facilitates access of academic and industry partners to the world-class capabilities and expertise at the national labs. These capabilities, such as (1) catalyst synthesis (including in particular high-throughput methods), (2) advanced characterization expertise, and (3) quantum chemical modeling address the salient knowledge gaps in PGM-free catalyst development: ascertaining the structure-function relationship, increasing active site density, improving electrode structure to maximize active site utilization and mitigate mass transport limitations, and improving durability. In the near-term, the consortium will work to develop PGM-free catalysts and electrodes to meet the 2025 DOE current density technical target of 0.044 A/cm2 at 0.9 VIR-free, analogous to the goal for low-PGM catalysts. In 2018, ElectroCat R&D efforts increased initial activity by 68% compared to the 2016 baseline of 16 mA/cm2. The presentation will highlight current R&D advancements, core national lab experimental and computational capabilities, as well as high-throughput and machine-learning approaches to expedite the development of PGM-free catalysts and electrodes.

(Invited) A New View of Fuel Cell Electrocatalysts through Multimodal Analytical Electron Microscopy

Significant advancements have been made in the performance of oxygen reduction reaction (ORR) electrocatalysts for polymer electrolyte membrane fuel cells. State-of-the-art Pt-alloy catalysts show exceptionally high mass activity and recent research into ordered nanoparticles shows promising improvements in durability. The activity of platinum group metal-free (PGM-free) catalysts, which will be required as world-wide demand for fuel cell stacks increases, have also improved substantially, although significant durability issues persist. Accelerated materials discovery and development is required to keep pace with ever-changing market and performance demands, such as the shift in focus towards heavy duty transportation where highly durable, cost-effective catalysts are required. Scanning transmission electron microscopy (STEM) has long played an important role in assessing the quality and durability of electrocatalysts and electrode structures, and the increasing demands on catalyst performance require a corresponding improvement in both the resolution and quantitative precision of analytical methods. Fortunately, recent advances in high-speed cameras and spectrometers allow for an unprecedented view of catalyst structure and composition in two, and in some cases, three dimensions. The newly installed JEOL NEOARM atomic-resolution STEM at the Center for Nanophase Materials Sciences combines the latest generation spectrometers with new imaging modes such as electron ptychography. The ability to acquire multiple signals simultaneously at high data acquisition speeds and across a range of accelerating voltages (30-200 kV) allows for a fresh look at the atomic structure of both Pt-based and PGM-free catalysts. The simultaneous application of electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy to study the beam sensitive surfaces of sub-5nm Pt-alloy nanoparticles (<5nm) will be discussed. The combination of dual EELS-EDS analysis using ultra-low voltage (30kV) electron beams under cryogenic cooling will also be demonstrated on atomically dispersed transition metal sites in PGM-free electrodes. Finally, recent developments in pixelated cameras and ptychographic analysis of 2D materials will be extended to both Pt-based and PGM-free electrocatalyst systems to explore the opportunities and limits of low-dose, high-speed imaging. Acknowledgements Research sponsored by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy (DOE), as part of the FC-PAD and ElectroCat Consortia, which is part of the Energy Materials Network. Microscopy performed as part of a user project at ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a U.S. DOE Office of Science User Facility.

Combined Synchrotron Nano X-Ray Computed Tomography and XANES to Understand PGM-Free Anodes of Alkaline Fuel Cells

Combined chemical information and X-ray CT or X-ray radiography is a new paradigm in energy-conversion and -storage characterization. During electrochemical experiment chemical mapping can be used to differentiate elements within fuel cell catalyst layer. Polymer electrolyte fuel cells (PEFCs) and alkaline exchange membrane (AEM) fuel cells are both low temperature, high efficiency technologies that use hydrogen and air (oxygen) to produce electricity. Most of the current efforts have focused on PEFCs, as commercial availability of polymer electrolyte membrane (PEM), such as Nafion enabled wide-spread research. Recently, AEM fuel cells have shown to produce current densities that are compatible to PEFCs 1-2. The promise of AEM fuel cells is to be more cost-effective as they can use Earth-abundant materials as electrocatalysts. In the past decade platinum group metal-free (PGM-free) catalysts have been developed. We reported novel class of NiMo catalysts, supported on carbon blacks, that showed unprecedented power densities for fully PGM-free fuel cells 3. Understanding chemical oxidation states of Ni, Mo and Cu under applied potentials in hydrogen environment within AEM fuel cell anode is critical to advance the knowledge of this domain of electrocatalysts. NiCu and NiMo bi-metallic catalysts are dispersed onto porous, high surface area supports to achieve high electrochemical surface area and good dispersion. The catalysts clusters are micrometer sized domains composed of nano particles. As hydrogen oxidation reaction proceeds in anode of these AEM fuel cells, the surface of NiCu and NiMo are reduced and thus oxidation state can change depending on local conditions. Our micro X-ray CT results indicate significant difference in electrochemical behavior and morphology between NiMo and NiCu electrocatalysts1, 3. The micro-CT technique provides large field-of-view (FOV) but it is lacking in spatial resolution. Designing experiment with combined nano X-ray computed tomography, X-ray absorption near edge structure (XANES) and/or fluorescence spatial mapping of the electrocatalyst under operating conditions will provide unprecedented insight into structure-function relations of these catalyst layers. In this work we use nano-CT imaging at adsorption edges of Ni and Cu to show electrocatalyst distribution and its XANES spectra at 18-ID National Synchrotron Light Source II (NSLS II) (Figure 1). We are the first group to use operando hardware with gas lines and electric inputs at the 18-ID NSLS II. Figure 1 shows our most recent updated operando cell at the beamline. Using our Gen. 3 operando fuel cell hardware, spatial 2D mapping of Ni and Cu was achieved as a function of applied potential and time with XANES probes. References Roy, A.; Talarposhti, M. R.; Normile, S. J.; Zenyuk, I. V.; De Andrade, V.; Artyushkova, K.; Serov, A.; Atanassov, P., Nickel–Copper Supported on a Carbon Black Hydrogen Oxidation Catalyst Integrated into an Anion-Exchange Membrane Fuel Cell. Sustainable Energy & Fuels 2018.Serov, A.; Zenyuk, I. V.; Arges, C. G.; Chatenet, M., Hot Topics in Alkaline Exchange Membrane Fuel Cells. Journal of Power Sources 375, 149-157 2018 Kabir, S.; Lemire, K.; Artyushkova, K.; Roy, A.; Odgaard, M.; Schlueter, D.; Oshchepkov, A.; Bonnefont, A.; Savinova, E.; Sabarirajan, D. C., Platinum Group Metal-Free Nimo Hydrogen Oxidation Catalysts: High Performance and Durability in Alkaline Exchange Membrane Fuel Cells. Journal of Materials Chemistry A 2017, 5, 24433-24443 Figure 1. a) Our Gen. 4 operando fuel cell at nano TXM beamline at 18-ID NSLS II, b) CAD design of modular operando cell and a photograph of our manufactured cell. c) XANES spectra and 2D projections of NiCu/KB catalyst. Figure 1

(Invited) Structural Characterization of Polymer Electrolyte Fuel Cell Electrodes Using Nano-Scale Synchrotron X-Ray Computed Tomography

With improvements in the intrinsic oxygen reduction reaction activity of both Pt-based and platinum group metal-free (PGM-free) catalysts for polymer electrolyte fuel cells, in recent years more attention is being focused on utilization of the active sites in these catalysts under conditions relevant to the automotive application. Taking full advantage of the performance offered by these high activity catalysts requires comprehensive knowledge of the electrode microstructure and the spatial distribution of the catalyst, support, ionomer, and pores. Nano-scale resolution X-ray computed tomography (nano-CT) has been used as a powerful tool for non-destructive quantification of three-dimensional pore morphology and estimation of ionomer distribution in both Pt-based and PGM-free cathode catalyst layers (CCL) [1, 2]. On the other hand, due to insufficient resolution (20 nm voxel size) this technique cannot resolve the detailed structures of support, ionomer, and catalyst. We developed the “hybrid computational method” [2] combining statistical information from transmission electron microscopy (TEM), ultra-small angle X-ray scattering (USAXS), and BET with nano-CT to reconstruct the features below the nano-CT resolution. In this study, an example will be given of application of this method to a Pt alloy-based electrode. Various shape, size and composition agglomerates are reconstructed to 1 nm voxel resolution using the hybrid methodology. It is shown that ionomer film thickness is spatially non-uniform inside the agglomerates and strongly correlates with ionomer to carbon weight ratio (I/C) while it is independent of the agglomerate size. Compared to experimental observations, the application of capillary condensation theory to the reconstructed agglomerate structure is shown to accurately represent the relative humidity (RH) dependence of the electrochemically-active surface area (ECA). Furthermore, direct numerical simulations are performed to quantify localized transport losses. Effectiveness factor correlations are derived for use in macroscale models. The effects of RH, agglomerate structural properties, and pores in the carbon support on the local oxygen transport resistance are examined. It is demonstrated that low catalyst utilization is responsible for the experimentally-observed high local transport resistance under dry conditions and that agglomerate shape and size affect the local oxygen transport resistance only if the primary pores are poorly accessible.

Layered PGM-Free Electrode for Improved Mass Transport

Increased interest in platinum group metal-free (PGM-free) catalysts as a low-cost replacement for PGM catalysts for the oxygen reduction reaction (ORR) has resulted in significant progress in activity and durability over the past decade (1-4). However, PGM-free catalysts still require high catalyst loadings to generate an acceptable level of performance due to their low active site density and turnover frequency. The thick catalyst layers suffer from both high ionic resistance and gas-transport limitations. Consequently, improvements to the morphology of PGM-free electrodes is as important to enhancing the ORR performance as is catalytic activity. Previously, Yin et al. were able to improve PGM-free electrode performance by developing a new fabrication technique for PGM-free membrane electrode assemblies (MEAs) and by optimizing ionomer loading and equivalent weight (5). Previous studies by Komini Babu et al. suggested that more substantial advancements in fuel cell performance could be accomplished by improving water management in the conventional PGM-free electrode (6, 7). Conventionally thick PGM-free electrodes suffer from mass transport resistance and challenging water management, limiting high catalyst utilization to only near the polymer membrane (6). Therefore, in order to increase catalyst utilization improved water management can be achieved by decreasing hydrophilicity in the catalyst layer (CL) near the microporous layer (MPL), which can be achieved by reducing the ionomer content or by replacing the low equivalent weight (EW) ionomer with a high-EW. In this work, we introduce PGM-free cathodes with improved high-current performance (mass transport-controlled range of current densities) by depositing two catalyst layers with varied ionomer loading (IL) or equivalent weight (EW) in order to decrease hydrophobicity near the MPL. For example, the EW effect was studied by preparing a MEA with two cathode layers consisting of 55 wt.% IL with EW of 1100 near the MPL and EW of 830 near the membrane. A different MEA with varying IL layers was prepared using 830 EW ionomer with 55wt. % IL near the membrane and 35 wt.% IL near the MPL. Our preliminary results (Figure 1) show that the MEA with the layered PGM-free cathode demonstrated improved performance when compared to the conventional single layer MEA, with the highest performance observed from the IL layered MEA. Acknowledgments This research was supported by DOE-EERE’s Fuel Cell Technologies Office (FCTO) through Electrocatalysis Consortium (ElectroCat). References G. Wu, K. L. More, C. M. Johnston and P. Zelenay, Science, 332, 443 (2011).H. T. Chung, D. A. Cullen, D. Higgins, B. T. Sneed, E. F. Holby, K. L. More and P. Zelenay, 484, 479 (2017).U. Martinez, S. Komini Babu, E. F. Holby, H. T. Chung, X. Yin and P. Zelenay, Advanced Materials, 1806545 (2019).U. Martinez, S. K. Babu, E. F. Holby and P. Zelenay, Current Opinion in Electrochemistry (2018).X. Yin, L. Lin, H. T. Chung, S. Komini Babu, U. Martinez, G. M. Purdy and P. Zelenay, ECS Transactions, 77, 1273 (2017).S. K. Babu, H. T. Chung, P. Zelenay and S. Litster, Journal of The Electrochemical Society, 164, F1037 (2017).S. Komini Babu, H. T. Chung, P. Zelenay and S. Litster, ACS Applied Materials & Interfaces, 8, acsami.6b08844 (2016). Figure 1

Nuclear Resonance Vibration Spectroscopy Study of 57-Fe-Enriched Atomically Dispersed (AD)Fe-N-C Oxygen Reduction Reaction Catalyst for Polymer Electrolyte Fuel Cells

Nuclear resonance vibration spectroscopy (NRVS) is a novel technique capable of probing vibrational modes of Fe atoms in samples enriched in the Mössbauer-active nucleus of 57Fe. Compared to other vibrational spectroscopies, such as infrared or Raman, NRVS is not subject to the optical selection rules, affording full vibrational spectrum with limited spectroscopic noise. The NRVS intensity is directly related to the magnitude and direction of the motion, adding a unique quantitative capability to the spectral analysis. The technique provides insights into the dynamics of Fe atoms in the Fe-N-C catalysts for oxygen reduction reaction (ORR), which is not offered by other spectroscopic methods such as x-ray absorption spectroscopy (XAS), most commonly used for characterizing the atomic structure of Fe-sites in this class of platinum group metal-free (PGM-free) ORR catalysts. For NRVS to be successful, it is critical for iron in the catalyst to remain in a single chemical form and to minimize contribution to the spectra from complex Fe compounds such as Fe-rich nanoparticles and clusters, routinely present in iron-based PGM-free catalysts. Since NRVS relies on the Mössbauer effect, a studied catalyst needs to be enriched in 57Fe to maximize the signal-to-noise ratio. In this study, an (AD)Fe-N-C catalyst, fully 57Fe-enriched and containing exclusively atomically dispersed Fe sites, was synthesized from a metal organic framework (MOF) precursor. As a heterogeneous electrocatalytic process, ORR is a surface reaction. Demonstrating the presence of surface Fe and providing its chemical characteristics thus represent a major step towards a better understanding of the origins of the catalytic activity of Fe-N-C catalysts. While fundamentally a bulk technique, NRVS can be made surface-specific when combined with molecules or ions capable of selectively interacting with Fe sites on the catalyst surface. The use of molecular or ionic surface probes such as nitric oxide (NO, an O2 analog) or nitrite anion (NO2 -) allows for the discrimination between the bulk iron and surface Fe species of interest to ORR electrocatalysis. In this talk, we will summarize our NRVS study of the (AD) Fe-N-C ORR catalyst for oxygen reduction, which provides new and important information about the nature of Fe sites as the most likely active centers for oxygen reduction reaction on Fe-based ORR catalysts. Acknowledgements This research has been supported by DOE Fuel Cell Technologies Office through Electrocatalysis Consortium (ElectroCat). It used resources of the Advanced Photon Source (APS) at sector 3, a U.S. Department of Energy (DOE) Office of Science User Facility. Microscopy was performed as part of a user project supported by Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences, a DOE Office of Science User Facility.

Correlations between Synthesis and Performance of Fe-Based PGM-Free Catalysts in Acidic and Alkaline Media: Evolution of Surface Chemistry and Morphology

The effects of the synthesis steps of a platinum group metal-free (PGM-free) catalyst on the surface chemistry, morphology, and electrochemical activity in acidic and alkaline media toward the oxyg...

Platinum group metal-free catalysts boost cost competitiveness of fuel cell vehicles

PGM-free catalysts for oxygen reduction represent a long-term, high-risk research and development approach with high potential impact on the single greatest cost contributor to automotive fuel cell stacks.

High-Throughput Synthesis and Characterization of PGM-Free PEFC Cathode Catalysts

The high cost of platinum, widely used in the electrode of polymer electrolyte fuel cells (PEFCs), is one of main barriers to the widespread use of fuel cell vehicles. Many studies have focused on reducing the excessive production cost of the fuel cell stack by either decreasing the platinum loading using more active Pt-based catalysts and nanostructured alloys or developing platinum group metal-free (PGM-free) catalysts based on low-cost transition metals (e.g., iron and cobalt), carbon, and nitrogen. This presentation will describe high-throughput techniques for synthesis, physical and chemical characterization, hydrodynamic activity screening, electrode fabrication, and fuel cell performance testing of PGM-free catalysts under the auspices of the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Offices’ Electrocatalysis Consortium (ElectroCat) [1]. Argonne’s robotic systems have been utilized to rapidly synthesize unique iron and transition metal-nitrogen-carbon (TM-N-C) catalysts exploring a large number of variables such as nitrogen content, ZIF-8 content, Fe content, TM dopant, and Fe precursor. These variables have been shown to be important factors in determining the activity and stability of the resulting catalysts. The catalysts were synthesized using multi-port ball-milling and multi-sample heat treatment. The ORR activity of synthesized materials was characterized using a newly-developed multichannel flow double electrode cell (m-CFDE), allowing the aqueous hydrodynamic measurement of multiple samples simultaneously and using removable electrode plugs to facilitate the quick change of catalyst [2]. A custom-designed system was utilized to deposit the catalyst-ionomer inks onto the glassy carbon electrodes of the m-CFDE electrode plugs. This system is equipped with a nano-liter injector with exchangeable micro-tips and a computer-controlled XY table mount for the electrode plug to uniformly distribute the ink across the 3 mm x 1 mm glassy carbon electrodes. The ORR activity is correlated with catalyst Fe speciation, as determined using Fe K-edge X-ray absorption spectroscopy (XAFS) and also with catalyst BET and electrochemically-determined surface areas. A segmented 25-electrode array cell was used for the high-throughput performance testing of 25 different types of catalyst in a single membrane-electrode assembly (MEA). The automated electrode deposition system has been used in combination with a custom-designed heated vacuum table and a multi-channel peristatic pump to deposit 25 circular electrodes onto the membrane in the pattern required by the NuVant Systems Inc. array fuel cell hardware [3]. This automated ink dispensing system can be used for precise and even catalyst ink deposition across the small circular array electrodes while minimizing the amount of material required to form those uniform electrodes. This presentation will correlate the performance of the catalysts in the MEA with the catalysts’ ORR activity, physical and chemical properties, and synthesis parameters. Acknowledgements This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office under the auspices of the Electrocatalysis Consortium (ElectroCat). This research used resources of the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract DE-AC02-06CH11357. Argonne National Laboratory is managed for the U.S Department of Energy by the University of Chicago Argonne, LLC, under contract DE-AC-02-06CH11357.

Electron Microscopy Study of Degradation Mechanisms in Platinum Group Metal-Free Catalysts

Commercial low-temperature polymer electrolyte membrane fuel cells (PEMFCs) rely on expensive platinum-based catalysts to drive the oxygen reduction reaction at the cathode. In order to overcome cost barriers limiting market penetration in both transportation and stationary applications, significant efforts are being made to develop suitable platinum group metal-free (PGM-free) catalysts. The latest classes of PGM-free catalysts show very promising mass activity, but the poor durability of these carbon-based materials is still a significant challenge that must to be addressed. In this effort, advanced analytical electron microscopy methods were employed to study the degradation pathways of PGM-free catalysts which were submitted to accelerated stress tests. Catalysts provided by four different suppliers were tested concurrently in a specially designed segmented fuel cell. Polarization curves were recorded both before and after a durability stress test which involved 10k square wave cycles between 0.95 – 0.60 V in air. Analytical electron microscopy performed across a wide length scale (sub-nanometer to micron) will be presented which links the observed performance losses with changes to the atomically dispersed active sites, carbon support, and ionomer distribution. The advantages and disadvantages of different catalyst precursors (metal organic frameworks, cyanamide/polyaniline, etc.) will also be discussed from a durability standpoint. This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office under the Electrocatalysis Consortium (ElectroCat). Microscopy was performed as part of a user project at ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility

(Invited) Progress Towards High Power Density and Durable PEFCs with PGM-Free Cathodes

This presentation will highlight recent advances in achieving high power density with polymer electrolyte fuel cells (PEFCs) that use platinum-group metal-free (PGM-free) catalysts for the cathode’s oxygen reduction reaction (ORR). PGM-free catalysts have the potential to radically reduce the cost of manufacturing PEFC cathodes. However, several technical challenges with PGM-free cathodes need to be addressed, including a need for increased catalyst activity and stability as well as improved catalyst integration into MEAs for higher power density and better durability. Electrode engineering is crucial to achieving high performance with PGM-free catalysts because the lower volumetric activity yields thicker electrodes that cause significant water flooding and large oxygen and proton transport losses. This talk will highlight recent advances in the field as well as our own recent achievements with PGM-free catalysts prepared from chemically-doped metal organic framework (MOF) precursors. The focus of the talk will be on the electrode integration of these catalysts, including a systematic optimization of the catalyst morphology for MEA integration that leverages the ability to carefully tune the size of the MOF-derived catalyst’s primary particles. A key aspect of our study was the control of particle morphology for effective ionomer distribution. Through combined computational simulation, experimental fuel cell characterization, and electron and high resolution X-ray imaging, we show the significant performance improvements that can be achieved. Furthermore, we present the current status of the MEA durability and will discuss current hypotheses regarding the degradation mechanisms and possible mitigation strategies. The authors gratefully acknowledge research support from the Electrocatalysis Consortium (ElectroCat), established as part of the Energy Materials Network under the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, under Contract Number DE-EE0008076.

Fe–N–C electrocatalyst with dense active sites and efficient mass transport for high-performance proton exchange membrane fuel cells

To achieve the US Department of Energy 2018 target set for platinum-group metal-free catalysts (PGM-free catalysts) in proton exchange membrane fuel cells, the low density of active sites must be overcome. Here, we report a class of concave Fe–N–C single-atom catalysts possessing an enhanced external surface area and mesoporosity that meets the 2018 PGM-free catalyst activity target, and a current density of 0.047 A cm–2 at 0.88 ViR-free under 1.0 bar H2–O2. This performance stems from the high density of active sites, which is realized through exposing inaccessible Fe–N4 moieties (that is, increasing their utilization) and enhancing the mass transport of the catalyst layer. Further, we establish structure–property correlations that provide a route for designing highly efficient PGM-free catalysts for practical application, achieving a power density of 1.18 W cm−2 under 2.5 bar H2–O2, and an activity of 129 mA cm−2 at 0.8 ViR-free under 1.0 bar H2–air. Iron single-atom catalysts are among the most promising fuel cell cathode materials in acid electrolyte solution. Now, Shui, Xu and co-workers report concave-shaped Fe–N–C nanoparticles with increased availability of active sites and improved mass transport, meeting the US Department of Energy 2018 target for platinum-group metal-free fuel cell catalysts.