PGM-free Cathode Research Articles

Degradation Analysis for Platinum Group Metal-Free Fuel Cell Cathodes

Recent advancements in platinum-group metal-free (PGM-free) catalysts have overwhelmingly prioritized improving initial electrocatalytic activity rather than focusing on catalyst stability. Unfortunately, this emphasis has led to less significant progress in addressing catalyst degradation, ultimately falling well below the level required for commercialization. Shifting the focus towards understanding and mitigating degradation unlocks the true potential of PGM-free catalysts and paves the way for their successful implementation in commercial applications. This work explores methods to mitigate degradation and expand models to predict PGM-free cathode fuel cell degradation due to voltage cycling, specifically current density loss over time. The tested PGM-free cathode fuel cells, using iron (Fe)-based catalyst, were electrochemically characterized before and after the accelerated stress tests (AST) using H2/N2 electrochemical impedance spectroscopy, cyclic voltammetry, and air polarization curves. This study reports the functions of each acceleration factor due to changes in temperature, cycling upper potential limits, and relative humidity are reported. The data indicates that degradation rates increase as temperature, upper potential limit, and relative humidity increase in Fe-based cathode proton exchange membrane fuel cells (PEMFCs). This finding is essential to more accurately quantify losses in active site density with respect to temperature, varying upper potential cycling limit, and relative humidity. More generally, these results can aid in predicting Fe-based cathode fuel cell degradation due to changes in operational conditions.

Atomically dispersed Fe-N-C catalyst with densely exposed Fe-N4 active sites for enhanced oxygen reduction reaction

Atomically dispersed Fe-N-C catalysts are a promising alternative to platinum group metal (PGM) catalysts for oxygen reduction reaction (ORR). However, attaining efficient ORR activity and superior stability in Fe-N-C catalysts is still crucial yet challenging. Herein, we report a rational SiO2-mediated two-step pyrolysis strategy for stabilizing densely exposed Fe-N4 active sites on hierarchically porous carbon (HP-Fe-N-C/2). Benefiting from the high density of accessible Fe-N4 sites and the high graphitization degree of carbon support, the obtained HP-Fe-N-C/2 catalyst demonstrates outstanding activity and stability for ORR in both alkaline and acidic media. When used as the cathode catalyst, the assembled Zn-air battery shows a high peak power density of 217 mW cm−2 and ultra-long cycling stability for 1342 h without noticeable degradation. As a PGM-free cathode in proton exchange membrane fuel cells, it delivers a maximum output power density of 0.66 W cm−2.

Projected Fuel Cell Stack Costs Using Non-PGM Cathode Electrocatalysts

Platinum is typically used at the cathode of the cathode of proton exchange membrane fuel cells (PEMFCs) to overcome the kinetic limitations of the oxygen reduction reaction (ORR) to water. Pt is a noble metal that is mined mainly in South Africa and Russia. At present costs of $965 per troy ounce ($31 per gram), the cost of Pt in the cathode is approximately 40% of the total cost of each PEMFC membrane electrode assembly (1,2). Supply chain concerns have been raised that there is insufficient Pt in the world to support the conversion of all automobiles to hydrogen, or that increasing the demand of Pt through mass commercialization of fuel cells would drive up its demand and then cost (1,3). Researchers have therefore looked for catalysts that do not contain Pt or other precious metals (3). Oxygen reduction without Pt is possible, and all living creatures and plants use cytochrome enzymes with reactive iron or copper sites as part of a catalytic cycle to reduce O2 to reactive oxygen. Significant improvements have been gained in the performance of fuel cells with non-PGM cathodes by improving the density of sites and improving the catalyst durability (4).We estimate the cost impact of eliminating Pt from the cathode of a heavy-duty fuel cell stack. The calculations are based on the comparison of Plug fuel cells with Pt cathodes, compared to literature polarization curves for cells and stacks containing PGM-free cathodes (note that the anodes still contain Pt). We carry out a sensitivity analysis using different costs to the non-PGM catalysts. Our primary finding is that eliminating Pt from the cathode does little to reduce stack costs because the performance of the fuel cells with non-PGM cathodes is low, and larger stacks are needed to meet performance requirements. We will also discuss how using a non-PGM cathode affects projected recycling costs.

Improved Activity and Durability of Carbon-Coated Pd and PdNi Anode Catalysts in AEMFC

Anion exchange membrane fuel cells (AEMFC) have received growing attention due to the strategic potential of using low content (or absence) of PGM catalysts in the electrodes.[1] Completely PGM-free cathode catalysts are well established in those conditions,[2] while the anode ones remain critical in terms of the beginning-of-life activity and long-term performance.[3] The widely Pt- and Pd-based catalysts degrade mainly by nanoparticles detachment and agglomeration in alkaline conditions.[4] These degradations may be slowed down by wrapping the nanoparticles in some carbon layers (called carbon-coated), that prevent also metal oxidation and leaching into the electrolyte in the RDE setup.[5] However, the performance in AEMFC of carbon-coated nanoparticle catalysts is still unclear.Here, two groups (previously characterized in RDE, XRD, IL-TEM, XPS and ICP-MS techniques[5]) of carbon-coated catalysts were investigated in 5 cm2 AEMFC: monometallic Pd (Pd@C/C) and bimetallic PdNi (PdNi@C/C) nanoparticles supported on carbon, and compared them to unprotected ones (Pd/C and PdNi/C). The four different anode catalysts were compared for a fixed Pd anode loading of 0.27 mgPd cm-2, and were combined with a ZIF-8 derived Fe-N-C cathode catalyst (previously developed Fe0.5-NH3). The Pd@C/C and PdNi@C/C reached superior initial performances (700 and 1300 mA cm-2) than the Pd/C and PdNi/C references (300 and 390 mA cm-2) at 0.6 V and Tcell = 60 °C. This translates into an initial activity trend of PdNi@C/C > PdNi/C > Pd@C/C > Pd/C. After 12 h durability test at a cell voltage of 0.3 V, the AEMFC with the PdNi@C/C based anode exhibits a dramatically higher retention of performance (95%) than the cells with Pd/C (55%) and PdNi/C (70%) anodes. These results evidence the robustness of bimetallic carbon-coated nanoparticle catalysts for AEMFC anodes. Figure 1

(Energy Technology Division Walter van Schalkwijk Award in Sustainable Energy Technology Address) Exploring New Electrode Designs with Nanofibers

In recent years, electrospinning has been gaining popularity as a robust and scalable technique for the fabrication of non-woven mats of sub-micron diameter polymer fibers. Although not as well studied, the technique can also be used to prepare particle/polymer fiber networks with high intra- and inter-fiber porosity. Such fibrous mats can be used as porous electrodes in fuel cells and batteries, where a high electrode/electrolyte interfacial area is needed. In this talk, particle/polymer electrospinning will be presented as a platform technique to fabricate a variety of electrode and membrane-electrode-assembly (MEA) products with unique and desirable compositions, architectures, and performance characteristics. Laboratory results will be presented for: (1) hydrogen/air fuel cell cathodes with Pt/C and PGM-free catalysts, (2) Pt/C hydrogen electrodes in a H2/Br2 redox flow battery, and (3) Si anodes for lithium-ion batteries.High particle content fibers containing Pt/C or Pt-alloy catalyst powder and a Nafion®perfluorosulfonic acid binder have shown great promise as electrodes in H2/air fuel cells. During electrospinning, Nafion ionomer migrates to the outer edges of the fiber, effectively creating a core-shell fiber morphology, with a low I/C ratio in the fiber interior. Additionally, the interior of the fiber contains nm-size pores and the outer surface of the Nafion shell exhibits enhanced hydrophobicity, due presumably to an accumulation of the polytetrafluoroethylene backbone chains (-CF2-CF2-) of Nafion. The resulting structure is advantageous in that the majority of catalyst particles are in a low I/C environment, nanopores in the fiber interior trap electro-generated water via capillary condensation, and the hydrophobic surface of the fibers facilitates electrogenerated water removal from the mat during fuel cell operation. The fuel cell power output using fiber mat anodes and cathodes is very high for a low Pt loading (0.10 -0.20 mg/cm2), near or above 1.0 W/cm2 for H2/air feeds in the range of 60-100% relative humidity.1,2 Additionally, the fiber mat cathodes exhibit excellent durability, with minimal power loss after accelerated voltage cycling tests. The hydrophobicity of fibers can be enhanced by adding polyvinylidene fluoride (PVDF) directly to the Nafion binder. Such blended binders in fiber/particle hybrid electrodes extend the operational lifetime of fuel cells with iron-based PGM-free cathode MEAs.As an extension of the fuel cell electrode work, electrospun Pt/C-Nafion nanofiber mat hydrogen electrodes have been made and tested for a H2-Br2 redox flow battery.3 The hydrophobic surface coating of core-shell fibers effectively blocks bromine species from poisoning the hydrogen electrode. In a 6.6 M HBr electrolyte lab-scale redox flow battery, the fiber mat electrodes worked extraordinarily well, with: (i) high power at low Pt loading, e.g., 650 mW/cm2 at 0.70 Vfor a loading of 0.11 mgPt/cm2 (6.4 kW/gPt) and 550 mW/cm2 at 0.65 V for a loading of 0.048 mgPt/cm2 (11.5 kW/gPt), (ii) no loss in power for more than 170 charge/discharge cycles, and (iii) a roundtrip current efficiency of 95%at both 0.2 and 0.3 A/cm2.For Li-ion battery applications, single and dual fiber anode mats have been prepared and evaluated, where the fibers contain a polymer binder (e.g., polyacrylic acid, polyamidimide, or polyvinylidene fluoride), Si nanoparticles as the electrochemically active material, and carbon powder as an electron conductor.4 Here, the gravimetric and areal capacities of the fiber anode are considerably higher than those of a conventional carbon particle anode. Inter-fiber voids allow for volume changes in Si during lithiation/delithiation, thus minimizing capacity fade during charge/discharge cycling (up to 600 cycles).AcknowledgmentsThis work was funded by the U.S. Department of Energy, Contract No. DE-EE0007653 from the Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability Consortium (Fuel Cells Program Manager: Dimitrios Papageoropoulos) and Contract No. DE-EE0007215 from the Vehicle Technology Office.

Highly durable platinum group metal-free catalyst fiber cathode MEAs for proton exchange membrane fuel cells

Fe-based platinum group metal (PGM)-free catalysts were incorporated into electrospun fiber mat or powder cathode membrane-electrode-assemblies (MEAs) with a Nafion 211 membrane and a Pt/C powder anode. Fabrication and characterization tests were performed on MEAs with: (1) a conventional powder cathode with a neat Nafion binder, (2) a fiber mat cathode with a Nafion/polyethylene oxide (PEO) binder, where PEO was extracted before MEA testing, (3) a powder cathode with a blended binder of Nafion and polyvinylidene fluoride (PVDF), and (4) a series of fiber mat cathodes with different Nafion/PVDF binder weight ratios. Cathode degradation occurred, with a loss in power output, in MEAs with a neat Nafion powder cathode or with a Nafion fiber cathode. In contrast, little or no power loss was observed for powder or fiber cathodes when the binder was a blend of Nafion and PVDF. The presence of hydrophobic PVDF drove water and electrogenerated peroxide out of the cathode, away from catalyst particles, which improved cathode durability, but PVDF also decreased the binder conductivity and slowed oxygen reduction kinetics, resulting in lower power densities. A 75:25 w:w Nafion:PVDF fiber cathode MEA was the best compromise for maximizing power and minimizing catalyst degradation. For such a cathode, with a PGM-free cathode catalyst loading of 3.0 mg cm−2, a power density of 88 mW cm−2 at 0.5 V, 80 °C, and 200 kPaabs pressure was maintained for 80 h of continuous operation.

Design of PGM-Free Cathode Catalyst Layers for PEMFC Applications: The Impact of Electronic Conductivity

The impact of the electronic resistance of platinum-group-metal-free cathode catalyst layers (PGM-free CCLs) for proton-exchange-membrane fuel cells (PEMFCs) was systematically investigated. Here we selected two different PGM-free catalysts (having high and low electronic conductivity) and integrated them into CCLs without and with conductive carbon fiber additives. To investigate the impact of the electronic resistivity () of PGM-free catalysts and CCLs, their through-plane values were quantified by an in situ electrochemical impedance spectroscopy (EIS) approach based on a one-dimensional transmission line model. The results indicate that the electronic conductivity of PGM-free CCLs can be increased by adding carbon additive, resulting in a significant improvement of the fuel cell performance (by ∼60 mV at 1 A cm−2 in H2/O2 configuration). Ex situ four-point probe measurements of the in-plane values of some of the CCLs were found to differ vastly from the through-plane values. This difference is attributed to the anisotropic morphology of the CCLs, caused by preferential fiber orientation and/or cracks in the CCLs. In the end, we suggest guidelines for the design and evaluation of PGM-free CCLs and for assessing and improving their electronic resistance.

Air Contamination of Platinum-Group Metal-free Fuel Cell Cathodes with Atomically Dispersed Iron Active Sites

In a wide variety of applications, proton exchange membrane (PEM) fuel cells must be tolerant to contaminants from the ambient air and the degradation of the system components. Previous work has demonstrated the effects of some of the most common air contaminants on platinum-group metal (PGM) cathode fuel cells. However, the results of air contamination on the degradation in a side-by-side comparison between PGM and PGM-free based cathode fuel cells has yet to be shown. Herein, we investigate the effects of air contamination by acetonitrile and sulfur dioxide (SO2) on operating PGM and PGM-free cathode fuel cells. This was done by monitoring the changes in current density at a constant voltage hold before, during, and after sulfur dioxide and acetonitrile contamination. In addition, air polarization curves, cyclic voltammetry, and electrochemical impedance spectroscopy characterized the overall contamination effects of each cell. It is found that acetonitrile at 20 ppm and SO2 at 4.6 ppm do not result in any additional degradation in iron (Fe)-metal–organic framework (MOF) derived cathode fuel cells during operation, while current density losses greater than 70% are seen in platinum (Pt)-based cathode fuel cells. This tolerance to contaminants is an often-overlooked advantage of PGM-free catalyst cathode fuel cells, which suffered no additional degradation due to the introduction of air contaminants in this study.

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

Platinum group metal (PGM)-free catalysts for oxygen reduction reaction (ORR) have attracted significant attention in the last two decades. These catalysts typically perform better in alkaline aqueous electrolytes than in their acidic counterparts.1, 2 However, the performance of PGM-free ORR catalysts in anion exchange membrane fuels cells (AEMFCs) have been consistently lower than in the acidic polymer electrolyte fuel cells (PEFCs). The most likely reasons for the sub-par behavior of PGM-free catalysts in AEMFCs has been often linked to difficulties in preparing electrodes with anion exchange ionomers and assuring efficient water management. These challenges have been amplified by the high-loading requirement for PGM-free ORR catalysts, resulting in electrodes by as much as an order of magnitude thicker than the PGM-based ones.In this presentation, we will demonstrate AEMFCs with much improved performance of the PGM-free cathode (Fe-N-C catalyst-based). The performance improvement has been achieved by optimizing the electrode fabrication process, including changes to the electrode configuration and catalyst ink preparation. These changes have allowed us to elevate the AEMFC performance, including the peak power density of > 0.8 W cm-2 in H2-O2 cells, to the level comparable to that of the corresponding PEFC, operating with a PGM-free cathode under the same operating conditions.

Half-cell electrode assessments of a crossover-tolerant direct methanol fuel cell with a platinum group metal-free cathode

In this work, a platinum group metal- free (PGM-free) Fe-N-C oxygen reduction reaction (ORR) catalyst derived from metal organic framework precursors (Fe-MOF) is evaluated at the cathode side of a direct methanol fuel cell (DMFC). A wide range of methanol concentrations were used, and the performance of the DMFC using air was evaluated and compared to a commercial Pt/C cathode. We conducted our tests with a custom, integrated H2 reference electrode that separately provided the anode and cathode overpotentials. This allowed us to perform a Tafel analysis on both electrodes and understand the dominant polarizations involved at the PGM-free cathode and the PtRu/C anode. Overall, we achieved a high cell performance using a Fe-N-C based catalyst under liquid methanol and air operation with a peak power density of 111 mW/cm2, where the cathode presented a markedly larger contribution to the voltage loss versus the anode. Additionally, we used nanoscale X-ray computed tomography (nano-CT) to image the Fe-N-C cathode and investigate the ionomer and pore size distributions allowing us to identify areas of improvement for catalyst layer fabrication.

Quantifying the electrochemical active site density of precious metal-free catalysts in situ in fuel cells

Advances in the development of precious-group metal-free (PGM-free) catalysts for the oxygen reduction reaction (ORR) in fuel cell cathodes have produced active catalysts that reduce the performance gap to the incumbent Pt-based materials. However, utilization of state-of-the-art PGM-free catalysts for commercial applications is currently impeded by their relatively low durability. Methods designed to study catalyst degradation in the operation of fuel cells are therefore critical for understanding durability issues and, ultimately, their solutions. Here we report the use of Fourier-transform alternating current voltammetry as an electrochemical method for accurate quantification of the electrochemically active site density of PGM-free cathode catalysts, and to follow their degradation in situ during the operation of polymer electrolyte fuel cells. Using this method, we were able to quantify the electrochemical active site density, which will enable the elucidation of degradation mechanisms of PGM-free ORR catalysts in situ in fuel cells.

PGM-Free Oxygen-Reduction Catalyst Development for Proton-Exchange Membrane Fuel Cells: Challenges, Solutions, and Promises

ConspectusProton-exchange membrane fuel cells (PEMFCs) are efficient and clean hydrogen energy technologies for transportation and stationary applications. Highly active and durable low-cost cathode catalysts for the oxygen-reduction reaction (ORR) under challenging acidic environments are desperately needed to address the cost and durability issues of PEMFCs. The most promising platinum group metal (PGM)-free catalysts for the ORR in acidic media are atomically dispersed and nitrogen-coordinated metal site catalysts denoted as M–N–C, M = Fe, Co, or Mn. Due to significant efforts in the past few decades, these catalysts have demonstrated much-improved ORR activity and promising initial fuel cell performance approaching traditional Pt/C catalysts. However, the insufficient long-term stability (up to 5000 h) under PEMFC operation represents a primary technical barrier to making current PGM-free catalysts less viable yet in PEMFCs. In this Account, we highlight recent advances in synthesizing efficient PGM-free catalysts for the ORR in PEMFCs, emphasizing effective strategies to improve mass and intrinsic activity and the possible degradation mechanisms. In particular, a chemical doping method based on the zeolitic imidazolate framework (ZIF)-8 represents the key to developing efficient M–N–C catalysts containing atomically dispersed and nitrogen-coordinated single metal active sites (i.e., MN4). The newly acquired understanding of the formation mechanism of MN4 active sites during the thermal activation and its correlation to catalytic properties guide the rational catalyst design rather than relying on current trial-and-error approaches. Considerable efforts have further been invested in increasing the active site density and enhancing intrinsic activity by regulating carbon-phase structures and the local coordination environment. These highly active catalysts usually suffer from significant activity loss during the ORR. Therefore, breaking the activity–stability trade-off is the key to simultaneously achieving activity and stability in one catalyst, which is discussed on the basis of our recent successes in regulating local carbon structures surrounding active single metal sites. Significant research efforts toward understanding the degradation mechanisms and improving the lifetime of PGM-free catalysts are still crucial for viable applications in the future. Novel electrode designing strategies are needed to translate the PGM-free catalysts’ ORR activity to solid-state electrolyte-based membrane electrode assemblies (MEAs) with robust three-phase (i.e., gas–liquid–solid) interfaces for efficient charge and mass transports for performance improvement. On the basis of our effort at the University at Buffalo supported by ElectroCat Consortium associated with U.S. DOE’s Hydrogen and Fuel Cell Technologies Office, we provide a perspective on PGM-free cathode catalysts concerning remaining bottlenecks and future opportunities, aiming to inspire the community in both mechanistic understanding and technological development.

On the Correlation between the Oxygen in Hydrogen Content and the Catalytic Activity of Cathode Catalysts in PEM Water Electrolysis

Altogether five platinum group metal (PGM) and PGM-free cathode catalysts were investigated in full PEM water electrolysis cells regarding their polarisation behaviour and their hydrogen and oxygen recombination properties. It was shown that the recombination activity of permeated oxygen and evolved hydrogen within the cathodic catalyst layer correlates with the activity of the oxygen reduction reaction (ORR) which was determined ex situ with linear sweep voltammetry. We found that the investigated PGM-free cathode catalysts had a low activity for the ORR resulting in higher measurable oxygen in hydrogen volume fractions compared to the PGM catalysts, which are more active for the ORR. Out of the three investigated PGM-free catalysts, only one commercially available material based on a Ti suboxide showed a similar good polarisation behaviour as the state of the art cathode catalyst platinum, while its recombination activity was the lowest of all catalysts. In addition to the recombination of hydrogen and oxygen on the electrocatalysts, we found that the prevalent carbon-based cathodic porous transport layers (PTL) also offer catalytically active recombination sites. In comparison to an inactive PTL, the measurable oxygen flux using carbon-based PTLs was lower and the recombination was enhanced by microporous coatings with high surface areas.

Understanding the Activity and Stability of Single-Atom Catalysts for the ORR in Anion Exchange Membrane Fuel Cell(AEMFCs)

To be considered the ultimate power source for transportation systems and portable devices, anion exchange membrane fuel cells (AEMFCs) need to overcome significant cost barriers1,2. For this, the U.S. Department of Energy (DOE) recently has set some challenging activity targets for AEMFCs; this includes the near-term target PGM loading is 0.2 mg/cm2 for 2021–2023, and by 2024 the total loading should be limited to ⩽ 0.125 mg/cm2.This final milestone is the first step in enabling a shift to completely platinum group metal (PGM)-free membrane electrode assemblies (MEAs) by 20303. Unfortunately, today’s high performing4 and long-life5,6 MEAs all rely on a high loading of PGM in both the anode and the cathode, resulting in a total PGM loading of ~1 mgPGM/cm2.To reduce the PGM loading in AEMFCs, it is important to transition to very low PGM electrodes, and eventually to catalysts that are PGM-free but do not contain bulk transition metals that passivate or dissolve in the electrolyte. One approach that can be used in both cases is to create atomically-dispersed ORR catalysts. On the PGM side, single atom catalysts (or even dual atom clusters) have the potential to drastically lower the PGM loading and cost while maximizing utilization7. The rational design of electrocatalyst, either through modification of commercial catalysts or entirely new structures, is very important to reach low loading high activity electrocatalusts for fuel cells.8–10. On the non-PGM side, the most common catalyst moiety has been single atoms of Fe in N4-funcitonal groups in carbon, or Fe-N-C. Though there have been Fe-N-C catlaysts that have shown good activity and durability in ex-situ tests, these materials are yet to be transitioned successfully to MEAs that can compete with their PGM-containing counterparts.In this study, two approaches will be discussed. The first synthesizes a high densities of atomically dispersed single-atom Pt catalysts, which were prepared in-house by a simple, and scalable synthesis method of chelate fixation (CheFi). These materials have been physically characterized using a wide array of techniques including x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution Cs aberration-corrected scanning transmission electron microscopy (STEM). The catalysts were also tested for their ORR activity and their in-situ behavior in operating AEMFCs. In addition to their performance data, these catalysts yielded interesting mechanistic insight that will be discussed. The second approach was to investigate the effect of structure on the behavior of Fe-N-C catalysts. In this part of the work, it was demonstrated that single atoms are indeed the most active and important componenes. It is also shown that the physical properties of the carbon structure (porosity and graphitization) play a role in determining not only the ORR activity, but also the ability to translate that activity to highly performing AEMFCs. In fact, it will be shown that it is possible to create AEMFCs with a PGM-free cathode that are able to achieve a peak power density > 2.0 W/cm2 and a kinetic current at 0.9 V (iR-free) of 100 mA/cm2. These cells are also able to out-perform and out-live their PEMFC counterparts.

On the Correlation between the Oxygen in Hydrogen Content and the Catalytic Activity of Cathode Catalysts in PEM Water Electrolysis

Hydrogen has a central role in future energy storage scenarios. In order to realise high market penetrations for proton exchange membrane water electrolyzers (PEMWE), the investment costs need to be reduced without compromising operational costs [1, 2, 3]. Today only the usage of precious platinum group metals (PGM) as electrocatalysts allows a good electrochemical polarisation behaviour and accordingly minimal operating costs. Platinum is known to be an outstanding catalyst for both hydrogen oxidation and evolution [1, 2, 3, 4]. However, platinum also catalyses the formation of reactive oxygen species when oxygen permeates from anode to cathode. The intermediates of the recombination reaction are sufficiently reactive to attack the polymeric backbone of the electrolyte membrane which leads to a performance loss due to material degradation and eventually to a cell failure [1, 5, 6].For this reason, not only the polarisation behaviour but also the hydrogen-oxygen recombination characteristics need to be evaluated in order to find new cathode catalysts for PEMWE. In this contribution, the polarisation behaviour of five different cathode catalysts was investigated. The ordinary electrochemical characterisation (polarisation curves, electrochemical impedance spectroscopy and linear sweep voltammetry) was extended by measuring the permeated oxygen in the cathodic product gas to draw conclusions on the recombination characteristics of the used catalysts.In general, the studied PGM-free catalysts showed lower activities for the oxygen reduction reaction and higher oxygen in hydrogen volume fractions compared to the PGM catalysts. However, all investigated electrocatalysts still show an activity towards the recombination reaction. Ex situ linear sweep voltammetry measurements have shown that the activity for the recombination reaction of oxygen and hydrogen correlates with the activity for the oxygen reduction reaction. In particular, a commercial PGM-free cathode catalyst based on a Ti suboxide showed a comparably good polarisation behaviour for the hydrogen evolution reaction similar to platinum and the lowest activity for the oxygen reduction reaction at the same time. Further, also the influence of the used PTL material on the measurable oxygen crossover was investigated. The results show that also carbon based PTL materials contain catalytically active sites for the recombination of permeated oxygen with hydrogen.Literature:[1] U. Babic, J. Elec. Soc., 2017, 164.[2] K. Ayers, Annu. Rev. Chem. Biomol. Eng., 2019, 10.[3] M. Carmo, Int. J. Hydrogen Energ., 2013, 38.[4] C. V. Pham, ChemElectroChem, 2018, 5.[5] M. Chandesris, Int. J. Hydrogen Energ., 2015, 3.[6] S.A. Grigoriev, Int. J. Hydrogen Energ., 2014, 35. Figure 1

A Durable Platinum Group Metal-Free Oxygen Reduction Catalyst for Polymer Electrolyte Fuel Cells

A Durable Platinum Group Metal-free Oxygen Reduction Catalyst for Polymer Electrolyte Fuel Cells Hanguang Zhang, Hoon T. Chung, Ulises Martinez, Edward F. Holby, and Piotr Zelenay Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA Performance of platinum group metal-free (PGM-free) catalysts for oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs), especially high-temperature metal-nitrogen-carbon (M-N-C) materials, has improved a lot in the last two decades,1, 2 however, their durability continues to present a big challenge. PGM-free catalysts typically suffer a 40-80% activity loss during the first 100 hours of operation in the low-temperature fuel cell cathode, which is way short of the 8,000-hour lifetime target for PEFCs.3, 4 Significant improvements to PGM-free catalyst performance durability are thus required before they can be considered for practical PEFCs.In this presentation, we will introduce a ‘dual-zone’ approach to high-temperature synthesis of PGM-free catalysts, which results in modifications to the catalyst surface and, ultimately, in very significant improvements to their durability. In H2-air fuel cell testing, the ‘dual-zone’ fuel cell catalyst has shown a minimal performance loss during up to 80,000 voltage cycles using standardized PGM-free durability test protocol developed in ElectroCat consortium and approved by DOE. The catalyst has also retained 70% of its initial activity in a 600-hour constant-voltage fuel cell test at 0.70 V.We will report fuel cell performance data in the first part of this presentation, followed by discussion of possible origins of the much improved performance durability imparted by the ‘dual-zone’ approach. Acknowledgement This research was supported by the U.S. DOE, Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office under the auspices of the Electrocatalysis Consortium (ElectroCat). References Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P., High-performance electrocatalysts for oxygen reduction derived from polyaniline, iron, and cobalt. Science 2011, 332, 443-447.Chung, H. T.; Cullen, D. A.; Higgins, D.; Sneed, B. T.; Holby, E. F.; More, K. L.; Zelenay, P., Direct atomic-level insight into the active sites of a high-performance PGM-free ORR catalyst. Science 2017, 357, 479-484.Shao, Y.; Dodelet, J.-P.; Wu, G.; Zelenay, P., PGM-Free Cathode Catalysts for PEM Fuel Cells: A Mini-Review on Stability Challenges. Mater. 2019, 31, 1807615.U.S. DRIVE Fuel Cell Technical Team Roadmap; Department of Energy, US, 2017.

Platinum group metal-free Fe-based (Fe N C) oxygen reduction electrocatalysts for direct alcohol fuel cells

In direct alcohol fuel cells (DAFCs), the oxidation of alcohols happens at the expenses of the oxygen reduction reaction at the cathode. DAFCs' cathodes use a significant amount of platinum that is an expensive critical raw material. Moreover, platinum oxidizes alcohols, a fact that combined with alcohol crossover, decreases significantly the performance of the cells. The use of Fe-based (FeNC) platinum group metal–free (PGM-free) cathodes is a convenient strategy to overcome these limitations. This review analyzes the application of PGM-free cathodes to DAFCs. The discussion focuses on acidic systems and covers the following subjects: (i) the breakdown of DAFC potential in its components, (ii) the analysis of the advantages from the use of the PGM-free cathode, and (iii) a review of the performance and durability of DAFCs. The review closes with a view of the authors of the future perspective for the research.

Understanding the Influence of Fe-N-C Cathode Catalyst Structure on Their Performance and Durability in High Performing Anion Exchange Membrane Fuel Cells

Anion exchange membrane fuel cells (AEMFCs) are a potentially very low-cost alternative to traditional proton exchange membrane fuel cells (PEMFCs)1,2 – as long as their promise to offer high performance and durability with no platinum group metal (PGM) catalysts is realized. From the perspective of performance and durabiltiy, AEMFCs are doing well quite well with some studies even able to achieve peak power densities of 3.5 W/cm2 3 with H2/O2 gas feeds and more than 2000 hours of continuous operation with less than 5% voltage decay4. However, these were achieved very high areal PGM loadings (anode + cathode) of 1.0-1.3 mgPGM/cm2.Recently, the U.S. Department of Energy (DOE) set some challenging activity targets for AEMFCs5. The near-term (2021-2023) targets all require a PGM loading £ 0.2 mg/cm2. The 2024 target is ⩽ 0.125 mg/cm2 and the final milestone of entirely PGM-free AEMFCs is slated for 2030. To get there, there is a need to develop PGM-free catalysts for both the anode and cathode6. At the anode, there are very few options for PGM-free catalysts. This means that the intermediate-term cell development should focus on using PGM-free cathodes and low-PGM anodes. Fe-N-C catalysts have emerged as the most promising candidates for the oxygen reduction reaction at the AEMFC cathode. However, though this family of materials has been of interest for several years, reported catalysts in the literature show quite different activity (in ex-situ experiments) and performance (in operating cells), despite reporting similar synthesis routes and precursors2. One reason behind this poor performance could be the catalyst's overall structure and the dispersion of catalytically-active single-atom reaction sites.In this study, multiple Fe-N-C catalysts were prepared and investigated. The main goal of the preparation was to prepare Fe-N-C cathodes with different levels of atomic dispersion and overall catalyst microstructure. After synthesis, catalysts were physically characterized using a suite of tools including x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution aberration-corrected scanning transmission electron microscopy (STEM). The electrochemical behavior of the catalysts was also studied – both ex-situ in a three electrode cell as well as in operating AEMFCs. From these results, our team was able to find important links between the catalyst structure and its in-cell performance, providing guidance for the design of future materials. Of particular note, AEMFCs were constructed with a Fe-N-C cathode and PtRu/C anode that were able to reach peak power densities over 2 W cm2 with H2/O2 reacting gases and stable operation for more than 100 h. These cells were also able to achieve an iR-corrected current density at 0.9 V as high as 124 mA/cm2, exceeding the U.S. Department of Energy target of 44 mA/cm2. In an alternate configuration, the Fe-N-C cathode was paired with a low-loading PtRu/C anode electrode to create AEMFCs with a total PGM loading of only 0.125 mgPt‐Ru cm−2 (0.08 mgPt/cm2 ). That configuration achieved 10.4 W mgPGM −1 (16.25 W/mgPt 1), which exceeds the U.S. Department of Energy 2022 milestone for AEMFC initial performance for the first time.

Layer Design for PGM-Free Catalysts for Pemfcs: Impact of Electrical Conductivity & Diagnostic Tools

Recently, the notable efforts placed on investigating platinum group metal (PGM)-free catalysts (M-N-C catalysts, M = Fe, Co, & Mn) for the oxygen reduction reaction (ORR) have led to the possible development of low-cost proton exchange membrane fuel cells (PEMFCs).1 However, compared to PGM ORR catalysts, the lower turnover frequency and active site density of PGM-free catalysts generally require high-surface-area structures and high catalyst loadings to achieve reasonable low-current density performance in PEMFCs, thereby resulting in significant ionic, mass, and electronic transport overpotentials. 2 With regards to these transport resistances, an intriguing observation from the PEMFC performance with PGM-free cathode catalysts is the significantly higher electronic overpotential (iR-drop) when compared to PGM-based cathode catalysts (e.g., Pt/C). This increased iR-drop results in an increase of the high frequency resistance (HFR) due to the high electrical resistivity (ρe- in Ω/cm) of the thick PGM-free cathode catalyst layers. While ρe- of conventional Pt/C electrodes is rather low (~0.5-1 Ω/cm) and results in negligible overpotential losses, ρe- of PGM-free cathode layers can be higher by a factor of ~10-20 due to poor electrical conductivity of the active site (e.g., Fe-N-C) and the high-surface area structures of these catalysts. While several studies have revealed the impact of ionic and mass transport resistances of PGM-free cathode electrodes,3,4 the impact of electrical resistance is still unclear. A quantification of this effect is required for the rational design of PGM-free cathode electrodes, whereby the following three points would be of major interest: (1) The values of PGM-free catalysts vary widely, so that in a comparative study catalysts with high and low should be investigated. (2) When preparing electrodes with PGM-free catalysts that have a high value ρe-, conductive-carbon additives are commonly added, but there is no simple diagnostic tool that could be used for optimizing the amount of additive. (3) A quantitative model to predict the cell voltage loss due to the impact of ρe- would be desired. This can be accomplished by in-situ AC impedance analysis based on a transmission line model. 5 However, owing to higher ratio of ρe- to the ionic resistivity (ρH+) in PGM-free electrodes (>0.1/1), a modified approach needs to be developed to precisely quantify the voltage losses.6 In the present work, we present small-active area (5 cm2) single-cell H2/O2 PEMFC performance (differential-flow operation) combined with in-situ AC impedance and ex-situ 4-point-probe resistance measurement in order to elucidate the impact of the electrical resistivity of PGM-free cathode catalyst layer on performance. Two different catalysts with low and high ρe- are investigated. The results indicate that the lower one shows no significant change in cell voltage upon the addition of 7.5 wt% of conductive carbon (Figure 1a). However, with same wt% of carbon additive, the cell voltage for catalyst with high ρe- can be increased by 70 mV at 1 A/cm2, resulting in a ~30% improvement in peak power density (Figure 1b). Furthermore, we will present an in-situ AC impedance technique under blocking condition (N2 feed to cathode) that we developed to quantify ρe- and ρH+ of PGM-free cathode electrodes with and without carbon additive. Using these values, the cell-voltage gains due to the reduction of ρe- of a PGM-free cathode by carbon additive can be predicted (Figure 1c). At the end, a comparison between in-situ AC-impedance and ex-situ 4-point-probe measurements for PGM-free cathode electrodes will be presented and discussed. Overall, the study not only provides a new insight and methodology for electrode optimization of PGM-free cathode catalysts, but also reveals in some cases show significant impact of electrical conductivity on fuel cell performance with PGM-free catalysts.ACKNOWLEDGEMENTSThis work is supported by the Fuel Cells and Hydrogen Joint Undertaking under grant agreement No. 779550 (PEGASUS). This Joint Undertaking receives support from the European Union’s Horizon 2020 research and innovation programme, Hydrogen Europe and Hydrogen Europe Research.REFERENCE T. Thompson, A. R. Wilson, P. Zelenay, D. J. Myers, K. L. More, K. C. Neyerlin, and D. Papageorgopoulos, Solid State Ionics319, 68-79 (2018)Banham, J. -Y. Choi, T. Kishimoto, and S. Ye, Advanced Materials 31, 1804846 (2019)Malko, T. Lopes, E. A. Ticianelli, and A. Kucernak, Journal of Power Sources 323, 189-200 (2016)G. Wang, L. Osmieri, A. G. Star, J. Pfeilsticker, and K. C. Neyerlin, Journal of The Electrochemical Society 167, 044519 (2020)Liu, M. Murphy, D. Baker, W. Gu, C. Ji, J. Jorne, and H. A. Gasteiger, ECS Transactions 11, 473 (2007)Landesfeind, M. Ebner, A. Eldiven, V. Wood, and H. A. Gasteiger, Journal of The Electrochemical Society 165, 469-476 (2018) Figure 1

Exploring Anion Exchange Membrane Fuel Cell (AEMFC) Performance with Different Classes of Cathode Electrocatalysts

AEMFCs as an alternative to acidic polymer electrolyte fuel cells have the potential to reduce cost and precious metal requirements of fuel cell systems.1 In the past decade, several highly conductive polymers have been developed and a number of these have demonstrated more than 1W∙cm-2 peak power density with hundreds of hours stability; however, most of these studies have focused on Pt containing cathodes with high loadings.2,3 While the promise of Pt and Pt group metal (PGM) free electrocatalysis has been a major selling point of AEMFCs, more work exploring the potential of PGM-free cathode electrocatalysis is needed. In this study we explore and compare the performance of three leading cathode electrocatalysts: Pt/C, Ag/C, and Fe-N-C. These electrocatalysts are tested in AEMFCs cathodes using both powder- and dispersion-based PF AEM Gen 2 ionomers.4 The ability to test the different catalyst and ionomer combinations leads to additional insight into performance and catalyst-ionomer interactions.The performance trends of the tested MEAs will be presented. Pt/C has been found to be slightly superior to the performance of the non-Pt electrocatalysts. The performance of both metal nanoparticle catalysts, Pt/C and Ag/C, exhibit strong differences in performance depending on whether or not a powder or dispersion-based ionomer is used, with the powder-based approach showing much higher performance. Unlike the metal electrocatalysts, Fe-N-C catalysts show almost no change of performance between powder and dispersion-based ionomers.These performance metrics are also reflected in high frequency resistance measurements (Figure 1) investigated as a function of relative humidity at 300 mA·cm- 2. This data clearly shows the expected trend of increasing HFR with decreasing humidification of reactant gasses. The Fe-N-C data shows the lowest HFR values and very small changes between powder and dispersion-based ionomers. Both, Pt/C and Ag/C show dramatic increases in HFR for the dispersion-based polymer compared to the powder-based ionomer. These large changes in HFR are unexpected and cannot be fully explained just by changes happening within the electrodes. They suggest that not only do the electrodes undergo variable electrochemical properties due to this processing, but water management within the cell is also impacted by the type of catalyst and ionomer used in processing the electrodes. These features and the impact they have on the performance of different AEMFC cathodes will be presented and discussed.