To increase the commercialization of fuel cell electric vehicles, it is imperative to improve the activity and performance of electrocatalysts through combined efforts focused on material ...characterization and device-level integration. Obtaining fundamental insights into the ongoing structural and compositional changes of electrocatalysts is crucial for not only transitioning an electrode from its as-prepared to functional state, also known as “conditioning”, but also for establishing intrinsic electrochemical performances. Here, we investigated several oxygen reduction reaction (ORR) electrocatalysts via in situ and ex situ characterization techniques to provide fundamental insights into the interfacial phenomena that enable peak ORR mass activity and high current density performance. A mechanistic understanding of a fuel cell conditioning procedure is described, which encompasses voltage cycling and subsequent voltage recovery (VR) steps at low potential. In particular, ex situ membrane electrode assembly characterization using transmission electron microscopy and ultra-small angle X-ray scattering were performed to determine changes in carbon and Pt particle size and morphology, while in situ electrochemical diagnostics were performed either during or after specific points in the testing protocol to determine the electrochemical and interfacial changes occurring on the catalyst surface responsible for oxygen transport resistances through ionomer films. The results demonstrate that the voltage cycling (break-in) step aids in the removal of sulfate and fluoride and concomitantly reduces non-Fickian oxygen transport resistances, especially for catalysts where Pt is located within the pores of the carbon support. Subsequent low voltage holds at low temperature and oversaturated conditions, i.e., VR cycles, serve to improve mass activities by a factor of two to three, through a combined removal of contaminants, surface-blocking species (e.g., oxides), and rearrangement of the catalyst atomic structure (e.g., Pt–Pt and Pt–Co coordination).
The successful development of high-performance, durable platinum group metal-free (PGM-free) electrocatalysts and electrodes for polymer electrolyte membrane fuel cells (PEMFCs) will ultimately ...improve the cost-competiveness of fuel cells in a wide range of applications. This is considered to be a critical development especially for automotive fuel cell applications in order to bring the system cost of an automotive fuel cell system down to the $30/kW cost target set by the U.S. Department of Energy (DOE). The platinum group metal (PGM) electrocatalysts are a major contributor to the system cost. Addressing the technical challenges to PGM-free electrocatalyst and electrode development, therefore, represents one of DOE's most pressing research and development (R&D) priorities. ElectroCat was formed by the DOE as part of the Energy Materials Network (EMN) in early 2016, and shares with other EMN consortia the goal of decreasing the time to market for advanced materials related to clean energy technologies, in the context of increasing U.S. fuel cell electric vehicle (FCEV) manufacturing competitiveness. To accomplish this, the consortium performs core research and development and provides universities and companies streamlined access to the unique, world-class set of tools and expertise relevant to early-stage applied PGM-free catalyst R&D of the member national laboratories. Moreover, ElectroCat fosters a systematic methodology by which prospective catalysts and electrodes are prepared and analyzed rapidly and comprehensively using high-throughput, combinatorial methods. Catalyst discovery is augmented by theory as well as foundational electrocatalysis and materials knowledge at the participating national laboratories. Furthermore, ElectroCat has developed a data sharing framework, requisite of all EMN consortia, for disseminating its findings to the public via a searchable database, to further expedite incorporation of PGM-free electrocatalysts into next-generation fuel cells by advancing the general understanding of the PGM-free electrocatalyst field.
•PGM-free catalysts essential to increase economic competitiveness of PEMFCs.•ElectroCat couples U.S. DOE national lab expertise with academic/industry partners for PGM-free catalyst and electrode R&D.•High-throughput, combinatorial methods expedite catalyst discovery and elucidate structure-function relationships.•Early experimental accomplishments include first direct observation of single-atom Fe sites in PGM-free catalyst.•Early computational accomplishments include an automated active site durability descriptor calculation.
We present a rheological investigation of fuel cell catalyst inks. The effects of ink parameters, which include carbon black-support structure, Pt presence on carbon support (Pt–carbon), and ionomer ...(Nafion) concentration, on the ink microstructure of catalyst inks were studied using rheometry in combination with ultrasmall-angle X-ray scattering (USAXS) and dynamic light scattering (DLS). Dispersions of a high-surface-area carbon (HSC), or Ketjen black type, demonstrated a higher viscosity than Vulcan XC-72 carbon due to both a higher internal porosity and a more agglomerated structure that increased the effective particle volume fraction of the inks. The presence of Pt catalyst on both the carbon supports reduced the viscosity through electrostatic stabilization. For carbon-only dispersions (without Pt), the addition of ionomer up to a critical concentration decreased the viscosity due to electrosteric stabilization of carbon agglomerates. However, with Pt–carbon dispersions, the addition of ionomer showed contrasting behavior between Vulcan and HSC supports. In the Pt–Vulcan dispersions, the effect of ionomer addition on the rheology was qualitatively similar to Vulcan dispersions without Pt. The Pt–HSC dispersions showed an increased viscosity with ionomer addition and a strong shear-thinning nature, indicating that Nafion likely flocculated the Pt–HSC aggregates. These results were verified using DLS and USAXS. Further, the observations of the effect of ionomer:carbon ratio and a comparison between carbons of different surface areas provided insights on the microstructure of the catalyst ink corresponding to the optimized I/C ratio for fuel cell performance reported in the literature.
A comparative study of the electrochemical stability of Pt
25Cu
75 and Pt
20Cu
20Co
60 alloy nanoparticle electrocatalysts in liquid electrolyte half-cell environment was conducted. The ...aforementioned catalysts were shown to possess improved resistance to electrochemical surface area (ECSA) loss during voltage cycling relative to commercially available pure Pt electrocatalysts. The difference in ECSA loss was attributed to their initial mean particle size, which varied depending on the temperature at which the alloy catalysts were prepared (e.g. 600, 800 and 950
°C). Higher preparation temperatures resulted in larger particles and lead to lower ECSA loss. Liquid electrolyte environment short-term durability testing (5000 voltages cycles) revealed the addition of cobalt to be beneficial as ternary compositions exhibited stability advantages over binary catalysts.
Oxygen reduction reaction (ORR) activity and catalyst stability tests were then performed for both Pt
25Cu
75 and Pt
20Cu
20Co
60 alloy catalysts in membrane electrode assemblies (MEA). ORR activity data, taken both prior to and at the conclusion of 30,000 voltage cycles from 0.5 to 1.0
V vs. reversible hydrogen electrode (RHE), revealed that both Pt
25Cu
75 and Pt
20Cu
20Co
60 were able to retain both their mass and Pt surface area-based activity advantage relative to Pt/C R. Srivastava, P. Mani, N. Hahn, P. Strasser, Angew. Chem. Int. Ed. 46 (2007), 8988; P. Mani, R. Srivastava, P. Strasser, J. Phys. Chem. C 112 (2008), 2770; S. Koh, P. Strasser, J. Am. Chem. Soc. 129 (2007), 12624. Further analysis revealed that the Pt surface area-based activity, measured at 0.9
V vs. RHE, of commercially available Pt catalysts, as well as that for both Pt
25Cu
75 and Pt
20Cu
20Co
60 increased on the order of tens of
μ
A
c
m
Pt
−
2
per 1000 voltage cycles. This increase in specific activity combined with a reduced ECSA loss resulted in a negligible change for the Pt mass-based activity of Pt
25Cu
75 alloys annealed at 950
°C.
In situ electrochemical diagnostics designed to probe ionomer interactions with platinum and carbon were applied to relate ionomer coverage and conformation, gleaned from anion adsorption data, with ...O2 transport resistance for low-loaded (0.05 mgPt cm–2) platinum-supported Vulcan carbon (Pt/Vu)-based electrodes in a polymer electrolyte fuel cell. Coupling the in situ diagnostic data with ex situ characterization of catalyst inks and electrode structures, the effect of ink composition is explained by both ink-level interactions that dictate the electrode microstructure during fabrication and the resulting local ionomer distribution near catalyst sites. Electrochemical techniques (CO displacement and ac impedance) show that catalyst inks with higher water content increase ionomer (sulfonate) interactions with Pt sites without significantly affecting ionomer coverage on the carbon support. Surprisingly, the higher anion adsorption is shown to have a minor impact on specific activity, while exhibiting a complex relationship with oxygen transport. Ex situ characterization of ionomer suspensions and catalyst/ionomer inks indicates that the lower ionomer coverage can be correlated with the formation of large ionomer aggregates and weaker ionomer/catalyst interactions in low-water content inks. These larger ionomer aggregates resulted in increased local oxygen transport resistance, namely, through the ionomer film, and reduced performance at high current density. In the water-rich inks, the ionomer aggregate size decreases, while stronger ionomer/Pt interactions are observed. The reduced ionomer aggregation improves transport resistance through the ionomer film, while the increased adsorption leads to the emergence of resistance at the ionomer/Pt interface. Overall, the high current density performance is shown to be a nonmonotonic function of ink water content, scaling with the local gas (H2, O2) transport resistance resulting from pore, thin film, and interfacial phenomena.
This work demonstrates the fabrication and processing steps required to produce high performance fuel cell membrane electrode assemblies (MEAs) based on spray-coated gas-diffusion electrodes (GDEs). ...It is demonstrated that coating the catalyst layer with a thin layer of ionomer and then hot pressing the GDEs to the membrane is required to achieve comparable catalyst activity and air performance to catalyst-coated-membrane MEAs. We show that there is a critical amount of ionomer required to achieve maximized performance. Using electron microscopy, we show that the combination of the ionomer overlayer and hot-pressing bonds the catalyst layer to the membrane, increasing the interfacial contact area and quality of this interface. We also find that the ionomer overlayer smooths the surface of the GDE and provides increased contact area between the GDE and the membrane. Additionally, we demonstrate that much less ionomer is required for high-performance than has been previously reported. Through model fitting of electrochemical impedance spectroscopy, we show that this improvement in the catalyst layer – membrane interface reduces the effective catalyst layer resistance, which reduces Ohmic losses and increases catalyst utilization.
•Goal was to fabricate high-performance gas-diffusion-electrode-based fuel cells.•The role of an ionomer overlayer and hot pressing were investigated.•Both are critical for high performance.•The overlayer and hot pressing reduce catalyst layer protonic resistance.•There is a critical ionomer overlayer loading for maximum performance.
Abstract
The electrochemical reduction of carbon dioxide to formic acid is a promising pathway to improve CO
2
utilization and has potential applications as a hydrogen storage medium. In this work, a ...zero-gap membrane electrode assembly architecture is developed for the direct electrochemical synthesis of formic acid from carbon dioxide. The key technological advancement is a perforated cation exchange membrane, which, when utilized in a forward bias bipolar membrane configuration, allows formic acid generated at the membrane interface to exit through the anode flow field at concentrations up to 0.25 M. Having no additional interlayer components between the anode and cathode this concept is positioned to leverage currently available materials and stack designs ubiquitous in fuel cell and H
2
electrolysis, enabling a more rapid transition to scale and commercialization. The perforated cation exchange membrane configuration can achieve >75% Faradaic efficiency to formic acid at <2 V and 300 mA/cm
2
in a 25 cm
2
cell. More critically, a 55-hour stability test at 200 mA/cm
2
shows stable Faradaic efficiency and cell voltage. Technoeconomic analysis is utilized to illustrate a path towards achieving cost parity with current formic acid production methods.
This work presents a study of the effects of ultrasonic dispersing methodology and time on catalyst agglomerate size in polymer electrolyte membrane fuel cell (PEMFC) catalyst ink dispersions. ...Cathode catalyst inks were prepared and characterized to elucidate the influences of ultrasonic dispersing method and time on catalyst ink particle size and CCL electrochemical properties. In-situ ultra-small-, small-, and wide-angle X-ray scattering (USAXS–SAXS–WAXS) analyses were used to study the impact of ultrasonication time and methodology on changes in the agglomerate, aggregate, and particle size and distribution during the dispersing process. Ex-situ transmission electron microscopy was also used to investigate the particle size of these inks. Fuel cell membrane electrode assemblies (MEAs) were prepared and tested to determine the influence of ink properties on CCL electrochemical properties, including the electrochemical active surface area (ECA), mass activity (MA), H2/air polarization curves, and oxygen mass-transport resistances. It was found that a combination of brief tip sonication followed by bath sonication was most effective at breaking up agglomerates, leading to maximum catalyst activity and MEA performance. Extended tip sonication was found to be too aggressive and resulted in detachment of the platinum nanoparticles from the carbon black support, which decreased electrochemical surface area and MEA performance. Quantification of oxygen mass transport resistance showed that electrodes with large catalyst agglomerates due to insufficient sonication had a higher non-Fickian (pressure independent) than properly dispersed catalyst. Through correlation of the performance with catalyst particle size, the desired CCL structure was proposed, which will provide insight into dispersion strategies for lab-scale spray coating and other processing techniques as well as for large-scale manufacturing.
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