Proton Exchange Membrane Fuel Cells Electrodes Research Articles

Investigation of Manufacturing Process Related Structure and Performance of the Fuel Cell Electrode Thanks to Small Angle Neutron Scattering

With climate change and fossil fuel sustainability concerns faced globally, proton exchange membrane fuel cell (PEMFC) based electric vehicles have been recognized as a promising alternative. However, large-scale commercialization requires progress in performance, cost and durability, for which the electrode (catalyst layer) is the limiting component. The electrode is composed of nanoparticles of platinum (Pt) catalyst on a carbon (C) support, embedded in a proton conductive polymer (ionomer). It is obtained from a slurry (catalyst ink) after evaporating solvent consisting of a water and alcohol mixture. The structural properties of the slurry, including the ionomer and carbon dispersions, catalyst aggregation and ionomer coverage onto the Pt/C composite strongly influence the performance and durability of the final electrode. Nevertheless, currently little is known of these structures, particularly with regards to the dispersion of the ionomer component onto the surface of porous carbon, despite the fact it is now established that it plays a crucial role in the operation of the fuel cell. The lack of information leaves research and development to be heavily reliant on a trial and error basis. Developing a detailed understanding of the dispersion and its correlation to fuel cell performance is key in advancing the PEMFC electrode.However, the heterogeneous nature of these inks, with only short-range order observed on both micro- and nano- scales, makes them difficult to study by conventional laboratory techniques. Small angle neutron scattering (SANS) has been identified as a suitable method for the study of the electrodes as it is not hindered by the ink opacity as in optical techniques (commonly used to study dispersions) and allows to probe the bulk structures in the 1-100 nm scale relevant for each component. In particular, contrast variation (CV) SANS exhibits a major advantage in studying such complex systems, where scattering signal from multiple components can be deconvoluted by matching the scattering length density of the solvent to that of a desired component. Nevertheless, data on the ink itself is missing and it has only been investigated in a handful of studies by SANS.1-3 Shibayama et al.1 proposed a structural model of Pt/C aggregates coated by an ionomer shell in a water-based ink. Yoshimune et al.2 demonstrated that the amount of Pt on the carbon surface alters the thickness and density of said ionomer layer. Harada et al.3 were able to distinguish adsorbed and deposited ionomer on the composite surface as well as quantize their distribution.Yet although ink composition is known to impact the dispersion greatly, no systematic investigation linking its many variables has been conducted. For example, the ionomer will conform differently in a water/alcohol solvent used commercially in contrast to a water-based one. The porosity of the carbon support will impact distribution of Pt on its surface as well as the degree of ionomer penetration into the support. Additionally, although the fundamental understanding of the ink structure is imperative, it has seldom been further linked to the performance of the electrode itself.Therefore, the aim of this project is to utilize CV-SANS to study the impact of each catalyst ink component on the structure of the dispersion, and their relation to each other, through altering only a single variable at a time. The variables include the components as well as their proportions, such as the carbon concentration or the ionomer to carbon ratio. The performance of the electrode manufactured from said ink compositions will then be evaluated using electrochemical studies. Recently we performed CV-SANS studies on the ink, investigating the extent of ionomer adsorption on the Pt/C composite as a function of both carbon type and presence of Pt in the system. Both carbon supports studied, high surface area carbon with surface nano-pores accessible to the ionomer, and graphitized carbon, which has high internal porosity, yet inaccessible to the ionomer, pose an interest in industrial applications. Indeed, from these preliminary results it is evident platinum plays a significant role in the ink structure, particularly in the case of graphitized carbon (figure 1). The ink system will be quantitatively characterized using a singular value decomposition method already employed in literature to give insight into parameters such as the amount of ionomer adsorbed onto the carbon surface or the structure of the layer. References 1 M. Shibayama, T. Matsunaga, T. Kusano, K. Amemiya, N. Kobayashi and T. Yoshida, J Appl Polym Sci, , DOI:10.1002/app.39842.2 W. Yoshimune and M. Harada, https://doi.org/10.1246/cl.190017, 2019, 48, 487–490.3 M. Harada, S. I. Takata, H. Iwase, S. Kajiya, H. Kadoura and T. Kanaya, ACS Omega, 2021, 6, 15257–15263. Figure 1

Molecular Weight Dependent Ionomer Volume Expansion and Its Effect on the Performance of PEM Fuel Cell Electrodes

Within the past years, proton exchange membrane (PEM) fuel cells have become more and more attractive due to their potential for the transition towards an environmentally friendly hydrogen economy. Especially by reducing the platinum catalyst loading, significant system cost reductions could be achieved, but low Pt loadings still lead to unassigned voltage losses during operation.[1,2] In order to overcome those losses, well-designed catalyst layers with optimized ionomer content and distribution are indispensable. The used perfluorosulfonic acid (PFSA) ionomers for PEM fuel cells are commonly characterized by the chemical structure of the ionomer and the equivalent weight (EW), which strongly affects the proton conductivity.[3,4] Current research focusses on the synthesis of modified ionomers with higher oxygen permeability to reduce mass transport losses, but also investigates the influence of the ionomer’s molecular weight (MW).[1,5,6] A high MW minimizes the water uptake of the ionomer/membrane in liquid water, which is desired for the use in PEM fuel cells to reduce mechanical stress during relative humidity (RH) cycling.[5] The exact MW of an ionomer is not easily accessible, but can be estimated by measuring the melt flow index (MFI), whereas a high MW is commonly reflected by a lower MFI if the ionomer chemistry and EW are the same.Within this study, a 3M ionomer modified to have a high MFI of 156 g/10 min (measured at 265 °C with a mass of 5 kg) is investigated and compared to a 3M standard ionomer with a low MFI of 6 g/10 min. Both PFSA ionomers have the same chemical structure and a similar EW of ~800. The significantly higher MFI of the modified ionomer (i.e., comprising a lower MW) is clearly reflected by a ~2 times higher water uptake compared to the 3M standard ionomer (measured for solution-cast membranes in liquid water at 80 °C). To evaluate the impact of the MFI within the cathode electrode, 5 cm² active area membrane electrode assemblies (MEAs) with these two ionomers at various ionomer/carbon (I/C) mass ratios in the cathode electrode (based on a 40 wt.% Pt/Vulcan catalyst at loadings of 0.11 mgPt/cm²) are manufactured and electrochemically characterized. At sufficiently high I/C ratios and under humid operation (i.e., I/C = 0.90 and 90 % RH), differential flow H2/air polarization curves of the MEAs reveal that the higher liquid water uptake of the 3M high-MFI ionomer can be correlated with a lower cell voltage (see Figure 1). Contrary to the MEAs with the 3M low-MFI standard ionomer, the larger ionomer swelling in MEAs with the 3M high-MFI ionomer reduces the void volume fraction in the electrode structure during operation. Most pronounced at high current densities, where oxygen mass transport plays an important role, the MEAs containing the 3M high-MFI ionomer show a significant voltage loss of ~50 mV at 2.5 A/cm² compared to the MEAs with the 3M low-MFI ionomer. This study demonstrates that the effect of ionomer volume expansion upon changes in the MFI and thus the MW might not be crucial for the MEA performance when using electrodes with a low I/C ratio, but is critical when aiming for electrode compositions with a high ionomer content, in which case a low-MFI (i.e., a high MW) ionomer should be favorable.

Design of graded cathode catalyst layers with various ionomers for fuel cell application

Proton exchange membrane fuel cells (PEMFCs) powered by green hydrogen (H2) have become a promising alternative to conventional hydrocarbon-fueled power generators. Despite technological advancements, further improvements in efficiency, durability, and low-cost production are required for the widespread adoption of PEMFCs. Though numerous approaches to improve PEMFC electrodes have been reported, most strategies utilize a single material set (e.g., one combination of catalyst and ionomer) to improve performance. Alternatively, anisotropic (graded) electrode structures with locally tunable properties may yield superior electrode performance due to improved ionic and gas phase transport. In this work, graded cathode catalyst layers (CCLs) incorporating different ionomers (Nafion D2020, Aquivion D79-25BS, and HOPI) were designed and prepared. Performance screening shows that some of these graded electrode structures have comparable performance to optimized single-ionomer electrode structures (D2020) suggesting some synergistic benefit. Additionally, we show that electrodes with lower equivalent weight (EW) D79 ionomer near the membrane and D2020 ionomer near the gas diffusion media outperformed electrodes with the inverted configuration. Finally, EIS analysis shows some graded ionomer structures (e.g. D79/D2020) have better than expected H+ conductivity, generally leading to better electrode performance. However, further optimization of ionomer content and catalyst ink formulations is needed to improve overall PEMFC performance.

Segmented tomographic evaluation of structural degradation of carbon support in proton exchange membrane fuel cells

The variation of the three-dimensional (3D) structure of the membrane electrode of a fuel cell during proton exchange cycling involves the corrosion/compaction of the carbon support. The increasing degradation of the carbon structure continuously reduces the electrocatalytic performance of proton exchange membrane fuel cells (PEM-FCs). This phenomenon can be explained by performing 3D tomographic analysis at the nanoscale. However, conventional tomographic approaches which present limited experimental feasibility, cannot perform such evaluation and have not provided sufficient structural information with statistical significance thus far. Therefore, a reliable methodology is required for the 3D geometrical evaluation of the carbon structure. Here, we propose a segmented tomographic approach which employs pore network analysis that enables the visualization of the geometrical parameters corresponding to the porous carbon structure at a high resolution. This approach can be utilized to evaluate the 3D structural degradation of the porous carbon structure after cycling in terms of local surface area, pore size distribution, and their 3D networking. These geometrical parameters of the carbon body were demonstrated to be substantially reduced owing to the cycling-induced degradation. This information enables a deeper understanding of the degradation phenomenon of carbon supports and can contribute to the development of stable PEM-FC electrodes.

Channel Diameter Effect of Porous Carbon Microparticles on PEMFC Performance for Highly Active Ultra-Low Pt Catalysts

Proton exchange membrane fuel cells (PEMFCs) have received much attention as environmentally benign automotive power sources. PEMFCs can offer a large amount of electricity required for autonomous vehicles that battery-powered systems may not be able to provide. However, because PEMFC electrodes are based on expensive and scarce Pt catalysts, Pt minimization is necessary to expand the PEMFC market. To reduce Pt usage, various attempts have been made to increase the intrinsic activity of Pt catalysts, particularly at the cathode where the oxygen reduction reaction (ORR) occurs. Despite decades of progress, high ORR activity was typically reported in half-cell setups and frequently failed to show corresponding performance in single-cell. Catalysts with low Pt content perform poorly in the high-current density region because the thick catalyst layer limits mass transport.[1] In addition, low Pt loading catalysts typically suffer more in long-term operation.[2] As a result, it is critical to develop PEMFC catalysts with low Pt content that can facilitate mass transport while also exhibiting high durability.Herein, we report highly active and durable PtFe@C catalysts with ultra-low amounts of Pt (1 wt%) on channeled mesoporous carbon (CMC) particles. These CMC particles are designed to have continuous channels with open porosity and a large surface area, facilitating the mass transport behavior and maximizing cell performance. Block-copolymer particles (BCPs) with different molecular weights were used to fabricate CMC particles with pore diameters ranging from 13 to 63 nm. Two steps of pre-crosslinking and hyper-crosslinking were conducted prior to the carbonization step to preserve the porous internal structure of the BCP-based carbon support during high-temperature treatment. After depositing Pt onto the support by facile incipient wetness impregnation method followed by reduction, thin layers of carbon shell were observed to encapsulate the PtFe alloy nanoparticles.The channel diameter effect on the mass transport was studied in both half-cell and single-cell. Interestingly, from the cell performance obtained with varying channel diameters and different oxygen concentrations, we concluded that both reactant (proton and oxygen) supply and product (water) removal were greatly enhanced with larger channel size. With the largest channel diameter of 63 nm, initial mass activity in the single-cell was obtained to be 3.5 A mgPt -1, which is the highest value reported to date to the best of our knowledge. Cell performance under H2-air flow, which is the industrially relevant condition, surpassed the commercial 20 wt% Pt/C with only 1/20 of the Pt loading. The origin of enhanced cell performance upon enlarging the channel diameter was investigated by separating kinetic, ohmic (electronic and ionic charge transport), and mass transport overpotentials. Both the proton and oxygen transport resistance were confirmed to be reduced with larger channel size. Moreover, carbon shell protected Fe from getting leached out in an acidic environment, resulting in preserved catalyst structure and high durability. The outstanding performance of 51 kW/gPt in H2/air condition after 30,000 cycles of accelerated degradation tests (ADTs) was observed. This work will open a new paradigm to develop PEMFC catalysts with much higher activity and durability while simultaneously minimizing Pt use.

(Invited) Novel Nature-Inspired Concepts to Design Ionomeric Nanomaterials for Energy Conversion and Storage Devices

Ion-conducting polymers (ionomers) are an integral part of renewable energy-driven technologies. Proton exchange membrane fuel cells (PEMFCs) have already been incorporated into low-duty vehicles and are now being adopted in heavy-duty vehicles. Therefore, we need more durable and efficient fuel cell materials than ever. A major technical challenge of PEMFCs is the ion transport limitation at the ionomer-catalyst interface which negatively impacts the efficiency of the cells. The current state-of-the-art fuel cell ionomers (both fluorocarbon- and hydrocarbon-based) conduct protons efficiently in bulk, several tens of micron thick, free-standing membranes, but poorly in sub-micron thick films. This needs attention as sub-micron thick ionomer layers are used as catalyst binders on electrodes of PEMFCs and other energy conversion and storage devices. However, the nanoscale behavior of these existing ionomers within complex hydration environment is not well-understood yet. More importantly, ionomers are rarely designed to improve the thin film ion conduction properties. To address and overcome these issues, we do not only design innovative nanoscale materials characterization techniques to explore the distribution of ion conduction environment across ionomer films/membrane, but also design novel classes of ionomers inspired by natural living systems to facilitate ion conduction in thin films. We experimentally measure the ion conductivity, morphology, and mechanical properties, and combine them with computational studies to elevate our understanding of ion conduction mechanism within 10-200 nm thick films of these newly designed ionomers. The results suggest the great promise of these new classes of ionomers as efficiently ion-transporting catalyst binders for fuel cells, electrolyzers, and batteries.

(Digital Presentation) Durable Silica Nanosheets/Carbon Black Supported Catalyst for Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells (PEMFCs) which have high efficiency have attracted attention as one of the most promising energy conversion devices. Currently the catalysts in PEMFCs are mainly carbon supported noble metals where the support can afford the electron transport environment to improve catalyst efficiency and decrease catalyst loss by strengthening catalyst–support bonding [1]. Due to high electrical conductivity and surface area, carbon-based catalyst supports are widely used in PEMFCs. However, the high electric fields applied to the fuel cell can easily cause carbon corrosion and further accelerate catalyst loss.New alternative support materials are needed to reduce costs and increase the lifetime of PEMFC electrodes. Recently, silica-based supports such as silicon oxide [2, 3] and silicon carbide [4] have been considered feasible as catalyst supports due to efficient catalyst utilization and their resistance to carbon corrosion. As a silica-based material, hydrophilic silica nanosheets (SN) derived from cheap natural vermiculite [5] is for the first time to be combined with carbon black (CB) as a catalyst support in this work. SN-CB supported platinum (Pt) catalysts were prepared according to different weight ratios of SN and CB using a modified polyol reduction method [6]. After 18000-cycle accelerated stress test (AST), as shown in Fig. 1a and b, the electrochemical surface area (ECSA) of Pt/SN2-CB3 (the weight ratio of SN to CB is 2: 3) decays from 23.7 to 22.5 m2/g (only 4.8% degradation) compared to Pt/CB (25.8% degradation from 16.0 to 11.9 m2/g). Meanwhile, Pt/SN2-CB3 shows comparable initial onset potential (0.850 V) and high half-wave potential (0.654 V) compared to 0.881 V onset potential and 0.625 V half-wave potential for Pt/CB as shown in Fig. 1c and d. The half-wave potential of Pt/SN2-CB3 after AST test demonstrated the addition of SN into the support can result in a stable oxygen reduction reaction (ORR) durability. The possible reason why the ECSA, ORR activity and durability of SN-CB supported platinum catalysts are enhanced is that the synergic interactions among carbon, silica nanosheets and Pt nanoparticles may hinder carbon corrosion, Pt agglomeration and dissolution [2]. Different weight ratios between SN and CB and the functionalized SNs are currently being tested and the real fuel cell data will be presented. Reference [1] Y.-J. Wang, D.P. Wilkinson, J. Zhang, Noncarbon support materials for polymer electrolyte membrane fuel cell electrocatalysts, Chemical reviews, 111 (2011) 7625-7651.[2] P. Dhanasekaran, A. Shukla, S.V. Selvaganesh, S. Mohan, S. Bhat, Silica-decorated carbon-Pt electrocatalyst synthesis via single-step polyol method for superior polymer electrolyte fuel cell performance, durability and stack operation under low relative humidity, Journal of Power Sources, 438 (2019) 226999.[3] P. Dhanasekaran, S.V. Selvaganesh, A. Rathishkumar, S. Bhat, Designing self-humidified platinum anchored silica decorated carbon electrocatalyst for boosting the durability and performance of polymer electrolyte fuel cell stack, International Journal of Hydrogen Energy, 46 (2021) 8143-8155.[4] L. Dong, J. Zang, J. Su, Y. Jia, Y. Wang, J. Lu, X. Xu, Oxidized carbon/nano-SiC supported platinum nanoparticles as highly stable electrocatalyst for oxygen reduction reaction, International journal of hydrogen energy, 39 (2014) 16310-16317.[5] Z. Guo, J. Chen, J.J. Byun, R. Cai, M. Perez-Page, M. Sahoo, Z. Ji, S.J. Haigh, S.M. Holmes, High-performance polymer electrolyte membranes incorporated with 2D silica nanosheets in high-temperature proton exchange membrane fuel cells, Journal of Energy Chemistry, 64 (2022) 323-334.[6] Z. Ji, M. Perez-Page, J. Chen, R.G. Rodriguez, R. Cai, S.J. Haigh, S.M. Holmes, A structured catalyst support combining electrochemically exfoliated graphene oxide and carbon black for enhanced performance and durability in low-temperature hydrogen fuel cells, Energy, 226 (2021) 120318. Figure 1

Durability of Platinum Overlayers formed By Self-Terminating Electrodeposition

In Proton Exchange Membrane Fuel Cell (PEMFC), the membrane electrode assembly (MEA), especially the electrode, is considered 'the heart' and is designed to accommodate the need for efficient transport of electrons, reactants, and heat as well as constraints imposed by the platinum cost. Advances in PEMFC electrode design over the years, which range from using Pt black films with a loading of 10 gpt/cm2 in the 1970s to present-day Pt/ PGM (Platinum Group Metal) nanoparticle coated carbon-black particles (Pt/C) that use about 0.3 mgpt/cm2, have led to significant cost-reduction and performance enhancement. However, durability problems associated with corrosion of carbon support and subsequent loss of active surface area during start-up or shut down cycles have led to renewed interest in carbon-free nanostructured electrodes that employ a thin coating of Platinum or PGM-based catalytic layer on mesostructured conductive support ((Debe 2012), (Liu et al. 2012)).In this context, we carried out systematic ex-situ investigations on the durability of platinum thin films comprising of atomic layers formed by Self-Terminating platinum electrodeposition (Liu et al. 2012). We characterized the sequential growth of platinum film using XPS and AFM measurements; and analyzed the electrocatalytic performance as a function of the number of self-terminating pulses. We found that eight pulses of electrodeposition, corresponding to ~10 nm thick platinum film, can meet stringent ex-situ durability targets set, by the Department of Energy (DOE), USA, for a potential electrocatalyst in Fuel Cell Vehicles (FCVs) application. Our results also show that platinum layers formed using four pulses (< 4 µg/cm2 loading) and even one pulse (< 1 µg/cm2 loading) are electrochemically active. For platinum samples formed using eight or more pulses, we observed ~ 20% loss in Electrochemically Active Surface Area (ECSA) during the first 3000 cycles and 10-15% ECSA loss in the remaining 27000 cycles, indicating stabilization of platinum activity over time. We will present the results of our studies on the minimum number of platinum pulses needed to form a complete overlayer of electrochemically active platinum and discuss the platinum loss during the ex-situ durability tests using ICP-OES and Cyclic Voltammetry measurements. These studies pave the way for an additive, roll-to-roll, and cost-effective manufacturing process of ultra-low platinum loaded MEAs for PEMFCs by building on our earlier results on printing silver nanostructures using a simple inkjet printer by Print-Expose-Develop technique (Parmar and Santhanam 2014) in conjunction with the ability to controllably form atomic layers of platinum on metallic substrates by Self-terminating electrodeposition. Figure 1

Spent Fuel Cell Electrocatalyst Recycling through Potentiodynamic Dissolution of Platinum

Large-scale commercialization and sustainable growth of proton exchange membrane fuel cells (PEMFCs) technology is throttled largely by high cost and scarce nature of Pt, the most preferred electrocatalyst for both the anode and cathode reactions. Recycling of the electrocatalyst from spent PEMFC electrodes through a commercially scalable and environmentally benign process is of significant interest. Here, we present technical details of a closed loop recycling of spent electrocatalyst from a PEMFC electrode. The process includes electrochemical dissolution of the Pt nanoparticles in dilute acidic bath, precipitation of the dissolved Pt in form of a chloride-free Pt precursor and synthesis of carbon supported Pt (Pt/C) electrocatalyst through reduction of the Pt precursor. The presentation will highlight some of the studies from our group aiming to enhance the Pt dissolution efficiency and to optimize the process steps for synthesis of the Pt/C electrocatalyst from the dissolved P. With a demonstrated recovery efficiency >90%, the process may be utilized for large-scale recycling of the spent electrocatalysts, especially suitable for PEMFC electrodes. References R Sharma, S Gyergyek, P Morgan and S M Andersen, Electrochem. Soc., 2020, 167 (2): p. 024521. R Sharma, K R Nielsen, P B Lund, S B Simonsen, L G Madsen, and S M Andersen, ChemElectroChem, 2019. 6(17): p. 4471-4482. R Sharma, S J Andreasen, J Chamier, and S M Andersen, Electrochem. Soc., 2019. 166(13): p. F963-F970. R Sharma, S B Simonsen, P Morgen, and S M Andersen, Acta, 2019. 321: p. 134662. R Sharma, S Gyergyek and S M Andersen, ChemSusChem 2018, 11(21), 3742-3750 R Sharma, Y Wang, F Li, J Chamier, and S M Andersen, ACS Appl. Energy Mater., 2019. 2(9): p. 6875-6882. R Sharma, P. Morgan and S M Andersen, PCCP, 2020 (Accepted manuscript), https://doi.org/10.1039/C9CP06066A Figure 1

Hydrogen Oxidation Artifact During Platinum Oxide Reduction in Cyclic Voltammetry Analysis of Low-Loaded PEMFC Electrodes

An artifact appearing during the cathodic transient of cyclic voltammograms (CVs) of low-loaded platinum on carbon (Pt/C) electrodes in proton exchange membrane fuel cells (PEMFCs) was examined. The artifact appears as an oxidation peak overlapping the reduction peak associated to the reduction of platinum oxide (PtOx). By varying the nitrogen (N2) purge in the working electrode (WE), gas pressures in working and counter electrode, upper potential limits and scan rates of the CVs, the artifact magnitude and potential window could be manipulated. From the results, the artifact is assigned to crossover hydrogen (H2X) accumulating in the WE, once the electrode is passivated towards hydrogen oxidation reaction (HOR) due to PtOx coverage. During the cathodic CV transient, PtOx is reduced and HOR spontaneously occurs with the accumulated H2X, resulting in the overlap of the PtOx reduction with the oxidation peak. This feature is expected to occur predominantly in CV analysis of low-loaded electrodes made of catalyst material, whose oxide is inactive towards HOR. Further, it is only measurable while the N2 purge of the WE is switched off during the CV measurement. For higher loaded electrodes, the artifact is not observed as the electrocatalysts are not fully inactivated towards HOR due to incomplete oxide coverage, and/or the currents associated with the oxide reduction are much larger than the spontaneous HOR of accumulated H2X. However, owing to the forecasted reduction in noble metal loadings of catalyst in PEMFCs, this artifact is expected to be observed more often in the future.Graphical abstract

Application of electrospinning for the fabrication of proton-exchange membrane fuel cell electrodes

Abstract Electrospun materials have been gaining great interest in the energy sector. Their tunability and robustness make them highly attractive, particularly for proton-exchange membrane fuel cell (PEMFC) electrodes. Conventional PEMFC electrodes, prepared by either spraying, painting, or slot-die coating, have not yet met the needs of large-scale PEMFC use. Electrospinning of fibrous materials has already shown great promise as an alternative methodology for electrode fabrication. Electrospinning has been used in fuel cell electrodes through two primary means: (1) segmented carbon or inorganic fibers to serve as precious metal catalyst support, and (2) high aspect ratio polymer/particle fibers to serve directly as the electrode. The use of electrospun fibrous electrodes has led to improved PEMFC durability and increased power output at low catalyst loadings, both of which are of paramount importance to large-scale commercialization of PEMFC electric vehicles.

Analysis of PEMFC Electrode Structure – Bridging the Mesoscale Gap

Proton exchange membrane fuel cell (PEMFC) electrodes are commonly composed of Pt nanoparticles supported on conductive carbon interspersed with ionomer, creating a porous network facilitating transport of electrons, protons, both reactant and product gasses, and liquid water. For the electrochemical reactions to occur at the metallic Pt active sites in the cathode, the electrons, protons, and oxygen all need to be present, which presumably requires the confluence of carbon, ionomer, and Pt all accessible at a pore. Following reaction at this 3-phase boundary, the pore network needs to remove product water from the electrode. Though the structure of PEMFC electrode layers has been studied for many years, the details are still unclear. The distribution of pore sizes, thickness and distribution of ionomer, and structure of carbon agglomerates are not well understood. This lack of understanding hinders rational design of catalyst layers that utilize reduced Pt loading while maintaining the performance and durability necessary for large-scale commercialization. Part of the difficulty in obtaining accurate morphological information on these complex structures is that it is unclear what techniques provide accurate analysis of the pore structures within these materials. Common pore size analysis techniques include nitrogen adsorption (BET), mercury intrusion (MIP), x-ray tomography (XCT), transmission electron microscopy (TEM), and focused ion beam tomography (FIB-SEM). When multiple techniques are applied, even to the same sample, the results for pore size distribution and connectivity can vary widely. It is unclear which, if any, of these techniques yield results representative of the true pore structure within the catalyst layer. Further, it is not known if the lack of agreement results from sample preparation methods and analysis conditions used for each technique, or fundamental limitations of the techniques when applied to PEMFC electrode materials. In this work, we examine results of pore analysis from multiple techniques. In addition to standard measurement techniques including BET, TEM, MIP, and FIB-SEM tomography, we employ techniques for measurement of pore size by analysis of the solid/liquid phase transition temperature using cryoporometry (Figure 1). We will discuss the results of these multiple techniques, address their utility and limitations, and attempt to present a pore size distribution in better agreement with reality that can be used as input for modeling efforts. Acknowledgement: This research is supported by the U.S. Department of Energy Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium (Technical Development Manager: Greg Kleen and Fuel Cells Program Manager: Dimitrios Papageoropoulos). Figure 1. Comparison of pore size distributions from nitrogen isotherm analysis using BJH1 (top) and cryoporometry (bottom). Top figure is PSD for TEC10E50E (CB), TEC10EA30E (GCB), and acetylene black supported Pt with specific surface areas of 779 and 219 m2 g-1 (AB800 and AB250 respectively). Bottom figure is PSD for TEC10E50E. 1) Young-Chul Park, Haruki Tokiwa, Katsuyoshi Kakinuma, Masahiro Watanabe, Makoto Uchida, Effects of carbon supports on Pt distribution, ionomer coverage and cathode performance for polymer electrolyte fuel cells, Journal of Power Sources, 315 (2016) pp.179-191 Figure 1

Preparation of Platinum Nanostructures Directly Grown on Microporous Layer By an Electrodeposition Technique for a Commercial-Scale PEMFC

Proton exchange membrane fuel cells (PEMFC) have a lot of attention due to their low operating temperature and quick startup. Pt-based catalysts, usually used in the PEMFC electrodes, are the main contributor to the high cost. An electrodeposition technique that could significantly improve the deposition of platinum (Pt) nanostructures onto on carbon cloths was adopted in this study. The electrolyte used for electrodeposition was a mixed solution of 0.1 M sulfuric acid and Pt precursor chemical of 0.5 mM hexachloroplatinic acid (H2PtCl6·6H2O) to fabricate a 3D structural catalyst layer as an electrode used in proton exchange membrane fuel cell (PEMFC). Without using any catalyst supports, the catalyst nanostructure would enhance the catalytic activity area. In this study, the dimensions of the working electrode were 1, 9, and 25 cm2. The sizes of the Pt nanostructures are 100-300 nm in diameter. This new technique could further enhance the catalyst activities and lead to a significantly improved Pt uniformly distribution on the carbon clothes.

Necessity to Avoid Titanium Oxide As Catalyst Support in PEM Fuel Cells

Although fuel cell electric vehicles (FCEVs) has become a commercial reality, overall cost reduction as well as durability improvement continue to remain as the key challenges facing fuel cell developers. One of the area of durability concern in proton exchange membrane (PEM) fuel cells relates to the degradation of carbon supported Pt-based electro catalysts. [1] Carbon-supported (e.g., Vulcan or Ketjen black) Pt or Pt-alloy (e.g., PtCo) catalysts are used in PEM fuel cells to catalyse the oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR). In an automotive environment, the catalysts must survive highly dynamic operations. State-of-the-art high surface area carbon support suffer from severe corrosion at potentials above 1.1 V. [1] A few transient operations that contribute to the corrosion of carbon support including vehicle start-up/shutdown, cell reversal due to fuel starvation, as well as load cycling. The degradation of carbon support and the electrocatalyst can induce significant performance decay. Therefore, material that is inherently more corrosion resistant has been actively investigated as alternative catalyst support to carbon in PEM fuel cell applications. In recent years, transition metal oxides are of interest as an alternate to carbon electro catalyst support due to their improved stability at higher potentials [2-3]. Furthermore, it is also believed that metal-oxide supports can possibly influence the intrinsic activity of the supported Pt (Pt-alloy) catalyst toward ORR due to strong metal-support interactions(SMSI). Titanium oxide has been demonstrated to be one of the suitable metal oxide, and was frequently reported as effective ORR catalyst support showing both significantly improved stability at high potential as well as improved ORR activity [3]. However, no long-term durability and possible negative effects of oxide supports on PEM fuel cell durability has been reported. In this study, for the first time we report that the use of titanium oxide (TiO2) as catalyst support in the PEM fuel cell electrode can lead to significant degradation of the polymeric membrane, causing membrane thinning and increased gas crossover. Experiments were conducted using Pt supported on titanium oxide (TiO2) as either the anode HOR catalyst or the cathode ORR catalyst. The impact on membrane chemical degradation is evaluated at open circuit voltage (OCV) condition at 115 oC and 25% RH for 100 hours. The exhaust water from both the anode and cathode of the fuel cell during the OCV hold was collected and analyzed using ion chromatography (IC) to quantify the concentration of fluoride and determine the fluoride release rate (FRR) which is a measure of the rate of membrane chemical degradation. With measured FRR, a total fluorine inventory loss of the membrane is also calculated. It was found that the total fluorine inventory loss is 26% and 15% for Pt/TiO2 used as anode and cathode catalyst, respectively, after 100hrs of OCV test. A comparable electrode with carbon supported Pt catalyst exhibit only about 0.3% total fluorine inventory loss at identical OCV conditions, which is more than 50X lower. Additional experiments were designed to confirm the impact of titanium oxide by adding titanium oxide only (without Pt) to either anode or cathode electrode while using Pt/C as anode or cathode catalyst. One reduced form of TiO2, i.e., Magneli phase (Ti4O7) which is shown to have improved electrical conductivity and has been investigated as Pt support is also tested in this experiment. The results show that by adding TiO2 or Ti4O7 to anode or cathode both increase the FRR compared titanium oxide free electrodes. The FRR release rate data with titanium oxide added in the cathode is shown in Fig. 1. In this presentation, we will discuss the details of the experimental study on the impact of titanium oxide on polymeric membrane chemical degradation, including fluoride release rate, fluorine inventory loss, as well as the change in hydrogen crossover rate. Additional analysis on the location of chemical degradation (membrane close to anode vs. membrane close to the cathode) has also been conducted which may shed additional light on the fundamental mechanisms of TiOx induced membrane degradation. Finally, the necessity to find alternative metal oxide support rather than TiOx in PEM fuel cell application is highlighted Figure 1

Minimizing mass-transport loss in proton exchange membrane fuel cell by freeze-drying of cathode catalyst layers

The widespread commercialisation of proton exchange membrane fuel cell (PEMFC) for either transportation or stationary application is still hindered by cost barriers owing to the use of precious metal catalysts, as well as performance and material related insufficient durability. Therefore, it is important to enhance the platinum utilization as well as optimize the fabrication method for the production of membrane electrode assemblies (MEAs). This study demonstrates that the drying step during the electrode manufacturing directly affects the microstructure of the catalyst layer, having inherent influence on the porosity and the platinum utilization during PEMFC operation that greatly affects the performance. Freeze-drying as a novel drying technique for PEMFC electrodes is proposed for preparation of low Pt-loaded cathodes (0.160 mgPt·cm−2). This technique possesses the unique feature of solvent removal via sublimation and not only generates 3.5-fold higher effective porosities but also increases the electrochemical surface area by 1.5 times when comparing to electrode dried by regular oven drying technique. Additionally, freeze-drying of electrode also improved ionomer distribution, as evident from a reduced resistance between the pores and a reduced electrolyte resistance of the catalyst layer. Consequently, we consider freeze-drying to be a highly promising technique for future production of MEAs.

Pt/C Electrocatalyst Synthesis from Recycling of the Spent PEMFC Membrane Electrode Assembly: A Closed Loop Circular Economy

Large-scale commercialization and sustainable growth of proton exchange membrane fuel cells (PEMFCs) technology depends largely on availability of the raw materials. Here, technical details of a closed loop recycling of spent electrocatalyst from a PEMFC electrode is demonstrated. The process includes electrochemical dissolution of the Pt nanoparticles in dilute acidic bath, precipitation of the dissolved Pt in form of a Pt-salt and synthesis of carbon supported Pt (Pt/C) electrocatalyst through reduction of the Pt-salt. Spent electrocatalysts from PEMFC electrodes, owing to their high activity, were subjected to dissolution through potentiodynamic treatment in dilute (0.1 M or 1 M) HCl bath to attain a Pt concentration of ∼0.7 mg/mL. Pt was recovered from the electrolyte by precipitation in form of ammonium hexachloroplatinate, which was converted to Pt/C electrocatalyst (20 wt% Pt) through reduction by refluxing in an ethylene glycol/water (1:1 v/v) mixture. Electrochemical surface area measurement and accelerated stress test of the synthesized Pt/C exhibit performances similar to or better than that of commercial Pt/C (20 wt% Pt). With a demonstrated recovery efficiency >90%, the process may be utilized for large-scale recycling of the spent electrocatalysts, especially suitable for PEMFC electrodes.

Degradations in the surface wettability and gas permeability characteristics of proton exchange membrane fuel cell electrodes under freeze-thaw cycles: Effects of ionomer type

Current state-of-the-art proton exchange membrane (PEM) fuel cell electrodes are typically comprised of either short-side-chain (SSC) or long-side-chain (LSC) ionomers, owing to their proven success in the electrode performance and durability under regular cell operation. However, the electrodes based on these two prominent ionomers have not been sufficiently investigated under sub-freezing conditions. In this study, the impact of ionomer type on the degradations of the surface wettability and gas permeability characteristics has been investigated for PEM fuel cell electrodes under freeze-thaw (F-T) cycles between 30 °C and −40 °C. The electrodes comprised of either SSC or LSC ionomers are manufactured with different catalyst loadings. It is found that the F-T cycles induce severe degradations in the electrodes, and the resulting surface morphologies differ greatly, depending on the ionomer type and catalyst loading. For a given catalyst loading, the SSC electrodes degrade more heavily than the LSC ones. Further, independent of the ionomer type, the high catalyst loading electrodes tend to degrade slower than their low catalyst loading counterparts. The SSC catalyst layers peel off from the electrodes virtually completely with the microporous layer largely degraded, inducing a highly corroded and heterogeneous surface morphology. The LSC electrodes experience relatively less degradations, thus the resulting surface morphologies are less corroded and more homogeneous. For all the electrodes, the morphological degradations cause a substantial increase in the gas permeability coefficients, but a decrease in the static contact angles. These increments and decrements correlate well with the severity of the surface degradations, and they are rapid and more substantial for the SSC electrodes.

Impact of PTFE distribution across the GDL on the water droplet removal from a PEM fuel cell electrode containing binder

By progress of the new generation of electrical powertrains and reducing of the fossil fuel resources, vehicle industry becomes more interested in utilizing proton exchange membrane (PEM) fuel cells. However, practical utilization of them is faced with some challenges including liquid water accumulation in the porous electrodes. The common belief for mitigating this issue is the treatment of electrodes' gas diffusion layers (such as carbon papers consisting of carbon fibers and binder for binding fibers) with a highly hydrophobic material such as poly-tetra-fluoro-ethylene (PTFE). In the current investigation, 3D stochastic reconstructions and 3D lattice Boltzmann simulations are employed to discover the impact of PTFE distribution as well as the role of binder content on the removal process of a water droplet from a PEM fuel cell electrode for the first time. Nine different simulations with three dissimilar PTFE distributions and three dissimilar binder contents are implemented. The results demonstrate that the PTFE distribution and the existence of binder can greatly affect the removal efficiency of water droplet from gas diffusion layer. Unexpectedly, for higher binder contents, the uniform distribution of PTFE is less effective. Besides, for a specific PTFE distribution adding binder can effectively hinder the removal process of droplet.

Polymeric graphitic carbon nitride nanosheet-coated amorphous carbon supports for enhanced fuel cell electrode performance and stability

Carbon blacks, which consist of the assemblies of small graphene clusters having a spherical concentric packing structure, are amorphous and have widely been used as catalyst supports for proton exchange membrane fuel cell (PEMFC) electrodes. However, poor electrochemical corrosion resistance during extended use and repeated cyclic operation of PEMFCs shortens their practical device lifetime; surface defects, such as nanosized disordered domains, have been established as the most influential factors. Herein, we coupled polymeric graphitic carbon nitride (pg-CN) with amorphous carbon black (a-CB) in a core@shell structure to produce an electrochemically efficient and stable electrocatalyst support. Our protocol allowed for the simultaneous optimization of uniformly thin graphitic shell structures for electrochemical corrosion stability and nitrogen-enriched functionalities on the carbon surfaces for electrocatalyst (e.g., platinum) nucleation. The newly prepared a-CB@pg-CNs enhanced the stable fuel cell devices operation and remained invariant even at high potential (1.2–1.7 V). Significantly, when used for constructing the PEMFC electrode, a-CB@pg-CN support-based Pt catalyst electrodes outperformed and exhibited superior stability over state-of-the-art commercial a-CB-based counterparts. Experimental observations and density functional theory (DFT) calculations revealed that the unusual electrocatalytic properties of as-prepared a-CB@pg-CNs originated from an intrinsic chemical and electronic coupling that synergistically reduced electrochemical corrosion kinetics and promoted catalyst–support interactions.

Evaluation of the Scaffolding Effect of Pt Nanowires Supported on Reduced Graphene Oxide in PEMFC Electrodes

The stacking and overlapping effect of two-dimensional (2D) graphene nanosheets in the catalyst coating layer is a big challenge for their practical application in proton exchange membrane fuel cells (PEMFCs). These effects hinder the effective transfer of reactant gases to reach the active catalytic sites on catalysts supported on the graphene surface and the removal of the produced water, finally leading to large mass transfer resistances in practical electrodes and poor power performance. In this work, we evaluate the catalytic power performance of aligned Pt nanowires grown on reduced graphene oxide (rGO) (PtNW/rGO) as cathodes in 16-cm2 single PEMFCs. The results are compared to Pt nanoparticles deposited on rGO (Pt/rGO) and commercial Pt/C nanoparticle catalysts. It is found that the scaffolding effect from the aligned Pt nanowire structure reduces the mass transfer resistance in rGO-based catalyst electrodes, and a nearly double power performance is achieved as compared with the Pt/rGO electrodes. However, although a higher mass activity was observed for PtNW/rGO in membrane electrode assembly (MEA) measurement, the power performance obtained at a large current density region is still lower than the Pt/C in PEMFCs because of the stacking effect of rGO.