Loss Of Surface Area Research Articles

Automotive Cold Start of a PEMFC System by Using a Methanol Solution as Antifreeze

Proton exchange membrane fuel cell vehicles are highly competitive due to their zero emissions, high efficiency and power density, quick response, low refuelling time and great autonomy. However, the lack of a sufficient hydrogen infrastructure and their required startup ability and durability at extreme winter conditions are limiting factors in their commercialisation. At subzero temperatures, the produced water during operation as well as the remained water in the cells after shutdown will freeze, hindering the cold start and causing several damages (e.g. cracks, pinholes, catalyst delamination or loss of electrochemical surface area). Commonly, the fuel cell system is purged with dried gases during shutdown to avoid that it freezes during the cold storage. Even though the dry gas purge methods have been extensively studied, thus reducing the degradation due to F/T cycles, there are still remaining unsolved issues, especially at very low temperatures. An alternative method that can mitigate this degradation and hence lower the temperature is the use of an antifreeze solution during the cold storage. Yet, it has been only tested in a single cell1. These results were quite promising, as the use of the antifreeze strongly diminished the degradation because of subzero temperatures and it was possible starting at -10°C. But they showed fairly poor cold start ability, since the residual methanol decreased the power density during the cold start. Therefore, the use of an antifreeze solution for a cold start should be further investigated in a stack.The aim of this study is to prove the cold start capability of a PEMFC system by using a methanol solution as antifreeze. For that, several cold starts at -10°C were carried out with a 4 kW commercial stack, a 25 vol% methanol solution to ensure no ice formation at -10°C and following next test protocol. A conditioning test and a polarization curve were conducted at room temperature. Then, the shutdown procedure, which involved soaking the stack with the methanol solution, was carried out before cooling down the system to -10°C. Following the cold storage, the antifreeze was removed from the stack and the cold start test was run. Finally, another polarization curve at room temperature was carried out. The cold start system ability with the antifreeze was compared with previous cold starts, which only used a dry gas purge method.All cold starts with antifreeze were successfully, thus proving the cold start capability of the PEMFC system by using the methanol solution. Indeed, they did not take longer than with the dry gas purge method. During the cold start the current generation did not decrease, which shows that the produced water did not freeze. Moreover, polarisation curves corroborated no performance losses after every cold start. These results are in concordance with those obtained at single cell level, as in both cases the antifreeze avoided ice formation during the cold storage. This significantly improvement with regard to the above results obtained with a single cell, is due to the ability of the stack to generate much more heat power and to minimize thermal losses, warming up the stack faster than a single cell.The obtained results allow confirming that a self-start of an automotive PEMFC system at subzero temperatures by using antifreeze solution is possible. This alternative method avoids ice formation during the cold storage, which reduces the degradation due to freezing and enables starting at lower temperatures. The decline in degradation increases the stack lifetime, thus can be achieved the strict automotive sector demands. The advantages of the antifreeze for automotive cold start can be used for others mobile applications too, such as trucks, trains, maritime or aircraft. In the future, cold starts with the antifreeze should be further investigated, and at temperatures down to -40°C.1. F. Knorr, D. Garcia, J. Schirmer, P. Gazdizcki, and KA. Friedrich. Appl. Energy, 238, 1 (2019).

Platinum-Nickel Nanotubes Array As Cathode for PEM Fuel Cell

Keywords: PEMFC, Anodic Alumina Oxide, Ni Nanowires, PtNi Alloy, Oxygen Reduction Reaction, Carbon-free electrode.Fuel cells technology have attracted great attention for future energy conversion and storage applications (1). The commercialization of polymer electrolyte membrane fuel cells (PEMFC) technologies is still a challenge due to durability and cost limitations of the catalyst (2). In order to enhance their achievement on the market, it is mandatory to optimize the high costly catalyst and improve its durability and stability or to innovate a new structure of electrodes.Commercial Pt/C catalysts (nanoparticles 3–5 nm) supported carbon black are considered as the current standard catalyst offering high surface areas and high specific activity. Unfortunately, they are limited by corrosion of the C and Pt dissolution/agglomeration through Electrochemical Ostwald ripening mechanism, which is reflected by a fast and significant loss of electrochemical surface area (ECSA) over time during fuel cell operation(3). This work focuses on reducing the amount of platinum and improving the performance and durability of membrane electrode assemblies (MEAs), aiming to reduce cost and thus encouraging the development of PEMFCs. Pt-based catalysts such as Pt-Co, Pt-Cu, and Pt-Ni etc. have shown to exhibit higher oxygen reduction reaction (ORR) activity than platinum for PEMFC. Accordingly, we aspire to develop and control a new carbon-free architecture of the cathode, made only with vertically aligned PtNi nanotubes supported onto a Nafion® membrane.Previous studies have demonstrated the benefits of platinum carbon-free nanostructures such as nanostructured thin films (NSTF) by 3M Company, USA (4), platinum opal nanostructures (5), or vertically aligned platinum nanotubes (6). They have shown very promising performance and stability for low Pt loading.A new carbon-free structure of electrodes made of vertically aligned PtNi nanotubes has been developed to improve performance and durability of PEMFC at low noble metal catalyst content. The process is based on an electrochemistry route of Ni electrodeposition inside anodic alumina oxide (AAO) mold, followed by a step of controlled spontaneous galvanic displacement (Figure 1), thermal treatment and acid treatment leading to the formation of nanotubes (~50nm in diameter and ~400nm in length). Ultraviolet spectrophotometric determination of platinum content showed a loading of ca. 0,1mg/cm². The latter are transferred onto a Nafion® membrane using an optimized hot pressing process. Finally, electrodes were integrated in a complete Membrane Electrode Assembly (MEA) in order to characterize their electrochemical and transport limitations in real operating conditions.The catalyst specific area estimated form the integration of Hupd desorption peaks at 30°C 100%RH in complete MEA, is approximately 6,7 cm²Pt/cm²geo. For comparison, we have elaborated a Pt/C standard cathode (Tanaka 50 wt.% catalyst onto Vulcan support) with low Pt loading (~20µg/cm²) which exhibits a close catalyst surface area of ~6,5 cm²Pt/cm²géo.The performance of the MEA was quantified by recording polarization curve under H2/O2 80°C (Figure 2 below). Promising results are obtained at a relative humidity of 80% at the anode side and 100% at the anode side, under a pressure of 1,38 bar / 1,5 bar respectively. Indeed, fully humified conditions at both anode and cathode lead to severe decay in performance as current density increases, ascribed to flooding. In addition, as shown in Figure 2, higher activities, estimated from the value of current density at 0.8V extracted from the polarization curve corrected from ohmic drop, are achieved with PtNiNTs comparing to Pt/C 20µg/cm². Figure 1

Quantifying the Impacts of Nafion Interfaces with PEMFC-Relevant Substrates through Neutron Reflectometry and Modeling

The critical electrochemical reactions within proton exchange membrane fuel cells (PEMFCs) take place in the heterogeneous catalyst layer (CL). This CL is comprised of three main materials: carbon, platinum, and thin film Nafion. The CL ionomer, Nafion, is particularly important with respect to PEMFC performance and durability. Ionic and charge-neutral reactants and products move through nano-thin Nafion films to reach the carbon-supported platinum catalyst. Increasing the rate of transport through these films reduces cell overpotentials. As the platinum loading of PEMFCs is reduced, to lower costs, the loss of catalyst surface area also decreases the performance; therefore, performance increases obtained from a better understanding of Nafion thin films become more vital in the establishment of low cost and high performance PEMFCs.Neutron reflectometry (NR) is a technique that can be used to simultaneously study the structure and chemical composition of Nafion at length scales relevant to PEMFC CLs. Relating the composition profiles from NR to transport properties across thin-film Nafion gives insight into the relationship between the material’s structure and properties. To date, a majority of NR studies involving Nafion have been performed on the native oxide surface of silicon substrates, where complex layered structures are observed with distinct water-rich and water-poor regions within the polymer [1,2]. Although these studies are useful, their relevance to common PEMFC CL materials (i.e. platinum and carbon) is indirect.In this presentation, we will show and discuss composition depth profiles for Nafion thin films (< 100 nm) on PEMFC-relevant substrates. Experiments performed in both humidified and dry environments allow for robust fits to the data sets. Data from the humidified environment provides insight into Nafion’s water uptake and structure within an operating PEMFC. In contrast to silicon interfaces, Nafion thin films on PEMFC-relevant materials demonstrate much less structuring, with a lower degree of phase segregation in the through-film direction [3,4]. We will provide a detailed look at polymer depth profiles, and discuss the implications of these on PEMFC performance, in particular their influence on ionic conductivity, oxygen diffusivity, and the resulting platinum surface kinetics. Predictions for these relationships and their impacts will also be discussed with the use of a physically based PEMFC model extended from our earlier work [5].[1] S. C. DeCaluwe, A. M. Baker, P. Bhargava, J. E. Fischer, and J. A. Dura, “Structure-property relationships at Nafion thin-film interfaces: Thickness effects on hydration and anisotropic ion transport,” Nano Energy, vol. 46, pp. 91–100, 2018.[2] S. A. Eastman, S. Kim, K. A. Page, B. W. Rowe, S. Kang, C. L. Soles, and K. G. Yager, “Effect of Confinement on Structure, Water Solubility, and Water Transport in Nafion Thin Films,” Macromolecules, vol. 45, no. 19, pp. 7920–7930, 2012.[3] J. A. Dura, V. S. Murthi, M. Hartman, S. K. Satija, and C. F. Majkrzak, “Multilamellar Interface Structures in Nafion,” Macromolecules, vol. 42, no. 13, pp. 4769–4774, 2009.[4] U. N. Shrivastava, H. Fritzsche, and K. Karan, “Interfacial and Bulk Water in Ultrathin Films of Nafion, 3M PFSA, and 3M PFIA Ionomers on a Polycrystalline Platinum Surface,” Macromolecules, vol. 51, no. 23, pp. 9839–9849, 2018.[5] C. R. Randall and S. C. DeCaluwe, “Physically Based Modeling of PEMFC Cathode Catalyst Layers: Effective Microstructure and Ionomer Structure–Property Relationship Impacts,” Journal of Electrochemical Energy Conversion and Storage, vol. 17, no. 4, Jan. 2020.

Influence of Flow Rate on Catalyst Layer Degradation in Polymer Electrolyte Fuel Cells

With the U.S. Department of Energy (DOE) 2020 durability target for transportation applications of 5000 hours in mind [1], several studies have been performed to understand the factors influencing load cycle durability of the cathode catalyst layer subjected to various operating conditions. In general, high upper potential limit (> 1 V vs RHE), high temperature (> 90 °C) and high relative humidity (RH) during potential cycling have been shown to exacerbate degradation [2]. It is known that Electrochemical surface area (ECSA) loss occurs by Pt dissolution attributed to particle size growth via modified Ostwald ripening, crystal migration, detachment from carbon support due to carbon corrosion, and precipitation in the membrane by chemical reduction due to hydrogen crossover [3].Accelerated stress tests (ASTs) are often used to mimic material component degradation similar to real automotive driving conditions. Different cycle profiles have been shown to cause varying rates of degradation depending on the operating conditions. The influence of flow rate of reactant gases on degradation, however, has received little attention. Stariha et al. [4], showed that a relatively high flow rate AST had the highest degradation acceleration factor and resulted in non-uniform decay rates when compared to low flow rate conditions of nitrogen gas (N2) at the cathode. However, this phenomenon has not been further studied in detail.The current study is aimed at elucidating the effect of flow rate on cathode catalyst layer degradation subjected to a standard DOE square-wave AST protocol. Single cell studies with 5-cm2 active area were performed using Nafion® XL membrane (Ion Power Inc.) and SGL-22 BB gas diffusion electrodes (GDEs) as MEA materials and 1-serpentine flow field. All the MEA samples used in the present study had nominally identical Pt loading and were subjected to a consistent conditioning procedure prior to AST. A DOE square-wave cycling protocol [4] was executed: H2/N2 flows at anode/cathode, voltage cycling between 0.6 V and 0.95 V, and 3 seconds hold at each voltage. 30,000 cycles (total test time of 50 hours) were performed at 100% RH, 80°C, and atmospheric pressure. Three different cathode N2 flow rates of 20 sccm, 40 sccm and 80 sccm were used during the AST with fixed H2 flow of 40 sccm at the anode. ECSA loss was tracked using cyclic voltammograms at several intervals during the AST. Complete electrochemical characterization at beginning and the end of life (BOL and EOL), along with ex-situ diagnostic techniques such as TEM and GIXRD, were used to obtain particle size distribution and spatial degradation profiles. The results indicate higher performance loss (at EOL) for 80 sccm flow of N2 at the cathode compared to 20 sccm and 40 sccm flow rates, most likely due to inhomogeneous Pt loss from inlet to the outlet due to variations in local flow conditions by fully saturated N2 flow at different flow rates.

Using Pore Former to Improve Performance of Anode Catalyst Layer of a PEM Water Electrolyzer

Polymer electrolyte membrane water electrolyzer (PEMWE) performance mostly depends on the catalyst layer (CL) composition and microstructure. The CL microstructure is influenced by catalyst ink composition and fabrication process [1]. Due to the use of an unsupported catalyst (IrOx) in PEMWE, the porosity of the CL is low, resulting in a low CL surface area and high activation loss. By increasing the porosity of the CL, it is hypothesized that more catalyst surface area will be exposed to the reactant and electrolyte. One method to increase the CL porosity is to use pore formers (PF): sacrificial particles that are added to the ink and then removed after CL fabrication, leaving additional pores. Limited work has been done on the use of PF in fuel cell applications [2–6] which resulted in an increase of the porosity [3,5] and the surface area [2–4]. No work has been done using PF in PEMWE application. In this study, a PF is used to study the effect of PF addition on PEMWE microstructure and performance.In order to fabricate the catalyst coated membranes (CCMs), an anode catalyst ink was prepared by dispersing IrOx powder (Tanaka Kikinzoku Kogyo, ELC-0110) and PF in a dispersion media as described in ref. [7]. The PF to IrOx volume ratio was varied between 0 to 0.25 (Table 1). The ionomer content was kept constant at 35 %wt. The cathode ink was prepared using the method and material described in ref. [8]. The CCMs were prepared by inkjet printing method as described in ref. [7] with catalyst loading as shown in Table 1.One CCM for each PF to IrOx volume ratio was tested and the results are reported. The effect of the PF on the electrochemical surface area (ECSA) was studied using cyclic voltammetry (CV). ECSA was estimated, after conditioning and obtaining nine polarization curves, using the method developed by Tan et al. [9]. The average ECSA of CCM-0.1 is increased by 57 % compared to the CCM-0 after nine polarization curves as shown in Figure 1b and Table 1. While CCM-0.1 shows the highest ECSA, a similar improvement is not seen from CCM-0.25, indicating that the removal of PF is having an impact beyond increasing the ECSA. To study the electrode performance without the effect of varying loading between electrodes, the normalized electrochemical performance with respect to IrOx loading is compared in Figure 1a. CCM-0.1 exhibits the highest performance and shows an improvement of 30 and 80 mV at 2 A/mg compared to the CCM-0.25 and CCM-0 respectively. The cell voltage decreased with an increase in the PF to IrOx volume ratio from 0 to 0.1. When the ratio increased further to 0.25, the cell voltage increased, showing an optimum cell voltage at 0.1 PF to IrOx volume ratio which is in line with the estimated ECSA. The PF has a complex impact on the CL microstructure. These preliminary results indicate that the PEMWE performance can be improved by using PF, and that there might be an optimal PF content. K.-H. Kim, K.-Y. Lee, H.-J. Kim, E. Cho, S.-Y. Lee, T.-H. Lim, S. P. Yoon, I. C. Hwang and J. H. Jang, Int. J. Hydrog. Energy, 35, 2119 (2010).Y. Song, Y. Wei, H. Xu, M. Williams, Y. Liu, L. J. Bonville, H. R. Kunz and J. M. Fenton, J. Power Sources, 141, 250 (2005).T. V. Reshetenko, H.-T. Kim and H.-J. Kweon, J. Power Sources, 171, 433 (2007).Q. Huang, J. Jiang, J. Chai, T. Yuan, H. Zhang, Z. Zou, X. Zhang and H. Yang, J. Power Sources, 262, 213 (2014).A. Fischer, J. Jindra and H. Wendt, J. Appl. Electrochem., 28, 277 (1998).Y.-H. Cho, N. Jung, Y. S. Kang, D. Y. Chung, J. W. Lim, H. Choe, Y.-H. Cho and Y.-E. Sung, Int. J. Hydrog. Energy, 37, 11969 (2012).M. Mandal, A. Valls, N. Gangnus and M. Secanell, J. Electrochem. Soc., 165, F543 (2018).S. Shukla, K. Domican, K. Karan, S. Bhattacharjee and M. Secanell, Electrochimica Acta, 156, 289 (2015).X. Tan, J. Shen, N. Semagina and M. Secanell, J. Catal., 371, 57 (2019). Figure 1

Catalyst Degradation in Polymer Electrolyte Fuel Cells with Multi-Modal Techniques: Understanding Phenomena Under Varied Gas and Relative Humidity

For widespread commercialization of polymer electrolyte fuel cells (PEFCs), cost and durability need to be addressed. Reduction in cost should not negatively affect PEFC durability and performance. At high volume production, platinum (Pt) electro catalyst contributes to 41% of the total fuel cell stack cost for light duty vehicles. This cost contribution increases to 53% for middle and heavy-duty vehicles. When membrane electrode assemblies (MEAs) are designed for durability, this cost will increase further. A complex balance exists between cost, performance and durability. With smaller nanoparticle size better Pt dispersion is achieved which gives improved performance and reduces Pt loading and cost. But this negatively affects durability, as smaller nanoparticles have higher surface energy which causes faster electrocatalyst degradation1. During an actual drive cycle, PEFCs need to operate over a wide range of operating conditions such as temperature (-40C to 120C), potential (0V to 1.5V) and relative humidity which can further accelerate Pt dissolution Under load cycling during typical drive cycle, repeated oxidation and reduction of Pt surface leads to Pt dissolution2. Dissolved Pt ions redeposit on nearby nanoparticles effectively increasing their particle size2. This electrochemical surface area (ECSA) loss via increase in particle size is well known as Ostwald ripening. Some Pt ions are reduced in the electrolyte membrane by crossover hydrogen which leads to formation of a Pt band, usually at the membrane and cathode catalyst layer interface3. At > 1V, carbon corrosion can lead to Pt detachment2. This can happen during start-up/shut down cycles when anode is exposed to hydrogen and air. Such degradation mechanisms need to be studied systematically with effect of operating conditions like gas flow rates, temperature and relative humidity under dynamic load cycling.In this study, we investigate the effect of cathode gas environment and gas flow rates on Pt particle size during aging experiment. A DOE adopted accelerated stress test (AST) protocol simulating the drive cycle operating conditions is used. It consists of a 0.6 V – 0.95 V square wave with 3s hold time at each potential. In situ electrochemical characterization at regular stages is used to identify ECSA loss trends and polarization performance. Figure 1 shows normalized ECSA loss for the four cells cycled in different environments during catalyst AST. Measurements were taken beginning of life (BOL), after 1,000, 5,000, 15,000 and 30,000 cycles. We observed that the cell that was cycled in N2 and 100 % RH degraded the most , having 25 % of ECSA left after 30,000 cycles compared to BOL. Electrochemical measurements are supplemented by detailed post-mortem ex-situ x-ray characterization by using micro x-ray fluorescence, x- ray computed tomography and micro x-ray diffraction, SEM-EDS. 1cm2 active areas of the catalyst layer at the inlet, middle and outlet of the flow field were mapped using micro x-ray diffraction to understand Pt particle size distribution under land/channel. Catalyst layer loading before and after the AST were confirmed using micro x-ray fluorescence.References(1) Cherevko, S.; Kulyk, N.; Mayrhofer, K. J. J. Durability of Platinum-Based Fuel Cell Electrocatalysts: Dissolution of Bulk and Nanoscale Platinum. Nano Energy 2016, 29, 275–298. https://doi.org/10.1016/j.nanoen.2016.03.005.(2) Meier, J. C.; Galeano, C.; Katsounaros, I.; Witte, J.; Bongard, H. J.; Topalov, A. A.; Baldizzone, C.; Mezzavilla, S.; Schüth, F.; Mayrhofer, K. J. J. Design Criteria for Stable Pt/C Fuel Cell Catalysts. Beilstein J. Nanotechnol. 2014, 5 (1), 44–67. https://doi.org/10.3762/bjnano.5.5.(3) Zhang, J.; Litteer, B. A.; Gu, W.; Liu, H.; Gasteiger, H. A. Effect of Hydrogen and Oxygen Partial Pressure on Pt Precipitation within the Membrane of PEMFCs. J. Electrochem. Soc. 2007, 154 (10), B1006. https://doi.org/10.1149/1.2764240. Figure 1

Ionic Liquid Modified Pt/C Electrocatalysts for the Oxygen Reduction Reaction in Polymer Electrolyte Fuel Cells

One of the main challenges of mass deployment of fuel cell electrical cars is the limited power density of the polymer electrolyte fuel cell (PEFC), requiring high loadings of precious Pt metal to drive the oxygen reduction reaction (ORR). Within the catalyst layer of PEFC electrodes, Pt nanoparticles are typically dispersed on porous Ketjenblack carbon due to its high surface area, and the ability of Pt to be deposited into the smaller mesopores of the carbon support; so that they can be protected from ionomer poisoning. Up to 62% of Pt nanoparticles can be buried in the interior of the mesopores1. As the cathode ORR takes place at the triple-phase boundary of the liquid electrolyte, oxygen gas, and solid Pt deposited on the carbon support, a sufficient diffusion of oxygen and protons in the liquid media is vital for the ORR reaction. However, Nafion as the proton conductor cannot penetrate into the micro and mesopores of the support due to the size exclusions; therefore, Pt nanoparticles buried within those pores will not be accessed by the reactants at dry conditions. Studies have shown that electric double layers formed at Pt-water interface can help mediate proton transport increasing proton conductivity by three orders of magnitude. Although, this conductivity is still about two orders of magnitude lower than that of Nafion2 , 3.The principle of electrocatalyst interface design in this work is to impregnate the high surface area (HSA) Ketjenblack carbon pores with a secondary phase, imidazolium-derived ionic liquids (ILs), that can fill the mesopores of the carbon support and provide satisfactory oxygen solubility and proton transfer due to high ΔpKa value and hydrogen-bonded network formation4 , 5. Pt/C-IL catalyst systems are prepared with 1-alkyl-3-metylimidazolium bis(trifluoromethylsulfonyl)imides with short cationic alkyl chain lengths including ([C2mim][NTf2]) and ([C4mim][NTf2]), along with 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([C4dmim][NTf2]) in order to study the effect of methyl group attachment to the carbon in the 2-position.The effect of the IL molecular structure on the electrochemical behavior of Pt/C catalyst is investigated in half-cell electrochemical set-up, rotating disk electrode (RDE). Figure 1a contains the cyclic voltammogram of pristine and IL-modified Pt/C samples. Pt/C catalyst exhibits electrochemical surface area (ECSA) loss after the IL modification. IL-modified Pt/C samples including Pt/C-[C2mim][NTf2], [C4mim][NTf2], and [C4dmim][NTf2] show ECSA values of 81.29, 78.1 and 78.9 m2 g-1 Pt’ respectively, which are lower than that of the pristine Pt/C. Moreover, IL molecules create a hydrophobic microenvironment that can lead to the repelling of the water molecules from Pt active sites protecting them from being oxidized; therefore, the onset potential of Pt-oxide formation is shifted to higher potentials and the coverage of the non-reactive oxygenated species is decreased, improving the specific activity of the electrocatalysts.Electrochemical impedance spectroscopy (EIS) is also implemented to measure overall effective proton diffusion resistance within the catalyst layers of Pt/C-IL-Nafion and Pt/C-Nafion. The linear portion of the Nyquist plot data at high frequencies shows a noticeably enhanced proton accessibility of the electrocatalyst in all Pt/C-Nafion-IL catalyst layers compared to that of Pt/C-Nafion (Figure 1b). References Huang, J., Li, Z. & Zhang, J. Review of characterization and modeling of polymer electrolyte fuel cell catalyst layer: The blessing and curse of ionomer. Front. Energy 11, 334–364 (2017). Zenyuk, I. V & Litster, S. Modeling ion conduction and electrochemical reactions in water films on thin-film metal electrodes with application to low temperature fuel cells. Electrochim. Acta 146, 194–206 (2014). Thompson, E. L. & Baker, D. Proton Conduction on Ionomer-Free Pt Surfaces. (2011). doi:10.1149/1.3635605 Miran, M. S. et al. Key factor governing the physicochemical properties and extent of proton transfer in protic ionic liquids: ΔpKa or chemical structure? Phys. Chem. Chem. Phys. 21, 418—426 (2018). Moschovi, A. M., Dracopoulos, V. & Nikolakis, V. Inter- and Intramolecular Interactions in Imidazolium Protic Ionic Liquids. J. Phys. Chem. B 118, 8673–8683 (2014). Figure 1

Fuel Cell Automotive Drive-Cycle Cathode Catalyst (Pt/C) Layer Durability: Dependence on Low Vs. High Carbon Supports and Ionomer-to-Carbon-Ratio

The typical urban automotive drive-cycle for proton exchange membrane fuel cells (PEMFC) consists of frequent idle-to-peak power cathode voltage transients, with extremes ranging from 1.2 V to 0.6 V, respectively. Understanding the platinum electrochemical surface area (ECSA) loss mechanisms dependence on material and operational conditions have been extensively investigated with the objective of minimizing performance losses over the lifetime of PEMFC powered automobiles. Commonly, automotive accelerated stress tests (AST) are performed ex-situ on thin catalyst films. The standard catalyst consists of carbon supported platinum nanoparticles (Pt/C). This study compares triangle waveform (TW) ASTs monitoring ECSA loss for ex-situ (thin film in 0.1 HClO4) and in-situ (catalyst layers bond with ionomeric electrolyte in a complete membrane electrode assembly (MEA)). The ionomer-to-carbon ratios (I/C) of the MEAs is varied from 0.6 to 1.5 on either low surface area carbon (LSAC) or high surface area carbon (HSAC) supports. The AST profiles included 25k cycles at 10 sec intervals, between the upper potential limit of either 0.9 V or 1.2 V and the lower potential limit of 0.6V, with intermediate ECSA evaluations from the H-desorption current (0.02 V → 0.4 V).The electrolyte environment alters, liquid vs. ionomer, the dominant AST decay mechanism, Figure 1A. The decay in the liquid electrolyte is most sever upon initiation of the AST and the rate of decay decrease after the first 2000 cycles. While the MEA decay is less initial, but maintains the same rate of initial decay over 5000 cycles resulting in a higher overall loss of ECSA. ECSA decay is exacerbated at temperatures of 70°C. Increasing the I/C increases the interfacial acidity at the surface of the Pt/C and results in exacerbated ECSA loss, Figure 1B. In accordance with the Pourbaix diagram for Pt, it is less stable in a more acidic environment. The relative rate of ECSA loss is reduced on HSAC supports. Figure 1

The Effect of Material Priorities on Oxygen Evolution Activity and Assessing Half-Cell Screening As a Predictive Tool in Electrolysis

While hydrogen as a chemical commodity is used today in transportation and agriculture, it is a relatively small contributor in overall energy pathways. Hydrogen produced by electrochemical water splitting has further been limited by cost. Electrolysis, however, when coupled directly with low-cost intermittent power has an opportunity for expanded use in the future in grid storage and offloading curtailed inputs to other sectors. Expanded use will require reduction in the power input and capital cost, and will likely involve component development.Within catalysis in proton exchange membrane electrolysis, this includes reductions in the platinum group metal loading and materials development to improve: performance to lessen load requirements; or stability to mitigate durability losses. Ex-situ testing, or rotating disk electrode half-cells, are commonly used to screen materials in the early stages of catalyst development. Understanding the limitations of the approach, however, is critical to predicting device-level performance and better guiding development efforts.In this effort, several iridium- and ruthenium-based catalysts were studied for oxygen evolution activity and stability in half-cell tests. Electrochemical surface area measurements were used to separate surface area from specific activity as well as approximate contributions for metals and oxides. Generally, catalysts with higher iridium metal and ruthenium composition tended to produce higher activity. Changes were observed following extended exposure to high potential, and the dissolution rates and surface area losses for iridium metal and ruthenium were typically higher. Catalyst options, however, were found that provided an optimum in activity and stability, and additional characterization was used to evaluate morphology, crystallinity, and alloying as potential contributors. Device-level examinations further assess the ability rotating disk electrodes as a predictive tool and test the approach across different material sets, compositions, and morphologies. Figure 1

Atomically Dispersed Dopants for Stabilizing Ceria Surface Area

Ceria is known to be a very good catalyst as well as a support for oxygen transfer (oxidation) as well as for hydrogen transfer (hydrogenation and dehydrogenation) reactions. Many of these reactions occur at high temperatures where ceria is known to sinter, leading to loss of surface area. The thermal stability of ceria can be improved by the addition of dopants, but the location of the dopant atoms and the mechanisms by which ceria stabilization occurs are poorly understood. We show here that dopants located on the surface of ceria are remarkably effective at stabilizing ceria surface area. Keeping metal loading constant at 0.88 mol%, we found that surface area of the ceria aged at 800 °C in air for 5 h ranged from 45 m2/g to 2 m2/g. Strongly bound dopants in atomically dispersed form help to pin surface sites and lower the mobility of ceria.

Unravelling the Structural Modification (Meso-Nano-) of Cu/ZnO-Al2O3 Catalysts for Methanol Synthesis by the Residual NaNO3 in Hydroxycarbonate Precursors

The effects of residual NaNO3 on the modification of Cu/ZnO-Al2O3 catalysts have been extensively documented, but the modification mechanism is so far unclear. This work studies in detail the influence of the residual sodium nitrate present in the hydroxycarbonate precursors on their decomposition during calcination and how it affects to the formation and configuration of the final active sites of the Cu/ZnO-Al2O3 catalysts. Different samples with varying sodium content after washing (from 0.01 to 7.3 wt%) were prepared and studied in detail after calcination and reduction steps. The results of this work demonstrated that NaNO3 affects the decomposition mechanism of the hydroxycarbonate precursors during calcination and produces its decarbonation at low temperature. The enhancement of the decarbonation by NaNO3 leads to segregation and crystallization of CuO and ZnO with loss of mesostructure and surface area in the calcined catalysts. The loss of mesostructure in calcined catalysts affects the subsequent reduction step, decreasing the reducibility and damaging the nanostructure of the reduced catalysts forming large Cu particles in poor contact with ZnOx that results in a significant decrease in the intrinsic activity of the copper active sites for methanol synthesis.

Electrochemical activation to enhance the volumetric performance of carbon nanotube electrodes

In recent years, the floating catalyst chemical vapor deposition (FCCVD) method has established itself as one of the most commercially viable methods to produce carbon nanotubes (CNTs). However, at the final stage of the FCCVD process, densification of the web of CNTs to form a sheet causes a significant loss in the active surface area of the CNTs. This loss of surface area reduces the density of active materials that can be infiltrated into the CNT sheet. Reducing the amount of active material in the sheet reduces the charge storage capacity and causes low volumetric performance for CNT sheet in energy storage applications. Here, we are reporting the use of an electrochemical activation technique to chemically functionalize the CNT surface and recover the lost surface area. Chemical functionalization separates the CNTs and thereby increases the density of active materials that can be integrated into the CNT sheet. In order to avoid deterioration of the CNT structure from excessive electrochemical activation, we have optimized the procedure by comparing the performance of four samples of CNT sheets prepared using different numbers of cycles of electrochemical activation (5, 10, 20, 40 cycles). The performance of the sheet was analyzed using specific capacitance measurement, electrochemical impedance spectroscopy (EIS) analysis, and UV–vis absorption spectroscopy. In addition, the Randle-Sevcik plot was used to select the CNTs with the largest activated surface area and electrochemical specific capacitance (150F/g). Further analysis such as x-ray photoelectron spectroscopy (XPS) was also carried out to unveil a 5% increase of the various oxygen-containing functional groups at the surface of the activated CNTs. Polyaniline (PANI) was then deposited on the surface of the electrochemically altered CNTs via an oxidation polymerization process to create a CNT-PANI composite material. A fully fabricated device with CNT-PANI electrodes and Polyvinylidene fluoride − 1-Ethyl-3-methylimidazolium tetrafluoroborate (PVDF-EMIMBF4) electrolyte showed a significant specific capacitance improvement over a device with Poly(vinyl alcohol) -Sulfuric acid (PVA-H2SO4) electrolyte. The PVDF-EMIMBF4 capacitor device had a volumetric energy density of 833.3 µWh/cm3 at a current density of 0.5 mA/cm2. The capacitor has good mechanical flexibility and high capacitance retention (75% after 5000 cycles), which show the advantages of the electrochemical activation method.

Investigations of molybdenum-promoted manganese-based solid sorbents for H2S capture

The promotion effect of Mo-addition to alumina-supported Mn-based sorbents for high-temperature desulphurization was explored. A series of Mn-based sorbents with fixed Mn-loading and different Mo-loadings were prepared by the wet impregnation method and both fresh and used sorbents were characterized with respect to their physical and chemical properties. The sorbents were active for H2S removal at 600 °C and could be regenerated by oxidation in diluted air. The sorbents were subjected to 10 repeated sorption/regeneration cycles, and some loss of capacity occurred during the cycles. The results show that Mo-addition promotes the Mn-based sulfur sorbent performance both in terms of capacity and stability. Over the range investigated (0–8 wt% Mo added to a 15 wt% Mn sorbent), the improvement increased with an increasing amount of Mo added. The sample with the highest Mo-addition (15Mn8Mo) also retained the capacity best, as over 90% of the capacity remained after 10 sorption-regeneration cycles, in spite of suffering the most from sintering (observed as loss in surface area, increased pore size, and growth in Mn particle size). Characterization of the fresh and used samples using XRD and Raman spectroscopy indicates that a mixed Mn–Mo oxide suggested to be MnMoO4 plays a role in the promotion mechanism. The sorbent, 15Mn8Mo, is suggested to be promising for high-temperature desulphurization of bio-syngas.

The Pyruvate Kinase Activator Mitapivat Ameliorates Anemia and Prevents Iron Overload in a Mouse Model of Hereditary Spherocytosis

Hereditary spherocytosis (HS) is the most common cause of inherited red cell membranopathy, due to mutations in genes encoding for membrane or cytoskeletal proteins, including band 3, ankyrin, spectrin, band 4.1 or band 4.2. Membrane instability results in membrane surface area loss and generation of spherocytic red cells with elevated MCHC, decreased cellular deformability and reduced red cell survival, due to splenic sequestration. Clinical management of the hemolytic anemia due to HS depends on the age of the patient and the severity of anemia. Splenectomy is indicated in children with symptomatic anemia. A classic diagnostic test for HS is the incubated osmotic fragility: this test highlights the crucial role that ATP content plays in maintenance of normal RBC function including membrane stability: it is generally believed that the increased fragility of HS is the result of abnormal ATP depletion over the 24hr incubation. We explored the hypothesis that pyruvate kinase activator, mitapivat, by modulating ATP content could have potential beneficial effects for HS RBCs in band 4.2-/- mice, a well-established model of HS (Peters LL et al JCI 103: 1527, 1999). 4.2-/- mice exhibit moderate anemia which recapitulates most of the features of typical human HS without showing the profound anemization seen in other mouse models. Oral AG-348 administration to band 4.2-/- mice at dosages of 200 mg/kg/day over 6 months resulted in (i) improvement of anemia with reduced reticulocyte count (Hb 11.6±0.035 g/dL, n=17 vs 13.104±0.09 g/dL, n=9; P&amp;lt;0.05; retics: 11.4±0.1%, n=16 vs 7.6±0.2%, n=7 % P&amp;lt;0.05) and decrease hemolytic indices (LDH, total bilirubin) with decreased spleen weight/mouse weight ratio (8.41±0.08 vs 6.53±0.06, n=8 P&amp;lt;0.05); (ii) reduced hepatic and splenic iron overload; (iii) reduction in the proportion of phosphatidylserine positive RBCs, measured with Annexin V binding (2.4±0.02 vs 1.5±0.06 % P&amp;lt;0.05); (iv) reduction of naturally occurring antibody (NAb) bound to band 4.2-/- RBC membrane. The decreased hemolysis was associated with reduction in serum erythropoietin (EPO: 1001.2±41.7 U/L vs 472 ± 12.5 U/L, n=7, P&amp;lt;0.05), supporting the beneficial effect of mitapivat on HS anemia. These data indicate that mitapivat ameliorates anemia of band 4.2-/- mice, reduces chronic hemolysis and improves band 4.2-/- mouse RBC features. Thus, mitapivat might represent an interesting therapeutic option for HS patients. Disclosures Kung: Agios Pharmaceuticals: Current Employment, Current equity holder in publicly-traded company. Kosinski:Agios Pharmaceuticals Inc: Current Employment, Current equity holder in publicly-traded company. Dang:Agios Pharmaceuticals Inc.: Current Employment, Current equity holder in publicly-traded company. Brugnara:Sysmex America Inc.: Consultancy; American Journal of Hematology: Other.

Microstructural Investigation of Coke Deposition in Pelleted Catalyst during Downhole Catalytic Upgrading of Heavy Crude Oil Using Porosimetry and X-ray Computed Tomography

Catalyst pore evolution due to coke and metals (e.g., Ni, V, etc.) deposition from heavy oil catalytic upgrading is studied using nitrogen adsorption–desorption, mercury porosimetry, X-ray computed tomography and Scanning electron microscope (SEM) techniques. These techniques probe the impact of coking on the global pore size distribution and the pore-scale connectivity of pores of different sizes. 24wt% coked NiMo/Al2O3 catalyst was studied. Coke deposition caused active site coverage and pore-mouth blockage making the core pore network inaccessible to reactants as reflected in the nearly loss of total surface area and pore volume observed from porosimetry, while the x-ray computed tomography image shows scanty coke deposits within the microstructure. The SEM image confirmed that pore-mouth blockage due to large coke deposition in the early hours of the upgrading reactions at the outer layer of the catalyst pellets is the major cause of deactivation. The spent catalyst experienced more than 90% drop in surface area with coke deposition on the outer layer of the catalyst far higher than in the centre. Therefore, one of the ways to enhance intra-particle diffusion and limit the impact of coke deposition on the outer layer of the catalyst is either to use nano-catalyst or engineered pore sizes.

Covalent Amine Tethering on Ketone Modified Porous Organic Polymers for Enhanced CO2 Capture.

Effective removal of excess greenhouse gas CO2 necessitates new adsorbents that can overcome the shortcomings of the current capture methods. To achieve that, porous materials are often modified post-synthetically with reactive amine functionalities but suffer from significant surface area losses. Herein, we report a successful amine post-functionalization of a highly porous covalent organic polymer, COP-130, without losing much porosity. By varying the amine substituents, we recorded a remarkable increase in CO2 uptake and selectivity. Ketone functionality, a rarely accessible functional group for porous polymers, was inserted prior to amination and led to covalent tethering of amines. Interestingly, aminated polymers demonstrated relatively low heats of adsorption, which is useful for the rapid recyclability of materials, due to the formation of suspected intramolecular hydrogen bonding.

The influence of spatially heterogeneous anthropogenic change on bill size evolution in a coastal songbird.

Natural history collections provide an unparalleled resource for documenting population responses to past anthropogenic change. However, in many cases, traits measured on specimens may vary temporally in response to a number of different anthropogenic pressures or demographic processes. While teasing apart these different drivers is challenging, approaches that integrate analyses of spatial and temporal series of specimens can provide a robust framework for examining whether traits exhibit common responses to ecological variation in space and time. We applied this approach to analyze bill morphology variation in California Savannah Sparrows (Passerculus sandwichensis). We found that bill surface area increased in birds from higher salinity tidal marshes that are hotter and drier. Only the coastal subspecies, alaudinus, exhibited a significant increase in bill size through time. As with patterns of spatial variation, alaudinus populations occupying higher salinity tidal marshes that have become warmer and drier over the past century exhibited the greatest increases in bill surface area. We also found a significant negative correlation between bill surface area and total evaporative water loss (TEWL) and estimated that observed increases in bill size could result in a reduction of up to 16.2% in daily water losses. Together, these patterns of spatial and temporal variation in bill size were consistent with the hypothesis that larger bills are favored in freshwater‐limited environments as a mechanism of dissipating heat, reducing reliance on evaporative cooling, and increasing water conservation. With museum collections increasingly being leveraged to understand past responses to global change, this work highlights the importance of considering the influence of many different axes of anthropogenic change and of integrating spatial and temporal analyses to better understand the influence of specific human impacts on population change over time.

S2888 Tetany and Carpopedal Spasm as the Initial Presentation of Celiac Disease

INTRODUCTION: Celiac disease (CD) presents with a wide variety of symptoms, with gastrointestinal manifestations being the most common. We discuss a rare case of celiac disease diagnosed by severe hypocalcemia and carpopedal spasm. CASE DESCRIPTION/METHODS: A 25 year old male with no significant medical history presented with complaints of paresthesias in his hands and arms and a few weeks of diarrhea. Physical exam revealed carpopedal spasm. Initial lab work was significant for severe hypocalcemia (ionized calcium level of 0.66), hypomagnesemia and undetectable vitamin D 25-hydroxy level, along with elevated PTH (583.2). His electrolytes were aggressively replaced intravenously, and after stabilization, transitioned to oral supplementation. Vitamin D supplementation was also initiated. Patient was found to have elevated tissue transglutaminase IgA antibodies and gliadin peptide levels. EGD demonstrated scalloped mucosa with complete flattening of villi in the duodenum consistent with celiac disease, which was also confirmed on biopsy. Patient was initiated on calcium and vitamin D replacement and a gluten-free diet on discharge. DISCUSSION: CD usually presents with diarrhea and weight loss. However, >40% of patients present with atypical manifestations, many of which are secondary to malabsorption from a loss of absorptive area and the presence of immature surface epithelial cells whose have impaired absorptive and secretory functions. Hypocalcemia in CD occurs due to intestinal malabsorption of calcium, both from loss of villous surface area and depletion of calbindin from enterocytes leading to decreased active intestinal calcium transport mechanisms. Concurrent malabsorption of vitamin D may also play a role, although there have been reported cases with hypocalcemia despite a normal vitamin D. Hypocalcemia in CD is common, with studies demonstrating that average serum calcium is 0.02 mmol/l lower in asymptomatic CD patients that the general population. However, there are few reports, especially in the developed world, of severe hypocalcemia and neurological symptoms being the initial presentation of CD. Management includes intravenous or oral calcium replacement, with replacement of other electrolytes (magnesium, phosphorous and potassium) and vitamin D as well. These levels should be monitored at follow up visits, but it is expected that they will normalize after management of CD. It is important for a clinician to recognize atypical presentations of CD, especially in countries with high prevalence.Figure 1.: Endoscopic view of duodenal bulb showing scalloped mucosa and complete flattening of villi.Figure 2.: Microscopic view showing villous blunting and increased intraepithelial lymphocytes, H&E staining.

Degradation study of a proton exchange membrane water electrolyzer under dynamic operation conditions

Understanding degradation phenomena of polymer electrolyte water electrolyzers operating under dynamic conditions is imperative for developing and implementing efficient and reliable means of energy storage from fluctuating and intermittent renewable energy sources. Herein, a commercial membrane electrode assembly with an amorphous IrOx anode is subjected to potential sweeping (1.4–1.8 V) with short and long holds and few steady-state interims for overall 830 h at 60 °C and ambient pressure to simulate frequent alternating idle and nominal operation regimes. Systematic electrochemical diagnostics and physicochemical methods are applied to identify degradation sources. Mild kinetic deactivation (2.6 μV/h) is observed independent on the dynamic protocol, due to loss of electrochemical surface area via crystallization. The amount of Ir dissolved and redeposited in the ionomer anode phase and in the membrane is negligible in comparison to the current state-of-the-art Ir loadings. As compared to kinetic losses, the irreversible resistive losses are one order of magnitude higher and are thought to be caused by the degradation of the membrane close to the anode catalyst layer. These resistive losses are associated specifically with dynamic operation. The two orders of magnitude higher, but recoverable, degradation during steady-state interims is attributed to the growth of inhibiting or long-lived species.

Phase development and pore stability of yttria‐ and ytterbia‐stabilized zirconia aerogels

AbstractHigh‐porosity yttria‐ and ytterbia‐stabilized zirconia aerogels offer the potential of extremely low thermal conductivity materials for high‐temperature applications. Yttria‐ and ytterbia‐doped zirconia aerogels were synthesized using a sol‐gel approach over the dopant range of 0‐20 atomic percent. Surface area, pore volume, and morphology of the as‐dried aerogels and materials thermally exposed for short periods of time to temperatures up to 1200°C were characterized by nitrogen physisorption, scanning and transmission electron microscopy, and X‐ray diffraction. The aerogels as supercritically dried all were X‐ray amorphous. At a 5% dopant level, a tetragonal structure with a smaller monoclinic phase developed on thermal exposure. Mixed tetragonal and cubic phases or predominantly cubic materials were observed at higher dopant levels, depending on the dopant level, temperature and exposure time. The formation of crystalline phases was accompanied by loss of surface area and pore volume, although some mesoporous structure was maintained on short‐term exposure to 1000°C. Incorporation of the smaller Yb atom into the lattice structure resulted in smaller lattice dimensions on crystallization than was seen with Y doping and favored a more highly equiaxed structure. Aerogels synthesized with 15% Y maintained the smallest particle size without evidence of sintering at 1100°C. Largest shrinkage and loss of pore volume occurred on crystallization from the amorphous phase, with further loss of pores at temperatures above 1000°C attributable to changes in lattice parameters.