VII.I.6 Fundamental Science for Performance, Cost and Durability (original) (raw)
Related papers
Solid acid proton conductors: from laboratory curiosities to fuel cell electrolytes
2007
The compound CsH 2 PO 4 has emerged as a viable electrolyte for intermediate temperature (200-300 1C) fuel cells. In order to settle the question of the high temperature behavior of this material, conductivity measurements were performed by two-point AC impedance spectroscopy under humidified conditions (p[H 2 O] = 0.4 atm). A transition to a stable, high conductivity phase was observed at 230 1C, with the conductivity rising to a value of 2.2 Â 10 À2 S cm À1 at 240 1C and the activation energy of proton transport dropping to 0.42 eV. In the absence of active humidification, dehydration of CsH 2 PO 4 does indeed occur, but, in contradiction to some suggestions in the literature, the dehydration process is not responsible for the high conductivity at this temperature. Electrochemical characterization by galvanostatic current interrupt (GCI) methods and three-point AC impedance spectroscopy (under uniform, humidified gases) of CsH 2 PO 4 based fuel cells, in which a composite mixture of the electrolyte, Pt supported on carbon, Pt black and carbon black served as the electrodes, showed that the overpotential for hydrogen electrooxidation was virtually immeasurable. The overpotential for oxygen electroreduction, however, was found to be on the order of 100 mV at 100 mA cm À2 . Thus, for fuel cells in which the supported electrolyte membrane was only 25 mm in thickness and in which a peak power density of 415 mW cm À2 was achieved, the majority of the overpotential was found to be due to the slow rate of oxygen electrocatalysis. While the much faster kinetics at the anode over those at the cathode are not surprising, the result indicates that enhancing power output beyond the present levels will require improving cathode properties rather than further lowering the electrolyte thickness. In addition to the characterization of the transport and electrochemical properties of CsH 2 PO 4 , a discussion of the entropy of the superprotonic transition and the implications for proton transport is presented.
Atomic-scale mechanisms of oxygen electrode delamination in solid oxide electrolyzer cells
International Journal of Hydrogen Energy, 2012
Solid oxide electrolyzer cell Hydrogen Delamination Defect Interface a b s t r a c t Materials used for different components (electrodes, electrolyte, steel interconnects, etc.) of solid oxide electrolyzer cell (SOEC) devices for hydrogen production have to function in aggressive, corrosive environments and in the presence of electric fields. This results in a number of degradation processes at interfaces between components. In this study, we used a combination of first-principles, density-functional-theory (DFT) calculations and thermodynamic modeling to elucidate the main processes that contribute into the oxygen delamination in typical SOEC device consisting of yttria-stabilized zirconia (YSZ) electrolyte and Sr-doped LaMnO 3 (LSM) oxygen electrode. We found that high temperature interdiffusion of different atoms across the LSM/YSZ interface significantly affects structural stability of the materials and their interface. In particular, we found that La and Sr substitutional defects positioned in ZrO 2 oxide and near LSM/YSZ interface significantly change oxygen transport which may develop pressure buildup in the interfacial region and eventually develop delamination process. Simple models for estimating these effects are proposed, and different possibilities for inhibiting and/or mitigating undesirable delamination processes are discussed.
Advanced Energy Materials, 2022
The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.202103559\. are largely dominating the market due to their unbeaten properties. Although the limitations of PFSAs are known in literature, [1,2] no reasonable alternative was reported until recent years. By using mechanical or chemical reinforcements, it was possible to keep PFSAs as material of choice for the current generation of electrochemical energy converters and achieve even the ambitious degradation goals for harsh automotive conditions. [3] Recently, ambitious milestones were set by fuel cell industry, for instance, Toyota or GM, and governmental institutions, such as the U.S. Department of Energy (DOE) or the New Energy and Industrial Technology Development Organization for mid-term development until 2030 or 2040: higher efficiency and operating potential, lower Ohmic and mass transport losses, and finally a higher operation temperature. A recent modeling study of Toyota [4] revealed the remarkable gap between the current state of the art and the ultimate performance goals for fuel cells (Figure 1). To achieve these very ambitious goals, some "early stage" materials were combined to estimate a polarization curve (dashed green line) that theoretically can exceed the 2030 target value (solid green line). Those materials include increased catalytic active sites and intrinsic activity [5,6] and catalyst layers with mesoporous Pt/C from GM R&D. [7] The optimal electrode ionomer should provide high proton conductivity at high temperatures (>100 °C) and low humidity (<50% relative humidity (RH)), thus high ion exchange capacity (IEC) [8] without poisoning the catalyst [9,10] and finally a high oxygen permeability. [11,12] The gas diffusion layers should also be improved to significantly reduce the gas transport losses. [13] The desired high operating temperatures (>100 °C) are a bottleneck for state-of-the-art PFSA membrane-electrode assemblies (MEAs), while being highly attractive to boost efficiency, increase cooling gradients, and increase CO tolerance. The low glass transition temperatures of conventional PFSAs [14] complicate cell operation above ≈110 °C, which is below the ultimate targets between 120 or even 150 °C. In addition to temperature limitations and gas crossover requirements, significantly higher proton conductivity as well as lower costs are demanded to fulfill the mid-term development goals. Based on those projections, it is striking that those targets might not be met with PFSA-based materials and novel, more proton conductive and temperature stable ionomers need to be developed.
Degradation Mechanisms in Oxygen Ion Conducting Materials
2010
We have developed a testing apparatus to characterize degradation mechanisms in oxygen ion conducting materials, with an emphasis on Solid Oxide Fuel Cell (SOFC) materials. While chemical potentials drive currents in SOFCs, we utilize a simple electrical potential to drive oxygen ionic currents through materials and interfaces. We can additionally adjust the temperature and gaseous environment of our experiment, enabling us to identify and characterize degradation mechanisms and their causes. Early performance results confirm multiple SOFC cathode degradation mechanisms driven by both high temperatures and ion currents. In particular, cation inter-diffusion is prevalent at interfaces such as those between La(0.6)Sr(0.4)Co(0.2)Fe(0.8)O(3) and Ga-doped CeO(2) resulting in an interfacial structure which is increasingly resistant to subsequent oxygen ion flow. By isolating and understanding various degradation mechanisms we can more effectively address those mechanisms to improve long t...
Effects of Cathode Corrosion on Through-Plane Water Transport in Proton Exchange Membrane Fuel Cells
Journal of the Electrochemical Society, 2013
The corrosion of carbon in the electrode supports of proton-exchange-membrane fuel cells leads to electrode collapse, reduced active catalyst area, and increased surface hydrophilicity. While these effects have been linked to performance degradation over cell lifetime, the role of corrosion in the evolving water balance has not been clear. In this study, neutron imaging was used to evaluate the through-plane water distribution within several cells over the course of accelerated stress testing using potential holds and square-wave cycling. A dramatic decrease in water retention was observed in each cell after the cathode was severely corroded. The increasing hydrophilic effect of carbon surface oxidation (quantified by ex situ x-ray photoelectron spectroscopy) was overwhelmed by the drying effect of increased internal heat generation. To evaluate this mechanism, the various observed electrode changes are included in a multiphase, non-isothermal one-dimensional cell model, and the simulated alterations to cell performance and water content are compared with those observed experimentally. Simulation results demonstrate that collapse and compaction of the catalyst layer is the dominant limitation to cell performance and not the lower amounts of active Pt surface area, and also show agreement with the higher temperature gradients resulting in drying out of the cell.
The Importance of Water Transport in High Conductivity and High-Power Alkaline Fuel Cells
High ionic conductivity membranes can be used to minimize ohmic losses in electrochemical devices such as fuel cells, flow batteries, and electrolyzers. Very high hydroxide conductivity was achieved through the synthesis of a norbornene-based tetrablock copolymer with an ion-exchange capacity of 3.88 meq/g. The membranes were cast with a thin polymer reinforcement layer and lightly cross-linked with N,N,N ,N-tetramethyl-1,6-hexanediamine. The norbornene polymer had a hydroxide conductivity of 212 mS/cm at 80°C. Light cross-linking helped to control the water uptake and provide mechanical stability while balancing the bound (i.e. waters of hydration) vs. free water in the films. The films showed excellent chemical stability with <1.5% conductivity loss after soaking in 1 M NaOH for 1000 h at 80°C. The aged films were analyzed by FT-IR before and after aging to confirm their chemical stability. A H 2 /O 2 alkaline polymer electrolyte fuel cell was fabricated and was able to achieve a peak power density of 3.5 W/cm 2 with a maximum current density of 9.7 A/cm 2 at 0.15 V at 80°C. The exceptionally high current and power densities were achieved by balancing and optimizing water removal and transport from the hydrogen negative electrode to the oxygen positive electrode. High water transport and thinness are critical aspects of the membrane in extending the power and current density of the cells to new record values. Anion-exchange membranes (AEMs) are a key component in alkaline exchange membrane fuel cells (AEMFCs), flow batteries and electrolyzers. 1 Alkaline conditions are attractive because of the facile electrochemical reaction kinetics at high pH for oxygen reduction and water oxidation. 2-7 Device operation at high pH allows for the use of non-precious metal catalysts, simpler design for the balance of plant, and reduced fuel crossover. 9-12 However, it is imperative that the AEMs are thin, have long-term alkaline stability, and high hy-droxide ion conductivity. 8 There have been issues in the past with AEMs showing low ionic conductivity, poor stability at high pH, and high water uptake (leading to dimensional change); however, these issues are being systematically addressed. 13-15 The formation of multi-block copolymers (BCP) are a means to achieve phase segregation within polymers in order to create high-mobility ion conduction channels within the hydrophilic phase of the polymer membrane. 16-20 Previously, we have reported AEMs consisting of poly(norbornene) BCPs with record high hydroxide conductivity , 198 mS/cm, and very high peak power density in a hy-drogen/oxygen fuel cell, 3.4 W/cm 2 at 80°C. 21-24 In addition, the poly(norbornene) polymer, as well as the membranes made from the polymer, were shown to have excellent thermal and mechanical properties. The S N 2 substitution and Hoffmann elimination degradation routes were suppressed by tethering the quaternary ammonium head-group to the all-hydrocarbon poly(norbornene) backbone via a long alkyl hydrocarbon chain. 25-28 Trimethyl quaternary ammonium cation head-groups have been shown to be stable cations in AEMs, and their low molecular weight enables high ion exchange capacity (IEC). 29 Water management is a key factor in achieving high AEMFC performance. The AEM plays a key role in balancing the water content and distribution during device operation. 30,31 It has been shown that a significant majority of the reacting water at the AEMFC cathode is provided by back-diffusion of water produced at the AEMFC anode. 32 This suggests that high AEM water permeability is bene-* Electrochemical Society Student Member. ficial in AEMFCs. However, excessive AEM water uptake can flood the ion conducting channels within the polymer and lead to membrane softening and mechanical failure. 33,34 Thus, high water permeability without high water solubility appears to be a critical feature for AEM-FCs. Waters of hydration are necessary for hydroxide ion conduction; however, excessive unbound (i.e. free) water leads to low hydroxide mobility and membrane distortion. 23 Hence, it is necessary to balance the amount of free and bound water inside the membranes to yield the maximum hydroxide mobility and water transport. 21-23 Cross-linking is an effective way to reduce water uptake and swelling. 24 However, AEMs with high cross-linking density can become too rigid, leading to poor ion mobility, mechanical properties and water diffusivity (i.e. high water solubility without high diffusivity). 35-37 In the case of polymers with high IEC, light cross-linking is an effective strategy to balance the high conductivity and water uptake (WU) without sacrificing IEC. 22,23 In addition, thinner membranes can enable rapid water transport without high water uptake, and enable high current density AEMFCs. Conveniently, light cross-linking also helps in the production of thin membranes with good mechanical properties. AEM carbonation upon exposure to CO 2 is another important factor in AEMFC performance. When CO 2-containing air is fed to an operating AEMFC, the hydroxide anions produced by the reduction of oxygen at the positive electrode react with carbon dioxide to produce carbonated anions with lower mobility than hydroxide (i.e. carbonate or bicarbonate), increasing ohmic-related losses. 38 Additionally, these carbonated anions can rapidly populate the AEM and AEMFC anode, leading to significant thermodynamic and kinetic-related losses. 32,39 It has been stated that the adverse effects of membrane carbonation could be minimized by using AEMs with very high ionic conductivity so that the decrease in mobility upon carbonation can be mitigated and cell decarbonation during operation through the so-called "self-purging" mechanism can occur more rapidly. Hence, AEMs with very high conductivity are most desirable. 23 In this study, the synthesis of chemically stable AEMs with record high conductivity, 212 mS/cm at 80°C, and their implementation into AEMFCs are described. This new AEM enables record performance in a hydrogen/oxygen AEMFC with a peak power density of 3.5 W/cm 2 and maximum current density of 9.7 A/cm 2 at 0.15 V at 80°C when) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.207.74.202 Downloaded on 2019-11-04 to IP
Journal of Electroceramics, 2007
The high-temperature electronic and ionic transport properties, thermal expansion and stability of dense Pr 2 NiO 4þδ ; Pr 2 Ni 0:9 Fe 0:1 O 4þδ and Pr 2 Ni 0:8 Cu 0:2 O 4þd ceramics have been appraised in comparison with K 2 NiF 4 -type lanthanum nickelate. Under oxidizing conditions, the extensive oxygen uptake at temperatures below 1073-1223 K leads to reversible decomposition of Pr 2 NiO 4 -based solid solutions into Ruddlesden-Popper type Pr 4 Ni 3 O 10 and praseodymium oxide phases. The substitution of nickel with copper decreases the oxygen content and phase transition temperature, whilst the incorporation of iron cations has opposite effects. Both types of doping tend to decrease stability in reducing atmospheres as estimated from the oxygen partial pressure dependencies of total conductivity and Seebeck coefficient. The steady-state oxygen permeability of Pr 2 NiO 4þd ceramics at 1173-1223 K, limited by both surface-exchange kinetics and bulk ionic conduction, is similar to that of La 2 NiO 4þd . The phase transformation on cooling results in considerably higher electronic conductivity and oxygen permeation, but is associated also with significant volume changes revealed by dilatometry. At 973-1073 K, porous Pr 2 Ni 0:8 Cu 0:2 O 4þd electrodes deposited onto lanthanum gallate-based solid electrolyte exhibit lower anodic overpotentials compared to La 2 Ni 0:8 Cu 0:2 O 4þd , whilst cathodic reduction decreases their performance.