Diffusion coefficients and VLE data of aqueous phosphoric acid

104

101

Synchrotron based X-ray tomographic microscopy (XTM) is used for imaging and quantifying the redistribution of phosphoric acid (PA) in high temperature polymer electrolyte fuel cells (HT-PEFC) in-operando. The main focus of this work is the redistribution of phosphoric acid under dynamic load conditions. Therefore, two different load cycling protocols were applied and the transient redistribution within the fuel cell components was imaged. XTM, for the first time, revealed that the examined PBI based membrane system exhibits extensive electrolyte migration from cathode to anode under high current density operation. PA flooding of anode gas diffusion layer (GDL) and flow field channels occurred. Implications for technical applications and fuel cell degradation are discussed. Quantification of the migrated electrolyte is made by correlating in-operando grayscale values to ex-situ reference samples. High temperature polymer electrolyte fuel cells (HT-PEFC) are operating at temperatures up to 200• C using phosphoric acid (PA) doped polybenzimidazole (PBI) based membranes. This fuel cell technology has long been in the focus of research 1 due to advantages based on the higher operation temperature as compared to standard PEFCs, operating between 60-90• C. At the higher operating temperatures, the tolerance to fuel gas impurities increases significantly and operation with CO levels up to 3% and H 2 S up to 10 ppm can be achieved.2 This renders HT-PEFC especially suitable for stationary combined heat and power (CHP) applications, where a fuel processing unit can easily be thermally integrated and used for reforming hydrocarbon-based fuels without the need of additional gas clean-up. The advantageous characteristics of HT-PEFCs are also determined by the physico-chemical properties of the phosphoric acid electrolyte. First of all, PA has a low vapor pressure at these operating temperatures. This principally allows for long term operation without major electrolyte loss, an important aspect also specifically defined in the US Department of Energy's 2015 targets 3 (50'000 operating hours for stationary CHP systems). Phosphoric acid also exhibits excellent proton conductivity owing to a fast proton hopping mechanism. 4 Due to this inherent difference to PFSA-type membranes (water assisted shuttle mechanism), additional humidification is not necessary, which significantly reduces the complexity of HT-PEFC systems. However, only ca. 2 PA molecules per PBI repeating unit (PA/PBI) are directly interacting with the basic pyridinic nitrogen of PBI.5 However, PA doping levels are typically significantly higher, e.g., 5-10 for PA imbibed PBI films 6 and 20-40 for PBI membranes produced through the so-called poly-phosphoric acid (PPA) process.7 It can, therefore, be expected that the bulk of this phosphoric acid is more or less mobile within the molecular pores of the membrane. Hence, movement and redistribution of PA within the porous components (membrane, catalyst layer, micro-porous and gas diffusion layers) of the cell are expec...

Section: Resultssupporting

confidence: 92%

Dynamic Operation of HT-PEFC: In-Operando Imaging of Phosphoric Acid Profiles and (Re)distribution

Eberhardt

Toulec

Marone

et al. 2015

104

101

“…For 85 wt% H 3 PO 4 , the chemical diffusion coefficient was found to be 1.7 × 10 −10 at room temperature. Similar values were reported by Ruiz-Beviá et al 39 This value was increased to about 8.6 × 10 −10 m 2 s −1 at 70 • C. 38 Taking the temperature effect into account, the diffusion coefficient of H 3 PO 4 through the polymer phase obtained in the present study is at least an order of magnitude lower.…”

Section: Modeling Results For Acid Migration and Back Diffusion-in Or-supporting

confidence: 91%

Determination of Anion Transference Number and Phosphoric Acid Diffusion Coefficient in High Temperature Polymer Electrolyte Membranes

Becker

Reimer

Aili

et al. 2018

The passage of an electrical current through phosphoric acid doped polymer membranes involves parasitic migration of the acid, which imposes a critical issue for long-term operation of the high temperature polymer electrolyte membranes fuel cell (HT-PEMFC). To elucidate the phenomenon, a three-layered membrane is constructed with embedded micro reference electrodes to measure phosphoric acid redistribution in a polybenzimidazole based membrane. Under a constant load, a concentration gradient develops due to the acid migration, which drives the back diffusion of the acid and eventually reaches a steady state between migration and diffusion. The acid gradient is measured as a difference in local ohmic resistances of the anode-and cathode-layer membranes by electrochemical impedance spectroscopy. The phosphoric acid diffusion coefficient through the acid doped membrane is about 10 −11 m 2 s −1 , at least one order of magnitude lower than that of aqueous phosphoric acid solutions. The anion (H 2 PO 4 − ) transference number is found to range up to 4% depending on current density, temperature and atmospheric humidity of the cell, implying that careful control of the operating parameters is needed in order to suppress the vehicular proton conduction as a degradation mitigation strategy.

“…• C. [6][7][8][9] Determining a phosphoric acid vapor pressure curve at the temperatures of interest for fuel cell operation (160-190…”

mentioning

confidence: 99%

Correlating Electrolyte Inventory and Lifetime of HT-PEFC by Accelerated Stress Testing

Eberhardt

Lochner

Büchi

et al. 2015

Phosphoric acid electrolyte evaporation in a polybenzimidazole based high temperature polymer electrolyte fuel cell is analyzed as a function of reactant gas stoichiometry and temperature. Based on these results a phosphoric acid vapor pressure curve is derived to predict the fuel cell liftetime with respect to electrolyte inventory. The predicted fuel cell life was validated by means of an accelerated stress test. Additionally, the correlation between electrolyte inventory and fuel cell performance was investigated by recording H 2 /air and H 2 /O 2 polarization curves during the course of the stress test to gain insight into the relation between acid inventory and the different degradation modes. © The Author High-temperature polymer electrolyte fuel cells (HT-PEFC) have the potential to become an important technology for small scale heat and power (CHP) applications. However, today, fuel cell based CHP applications are dominated by low-temperature PEFC (LT-PEFC), 1 even though the possibility to sustain high CO levels of up to 3%, 2 thermal integration of the fuel processing unit and no need of additional gas clean-up render HT-PEFCs especially suitable for operation on hydrocarbon-based fuels, i.e. natural gas. The high operating temperature of 160-200• C, reduced system complexity, due to the absence of additional gas humidification, and high system efficiencies are ideal properties of HT-PEFC for stationary CHP applications.Fuel cell durability, efficiency and cost are essential factors for commercialization. Durability is mainly determined by membrane electrode assembly (MEA) degradation. Amongst other degradation modes that HT-PEFCs share with low temperature PEFC, 3 electrolyte loss by evaporation and migration is exclusive to HT-technology and a limiting factor for CHP applications. We have recently demonstrated that PBI based membrane systems exhibit extensive electrolyte migration from cathode to anode under high current operation. 4 This was attributed to the high mobility of free hydrogen phosphate anions which carry part of the ionic current. While this work focuses on phosphoric acid loss by evaporation and its implication on lifetime and fuel cell performance, it cannot be excluded that the high PA mobility has an effect on electrolyte evaporation as it can influence the PA resupply and saturation of the electrodes.With respect to electrolyte evaporation, the phosphoric acid vapor pressure below temperatures of 300• C is extremely low, nevertheless it is expected to be significant considering the targeted lifetime of 50,000 h for CHP systems set out by the US Department of Energy (DOE) for 2015.5 Up to now, no literature data is available for the vapor pressure of phosphoric acid for temperatures below 200• C. 6-9Determining a phosphoric acid vapor pressure curve at the temperatures of interest for fuel cell operation (160-190• C) is a tedious task, due to the low phosphoric acid concentration in the gas phase and the accompanied analytical measurement complexity. Furthermore, phosphoric acid, b...