2-Sulfoethylammonium trifluoromethanesulfonate as an Ionic Liquid for High Temperature PEM Fuel Cells

2-Sulfoethylammonium trifluoromethanesulfonate ([2-Sea+][TfO−]) represents a novel class of proton-conducting ionic liquids (PILs) based on aminoalkylsulfonic acids. The fundamental suitability of [2-Sea+][TfO−] for application as a protic electrolyte in high temperature PEM fuel cells (HT-PEFCs) was investigated up to a temperature of 130°C. A comparison was made against a state-of-the-art electrolyte, phosphoric acid. [2-Sea+][TfO−] is electrochemically and thermally stable up to 140°C. The specific conductivity of 95 wt% [2-Sea+][TfO−] aqueous solution at 130°C is ≈20 times lower compared to 95 wt% H3PO4. The strong coupling of ion transport and viscous flow suggests a vehicular ion (proton) transport in [2-Sea+][TfO−]. 95 wt% [2-Sea+][TfO−] shows superior kinetics in terms of oxygen reduction reaction (ORR) on polycrystalline Pt compared to 95 wt% H3PO4 at temperatures greater than 90°C in a fuel cell-applicable potential range. Double layer capacitances suggest a complex double layer structure, including adsorbed [2-Sea+][TfO−] and water, as well as intermediates of oxygen reduction and Pt oxidation. Potential and temperature-dependent ORR kinetics in the presence of 95 wt% [2-Sea+][TfO−] yield different Tafel slopes (b = 82–139 mV) and symmetry factors (β = 0.46–0.96), indicating changes in surface coverages of the adsorbed species and possibly also a change in the reaction mechanism

Electrical conductivity and chemical equilibria of the phosphoric acid \u2013 water system at HT-PEM fuel cells relevant condition

High temperature polymer electrolyte membrane (HT-PEM) fuel cells typically work at 120-200\ub0C and are mainly based on phosphoric acid (PA) swollen basic polymer membranes like phosphoric acid doped polybenzimidazole. An overview can be found in [1-7]. Although PA is a widely used material even outside the field of electrochemical transformers, only little is known about the thermodynamical but also physical properties at temperatures above 100\ub0C. Especially the correlation between different parameters provided from different sources in literature is often demanding due to different experimental approaches. In this work, an alternative approach is used simulating directly the conditions inside an operational HT-PEM. The obtained physical and thermodynamic data are critically compared to results from other laboratories

Adsorption process of phosphoric acid on polybenzimidazole membranes: a crucial step inside operational high temperature PEM fuel cells

High temperature polymer electrolyte fuel cells (HT-PEFC) typically work at 120-200\ub0C and are mainly based on phosphoric acid (PA) swollen basic polymer membranes like poly\uacbenzimidazole (PBI). Although PA doped PBI membranes were investigated in several ex\uacperimental studies, the kinetic of the adsorption process, the mo\uaclecular interactions bet\uacween the PA molecules and the polymer chains, poly\uaccon\uacdensation equilibria of the PA molecules as well as the implications on the proton conductivity is not finally illuminated. In this work, we have investigated the adsorption process of PA on m-PBI and a commercial AB-PBI derivative (Fumapem AM-55). A number of membranes (cross and uncross-linked) has been prepared at different doping level and analysed to elucidate the adsorption process of the PA as function of temperature, acid concentration and chemical equilibria between PA derivatives. Impedance measurements, Karl Fischer titration method and RAMAN spectra have been used to characterise the membranes. The thermodynamic aspects related to the adsorption process have been analysed with different kinetic models. Considering own and literature data on non-crosslinked m-PBI, a BET-like adsorption kinetics explains the behaviour of this polymer type satisfactorily. Using the RAMAN data, different regions in the BET-like isotherm can be correlated to the protonation of the polymer chain, formation of H-bonds directly to the chain and indirectly to still adsorbed PA molecules

Liquid \u2013 gas phase equilibria of phosphoric acid at high temperature electrolyte polymer fuel cell condition

Phosphoric acid is used in most high temperature polymer electrolyte membrane fuel cells (HT-PEFC) especially because it exhibits high proton conductivity together with high availability. The nature of the proton conductivity in phosphoric acid and the interaction between the polymer matrix and phosphoric acid were investigated recently by MD simulations [1]. Experimental conductivity data are available extensively for temperatures up to 100\ub0C and different water contents, while it gets scares for higher temperatures [2, 3]. The reason for this is the complex nature of the phosphoric acid, tending to polymerize from its monomeric form at low temperature and surplus of water towards chain or even ring structures at temperatures above 100\ub0C with depletion of water [4]. However, the different species have different chemical but also physical properties. This circumstance is either neglected or not considered for most examinations. In contrast to most published methods starting with P2O5 and defined water content, we start from aqueous orthophosphoric acid (85w%) and impose a fixed water vapour pressure using a climate chamber. The development of the equilibrium state with the gas phase is investigated using time sequenced impedance measurements. Temporal snapshot samples are taken and examined using Karl-Fischer titration method to determine the content of unbound water and Raman spectroscopy to determine changes in molecular interactions. The transitions between the different species and adducts of phosphoric acid can be precisely determined

Investigations on the H3PO4-Uptake of Polybenzimidazole type Polymers using RAMAN Spectroscopy — Correlations between Adsorption Process and Electrolyte - Polymer Molecular Interactions

An important application of phosphoric acid doped polybenzimidazol (PBI) is the use as proton conducting electrolyte membrane in high temperature polymer electrolyte fuel cells (HT-PEFC). At typical operation temperatures of 120 to 200 °C and very low humidity ionic conductivities of 10-1 to 10-2 S cm-1 can be measured. The H3PO4-doping process of different PBI-type materials were still investigated in se¬ve¬ral experimental studies, e.g. [1,2]. Unfortunately, no general applicable model for the kinetic of the adsorption process available which is able to describe the whole acces¬sible range of doping degrees has been published yet. The correlation between the interactions between H3PO4 molecules and polymer chains, the ad-sorp¬tion iso¬therm as well as the polycondensation equilibria of H3PO4 and the corresponding implications on the pro¬ton conductivity are also not finally illuminated. We have investigated the ad¬sorption process of H3PO4 on a com¬mer¬cial cross-linked PBI deri¬vative (Fuma¬pem AM-55). A num¬ber of mem¬bra¬nes has been pre¬pared at dif¬fe¬rent do¬ping levels and analysed to elucidate the ad¬sorp¬tion process of H3PO4 as func¬tion of temperature and con¬cen¬tra¬tion. Karl-Fischer-, pH-titration and RAMAN spec¬tro¬scopy are used to cha¬rac¬terise the mem¬branes [3,4]. The ad¬sorp¬tion equi¬li¬bria of the up¬take pro¬cess have been ana¬ly¬sed with dif¬fe¬rent ki¬ne¬tic mo¬dels for own and for lite¬ra¬ture data on non-cross¬lin¬ked m-PBI and AB-PBI. The be¬ha¬viour of all PBI-type poly¬mers can be de¬scri¬bed sa¬tis¬fac¬torily with a BET-like ad¬sorp¬tion iso¬therm. Using the RAMAN data, re-gions in the isotherm can be cor¬re¬la¬ted to the pro¬tonation of the poly¬mer chains, for¬ma¬tion of H-bonds di¬rec¬tly to the chains and to still adsorbed H3PO4 mole¬cules. [1] Q. Li et al., Solid State Ionics, 2004, 168, 177-185 [2] J. A. Asensio et al., Chem. Soc. Rev., 2010, 39, 3210-3239 [3] F. Conti et al., submitted to Fuel Cells, Jan. 2014 [4] C. Korte et al., to be submitted 201

Uptake of Protic Electrolytes by Polybenzimidazole Type Polymers: Adsorption Isotherm and Electrolyte/Polymer Interactions

Polybenzimidazol (PBI)membranes doped with phosphoric acid are used as protonconducting membrane in high-temperature polymer electrolyte fuel cells (HTPEFCs). There is no general model for the thermodynamics of the adsorption process over the whole accessible doping range for all published data. The interactions between H\u2083PO\u2084 and polymer chains, the polycondensation equilibria of H\u2083PO\u2084 and the implications on proton conductivity are still largely unknown. In this study, we demonstrate that the uptake of a protic electrolyte by a polymer with basic moieties, i.e. a PBI-type polymer, can be described satisfactorily with a BET-like adsorption isotherm. The adsorption equilibria of the uptake process are analysed using literature data on the H\u2083PO\u2084, H\u2082SO\u2084 and HClO\u2084 uptake of non-crosslinked m-PBI and AB-PBI as well as using our own investigations on the H\u2083PO\u2084 uptake of a commercial crosslinked PBI derivative, Fumapem AM-55. In addition to the thermodynamic data of the adsorption process, Raman data on m-PBI taken from the literature and our own Raman investigations on Fumapem AM-55 are taken into account. It is possible to correlate domains in the adsorption isotherms with specific features in the Raman spectra. Three stages for the uptake of a protic electrolyte can be distinguished: i) the protonation of the polymer chains, ii) the formation of H bonds directly with the chains and iii) the formation of H bonds with electrolyte molecules that are still adsorbed