Dirk Henkensmeier

Effect of morphology of electrodeposited Ni catalysts on the behavior of bubbles generated during the oxygen evolution reaction in alkaline water electrolysis

We have investigated the release of active sites blocked by bubbles attached on the surface of catalysts during the oxygen evolution reaction (OER) in alkaline water electrolysis, via the modulation of the wetting properties of the four different morphologies of a nickel catalyst.

Thermal crosslinking of PBI/sulfonated polysulfone based blend membranes

Crosslinked polybenzimidazole (PBI) membranes are most often obtained by reacting the nitrogen atoms of PBI with an alkylating agent. These links can be attacked by nucleophiles at elevated temperatures. To avoid N–CH2-links we introduce a new method to crosslink PBI, starting from ionically crosslinked acid/base blend membranes. By heating them to temperatures above say 200 °C, a Friedel–Crafts reaction between sulfonic acid groups and electron rich phenyl groups covalently crosslinks the acid and base components in the blend by chemically stable aromatic sulfone bonds. According to the literature pure PBI can also be cured and a radical mechanism involving air was suggested. We show that PBI can also be cured in an inert atmosphere. We propose that the thermal curing of pure PBI, which necessitates slightly higher temperatures than blend membranes, proceeds via hydrolysis of imidazole to –COOH and diamine, followed by a Friedel–Crafts reaction of the acid. While crosslinks cannot be directly analysed by nmr or IR, our data support the mentioned mechanism. We show the effect of curing temperature and time on membrane properties like solubility, phosphoric acid uptake and mechanical properties, and test a membrane in a fuel cell, proving that the membranes are gas tight and show a good performance.

Novel sulfonated poly(arylene ether sulfone) containing hydroxyl groups for enhanced proton exchange membrane properties

We here report a new sulfonated poly(arylene ether sulfone) modified with hydroxyl side groups for a proton exchange membrane fuel cell (PEMFC). The polymer was synthesized via 4-step reaction including direct copolymerization, sulfochlorination, demethylation, and hydrolysis. By adding the hydroxyl group into the polymer backbone, strong intermolecular hydrogen bonds could be generated between polymer chains. The hydroxyl sulfonated polymer demonstrated high proton conductivities at a variety of temperatures and relative humidities due to its enhanced phase-separated morphology. Moreover, it showed an improved fuel cell performance compared to the non-hydroxylated analog with the same sulfonation degree. The introduction of hydroxyl groups to the sulfonated poly(arylene ether sulfone) provides a potential candidate for an improved PEMFC membrane.

Sulfonation of PIM-1 — towards highly oxygen permeable binders for fuel cell application

AbstractThe development of alternative, non-fluorinated membranes for polymer electrolyte membrane fuel cells necessitates the co-development of a non-fluorinated electrode catalyst binder to ensure compatibility between membrane and electrode. However, most hydrocarbon based polymers have lower gas permeability than perfluorinated Nafion. In this work we tried to obtain a sulfonated, non-fluorinated binder based on PIM-1 (polymer of intrinsic microporosity 1) which has up to 2000 times higher permeability than Nafion. However, sulfonation was not straightforward and often led to degradative side reactions. Sulfonated polymers were too brittle to give stable membranes and the highest experimental IEC was 1.03 meq/g, significantly lower than the theoretical IEC of 3.2 meq/g (2 sulfonic acid groups per repeat unit).

Tetrazole substituted polymers for high temperature polymer electrolyte fuel cells

While tetrazole (TZ) has much lower basicity than imidazole and may not be fully protonated in the presence of phosphoric acid (PA), DFT calculations suggest that the basicity of TZ groups can be increased by the introduction of a 2,6-dioxy-phenyl-group in position 5 of TZ. This structure allows hydrogen bonds between TZ protons and ether oxygen atoms, and thereby establishes a resonance stabilised, co-planar structure for tetrazolium ions. Molecular electrostatic potential (MEP) calculations also indicate that tetrazolium ions possess two sites for proton hopping. This makes such materials interesting for use in a high temperature fuel cell (HT PEMFC). Based on these findings, two polymers incorporating the proposed TZ groups were synthesised, formed into membranes, doped with PA and tested for fuel cell relevant properties. At room temperature, TZ-PEEN and commercial meta-PBI showed an equilibrium uptake of 0.5 and 4.7 mol PA per mol heterocycle, respectively, indicating that PBI has higher affinity for PA than TZ-PEEN. The highest achieved PA uptake was ca. 110 wt%, resulting in a proton conductivity of 25 mS cm−1 at 160 °C with a low activation energy of about 35 kJ mol−1. In a first HT PEMFC test at 160 °C, a peak power density of 287 mW cm−2 was achieved.

Vanadium Redox Flow Batteries Using meta-Polybenzimidazole-Based Membranes of Different Thicknesses

15, 25, and 35 μm thick meta-polybenzimidazole (PBI) membranes are doped with H2SO4 and tested in a vanadium redox flow battery (VRFB). Their performances are compared with those of Nafion membranes. Immersed in 2 M H2SO4, PBI absorbs about 2 mol of H2SO4 per mole of repeat unit. This results in low conductivity and low voltage efficiency (VE). In ex-situ tests, meta-PBI shows a negligible crossover of V3+ and V4+ ions, much lower than that of Nafion. This is due to electrostatic repulsive forces between vanadium cations and positively charged protonated PBI backbones, and the molecular sieving effect of PBIs nanosized pores. It turns out that charge efficiency (CE) of VRFBs using meta-PBI-based membranes is unaffected by or slightly increases with decreasing membrane thickness. Thick meta-PBI membranes require about 100 mV larger potentials to achieve the same charging current as thin meta-PBI membranes. This additional potential may increase side reactions or enable more vanadium ions to overcome the electrostatic energy barrier and to enter the membrane. On this basis, H2SO4-doped meta-PBI membranes should be thin to achieve high VE and CE. The energy efficiency of 15 μm thick PBI reaches 92%, exceeding that of Nafion 212 and 117 (N212 and N117) at 40 mA cm-2.

Effect of membrane electrode assembly fabrication method on the single cell performances of polybenzimidazole-based high temperature polymer electrolyte membrane fuel cells

Membrane electrode assemblies (MEAs) for a high temperature polymer electrolyte membrane fuel cell (HTPEMFC) were fabricated using acid-doped polybenzimidazole (PBI) as the electrolyte membrane and polytetrafluoroethylene (PTFE) as the electrode binder. PTFE concentrations of 20, 30, and 45 wt% in the electrode were evaluated to determine the optimal binder content. Additionally, the influence of applying a pressing process during MEA fabrication on the electrode performance was examined. When MEA was prepared without the pressing process, the electrode containing 20 wt% PTFE exhibited the best cell performance (338 mA cm−2 at 0.6 V). However, when MEA was prepared with the pressing process, the electrode containing 45 wt% PTFE exhibited the best cell performance (281 mA cm−2 at 0.6 V). This result is because of the inclusion of the pressing process, as gas permeability is hindered by the transfer of excess phosphoric acid from the electrolyte membrane to the electrodes.

One-Step Cationic Grafting of 4-Hydroxy-TEMPO and its Application in a Hybrid Redox Flow Battery with a Crosslinked PBI Membrane

By using a one-step epoxide ring-opening reaction between 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (4-hydroxy-TEMPO) and glycidyltrimethylammonium cation (GTMA+ ), we synthesized a cation-grafted TEMPO (g+ -TEMPO) and studied its electrochemical performance against a Zn2+ /Zn anode in a hybrid redox flow battery. To conduct Cl- counter anions, a crosslinked methylated polybenzimidazole (PBI) membrane was prepared and placed between the catholyte and anolyte. Compared to 4-hydroxy-TEMPO, the positively charged g+ - TEMPO exhibits enhanced reaction kinetics. Moreover, flow battery tests with g+ -TEMPO show improved Coulombic, voltage, and energy efficiencies and cycling stability over 140 cycles. Crossover of active species through the membrane was not detected.

Facile preparation of a long-term durable nano- and micro-structured polymer blend membrane for a proton exchange membrane fuel cell

The blend of sulfonated poly(ether sulfone)s (PESs) with different degrees of sulfonation is used to prepare a membrane for fuel cell applications. Considering their facile preparation and excellent performance, such sulfonated PES blends constitute good candidates as polymer electrolyte for fuel cell applications. The long-term durability of the blend membrane is improved compared to unblended sulfonated polymer membranes with one degree of sulfonation. The nano-to-micron scale morphology of the polymer blend is investigated by transmission electron microscopy. The long-term stability of the blend membrane is related to its morphology, as confirmed by ultra small angle and small angle neutron scattering (USANS and SANS, respectively) analyses. The blend membrane comprises better-defined interconnected nano-sized ionic phases that enhance proton conductivity and water flow and micro heterogeneous domains, which could be reasons for the long-term durability.

Shape memory effect in radiation grafted ion exchange membranes

When swollen radiation grafted ion exchange membranes are dried in a fixed geometry, e.g. by clamping on a frame, dry membranes memorise the wet shape and show reduced area swelling and increased swelling in the thickness direction upon rehydration. In one case, the ratio thickness/area swelling increased from 0.6 to 35.