Bilayer electrolyte membrane and a redox flow battery comprising a bilayer electrolyte membrane

Abstract

An electrolyte membrane and method for generating the membrane provide a resistance as low as possible to minimize ohmic losses. The membrane has a low permeability for redox-active species. If redox-active species still cross the membrane, this transport is balanced during charge and discharge preventing a net vanadium flux and associated capacity fading. The membrane is mechanically robust, chemically stable in electrolyte solution, and low cost. A family of ion exchange membranes including a bilayer architecture achieves these requirements. The bilayer membrane includes two polymers, i) a polymer including N-heterocycles with electron lone pairs acting as proton acceptor sites and ii) a mechanically robust polymer acting as a support, which can be a dense cation exchange membrane or porous support layer. This bilayer architecture permits a very thin polymer film on a supporting polymer to minimize ohmic resistance and tune electrolyte transport properties of the membrane.

Claims

1. A bilayer ion exchange membrane for use in electrochemical cells, the membrane comprising: a) an ion exchange polymer layer comprising N-heterocycles with electron lone pairs acting as proton acceptor sites; b) a mechanically robust polymer substrate as support layer; c) the ion exchange polymer layer with proton acceptor sites attached to the support layer by solution-coating or spray-coating; d) the ion exchange polymer layer with proton acceptor sites is a polybenzimidazole-class polymer comprising one or more of the following compounds: poly (2,2′-m-phenylene-5,5′-bibenzimidazole) (meta-PBI), poly (2,2′-p-phenylene-5,5′-bibenzimidazole) (para-PBI), poly (2,5′-p-benzimidazole) (AB-PBI), poly (p-phenylene benzobisimidazole) (PBDI), poly-2,2-(X, Y-pyridine)-5,5′-bibenzimidazole (P-PBI) where (X,Y) is (2,5), (3,5), (2,6) or (2,4), and poly-[(1-(4,4′-diphenylether)-5-oxybenzimidazole)-benzimidazole] (PBI-OO); and e) the polybenzimidazole-class polymer is functionalized at the nitrogen-hydrogen (NH) site of the imidazole and/or benzimidazole and/or benzobisimidazole.

2. The membrane according to claim 1, wherein the polymer layer with proton acceptor sites consists of a polymer comprising at least 50 mol % of an imidazole and/or benzimidazole and/or benzobisimidazole unit.

3. The membrane according to claim 1, wherein the polymer layer with proton acceptor sites consists of a polymer comprising pyridine and/or imidazole units in a main chain or as pendant groups with a content of less than 50 mol %.

4. The membrane according to claim 1, wherein the functionalization includes deprotonation of the nitrogen-hydrogen NH site of the imidazole and/or benzimidazole and/or benzobisimidazole unit with an alkali hydride followed by alkylation with an R—X compound, wherein X is a halogen or a cyclic compound which opens upon reaction.

5. The membrane according to claim 4, wherein said R—X compound used for said functionalization yields one or more of the following: a) crosslinking; b) cation exchange functionalities; c) protected acid groups or their alkali salts; and d) anion exchange functionalities.

6. The membrane according to claim 1, wherein the polybenzimidazole-class layer has a thickness which is below 15 μm.

7. The membrane according to claim 1, wherein the support layer is a microporous polyolefin or a combination of microporous polyolefins.

8. The membrane according to claim 1, wherein the support layer is a dense cation exchange membrane containing sulfonate exchange sites (—SO.sub.3−).

9. The membrane according to claim 8, wherein the cation exchange membrane is a perfluoroalkylsulfonic acid (PFSA) type membrane.

10. The membrane according to claim 8, wherein the cation exchange membrane is a partially fluorinated or non-fluorinated sulfonic acid type membrane or a radiation grafted membrane comprising styrene type sulfonic acid groups.

11. The membrane according to claim 1, wherein the proton accepting layer consists of a polybenzimidazole class polymer with a thickness of less than 15 μm and the support layer consists of a cation exchange membrane with a thickness between 15 and 150 μm.

12. The membrane according to claim 11, wherein the support layer is a microporous polyolefine modified at a surface of a film to improve the wettability of the material and/or increase adhesion with the proton accepting layer.

13. The membrane according to claim 12, wherein the modification of the microporous polyolefin support comprises a plasma-, corona discharge- or ionizing radiation-induced graft copolymerization introducing proton-donating- or accepting groups.

14. The membrane according to claim 12, wherein the surface of the microporous polyolefin support is ozone or corona treated or ionized with any radiation method and has a surface energy of at least 0.25 mN/m.sup.−2 but not more than 8.5 mN/m.sup.−2.

15. The membrane according to claim 11, wherein the proton accepting polymer consists of a polybenzimidazole class polymer with a thickness of less than 15 μm and the support layer consists of a microporous polymer.

16. A method for generating a membrane or a bilayer membrane according to claim 1, the method comprising the following steps: dissolving a polybenzimidazole class polymer in a suitable solvent or dimethylacetamide (DMAC) to form a polymer solution at a polymer concentration between 0.5 and 35 wt-%; and: a) casting onto a flat substrate or a glass plate followed by drying and curing of a film, release of said film from said substrate and hot-pressing it together with said support layer to form the bilayer membrane, or b) casting onto said support layer, followed by drying and curing of said film to form the bilayer membrane; or c) spraying onto said support layer, followed by drying and curing of said film to form the bilayer membrane.

17. The method according to claim 16, wherein said polymer solution includes a mixture of a pristine polybenzimidazole class polymer and a modified polybenzimidazole class polymer yielding a polymer blend after film formation.

18. A redox flow battery, comprising a membrane according to claim 1 forming a membrane electrolyte.

19. The redox flow battery according to claim 18, wherein the redox flow battery is a vanadium redox flow battery, and said proton accepting layer has a thickness determining an amount and a direction of a net vanadium flux across the bilayer membrane.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) FIG. 1 Key properties of membranes for redox flow applications, consisting of the anion and cation exchange capacity, the net vanadium flux, the capacity fading rate, the ohmic resistance R.sub.Ω in the vanadium redox flow cell and the round-trip efficiency at 40, 120 and 200 mA cm.sup.−2;

(2) FIG. 1A a sketch of a state of the art redox flow battery (RFB), comprising an ion-exchange polymer as electrolyte membrane;

(3) FIG. 2 a schematic of the bilayer membranes described in the present invention, comprising a thin (<15 μm) ion exchange polymer layer (1) and a polymeric support of >25 μm thickness; (a) bilayer membrane using a non-porous cation exchange membrane as support; (b) bilayer membrane using a porous non-ion-conducting layer as support;

(4) FIG. 3 a non-exhaustive list of polymers containing the benzimidazole motif which form anion exchange ionomers upon exposure to an acid;

(5) FIG. 4 pyridine and imidazole units contained in a polymer structure, presenting basic nitrogen atoms that are protonated in the presence of an acid;

(6) FIG. 5 schematically a functionalization of benzimidazole units in a polymer with the aim to introduce pendant groups R, which may comprise ion exchange groups, via the lithium hydride route;

(7) FIG. 6 a non-exhaustive list of alkylation agents to modify a polymer comprising imidazole or benzimidazole groups; (a) 1,3-propane sultone, (b) halo-alkyl sulfonates, (c) halo-alkyl phosphonic ester, (d) quaternary halo-alkyl ammonium salts;

(8) FIG. 7 schematically the protonation of an imidazole group in a polymer chain in the presence of an acid, yielding a benzimidazolium group, which acts as anion exchange group;

(9) FIG. 8 shows the round-trip efficiency (energy efficiency) of a vanadium flow battery equipped with the bilayer membranes of the following Example 1, Example 2 and the comparative examples 1 and 2 over a usage range of 40-200 mA cm.sup.−2;

(10) FIG. 9 shows the discharge capacity of a vanadium flow battery equipped with the bilayer membranes of Example 1 and Example 2 and the comparative example 1 at a current density of 160 mA cm.sup.−2 over 50 charge discharge cycles;

(11) FIG. 10 shows the round-trip efficiency (energy efficiency) of a vanadium flow battery equipped with the bilayer membranes of Example 3 and Example 4 and the comparative examples 1 and 2 over a usage range of 40-200 mA cm.sup.−2; and

(12) FIG. 11 shows the area resistance of a vanadium flow battery equipped with the bilayer membranes of Example 1-5 and of the Comparative Examples 1-2 in the fully discharged state.

DETAILED DESCRIPTION OF THE INVENTION

Examples

(13) The following Examples and Comparative Examples are provided for the purpose of further illustrating the present invention but are in no way to be taken as limiting.

Example 1

(14) A Nafion®/meta-polybenzimidazole (mPBI) bilayer membrane with a PBI-layer thickness of 3 μm was prepared by hot-pressing a PBI- and a Nafion® NR212-film onto each other at 80° C. for 3 min. Nafion® NR212 is used as received, the 3 μm PBI film is previously prepared by casting a 10 wt % solution of mPBI in dimethylacetamide onto a glass plate. The cast film was dried at 110° C. for 5 min, cured at 150° C. for 10 min and subsequently placed into a deionized water bath to remove the PBI-film from the glass plate.

Example 2

(15) A Nafion®/meta-polybenzimidazole (mPBI) bilayer membrane with a PBI-layer thickness of 4 μm was prepared by hot-pressing a PBI- and a Nafion® NR212-film onto each other at 80° C. for 3 min. Nafion® NR212 was used as received, the 4 μm PBI film was previously prepared by casting a 10 wt % solution of mPBI in dimethylacetamide onto a glass plate. The cast film was dried at 110° C. for 5 min, cured at 150° C. for 10 min and subsequently placed into a deionized water bath to remove the PBI-film from the glass plate.

Example 3

(16) A Treopore®/meta-polybenzimidazole (mPBI) bilayer membrane with a PBI-layer thickness of 3 μm was prepared by hot-pressing a PBI- and a 20 μm thick Treopore® PDA-film onto each other at 80° C. for 3 min. Treopore® PDA was used as received, the 3 μm PBI film was previously prepared by casting a 10 wt % solution of mPBI in dimethylacetamide onto a glass plate. The cast film was dried at 110° C. for 5 min and subsequently cured at 150° C. for 10 min.

Example 4

(17) A Treopore®/meta-polybenzimidazole (mPBI) bilayer membrane with a PBI-layer thickness of 1 μm was prepared by spray-coating of a PBI solution in dimethylacetamide (1 w %) directly onto the Treopore® PDA-film (20 μm thickness). The coated film was dried at 110° C. for 5 min and subsequently cured at 150° C. for 10 min.

Example 5

(18) To prepare a functionalized PBI-film, 1 mole equivalent of LiH was added to a 5 w % solution of mPBI in dimethylacetamide in Argon atmosphere. The mixture was stirred for 6 h at 90° C. until no more gas bubbles were observed (indicating the end of the deprotonation). Afterwards, 1 mole equivalent diethyl(bromodifluoromethyl)phosphonate is added to the solution and stirred for another 16 h at 90° C. The functionalized polymer solution was filtered and cast on a glass plate, dried at 110° C. for 5 min and subsequently cured at 150° C. for 10 min. The membrane was placed into 2 H.sub.2SO.sub.4 in order to hydrolyze the phosphonate ester.

Comparative Example 1

(19) The cation exchange membrane Nafion® NR212 (Chemours) is used as Comparative Example 1. The membrane was pretreated by immersion in water or electrolyte depending on the performed analysis.

Comparative Example 2

(20) The anion exchange membrane Fumasep® FAP-450 (fumatech) is used as Comparative Example 2. The membrane was pretreated by immersion in water or electrolyte depending on the performed analysis.

(21) Membrane Characterization

(22) The membranes were characterized for their key properties in the context of the application in redox flow batteries.

(23) The ion exchange capacity was determined with titration. For that, all membranes were fully protonated in 2 M H.sub.2SO.sub.4 overnight and subsequently rinsed with water until a neutral pH was observed. Afterwards, the fully protonated membrane samples were immersed in 1 M KCl solution in order to exchange the acidic protons (corresponding to the sulfonic acid groups of NR212). The cation exchange capacity (CEC.sup.exp) was determined from direct titration of the exchanged protons with 0.05 M KOH and calculated according to:

(24) ${\mathrm{CEC}}^{\exp }(\mathrm{bilayer})=\frac{c\left(m\mathrm{mol}{L}^{-1}\right).Math.F.Math.V\left(L\right)}{m_{\mathrm{dry}}\left(g\right)}$
where c, F, V and m.sub.dry are the concentration of KOH, the titration factor, the volume of KOH added at the equivalent point and the dry mass of the protonated membrane, respectively.

(25) To determine the anion exchange capacity, the samples were removed from the KCl-solution and again rinsed with water until a neutral pH was observed. Titration was then performed indirectly by adding 2 mL KOH (0.05 mol L.sup.−1) to the samples. After 2 h, this solution was titrated with 0.05 M HCl and the anion exchange capacity (AEC.sup.exp) was calculated according to:

(26) $\mathrm{AE}{C}^{exp}\left(\mathrm{bilayer}\right)=\frac{\begin{array}{c}\left[c^{KOH}\left(m\mathrm{mol}{L}^{-1}\right).Math.F^{KOH}.Math.V^{KOH}\left(L\right)\right]-\\ [c^{HCl}\left(m\mathrm{mol}{L}^{-1}\right).Math.F^{HCl}.Math.V^{HCl}\left(L\right)\end{array}}{m_{\mathrm{dry}}\left(g\right)}$
including the concentration c, the titration factor F and the volume V of the titration solution.

(27) The net flux of vanadium between the negative and the positive electrolyte was determined from the vanadium concentration and the volume of the fully discharged electrolyte over 50 cycles. For all measurements, the first 20 cycles were used for conditioning and neglected in the analysis. In this initial phase, the vanadium transport is superimposed by electrolyte flux resulting from an osmotic pressure gradient formed during the initial charging reaction (when using vanadium electrolyte of an average valence of 3.5 (−50% state of charge) a proton gradient evolves during the first charging that is not fully reversed during discharge). A negative flux represents transport towards the negative electrolyte. To measure the concentration, 300 μL samples were taken periodically over time once the applied potential was close to the lower termination voltage (0.8 V), corresponding to a state of charge of 0-5%. The sample was divided into 3 aliquots and a redox titration with KMnO.sub.4 in 2 M H.sub.2SO.sub.2 was performed. The total volume of the electrolyte was constantly decreasing due to the continuous argon purging of the two electrolyte compartments. The net vanadium flux J.sub.V can be calculated using the vanadium concentration c μmol L.sup.−1), the volume V (L), the operating time t (h) and the cell area A.sub.cell (cm.sup.2):

(28) $J_V=\frac{\left[c_{pos}.Math.V_{pos}\right]-\left[c_{neg}.Math.V_{neg}\right]}{t.Math.A_{cell}}$

(29) To determine the rate of capacity fading due to imbalanced vanadium crossover, assembled cells were operated at a constant current density of 160 mA cm.sup.−2 for 50 cycles (within termination voltages of 0.8 and 1.65 V). After each cycle, the discharge capacity was obtained from the discharge time at constant current.

(30) The ohmic resistance of a redox flow cell is largely influenced by the conductivity of the membrane in the respective electrolyte solution(s). Therefore, the ohmic resistance was measured in an assembled all-vanadium redox flow cell at room temperature comprising carbon felt electrodes (SGL SIGRACELL® GFD4.6 EA) and the respective membrane as polymer electrolyte. A redox flow test system (Model 857) from Scribner Associates was used to operate the cell. Both the negative and the positive electrolyte compartment were filled with an aqueous solution containing commercially available (Oxkem, Reading, UK) 1.6 M vanadium sulfate solution (average oxidation state is 3.5) in 2 M H.sub.2SO.sub.4 and 0.05 M H.sub.3PO.sub.4. The ohmic area specific resistance R.sub.Ω of the cell was determined in the uncharged state at open circuit potential from the intercept of the impedance spectrum in Nyquist representation with the real axis at the high frequency end.

(31) The description of the cell test results refer to Table 1 and the FIGS. 8 to 10.

(32) The combination of a robust supporting layer with a low area resistance and a thin (<15 μm) PBI-layer improves the performance of vanadium flow batteries mainly by increasing the round-trip efficiency and by stabilizing capacity fading. The performance relevant characteristics of PFSA membranes coated with a thin PBI-layer are reported in Table 1. The moderate area resistance and concurrently reduced vanadium crossover yields an improved round-trip efficiency especially at current densities<120 mA cm-2 (FIG. 8). By balancing the ratio of anion and cation exchange capacity, the net vanadium flux was decreased to 30% reducing capacity fading by 93% for a PBI/Nafion® bilayer with a PBI thickness of 4 μm (at 160 mA cm-2). This stabilized capacity fading is further illustrated in FIG. 9.

(33) When using a porous Treopore® PDA 30 film as a supporting layer in combination with a thin (<15 μm) PBI-layer that can either be hot-pressed or spray-coated onto the supporting layer, the round-trip efficiency of a vanadium flow battery was improved by up to 10% in the technical relevant range of current densities (40-120 mA cm-2) (FIG. 10).

(34) To minimize voltaic efficiency losses, a low area resistance is favorable. All embodiments of the present invention have an area resistance that is lower compared to the benchmark membrane FAP-450 (Comparative Example 2). A further improvement of the area resistance of bilayered membranes comprising a supporting layer and a thin (<15 μm) PBI-layer is possible by introducing cation exchange groups into the PBI-layer. When attaching phosphonic acid groups to the imidazole-unit of a 12 μm PBI-layer (Example 5), the area resistance was reduced to 0.5 Ωcm2 (FIG. 11).