METHOD FOR REMOVING NON-PROTON CATIONIC IMPURITIES FROM AN ELECTROCHEMICAL CELL AND AN ELECTROCHEMICAL CELL

Abstract

Non-proton cationic impurities are removed from the ionomer in a proton exchange membrane of an electrochemical cell and from the anode side and cathode side catalyst layers. A supply path for an anode feed to the ionomer on the anode side of the proton exchange membrane and a supply path for a cathode feed to the ionomer on the cathode side of the proton exchange membrane are provided. A regenerating fluid with acidic pH is brought into contact with the ionomer on the cathode side of the proton exchange membrane to accomplish an ion exchange of the non-proton cationic impurities with protons and thus remove the non-proton cationic impurities from the ionomer into the regenerating fluid. This removes the non-proton cationic impurities from the ionomer of the electrochemical cell without increasing the risk of corrosion and without interrupting the process of the electrochemical cell.

Claims

1-15. (canceled)

16. A method of removing non-proton cationic impurities from an ionomer associated with a proton exchange membrane and respective anode side and cathode side catalyst layers of an electrochemical cell, the method comprising the steps of: a) providing a supply path for an anode feed to the ionomer on the anode side of the proton exchange membrane; b) providing a supply path for a cathode feed to the ionomer on the cathode side of the proton exchange membrane; and c) bringing a regenerating fluid having an acidic pH in contact with the ionomer on the cathode side of the proton exchange membrane to cause an ion exchange of non-proton cationic impurities with protons and to remove the non-proton cationic impurities from the ionomer into the regenerating fluid.

17. The method according to claim 16, which comprises bringing a regenerating fluid with acidic pH in contact with the ionomer on the anode side of the proton exchange membrane to cause an ion exchange of the non-proton cationic impurities with protons and to remove the non-proton cationic impurities from the ionomer into the regenerating fluid.

18. The method according to claim 16, which comprises acidifying a regenerating solution by injecting acid into an aqueous cathode feed and optionally into an aqueous anode feed.

19. The method according to claim 18, which comprises acidifying the regenerating solution by injecting an acid selected from the group consisting of sulfuric acid, nitric acid, carbonic acid, and hydrochloric acid,

20. The method according to claim 16, which comprises acidifying the regenerating solution by introducing carbon dioxide CO.sub.2 to an aqueous cathode feed triggering a formation of carbonic acid, and optionally looping the aqueous cathode feed.

21. The method according to claim 16, which comprises discarding the regenerating solution after a regeneration process, or restoring the regenerating solution by acid refilling or by passing the regenerating solution through an ion exchange bed to scavenge non-proton cations and maintain the acidic pH for recirculation.

22. The method according to claim 16, which comprises introducing the regenerating solution into the ionomer on the cathode side of the proton exchange membrane while the electrochemical cell is operating.

23. The method according to claim 16, which comprises using carbon dioxide CO.sub.2 to acidify the regenerating solution and form carbonic acid at a process pressure and a temperature favorable for increasing a solubility of carbon dioxide CO.sub.2 in the regenerating solution.

24. The method according to claim 16, which comprises dissolving carbon dioxide CO.sub.2 in water in a mixing chamber external to an electrochemical device having the electrochemical cell, in order to produce a regenerating solution which is fed to the electrochemical cell.

25. The method according to claim 16, which comprises feeding carbon dioxide CO.sub.2 in gas phase to the electrochemical device, and dissolving the carbon dioxide CO.sub.2 in water inside the electrochemical device in order to produce the regenerating solution.

26. The method according to claim 16, which comprises feeding carbon dioxide CO.sub.2 in gas phase to the ionomer on the cathode side of the operating electrochemical cell and transporting the carbon dioxide CO.sub.2, dissolved in water, to the cathode side by electroosmotic drag.

27. The method according to according to claim 16, which comprises operating the electrochemical cell as a proton exchange membrane water electrolyzer.

28. An electrochemical cell, comprising: a) a proton exchange membrane, an anode side catalyst layer, a cathode side catalyst layer, and an ionomer for providing an ionic phase throughout the electrochemical cell; b) a supply path for an anode feed to the ionomer on an anode side of said proton exchange membrane; c) a supply path for a cathode feed to the ionomer on a cathode side of said proton exchange membrane; and d) an apparatus for bringing a regenerating fluid with acidic pH in contact with the ionomer on the cathode side of said proton exchange membrane, and optionally on the anode side of said proton exchange membrane, to accomplish an ion exchange of non-proton cationic impurities with protons and thus to remove the non-proton cationic impurities from the ionomer into the regenerating fluid and for extracting the regenerating solution comprising the non-proton cationic impurities from the ionomer.

29. The electrochemical cell according to claim 28 being a proton exchange membrane water electrolyzer.

30. The electrochemical cell according to claim 28, wherein at least one of the anode side or the cathode side of said proton exchange membrane comprises a gas diffusion layer having areas with modified water management properties.

31. The electrochemical cell according to claim 30, wherein said gas diffusion layer is a porous gas diffusion layer with modified mechanical structures forming separated gas and water transport pathways in the gas diffusion layer.

32. The electrochemical cell according to claim 28, wherein said gas diffusion layer has separated gas and water transport pathways formed by localized changes in a hydrophilicity thereof.

Description

[0032] Preferred embodiments of the present invention are hereinafter explained in more detail with reference to the attached drawings which depict in:

[0033] FIG. 1 a schematic representation of a PEMWE cell cross-section, and the electrochemical reactions taking place at the anode and cathode catalyst, including the overall water splitting reaction;

[0034] FIG. 2 a schematic representation of the operating cell voltage (left) and the overpotential situation;

[0035] FIG. 3 a schematic representation of the PEMWE ionomer regeneration procedure in the cathodic water recirculation loop;

[0036] FIG. 4 a schematic representation of the a PEMWE electryser cell including regeneration circuits for the regeneration of the cathionic ionomer load; and

[0037] FIG. 5 polarization curves of a pristine, contaminated and a regenerated electrolyzer measured at 40° C. and ambient pressure.

[0038] The invention introduced here presents a viable way of operando regeneration of proton exchange membrane water electrolysers by extracting the contaminants from the electrolyte on the cathode side. The presented solution provides a way to achieve the reprotonation of the ion-exchange groups in the electrolyte during the operation of the device by indirectly forming an acidic environment in the cell, bypassing possible problems with corrosion of the system components. Further, the invention presents a viable way of operando regeneration of the electrolyzer CCM without the necessity of stopping the stack operation and component disassembly. FIG. 1 shows schematically a representation of a PEMWE cell cross-section, and the electrochemical reactions taking place at the anode and cathode catalyst, including the overall water splitting reaction. FIG. 2 shows the problem of the overvoltage having a negative impact on the cell performance.

[0039] During operation, the cationic species in the ionomer will move towards the negative electrode (cathode), and accumulate in the cathodic region of the catalyst coated membrane (CCM) which in general comprises an ionomer comprising the catalyst that is deposited on the polymer electrolyte membrane. In order to get rid of cations from the CCM it is necessary to introduce a counter ion which will attract the cationic contaminants from the ionomer and bind them. It is proposed here to saturate the cathode water recirculation loop with CO.sub.2, which will lead to the formation of carbonic acid (H.sub.2CO.sub.3). Another possible embodiment is the feeding of pure CO2 to the cathode compartment at pressure.

Carbonic acid formation CO.sub.2(aq)+H.sub.2O.Math.H.sub.2CO.sub.3

[0040] Carbonic acid may deprotonate and decrease the pH of the water in the cathode cell compartment and form bicarbonates, facilitating the ion-exchange with the contaminants.

Deprotonation of carbonic acid H.sub.2CO.sub.3.fwdarw.HCO.sub.3.sup.−+H.sup.+

[0041] Since the metallic contaminants are accumulated at the cathode, they will undergo an ion-exchange process and thus the metal ion will be removed from the ionomer:

Ionomer regeneration —SO.sub.3.sup.− . . . Me.sup.n++H.sup.+.fwdarw.—SO.sub.3.sup.− . . . H.sup.++Me.sup.n+

where —SO.sub.3.sup.− . . . Me.sup.n+ indicates a metal cation Me.sup.n+ counter-ion in the ionomer with sulfonate anion fixed groups. The solubility of CO.sub.2 is relatively low (around 0.005% molar fraction of CO.sub.2 in liquid phase) at normal conditions. Therefore, the regeneration should be done at elevated pressures and low temperatures allowing the decrease of the pH value of the exchange solution significantly. For example, a system of carbon dioxide and water at 35° C. and 4 bar has been demonstrated to yield a pH of approximately 3.7. Increasing the pressure to 150 bar at the same temperature would result in a more acidic solution with a pH of approximately 3. State-of-the-art electrolyzer membranes are around 0.2 mm thick, and allow a safe differential operation at pressures in the range of 50 to 100 bar, thereby making the regeneration solution viable in a technical system. The cathodic recirculation loop can be additionally equipped with a heat exchanger to reduce the temperature of the solution below ambient temperature and decrease the pH even further.

[0042] As the water in the regeneration loop is diluted with the electro-osmotically dragged water and saturated with metal bicarbonates, a cation exchange resin can be optionally introduced in the regeneration loop to scavenge the metallic species and protonate the solution to maintain the pH. A schematic of an electrolyzer system equipped with the cathodic regeneration process in outlined in FIG. 4. The anode compartment, which is more sensitive to impurities and contains particularly expensive components, would be unaffected. The proposed regeneration solution involves introducing agents on the cathode side alone, which will not disturb normal operation of the stack. FIG. 3 shows a schematic representation of the PEMWE ionomer regeneration procedure in the cathodic water recirculation loop.

[0043] The proposed method of regenerating the performance of the contaminated electrolyzer CCM has been tested using a system equipped with a peristaltic pump for the recirculation of the feed, contamination and the regeneration solution through a PEMWE cell, a mass flow meter and a pressure controller to regulate the cathodic process parameters, and a laboratory potentiostat to drive the water splitting reaction and measure the polarization curve.

[0044] After the conditioning of the cell and the baseline measurements, the electrolyzer was contaminated with a 2 mM FeSO.sub.4 solution introduced into anode feed water loop for 60 min. The effect on the polarization curve is shown in FIG. 5. The anodic loop of the electrolyzer was then replaced with fresh miliQ water to avoid further contamination, and polarization data was collected after the potential had stabilized. The cathodic loop was connected to the CO.sub.2 feed and pressurized to 21 bar. The temperature of the cathodic loop was kept at 30° C. CO.sub.2 saturated regeneration solution was circulated through the cathodic cell compartment of an operating contaminated PEMWE cell at a flow rate of 600 mL/min. The PEMWE cell was operated at a current density of 1 A cm.sup.−2 for 25 h during the regeneration process. The cathodic loop was afterwards purged and the polarization curves were recorded after stabilization of the cell potential. The regeneration yielded a ˜98% restored voltage efficiency in the current density range of 1 to 2 A cm.sup.−2. The approach is thereby validated for the pH controlling parameters (21 bar and 30° C.). FIG. 5 shows the respective polarization curves of a pristine, contaminated and a regenerated electrolyzer measured at 40° C. and ambient pressure.