Breathable electrode structure and method for use in water splitting

Abstract

The present invention relates to a water splitting cell having at least one electrode comprising a porous membrane, wherein gas produced at the electrode diffuses out of the cell via the porous membrane, separating the gas from the reaction at the electrode without bubble formation.

Claims

1. A method of splitting water into hydrogen and oxygen in a water-splitting electrolysis cell, the method comprising: combining a first electrode having a conducting layer on a surface of a porous gas-permeable hydrophobic polymer membrane and a catalyst over the conducting layer with a counter-electrode in an electrochemical cell containing an aqueous electrolyte; applying a current to the electrochemical cell to produce hydrogen gas and oxygen gas from the aqueous electrolyte; and separately collecting the produced hydrogen gas directly from the first electrode and oxygen gas from the counter-electrode.

2. The method of claim 1, wherein collecting the hydrogen gas comprises collecting the hydrogen gas directly from the porous gas-permeable hydrophobic polymer membrane.

3. The method of claim 1, wherein the conducting layer is gold.

4. The method of claim 1, further comprising performing a surface treatment on the porous gas-permeable hydrophobic polymer membrane prior to applying the conducting layer to the surface.

5. The method of claim 4, wherein the surface treatment comprises a plasma treatment.

6. The method of claim 4, wherein the surface treatment comprises a polyacid treatment.

7. The method of claim 6, wherein the surface treatment comprises treating the membrane with maleic anhydride.

8. The method of claim 1, wherein the catalyst layer comprises platinum.

9. The method of claim 1, wherein the catalyst is a metal or a metal compound.

10. The method of claim 9, wherein the catalyst comprises one or more of the following: Pd, Ru, Ir, Mn, Fe, Ni, Co, NiO.sub.x, Mn complexes, Fe complexes, MoS.sub.x, CdS, CdSe, and GaAs.

11. The method of claim 1, wherein the polymer membrane is a polytetrafluoroethylene (PTFE) membrane.

12. The method of claim 1, wherein the polymer membrane comprises one or more of polyethylene (PE), polypropylene (PP), poly n-butyl methacrylate (PnBMA), polytrifluoroethylene, nylon, polybutadiene, and polychlorotrifluoroethylene (PCTFE).

13. The method of claim 1, wherein the polymer membrane comprises one or more of polydimethylsiloxane (PDMS), poly t-butyl methacrylate (PtBMA), fluorinated ethylene propylene (FEP), hexatriacontane, paraffin, poly(hexafluoropropylene), and polyisobutylene (PIB, butyl rubber).

14. The method of claim 1, wherein the porous membrane has a pore size of less than 0.5 m.

15. The method of claim 14, wherein the porous membrane has a pore size of less than 0.1 m.

16. The method of claim 15, wherein the porous membrane has a pore size of less than 0.05 m.

17. The method of claim 1, wherein the porous membrane has a contact angle greater than 90.

18. The method of claim 1, wherein the porous membrane has a contact angle from 100 to 115.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further disclosure, objects, advantages and aspects of preferred and other embodiments of the present application may be better understood by those skilled in the relevant art by reference to the following description of embodiments taken in conjunction with the accompanying drawings, which are given by way of illustration only, and thus are not limitative of the disclosure herein, and in which:

(2) FIG. 1a is a schematic of the experimental set-up showing the reference electrode 1, anode 3, cathode 4, and oxygen probe 5 relative to the electrolysis chamber 6 on the left, attached to the gas collection chamber 7 on the right (half breathing cell); FIG. 1b illustrates gas and ion movements in an aqueous electrolyte 11 relative to the cathode 10 and anode 12 corresponding to the set-up shown in FIG. 1a; and FIG. 1c illustrates gas and ion movements in a full breathing cell. The black rectangles in FIG. 1b and FIG. 1c indicated the Micro-Oxygen Electrode position;

(3) FIG. 2 illustrates O.sub.2 measurement behind the different Pt-coated membranes (Au/Goretex 21, Au/Mitex 10 m 22 and GDE 23);

(4) FIG. 3 illustrates O.sub.2 measurements above the electrolyte (O.sub.2 front 30) and behind the membrane in the adjacent chamber (O.sub.2 back 32) after commencement of the application of 10 mA 34;

(5) FIG. 4 is a series of scanning electron micrographs (SEM) of Pt-coated Au/Goretex (FIG. 4(c)), Au/Mitex 10 m (FIG. 4(b)) and GDE (FIG. 4(a)). (Scale bars: left column100 m, middle column10 m and right column10 nm).

(6) FIG. 5 illustrates O.sub.2 measurement behind the different membranes coated with Pt: polyethylene Celgard 880 (40), polypropylene mesh (41) and non-woven polypropylene (42). 10 mA is applied where indicated (45).

(7) FIG. 6 is a plot of DO (mV) against time (min) illustrating O.sub.2 evolution of the back chamber during shining of light and during evacuation using CdS/Ti/Au/Goretex. The peaks appearing in the graph correspond to 13 min DO 42 mV (50), 13 min DO 40 mV (51), 18 min DO 49 mV (52), 12 min DO 47 mV (53), 12 min DO 52 mV (54) and 12 min DO 53 mV (55). Measurements were taken with light off and N.sub.2 and O.sub.2 admitted to the chamber (56), with the light on and N.sub.2 out (57) then O.sub.2 out (58)

(8) FIG. 7 is a plot of O.sub.2 evolution rate over light exposed time (min) for CdS/Ti/Au/Goretex membrane (60) and Ti/Au/Gortex membrane (61).

EXAMPLES

(9) The invention will be further described with reference to the following non-limiting examples. More specifically, three membrane electrodes with different morphology and pore sizes and shapes were prepared and studied. Platinum, the most well studied catalyst was used as the model catalyst material. However the electrodes of the present invention should not be interpreted as being limited to this catalyst and can be operated with many catalysts.

(10) Membrane Treatment and Pt Coating

(11) PTFE membranes (Goretex) was obtained from Gore Inc and Mitex (10 m) was obtained from Millipore. Au mylar (2.5 Ohm/square) was purchased from CPFilms Inc. Maleic anhydride was obtained from Sigma-Aldrich. Preparation of the Goretex, Mitex, polyethylene (PE) and polypropylene (PP) membranes prior to Pt coating was similar to previous work described by Winther-Jensen et al entitled High rates of oxygen reduction over a vapor phase-polymerized PEDOT electrode in Science 2008; 321:671-4. Maleic anhydride was grafted onto the hydrophobic surface of the membranes to ensure good bonding to the gold conducting layer, using plasma polymerisation as earlier reported in the aforementioned article and by Ademovic Z et al, in and article entitled Surface modification of PET films using pulsed AC plasma polymerisation aimed at preventing protein adsorption in Plasma Processes Polym 2005; 2:53-63. The gold was sputtered onto the plasma treated membranes and its thickness was optimised to give a surface resistance 5 Ohm/sq. The Pt was then sputtered on top of the gold layer at 28-30 mA for 60 sec. A traditional GDE was also studied for comparison; this was an ionomer free (LT-140EW-30% Pt on Vulcan XC-72, 0.5 mg cm.sup.2) from E-TEK and used as supplied. SEM images were obtained using a JEOL 7100F Field Emission Gun Scanning Electron Microscope at 5 kV.

(12) Electrode Assembly

(13) The membrane was sandwiched with a gold strip using a conventional laminator. A 0.7 cm.sup.2 window in the laminate allowed access for electrolyte to the Pt coated side of the membrane and for the gas to breathe out to the adjacent chamber when mounted on the test cell with double-sided adhesive tape (FIG. 1).

(14) Experimental Set-Up and Gas Measurement

(15) Sodium p-toluene sulphonate (from Sigma Aldrich) 0.05 M pH 4 was used as an electrolyte. 30 ml of electrolyte was used in the test cell leaving 30 ml gas space above the electrolyte. A three electrode cell was set-up using a saturated calomel reference electrode (SCE) and carbon rod or Pt counter electrode. A multi-channel potentiostat (VMP2 from Princeton Applied Research) was used for the constant-current electrolysis. The distance between the electrodes is 1.5 cm and the potential during operation of all working electrodes was typically 2-2.4 V vs SCE.

(16) The Micro-Oxygen Electrode was purchased from eDAQ and used to monitor O.sub.2 evolution from the electrolysis reactions. It was calibrated at 21% O.sub.2 in air and 0% O.sub.2 in pure nitrogen gas. The slope from the calibration was 10.3 mV equals 1% O.sub.2. The amount of H.sub.2 was measured using gas chromatography (SRI 310C, MS-5A column, TCD, Ar carrier).

(17) Results

(18) The test cell was set up as shown in Scheme 1. Firstly, the experiments were focused on WO. Pt coated membrane was used as the anode and the liberated O.sub.2 was monitored using a Micro-Oxygen Electrode placed in the chamber (60 ml) on the back side of the membrane (Scheme 1). Several seconds after 10 mA current was applied to the cell, bubbles started to form on the counter electrode (carbon rod). On the anode side, bubbles were not observed on the working area when Goretex membrane was used. This suggested that the major portion of the O.sub.2 was able to escape to the back side of the membrane. Some bubble formation was observed on the working area when the other membranes were used. The O.sub.2 content of the back side chamber steadily increased during electrolysis for both Pt-coated Au/Goretex and Au/Mitex electrodes, but remained unchanged for the GDE (FIG. 2), suggesting no O.sub.2 production in the latter case. The studies by Chaparro et al (Chaparro A M, Mueller N, Atienza C, Daza L. Study of electrochemical instabilities of PEMFC electrodes in aqueous solution by means of membrane inlet mass spectrometry. J Electroanal Chem 2006; 591:69-73) and Jang and Kim (Jang S E, Kim H. Effect of water electrolysis catalysts on carbon corrosion in polymer electrolyte membrane fuel cells. J Am Chem Soc 2010; 132:14700-1) support this observation with the GDE as they showed the electrochemical oxidation of carbon on the GDE in presence of water at oxidative potentials (Chaparro A M, Mueller N, Atienza C, Daza L. Study of electrochemical instabilities of PEMFC electrodes in aqueous solution by means of membrane inlet mass spectrometry. J Electroanal Chem 2006; 591:69-73; and Jang S E, Kim H. Effect of water electrolysis catalysts on carbon corrosion in polymer electrolyte membrane fuel cells. J Am Chem Soc 2010; 132:14700-1).

(19) The O.sub.2 evolution rate from the Pt-coated Au/Goretex electrode was the highest, indicating that the coated Goretex electrode is the most efficient in emitting gaseous O.sub.2 from the WO reaction.

(20) Further investigation was performed by monitoring the O.sub.2 evolution in the head space above the electrolyte, in the front chamber, during water splitting with the Pt-coated Au/Goretex electrode. The result (FIG. 3) showed no measurable increase in O.sub.2 above the electrolyte, indicating a very high efficiency in removing it into the back chamber. The Faradaic efficiency in these experiments was 903%.

(21) In order to understand the breathing ability of each membrane, scanning electron microscopy was performed as shown in FIG. 4.

(22) As expected, Pt nanoparticles were well distributed on the membrane surfaces. The images of the GDE showed dense, packed structure with Pt nanoparticles ranging from 65 to 100 nm. The size of the sputtered Pt nanoparticles was in the range of 30-40 nm on Mitex and Goretex membranes. The Mitex 10 m images showed inconsistent pore size and distribution, whereas Goretex has a fine pore size (110 m) with consistent distribution. The Goretex structure is believed to contribute to its higher performance observed in the water splitting experiments.

(23) As a control experiment, a non-porous substrate consisting of Pt-coated Au mylar was used as an anode in a single chamber set-up with the oxygen probe placed above the electrolyte. The O.sub.2 produced in this experiment was much lower (0.48 mol/min) than when using the Pt-coated Au/Goretex (1.35 mol/min) in the two chambers set-up. The Faradaic efficiency from this control experiment was only 31%. This indicates the degree of oxygen shuttling between the electrodes that are present in this cell configuration, having no separator, when a non-porous electrode is used.

(24) In another experiment the Pt-coated Au mylar was used as the anode and Pt-coated Au/Goretex as the cathode, ie as the H.sub.2 producing electrode. There was no H.sub.2 bubble formation observed on the cathode. The Faradaic efficiency of O.sub.2 evolution in this experiment was 61%. When Pt-coated Au/Goretex electrodes were used for both anode and cathode, so that both gases were removed from the cell, the Faradaic efficiency was increased to 92%. H.sub.2 detected in this experiment was found to be close to 2:1 stoichiometric ratio within measurement error (7%). This suggests that in an optimized cell and gas flow configuration it may be practical to avoid the use of a separator in these cells.

(25) Although Goretex initially was found to be the best among the three membranes tested, certainly there are membranes with different hydrophobicity and various pore sizes and shapes which can be used. A number of these possibilities were tested in an additional experiment. Here polyethylene (PE, Celgard 880 (0.11 poresize)) and polypropylene (PP) mesh (5 poresize) and PP non-woven (5 poresize) membranes were tested in similar way as described above (see FIG. 5). The Celgard 880 performed nearly as good as the Goretex as seen from the increase in oxygen measured on the back chamber of the setup, which correspond to a faradaic efficiency of 82%. The two PP membranes were less efficient (51% and 41% respectively), however clearly showed that this material can be used for the membrane structure.

(26) Stability Test of CdS on Ti/Au/Goretex and Baseline Test Using Ti/Au/Goretex.

(27) CdS/Ti/Au/Goretex or Ti/Au/Goretex (0.5 cm.sup.2) was laminated and sandwiched between two plastic bottles. The front chamber was filled up with 0.05 M NaPTS pH 6.75 30 ml. An oxygen sensor was placed in the gas chamber. Black cloth was used to cover the plastic chamber to protect the light directly shining on the DO probe. Asahi lamp was used to shine the light on the sample. Each data point was collected after the following procedure: N.sub.2 gas was used to purged the electrolyte for about 15 min or until stable baseline was achieved and in the same time O.sub.2 was flushed into the back chamber, immediately after removal of N.sub.2 (and the hole was sealed) the light was shone on the sample for 7 min, O.sub.2 was then removed (and the hole was sealed) with the light continued to shine for another 5 min. This process has been repeated for 39 cycles. The O.sub.2 increased was monitored and typical graph was shown in FIG. 6.

(28) The data was then plotted as the rate of O.sub.2 increased (increased in O.sub.2 reading over, typically, 12 min light exposure) versus light exposed time (FIG. 7). From FIG. 7 it can be seen that the O.sub.2 evolution rate from CdS/Ti/Au/Goretex electrode was higher than from the Ti/Au/Goretex baseline and stable for more than 8 hours. This result should be compared to the usual degradation of CdS within several minutes under light/oxygen evolution.

(29) The surface treatment, using polyacid and plasma polymerisation, is also a vital step to ensure a good cohesion between the catalyst and the membrane. It also opens the route to deposit the catalyst onto hydrophobic membranes. The possibility of merging this technology with some of the non-precious metal and metal oxide catalysts (Pletcher D, Li X. Prospects for alkaline zero gap water electrolysers for hydrogen production. Int J Hydrogen Energy 2011; 36:15089-104) that have limited possible use in PEM electrolysers will lead to a facile and cost efficient water splitting device. It is also possible to use this approach to enhance the lifetime of photo-active electro-catalysts, many of which are sensitive to the presence of oxygen bubbles.

(30) While this invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modification(s). This application is intended to cover any variations uses or adaptations of the invention following in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth.

(31) As the present invention may be embodied in several forms without departing from the spirit of the essential characteristics of the invention, it should be understood that the above described embodiments are not to limit the present invention unless otherwise specified, but rather should be construed broadly within the spirit and scope of the invention as defined in the appended claims. The described embodiments are to be considered in all respects as illustrative only and not restrictive.

(32) Various modifications and equivalent arrangements are intended to be included within the spirit and scope of the invention and appended claims. Therefore, the specific embodiments are to be understood to be illustrative of the many ways in which the principles of the present invention may be practiced. In the following claims, means-plus-function clauses are intended to cover structures as performing the defined function and not only structural equivalents, but also equivalent structures.

(33) Comprises/comprising and includes/including when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. Thus, unless the context clearly requires otherwise, throughout the description and the claims, the words comprise, comprising, includes, including and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of including, but not limited to.