Membrane electrode assembly for proton exchange membrane water electrolyzer and method of preparing membrane electrode assembly for proton exchange membrane water electrolyzer

Abstract

Provided is a membrane electrode assembly for a proton exchange membrane water electrolyzer, including: an oxygen electrode including an iridium oxide (IrO.sub.2) layer which is an electrodeposited oxygen electrode catalyst layer on a titanium (Ti) layer which is a diffusion layer; a hydrogen electrode in which a hydrogen electrode catalyst layer is formed on a diffusion layer; and an electrolyte membrane placed between the oxygen electrode catalyst layer and the hydrogen electrode catalyst layer, in which a portion of the pores of the Ti diffusion layer are filled with an electrolyte of the electrolyte membrane.

Claims

1. A method of preparing a membrane electrode assembly for a proton exchange membrane water electrolyzer, the method comprising: preparing an oxygen electrode comprising a titanium (Ti) layer which is a diffusion layer and an IrO.sub.2 layer which is an oxygen electrode catalyst layer by electrodeposition of iridium oxide (IrO.sub.2) onto the Ti layer to form the IrO.sub.2 layer; stacking an electrolyte membrane onto the IrO.sub.2 layer of the oxygen electrode; forming an oxygen electrode-electrolyte membrane assembly by performing a compression process on the oxygen electrode and the electrolyte membrane, wherein the Ti layer on which the IrO.sub.2 is deposited comprises a plurality of pores and is divided into an upper zone and a lower zone, wherein pores of the upper zone of the Ti layer are partially filled with the electrolyte of the electrolyte membrane, and pores of the lower zone of the Ti layer are not filled with the electrolyte of the electrolyte membrane, and wherein, in order to increase an interface between the oxygen electrode and the electrolyte while preventing a disturbance of mass transfer of water, an M-E zone where pores are partially filled with the electrolyte of the electrolyte membrane in the upper zone of the Ti layer on which the IrO.sub.2 is deposited has a thickness of 25 to 35 μm; and preparing the membrane electrode assembly by assembling a hydrogen electrode in which a hydrogen electrode catalyst layer is formed on a diffusion layer with one surface of the electrolyte membrane after the compression process.

2. The method according to claim 1, wherein the compression process is performed under a temperature condition of 120 to 160° C. for 1 min to 5 min.

3. The method according to claim 1, wherein the compression process is performed under a pressure condition of 4 to 20 MPa.

4. The method according to claim 1, wherein the Ti layer is titanium mesh or titanium paper.

5. The method according to claim 1, wherein a process of electrodepositing IrO.sub.2 is performed under a deposition potential condition of 0.5 to 0.9 V.sub.SCE for 1 min to 10 min.

6. The method according to claim 1, wherein the IrO.sub.2 layer comprises iridium oxide loaded at 0.01 to 1.05 mg/cm.sup.2 onto the Ti layer.

7. The method according to claim 1, wherein prior to electrodeposition of IrO.sub.2, the titanium layer is immersed into oxalic acid, and then rinsed with ionized water.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1A to 1D are a schematic view showing a process of preparing a membrane electrode assembly according to an electrode in the related art.

(2) FIG. 2 is a schematic view showing a process of preparing a membrane electrode assembly according to the present disclosure.

(3) FIG. 3 shows a result comparing the surfaces of the oxygen electrode-electrolyte membrane assembly before and after a compression (hot press) process according to the present disclosure.

(4) FIGS. 4A to 4C are photographs showing a surface morphology of an electrodeposited IrO.sub.2 layer (FIG. 4A) and its enlarged image (FIG. 4B), and a corresponding EPMA mapping result (FIG. 4C), and FIG. 4D is a graph showing a change in IrO.sub.2 loading according to a change in t.sub.dep measured by ICP-MS.

(5) FIG. 5A shows iV curves of various electrodes (IrO.sub.2/CP, IrO.sub.2/TP, and membrane-oxygen electrode assemblies including IrO.sub.2/TP electrodes denoted as P1 and P2), and FIG. 5B shows a Nyquist plot at 1.6 V for the electrodes.

(6) FIG. 6 shows current densities at voltages of 1.6 V, 1.8 V, and 2.0 V and a change in R.sub.ohm according to a change in applied voltage during a compression (hot press) process. The compression (hot press) process was proceeded as in the P2 in FIG. 5A.

(7) FIG. 7A shows the individual components of R.sub.ohm for a full cell, and FIG. 7B shows the individual components of R.sub.ohm for a membrane-free cell. FIG. 7C shows the contributions by R.sub.t, R.sub.m and R.sub.i to total R.sub.ohm for various electrodes. Meanwhile, the upper right inset of FIG. 7C describes the iV characteristics of membrane-free cell equipped with CP and TP as anode DL.

(8) FIG. 8A shows a change in thickness of TP, membrane, and TP-membrane assembly after a compression (hot press) process under a pressure condition of 0 to 20 MPa, and FIGS. 8B and 8C show cross-sectional images of IrO.sub.2/TP-membrane assembly after hot press (FIG. 8B) and the corresponding EPMA result for F, Ti, and Ir (FIG. 8C).

(9) FIG. 9A shows thickness of M-E zone and corresponding contact area (Ac) between membrane-electrode in a change in pressure of the compression (hot press) process, and FIG. 9B describes R.sub.i,m according to a change in 1/Ac.

(10) FIG. 10 shows current densities at 2.0 V as a function of M-E zone for various electrodeposited IrO.sub.2/TP electrodes.

(11) FIG. 11 shows cell voltages for the electrodeposited IrO.sub.2/TP according to the catalyst loading at anode, and also describes matters on the related art.

DETAILED DESCRIPTION

(12) Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings.

(13) The Examples of the present disclosure disclosed herein are exemplified for the purpose of describing the Examples of the present disclosure only, and the Examples of the present disclosure may be carried out in various forms and should not be construed to be limited by the Examples described herein.

(14) Since the present disclosure may have various changes and different forms, it should be understood that the Examples are not intended to limit the present disclosure to specific disclosure forms and they include all the changes, equivalents and replacements included in the spirit and technical scope of the present disclosure.

Definition of Terms

(15) In the present specification, the term ‘titanium paper’ refers to a substrate in the form of paper formed by closely weaving cylindrical or polygonal drum-like titanium (Ti) fiber. The titanium fiber may have a cross-sectional area of 50 to 10,000 μm.sup.2.

(16) In the present specification, the ‘oxygen electrode’ is used as a term referring to an electrode which is the same as ‘anode’, and the ‘hydrogen electrode’ is used as a term referring to an electrode which is the same as ‘cathode’.

(17) Membrane Electrode Assembly for Proton Exchange Membrane Water Electrolyzer and Proton Exchange Membrane Water Electrolyzer

(18) An exemplary embodiment of the present disclosure provides a membrane electrode assembly for a proton exchange membrane water electrolyzer. The membrane electrode assembly for the proton exchange membrane water electrolyzer includes: an oxygen electrode including an iridium oxide (IrO.sub.2) layer which is an electrodeposited oxygen electrode catalyst layer on a titanium (Ti) layer which is a diffusion layer; a hydrogen electrode in which a hydrogen electrode catalyst layer is formed on a diffusion layer; and an electrolyte membrane placed between the oxygen electrode catalyst layer and the hydrogen electrode catalyst layer, and a portion of pores of the Ti diffusion layer may be filled with an electrolyte of the electrolyte membrane.

(19) Specifically, the oxygen electrode includes a Ti layer as a diffusion layer.

(20) Herein, the Ti layer is a porous Ti layer, and may include titanium mesh or titanium paper (TP). In an exemplary embodiment, the use of titanium paper may be preferred because the area ratio of Ti exposed per unit volume is high, and a short circuit is less likely to occur in a hot press procedure.

(21) Meanwhile, the Ti layer may include a plurality of pores, and the Ti layer may have a porosity of about 50% to about 80%, preferably about 60% to about 70%.

(22) In an exemplary embodiment, the Ti layer may have a thickness of 60 to 500 μm. When the Ti layer has a thickness of less than 60 μm, the pore of titanium paper is blocked after a compression process, and as a result, the performance may deteriorate, and when the Ti layer has a thickness of more than 500 μm, the mass transfer route of water is lengthened, and as a result, the performance may deteriorate.

(23) Meanwhile, a portion of the pores of the Ti layer may filled with an electrolyte of an electrolyte membrane placed between oxygen electrode catalyst layer and hydrogen electrode catalyst layer, and this is because the electrolyte of the electrolyte membrane penetrates into an upper zone of the Ti layer when a compression process is performed on the oxygen electrode and the electrolyte membrane. In this case, the electrolyte/electrode interface may be enlarged, and the performance of a proton exchange membrane water electrolyzer including the same may be excellent compared to that of a particle-type electrode in the related art.

(24) Meanwhile, a portion of the pores of the lower portion of the Ti layer may not be filled with the electrolyte of the electrolyte membrane, and for example, when the Ti layer is divided into an upper zone and a lower zone, the pore of the upper zone of the Ti layer may be filled with the electrolyte of the electrolyte membrane, and the pore of the lower zone may not be filled with the electrolyte of the electrolyte membrane.

(25) In an exemplary embodiment, the Ti layer may be a Ti layer which is immersed into oxalic acid, and then rinsed with de-ionized water, and in this case, a natural oxide film on the surface of the Ti layer may be removed, and IrO.sub.2 may be uniformly impregnated on the entire surface, and as a result, the performance may be further enhanced.

(26) Meanwhile, the IrO.sub.2 layer may function as an oxygen electrode catalyst layer (i.e., oxygen electrode catalyst layer). The IrO.sub.2 layer is electrodeposited, and has an inherent property (or morphology) and structure of electrodeposition, which is differentiated from a case where another preparation method (for example, a spraying method or a decal method, and the like) for forming IrO.sub.2 on the corresponding Ti layer is used.

(27) In an exemplary embodiment, the IrO.sub.2 layer covers the surface of the titanium layer, and it is preferred that the IrO.sub.2 layer has a crack-free structure.

(28) Meanwhile, in the present disclosure, the IrO.sub.2 layer may exhibit an effect which is the same as or more improved than that of an existing anode even though a small amount of IrO.sub.2 is loaded onto the Ti layer. For example, even though IrO.sub.2 at 0.01 to 1.05 mg/cm.sup.2, specifically, 0.075 to 0.2 mg/cm.sup.2 is loaded onto the Ti layer, it is possible to exhibit an effect which is improved as compared to that of an existing proton exchange membrane water electrolyzer.

(29) Meanwhile, still another exemplary embodiment of the present disclosure provides a proton exchange membrane electrolyzer including the membrane electrode assembly for the proton exchange membrane water electrolyzer.

(30) In the present disclosure as aforementioned, an oxygen electrode including the IrO.sub.2/Ti layer is prepared, and then a one-side compression process is performed by using an electrolyte membrane. Therefore, the pore of IrO.sub.2/Ti layer may be partially filled with an electrolyte of the electrolyte membrane by allowing the electrolyte of the electrolyte membrane to penetrate into the Ti layer of the oxygen electrode. The average thickness of zone where the pore of the Ti layer is filled affects an active area, and within an optimal thickness range, the cell voltage required at 1.0 A/cm.sup.2 and 80° C. may be reduced from 1.78 V to 1.64 V. This is a value which is comparable to those when a PGM-based catalyst is loaded (for example, 1 to 3 mg). Therefore, the proton exchange membrane water electrolyzer may be widely used for water electrolyzers.

(31) Method of Preparing Membrane Electrode Assembly for Proton Exchange Membrane Water Electrolyzer

(32) In an exemplary embodiment, the present disclosure provides a method of preparing a membrane electrode assembly for a proton exchange membrane water electrolyzer, the method including: preparing an oxygen electrode including a titanium (Ti) layer which is a diffusion layer and an IrO.sub.2 layer which is an oxygen electrode catalyst layer by electrodeposition of iridium oxide (IrO.sub.2) onto the Ti layer to form the IrO.sub.2 layer; stacking an electrolyte membrane onto the IrO.sub.2 layer of the oxygen electrode; forming an oxygen electrode-electrolyte membrane assembly by performing a compression process on the oxygen electrode and the electrolyte membrane; and preparing the membrane electrode assembly by assembling a hydrogen electrode with one surface of the exposed electrolyte membrane after the compression process. FIG. 2 schematically describes the corresponding preparing method. Hereinafter, the present disclosure will be described in detail for each step.

(33) First, an oxygen electrode including a Ti layer and an IrO.sub.2 layer is prepared by electrodepositing iridium oxide (IrO.sub.2) on a titanium (Ti) layer to form the IrO.sub.2 layer. The Ti layer may function as a diffusion layer, and the IrO.sub.2 layer may function as an electrode catalyst layer.

(34) In an exemplary embodiment, the Ti layer is not limited, but may include titanium mesh or titanium paper. However, titanium paper may be preferred because the area ratio of Ti exposed per unit volume is high, and a short circuit is less likely to occur in a hot press procedure.

(35) Meanwhile, the Ti layer is a porous Ti layer, and may include a plurality of pores. In an aspect, the porous Ti layer may have a porosity of about 50% to about 80%, preferably about 60% to about 70%.

(36) In an exemplary embodiment, the Ti layer may have a thickness of 60 to 500 μm. When the Ti layer has a thickness of less than 60 μm, the pore of titanium paper is blocked after a compression process, and as a result, the performance may deteriorate, and when the Ti layer has a thickness of more than 500 μm, the mass transfer route of water is lengthened, and as a result, the performance may deteriorate.

(37) In an exemplary embodiment, a process of immersing the Ti layer into oxalic acid, and then rinsing the Ti layer with ionized water may be further performed before IrO.sub.2 is electrodeposited, and in this case, a natural oxide film on the surface of the Ti layer may be removed, and IrO.sub.2 may be uniformly impregnated on the entire surface.

(38) Meanwhile, in the present disclosure, when an iridium oxide (IrO.sub.2) layer is electrodeposited, the iridium oxide (IrO.sub.2) layer has an inherent property (or morphology) and structure of electrodeposit, which is differentiated from a case where another preparation method (for example, a spraying method or a decal method, and the like) for forming IrO.sub.2 on the corresponding Ti layer having a mesh shape is used. That is, when an IrO.sub.2 layer is formed on a Ti layer by electrodeposition, while IrO.sub.2 particles (for example, spherical particles) begin to be formed, the IrO.sub.2 particles are produced on the surface of the Ti layer and grown in the form of a film, and as a result, the surface of the Ti layer is covered (that is, surface-covered) with an IrO.sub.2 layer (or may also be expressed as an IrO.sub.2 film), and the surface of the Ti layer has a form (structure) in which IrO.sub.2 particles further formed as the deposition time passes are attached (or deposited) onto the corresponding IrO.sub.2 layer.

(39) In an exemplary embodiment, the IrO.sub.2 layer covers the surface of the Ti layer, and it is preferred that the IrO.sub.2 layer has a crack-free structure.

(40) In an aspect, the IrO.sub.2 layer may cover the surface of titanium paper.

(41) Meanwhile, in the present disclosure, the IrO.sub.2 layer may exhibit an effect which is the same as or more improved than that of an existing anode even though a small amount of IrO.sub.2 is loaded onto the Ti layer. For example, even though IrO.sub.2 at 0.01 to 1.05 mg/cm.sup.2, specifically, 0.075 to 0.2 mg/cm.sup.2 is loaded onto the Ti layer, it is possible to exhibit an effect which is improved as compared to that of an existing proton exchange membrane water electrolyzer.

(42) Meanwhile, in an exemplary embodiment, the electrodeposition process may be proceeded under a deposition potential condition of 0.5 to 0.9 V.sub.SCE, more specifically, 0.6 to 0.9 V.sub.SCE. When the electrodeposition is performed at less than 0.5 V.sub.SCE, cracks may be produced on the IrO.sub.2 layer, and when the electrodeposition is performed at more than 0.9 V.sub.SCE, the deposition efficiency is not excellent, and as a result, the range may not be preferred in terms of economic efficiency.

(43) In an exemplary embodiment, the electrodeposition may be performed for 1 min or more and less than 20 min, specifically, 5 min or more and 15 min or less. When the electrodeposition is performed for less than 1 min, the IrO.sub.2 layer may not completely cover the Ti layer, and when the electrodeposition is performed for more than 10 min, cracks may be produced on the IrO.sub.2 layer.

(44) Thereafter, an electrolyte membrane may be stacked on the IrO.sub.2 layer of the oxygen electrode.

(45) In an exemplary embodiment, as the electrolyte membrane, for example, nafion, polybenzimidazole (PBI), Aquivion, or a polymer membrane thereof, and the like may be used. Further, when the electrolyte membrane is stacked, the electrolyte membrane may be stacked by using a typical technology which is not limited.

(46) Subsequently, an oxygen electrode-electrolyte membrane assembly is formed by performing a compression process on the oxygen electrode and the electrolyte membrane. Through the compression process, pressure is applied on the oxygen electrode and the electrolyte membrane, and as a result, the oxygen electrode and the electrolyte membrane may be compressed.

(47) When a compression process is performed on the oxygen electrode and the electrolyte membrane, the pore of the Ti layer may be partially filled with an electrolyte of the electrolyte membrane. Accordingly, the interface between electrode and electrolyte may be enlarged, and an additional interface between electrode and electrolyte may be provided.

(48) In an exemplary embodiment, the compression process may be performed under a temperature condition of 120 to 160° C. for 1 min to 5 min, and may be specifically performed under a temperature condition of 135 to 150° C. for 2 min to 4 min. When the temperature condition is out of the range, the penetration depth of the electrolyte of the electrolyte membrane into the pore of the Ti layer of the oxygen electrode becomes extremely small or large, so that the performance may not be sufficiently improved.

(49) In an exemplary embodiment, the compression process may be performed under a pressure condition of 4 to 20 MPa. The higher the pressure is, the larger the number of pores of the Ti layer of the oxygen electrode to be filled with an electrolyte of the electrolyte membrane is, and thus the active area may be increased, but when the pore is filled with the electrolyte as described above, the number of reaction sites with which water, which is a raw material for a proton exchange membrane water electrolyzer, may be reacted is decreased, so that only the pore of an upper zone adjacent to IrO.sub.2 in the Ti layer of the oxygen electrode is filled with the electrolyte of the electrolyte membrane.

(50) In an exemplary embodiment, the compression process may be performed under a pressure condition of 5 to 10 MPa.

(51) In an exemplary embodiment, the compression process may be a hot press process.

(52) Through the compression process, a portion of the pores of the Ti layer may be filled with the electrolyte of the electrolyte membrane. For example, the Ti layer is divided into an upper zone and a lower zone, and herein, the pore of the upper zone of the Ti layer may be filled with the electrolyte of the electrolyte membrane, whereas the pore of the lower zone of the Ti layer may not be filled with the electrolyte of the electrolyte membrane.

(53) In this case, the upper zone of the Ti layer may have a thickness (vertical) of 20 to 40 jam, for example, a thickness of 25 to 35 μm.

(54) When the upper zone of the Ti layer has a thickness of less than 20 μm, the interface between electrode and electrolyte is enlarged, and as a result, an effect in which the 3-phase boundary is enlarged may be slight, and when the upper zone of the Ti layer has a thickness of more than 40 μm, the number of reaction sites with which water, which is a raw material for a proton exchange membrane water electrolyzer, may be reacted is decreased, and as a result, the performance of the electrolyzer may deteriorate.

(55) Thereafter, a membrane electrode assembly is prepared by preparing a hydrogen electrode, and then assembling the hydrogen electrode with one surface of the exposed electrolyte membrane. In an exemplary embodiment, the hydrogen electrode may also include a diffusion layer and a hydrogen electrode catalyst layer (i.e., hydrogen electrode catalyst layer) formed on the diffusion layer. In an example, the hydrogen electrode may be prepared so as to have a configuration of Pt/C.

(56) Meanwhile, the hydrogen electrode needs to be assembled with an electrolyte membrane after the above-described compression process, and the reason is because a carbon material for a hydrogen electrode may be damaged when a hydrogen electrode and an oxygen electrode are placed with an electrolyte membrane therebetween, and then a compression process is performed as in FIG. 1D.

(57) In the present disclosure as described above, an oxygen electrode including the IrO.sub.2/Ti layer is prepared, and then a one-side compression process is performed by using an electrolyte membrane. Therefore, the pore of IrO.sub.2/Ti layer may be partially filled with an electrolyte by allowing the membrane electrolyte to penetrate into the Ti layer of the oxygen electrode. The average thickness of zone where the pore of the Ti layer is filled affects an active area, and within an optimal thickness range, the cell voltage required at 1.0 A/cm.sup.2 and 80° C. may be reduced from 1.78 V to 1.64 V. Therefore, the proton exchange membrane water electrolyzer may be widely used for water electrolyzers.

(58) Hereinafter, the present disclosure will be described in more detail through Examples. These Examples are only for exemplifying the present disclosure, and it will be obvious to those skilled in the art that the scope of the present disclosure is not interpreted to be limited by these Examples.

EXAMPLES

(59) Preparation Examples

(60) Preparation of IrO.sub.2/TP

(61) After a bath including an electrodeposition solution consisting of 10 mM iridium chloride hydrate (IrCi.sub.4.H.sub.2O), 100 mM hydrogen peroxide (H.sub.2O.sub.2), 40 mM oxalic acid (COOH.sub.2.2H.sub.2O), and 340 mM potassium carbonate (KCO.sub.3) was prepared, IrO.sub.2 catalyst was anodically electrodeposited. The bath was aged at least 3 days after electrodeposition for stabilization of Ir complex. Titanium paper (berkit, ST/Ti/20/450/60) with an active area of 6.25 cm.sup.2 was used as a working electrode, while Ti mesh and standard calomel electrode (SCE) were employed as counter and reference electrodes, respectively. Prior to electrodeposition, the working electrode was immersed into 5 wt % oxalic acid at 60° C. for 30 min and then rinsed with pure D.I. water. After then, the working electrode was transferred to a teflon cell, and potential of 0.7 V.sub.SCE was constantly applied for 10 min in room temperature.

(62) Preparation of IrO.sub.2/CP

(63) For comparison, IrO.sub.2 was electrodeposited on carbon paper with the same experimental conditions as in the preparation of IrO.sub.2/TP.

(64) Preparation of HP—IrO.sub.2/TP

(65) As described above, IrO.sub.2/TP was prepared by electrodepositing IrO.sub.2 onto TP. After then, HP—IrO.sub.2/TP (HP—IrO.sub.2/TP is denoted as P2 in FIG. 5A) was prepared by subjecting the electrolyte membrane and the oxygen electrode to hot press at 140° C. and 4 to 20 MPa for 2 min.

(66) Furthermore, in the following Example, HP—IrO.sub.2/TP was prepared by varying the pressure conditions during the hot press process. Among them, the case where pressure was adjusted to 4 MPa, 8 MPa, and 20 MPa was expressed as HP—IrO.sub.2/TP@4 MPa, HP—IrO.sub.2/TP@8 MPa, and HP—IrO.sub.2/TP@20 MPa, respectively.

(67) FIG. 3 shows a result comparing the surfaces of the oxygen electrode-electrolyte membrane assembly before and after a compression (hot press) process.

(68) In FIG. 3, IrO.sub.2/TP are depicted to have two parts, i.e., IrO.sub.2 electrodeposited TP part (1) and non-electrodeposited TP part (2). After a compression (hot press), M-E zone (3) where the pore of TP was partially filled with the membrane is formed in the IrO.sub.2 electrodeposited TP part (1).

(69) Preparation of HP′—IrO.sub.2/TP

(70) For comparison, an oxygen electrode and a hydrogen electrode were prepared, respectively, and then a nafion membrane (NR-212, Dupont) was placed in the middle thereof, and then hot-pressed. The oxygen electrode was prepared as described above, and the hydrogen electrode was prepared by spray-coating a catalyst ink composed of 46.5 wt % Pt/C (Johnson Matthey), 5 wt % nation solution, isopropyl alcohol (IPA, J. T. Baker), and deionized water on commercial carbon paper (39BC, SGL carbon). The Pt loading was 0.4 mg/cm.sup.2, the nafion content was 30 wt %, and the hot press process was performed at 140° C. and 4 to 20 MPa for 2 min. Therefore, MEA including HP′—IrO.sub.2/TP (HP′—IrO2/TP is denoted as P1 in FIG. 5A) was prepared.

(71) Analysis of IrO.sub.2/CP Electrode and IrO.sub.2/TP Electrode Characteristics

(72) All processes were carried out with potentiostat equipment (AUT302N, AUTO LAB Ltd.). IrO.sub.2 distribution at the electrode surface and IrO.sub.2 loading were examined with EPMA and inductively coupled plasma mass spectroscopy (ICP-MS) analysis, respectively.

(73) Preparation of Membrane Electrode Assembly and PEMWE Single Cell

(74) The above-described electrodes (IrO.sub.2/CP, IrO.sub.2/TP, HP—IrO.sub.2/TP, and the like) and the hydrogen electrode were prepared, respectively, and then MEA was prepared. The hydrogen electrode was prepared through the same process as described above. When HP—IrO.sub.2/TP was used, the MEA was prepared by placing HP—IrO.sub.2/TP and the hydrogen electrode (Pt/C) onto either sides of nafion membrane (NR-212, Dupont), and then assembling the electrodes with the nafion membrane. Subsequently, a single cell for a proton exchange membrane water electrolyzer (PEWME) was prepared by loading the MEAs on a home-made single cell that is equipped with graphite (cathode) and an Au/Ti bipolar plate (anode). At this time, an exposed active area of the MEA was fixed at 6.25 cm.sup.2.

(75) Analysis of Membrane Electrode Assembly and PEMWE Single Cell Characteristics

(76) iV curves and electrochemical impedance spectroscopy (EIS) results were obtained from the potentiostat (HCP-803, biologic) combined with a power booster. The constant voltages from 1.35 V to 2.0 V were applied for 1 min each with an interval of 0.05 V. D. I. water was fed into the anode side of the cell with a flow rate of 15 mL/min. Temperatures of the ionized water and the cell were fixed at 80° C.

(77) Results and Discussion

(78) Review of IrO.sub.2/TP and IrO.sub.2/CP Electrode Characteristics

(79) The surface morphology of an IrO.sub.2 film electrodeposited onto titanium paper (TP) at E.sub.dep of 0.7 V for 10 min is shown in FIGS. 4A to 4C. It could be confirmed that deposited IrO.sub.2 was uniformly distributed without any noticeable defect or agglomeration. A magnified image showed that the IrO.sub.2 particles with submicron diameter were formed on the cracked IrO.sub.2 films, providing a high surface area. Such morphology is similar to electrodeposited IrO.sub.2 film on carbon paper. The ICP-MS analysis results in FIG. 4D exhibited that the IrO.sub.2 loading was nearly proportional to t.sub.dep with a slope of 0.0148 mg/cm.sup.2.Math.min. It could be confirmed that for t.sub.dep of 10 min, the IrO.sub.2 loading reached 0.13 mg/cm.sup.2, which is almost consistent with those on CP (0.1 mg/cm.sup.2).

(80) Thereafter, the performance of a water electrolyzer including the electrodes was reviewed.

(81) FIG. 5A shows the iV curves of IrO.sub.2 electrodes (that is, IrO.sub.2/CP and IrO.sub.2/TP) deposited on CP and TP with E.sub.dep of 0.7 V for 10 min. Despite their similar IrO.sub.2 loading, it could be confirmed that the IrO.sub.2/TP electrode yielded lower voltage efficiency than IrO.sub.2/CP for all current density (However, there is a problem in that the IrO.sub.2/CP electrode could not be used because of its stability issue). The cell voltage at 1 A/cm.sup.2 for IrO.sub.2/TP was 1.78 V, which was 0.13 V higher than that for IrO.sub.2/CP (1.65 V). EIS results at 1.6 V revealed that both higher ohmic resistance (R.sub.ohm) as well as lower kinetics contributed to inferior performance of IrO.sub.2/TP (FIG. 5B). iV curves of HP—IrO.sub.2/TP and HP′—IrO.sub.2/TP electrodes are also presented in FIG. 5A (in FIG. 5A, HP—IrO.sub.2/TP is represented by P2 and HP′—IrO.sub.2/TP is represented by P1). It could be confirmed that HP′—IrO.sub.2/TP yielded an improvement in performance of electrode due to reduction of R.sub.kin, whereas in the case of HP—IrO.sub.2/TP, the compression process provides not only better kinetic but also the lower R.sub.ohm (FIG. 5B). The reduction of R.sub.kin indicated that the application of the compression process enlarged the interface between electrode and electrolyte, and thus 3-phase boundary. Enlarged interface might give lower R.sub.ohm as well by providing an additional interface for proton transfer, as shown in the case of HP—IrO.sub.2/TP. Meanwhile, when a cathode is prepared, and then a compression process is performed as in HP′—IrO.sub.2/TP, the carbon fiber structure at cathode side could be destructed, leading to an increase in R.sub.ohm. Therefore, it could be confirmed that a method of preparing an anode, and then performing a compression process with an electrolyte membrane, and then assembling a cathode with the anode and the electrolyte membrane is the most effective way to improve the cell performance.

(82) Experiments were carried out by varying pressure in the compression process performed when the HP—IrO.sub.2/TP was prepared, and the performance results are shown in FIG. 6. Further, the R.sub.ohm measured with high frequency intercept of EIS at 1.6 V is also shown in FIG. 6. When FIG. 6 is examined, R.sub.ohm was reduced by half (from 0.238 Ω.Math.cm.sup.2 at 0 MPa to 0.118 Ω.Math.cm.sup.2 at 20 MPa) when the applied pressure during the compression process exceeded 8 MPa. It could be confirmed that the increase in current density was observed at low voltage (1.6 V) and medium voltage (1.8 V) where the activation and ohmic losses are significant, and at high voltage (2.0 V), the highest current density was observed near 8 MPa, and the applying more pressure led to performance loss. This result implied that excessively high pressure during the compression process imposes positive impact on reaction kinetics and ohmic loss but is detrimental to mass transfer.

(83) As shown in FIG. 6, R.sub.ohm may be represented by a function of anode substrate and the applied pressure during hot press, and in order to clarify its origin, R.sub.ohm was represented by a sum of outer cell resistance (R.sub.O), membrane resistance (R.sub.m), cathode resistance (R.sub.c), anode resistance (R.sub.a), and interfacial resistance (R.sub.i,cm, R.sub.i,am, R.sub.i,cb, R.sub.i,ab) as presented at the following [Equation 1].
R.sub.ohm=R.sub.O+R.sub.m+R.sub.i,cm+R.sub.i,am+R.sub.c+R.sub.a+R.sub.i,cb+R.sub.i,ab  [Equation 1]

(84) FIG. 7A is a schematic view showing R.sub.ohm of a cell including an electrolyte membrane, and FIG. 7B is a schematic view showing R.sub.ohm of a membrane-free cell.

(85) When FIG. 7B is examined, the contributions of R.sub.O, R.sub.c, R.sub.a, R.sub.i,cb, and R.sub.i,ab are roughly approximated from the R.sub.ohm of the membrane-free cell with assuming that R.sub.i,ca is negligible. Meanwhile, the R.sub.m can be calculated from conductivity of a nafion membrane in fully wetted condition at 80° C. (0.16 S/cm.sup.2). Accordingly, when this is reflected, and R.sub.i,cm and R.sub.i,am are combined to represent Rim, the [Equation 1] may be represented by the following [Equation 2].
R.sub.ohm=0.031Ω.Math.cm.sup.2+R.sub.t+R.sub.i,m  [Equation 2]
The individual contribution of R.sub.t, R.sub.m and R.sub.i,m to total R.sub.ohm for various electrodes (IrO.sub.2/CP, IrO.sub.2/TP, HP—IrO.sub.2/TP(@4 MPa), HP—IrO.sub.2/TP(@8 MPa) and HP—IrO.sub.2/TP(@20 MPa) are shown in FIG. 7C. When TP was used instead of CP, two noticeable changes were accompanied; the increase in R.sub.t from 0.034 Ω.Math.cm.sup.2 to 0.078 Ω.Math.cm.sup.2 because of nonconductive TiO.sub.2 layers that was naturally formed on TP, and the increase in R.sub.i,m was increased from 0.028 Ω.Math.cm.sup.2 to 0.129 Ω.Math.cm.sup.2. It is believed that the difference in R.sub.i,m between CP and TP could be related with the roughness factor of substrate (CP: 35.7, TP: 20.0) that influences on the contact area between electrode and electrolyte. In addition, it was found that the application of hot press under a predetermined pressure condition significantly reduces the from 0.129 to 0.009 Ω/cm.sup.2, and in order to confirm the changes of electrode, the thicknesses of IrO.sub.2/TP, TP and a membrane were measured by performing a hot press process on IrO.sub.2/TP, TP and the membrane under various pressure conditions. Unlike TP and membrane which were not compressed, the thickness of the membrane-TP assembly formed after the hot press for IrO.sub.2/TP is significantly reduced, indicating the penetration of rigid TP body into the membrane electrolyte. The penetration depth was roughly proportional to applied pressure, and it could be confirmed that the compression process proceeded under a pressure condition of 4 MPa, 8 MPa, and 20 MPa resulted in the thickness reductions of 13.4, 20.4, and 37.2 μm, respectively. The inset of FIG. 8A schematically exhibited the assembly of TP and a membrane after the compression process, and it could be confirmed that when the TP body penetrated into the membrane, the zone (hereinafter, referred to as M-E zone) where the pore of TP was partially filled with the membrane was formed. The M-E zone provides an additional interface between electrode and electrolyte, which directly affects the R.sub.if and the 3-phase boundary, and the cross-sectional images and EPMA results of a membrane-TP assembly hot-pressed with 8 MPa are shown in FIGS. 8B and 8C. When FIGS. 8B and 8C are examined, it can be clearly seen that TP penetrates into the membrane and thus, the M-E zone is formed. An average vertical thickness of the M-E zone (T.sub.M-E) and the corresponding electrode/electrolyte contact area (Ac) can be represented by [Equation 3] and [Equation 4].

(86) $\begin{array}{cc}{t}_{M-E}=\Delta t_a/p& \left[\mathrm{Equation}3\right]\\ A_C=\frac{A_r}{A_{\mathrm{geo}}}*\frac{\Delta t_a}{{\mathrm{pt}}_{\mathrm{Ti}}}\cong \frac{4t_{\mathrm{Ti}}\left(1-p\right)}{D}*\frac{\Delta t_a}{{\mathrm{pt}}_{\mathrm{Ti}}}\cong \frac{4\Delta t_a\left(1-p\right)}{\mathrm{pD}}& \left[\mathrm{Equation}4\right]\end{array}$

(87) Where Δt.sub.a denotes the thickness change of IrO.sub.2/membrane assembly after a compression process, p denotes the porosity of TP, t.sub.Ti denotes the thickness of TP, and D is the diameter of Ti fiber. FIG. 9A shows a contact area corresponding to t.sub.M-E as a function of applied pressure. When 4 MPa, 8 MPa, and 20 MPa were applied, it could be confirmed that the average contact area was enlarged to 1.23, 2.16, and 4.40 cm.sup.2/cm.sup.2.sub.geo. With an assumption that the is inverse proportional to Ac, the correlation between R.sub.i,m and Ac can be expressed as Equation 5.

(88) $\begin{array}{cc}{R}_{i,m}=\frac{\sigma }{A_c}& \left[\mathrm{Equation}5\right]\end{array}$

(89) Meanwhile, FIG. 9B shows the R.sub.if as the function of 1/Ac, revealing the specific contact resistance (σ) to b e 0.024 Ω.Math.cm.sup.4.

(90) As described above, it could be confirmed that applying a hot press gave an additional electrode/electrolyte interface, but also caused disturbance of mass transfer. For generalization, electrodes were formed by electrodepositing IrO.sub.2 onto Ti papers having porosity of 56%, 60%, and 73%, and then the current density at 2.0 V was calculated as a function of t.sub.M-E (FIG. 10). The highest current density was found at t.sub.M-E of about 30 μm. Under that value, the limited electrode/electrolyte interface may be provided, and Rohm as well as R.sub.kin may be increased. When t.sub.M-E exceeded 30 μm, it seemed that the access of H.sub.2O to electrolyte/electrode interface was blocked by membrane scraps in the pore of TP. It was confirmed that the best performance might be obtained when the electrode/electrolyte contact and the mass transfer of water met the balance (for example, t.sub.M-E of 30 μm).

(91) Meanwhile, the performance of IrO.sub.2/TP electrode after the compression process is compared with various electrodes reported in the reference documents. As in FIG. 11, it can be confirmed that the current density of IrO.sub.2/TP electrode under 80° C. and 1.7 V conditions prior to hot press exhibited a value similar to the results of the reference document having a similar catalyst loading (Non-Patent Document 2), and the current density of IrO.sub.2/TP electrode under 80° C. and 1.7 V conditions after hot press exhibited a value similar to those of cells having a catalyst loading of 1 to 1.5 mg/cm.sup.2 (Non-Patent Documents 6, 7, and 8).

(92) In the present disclosure as described above, effects of the one-side compression process will be described in an electrodeposited IrO.sub.2 electrode for a proton exchange membrane water electrolyzer (PEMWE). An M-E zone where the pore of an IrO.sub.2/diffusion layer was partially filled with electrolyte was formed by stacking an electrolyte membrane on the upper surface of the IrO.sub.2/TP electrode, and then applying a compression process to allow the membrane electrolyte to penetrate into the IrO.sub.2/TP electrode. A high thickness of the M-E zone provides a better electrode/electrolyte interface, but limits the access of H.sub.2O. In an optimal M-E thickness, the cell voltage of the IrO.sub.2/TP electrode under 1.0 A/cm.sup.2 and 80° C. conditions was significantly reduced from 1.78 V to 1.64 V, and the value was comparable with those of a recent particle-type electrode with more catalyst loading of IrO.sub.2 (1 to 3 mg/cm.sup.2). Therefore, it could be confirmed that for a PEMWE, an excellent film-type electrode could be prepared with a small amount of PGM metal.

(93) The Examples of the present disclosure previously described should not be interpreted to limit the technical spirit of the present disclosure. The scope of the present disclosure to be protected is limited only by the matters described in the claims, and those skilled in the art of the present disclosure can improve and change the technical spirit of the present disclosure in various forms. Therefore, such improvements and changes would fall within the scope of the present disclosure to be protected as long as they are obvious to those skilled in the art.