Proton conductors and electrochemical devices

Abstract

To provide a proton conductor that can ensure high proton conductivity over a wide range of temperature and humidity. [Solution] The proton conductor is used in environments above 100°C. The proton conductor is a composite of polymerized zwitterions and phosphonic acid. Under high temperature and low humidity conditions, the phosphonic acid is partially anionized by the zwitterions and becomes a proton carrier. Therefore, high proton conductivity can be obtained even without humidity. Furthermore, under low temperature and high humidity conditions, proton conduction by phosphonic acid alone decreases according to the activation energy, but water is retained by the dipole of the zwitterion. Therefore, under high humidity conditions, protons are donated to water by the phosphonic acid, thus improving proton conductivity. Thus, it is possible to ensure high proton conductivity over a wide temperature and humidity range.

Description

[Technical Field]

[0001] This disclosure relates to proton conductors and electrochemical devices. [Background technology]

[0002] Proton conductors are solid materials through which protons can move, and are used in fuel cells, hydrogen sensors, hydrogen pumps, and other applications. Examples of proton conductors include proton-conducting polymers such as Nafion®. However, because Nafion® contains water as a proton carrier, its conductivity decreases significantly above 100°C when the water evaporates.

[0003] Incidentally, in fuel cells, temperatures exceeding 100°C are suitable for the catalyst on the electrodes. Furthermore, operating a fuel cell below 100°C reduces the temperature difference with the ambient air, requiring a large cooling system. From this perspective as well, it is desirable to operate fuel cells at temperatures exceeding 100°C. For this reason, in order to optimize fuel cell performance, there is a need for proton conductors that can operate at high temperatures and low humidity, i.e., even with the amount of water produced by the electrode reaction.

[0004] In contrast, Patent Document 1 discloses an electrolyte that can operate at high temperature and low humidity, which is an electrolyte combining a zwitterionic ionic liquid and a proton donor. However, because the electrolyte described in Patent Document 1 is a liquid, measures to prevent liquid leakage are necessary when manufacturing the fuel cell stack, and measures to prevent leaching into liquid water are also necessary.

[0005] In contrast, Patent Document 2 discloses an electrolyte obtained by impregnating polymerized zwitterions with sulfuric acid, hydrochloric acid, or trifluoromethanesulfonic acid. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] International Publication No. 2006 / 025482 [Patent Document 2] Patent No. 4431939 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, in the electrolyte described in Patent Document 2, the polymerized zwitterions are gel-like, which can lead to immobilization of the zwitterions and a decrease in proton conductivity. For this reason, it has been difficult to increase conductivity in low-humidity environments.

[0008] In view of the above points, this disclosure aims to provide a proton conductor that can ensure high proton conductivity over a wide range of temperature and humidity. [Means for solving the problem]

[0009] To achieve the above objective, the proton conductor described in claim 1 is a proton conductor used in an environment of 100°C or higher, It is a complex of polymerized zwitterions and phosphonic acid.

[0010] According to this, under high temperature and low humidity conditions, phosphonic acid is partially anionized by zwitterions, and the phosphonic acid anions become proton carriers. Therefore, high proton conductivity can be obtained even without humidification. Furthermore, under low temperature and high humidity conditions, proton conduction by phosphonic acid alone decreases according to the activation energy, but water is retained by the dipole of the zwitterion. Therefore, under high humidity, protons are donated to water by phosphonic acid, thus improving proton conductivity. Consequently, it is possible to ensure high proton conductivity over a wide temperature and humidity range. [Brief explanation of the drawing]

[0011] [Figure 1] This is a conceptual diagram of a fuel cell cell according to one embodiment. [Figure 2]This characteristic diagram shows the relationship between temperature and proton conductivity when the type of phosphonic acid contained in the proton conductor is changed. [Figure 3] This is a characteristic diagram showing the relationship between temperature and proton conductivity in the examples and comparative examples. [Figure 4] This is a characteristic diagram showing the relationship between the phosphonic acid group excess and proton conductivity. [Figure 5] This is a characteristic diagram showing the relationship between the excess phosphonic acid group ratio and the activation energy. [Figure 6] This is a characteristic diagram showing the relationship between proton conductivity and the use of polysulfobetaine and polypyridine as proton donors. [Modes for carrying out the invention]

[0012] Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. Note that at least one of the members or parts described with reference numerals is provided, unless otherwise specified as "one" or similar.

[0013] In this embodiment, the proton conductor of the present disclosure is applied to a fuel cell cell 100 of a fuel cell. The fuel cell cell 100 is an electrochemical device that transports protons in a predetermined proton transport direction.

[0014] The fuel cell cell 100 of this embodiment may reach temperatures exceeding 100°C during use. That is, the fuel cell cell 100 is used in an environment of 100°C or higher. In this specification, "used in an environment of 100°C or higher" does not mean that it is always used in an environment of 100°C or higher, but rather that it may be used in an environment of 100°C or higher.

[0015] As shown in FIG. 1, the fuel cell 100 includes a membrane electrode assembly (MEA) composed of an electrolyte membrane 110 and a pair of electrodes 120 and 130 sandwiching the electrolyte membrane 110. The pair of electrodes 120 and 130 consists of an anode electrode 120 and a cathode electrode 130. Note that the anode electrode 120 is also referred to as a hydrogen electrode, and the cathode electrode 130 is also referred to as an air electrode.

[0016] The fuel cell 100 outputs electrical energy by utilizing an electrochemical reaction between hydrogen and oxygen in the air. It can be used in a stack structure in which a plurality of fuel cells 100 are stacked as basic units. Hydrogen is the fuel gas, and oxygen in the air is the oxidant gas.

[0017] When hydrogen is supplied to the anode electrode 120 and air is supplied to the cathode electrode 130, as shown below, hydrogen and oxygen undergo an electrochemical reaction to output electrical energy.

[0018] (Anode electrode side) H2 → 2H ,

[0019] + 2e - (Cathode electrode side) 2H + + 1 / 2O2 + 2e - → H2O At this time, on the anode electrode 120, hydrogen is ionized into electrons (e - ) and protons (H + ) by a catalytic reaction, and the protons (H + ) move through the electrolyte membrane 110. On the other hand, on the cathode electrode 130, the protons (H + ) that have moved from the anode electrode 120 side, the electrons that have flowed in from the outside, and the oxygen (O2) in the air react to generate water (H2O).

[0019] The anode electrode 120 includes an anode-side catalyst layer 121 disposed in close contact with the anode-side surface of the electrolyte membrane 110, and an anode-side diffusion layer 122 disposed outside the anode-side catalyst layer 121. The cathode electrode 130 includes a cathode-side catalyst layer 131 disposed in close contact with the cathode-side surface of the electrolyte membrane 110, and a cathode-side diffusion layer 132 disposed outside the cathode-side catalyst layer 131. The diffusion layers 122 and 132 are formed of a carbon cloth or the like.

[0020] The catalyst layers 121 and 131 include catalysts 121a and 131a, and ionomers 121b and 131b that coat the catalysts 121a and 131a. As an example, in the present embodiment, as the catalysts 121a and 131a, catalyst-supported carbon on which a platinum catalyst that promotes an electrochemical reaction is supported on a carbon carrier is used. The platinum catalyst only needs to contain platinum, and for example, pure platinum or a platinum-cobalt alloy composed of platinum and cobalt can be used.

[0021] The ionomers 121b and 131b include a proton conductor and a polymer as a binder. As the binder, for example, polytetrafluoroethylene (PTFE) can be used.

[0022] The electrolyte membrane 110 is a proton conductor. The proton conductor constituting the electrolyte membrane 110 includes a polymer as a binder and a proton carrier that is a proton-conductive substance.

[0023] The proton conductor of the present embodiment is a complex of a polymerized zwitterion and phosphonic acid. As an example, in the present embodiment, as the zwitterion of the proton conductor, sulfobetaine in which the anion is SO3 - is used.

[0024] In the present embodiment, the proton carrier at a high temperature of 100 °C or higher is a phosphonic acid anion, and the proton carrier at a low temperature below 100 °C is water. Further, the proton donor is neutral phosphonic acid.

[0025] The proton conduction action in the proton conductor of this embodiment will be described in detail below.

[0026] Neutral phosphonic acid has two protons. Neutral phosphonic acid loses one proton to sulfobetaine, becoming a phosphonic acid anion. Then, the phosphonic acid anion accepts one proton from another adjacent neutral phosphonic acid to become a neutral phosphonic acid, but then transfers a proton to yet another phosphonic acid anion. In this way, protons are relayed (transferred) alternately between neutral phosphonic acid and phosphonic acid anions, thereby conducting protons within the electrolyte membrane 110. For this reason, in the proton conductor of this embodiment, the phosphonic acid anion functions as a proton carrier, and the neutral phosphonic acid functions as a proton donor.

[0027] Here, if the sulfonate groups of the sulfobetaine are greater than the phosphonic acid groups of the phosphonic acid, the sulfobetaine will accept one proton from every neutral phosphonic acid. As a result, all the neutral phosphonic acids will become phosphonic anions, and proton transfer will cease.

[0028] Therefore, it is desirable to have an excess of phosphonic acid groups in the ratio of sulfonate groups of sulfobetaine to phosphonic acid groups of phosphonic acid. In other words, it is desirable to have more phosphonic acid groups in phosphonic acid than sulfonate groups of sulfobetaine.

[0029] Furthermore, sulfobetaine has a dipole. Since water also has a dipole, water is attracted to sulfobetaine. As a result, sulfobetaine can retain water. Therefore, even in environments with little water, sulfobetaine can retain water, thereby increasing conductivity. Consequently, conductivity can be ensured even in low-humidity environments such as during startup.

[0030] As explained above, the fuel cell cell 100 of this embodiment uses a composite of polymerized zwitterionic sulfobetaine and phosphonic acid as a proton conductor. Under high temperature and low humidity conditions, the phosphonic acid is partially anionized by the sulfobetaine to form phosphonic acid anions, which then act as proton carriers. Therefore, high proton conductivity can be obtained even without humidification. Furthermore, under low temperature and high humidity conditions, proton conduction by phosphonic acid alone decreases according to the activation energy, but water is retained by the dipole of sulfobetaine. Therefore, under high humidity conditions, protons are donated to water by phosphonic acid, thereby improving proton conductivity. Consequently, it is possible to ensure high proton conductivity over a wide range of temperature and humidity.

[0031] Furthermore, in this embodiment, sulfobetaine is used as the zwitterion in the proton conductor. Sulfobetaine has excellent thermal stability and water retention properties among zwitterions, so it can more reliably improve proton conductivity.

[0032] Furthermore, in the proton conductor of this embodiment, the ratio of sulfonate groups of sulfobetaine to phosphonic acid groups of phosphonic acid is set to an excess of phosphonic acid groups. This prevents sulfobetaine from accepting one proton from all neutral phosphonic acid. As a result, protons are transferred more reliably between neutral phosphonic acid and phosphonic acid anions, thereby improving proton conductivity. [Examples]

[0033] The present disclosure will be described in more detail below with reference to examples, but the present disclosure is not limited to the examples described later.

[0034] (Procedure for preparing the evaluation film) P4VP (poly(4-vinylpyridine)) and propanesultone were dissolved in DMF (N,N-dimethylformamide) and refluxed at 90°C for 24 hours. After refluxing, the reaction mixture was washed with water, a phosphonic acid solution was added, and ultrasonic dispersion was performed. The ultrasonically dispersed solution was cast and dried in a vacuum at 150°C to prepare the evaluation film.

[0035] In the evaluation film of the example, HEDP (1-hydroxyethylidene 1,1-diphosphonic acid) was used as the phosphonic acid in the above-described film preparation procedure. As a result, in the evaluation film of the example, a composite of sulfobetaine and HEDP was used as the proton conductor.

[0036] In the comparative example's evaluation film, PTSA (p-toluenesulfonic acid), a sulfonic acid, was used instead of phosphonic acid in the above evaluation film preparation procedure. As a result, in the comparative example's evaluation film, a composite of sulfobetaine and PTSA was used as the proton conductor.

[0037] (Measurement of proton conductivity) The proton conductivity of each evaluation film was measured by varying the type of phosphonic acid used in the film. Specifically, the proton conductivity was measured for evaluation films using EPA (ethylphosphonic acid), PhPA (phenylphosphonic acid), or HEDP (1-hydroxyethylidene 1,1-diphosphonic acid). The results are shown in Figure 2.

[0038] In Figure 2, the solid line shows the results when using EPA, the dashed line shows the results when using HEDP, and the dotted line shows the results when using PhPA. Proton conductivity was measured both under unhumidified and humidified conditions (dew point temperature: 74.5°C).

[0039] As is clear from Figure 2, regardless of the type of phosphonic acid, the proton conductivity increased with increasing temperature in the absence of humidity. This indicates that protons are propagating through the alternating transfer of protons between neutral phosphonic acid and phosphonic acid anions. Hereafter, the state in which "protons are propagating through the alternating transfer of protons between neutral phosphonic acid and phosphonic acid anions" will also be referred to as "protons moving between phosphonic acids."

[0040] On the other hand, during humidification, particularly in the low-temperature range, the proton conductivity increased as the temperature decreased. This indicates that the sulfobetaine in the evaluated film contributes to the improvement of proton conductivity by retaining water.

[0041] (Comparison of proton conductivity) The proton conductivity was measured for the evaluation film of the example and the evaluation film of the comparative example. The results are shown in Figure 3. In Figure 3, the solid line shows the results of the example, and the dashed line shows the results of the comparative example. The proton conductivity was measured both under unhumidified and humidified conditions (dew point temperature: 74.5°C).

[0042] As is clear from Figure 3, the proton conductivity of the evaluation film in the examples was shown to be higher than that of the evaluation film in the comparative example. In particular, the difference between the proton conductivity of the evaluation film in the examples and the evaluation film in the comparative example was large at low humidity (i.e., without humidification). This is thought to be because, at low humidity, the phosphonic acid in the evaluation film in the examples functioned as a proton carrier, whereas the sulfonic acid in the evaluation film in the comparative example did not function as a proton carrier. In other words, the sulfonic acid contained in the evaluation film in the comparative example contributes to proton donation but does not accept protons, so proton conduction is difficult to achieve.

[0043] (Ratio of sulfobetaine to phosphonic acid) For the evaluation films in the examples, the proton conductivity and activation energy were measured by changing the charging ratio of sulfobetaine and phosphonic acid (HEDP). Figure 4 shows the measurement results for proton conductivity, and Figure 5 shows the measurement results for activation energy. Proton conductivity was measured both under unhumidified and humidified conditions (dew point temperature: 74.5°C). In Figures 4 and 5, the horizontal axis represents the excess ratio of phosphonic acid groups relative to the sulfonate groups of sulfobetaine.

[0044] As is clear from Figure 4, the proton conductivity changes depending on the charging ratio of sulfobetaine to phosphonic acid (i.e., the excess ratio of phosphonic acid groups), and it was shown that an optimal value exists.

[0045] As shown in Figure 5, when the activation energy is 0.6 eV or less, protons are propagated by moving between phosphonic acid groups. In the evaluation film of the examples, it was shown that if the excess ratio of phosphonic acid groups is 2 or more, protons can move between phosphonic acid groups stably.

[0046] In Figures 4 and 5, the proton conductivity is highest and the activation energy is lowest when the excess phosphonic acid group ratio is 4. However, when HEDP is used as the phosphonic acid as in this embodiment, the optimal value for the excess phosphonic acid group ratio varies depending on the degree of dissociation.

[0047] (Comparison between PSB and P4V4) The proton conductivity was measured for evaluation films using polysulfobetaine (PSB) as the substance that accepts protons from phosphonic acid, and for evaluation films using polypyridine (P4VP). The results are shown in Figure 6. The proton conductivity was measured both under unhumidified and humidified conditions (dew point temperature: 74.5°C).

[0048] As shown in Figure 6, the proton fraction of the evaluation film using PSB was higher than that of the evaluation film using P4VP, both under unhumidified and humidified conditions. This is thought to be because polypyridine can accept protons but does not release them once accepted, thus not contributing much to proton propagation. For this reason, the evaluation film using PSB has a higher proton conductivity under unhumidified conditions than the evaluation film using P4VP.

[0049] Furthermore, as mentioned above, sulfobetaine can retain water. Therefore, even when humidified, the evaluation film using PSB can retain water up to high temperatures, resulting in high proton conductivity in the high-temperature range.

[0050] (Other embodiments) This disclosure is not limited to the embodiments described above, and can be modified in various ways without departing from the spirit of this disclosure, as follows.

[0051] For example, in the above embodiment, an example was described in which a proton conductor was applied to a fuel cell cell 100 of a fuel cell, but the application of the proton conductor is not limited to this embodiment. For example, a proton conductor may be applied to the electrolyte membrane of a water electrolysis device, which is an electrochemical device.

Claims

1. A proton conductor used in environments above 100°C, A proton conductor is a complex of polymerized zwitterions and phosphonic acid.

2. The aforementioned zwitterion has an anion SO ３ － The proton conductor according to claim 1, which is sulfobetaine.

3. The proton conductor according to claim 2, wherein the ratio of sulfonate groups of the sulfobetaine to phosphonic acid groups of the phosphonic acid is in excess of phosphonic acid groups.

4. An electrochemical device comprising any one of claims 1 to 3, which is a proton conductor.

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