A method for preparing an electrode for evaluating oxygen reduction activity in a fuel cell catalyst half-cell

Abstract

This invention provides a method for preparing an electrode to evaluate the oxygen reduction activity in a half-cell of a fuel cell catalyst, comprising the following steps: dispersing a catalyst, ultrapure water, alcohol, liquid acid, and a proton-conducting solution to form a stable and uniform catalyst ink; coating the catalyst ink onto a transfer intermediate plate and drying it to form a laminate consisting of a catalyst layer and the transfer intermediate plate; laminating the obtained laminate with an electrolyte membrane and hot-pressing it; removing the transfer intermediate plate to obtain an electrolyte membrane covered with a catalyst layer on one side, i.e., a catalyst-coated membrane; cutting the obtained catalyst-coated membrane to the required size; cutting a gas diffusion layer to the required size; and hot-pressing a microporous layer onto the cut catalyst-coated membrane to obtain the electrode. The electrode preparation method of this invention is simple and easy to implement, suitable for scale-up production, and consistent with actual production processes. The half-cell evaluation patterns can be directly correlated with the actual membrane electrode production process, shortening the R&D cycle and saving R&D costs.

Description

Technical Field

[0001] This invention belongs to the field of fuel cell technology, specifically relating to an electrode preparation method for evaluating the oxygen reduction activity in a fuel cell catalyst half-cell. Background Technology

[0002] Proton exchange membrane fuel cells (PEMFCs) are high-efficiency devices that are not limited by the Carnot cycle of internal combustion engines. They directly convert the chemical energy in fuel hydrogen (pure hydrogen or reformed gas) and oxidant (pure oxygen or air) into electrical energy, and their only byproduct is water, making them very environmentally friendly. Therefore, PEMFCs are considered one of the most promising clean energy sources and are being widely promoted in the field of vehicle energy. They help my country achieve its goals of energy conservation, emission reduction, and low-carbon environmental protection.

[0003] For the development of any product, evaluation methods are crucial. Currently, there are two main methods commonly used for evaluating the performance of PEMFC catalysts: rotating disk electrode (RDE) measurement (half-cell) and direct measurement of single cells (full cell) using membrane electrode assemblies. RDE evaluation methods use very few catalysts, have well-developed schemes and methods, low test bench costs, short evaluation time, and low energy consumption. However, the catalyst environment of RDE differs from that of actual fuel cell catalysts; for example, it can only be conducted in dilute acid and at low temperatures. Furthermore, the reactant gases participate in the reaction by dissolving in the liquid electrolyte, and their concentration is limited by solubility, resulting in extremely low reaction rates. Therefore, in practical development, a persistent problem for researchers is the mismatch between the results of RDE evaluation (half-cell) and single-cell evaluation (full cell), with no discernible pattern. The results of half-cell evaluation cannot reflect the catalyst's performance in real-world operating environments, leading to a significant amount of evaluation work being devoted to single-cell testing. However, single-cell test benches are expensive and time-consuming (depending on the activation method, activation and simple polarization performance measurement of a single sample can take 3-72 hours). Fabrication of membrane electrodes requires a large amount of catalyst, resulting in high energy consumption and labor costs. Patent CN111257385A discloses a device and method for evaluating the oxygen reduction activity of a gas diffusion electrode. Although it fabricates a gas diffusion electrode, increases the gas transport process, and better constructs the three-phase reaction interface of the catalyst, avoiding direct contact between the catalyst and the liquid electrolyte, making the experiment closer to actual conditions and the test results more accurate and reliable, it still uses half-cell measurements.

[0004] Extensive research revealed that existing methods for preparing gas diffusion electrodes for evaluating catalyst oxygen reduction activity primarily involve coating a catalyst slurry onto a cathode gas diffusion layer (CGDL) using methods such as spraying, sputtering, or electrochemical deposition. This slurry is then hot-pressed onto a proton exchange membrane (PEM) to form a stacked electrode structure with the required reaction area, consisting of a cathode gas diffusion layer, a cathode catalyst layer, and a proton exchange membrane (this method is generally called the GDE method). The electrode is then assembled into a device similar to that described in CN111257385A, and the catalyst activity is tested using a classic three-electrode electrochemical method with cyclic voltammetry and oxygen reduction curves. While these electrode preparation methods are simple and easy to implement, the process of coating the catalyst slurry onto the GDL results in insufficient bonding between the catalyst layer and the PEM, leading to increased contact resistance. Furthermore, the GDE method is not suitable for large-scale production and is almost obsolete. This inconsistency between the electrode preparation method used for half-cell evaluation and the production line electrode preparation method results in biased evaluation results, making it difficult to accurately correlate the evaluation patterns of the half-cell with the actual scaled-up electrode process.

[0005] Therefore, it is necessary to develop a method for preparing an electrode for evaluating oxygen reduction activity in catalyst half-cells that is easy to scale up for production. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing technologies and provide an electrode preparation method for evaluating the oxygen reduction activity in the half-cell of a fuel cell catalyst. This invention employs a thermal transfer method during electrode preparation. By adjusting slurry parameters such as the I / C ratio, slurry pH, and slurry viscosity, as well as thermal transfer parameters such as transfer temperature, pressure, and time, the bonding force between the catalyst layer and the proton exchange membrane, and the microstructure of the electrode, can be adjusted. This facilitates the preparation of a well-structured and high-performance electrode, constructs a good three-phase interface reaction zone, and avoids interference from differences in electrode preparation on the catalyst's evaluation structure, making the evaluation results more accurate and reliable.

[0007] To achieve the above technical objectives, the technical solution adopted in the embodiments of the present invention is as follows:

[0008] A method for preparing an electrode for evaluating the oxygen reduction activity in a fuel cell catalyst half-cell includes the following steps:

[0009] (1) A certain amount of conductive and catalytically active catalyst, ultrapure water, alcohol, liquid acid, and proton-conducting solution are added sequentially to the centrifuge tube to form a slurry;

[0010] (2) The slurry in step (1) is dispersed to form a stable and uniform catalyst ink, wherein the solid content of the catalyst ink is 1%-20%;

[0011] (3) The catalyst ink from step (2) is coated onto the transfer intermediate plate and dried at 40-120°C to form a laminate consisting of a catalyst layer and a transfer intermediate plate.

[0012] (4) Hot pressing process: The laminate obtained in step (3) is stacked with the electrolyte membrane and hot pressed to remove the transfer intermediate plate, thereby obtaining an electrolyte membrane covered with a single-sided catalyst layer, namely a catalyst coated membrane (CCM).

[0013] (5) Cut the catalyst coating film obtained in step (4) into the required size;

[0014] (6) Cut the gas diffusion layer (GDL) to the required size and heat-press it with the catalyst coating film cut in step (5) to obtain the electrode.

[0015] Further, the catalyst mentioned in step (1) is one or more of the following: platinum / carbon catalyst, platinum alloy / carbon catalyst, and non-platinum / carbon catalyst;

[0016] The non-platinum / carbon catalyst is one or more of the following: transition metal-nitrogen-carbon catalyst, transition metal oxide-carbon catalyst, transition metal sulfide-carbon catalyst, transition metal nitride-carbon catalyst, and transition metal oxynitrogen compound.

[0017] Further, the alcohol mentioned in step (1) is one or more of a monohydric alcohol or a polyhydric alcohol; the volume ratio of water to alcohol is 1:99~99:1.

[0018] Further, the proton-conducting solution mentioned in step (1) is a resin solution. Preferably, the proton-conducting solution is a Nafion solution.

[0019] Further, the liquid acid in step (1) is an organic acid or an inorganic acid, the organic acid is citric acid and / or glacial acetic acid, the inorganic acid is one or more of phosphoric acid, nitric acid, and perchloric acid, and the amount of liquid acid added ensures that the pH value of the slurry in step (1) is 1-5.

[0020] Further, the I / C ratio in step (1) is 0.1-2.8, where the I / C ratio refers to the mass ratio of the resin in the proton-conducting solution to the carbon in the catalyst.

[0021] Furthermore, in step (2), one or more of the following methods are used for dispersion: stirring, ultrasonic cell disruption, and homogenization;

[0022] In step (3), the catalyst ink is coated onto the transfer intermediate plate by one of the following methods: brushing, screen printing, scraping, and spraying.

[0023] Further, the transfer intermediate plate in step (3) is a metal material or resin, wherein the metal material is one of stainless steel, iron, titanium, aluminum, tin, and copper, and the resin is polytetrafluoroethylene (PTFE) or polyimide (PI).

[0024] Furthermore, the hot pressing temperature in step (4) is higher than the softening point of the electrolyte membrane. Preferably, the hot pressing temperature is 80-160°C, the hot pressing pressure is 5-150 bar, and the hot pressing time is 1-60 minutes. The above hot pressing pressure can ensure that the catalyst layer and the proton exchange membrane are bonded firmly, and will not cause the catalyst layer to over-bite into the electrolyte membrane. The above hot pressing time can achieve a better transfer effect.

[0025] Furthermore, the thickness of the catalyst coating film in step (4) is 8-40 μm, and the noble metal loading in the catalyst coating film is 0-0.1 mg / cm³. 2 When a non-precious metal catalyst is used, the loading of precious metals in the catalyst coating film is 0.

[0026] The beneficial effects of the technical solution provided by the embodiments of the present invention are as follows:

[0027] 1. The electrode preparation method of the present invention is simple and easy to implement, easy to scale up for production, and consistent with the actual production process. The half-cell evaluation rules can be directly linked to the actual membrane electrode production process, which can shorten the research and development cycle and save research and development costs.

[0028] 2. This invention employs a thermal transfer method during electrode preparation. By adjusting slurry parameters such as I / C ratio, slurry pH, and slurry viscosity, as well as thermal transfer parameters such as transfer temperature, pressure, and time, the bonding force between the catalyst layer and the proton exchange membrane, as well as the microstructure of the electrode, can be adjusted. This facilitates the preparation of an electrode with a good structure and excellent performance, constructs a good three-phase interface reaction zone, avoids interference from differences in electrode preparation on the evaluation structure of the catalyst, and makes the evaluation results more accurate and reliable.

[0029] 3. The electrode reaction region prepared by the method of the present invention is controllable in size and can be adjusted according to the experimental purpose. For example, if you want to verify the activity and consistency of a catalyst in half-cell and full-cell at the same time, you can prepare more slurry and prepare a large electrode at the same time, and then cut it according to the needs, thus avoiding the experimental error introduced by multiple sample preparations. Attached Figure Description

[0030] Figure 1 This is a schematic cross-sectional view of one morphological example of the electrode obtained according to an embodiment of the present invention.

[0031] Figure 2 This is a schematic diagram of one form of the fixture used in the hot pressing process of this invention.

[0032] Figure 3 This is a comparison chart of the cyclic voltammetry (CV) curves of the electrodes in Embodiments 1-2 of the present invention.

[0033] Figure 4 This is a comparison diagram of the polarization curves (Pol) of the electrodes in Embodiment 1 and Comparative Example 1 of the present invention.

[0034] Explanation of reference numerals in the attached figures: 10-proton exchange membrane; 11-catalyst layer; 12-gas diffusion layer; 100-clamp; 110-pressure plate; 120-buffer material; 130-transfer intermediate plate; 140-protective film. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0036] Example 1

[0037] A method for preparing an electrode for evaluating the oxygen reduction activity in a fuel cell catalyst half-cell includes the following steps:

[0038] (1) Weigh 4 mg of Johnson Matthey JM9100 catalyst, add 75 μL of deionized water, 15 μL of ethanol, 5 μL of 0.1 M acetic acid, and 10 μL of 5% Nafion solution in sequence, and ultrasonically disperse for 30 min to obtain a uniformly dispersed catalyst ink.

[0039] (2) The catalyst ink was coated onto a 3cm*3cm PI film in four layers using a wire rod, and then dried at 80℃.

[0040] (3) After drying, cover the PI membrane with the Gore M765.12 proton exchange membrane and press it onto the membrane. Figure 2 The assembled fixture was placed on a hot press and subjected to hot pressing at 150°C and 30 bar for 6 minutes to transfer the catalyst layer onto the proton exchange membrane. The PI membrane was then peeled off to obtain a Pt loading of 40 μg / cm³. 2 CCM (Pt load was measured by XRF);

[0041] (4) Cut the CCM to 1.4cm*1.4cm, and use a tooling to hot-press it onto a pre-cut gas diffusion layer (Frederick H24CX483) with dimensions of 1.2cm*1.1cm to obtain the required electrode, such as Figure 1 As shown, electrode 20 includes a proton exchange membrane 10, a catalyst layer 11, and a gas diffusion layer 12 arranged sequentially.

[0042] The electrode was assembled into the specialized device described in CN111257385A, forming a reaction area of ​​1.3 cm². 2 The working electrode.

[0043] Example 2

[0044] A method for preparing an electrode for evaluating the oxygen reduction activity in a fuel cell catalyst half-cell includes the following steps:

[0045] (1) Weigh 8 mg of JM9100 catalyst, add 75 μL of deionized water, 15 μL of ethanol, 5 μL of 0.1 M acetic acid, and 10 μL of 5% Nafion solution in sequence, and ultrasonically disperse for 30 min to obtain a uniformly dispersed catalyst ink.

[0046] (2) The catalyst ink was coated onto a 3cm*3cm PI film in four layers using a wire rod, and then dried at 80℃.

[0047] (3) After drying, cover the PI membrane with the Gore M765.12 proton exchange membrane and press it onto the membrane. Figure 2 The assembled fixture was placed on a hot press and hot-pressed at 150℃, 30 bar, and for 6 minutes to transfer the catalyst layer onto the proton exchange membrane. The PI membrane was then peeled off, yielding a Pt loading of 80 μg / cm³. 2 CCM.

[0048] (4) Cut the CCM to 1.4cm*1.4cm, and use a tooling to hot-press it onto a pre-cut gas diffusion layer (Frederick H24CX483) with dimensions of 1.2cm*1.1cm to obtain the required electrode, such as Figure 1 As shown, electrode 20 includes a proton exchange membrane 10, a catalyst layer 11, and a gas diffusion layer 12 arranged sequentially.

[0049] The electrode was assembled into a specialized device as described in CN111257385A, forming a reaction area of ​​1.3 cm². 2 The working electrode.

[0050] Example 3

[0051] A method for preparing an electrode for evaluating the oxygen reduction activity in a fuel cell catalyst half-cell includes the following steps:

[0052] (1) Weigh 4 mg of Johnson Matthey JM9100 catalyst, add 75 μL of deionized water, 15 μL of ethanol, 5 μL of 0.1 M acetic acid and 10 μL of 5% Nafion solution in sequence, and sonicate for 30 min to obtain a uniformly dispersed catalyst ink.

[0053] (2) The catalyst ink was coated onto a 3cm*3cm PI film in four layers using a wire rod, and then dried at 80℃.

[0054] (3) After drying, cover the PI membrane with the Gore M765.12 proton exchange membrane and press it onto the membrane. Figure 2 The assembled fixture was placed on a hot press and hot-pressed at 100℃, 110 bar, and for 6 minutes to transfer the catalyst layer onto the proton exchange membrane. The PI membrane was then peeled off, yielding a Pt loading of 40 μg / cm³. 2 CCM (Pt load was measured by XRF);

[0055] (4) Cut the CCM to 1.4cm*1.4cm, and use a tooling to hot-press it onto the cut Freudenberg H24CX483 electrode with dimensions of 1.2cm*1.1cm to obtain the required electrode, such as Figure 1 As shown, electrode 20 includes a proton exchange membrane 10, a catalyst layer 11, and a gas diffusion layer 12 arranged sequentially.

[0056] The electrode was assembled into a specialized device as described in CN111257385A, forming a reaction area of ​​1.3 cm². 2 The working electrode.

[0057] Figure 2 This is a schematic diagram of a type of fixture used in a hot pressing process. The fixture 100 transfers the catalyst layer 11 from the transfer intermediate plate 130 to the electrolyte membrane 10 by hot pressing with a press, thereby forming a CCM.

[0058] Figure 2 The fixture 100 uses two pressure plates 110 (e.g., stainless steel plates) to hot-press the catalyst layer 11 and the electrolyte membrane (e.g., PEM) 10 formed on the transfer intermediate plate (e.g., PI) 130. Preferably, the fixture 100 also includes a cushioning material (e.g., flexible graphite plate) 120 and a protective film (e.g., PTFE) 140, wherein the protective film (e.g., PTFE) 140 protects the electrolyte membrane (e.g., PEM) 10 from being contaminated, and the cushioning material (e.g., flexible graphite plate) 120 acts as a buffer to prevent the structure of the electrolyte membrane (e.g., PEM) 10 from being damaged.

[0059] To better understand the difference between the new evaluation method involved in the embodiments of the present invention and the traditional RDE method, a catalyst layer was prepared on a glassy carbon electrode, and the activity of the catalyst was evaluated using a traditional three-electrode evaluation method. The electrode preparation method is shown in Comparative Example 1.

[0060] Comparative Example 1

[0061] (1) Weigh 2 mg of Johnson Matthey JM9100 catalyst, add 112.5 μL of deionized water, 22.5 μL of ethanol, 7.5 mL of 0.1 M acetic acid, and 15 μL of 5% Nafion solution in sequence, and ultrasonically disperse for 30 min to obtain a uniformly dispersed catalyst ink.

[0062] (2) Measure 10.3 μL of the above-mentioned uniformly dispersed catalyst ink and add it dropwise in three portions using a pipette until the active area is 1.96 cm². 2 The reaction was dried on a glassy carbon electrode, resulting in a reaction area of ​​1.96 cm². 2 The Pt loading capacity is 40 μg / cm³. 2 The working electrode.

[0063] To better understand the present invention, the electrodes prepared in Examples 1 and 2 were subjected to cyclic voltammetry tests, and the oxygen reduction polarization curves of Examples 1 and Comparative Example 1 were tested.

[0064] Specifically, the above-mentioned test conditions are: room temperature, nitrogen or oxygen with a purity of not less than 99.99%, 100% humidified gas, high flow rate, and the amount of gas introduced is much greater than the amount of gas consumed in the reaction. The purpose of the high flow rate is to deduct the influence of gas transmission resistance. The high flow rate means the highest possible flow rate without damaging the sample. In the embodiments of this invention, the gas flow rate is 300 mL / min and the sulfuric acid concentration is 0.5 M.

[0065] More specifically, cyclic voltammetry testing: The gas inlet channel of the testing device was connected to a nitrogen cylinder, and high-purity nitrogen was introduced for 30 minutes. The electrochemical workstation was started, and cyclic voltammetry curves of the electrodes prepared in Examples 1 and 2 were obtained at a scan rate of 50 mV / s. The total duration of each test was controlled within 45 minutes. Specific results are as follows: Figure 3 As shown in Figure 3, the Pt loading is calculated to be 40 μg / cm³. 2 and 80μg / cm 2 The electrochemical active areas of the catalysts are 26m². 2 / g and 56 m 2 / g, thus it can be seen that 80μg / cm 2 The catalyst utilization rate is higher with increased loading, which is consistent with existing observational patterns.

[0066] More specifically, oxygen reduction polarization curve testing: The gas inlet channel of the testing device was connected to an oxygen cylinder, and high-purity oxygen was introduced for 30 minutes. The electrochemical workstation was then started to obtain the oxygen reduction polarization curves of Example 1 and Comparative Example 1. The total duration of each test was controlled within 45 minutes. Specific results are shown in [link to results]. Figure 4 .from Figure 4It can be seen that the current density achievable by the new evaluation method (Example 1) is much higher than that of the traditional RDE method (Comparative Example 1). This is because the mass transfer conditions of the new evaluation method are closer to real-world conditions, allowing for the measurement of a higher current density. In contrast, the traditional RDE method suffers from extremely low current density due to the very low solubility of oxygen in the liquid electrolyte, resulting in significant mass transfer resistance. A typical example is comparing the current density at 0.9 V / VRHE to evaluate catalyst activity. However, the actual operating voltage range of PEMFCs is 0.6-0.8 V. Figure 4 The advantages of the new assessment method are obvious.

[0067] The electrode provided by the present invention is not limited to Figure 1 The morphology shown, for example, the catalytic layer of the electrode can be a single-layer structure or a multi-layer structure.

[0068] Furthermore, this invention is described with a focus on electrodes for fuel cell half-cells in this specification, but the electrode manufacturing method of this invention is not limited to the field of fuel cells, and can also be applied to the manufacture of electrodes for other fields.

[0069] Finally, it should be noted that the above specific embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to examples, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing an electrode to evaluate the oxygen reduction activity in a half-cell of a fuel cell catalyst, characterized in that, Includes the following steps: (1) A certain amount of conductive and catalytically active catalyst, ultrapure water, alcohol, liquid acid, and proton-conducting solution are added sequentially to the centrifuge tube to form a slurry; (2) The slurry in step (1) is dispersed to form a stable and uniform catalyst ink, wherein the solid content of the catalyst ink is 1%-20%; (3) The catalyst ink from step (2) is coated onto the transfer intermediate plate and dried at 40-120°C to form a laminate consisting of a catalyst layer and a transfer intermediate plate. (4) Hot pressing process: The laminate obtained in step (3) is stacked with the electrolyte membrane and hot pressed to remove the transfer intermediate plate, thereby obtaining an electrolyte membrane covered with a single-sided catalyst layer, i.e., a catalyst coating membrane. (5) Cut the catalyst coating film obtained in step (4) into the required size; (6) Cut the gas diffusion layer to the required size and hot-press it with the catalyst coating film cut in step (5) to obtain the electrode; The volume ratio of water to alcohol in step (1) is 1:99~99:1, the I / C ratio in step (1) is 0.1-2.8, and the amount of liquid acid added ensures that the pH value of the slurry in step (1) is 1-5; The hot pressing temperature in step (4) is higher than the softening point of the electrolyte membrane, the hot pressing pressure is 5-150 bar, and the hot pressing time is 1-60 minutes. The thickness of the catalyst coating film in step (4) is 8-40 μm, and the noble metal loading in the catalyst coating film is 0-0.1 mg / cm³. 2 .

2. The electrode preparation method for evaluating the oxygen reduction activity in a fuel cell catalyst half-cell according to claim 1, characterized in that, The catalyst mentioned in step (1) is one or more of the following: platinum / carbon catalyst, platinum alloy / carbon catalyst, and non-platinum / carbon catalyst; The non-platinum / carbon catalyst is one or more of the following: transition metal-nitrogen-carbon catalyst, transition metal oxide-carbon catalyst, transition metal sulfide-carbon catalyst, transition metal nitride-carbon catalyst, and transition metal oxynitrogen compound.

3. The electrode preparation method for evaluating the oxygen reduction activity in a half-cell of a fuel cell catalyst according to claim 1, characterized in that, The alcohol mentioned in step (1) is one or more monohydric alcohols or a mixture of polyhydric alcohols.

4. The electrode preparation method for evaluating the oxygen reduction activity in a half-cell of a fuel cell catalyst according to claim 1, characterized in that, The proton-conducting solution mentioned in step (1) is a resin solution.

5. The electrode preparation method for evaluating the oxygen reduction activity in a half-cell of a fuel cell catalyst according to claim 1, characterized in that, The liquid acid mentioned in step (1) is an organic acid or an inorganic acid. The organic acid is citric acid and / or glacial acetic acid, and the inorganic acid is one or more of phosphoric acid, nitric acid, and perchloric acid.

6. The electrode preparation method for evaluating the oxygen reduction activity in a half-cell of a fuel cell catalyst according to claim 1, characterized in that, In step (2), one or more of the following methods are used for dispersion: stirring, ultrasonic cell disruption, and homogenization; In step (3), the catalyst ink is coated onto the transfer intermediate plate by one of the following methods: brushing, screen printing, scraping, and spraying.

7. The electrode preparation method for evaluating the oxygen reduction activity in a half-cell of a fuel cell catalyst according to claim 1, characterized in that, The transfer intermediate plate mentioned in step (3) is a metal material or resin, wherein the metal material is one of stainless steel, iron, titanium, aluminum, tin and copper, and the resin is polytetrafluoroethylene PTFE or polyimide PI.

8. The electrode preparation method for evaluating the oxygen reduction activity in a half-cell of a fuel cell catalyst according to claim 1, characterized in that, In step (4), the hot pressing temperature is 80-160℃.

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