Control method and control system for stop mode of a fuel cell

Abstract

A method for controlling a stop mode of a fuel cell (10), the method comprising: Calculating (S100) a degree of deterioration of the fuel cell (10); Determining (S200) a stop voltage of the fuel cell (10) according to the calculated degree of deterioration; Determining (S300) whether the fuel cell (10) has entered the stop mode; and Controlling (S400) an output voltage of the fuel cell (10) to be the determined stop voltage when it is determined that the fuel cell (10) has entered the stop mode, wherein, when calculating a degree of deterioration, the degree of deterioration of the fuel cell (10) is calculated using an oxidation degree of a cathode catalyst of the fuel cell (10) according to the following equations: d θ P t O xdt = k P t O x ( ( 1 − θ P t O x ) exp ( α ' α FRT η P t O x ) − θ P t O x exp ( − α ' c FRT η P t O x ) ) η P t O x = Φ C − Φ ion − UP t O x with θ Ptox : Oxidation degree of the platinum catalyst of the cathode (0~1) n Ptox : Potential difference at cathode k Ptox : Reaction rate for PtOx formation α' α , α' c : Anodic and cathodic transfer coefficient for PtOx formation U Ptox : PtOx equilibrium potential ϕ c : Measured voltage of the fuel cell cell (average for a large number of cells) ϕ ion : Potential loss of the electrolytic membrane U PtOX : equilibrium voltage F: Faraday constant R: Ideal gas constant T: Temperature (K).

Description

BACKGROUND OF THE INVENTIONField of the InventionThe present invention relates to a control method and system for a stop mode of a fuel cell, the method and system being capable of improving durability and operability by varying a stop voltage in a stop mode of a fuel cell according to the degree of deterioration of the fuel cell.DESCRIPTION OF THE RELATED ARTThe specifics described as related to the present invention herein are merely for aiding in the understanding of the background of the present invention and should not be construed as belonging to the prior art already known to those skilled in the art.A fuel cell, which is a type of device that converts chemical energy from a fuel into electric energy by an electrochemical reaction in a fuel cell stack without converting the chemical energy into heat by burning the fuel, can be used not only for power supply for industry, households, and vehicles, but also for power supply for small-sized electric / electronic products, particularly mobile devices.When a fuel cell is stopped or the required power is a predetermined value or less, it is necessary to stop supplying a reactive gas to the fuel cell, but the fuel cell outputs power by a reaction of the gases remaining in a reaction layer, and therefore the electrodes of the fuel cell are exposed to a high potential.However, when an electrode, particularly the platinum (Pt) catalyst at the cathode is exposed to a high potential near an open circuit voltage (OCV), the catalyst oxidizes by reacting with the oxygen in the supplied air or the moisture in the moist air. The oxidized catalyst does not react in the fuel cell, and therefore, the performance of the fuel cell is deteriorated, causing an output power decrease of the fuel cell when the fuel cell is operated with the current in this case, resulting in a decrease in system efficiency.For this problem, a technology has been disclosed in the art that sets an upper limit voltage lower than the OCV and controls a fuel cell below the upper limit voltage in a stop mode to reduce a decrease in performance of the electrodes at a high potential.However, the setting of an upper limit voltage is not sufficient to compensate for the deterioration, and catalyst separation is caused by the deterioration, which causes permanent deterioration of the performance.Furthermore, DE 11 2008 00 096 T5 discloses a fuel cell system comprising a fuel cell which receives a supplied reaction gas for power generation, and a control device which, when a power demand for the fuel cell is less than a predetermined value, controllably stops the supply of the reaction gas to the fuel cell and maintains an output voltage of the fuel cell equal to a high potential avoidance voltage which is lower than an open circuit voltage, and, when the power demand for the fuel cell is equal to or greater than a predetermined value, controls the output voltage of the fuel cell, wherein the high potential avoidance voltage is set as an upper limit value.The foregoing is intended merely to aid in understanding the background of the present invention and is not intended to mean that the present invention is within the scope of the related art that is already known to one of skill in the art.SUMMARY OF THE INVENTIONThe present invention has been made in an effort to solve the problems, and an object of the present invention is to provide a method and a system for controlling a fuel cell to maintain a stop voltage determined based on the degree of deterioration of an electrode.In order to achieve the above-mentioned object, according to an aspect of the present invention, there is provided a method of controlling a stop mode of a fuel cell, the method including: calculating a degree of deterioration of the fuel cell; determining a stop voltage of the fuel cell according to the calculated degree of deterioration; determining whether the fuel cell has entered the stop mode; and controlling an output voltage of the fuel cell to be the determined stop voltage when it is determined that the fuel cell has entered the stop mode, wherein in calculating a degree of deterioration, the degree of oxidation of the fuel cell is calculated using a degree of deterioration of a cathode catalyst of the fuel cell by equations:θ Ptox: Oxidation Degree of Platinum Catalyst of Cathode (0-1)η Ptox: potential difference at cathodek Ptox: Reaction Rate for PtOx Formationα'α, α' c: Anodic and Cathodic Transfer Coefficient for PtOxformationU PtOx: PtOx equilibrium potentialφ C: Measured voltage of the cell of the fuel cell (average for a plurality of cells)φ ion: Potential loss of electrolytic membraneU Ptox: equilibrium tensionF: Faraday constantR: Ideal gas constantT: Temperature (K)In calculating a degradation level, the oxidation level of the cathode catalyst of the fuel cell may be updated and stored in a non-volatile memory.In determining a stop voltage, the calculated degree of degradation may be compared with a predetermined value, and a stop voltage may be selected from first and second stop voltages stored in advance according to the comparison result.In determining a stop voltage, the stop voltage may be determined such that when the calculated degree of degradation is greater than the predetermined value, the stop voltage is set to the first stop voltage.In determining a stop voltage, the stop voltage may be determined such that when the calculated degree of degradation is equal to or less than the predetermined value, the stop voltage is set to the second stop voltage.In determining whether the fuel cell has entered the stop mode, whether a transition to the stop mode has occurred may be determined in accordance with whether there is a requested output.In order to achieve the above-mentioned object, according to another aspect of the present invention, there is provided a system for controlling a stop mode of a fuel cell, the system including: a deterioration calculation unit that calculates a deterioration degree of a fuel cell; a stop voltage determination unit that determines a stop voltage of the fuel cell based on the deterioration degree of the fuel cell calculated by the deterioration calculation unit; a stop mode transition determination unit that determines whether the fuel cell has entered the stop mode; A power distribution control unit that controls an output voltage of the fuel cell to be the stop voltage determined by the stop voltage determining unit when the stop mode transition determining unit determines that the fuel cell has entered the stop mode, and an oxidation estimating unit that estimates a degree of oxidation of a cathode catalyst of the fuel cell, wherein the deterioration calculating unit calculates the degree of deterioration of the fuel cell from the degree of oxidation of the cathode catalyst of the fuel cell by the following equations:θ Ptox: Oxidation Degree of Platinum Catalyst of Cathode (0-1)η Ptox: potential difference at cathodek Ptox: Reaction Rate for PtOx Formationα'α, α' c: Anodic and Cathodic Transfer Coefficient for PtOxformationU Ptox: PtOx equilibrium potentialφ c: Measured voltage of the cell of the fuel cell (average for a plurality of cells)φ ion: Potential loss of electrolytic membraneU Ptox: equilibrium tensionF: Faraday constantR: Ideal gas constantT: Temperature (K)The oxidation estimation unit may include a nonvolatile memory for updating and storing the oxidation degree of the cathode catalyst.Further, a reference deterioration degree and first and second stop voltages may be stored in advance in the stop voltage determination unit, the second stop voltage being larger than the first stop voltage, and the stop voltage determination unit may determine the stop voltage by comparing the oxidation degree of the fuel cell calculated by the deterioration calculation unit with the reference deterioration degree and selecting a stop voltage from the first and second stop voltages stored in advance according to the comparison result.The stop voltage determination unit may determine the stop voltage such that when the degree of deterioration calculated by the deterioration calculation unit is greater than the predetermined value, the stop voltage is set to the first stop voltage, and may determine the stop voltage such that when the calculated degree of deterioration is equal to or less than the predetermined value, the stop voltage is set to the second stop voltage.According to the method and system for varying a stop voltage in a stop mode of a fuel cell, the following effects can be obtained.First, it is possible to prevent a degradation in performance by preventing deterioration of a fuel cell that may be caused during operation.Secondly, it is possible to achieve an effect of improving durability that reduces a possibility of causing a permanent drop in performance that is unrecoverable due to separation of a platinum (Pt) catalyst of the cathode from an electrode upon continuous exposure to a high voltage by minimizing oxidation of the catalyst during operation.Third, the efficiency of the system is increased by compensating for the deterioration during the operation of the fuel cell, and therefore it is possible to improve the fuel efficiency of a vehicle compared to the related art.Fourth, since the stop voltage of the fuel cell is selectively lowered only when necessary, it is possible to minimize a delay in output when the stop mode is ended.BRIEF DESCRIPTION OF THE DRAWINGSThe above and other objects, features and other advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: FIG. 1 shows a flow chart of a process flow of an embodiment of the present application; FIG. 2 is a flowchart illustrating a method of controlling a stop mode of a fuel cell according to an embodiment of the present invention; FIG. 3 is a diagram illustrating the configuration of a system for controlling a stop mode of a fuel cell according to an embodiment of the present invention; and FIG. 4 is a diagram illustrating the configuration of a system for controlling a stop mode of a fuel cell according to another embodiment of the present invention.DETAILED DESCRIPTION OF THE INVENTIONWhile the present invention has been described with reference to particular embodiments shown in the drawings, it will be apparent to one skilled in the art that the present invention may be changed and modified in various ways without departing from the scope of the present invention which is described in the following claims.Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. FIGS. 1 and 2 are flowcharts illustrating methods for controlling a stop mode of a fuel cell 10 according to an embodiment of the present invention. FIG. 3 is a diagram illustrating the configuration of a system for controlling a stop mode of a fuel cell 10 according to an embodiment of the present invention.Referring to FIG. 2, a method for controlling a stop mode of a fuel cell 10 according to an embodiment of the present invention includes: calculating the degree of deterioration of a fuel cell 10 (S 100); determining a stop voltage of the fuel cell 10 according to a calculated degree of deterioration (S 200); determining whether a transition to the stop mode of the fuel cell has occurred (S 300); and controlling an output voltage of the fuel cell 10 to be the determined stop voltage when it is determined that a transition to the stop mode of the fuel cell 10 has occurred (S 400).According to the method of controlling a stop mode of a fuel cell 10, the stop voltage is determined based on the calculated degree of deterioration, wherein when the degree of deterioration is large, compensation of the deterioration is promoted, and when the degree of deterioration is small, the stop voltage is determined such that output deceleration is minimized when the stop mode of the fuel cell 10 is terminated. Consequently, it is possible to both increase the efficiency of a system and minimize the output delay.Referring to FIG. 3, a system for controlling a stop mode of a fuel cell 10 according to an embodiment of the present invention includes: a deterioration calculation unit 20 that calculates the degree of deterioration of a fuel cell 10; a stop voltage determination unit 30 that determines a stop voltage of the fuel cell based on the degree of deterioration of the fuel cell 10 calculated by the deterioration calculation unit 20; a stop mode transition determination unit 40 that determines whether the fuel cell 10 has entered the stop mode; and a power distribution control unit 50 that controls an output voltage of the fuel cell 10 to be the stop voltage determined by the stop voltage determining unit 30 when the stop mode transition determining unit determines that the fuel cell 10 has entered the stop mode.Further, the method of controlling a stop mode of a fuel cell 10 according to an embodiment of the present invention shown in FIG. 2 may be performed by the system of controlling a stop mode of a fuel cell 10 shown in FIG. 3.Thus, referring to FIGS. 2 and 3, the calculation of the degree of deterioration of a fuel cell (S 100) is a step in which the deterioration calculation unit 20 calculates the degree of deterioration of the fuel cell 10.An oxidation estimating unit 60 estimates the oxidation degree of platinum (Pt) which is a cathode catalyst of the fuel cell 10, and it is possible to calculate the deterioration degree of the fuel cell 10 from the oxidation degree of the catalyst estimated by the deterioration estimating unit 60. The degree of deterioration of the fuel cell 10 may be obtained by scaling the degree of oxidation of the cathode catalyst or may be obtained from an equation that substitutes the degree of oxidation of the cathode catalyst as a variable.The degree of oxidation of the cathode catalyst of the fuel cell 10 estimated by the oxidation estimation unit 60 may be updated and stored in a nonvolatile memory 61 to store the latest degree of oxidation value.In calculating the degree of deterioration, the oxidation degree of the cathode catalyst may be obtained from the following equation. In the following equation, the Faraday constant may be 96485, the ideal gas constant may be 8.314, and k Ptox, α'α, α', c and U Ptox, which are characteristics based on the material and the configuration of an electrode, may be calculated from test results on the material or other documents. Further, a potential loss of the diaphragm may be obtained by multiplying the current of a fuel cell 10 by a diaphragm resistance determined in advance, calculated, or estimated.θ Ptox: Oxidation Degree of Platinum Catalyst of Cathode (0-1)η Ptox: potential difference at cathodek Ptox: Reaction Rate for PtOx Formationα'α, α' c: Anodic and Cathodic Transfer Coefficient for PtOxformationU Ptox: PtOx equilibrium potentialφ c: Measured voltage of the cell of the fuel cell (average for a plurality of cells)φ ion: Potential loss of electrolytic membraneU Ptox: equilibrium tensionF: Faraday constantR: Ideal gas constantT: Temperature (K)Further, a previously calculated value is required to calculate the oxidation degree of the electrode catalyst. The previously calculated value is used when the fuel cell 10 is operating, and may be calculated from the values stored in the nonvolatile memory 61 when there is no previously calculated value, for example, immediately after the fuel cell 10 is operated.In detail, when the fuel cell 10 is initially operated, a new degree of oxidation of the electrode catalyst is calculated on the assumption that the voltage of the fuel cell 10 for a stop time is 0 [V], by reading out the degree of oxidation of the electrode catalyst stored in the nonvolatile memory 61 and measuring the stop time from the stop to the start of the fuel cell 10.The determination of a stop voltage according to the degree of deterioration (S 200) may determine the stop voltage by comparing the degree of oxidation calculated by the stop voltage determination unit 30 with a predetermined value and selecting a stop voltage from among first and second stop voltages stored in advance according to the comparison result.A reference deterioration degree and first and second stop voltages are stored in advance in the stop voltage determination unit 30, and the stop voltage determination unit 30 may determine the stop voltage by comparing the oxidation degree of the fuel cell 10 calculated by the deterioration calculation unit with the reference deterioration degree and selecting a stop voltage of the stop voltages stored in advance according to the comparison result.For example, the calculated degree of deterioration may be compared with a predetermined value (S 210), the stop voltage may be determined as V 1 (S 222) when the degree of deterioration is greater than the predetermined value, and the stop voltage may be determined as V 2 (S 221) when the degree of deterioration is equal to or less than the predetermined value. V2 is set to be larger than V1.By determining the stop voltage such that the calculated degree of deterioration is greater than the predetermined value, it is possible to quickly compensate for the deterioration by setting the stop voltage to the first stop voltage.On the other hand, by determining the stop voltage such that the smaller the calculated degree of deterioration, the greater the stop voltage, it is possible to minimize a delay in output power compensation when the stop mode of the fuel cell 10 is terminated.The determination of whether a transition to a stop mode of the fuel cell 10 has been made (S 300) may be performed by the stop mode transition determination unit 40.Depending on whether there is a requested output, it is possible to determine whether the fuel cell 10 has entered the stop mode. When the requested output of the fuel cell 10 is 0 (S 300), there is no need to operate the fuel cell 10 longer, and thus, a transition is made to the stop mode, but when the requested output is not 0, the process for determining the degree of deterioration (S 100) may return.When the fuel cell 10 has entered the stop mode, the operation of the fuel cell 10 is ended, and the control of an output voltage of the fuel cell 10 to be the determined stop voltage (S 400) may be performed by the power distribution control unit 50.The power distribution control unit 50 may control the fuel cell 10 and a DC-DC converter (not shown) provided between a drive motor 51, electric devices 52, and a high-voltage battery 53.In detail, when the output voltage of the fuel cell 10 is higher than the stop voltage when the operation of the fuel cell 10 is stopped, the DC-DC converter (not shown) is controlled to maintain the output voltage of the fuel cell 10 at the determined stop voltage by distributing power to the electric devices 52 or the high-voltage battery 53.Referring to FIG. 4, a system for controlling a stop mode of a fuel cell 10 according to another embodiment of the present invention includes: a deterioration calculation unit 20 that calculates the degree of deterioration of a fuel cell 10; a stop voltage determination unit 30 that determines a stop voltage of the fuel cell 10 based on the degree of deterioration of the fuel cell 10 calculated by the deterioration calculation unit 20; a stop mode transition determination unit 40 that determines whether the fuel cell 10 has entered the stop mode; and a power sharing control unit 50 that controls an output voltage of the fuel cell 10 to be the stop voltage determined by the stop voltage determining unit 30 when the stop mode transition determining unit determines that the fuel cell 10 has entered the stop mode. In this embodiment, the fuel cell 10 is not directly connected to the power distribution control unit 50.While a preferred embodiment of the present invention has been described for illustrative purposes, it will be appreciated by those skilled in the art that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention disclosed in the appended claims.

Claims

A method for controlling a stop mode of a fuel cell (10), the method comprising: calculating (S100) a degree of deterioration of the fuel cell (10); determining (S200) a stop voltage of the fuel cell (10) according to the calculated degree of deterioration; determining (S300) whether the fuel cell (10) has entered the stop mode; and controlling (S400) an output voltage of the fuel cell (10) to be the determined stop voltage when it is determined that the fuel cell (10) has entered the stop mode, wherein, in calculating a deterioration degree, the deterioration degree of the fuel cell (10) is calculated using a deterioration degree of a cathode catalyst of the fuel cell (10) from the following equations: d θ P t O x d t = k P t O x ((1 - θ P t O x) exp (α ' α F R T η P t O x) - θ P t O x exp (- α ' c F R T η P t O x) ) η P t O x = φ C - φ i o n - U P t O x where θ Ptox: the oxidation degree of the platinum catalyst of the cathode (0~1) η Ptox: Cathode potential difference k Ptox: Reaction rate for PtOxformation α'α, α' c: Anodic and cathodic transfer coefficient for PtOxformation U Ptox: PtOxequilibrium potential φ c: Measured voltage of the cell of the fuel cell (average for a plurality of cells) φ ion: Potential loss of the electrolytic membrane U PtOX: Equilibrium voltage F: Faraday constant R: Ideal gas constant T: temperature (K).The method according to claim 1, wherein in calculating a deterioration degree, the oxidation degree of the cathode catalyst of the fuel cell (10) is updated and stored in a nonvolatile memory (61).The method according to claim 1, wherein in determining a stop voltage, the calculated degree of degradation is compared (S210) with a predetermined value, and a stop voltage is selected from first and second stop voltages stored in advance according to the comparison result.The method according to claim 3, wherein in the determining a stop voltage, the stop voltage is determined such that when the calculated degree of degradation is greater than the predetermined value, the stop voltage is set to the first stop voltage (S222).The method according to claim 3, wherein in the determining a stop voltage, the stop voltage is determined such that when the calculated degree of degradation is equal to or less than the predetermined value, the stop voltage is set to the second stop voltage (S221).The method of claim 1, wherein in determining whether the fuel cell (10) has entered the stop mode, it is determined whether a transition to the stop mode has occurred in accordance with whether there is a requested output power.A system for controlling a stop mode of a fuel cell (10), comprising: a deterioration calculation unit (20) that calculates a deterioration degree of a fuel cell (10); a stop voltage determination unit (30) that determines a stop voltage of the fuel cell (10) based on the deterioration degree of the fuel cell (10) calculated by the deterioration calculation unit (20); a stop mode transition determination unit (40) that determines whether the fuel cell (10) has entered the stop mode; a power distribution control unit (50) that controls an output voltage of the fuel cell (10) to be the stop voltage determined by the stop voltage determination unit (30) when the stop mode transition determination unit (40) determines that the fuel cell (10) has entered the stop mode; and an oxidation estimation unit that estimates an oxidation degree of a cathode catalyst of the fuel cell (10), wherein the deterioration calculating unit (20) calculates the deterioration degree of the fuel cell (10) from the oxidation degree of the cathode catalyst of the fuel cell (10) from the following equations: d θ P t O x d t = k P t O x ((1 - θ P t O x) exp (α ' α F R T η P t O x) - θ P t O x exp (- α ' c F R T η P t O x) ) η P t O x = φ C - φ i o n - U P t O x where θ Ptox: oxidation degree of the platinum catalyst of the cathode (0~1) η Ptox: Cathode potential difference k Ptox: Reaction rate for PtOxformation α'α, α' c: Anodic and cathodic transfer coefficient for PtOxformation U Ptox: PtOxequilibrium potential φ c: Measured voltage of the cell of the fuel cell (average for a plurality of cells) φ ion: Potential loss of the electrolytic membrane U Ptox: Equilibrium voltage F: Faraday constant R: Ideal gas constant T: temperature (K)The system according to claim 7, wherein the oxidation estimating unit includes a nonvolatile memory (61) for updating and storing the oxidation degree of the cathode catalyst.The system according to claim 7, wherein a reference deterioration degree and first and second stop voltages are stored in advance in the stop voltage determination unit (30), the second stop voltage being larger than the first stop voltage, and the stop voltage determination unit (30) determines the stop voltage by comparing the oxidation degree of the fuel cell (10) calculated by the deterioration calculation unit (20) with the reference deterioration degree and selecting a stop voltage from the stored first and second stop voltages in advance according to the comparison result.The system according to claim 9, wherein the stop voltage determination unit (30) determines the stop voltage such that when the degree of deterioration calculated by the deterioration calculation unit (20) is greater than the predetermined value, the stop voltage is set to the first stop voltage, and determines the stop voltage such that when the calculated degree of deterioration is equal to or less than the predetermined value, the stop voltage is set to the second stop voltage.

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