Fuel Cell Control Device

Abstract

To provide a fuel cell control device capable of minimizing deterioration of a cathode catalyst without sacrificing output density or power generation efficiency when a load is required for a fuel cell.SOLUTION: A fuel cell control device includes a load requirement acquisition unit that acquires a load requirement PFC for a solid polymer fuel cell, a deterioration amount estimation unit that estimates the amount of deterioration of a cathode catalyst contained in the solid polymer fuel cell, a potential range setting unit that changes the potential range Vr of the solid polymer fuel cell, which is determined by an upper potential VH and a lower potential VL, in accordance with the amount of deterioration, and a potential control unit that controls the potential of the solid polymer fuel cell such that the actual potential Vreal of the solid polymer fuel cell corresponding to the PFC is within the Vr.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present invention relates to a fuel cell control device, and more specifically, to a fuel cell control device that can minimize deterioration of the cathode catalyst without sacrificing power density or power generation efficiency when a load is required for a fuel cell. [Background technology]

[0002] A polymer electrolyte fuel cell is equipped with a membrane electrode assembly (MEA) in which catalyst-containing catalyst layers are bonded to both sides of an electrolyte membrane. The catalyst layer is the reaction field for the electrode reaction, and generally consists of a composite of carbon carrying catalyst particles such as platinum and a solid polymer electrolyte (catalyst layer ionomer). In a polymer electrolyte fuel cell, a gas diffusion layer is usually arranged on the outside of the catalyst layer. A current collector (separator) equipped with a gas flow path is also arranged on the outside of the gas diffusion layer. A polymer electrolyte fuel cell usually has a structure (fuel cell stack) in which a plurality of unit cells each consisting of such an MEA, gas diffusion layer, and current collector are stacked.

[0003] When a polymer electrolyte fuel cell is used as an on-board power source, the voltage of the polymer electrolyte fuel cell varies greatly depending on the driving conditions of the vehicle. When the polymer electrolyte fuel cell is in a low-load state, the power generation efficiency is high, but the cathode catalyst is exposed to a high potential state, so that the catalytic components are easily eluted from the cathode catalyst. On the other hand, when the polymer electrolyte fuel cell is in a high-load state, the power generation efficiency is low, but the cathode catalyst is exposed to a low potential state, so that the eluted catalytic components are easily redeposited on the surface of the cathode catalyst. Therefore, when the cathode catalyst is repeatedly exposed to high and low potential states, there is a problem that the cathode catalyst gradually deteriorates.

[0004] In order to solve this problem, various proposals have been made in the past. For example, Patent Document 1 discloses a fuel cell system that, when it is determined that the output voltage at an operating point of the fuel cell will exceed an upper limit voltage value, causes the fuel cell to generate electricity so that the upper limit potential becomes the output voltage of the fuel cell instead of the output voltage at the operating point. Patent document 2 discloses a fuel cell system that sets the current drawn from the fuel cell stack so that the voltage of the fuel cell stack is below a predetermined voltage when the idle stop state is released and the fuel cell system transitions to a low-load idle state.

[0005] Patent Document 3 discloses a control device for a fuel cell system that limits the output voltage of the fuel cell to a predetermined upper limit or lower if it is determined that oxidant gas is present at the oxidant electrode when the fuel cell is started up. Patent Document 4 discloses a fuel cell system equipped with a control means for controlling the output voltage of the fuel cell to avoid high potential, with an upper limit of a high potential avoidance voltage that is lower than the open end voltage of the fuel cell.

[0006] Patent Document 5 discloses a fuel cell system in which, when the fuel cell system is in an idle-stop state, an oxidant gas supply means is operated so that the maximum cell voltage of the fuel cell unit is equal to or lower than a preset upper limit voltage. Patent document 6 discloses a fuel cell system that prevents the total voltage of the fuel cell from exceeding a predetermined high-potential avoidance voltage threshold by maintaining the converter command voltage at a high-potential avoidance voltage that is lower than the open circuit voltage of the fuel cell.

[0007] Patent Document 7 discloses a power generation amount control device for a fuel cell that uses the smaller of the output of an output upper limit value calculation means or the output of a target power generation amount calculation means as a command value for the output to be taken from the fuel cell. Patent Document 8 discloses a fuel cell system that judges the predicted state of deterioration of an electrode catalyst layer in a fuel cell, and determines an upper limit of the output voltage of the fuel cell according to the judged state of deterioration.

[0008] Patent document 9 discloses a fuel cell system equipped with a control means for controlling the operation of the fuel cell with the output voltage of the fuel cell set at an upper limit of a high potential avoidance voltage that is lower than the open end voltage, and a high potential avoidance voltage setting means for variably setting the high potential avoidance voltage according to the charging state of a power storage device.

[0009] Furthermore, Patent Document 10 states: A fuel cell that receives a supply of reactant gas and generates electricity; a power storage device that stores at least a portion of the power generated by the fuel cell; a control unit that controls a lower limit potential of the fuel cell when it is predicted that the charge amount of the power storage device will reach an upper limit charge threshold so that the charge amount of the power storage device does not exceed the upper limit charge threshold; A fuel cell system comprising:

[0010] Deterioration of a fuel cell can be suppressed to some extent by defining the range (upper and lower limit potential) in which the fuel cell is operated. This is also expected to improve durability. However, in Patent Documents 1 to 7, the range in which the fuel cell is operated is fixed, which results in a tradeoff between a decrease in power generation efficiency and a decrease in the output density of the fuel cell. The reason for the decrease in power generation efficiency is that the lower the load on a fuel cell, the higher its power generation efficiency. If an upper limit potential is set, the fuel cell cannot be used at the most efficient power generation point, and power generation efficiency decreases. Moreover, the reason why the output density is reduced is that when a lower limit potential is set, the range of power that can be swept is limited.

[0011] On the other hand, Patent Document 8 discloses a method for varying the upper limit potential depending on the deterioration state of the electrode catalyst layer. However, the method described in this document only extends the life of the fuel cell by changing the upper limit potential depending on the number of fluctuations, and is unable to achieve both high power generation efficiency and high output density. Furthermore, the methods described in Patent Documents 9 and 10 are methods for suppressing deterioration (overcharging) of a power storage device, and are not methods for suppressing deterioration of a fuel cell. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] JP 2012-094257 A [Patent Document 2] JP 2006-309971 A [Patent Document 3] JP 2008-165994 A [Patent Document 4] JP 2009-129639 A [Patent Document 5] JP 2007-109569 A

[0013] [Patent Document 6] JP 2009-104977 A [Patent Document 7] JP 2003-036871 A [Patent Document 8] JP 2012-129069 A [Patent Document 9] JP 2009-129647 A [Patent Document 10] JP 2013-098052 A Summary of the Invention [Problem to be solved by the invention]

[0014] The problem that the present invention aims to solve is to provide a fuel cell control device that can minimize deterioration of the cathode catalyst without sacrificing output density or power generation efficiency when a load is required for the fuel cell. [Means for solving the problem]

[0015] In order to solve the above problems, the fuel cell control device according to the present invention comprises: Load requirement P for polymer electrolyte fuel cells FC A load request acquisition unit that acquires a load request; a deterioration amount estimation unit that estimates a deterioration amount of a cathode catalyst of the solid polymer electrolyte fuel cell; Depending on the amount of deterioration, the upper limit potential V H and the lower limit potential V L A potential range setting unit that changes the potential range Vr of the polymer electrolyte fuel cell, which is determined by P FC The real potential V of the polymer electrolyte fuel cell corresponding to real a potential control unit that controls the potential of the polymer electrolyte fuel cell so that the potential Vr is within the range of Vr; It is equipped with:

[0016] The deterioration amount estimation unit The electrochemically effective surface area A of the cathode catalyst at time [i] ECS an ECS estimation unit for estimating [i]; The above A ECS Based on [i], the average particle diameter D of the cathode catalyst at the time [i] CAT [i] is estimated by the average particle size estimation part. It is preferable that the above configuration is provided. The potential range setting unit is D CAT The larger [i], the greater the V H and / or a first setting portion that increases D CAT The larger [i], the greater the V L Increasing the second setting part, It is preferable that the above configuration is provided. Effect of the Invention

[0017] The equilibrium potential of the electrochemical reaction (reaction A) that dissolves the components of the cathode catalyst is the average particle diameter D of the cathode catalyst. CAT It depends on [i]. Specifically, D CAT The larger [i] is, the higher the potential required to cause reaction A. Therefore, D CAT When [i] is large, reaction A does not proceed easily even if the potential is increased, so the upper limit potential V H Even if the D is increased, the durability is not affected. CAT The larger [i] is, the greater VH Increasing the potential increases the frequency at which the fuel cell is maintained in a high potential state with high efficiency, thereby improving the efficiency of the fuel cell system.

[0018] On the other hand, the equilibrium potential of the electrochemical reaction (reaction B) in which an oxide is formed on the surface of the cathode catalyst is also D CAT It depends on [i]. Specifically, D CAT The larger [i] is, the higher the potential required to cause reaction B. Therefore, D CAT Even though [i] is large, V L If D is kept low, the oxides are easily reduced. CAT The larger [i] is, the greater V L Increasing the amount of oxygen makes it easier to retain oxides, improving the durability of the fuel cell. [Brief description of the drawings]

[0019] [Figure 1] The results are shown in Fig. 13. The AECS calculation results for catalyst particles with different average particle diameters and dispersions. [Diagram 2] 1 is a diagram showing the relationship between an output voltage VP and an output power P with respect to an output current IP. [Diagram 3] FIG. 2 is a schematic cross-sectional view of a Pt particle on which an oxide film is formed. [Figure 4] 4 is a flowchart of a potential control method according to the present invention. [Diagram 5] FIG. 2 is a graph showing a comparison of the deterioration amount and power generation efficiency of the fuel cell systems of Example 1 and Comparative Example 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] [Configuration 1] Load requirement P for polymer electrolyte fuel cells FC A load request acquisition unit that acquires a load request; a deterioration amount estimation unit that estimates a deterioration amount of a cathode catalyst of the solid polymer electrolyte fuel cell; Depending on the amount of deterioration, the upper limit potential V H and the lower limit potential V LA potential range setting unit that changes the potential range Vr of the polymer electrolyte fuel cell, which is determined by P FC The real potential V of the polymer electrolyte fuel cell corresponding to real a potential control unit that controls the potential of the polymer electrolyte fuel cell so that the potential Vr is within the range of Vr; A fuel cell control device comprising:

[0021] [Configuration 2] The deterioration amount estimation unit The electrochemically effective surface area A of the cathode catalyst at time [i] ECS an ECS estimation unit for estimating [i]; The above A ECS Based on [i], the average particle diameter D of the cathode catalyst at the time [i] CAT [i] is estimated by the average particle size estimation part. Equipped with The potential range setting unit is D CAT The larger [i], the greater the V H and / or a first setting portion that increases D CAT The larger [i], the greater the V L Increasing the second setting part, 2. The fuel cell control device according to claim 1,

[0022] [Configuration 3] The ECS estimation unit calculates the A based on the formula (1) described later. ECS 3. The fuel cell control device according to configuration 2, further comprising a first calculation unit that calculates [i].

[0023] [Configuration 4] The ECS estimation unit is an information acquisition unit that sequentially acquires the voltage V[i] of the polymer electrolyte fuel cell at least at the time [i]; a catalyst potential calculation unit that calculates a catalyst potential Vcat[i] of the cathode of the polymer electrolyte fuel cell at the time [i] using at least the V[i]; a surface utilization rate calculation unit that calculates an effective surface utilization rate θact[i] of the cathode catalyst at the time [i] using the Vcat[i]; Using the formula (2) and / or formula (3) described later, ECS A second calculation unit for calculating [i]; 4. The fuel cell control device according to claim 2 or 3, comprising:

[0024] [Configuration 5] The first setting unit determines the V H

[0023] The present invention includes a device for setting The second setting unit determines the V L Including a device for setting 5. A fuel cell control device according to any one of configurations 2 to 4.

[0025] [Configuration 6] The potential control unit is P FC The output current I P and output potential V P A control point setting unit for acquiring a control point determined by (a) The above V P When is within the range of Vr, real As said V P Select (b) The above V P V H When it exceeds the above V real As said V H Select (c) Said V P V L When it is less than V real As said V L Select Potential output section 6. The fuel cell control device according to any one of configurations 1 to 5, comprising:

[0026] An embodiment of the present invention will be described in detail below. [1. Fuel cell control device] The fuel cell control device according to the present invention comprises: Load requirement P for polymer electrolyte fuel cells FC A load request acquisition unit that acquires a load request; a deterioration amount estimation unit that estimates a deterioration amount of a cathode catalyst of the solid polymer electrolyte fuel cell; Depending on the amount of deterioration, the upper limit potential V H and the lower limit potential V L A potential range setting unit that changes the potential range Vr of the polymer electrolyte fuel cell, which is determined by P FC The real potential V of the polymer electrolyte fuel cell corresponding to real a potential control unit that controls the potential of the polymer electrolyte fuel cell so that the potential Vr is within the range of Vr; It is equipped with:

[0027] [1.1. Load request acquisition part] The load request acquisition unit acquires a load request P FC The device that obtains the obtained P FC is stored in memory. The configuration of the load request acquisition unit is not particularly limited as long as it provides the above-mentioned functions, and an optimal configuration can be selected depending on the purpose.

[0028] The fuel cell control device according to the present invention is used to control the potential of a fuel cell in a fuel cell system including a fuel cell and a secondary battery. The total load demand Ptotal for the system is usually set to the load demand Ptotal for the fuel cell so that the efficiency of the entire system is maximized. FC and the load demand P for the secondary battery BAT The P obtained in this way is FC is the actual potential V of the fuel cell in the potential control section. real This is used to control the

[0029] [1.2. Deterioration amount estimator] [1.2.1. Overview] The deterioration amount estimating unit is a device for estimating the deterioration amount of the cathode catalyst of the polymer electrolyte fuel cell. The estimated deterioration amount is stored in a memory. The configuration of the deterioration amount estimating section is not particularly limited as long as it can perform the above-mentioned functions, and an optimal configuration can be selected depending on the purpose. Furthermore, the type of "deterioration amount", that is, "parameter representing the degree of deterioration of the cathode catalyst" is not particularly limited, and an optimal parameter can be selected depending on the purpose.

[0030] The deterioration of the cathode catalyst occurs due to repeated elution and reprecipitation of the catalyst components, and the average particle size D CAT This is caused by an increase in D CAT As the electrochemically effective surface area A of the cathode catalyst increases, ECS Therefore, the deterioration amount estimation unit estimates the average particle diameter D of the cathode catalyst at time [i]. CAT [i] and / or the electrochemically effective surface area A of the cathode catalyst at time [i] ECS A device capable of estimating [i] is preferred.

[0031] Specifically, the deterioration amount estimation unit Electrochemically effective surface area A of the cathode catalyst at time [i] ECS an ECS estimation unit for estimating [i]; A ECS Based on [i], the average particle diameter D of the cathode catalyst at time [i] CAT [i] is estimated by the average particle size estimation part. It is preferable that the above configuration is provided.

[0032] In actual fuel cells, non-destructive testing has confirmed CAT On the other hand, the electrochemically effective surface area A of the cathode catalyst is ECS can be estimated based on the operating history and operating conditions of the fuel cell. CAT A ECS Generally, D CAT The larger the A ECSTherefore, the electrochemically effective surface area A at time [i] ECS If we can estimate [i] one by one, we can estimate the estimated A ECS Based on [i], the average particle diameter D of the cathode catalyst at time [i] CAT [i] can be estimated.

[0033] [1.2.2. ECS estimation part: A ECS [Estimation of [i]] The ECS estimation part is composed of A ECS As long as it is capable of estimating [i], there is no particular limitation, and an optimal configuration can be selected according to the purpose. Specific examples of the ECS estimation unit are as follows. The ECS estimation unit may include one of the following devices, or may include two or more of them.

[0034] [A. First Example: A using integrated time ECS [Estimation of [i]] The ECS estimation part calculates A based on the following equation (1). ECS The calculated AECS[i] may be stored in the memory.

[0035]

number

[0036] however, A ECS0 is the initial value of the electrochemically effective surface area, B1 is the fit coefficient, Ts is the calculation interval. In addition, A ECS0 It is preferable that initial performance tests and durability tests are carried out in advance on a fuel cell having the same specifications as the controlled object, and that B1 and B2 are set so as to match the test results.

[0037] Equation (1) is expressed as A using the cumulative time (=ΣTs) of fuel cell power generation. ESCThis is the formula for calculating [i]. Generally, the longer the cumulative time of power generation in a fuel cell, the greater the number of repeated dissolution and reprecipitation of catalyst components. ECS [i] decreases monotonically with the accumulation time. Equation (1) is ECS This is an approximation formula that approximates the change in [i] as a linear function of the integrated time. Equation (1) has low estimation accuracy, but has the advantage of reducing calculation costs.

[0038] [B. Second Example: A using the voltage of a fuel cell ECS [Estimation of [i]] The ECS estimation part is an information acquisition unit that sequentially acquires the voltage V[i] of the polymer electrolyte fuel cell at least at time [i]; a catalyst potential calculation unit that calculates a catalyst potential Vcat[i] of the cathode of the polymer electrolyte fuel cell at time [i] using at least V[i]; a surface utilization rate calculation unit that calculates an effective surface utilization rate θact[i] of the cathode catalyst at time [i] using Vcat[i]; Using the formula (2) and / or formula (3) described below, A ECS A second calculation unit for calculating [i]; The above may also be provided.

[0039] [B.1. Information acquisition section] The information acquisition unit is a device for acquiring information necessary for calculating θact[i] and Vcat[i]. The acquired information is stored in a memory. The configuration of the information acquisition unit is not particularly limited as long as it performs such functions, and an optimal configuration can be selected depending on the purpose.

[0040] In order to calculate θact[i] and Vcat[i], the information acquisition unit must be capable of sequentially acquiring at least the fuel cell voltage V[i]. The information acquisition unit may be capable of sequentially acquiring other information in addition to V[i]. When the information acquisition unit is capable of acquiring other information, the calculation accuracy of θact[i] and Vcat[i] may be further improved. Other information includes, for example, the fuel cell current I[i] at time [i], the high-frequency impedance R[i], and the temperature T FC [i], the humidity of the cathode or anode RH[i], the gas flow rate of the cathode or anode Q[i], and the gas pressure of the cathode or anode Pr[i].

[0041] [B.2. Catalyst potential calculation section] The catalytic potential calculation unit is a device that uses at least V[i] to calculate the catalytic potential Vcat[i] of the cathode of the polymer electrolyte fuel cell at time [i]. The calculated Vcat[i] is stored in a memory. The configuration of the catalytic potential calculation unit is not particularly limited as long as it performs such a function, and an optimal configuration can be selected depending on the purpose. The method of calculating Vcat[i] will be described in detail later.

[0042] [B.3. Surface utilization rate calculation section] The surface utilization rate calculation unit is a device that uses Vcat[i] to calculate the effective surface utilization rate θact[i] of the cathode catalyst at time [i]. The calculated θact[i] is stored in a memory. The configuration of the surface utilization rate calculation unit is not particularly limited as long as it performs such functions, and an optimal configuration can be selected depending on the purpose. Details of the calculation method of θact[i] will be described later.

[0043] [B.4. Second calculation part] The second calculation unit calculates A using the following formula (2) and / or formula (3): ECS [i] is calculated. ECS [i] is stored in the memory. The configuration of the second calculation unit is not particularly limited as long as it has the above function, and an optimal configuration can be selected depending on the purpose.

[0044]

number

[0045] however, A ECS0 is the initial value of the electrochemically effective surface area, D1 to D4 are the compatibility coefficients. Ts is the calculation interval. In addition, A ECS0 It is preferable that initial performance tests and durability tests are carried out in advance on a fuel cell having the same specifications as the controlled object, and that the values ​​of D1 to D4 are set to match the test results.

[0046] Equation (2) is expressed as A using Ts, θact[i], and Vcat[i]. ECS This is the formula for calculating [i]. Compared to formula (1), formula (2) is ECS In equation (1), A is calculated based on the assumption that dissolution and precipitation of the catalyst components occurs regardless of Vcat[i]. ECS [i] is calculated, but A ECS [i] should actually depend on Vcat[i]. In formula (2), we focus on the phenomenon during elution and assume that the amount of elution is proportional to an exponential function with e as the base and Vcat[i] as the power exponent. We also assume that the elution phenomenon of the catalyst component occurs only in areas not covered by oxides, and multiply the above exponential function by θact[i]. On the other hand, formula (2) has the disadvantage of being more expensive to calculate than formula (1).

[0047] Equation (3) is expressed as A using Ts, θact[i], and V[i]. ECS [i]. Equation (3) uses V[i] instead of Vcat[i]. Therefore, the accuracy of equation (3) is slightly lower than that of equation (2).

[0048] [C. Other Specific Examples] A ECS [i] can also be calculated using the degradation prediction method described in Reference 1 below. ECS The physical model for calculating [i] is also reported in References 2 and 3 below. [Reference 1] JP 2010-236989 A [Reference 2] Darling, RM and JP Meyers(2003), "Kineti model of platinum dissolution in PEMFCs," Journal of the Electrochemical Society 150(11): A1523-A1527 [Reference 3] Sekine, S. (2020), "PtCo Catalyst Dissolution and Oxidation Modeling for Durability Improvement of Automotive Fuel Cell," ECS transaction

[0049] [1.2.3. Average particle size estimation part: D CAT [Estimation of [i]] The average particle size estimation section is A ECS Based on [i], the average particle diameter D of the cathode catalyst at time [i] CAT [i] is an estimation device. The estimated D CAT [i] is stored in the memory. The configuration of the average particle size estimation unit is not particularly limited as long as it has the above function, and an optimal configuration can be selected depending on the purpose.

[0050] for example, (a) The catalyst particles have a perfectly circular shape; (b) The particle size distribution of the catalyst particles follows a semi-log normal distribution. (c) The particle size variance σ 2 does not change Assuming that, the theoretical calculations show that the A of catalyst particles with different average particle size and dispersion ECS can be calculated.

[0051] Figure 1 shows the A of catalyst particles with different average particle size and dispersion. ECS The calculation results of the initial dispersion σ of the cathode catalyst used in the fuel cell are shown below. 2 is known. Therefore, even if the average particle size changes, σ 2 Assuming that does not change, AECS Once we know the estimate of [i], we can use Figure 1 to ECS D corresponding to [i] CAT You can know [i].

[0052] In addition, in Figure 1, some of the average particle size -A ECS The curve is upwardly convex. Therefore, for example, the initial variance σ 2 is 2.0×10 -18 [m 2 ] and A ECS [i] is 50[m 2 / g, there are two values ​​(approximately 1.3 nm and approximately 4.6 nm) that satisfy this condition. In this case, the larger value (approximately 4.6 nm) is used as D CAT This is because it is extremely difficult to actually produce catalyst particles that satisfy the smaller value (approximately 1.3 nm).

[0053] [1.3. Potential range setting section] The potential range setting section adjusts the upper limit potential V H and the lower limit potential V L This is a device for changing the potential range Vr of the polymer electrolyte fuel cell, which is determined by the following equation: Vr after the change is stored in memory. The configuration of the potential range control unit is not particularly limited as long as it performs the above-mentioned function, and an optimal configuration can be selected depending on the purpose.

[0054] In order to suppress deterioration of the cathode catalyst, the potential range setting unit: D CAT The larger [i], the greater the V H and / or a first setting portion that increases D CAT The larger [i], the greater the V L Increasing the second setting part, It is preferable that the sensor further comprises:

[0055] In addition, "D CAT The larger [i], the greater the V H (or V L) to increase the CAT The smaller [i], the greater the V H (or V L This also includes "reducing As the cathode catalyst deteriorates, D CAT [i] is the average particle diameter D of the cathode catalyst at time [i-1] CAT [i-1]. Therefore, usually, V H and / or V L Only operations that increase However, for example, when the fuel cell is replaced with a new one of the same specifications, or when the fuel cell is replaced with a fuel cell of different specifications, D CAT [i] is D CAT [i-1]. In such a case, D CAT The smaller [i], the greater the V H and / or V L This may reduce the

[0056] V H affects the chemical reaction (reaction A) that dissolves the components of the cathode catalyst. The equilibrium potential of reaction A is D CAT The larger [i] is, the higher the value becomes. Therefore, D CAT When [i] is large, V H Even if the reaction A is increased, it becomes difficult for the reaction A to proceed. CAT The larger [i] is, the greater V H Increasing the potential increases the frequency at which the fuel cell is maintained in a high potential state with high efficiency, thereby improving the efficiency of the fuel cell system.

[0057] V L affects the reaction (reaction B) in which an oxide is formed on the surface of the cathode catalyst. The equilibrium potential of reaction B is also CAT The larger [i] is, the higher the value becomes. Therefore, D CAT Even though [i] is large, V L If D is kept low, the oxides are easily reduced. CAT The larger [i] is, the greater V LIncreasing the amount of oxygen makes it easier to retain oxides, improving the durability of the fuel cell.

[0058] The first setting unit and the second setting unit are not particularly limited as long as they perform such functions. H Similarly, the second setting unit preferably includes a device for setting the V based on the following formula (5): L It is preferable that the device includes a device for setting

[0059]

number

[0060] Equations (4) and (5) are, respectively, D CAT [i] and V H Or V L Equation (4) and Equation (5) are empirical formulas that express the relationship between V H Or V L (1 / D CAT Since it is an approximation formula using a linear function of [i]), it has the advantage of requiring little computational load. β1 to β4 are determined based on the durability required for the fuel cell system. When the signs and absolute values ​​of β1 to β4 are optimized, D CAT The larger [i], the greater the V H and / or V L So that V r can be set.

[0061] [1.4. Potential control section] The potential control section is P FC The actual potential V of the polymer electrolyte fuel cell corresponding to real is within Vr. The configuration of the potential control unit is not particularly limited as long as it performs the above function, and an optimal configuration can be selected depending on the purpose. It is particularly preferable that the potential control unit includes a control point setting unit and a potential output unit.

[0062] [1.4.1. Control point setting section] The control point setting section is P FC The output current I P and output potential V P This is a device that obtains the control points determined by the obtained V P is stored in the memory. P is stored in the memory as necessary. The configuration of the control point setting unit is not particularly limited as long as it has the above-mentioned functions, and an optimal configuration can be selected depending on the purpose.

[0063] Figure 2 shows the output current I P Output voltage I P The relationship between the I and the output power P is shown. Fuel cells generally show the IV characteristics shown in FIG. 2. P is also expressed as I P and V P Therefore, the load demand P FC Once this is decided, the V to achieve this is P and I P is uniquely determined.

[0064] [1.4.2. Potential output section] The potential output section is (a)V P When is within the range of Vr, V real As said V P Select (b)V P V H When it exceeds V real As said V H Select (c)V P V L If less than V real As said V L Select It is a device. The configuration of the potential output section is not particularly limited as long as it performs the above-mentioned function, and an optimal configuration can be selected depending on the purpose.

[0065] Vreal Once selected, the fuel cell potential is set to the selected V real The operating conditions of the fuel cell are controlled so that the cathode catalyst deterioration can be minimized without sacrificing the power density or power generation efficiency. In addition, V real As V P If not selected, the actual power output of the fuel cell P real , P FC In such a case, P real and P FC The difference between these is offset by charging and discharging the secondary battery.

[0066] [2. How to calculate Vcat[i]] Accurate A ECS To calculate [i], it is necessary to know the catalyst potential Vcat[i]. Specifically, Vcat[i] can be calculated by the following formula (6) or formula (7). Either method may be used in the present invention.

[0067]

number

[0068] however, N cell is the number of stacked cells in the polymer electrolyte fuel cell, A cell is the area of ​​the cell, I[i] is the current of the polymer electrolyte fuel cell at time [i]. R[i] is the high-frequency impedance of the polymer electrolyte fuel cell at time [i].

[0069] Vcat[i] expressed by formula (6) is an approximation of Vcat[i] that ignores the potential drop caused by the internal resistance. Formula (6) is less accurate than formula (7). However, by using formula (6), Vcat[i] can be calculated without using I[i] and R[i], so the calculation of Vcat[i] can be simplified.

[0070] Strictly speaking, Vcat[i] is expressed by the formula (7). In the formula (7), the first term on the right side represents the potential difference (cell voltage) between both ends of the single cell. In the first term on the right side, V[i] is multiplied by N cell The potential per cell is calculated by dividing by . The second term on the right side represents the potential drop per unit cell due to internal resistance. In the second term on the right side, I[i] and R[i] are converted to values ​​per area or per cell, respectively. Using equation (7), Vcat[i] can be calculated accurately. A ECS In order to accurately calculate [i], it is preferable to use equation (7) for calculating Vcat[i].

[0071] [3. How to calculate θcat[i]] The "effective surface utilization rate θact[i] of the cathode catalyst" refers to the ratio of the area of ​​the surface utilized for the oxygen reduction reaction (ORR) (i.e., the surface not covered with an oxide film) to the surface area of ​​the cathode catalyst (noble metal catalyst particle). A ECS To calculate [i], it is necessary to know the effective surface utilization rate θcat[i]. Specifically, θact[i] can be calculated as follows.

[0072] [3.1. Noble metal catalyst particles] In the present invention, the term "cathode catalyst" refers to precious metal-based catalyst particles (hereinafter, simply referred to as "catalyst particles") made of a metal or alloy containing a precious metal element, and which have activity in the oxygen reduction reaction (ORR). In the present invention, the material of the catalyst particles is not particularly limited as long as it exhibits ORR activity. Examples of the material of the catalyst particles include: (a) Precious metals (Au, Ag, Pt, Pd, Rh, Ir, Ru, Os), (b) An alloy containing two or more precious metal elements; (c) Alloys containing one or more precious metal elements and one or more base metal elements (e.g., Fe, Co, Ni, Cr, V, Ti, etc.) etc.

[0073] [3.2. Types of oxides] When catalyst particles are exposed to a high potential, the catalyst components are more likely to dissolve from the catalyst particles. On the other hand, when catalyst particles are exposed to a high potential, an oxide film (including hydroxide) is formed on the surface of the catalyst particles, which inhibits the dissolution of the catalyst components from the catalyst particles. However, because the rate at which the oxide film forms is slow, if the cathode potential fluctuates suddenly, the formation of the oxide film is delayed, and the catalyst components are more likely to dissolve from the catalyst particles. In other words, if a fuel cell is used continuously in an environment where sudden potential fluctuations are repeated, the catalyst particles will eventually deteriorate. In other words, the durability of the cathode catalyst depends on the total amount of the precious metal oxide and precious metal hydroxide present on the surface of the catalyst particles. On the other hand, the surface of the catalyst particle covered with an oxide film has a lower ORR activity than the surface that is not covered with an oxide film. Therefore, the IV characteristics of a polymer electrolyte fuel cell depend on the θact[i] of the catalyst particle.

[0074] The precious metal oxide present on the surface of the catalyst particle is (a) precious metal hydroxide adsorbed on the surface of the catalyst particle; (b) a precious metal oxide A adsorbed on the surface of the catalyst particle; and (c) Precious metal oxide B formed just below the surface of the catalyst particle due to oxygen diffusion inside the particle. They can be broadly divided into:

[0075] A schematic cross-sectional view of a Pt particle on which an oxide film has been formed is shown in Figure 3. When a Pt particle is exposed to a high potential, an oxide (including a hydroxide) is formed on the surface of the Pt particle. In this case, the oxide on the surface of the Pt particles is (a) Pt hydroxide (PtOH) adsorbed on the surface of Pt particles ad ), (b) Pt oxide (PtO ad ), and (c) Oxygen diffuses into the interior of Pt particles, forming Pt oxide (PtO sub ) It consists of:

[0076] Where PtOH ad The coverage of the precious metal hydroxide adsorbed on the surface of the catalyst particle at time [i] is defined as θox1[i]. θox1[i] is expressed as the ratio (=S1 / S0) of the area (S1) of the precious metal hydroxide adsorbed on the surface of the catalyst particle to the surface area (S0) of the catalyst particle. Similarly, PtO ad The coverage of the precious metal oxide A adsorbed on the surface of the catalyst particle at time i is defined as θox2[i]. θox2[i] is expressed as the ratio (=S2 / S0) of the area (S2) of the precious metal oxide A adsorbed on the surface of the catalyst particle to S0. Similarly, PtO sub The coverage of the precious metal oxide B present inside the catalyst particle at time i is defined as θox3[i]. θox3[i] is expressed as the ratio (=S3 / S0) of the area (S3) of the precious metal oxide B present inside the catalyst particle to S0.

[0077] As shown in Figure 3, PtOH was applied to the surface of the Pt particles. ad or PtO ad Just below the area where PtO is adsorbed sub Therefore, the coverage of the entire Pt particle does not necessarily coincide with the sum of θox1[i] to θox3[i]. Each of θox1[i] to θox3[i] can be calculated sequentially using a reaction model based on the reaction rate equation. Also, once θox1[i] to θox3[i] are known, they can be used to calculate θact[i].

[0078] [3.3. Reaction model] There are various methods for calculating θact[i]. In the present invention, the method for calculating θact[i] is not particularly limited, and an optimal method can be used depending on the purpose. Specifically, θact[i] can be calculated by the following formula (8) or formula (9). Either method may be used in the present invention.

[0079]

number

[0080] however, θox1[i] is the coverage of the precious metal hydroxide adsorbed on the surface of the catalyst particle at time [i], θox2[i] is the coverage of the precious metal oxide A adsorbed on the surface of the catalyst particle at time [i]. θox3[i] is the coverage of the precious metal oxide B present inside the catalyst particle at time [i], Γ is the maximum amount of oxygen coverage per unit surface area (constant), Ts is the calculation step size. α1~α4, α 11 ~α 17 , α 21 ~α 27 , α 31 ~α 37 are the fitting coefficients, respectively.

[0081] Specifically, Ts represents the time from time [i-1] to time [i]. The value of Ts is not particularly limited, and an optimal value can be set depending on the purpose. Ts is usually set in the range of 0.01 s to 100 s. α1~α 37 are preferably determined so as to fit the actual IV characteristics and test results obtained by cyclic voltammetry (CV), respectively. v1 to v3 are each oxide or hydroxide (MO ad , M.O.H. ad , M.O. sub ) reaction rate. G1 to G3 represent the free energies of the reactions of v1 to v3.

[0082] θox1[i-1], θox2[i-1], and θox3[i-1] are the coverages at time [i-1], and are already stored in the memory. θox1[i-1], θox2[i-1], and θox3[i-1] can be calculated by sequential calculations if the initial values ​​are known. The initial values ​​may be held at the time of the previous stop, and may be used as the initial values. In general, when the fuel cell is stopped, it is often held at a low potential, and at that time, all oxides are reduced. Therefore, the initial values ​​after stopping may be θox1=θox2=θox3=0. Therefore, if Vcat[i] is obtained, θact[i] can be calculated from equation (8) or equation (9).

[0083] Equation (9) calculates θact[i] by subtracting the coverage multiplied by each coefficient (α2 to α4) from the total surface (α1). θox1[i] represents the coverage of hydroxide by one-electron reaction. θox2[i] and θox3[i] represent the coverage of oxide by two-electron reaction, respectively. Assuming that one platinum surface site is destroyed per one-electron reaction, α1=1, α2=1, α3=2, and α4=2. In actual use, since the platinum surface is not uniform, α1 to α4 are determined to fit the test results.

[0084] However, formula (9) does not take into account that the surface oxidized species (coverage θox1[i], θox2[i]) and the internal oxidized species (coverage θox3[i]) occur at the same platinum site, and in such a case, there is a concern that θact[i] may be underestimated. For example, when Vcat[i] remains high continuously, the above problem becomes more pronounced as θox1[i], θox2[i], and θox3[i] each become large, and there is a concern that the accuracy may decrease. In contrast, equation (8) has the advantage that it can estimate with high accuracy even in the above case by taking the ratio of the surface oxidized species to the internal oxidized species. On the other hand, in other cases, there is a concern that the accuracy of equation (8) will be lower than that of equation (9).

[0085] [4. Flowchart] 4 shows a flow chart of the potential control method according to the present invention. First, in step 1 (hereinafter simply referred to as "S1"), a load demand P FC (load request acquisition unit).

[0086] Next, in S2, the detection values ​​of various sensors are acquired, and the electrochemical surface area A of the cathode catalyst included in the fuel cell is calculated. ECS [i] is estimated (deterioration amount estimator, ECS estimator). Detected values ​​include, for example, the fuel cell voltage V[i], current I[i], and high-frequency impedance R[i] at time [i]. Next, in S3, A ECS Based on [i], the average particle diameter D of the cathode catalyst at time [i] CAT [i] is estimated (deterioration amount estimation section, average particle size estimation section).

[0087] Next, in S4, the deterioration amount (i.e., A ECS [i], D CAT [i]) depending on the upper limit potential V H and the lower limit potential V L The potential range Vr of the polymer electrolyte fuel cell, which is determined by the above formula (4) and formula (5), is changed (potential range setting unit). Specifically, Vr is preferably set using the above formula (4) and formula (5) (first setting unit, second setting unit).

[0088] Next, in S5, P FC Based on the control point V P Next, proceed to S6. In S6, V P V H It is judged whether it is less than V. P V H If it is not less than V (S6: NO), proceed to S7. real V H The fuel cell is controlled so that the above-mentioned condition is satisfied. Then, the process proceeds to S8. In S8, it is determined whether or not the control should be continued. If the control should be continued (S8: YES), the process returns to S1, and the above-mentioned steps S1 to S8 are repeated.

[0089] On the other hand, in S6, V P V H If it is less than the threshold (S6: YES), proceed to S9. P V L It is judged whether or not the value exceeds V. P V L If it does not exceed V (S9: NO), proceed to S10. real V L The fuel cell is controlled so that the above-mentioned condition is satisfied. Then, the process proceeds to S8. In S8, it is determined whether or not the control should be continued. If the control should be continued (S8: YES), the process returns to S1, and the above-mentioned steps S1 to S10 are repeated.

[0090] Furthermore, in S9, V P V L If it exceeds V (S9: YES), proceed to S11. real V P The fuel cell is controlled so that the potential is increased. Then, the process proceeds to S8. In S8, it is determined whether or not the control should be continued. If the control should be continued (S8: YES), the process returns to S1, and the above-mentioned steps S1 to S11 are repeated. On the other hand, if the control should not be continued (S8: NO), the control is ended. In the case of FIG. 1, S6 to S7 and S9 to S11 correspond to the potential control section.

[0091] [5. Effect] Fuel cell deterioration is mainly caused by the elution and redeposition of the cathode catalyst components. The higher the potential of the cathode catalyst, the more the elution reaction of the catalyst components proceeds. On the other hand, when the potential is high, oxides are formed on the surface of the cathode catalyst, suppressing the elution reaction, but the reaction rate of the reaction to form the oxides is slow. Furthermore, when the potential is low, the catalyst components eluted from the fine cathode catalyst particles are redeposited on the surface of the coarse cathode catalyst particles. During the use of a fuel cell, large potential fluctuations occur, and the elution and re-precipitation of the catalyst components are repeated. This leads to a decrease in the number of cathode catalyst particles and / or a decrease in the effective reaction area (electrochemical effective surface area A ECS) decreases, resulting in a decrease in fuel cell performance.

[0092] In contrast, the equilibrium potential of the electrochemical reaction (reaction A) that dissolves the components of the cathode catalyst is the average particle diameter D of the cathode catalyst. CAT It depends on [i]. Specifically, D CAT The larger [i] is, the higher the potential required to cause reaction A. Therefore, D CAT When [i] is large, reaction A does not proceed easily even if the potential is increased, so the upper limit potential V H Even if the D is increased, the durability is not affected. CAT The larger [i] is, the greater V H Increasing the potential increases the frequency at which the fuel cell is maintained in a high potential state with high efficiency, thereby improving the efficiency of the fuel cell system.

[0093] In addition, in order to suppress the deterioration of the cathode catalyst, it is important to maintain the oxide on the surface of the cathode catalyst. L It is preferable to operate the fuel cell at a potential always higher than the potential at which the oxide begins to be reduced. On the other hand, the equilibrium potential of the electrochemical reaction (reaction B) in which an oxide is formed on the surface of the cathode catalyst is also set to D CAT It depends on [i]. Specifically, D CAT The larger [i] is, the higher the potential required to cause reaction B. Therefore, D CAT Even though [i] is large, V L If D is kept low, the oxides are easily reduced. CAT The larger [i] is, the greater V L Increasing the amount of oxygen makes it easier to retain oxides, improving the durability of the fuel cell. EXAMPLES

[0094] (Example 1, Comparative Example 1) [1. Test method] The deterioration of the cathode catalyst and the power generation efficiency were calculated by simulation when a fuel cell system in which the cathode catalyst was Pt particles was operated under specified conditions. The simulation is (a) According to the flowchart in Figure 4, the upper limit potential V H is increased from 0.8V to 1.0V, and the lower limit potential V L When the voltage is increased from 0.4V to 0.7V (Example 1), and (b)V H is fixed at 1.0V, and V L When fixed at 0.4V (Comparative Example 1) I followed him.

[0095] The fuel cell was operated under the LA#4 mode, which is a commonly used pattern for measuring exhaust gas emissions. The power generation efficiency was calculated as the ratio of the ideal energy of hydrogen, estimated from the integrated amount of hydrogen used, which can be derived from the amount of current, to the amount of work actually used during driving.

[0096] [2. Results] Fig. 5 shows a comparison of the deterioration amount and power generation efficiency of the fuel cell systems of Example 1 and Comparative Example 1. Fig. 5 shows that by using the method according to the present invention, the deterioration of the cathode catalyst can be suppressed without sacrificing the power generation efficiency.

[0097] Although the embodiment of the present invention has been described in detail above, the present invention is not limited to the above embodiment, and various modifications are possible without departing from the gist of the present invention. [Industrial Applicability]

[0098] The fuel cell potential control device according to the present invention can be used for controlling power generation in a fuel cell vehicle.

Claims

1. Load requirement P for polymer electrolyte fuel cells FC A load request acquisition unit that acquires a load request; a deterioration amount estimation unit that estimates a deterioration amount of a cathode catalyst of the solid polymer electrolyte fuel cell; Depending on the amount of deterioration, the upper limit potential V H and the lower limit potential V L a potential range setting unit for changing a potential range Vr of the polymer electrolyte fuel cell, the potential range Vr being determined by The P FC The real potential V of the polymer electrolyte fuel cell corresponding to real a potential control unit that controls the potential of the polymer electrolyte fuel cell so that the potential Vr is within the range of Vr; Equipped with The deterioration amount estimation unit An ECS estimation unit that estimates an electrochemically effective surface area A ECS [i] of the cathode catalyst at time [i]; an average particle diameter estimation unit that estimates an average particle diameter D CAT [i] of the cathode catalyst at the time [i] based on the A ECS [i]; Equipped with The potential range setting unit is a first setting unit that increases the V H as the D CAT [i] increases; and / or a second setting unit that increases V L as D CAT [i] increases; Equipped with Fuel cell control device.

2. The ECS estimation unit calculates the A based on the following equation (1). ECS The fuel cell control device according to claim 1 , further comprising a first calculation unit that calculates [i]. [0010] however, A ECS0 is the initial value of the electrochemically effective surface area, B 1 is the fitting coefficient, Ts is the calculation interval.

3. The ECS estimation unit is an information acquisition unit that sequentially acquires the voltage V[i] of the polymer electrolyte fuel cell at least at the time [i]; a catalyst potential calculation unit that calculates a catalyst potential Vcat[i] of the cathode of the polymer electrolyte fuel cell at the time [i] using at least the V[i]; a surface utilization rate calculation unit that calculates an effective surface utilization rate θact[i] of the cathode catalyst at the time [i] using the Vcat[i]; The A is calculated using the following formula (2) and / or formula (3). ECS A second calculation unit that calculates [i]; The fuel cell control device according to claim 1 , further comprising: [0025] however, A ECS0 is the initial value of the electrochemically effective surface area, D 1 ~D 4 is the fitting coefficient, Ts is the calculation interval.

4. The first setting unit determines the V based on the following formula (4): H [0023] The present invention includes a device for setting The second setting unit determines the V based on the following formula (5): L Including a device for setting The fuel cell control device according to claim 1 . [0030] However, β 1 ~β 4 is the fit coefficient.

5. The potential control unit is The P FC The output current I P and the output potential V P A control point setting unit for acquiring a control point determined by (a) The V P When is within the range of Vr, real As the V P Select (b) the V P is the V H When it exceeds the above V real As the V H Select (c) Said V P is the V L When it is less than V real As the V L Select Potential output section The fuel cell control device according to claim 1 , further comprising:

Images