JP7669776B2 - Fuel cell potential control device - Google Patents

Description

The present invention relates to a fuel cell potential control device, and more specifically, to a fuel cell potential control device capable of controlling a fuel cell so that when a load is required for the fuel cell, the requested power is output, the power generation efficiency is relatively high, and deterioration of the cathode catalyst is minimized.

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 is generally made 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) having a gas flow path is further 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.

When a polymer electrolyte fuel cell is used as an on-board power source, the voltage of the polymer electrolyte fuel cell fluctuates 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, making it easier for catalytic components to dissolve 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, making it easier for the dissolved catalytic components to re-deposit on the surface of the cathode catalyst. Therefore, if the cathode catalyst is repeatedly exposed to high and low potential states, there is a problem in that the cathode catalyst gradually deteriorates.

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.

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 value 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 set to a high potential avoidance voltage that is lower than the open end voltage of the fuel cell.

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.

Patent Document 7 discloses a power generation 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 value for the output voltage of the fuel cell depending on the judged state of deterioration.

Patent Document 9 discloses a fuel cell system in which the lower limit voltage and the upper limit voltage of the fuel cell during system operation are set taking into consideration the progression of deterioration of the carbon support due to potential fluctuations.
Patent Document 10 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.

Furthermore, Patent Document 11 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;
The present invention discloses a fuel cell system including a control unit that controls the 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.

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 for the decrease in the power density is that when a lower limit potential is set, the power that can be swept from the fuel cell is limited.

On the other hand, Patent Document 8 discloses a method of 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.
Patent Document 9 discloses a method for varying the upper and lower limit potentials depending on the deterioration state of the carbon support. However, the method described in this document, like Patent Document 8, is unable to achieve both power generation efficiency and output density.
Furthermore, the methods described in Patent Documents 10 and 11 are methods for suppressing deterioration (overcharging) of a power storage device, and are not methods for suppressing deterioration of a fuel cell.

JP 2012-094257 A JP 2006-309971 A JP 2008-165994 A JP 2009-129639 A JP 2007-109569 A

JP 2009-104977 A JP 2003-036871 A JP 2012-129069 A JP 2010-021072 A JP 2009-129647 A JP 2013-098052 A

The problem that this invention aims to solve is to provide a fuel cell potential control device that can control a fuel cell so that deterioration of the cathode catalyst is minimized without sacrificing output density or power generation efficiency when a load is required for the fuel cell.

In order to solve the above problems, the fuel cell potential control device according to the present invention comprises:
(A) a first means for acquiring a load requirement P for a polymer electrolyte fuel cell and storing said P in a memory;
(B) a second means for acquiring a total voltage V(i) of at least the solid polymer electrolyte fuel cell at time i and storing the V(i) in the memory;
(C) a third means for calculating a total amount M(i) of precious metal oxide and precious metal hydroxide contained in the cathode-side precious metal-based catalyst particles at the time i based on at least the V(i) and storing the M(i) in the memory;
(D) when the M(i) is less than the first threshold ε ₁ (or the M(i) is equal to or less than the ε ₁ ), decreasing the upper limit potential _VH of the potential range _Vr of the polymer electrolyte fuel cell and/or increasing the lower limit potential _VL of the potential range _Vr ;
When M(i) is equal to or greater than a second threshold ε ₂ (≧ε ₁ ) (or when M(i) is greater than ε ₂ ), increasing V _H and/or decreasing V _L ;
A fourth means for storing the increased or decreased _VH and/or _VL in the memory;
(E) a fifth means for setting a control point determined by an output current I _P and an output potential V _P based on the P, and storing the control point in the memory;
(F) when V _H >V _P is not satisfied (or when V _H ≧V _P is not satisfied), output the V _H as the voltage V(i+1) at time (i+1);
When V _L <V _P is not satisfied (or when V _L ≦V _P is not satisfied), output V _L as V(i+1);
and sixth means for outputting V _P as V(i+1) when V _L <V _P <V _H (or when V _L ≦V _P ≦V _H ).

When the cathode-side precious metal-based catalyst particles are exposed to a high potential, the precious metal components in the precious metal-based catalyst particles are oxidized, and a coating containing precious metal oxides and precious metal hydroxides is formed on the surfaces of the precious metal-based catalyst particles. The total amount M(i) of precious metal oxides and precious metal hydroxides at time i depends on the operating history of the fuel cell (mainly the voltage V(i) of the fuel cell at time i).
Therefore, by successively acquiring the operation history of the fuel cell, it is possible to know M(i) at the current time, that is, the total amount of oxides and hydroxides covering the surfaces of the precious metal-based catalyst particles.

A small M(i) means that the total amount of oxides and hydroxides covering the surface of the precious metal catalyst particles is small. In such a case, when a sudden potential change acts on the precious metal catalyst particles, dissolution and precipitation of the precious metal components progresses, which causes the catalyst to deteriorate.
Therefore, when M(i) is small, narrowing the potential range _Vr of the fuel cell (i.e., decreasing the upper limit potential _VH and/or increasing the lower limit potential _VL ) suppresses excessive potential fluctuations, and as a result, deterioration of the precious metal-based catalyst particles can be suppressed.

On the other hand, a large M(i) means that the total amount of oxides and hydroxides covering the surface of the precious metal catalyst particles is large. In such a case, even if a sudden potential change acts on the precious metal catalyst particles, dissolution and precipitation of the precious metal components are unlikely to proceed.
Therefore, when M(i) is large, by widening the potential range _Vr of the fuel cell (i.e., by increasing the upper limit potential _VH and/or decreasing the lower limit potential _VL ), it is possible to output the required electric power while suppressing the dissolution and deposition of the precious metal components, or to select operating conditions that provide high power generation efficiency.

FIG. 2 is a schematic cross-sectional view of a Pt particle on which an oxide film is formed. FIG. 13 is a graph showing the effect of the coverage ratio θ _ad on the dissolution rate of a catalyst component. FIG. 1 is a graph showing the relationship between fuel cell potential and redox current. 13 is a diagram showing an example of the relationship between the coverage θ _ad and the correction amount of the upper limit potential V _H. FIG. 1 is a diagram showing the relationship between an output current I and an output potential V, and the relationship between the output current I and an output power P. FIG. 4 is a flowchart of a potential control method according to the present invention. 1 shows a comparison of the deterioration amount, power generation efficiency, and output density of the fuel cell systems of Example 1 and Comparative Example 1.

An embodiment of the present invention will be described in detail below.
[1. Fuel cell potential control device]
The fuel cell potential control device according to the present invention comprises:
(A) a first means for acquiring a load requirement P for a polymer electrolyte fuel cell and storing said P in a memory;
(B) a second means for acquiring a total voltage V(i) of at least the solid polymer electrolyte fuel cell at time i and storing the V(i) in the memory;
(C) a third means for calculating a total amount M(i) of precious metal oxide and precious metal hydroxide contained in the cathode-side precious metal-based catalyst particles at the time i based on at least the V(i) and storing the M(i) in the memory;
(D) when the M(i) is less than the first threshold ε ₁ (or the M(i) is equal to or less than the ε ₁ ), decreasing the upper limit potential _VH of the potential range _Vr of the polymer electrolyte fuel cell and/or increasing the lower limit potential _VL of the potential range _Vr ;
When M(i) is equal to or greater than a second threshold ε ₂ (≧ε ₁ ) (or when M(i) is greater than ε ₂ ), increasing V _H and/or decreasing V _L ;
A fourth means for storing the increased or decreased _VH and/or _VL in the memory;
(E) a fifth means for setting a control point determined by an output current I _P and an output potential V _P based on the P, and storing the control point in the memory;
(F) when V _H >V _P is not satisfied (or when V _H ≧V _P is not satisfied), output the V _H as the voltage V(i+1) at time (i+1);
When V _L <V _P is not satisfied (or when V _L ≦V _P is not satisfied), output V _L as V(i+1);
and sixth means for outputting V _P as V(i+1) when V _L <V _P <V _H (or when V _L ≦V _P ≦V _H ).

[1.1. First means]
The first means is a means for acquiring a load requirement P for a polymer electrolyte fuel cell (hereinafter also simply referred to as a "fuel cell") and storing in a memory the load requirement P. In the case of a fuel cell vehicle, the load P corresponds to a load required of the fuel cell by the system due to accelerator operation or the like.
In the present invention, the method of acquiring P is not particularly limited, and an optimal method can be selected depending on the purpose. The acquired P is used to determine the output current I _P and the output voltage V _P of the fuel cell. This will be described later.

[1.2. Second means]
The second means is a means for acquiring at least the total voltage V(i) of the polymer electrolyte fuel cell at time i, and storing V(i) in a memory.
The second means may include means for acquiring, in addition to V(i), the current I(i) and DC resistance R(i) of the polymer electrolyte fuel cell at time i, and storing I(i) and R(i) in a memory.
Furthermore, the second means may further include means for acquiring the temperature T _e (i) of the polymer electrolyte fuel cell at time i and storing it in a memory.

In the present invention, the method of acquiring V(i) is not particularly limited, and an optimal method can be selected depending on the purpose. The acquired V(i) is used to calculate the catalytic potential Vcat(i) of the cathode catalyst at time i.
In addition, when I(i) and R(i) are further acquired, these are also used in the calculation of Vcat(i). The method of measuring the DC resistance R(i) is not particularly limited, but it is preferable to superimpose a high frequency on the current I(i) or the total voltage V(i) using an FDC or the like and measure it.
Furthermore, when T _e (i) is acquired, T _e (i) is used to correct the coverage factors θ ₁ (i) to θ ₃ (i), that is, to correct M(i), because θ ₁ (i) to θ ₂ (i) also depend on T _e (i), strictly speaking.
In addition, since T _e (i) also affects the deterioration (dissolution, precipitation) rate, V _L and V _H may be corrected according to T _e (i), as will be described later.

Here, "total voltage V(i)" refers to the potential difference across the stack at time i.
Strictly speaking, the "catalytic potential Vcat(i) of the cathode catalyst" refers to the value obtained by adding the potential drop caused by the internal resistance to the potential of the cathode of each single cell. Strictly speaking, Vcat(i) is calculated based on V(i), I(i), and R(i), but when R(i) cannot be obtained, it may be calculated by an approximation calculation using only V(i). Details of the calculation method of Vcat(i) will be described later.

[1.3. Third means]
The third means is a means for calculating the total amount M(i) of precious metal oxide and precious metal hydroxide contained in the cathode-side precious metal-based catalyst particles at time i based on at least V(i) and storing M(i) in memory.

[1.3.1. Precious metal catalyst particles]
In the present invention, the term "noble metal-based catalyst particles (hereinafter, simply referred to as "catalyst particles")" refers to particles made of a metal or alloy containing a noble 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.
(a) Noble 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.).

Calculation of Vcat(i)
M(i) depends on the operation history of the catalyst particles, i.e., Vcat(i). Therefore, in order to calculate M(i), it is necessary to first calculate Vcat(i). Since Vcat(i) is a function of V(i), Vcat(i) can be calculated by obtaining at least V(i). Specifically, Vcat(i) is expressed by the following formula (1) or formula (2).

但し、
Ｎ_cellは、前記固体高分子形燃料電池のセルの積層数、
Ａ_cellは、前記セルの面積。

however,
N _cell is the number of stacked cells of the polymer electrolyte fuel cell,
A _cell is the area of the cell.

Strictly speaking, Vcat(i) is expressed by formula (1). In formula (1), the first term represents the potential difference (cell voltage) across the single cell. The second term represents the potential drop per single cell due to internal resistance. By using formula (1), Vcat(i) can be calculated accurately. In order to accurately calculate M(i), it is preferable to use formula (1) for calculating Vcat(i).
Equation (2) is an approximation of Vcat(i) that ignores the potential drop caused by the internal resistance. Equation (2) is less accurate than Equation (1). However, by using Equation (2), calculation can be performed without acquiring R(i), so that the calculation of Vcat(i) can be simplified.

[1.3.3. Types of oxides]
When the catalyst particles on the cathode side are exposed to a high potential, the catalyst components are more likely to dissolve from the catalyst particles. On the other hand, when the catalyst particles are exposed to a high potential, an oxide film (including hydroxide) is formed on the surface of the catalyst particles, suppressing the dissolution of the catalyst components from the catalyst particles. However, since the rate at which the oxide film forms is slow, if the potential of the cathode 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 the fuel cell is continued to be used in an environment where sudden potential fluctuations are repeated, the catalyst particles will eventually deteriorate.
In other words, the durability of the cathode-side precious metal-based catalyst particles depends on the total amount M(i) of the precious metal oxide and precious metal hydroxide present on the surface of the precious metal-based catalyst particles.

The precious metal oxide present on the surface of the precious metal catalyst particles is
(a) a precious metal oxide adsorbed on the surface of a precious metal-based catalyst particle;
(b) a precious metal hydroxide adsorbed on the surface of the precious metal-based catalyst particles; and
(c) precious metal oxides formed just below the surface of the particles as a result of oxygen diffusing into the interior of the precious metal catalyst particles.

A schematic cross-sectional view of a Pt particle on which an oxide film is formed is shown in Fig. 1. 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 oxide (PtO _ad ) adsorbed on the surface of Pt particles;
(b) Pt hydroxide (PtOH _ad ) adsorbed on the surface of the Pt particles; and
(c) Pt oxide (PtO _sub ) formed just below the surface of the Pt particle due to oxygen diffusion into the Pt particle.
It consists of:

Here, the coverage of the precious metal oxide adsorbed on the surface of a precious metal-based catalyst particle such as _PtOad at time _i is denoted as _θ1 (i), which is expressed as the ratio (=S1/ _S0 ) of the area ( _S1 ) of the precious metal oxide adsorbed on the surface of the precious metal-based catalyst particle to _the surface area ( _S0 ) of the precious metal-based catalyst particle.
Similarly, the coverage of the precious metal hydroxide adsorbed on the surface of a precious metal-based catalyst particle such as PtOH _ad at time _i is denoted as _θ2 (i), which is expressed as the ratio (= _S2 / _S0 ) of the area (S2) of the precious metal hydroxide adsorbed on the surface of the precious metal-based catalyst particle _{to S0} .
Similarly, the coverage of the precious metal oxide present inside a precious metal-based catalyst particle such as PtO _sub at time i is defined as _θ3 (i), which is expressed as the ratio (= _S3 / _S0 ) of the area ( _S3 ) of the precious metal oxide present inside the precious metal-based catalyst particle _{to S0} .
Furthermore, in particular, the sum of θ ₁ (i) and θ ₂ (i) is denoted as θ _ad (i).

When a precious metal oxide and/or a precious metal hydroxide is adsorbed on the surface of a precious metal-based catalyst particle, the elution of the catalyst components from the particle surface is suppressed, which means that when θ _ad (i) is large, the catalyst particles are less likely to deteriorate even when exposed to a high potential.
FIG. 2 shows the effect of the coverage θ _ad on the dissolution rate of the catalyst component.
(a) When catalyst particles are exposed to a high potential, the dissolution rate increases; and
(b) It can be seen that the larger θ _ad is, the slower the dissolution rate is.

On the other hand, when a precious metal oxide is formed inside a precious metal catalyst particle, the catalyst particle becomes stable and the precious metal oxide becomes difficult to reduce. This means that when _θ3 is large, the oxide on the catalyst particle surface is maintained even if the catalyst particle is exposed to a low potential.
Figure 3 shows the relationship between the fuel cell potential and the redox current. The vertical axis in Figure 3 is the reduction current of oxides (including hydroxides; the same applies below), and the smaller this value (the lower down on the graph), the more the oxides adsorbed on the surface are reduced and θ _ad (i) is reduced. Figure 3 shows that the larger θ ₃ (i) is, the more the potential at which the reduction current begins to decrease (the potential at which θ _ad (i) begins to decrease) shifts to the lower potential side. This is because the larger θ ₃ (i) is, the more stable the catalyst particles become, making it more difficult for oxides to be reduced.

That is, in order to suppress deterioration (=dissolution) of the catalyst particles, it is preferable to prevent them from being exposed to a high potential when the proportion of the non-oxidized state is large (when θ _ad (i) is small). For this purpose, it is preferable to set an upper limit potential _VH of the fuel cell.
In addition, since the power generation efficiency is better at higher potentials, in order to increase the power generation efficiency without degrading the catalyst particles, it is preferable to prevent the catalyst particles from being exposed to a low potential for a long period of time (i.e., to prevent _θ3 from becoming excessively small). For this purpose, it is preferable to set a lower limit potential _VL of the fuel cell.

In order to maximize accuracy and effectiveness, it is desirable to calculate θ ₁ (i), θ ₂ (i), and θ ₃ (i) individually at time i. However, if the total amount M(i) of precious metal oxides and precious metal hydroxides at time i is generalized using θ ₁ (i) to θ ₃ (i), the potential range _Vr of the fuel cell can be easily controlled using the generalized M(i).

1.3.4. Calculation of M(i)
Specifically, the total amount M(i) of the precious metal oxide and precious metal hydroxide contained in the cathode-side precious metal catalyst particles at time i is preferably calculated based on the following formula (3).

however,
θ ₁ (i) is the coverage of the precious metal oxide adhering to the surface of the precious metal-based catalyst particle at time i;
θ ₂ (i) is the coverage of the precious metal hydroxide adhering to the surface of the precious metal-based catalyst particle at time i;
θ ₃ (i) is the coverage of the precious metal oxide present inside the precious metal-based catalyst particle at time i;
Γ is the maximum surface coverage of oxygen per unit surface area (constant),
ΔT is the calculation step size,
α ₁₁ to α ₃₇ are the fit coefficients,
A _PM is the total surface area of precious metal catalyst particles.

Formula (3) represents the total amount of oxides and hydroxides formed on the surface of the precious metal-based catalyst particles.
v ₁ to v ₃ represent the reaction rates of the formation and disappearance of each oxide or hydroxide (MO _ad , MOH _ad , MO _sub ).
G ₁ to G ₃ represent the free energies of the reactions of v ₁ to v ₃ .
θ ₁ (i-1), θ ₂ (i-1), and θ ₃ (i-1) are the coverage rates at time (i-1), and are already stored in memory. θ ₁ (i-1), θ ₂ (i-1), and θ ₃ (i-1) can be calculated by sequential calculations once the initial values are known. The initial values may be retained and used as the values from the previous shutdown. Generally, when a fuel cell is shut down, it is often held at a low potential, and at that time, all oxides are reduced. Therefore, the initial values after shutdown may be θ ₁ = θ ₂ = θ ₃ = 0.
α ₁₁ to α ₃₇ can be calculated from cyclic voltammetry.
A _PM can be calculated by integrating the amount of reaction of the catalyst from the reaction current by cyclic voltammetry.
Therefore, if Vcat(i) is obtained, M(i) can be calculated from equation (3).

Although α ₁₂ , α ₁₃ , α ₂₂ , α ₂₃ , α ₃₂ , and α ₃₃ may be regarded as constants, strictly speaking, they depend on the temperature T _e (i). Therefore, when the second means acquires the temperature T _e (i) of the fuel cell at time i, these compatibility coefficients may be corrected according to T _e (i). The following formula (3.1) shows an example of the correction formula.
α _ij =α _ij ⁰ /T _e (i) …(3.1)
however,
α _ij ⁰ is the reference value of the fitting coefficient α _ij ,
i = 1, 2, or 3, j = 2 or 3.

Similarly, α ₁₁ , α ₂₁ , and α ₃₁ may be regarded as constants, but strictly speaking they depend on the temperature T _e (i). Therefore, when the second means acquires the temperature T _e (i) of the fuel cell at time i, these adaptation coefficients may be corrected according to T _e (i). An example of the correction formula is shown in the following formula (3.2).
α _ij =k _i T _e (i)exp(α _ij ⁰ /T _e (i)) …(3.2)
however,
α _ij ⁰ is the reference value of the fitting coefficient α _ij ,
_k is the rate constant,
i=1, 2, or 3, j=1.
Both α _ij ⁰ and k _i can be calculated by cyclic voltammetry at different temperatures.

[1.4. Fourth means]
The fourth means is
When M(i) is less than the first threshold ε ₁ (or M(i) is equal to or less than ε ₁ ), the upper limit potential V _H of the potential range V _r of the polymer electrolyte fuel cell is decreased and/or the lower limit potential V _L of V _r is increased;
When M(i) is equal to or greater than a second threshold ε ₂ (≧ε ₁ ) (or M(i) is greater than ε ₂ ), increasing V _H and/or decreasing V _L ;
The increased or decreased V _H and/or V _L are stored in the memory.

[1.4.1. First Threshold]
The "first threshold value ε ₁ " refers to the threshold value of M(i) for reducing the potential range _Vr of the fuel cell. The value of _{ε 1} is not particularly limited, and it is preferable to select an optimal value depending on the composition of the catalyst particles, the required durability, etc.
A relatively small M(i) means that there is less oxide protecting the surface of the catalyst particle. In such a case, if a sudden potential change acts on the catalyst particle, the catalyst particle is likely to deteriorate. Therefore, when M(i) is less than ε ₁ (or ε ₁ or less), it is preferable to reduce _Vr . The reduction of _Vr may be performed by either decreasing the upper limit potential _VH or increasing the lower limit potential _VL , or both.
In the present invention, when "M(i) is less than _ε1 (or equal to or less than _ε1 )," it means that the boundary value (in this case, M(i)= _ε1 ) belongs to either the "true" side or the "false" side of the proposition. The same applies when judging the truth or falsity of other variables.

[1.4.2. Second Threshold]
The "second threshold value _ε2 " refers to the threshold value of M(i) for expanding the potential range _Vr of the fuel cell. The value of _ε2 is not particularly limited, and it is preferable to select an optimal value depending on the composition of the catalyst particles, the required durability, etc. _ε2 may be the same value as _ε1 , or may be a value larger than _ε1 .
A relatively large M(i) means that the catalyst particles are protected by a relatively large amount of oxide. In such a case, even if a sudden potential change acts on the catalyst particles, the catalyst particles are unlikely to deteriorate. Therefore, when M(i) is equal to or greater than _ε2 (or exceeds _ε2 ), it is preferable to increase _Vr . The increase in _Vr may be performed by either increasing the upper limit potential _VH or decreasing the lower limit potential _VL , or both.

[1.4.3. Setting of upper limit potential _VH and lower limit potential _VL ]
As described above, when M(i) is large, it is preferable to increase _{Vr in order to improve the power generation efficiency or improve the output density. On the other hand, when M(i) is small, it is preferable to reduce Vr in} order to suppress the deterioration of the catalyst particles. Furthermore, the increase and decrease of _Vr may be performed by increasing or decreasing either _VH or _VL , or may be performed by increasing or decreasing both. In this case, the correction amount of _VH and the correction amount of _VL are not particularly limited, and the optimal correction amount can be selected according to the purpose.

In general, the larger Vcat(i) and/or the smaller _θad (i) is, the greater the amount of eluted catalytic components becomes, and therefore, it is preferable to set the correction amount of _VH to a value that does not increase the amount of eluted catalytic components compared to the reference value even if _VH is increased.
Fig. 4 shows an example of the relationship between the coverage θ _ad and the correction amount of the upper limit potential V _H. Fig. 4 shows the correction amount of the upper limit potential V _H that can obtain the same dissolution rate as the dissolution rate when the coverage θ _ad is zero. In other words, for each coverage θ _ad , the dissolution rate at the corrected upper limit potential is the same.
The correction amount of the lower limit potential _VL is similar, and it is preferable to set the correction amount of _VL to a value that does not increase the amount of reduction of _oxides compared to a reference value (for example, the amount of reduction when _θ3 is zero) even if VL is reduced.

The fourth means, in particular,
A means for determining the _VH based on the following formula (4); and/or
It is preferable that the device includes a means for determining V _L based on the following formula (5):

但し、
Ｖ_H0は、上限電位の基準値、
Ｖ_L0は、下限電位の基準値、
β₁～β₄は、それぞれ、適合係数。

however,
V _H0 is the reference value of the upper limit potential,
V _L0 is the reference value of the lower limit potential,
β ₁ to β ₄ are the fit coefficients, respectively.

The higher _VHO , the higher the efficiency of the system but the lower the durability. Conversely, the lower _VHO , the lower the efficiency of the system but the higher the durability. Therefore, it is preferable to determine _VHO based on the efficiency and durability required by the system.
The higher _VLO is, the lower the maximum output (i.e., the larger the body size is to obtain the same output), but the durability is improved. Conversely, the lower _VLO is, the higher the maximum output is, but the durability is reduced. Therefore, it is preferable to determine _VLO based on the body size and durability required by the system.
It is preferable to determine β ₁ to β ₄ based on the durability required for the system.

Equation (4) represents the upper limit potential at which the dissolution rate of the catalyst component becomes the same as that at the reference value V _H0 . It can be seen from equation (4) that the larger θ _ad (i) becomes, the larger V _H increases.
Moreover, formula (5) represents the lower limit potential at which the amount of oxide reduction is the same as that at the reference value V _L0 . From formula (5), it can be seen that the larger θ _ad (i) and/or the larger θ ₃ , the more V _H decreases.
Therefore, by setting β ₁ to β ₄ to appropriate values, it is possible to appropriately increase or decrease V _H and/or V _L according to the values of θ ₁ to θ ₃ .

Furthermore, in formulas (4) and (5), _VLO and _VHO may be regarded as constants, but strictly speaking they depend on the temperature _Te (i). Therefore, in the second means, if the temperature _Te (i) of the fuel cell at time i is acquired, _VHO and/or _VLO may be corrected according to _Te (i). The following formulas (6) and (7) show examples of correction formulas for _VHO and _VLO , respectively.
V _H0 =V _H0 '+β ₅ T _e (i)/T _H0 ...(6)
V _L0 =V _L0 '+β ₆ T _e (i)/T _L0 ...(7)
however,
V _H0 ' is a reference value (constant) of the upper limit potential at temperature T _H0 ,
V _L0 ' is a reference value (constant) of the lower limit potential at temperature T _L0 ,
_T is the reference temperature (constant) of the upper limit potential.
T _L0 is the reference temperature (constant) of the lower limit potential.
β ₅ and β ₆ are the fit coefficients, respectively.

[1.5. Fifth means]
The fifth means is a means for setting a control point determined by the output current I _P and the output potential V _P on the basis of P, and storing the control point in a memory.
The "control point" refers to a combination of output current I _P and output voltage V _P that provides power equivalent to the load request P and maximizes power generation efficiency.

5 shows the relationship between the output current I and the output potential V, and the relationship between the output current I and the output power P. As shown in Fig. 5, when the output power (load requirement P) is determined, the combination of the output current I _P and the output potential V _P required to generate the required power is uniquely determined.

[1.6. Sixth means]
The sixth means is
When V _H >V _P is not satisfied (or when V _H ≧V _P is not satisfied), V _H is output as a voltage V(i+1) at time (i+1);
When V _L <V _P is not satisfied (or when V _L ≦V _P is not satisfied), output V _L as V(i+1);
When V _L <V _P <V _H (or when V _L ≦V _P ≦V _H ), this is a means for outputting the V _P as the V(i+1).

"When _VH > _VP is not satisfied (or when _VH ≥ _VP is not satisfied)" means that _VP is equal to or greater than _VH (or _VP exceeds _VH ). In this case, if the fuel cell is operated at _VP , there is a high possibility that deterioration of the catalyst particles will progress. In such a case, it is preferable to operate the fuel cell at _VH , although the efficiency may decrease slightly.
"When _VL < _VP is not satisfied (or when _VL ≦ VP is not _satisfied )" means that _VP is equal to or less than _VL (or _VP is less than _VL ). In this case, if the fuel cell is operated at _VP , there is a high possibility that the reduction of oxides present on the catalyst particle surface will proceed excessively. In such a case, it is preferable to operate the fuel cell at _VL , although the power density may decrease slightly.
"When _VL < _VP < _VH (or when _VL ≦ _VP ≦ _VH )" means that _VP is within the allowable potential range Vr. In such a case, it is preferable to operate the fuel cell _{at VP .} Note that the boundary values ( _VL and _VH ) only need to belong to either the "true" side or the "false" side of the proposition, and this means that, for example, even when "when _VL ≦ _VP < _VH " or "when _VL < _VP ≦ _VH" , the fuel cell may be operated at _VP .

[2. Potential Control Method]
6 is a flowchart of the potential control method according to the present invention. First, in step 1 (hereinafter simply referred to as "S1"), a load requirement P for the polymer electrolyte fuel cell is obtained and stored in memory (first procedure).

Next, in S2, at least the total voltage V(i) of the polymer electrolyte fuel cell at time i is obtained, and V(i) is stored in memory (second procedure). As described above, in S2, in addition to V(i), detection values of various sensors (e.g., I(i), R(i), T(i), _{etc. )} required for calculating the coverages _θ1 (i) _-θ3 (i), the total amount M(i) of oxides and hydroxides, the upper potential _VH , and/or the lower potential VL may be further obtained.

Next, in S3, the total amount M(i) of precious metal oxides and precious metal hydroxides contained in the cathode-side precious metal-based catalyst particles at time i is calculated based on at least V(i), and M(i) is stored in memory (third step).
It is preferable that M(i) is calculated based on the above-mentioned formula (3). When M(i) is calculated based on the formula (3), θ ₁ (i) to θ ₃ (i) are also calculated at the same time. θ ₃ and θ _ad (i) (= θ ₁ (i) + θ ₂ (i)) are used when determining V _H and/or V _L. Details of the formula (3) are as described above, so a description thereof will be omitted.

Next, in S4,
When M(i) is less than the first threshold ε ₁ (or M(i) is equal to or less than ε ₁ ), the upper limit potential V _H of the potential range V _r of the polymer electrolyte fuel cell is decreased and/or the lower limit potential V _L of V _r is increased;
When M(i) is equal to or greater than a second threshold ε ₂ (≧ε ₁ ) (or M(i) is greater than ε ₂ ), increasing V _H and/or decreasing V _L ;
The increased or decreased _VH and/or _VL are stored in memory (fourth step).
It is preferable to calculate _VH and _VL using the above-mentioned formula (4) and formula (5), respectively. _VH and _VL can be calculated by substituting _θad (i) and _θ3 (i) calculated in S3 into formula (4) and formula (5). Details of formula (4) and formula (5) are as described above, so their explanation will be omitted.

Next, in S5, a control point determined by the output current I _P and the output potential V _P is set based on P, and the control point is stored in memory (fifth procedure). The method of setting the control point is as described above, and therefore will not be described here.

Next, the process proceeds to S6. In S6, it is determined whether or not V _H > V _P. If _{V H} > V _P is not the case (S6: NO), the process proceeds to S7. In S7, V _H is output as the voltage V(i+1) at time (i+1) (sixth procedure). After that, the process proceeds to S8. In S8, it is determined whether or not to continue the control. If the control is to be continued (S8: YES), the process returns to S1, and the above-mentioned steps S1 to S8 are repeated.

On the other hand, if V _H > V _P in S6 (S6: YES), the process proceeds to S9. In S9, it is determined whether or not V _L < V _P. If V _L < V _P is not true (S9: NO), the process proceeds to S10. In S10, V _L is output as V(i+1) (sixth procedure). After that, the process proceeds to S8. Then, if control is to be continued (S8: YES), the process returns to S1, and the above-mentioned steps S1 to S10 are repeated.

Furthermore, if V _L <V _P is satisfied in S9 (S9: YES), the process proceeds to S11. In S11, V _P is output as V(i+1) (sixth procedure). Then, the above-mentioned steps S1 to S11 are repeated until the control is completed (S8: NO).

[3. Action]
When the cathode-side precious metal-based catalyst particles are exposed to a high potential, the precious metal components in the precious metal-based catalyst particles are oxidized, and a coating containing precious metal oxides and precious metal hydroxides is formed on the surfaces of the precious metal-based catalyst particles. The total amount M(i) of precious metal oxides and precious metal hydroxides at time i depends on the operating history of the fuel cell (mainly the voltage V(i) of the fuel cell at time i).
Therefore, by successively acquiring the operation history of the fuel cell, it is possible to know M(i) at the current time, that is, the total amount of oxides and hydroxides covering the surfaces of the precious metal-based catalyst particles.

A small M(i) means that the total amount of oxides and hydroxides covering the surface of the precious metal catalyst particles is small. In such a case, when a sudden potential change acts on the precious metal catalyst particles, dissolution and precipitation of the precious metal components progresses, which causes the catalyst to deteriorate.
Therefore, when M(i) is small, narrowing the potential range _Vr of the fuel cell (i.e., decreasing the upper limit potential _VH and/or increasing the lower limit potential _VL ) suppresses excessive potential fluctuations, and as a result, deterioration of the precious metal-based catalyst particles can be suppressed.

On the other hand, a large M(i) means that the total amount of oxides and hydroxides covering the surface of the precious metal catalyst particles is large. In such a case, even if a sudden potential change acts on the precious metal catalyst particles, dissolution and precipitation of the precious metal components are unlikely to proceed.
Therefore, when M(i) is large, by widening the potential range _Vr of the fuel cell (i.e., by increasing the upper limit potential _VH and/or decreasing the lower limit potential _VL ), it is possible to output the required electric power while suppressing the dissolution and deposition of the precious metal components, or to select operating conditions that provide high power generation efficiency.

(Example 1, Comparative Example 1)
1. Test Method
The deterioration amount of the cathode catalyst, the power generation efficiency, and the output density when a fuel cell system in which the cathode catalyst is Pt particles is operated under specified conditions were calculated by simulation.
The simulation is
(a) In accordance with the flowchart of FIG. 6, the upper limit potential _VH of the fuel cell is variably controlled within a range of 0.8 V to 1.0 V, and the lower limit potential _VL is variably controlled within a range of 0.4 V to 0.7 V (Example 1); and
(b) When _VH is fixed at 1.0 V and _VL is fixed at 0.4 V (Comparative Example 1)
I followed him.

The fuel cell was operated under the LA#4 mode, which is a commonly used pattern for measuring exhaust gas emissions.
The deterioration amount of the cathode catalyst was obtained by successively calculating the amount of elution from the relationship between the potential, θ _ad , and the elution rate shown in FIG. 2, and integrating these amounts.
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.
Furthermore, the output density was determined from the output (maximum output) at the lower limit potential V _L .

2. Results
Fig. 7 shows a comparison of the deterioration amount, power generation efficiency, and power density of the fuel cell systems of Example 1 and Comparative Example 1. It can be seen from Fig. 7 that when the method according to the present invention is used, the deterioration amount of the cathode catalyst increases slightly, but the power generation efficiency and power density are improved.

The above describes in detail the embodiments of the present invention, but the present invention is not limited to the above embodiments, and various modifications are possible without departing from the spirit of the present invention.

The fuel cell potential control device of the present invention can be used to control power generation in fuel cell vehicles.

Claims (6)

(A) a first means for acquiring a load requirement P for a polymer electrolyte fuel cell and storing said P in a memory;
(B) a second means for acquiring a total voltage V(i) of at least the solid polymer electrolyte fuel cell at time i and storing the V(i) in the memory;
(C) a third means for calculating a total amount M(i) of precious metal oxide and precious metal hydroxide contained in the cathode-side precious metal-based catalyst particles at the time i based on at least the V(i) and storing the M(i) in the memory;
(D) when the M(i) is less than the first threshold ε ₁ (or the M(i) is equal to or less than the ε ₁ ), decreasing the upper limit potential _VH of the potential range _Vr of the polymer electrolyte fuel cell and/or increasing the lower limit potential _VL of the potential range _Vr ;
When M(i) is equal to or greater than a second threshold ε ₂ (≧ε ₁ ) (or when M(i) is greater than ε ₂ ), increasing V _H and/or decreasing V _L ;
A fourth means for storing the increased or decreased _VH and/or _VL in the memory;
(E) a fifth means for setting a control point determined by an output current I _P and an output potential V _P based on the P, and storing the control point in the memory;
(F) when V _H >V _P is not satisfied (or when V _H ≧V _P is not satisfied), output the V _H as the voltage V(i+1) at time (i+1);
When V _L <V _P is not satisfied (or when V _L ≦V _P is not satisfied), output V _L as V(i+1);
and sixth means for outputting V _P as V(i+1) when V _L <V _P <V _H (or when V _L ≦V _P ≦V _H ). the second means includes means for acquiring, in addition to V(i), a current I(i) and a DC resistance R(i) of the polymer electrolyte fuel cell at the time i, and storing the I(i) and the R(i) in the memory;
The third means is
A catalyst potential Vcat(i) of the polymer electrolyte fuel cell at the time i is calculated based on the following formula (1):
2. The fuel cell potential control device according to claim 1, further comprising a means for calculating M(i) based on the following equation (3):

however,
N _cell is the number of stacked cells of the polymer electrolyte fuel cell,
A _cell is the area of the cell.

however,
θ ₁ (i) is the coverage of the precious metal oxide adsorbed on the surface of the precious metal-based catalyst particle at the time i;
θ ₂ (i) is the coverage of the precious metal hydroxide adsorbed on the surface of the precious metal-based catalyst particle at the time i;
θ ₃ (i) is the coverage of the precious metal oxide present inside the precious metal-based catalyst particle at the time i;
Γ is the maximum surface coverage of oxygen per unit surface area (constant),
ΔT is the calculation step size,
α ₁₁ to α ₃₇ are the fit coefficients,
A _PM is the total surface area of precious metal catalyst particles. The third means is
Calculating a catalytic potential Vcat(i) of the polymer electrolyte fuel cell at the time i based on the following formula (2):
2. The fuel cell potential control device according to claim 1, further comprising a means for calculating M(i) based on the following equation (3):

Here, N _cell is the number of stacked cells of the polymer electrolyte fuel cell.

however,
θ ₁ (i) is the coverage of the precious metal oxide adsorbed on the surface of the precious metal-based catalyst particle at the time i;
θ ₂ (i) is the coverage of the precious metal hydroxide adsorbed on the surface of the precious metal-based catalyst particle at the time i;
θ ₃ (i) is the coverage of the precious metal oxide present inside the precious metal-based catalyst particle at the time i;
Γ is the maximum surface coverage of oxygen per unit surface area (constant),
ΔT is the calculation step size,
α ₁₁ to α ₃₇ are the fit coefficients,
A _PM is the total surface area of precious metal catalyst particles. the second means further includes means for acquiring a temperature T _e (i) of the polymer electrolyte fuel cell at the time i and storing the temperature T e (i) in the memory;
4. The fuel cell potential control device according to claim 2, wherein the third means includes means for correcting the M(i) using the T _e (i). The fourth means is
A means for determining the _VH based on the following formula (4); and/or
5. The fuel cell potential control device according to claim 2 , further comprising a means for determining V _L based on the following equation (5):

however,
V _H0 is the reference value of the upper limit potential,
V _L0 is the reference value of the lower limit potential,
β ₁ to β ₄ are the fit coefficients, respectively. the second means further includes means for acquiring a temperature T _e (i) of the polymer electrolyte fuel cell at the time i and storing the temperature T e (i) in the memory;
6. The fuel cell potential control device according to claim 5, wherein the fourth means includes means for correcting the V _H0 and/or the V _L0 using the T _e (i).

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