WO2016095238A1 - Method and device for detecting pile consistency of proton exchange membrane fuel cell - Google Patents

Abstract

A method for detecting the fluid distribution consistency of a proton exchange membrane fuel cell pile (620). The proton exchange membrane fuel cell pile (620) consists of more than two sequentially stacked single cells. The proton exchange membrane fuel cell pile (620) comprises an anode inlet and a cathode inlet. A gas mixed with hydrogen and an inert gas is introduced into the anode inlet or the cathode inlet of the proton exchange membrane fuel cell pile (620), a same voltage used for hydrogen oxidation is applied to two electrodes of each cell of the proton exchange membrane fuel cell pile (620), and the fluid distribution consistency of the proton exchange membrane fuel cell pile (620) is determined by testing and comparing oxidation currents In of each single cell. Compared with the prior art, the method for detecting the fluid distribution consistency and the pile consistency of the proton exchange membrane fuel cell (620) is simple and easy to implement, the detection device is simple and easy to assemble, and the detection method and the detection device are of great significance to accurate detection of performance of each cell of the proton exchange membrane fuel cell pile (620).

Description

Proton exchange membrane fuel cell stack consistency detection method and detection device Technical field

The invention belongs to the field of fuel cells, and in particular relates to a test method for consistency of fluid distribution in a fuel cell stack.

Background technique

Proton exchange membrane fuel cell is an energy conversion device that directly converts fuel chemical energy into electrical energy. It has the advantages of high power generation efficiency and environmental friendliness, and has broad application prospects in the fields of power, carrying and backup power.

A stack is the "heart" of a fuel cell and is the site where electrochemical reactions occur to convert the chemical energy of a fuel into electrical energy. A stack consists of tens to hundreds of cells, which consist of a membrane electrode and a plate with flow channels on either side of the membrane electrode. Inconsistent performance of the single battery will affect the performance of the stack and reduce the service life.

There are two main reasons for the inconsistency of the reactor: 1) the inconsistency of the membrane electrode, the performance difference caused by the process and material uniformity in the process of manufacturing the membrane electrode; 2) the inconsistency of the fluid distribution due to the plate The processing process and the stack assembly process cause different resistance drops in the flow field, resulting in inconsistent reactant flow rates into the individual cells, resulting in a difference in performance of the cells.

At present, there is little satisfactory method for the detection of stack consistency. We can detect the voltage of each cell in the stack during operation, but this voltage difference is caused by the difference in performance of the membrane electrode itself or due to insufficient supply (inconsistent reactant flow), which is difficult to judge. Patent CN102981124A proposes a method for judging the consistency of the inner membrane electrode of the stack by electrochemically active surface area. However, the relationship between the battery performance and the electrochemical active surface area is still unclear, and the decrease of the electrochemical active surface area does not necessarily cause battery performance. The decline, so this method is only a reference value for the consistency detection of the reactor.

In view of the above problems, the present invention provides a simple method for detecting the consistency of fluid distribution in a fuel cell. A cathode gas (or anode) of the fuel cell is introduced with a mixture of H ₂ and an inert gas, and a positive voltage is applied to the cathode (or anode) (sufficient to completely oxidize H ₂ ), and the magnitude of the measured oxidation current can be reflected in the flow channel. The flow rate of the reactants. During the test, H _{2 is} electrochemically oxidized at the cathode (or anode), and electrons generated by the protons passing through the ion exchange membrane to the anode (or cathode) react with the external circuit to form H ₂ . Since the electrochemical oxidation rate of H ₂ on the surface of the Pt-based catalyst is faster, the reaction is controlled by mass transfer, that is, the oxidation current is related to the H ₂ mass transfer flux, and the mass transfer flux of H ₂ is the flow rate of the mixed gas. Closely connected. Therefore, the magnitude of the oxidation current can reflect the level of the gas flow.

Summary of the invention

In view of the above problems, an object of the present invention is to provide a simple and easy method for detecting the consistency of a stack.

In order to achieve the above object, the technical solution adopted by the present invention is: a method for detecting the consistency of a proton exchange membrane fuel cell stack, wherein the proton exchange membrane fuel cell stack is composed of two or more cells stacked in series and connected in series with a gas path. The proton exchange membrane fuel cell stack includes an anode inlet and a cathode inlet;

Applying a mixture of hydrogen and inert gas to the anode inlet or the cathode inlet of the proton exchange membrane fuel cell stack, and applying the same voltage for hydrogen oxidation between the two poles of the proton exchange membrane fuel cell stack, and passing the test The consistency of the oxidation current I _n of each cell is compared to determine the consistency of the fluid distribution of the proton exchange membrane fuel cell stack.

The theoretical basis is that H _{2 is} electrochemically oxidized at the cathode (or anode), and electrons generated by the protons passing through the ion exchange membrane to the anode (or cathode) react with the external circuit to form H ₂ . Since the rate of electrochemical oxidation of H ₂ Pt or Pd based catalyst surface faster group, the reaction is controlled by mass transfer, i.e. oxidation current magnitude associated with the mass transfer flux of H ₂ and H ₂ mass transfer flux is mixed with The flow of gas is closely connected. Therefore, the magnitude of the oxidation current can reflect the level of the gas flow.

Applying a voltage for hydrogen oxidation between the two electrodes of the proton exchange membrane fuel cell stack is specifically a mixture of hydrogen and an inert gas at the anode inlet of the fuel cell stack, and the proton exchange membrane fuel cell is electrically charged. The same positive voltage is applied to each of the anode cells of the stack;

Or a mixture of hydrogen and an inert gas is introduced into the cathode inlet of the proton exchange membrane fuel cell stack, and a same positive voltage is applied to each of the cathode cells of the proton exchange membrane fuel cell stack;

Judging the consistency of the fuel cell stack fluid distribution by testing and comparing the consistency of the oxidation currents of the individual cells; if the oxidation currents are the same, the fluid flows into the cells in the stack are consistent, if If the oxidation currents are inconsistent, the fluid flow rates of the individual cells entering the stack are inconsistent.

The optimization condition in the detection process is that the membrane electrode in each unit cell of the proton exchange membrane fuel cell stack is intact; the optimal condition is that the membrane electrode performance in each unit cell of the proton exchange membrane fuel cell stack is consistent.

The voltage for hydrogen oxidation is from 0.3 to 0.9V.

The positive voltage is 0.3-0.9V.

The volume concentration of hydrogen in the mixed gas is from 0.1 to 10%.

The inert gas is one or a mixture of two or more of nitrogen, helium, and argon.

The unit cell includes a membrane electrode and an anode plate and a cathode plate disposed on both sides thereof; the membrane electrode includes an electrolyte membrane and an electrocatalyst attached to both side surfaces thereof; the electrocatalyst is a Pt group or Pd-based electrocatalyst.

A method for detecting the consistency of a proton exchange membrane fuel cell stack, wherein the proton exchange membrane fuel cell stack is composed of two or more cells stacked in series and connected in series with a gas path; the proton exchange membrane fuel cell stack includes an anode Inlet and a cathode inlet;

The detecting method includes the following steps,

(1) measuring the voltage V _n of each cell during the operation of the fuel cell stack;

(2) introducing a mixture of hydrogen and an inert gas at the anode inlet or the cathode inlet of the fuel cell stack, and applying a voltage for hydrogen oxidation between the two poles of each unit of the fuel cell stack to test each unit cell oxidation current I _n, while identifying the sections is less than the cell current corresponding to an average oxidation unit cell K, and calculates the current I _n corresponding to the oxidation gas flow rate Q _n;

(3) Theoretical calculation determines the corresponding voltage range V _n ^ο when the gas flow rate is Q _n under the operating conditions of the qualified membrane electrode;

(4) Comparing V _n and V _n ^ο , if V _{n is} within the range of V _n ^ο , the difference in performance of the single cell K is due to inconsistent fluid distribution of the stack, and if V _{n is} less than V _n ^ο , the difference in performance of the single cell K It is also related to the membrane electrode itself.

The step (2) test process is specifically: introducing a mixture of hydrogen and an inert gas into the anode inlet of the proton exchange membrane fuel cell stack, and applying a same positive voltage between the anodes of the single cell of the proton exchange membrane fuel cell stack. Or a mixture of hydrogen and inert gas is introduced into the cathode inlet of the proton exchange membrane fuel cell stack, and an identical positive voltage is applied between the cathode cells of the proton exchange membrane fuel cell stack.

The voltage for hydrogen oxidation is from 0.3 to 0.9V.

The applied positive voltage is 0.3-0.9V.

The volume concentration of hydrogen in the mixed gas is from 0.1 to 10%.

The inert gas is one or a mixture of two or more of nitrogen, helium, and argon.

The unit cell includes a membrane electrode and an anode plate and a cathode plate disposed on both sides thereof; the membrane electrode includes an electrolyte membrane and an electrocatalyst attached thereto; and the electrocatalyst is a Pt-based or Pd-based group catalyst.

A specific test procedure for a method for detecting the consistency of a proton exchange membrane fuel cell stack is:

(1) introducing a mixture of H ₂ and an inert gas into a cathode (or anode) chamber of the fuel cell stack;

The inert gas is one or more of N ₂ , Ar, He;

The concentration of hydrogen in the mixed gas is 0.1-10%;

The mixed gas may or may not be humidified;

The catalyst used in the proton exchange membrane fuel cell is a Pt or Pd based catalyst.

(2) applying a positive potential to the cathode (or anode) of each single cell, as shown in Figures 3 and 4, recording the current magnitude;

The positive potential has a magnitude of 0.3-0.9V;

The other chamber of the test stack may be supplied with an inert gas (one or two or more of N ₂ , Ar, He) or deionized water, or may be unreachable.

(3) According to the magnitude of the test current, the flow ratio into each cell is obtained, that is, i ₁ : i ₂ : i ₃ : ... i _n = Q ₁ : Q ₂ : Q ₃ : ... Q _n ;

Where i _n is the nth cell test current value, and Q _n is the nth cell gas flow rate. Knowing the total flow of gas into the stack, the flow of gas into each cell can be calculated according to the above formula.

The principles of the present invention are set forth below in conjunction with Figures 1 and 2. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram of a single cell in which a proton exchange membrane; 2 and 2' are catalytic layers; 3 and 3' are diffusion layers; and 4 and 4' are plates with flow channels.

During the test, a mixture of H ₂ and N _{2 is introduced} into the cathode 4' (or anode) of the fuel cell, and then a positive potential is applied to the cathode (or anode), at which time H _{2 is} electrically oxidized in the catalytic layer 2' to generate The protons reach the catalytic layer 2 through the proton exchange membrane 1, and the protons in the catalytic layer 2 are electrically reduced by electrons from the external circuit to form H ₂ . The detected current is calculated according to formula (1).

i=J×nF (1)

In the formula (1), i is a current, J is a hydrogen molar flux, F is a Faraday constant, and n is a reaction transfer electron number, here is 2. The mass transfer flux J is related to the gas flow rate Q entering the battery, which is illustrated below with reference to FIG. 2 is a schematic view showing the distribution of H ₂ concentration during the test, in which 2 is a catalytic layer; 3 is a diffusion layer; and 4 is a plate with a flow path. In steady state

J=k(C _ch -C _cl ) (2)

Wherein C _ch is the concentration of H ₂ in the flow channel, C _cl is the concentration of H ₂ in the catalytic layer, and the mass transfer coefficient of k. Since the applied potential is sufficiently high, the hydrogen gas is completely oxidized, that is, the concentration of H ₂ in the catalytic layer C _cl =0, so that the mass transfer flux J is proportional to the mass transfer coefficient from the formula 2, and the mass transfer correlation is known according to the dimensional analysis. ,

k=f(Re,Sc) (3)

Where Re is the Reynolds number and Sc is the Schmidt number (only related to fluid properties, a constant in this experiment). It can be seen that k is closely related to the Reynolds number, and

Re=dvρ/μ∝Q (4)

Where d is the hydraulic diameter of the flow channel, v is the flow velocity, ρ is the density, and μ is the viscosity. When the fluid is determined, Re changes only with the flow rate v, and the flow rate is proportional to the flow rate, so Re is proportional to the fluid flow rate Q. The integrated equations (1)-(4) show that the current i is closely related to the fluid flow rate Q, so the magnitude of the flow into the flow channel can be reflected by the magnitude of the current. Through experiments, it is found that in the common gas flow range, that is, when the gas flow metering ratio is lower than 5, the current is linear with the flow rate (as shown in Fig. 5), that is,

i∝Q (5)

At this time, the gas flow rate of the _nth cell Q _n can be calculated by the following formula

m represents the mth cell in the stack, i _m is the m-cell cell current value, and Q _m is the m-th cell gas flow.

m,n is a positive integer >2.

Of course, the relationship between current and flow rate is not limited to the above linear relationship.

A detecting device for detecting a consistency of a fluid distribution of a proton exchange membrane fuel cell stack, comprising a mixed gas supply device, a DC power source and a current detecting module;

The mixed gas supply device is configured to supply a mixed gas to a anode inlet or a cathode inlet of a proton exchange membrane fuel cell stack; the DC power supply and current detecting module is configured to apply voltage to each of the cells of the proton exchange membrane fuel cell stack and simultaneously The current of each cell of the proton exchange membrane fuel cell stack is detected.

The power source and current detecting module may be an integrated device, or may be independent power supply devices and current detecting devices.

The power source and current detecting module is an electrochemical workstation or a programmable power source when the integrated device is used.

When the power source and current detecting module are independent power supply devices and current detecting devices, the power device is one of a controllable switching power supply and a power module, and the current detecting device is an ammeter, a Hall current sensor, and a current detecting module. One.

Taking FIG. 6 as an example, the testing device is provided with: 1. gas flow control device, including valves 601, 602, 603, mass flow meters 604, 605, 606; 2 gas mixer 610; 3 isolated power supply 650, data acquisition and processing Unit 640.

The isolated power supply is composed of a plurality of power sources that can independently apply voltage to the stack, and the isolated power supply can be a plurality of constant voltage sources with current measurement functions, as shown in FIG. It can be one of an electrochemical workstation, a programmable power supply, and a switching power supply.

The isolated power supply is composed of an independent module power supply and a detection current device. As shown in FIG. 8, it may be one of an ammeter, a Hall current sensor, and a current detecting module.

Compared with the prior art, the method for detecting fluid distribution consistency and stack consistency of a proton exchange membrane fuel cell according to the present invention is simple and easy to implement, and the detecting device is simple and easy to assemble, for accurately detecting a proton exchange membrane fuel cell stack. The performance of each single battery is significant.

DRAWINGS

1 is a schematic view of a proton exchange membrane fuel cell unit.

Wherein, 1 proton exchange membrane; 2 and 2' are first and second catalytic layers; 3 and 3' are first and second diffusion layers, respectively; 4 and 4' are first and second with flow paths, respectively Two plates.

2 is a schematic view showing the distribution of H ₂ concentration in the membrane electrode during the test, wherein 2 is a catalytic layer; 3 is a diffusion layer; and 4 is a plate with a flow channel.

Figure 3 is a schematic diagram of the stack test, in which a mixture of H ₂ and N _{2 is introduced} into the cathode chamber of the stack, and a positive potential is applied to the cathode of the unit cell.

Figure 4 is a schematic diagram of the stack test. A mixture of H ₂ and N _{2 is introduced} into the anode chamber of the stack. At this time, a positive potential is applied to the anode of the unit cell.

Figure 5 is the relationship between the test current and the gas flow rate under different membrane electrode compression conditions.

Figure 6 is a schematic illustration of a detection device. Among them, 601, 602, and 603 are valves, 604, 605, and 606 are mass flow meters, 610 is a mixer, 620 is a fuel cell stack, 640 is a data acquisition and processing unit, and 650 is a power source.

7 is a schematic diagram of an isolated power supply 650, wherein 701, 702, 703, 704, 705, and 706 are constant voltage sources with current measurement functions, and may be one of an electrochemical workstation, a programmable power supply, and a switching power supply.

8 is a schematic diagram of an isolated power supply 650, wherein 801, 802, 803, 804, 805, and 806 are module power supplies, and 811, 812, 813, 814, 815, and 816 are current detecting devices, which are current meters, Hall current sensors, and sampling. One of a resistor and a high-side current sense amplifier.

detailed description

The invention will now be described in detail in connection with the embodiments. Of course, the invention is not limited to the specific embodiments described below.

Example 1

The stack used contains three single cells. The cell area was 25 cm ² , the proton exchange membrane used was Nafion 115, and the catalyst was Pt/C. During the test, a mixture of H ₂ and N ₂ was introduced into the cathode at a total flow rate of 1 L/min and a H ₂ volume concentration of 1.5%. The anode chamber was free of any fluid but remained in communication with the atmosphere. Thereafter, a 0.6 V positive potential was applied to each of the single cell cathodes using an electrochemical workstation Soltron 1287, and the current was recorded. The test schematic is shown in Figure 3. The test results are as follows:

Battery number	Current size mA
		1	517.4
2	446.4
		3	494.3

Example 2

The stack used contains three single cells. The cell area was 25 cm ² , the proton exchange membrane used was Nafion 115, and the catalyst was Pt/C. During the test, a mixture of H ₂ and N ₂ was introduced into the anode chamber, the volume of H ₂ was 1.5%, and the cathode chamber was purged with deionized water. Then, a DC power supply, Aetna PPS3005, was used to apply a positive potential of 0.9 V to the cathode of each single cell, and the current was recorded. The test schematic is shown in Figure 4. The test results are as follows:

Battery number	Current size mA
		1	561.2
2	376.4
		3	525.6

Example 3

The stack used contains six single cells. The cell area was 25 cm ² , the proton exchange membrane used was Nafion 212, and the catalyst was Pt/C. During the test, a saturated humidified H ₂ , N ₂ mixture gas was introduced into the cathode chamber, the total flow rate was 1 L/min, the H ₂ volume concentration was 2%, and the anode chamber was passed through saturated humidification N ₂ . Then, a positive potential of 0.8 V was applied to each of the single cell cathodes using the isolation module power supply as shown in Fig. 8, and the current was recorded. The performance of the hydrogen-air fuel cell at 65 ° C saturated humidification was then tested at a flow rate of 7.5 L/min, and the cell voltages were recorded at 1 A/cm ² constant current discharge. For a hydrogen-air fuel cell, the hydrogen oxidation rate of the anode side is extremely fast, and the overpotential of the battery is mainly caused by the mass transfer reaction process of the cathode oxygen. Therefore, the difference in main cathode performance of a proton exchange membrane fuel cell using H ₂ as a fuel. The test and calculation results are shown in the table below.

It can be seen that the voltage of the batteries 3 and 4 is low. It can be seen from the analysis that for the battery 4, its V _n <V _n ^ο , the performance of the membrane electrode itself is slightly inferior. On the other hand, for the battery 3, the V _n value falls within the acceptable voltage range, and the reason why the voltage is low is that the air supply amount is small.

Claims (19)

1. A method for detecting the consistency of a proton exchange membrane fuel cell stack, characterized in that the proton exchange membrane fuel cell stack is composed of two or more cells stacked in series and connected in series with a gas path; the proton exchange membrane fuel cell stack includes An anode inlet and a cathode inlet;

   Applying a mixture of hydrogen and inert gas to the anode inlet or the cathode inlet of the proton exchange membrane fuel cell stack, and applying the same voltage for hydrogen oxidation between the two poles of the proton exchange membrane fuel cell stack, and passing the test The consistency of the oxidation current I _n of each cell is compared to determine the consistency of the fluid distribution of the proton exchange membrane fuel cell stack.
2. The detecting method according to claim 1, wherein said applying a voltage for hydrogen oxidation between the electrodes of each of the cells of the proton exchange membrane fuel cell stack is specifically: introducing hydrogen into the anode inlet of the fuel cell stack. a mixture of inert gases, applying the same positive voltage to the anodes of the single cells of the proton exchange membrane fuel cell stack;

   Or a mixture of hydrogen and an inert gas is introduced into the cathode inlet of the proton exchange membrane fuel cell stack, and a same positive voltage is applied to each of the cathode cells of the proton exchange membrane fuel cell stack;

   Judging the consistency of the fuel cell stack fluid distribution by testing and comparing the consistency of the oxidation currents of the individual cells; if the oxidation currents are the same, the fluid flows into the cells in the stack are consistent, if If the oxidation currents are inconsistent, the fluid flow rates of the individual cells entering the stack are inconsistent.
3. The detecting method according to any one of claims 1 to 2, wherein the optimization condition in the detecting process is that the membrane electrode in each unit cell of the proton exchange membrane fuel cell stack is intact; the optimal condition is proton The membrane electrodes in the individual cells of the exchange membrane fuel cell stack have the same performance.
4. The detecting method according to claim 1, wherein said voltage for hydrogen oxidation is from 0.3 to 0.9V.
5. The detecting method according to claim 2, wherein said positive voltage is 0.3 to 0.9V.
6. The detecting method according to any one of claims 1 to 2, characterized in that the volume concentration of hydrogen in the mixed gas is from 0.1 to 10%.
7. The detecting method according to any one of claims 1 to 2, wherein the inert gas is one or a mixture of two or more of nitrogen, helium and argon.
8. The detecting method according to claim 1, wherein said unit cell comprises a membrane electrode and an anode plate and a cathode plate disposed on both sides thereof; said membrane electrode comprising an electrolyte membrane and attached thereto An electrocatalyst on the side surface; the electrocatalyst is a Pt-based or Pd-based electrocatalyst.
9. A method for detecting the consistency of a proton exchange membrane fuel cell stack, characterized in that: a proton exchange membrane fuel cell stack consists of two or more cells stacked in series with a gas path in series; the proton exchange membrane fuel cell stack An anode inlet and a cathode inlet are included;

   The detecting method includes the following steps,

   (1) measuring the voltage V _n of each cell during the operation of the fuel cell stack;

   (2) introducing a mixture of hydrogen and an inert gas at the anode inlet or the cathode inlet of the fuel cell stack, and applying a voltage for hydrogen oxidation between the two poles of each unit of the fuel cell stack to test each unit cell oxidation current I _n, while identifying the sections is less than the cell current corresponding to an average oxidation unit cell K, and calculates the current I _n corresponding to the oxidation gas flow rate Q _n;

   (3) Theoretical calculation determines the corresponding voltage range V _n ° when the qualified membrane electrode is under operating conditions when the gas flow rate is Q _n ;

   (4) Comparing V _n and V _n °, if V _{n is} within V _n °, the difference in performance of single cell K is due to inconsistent fluid distribution of the stack. If V _{n is} less than V _n °, the performance difference of single cell K It is also related to the membrane electrode itself.
10. The detecting method according to claim 9, wherein the step (2) testing process is specifically Passing a mixture of hydrogen and inert gas at the anode inlet of the proton exchange membrane fuel cell stack, and applying the same positive voltage between the anodes of the single cell of the proton exchange membrane fuel cell stack; or on the proton exchange membrane fuel cell stack The cathode inlet is supplied with a mixture of hydrogen and an inert gas, and an identical positive voltage is applied between the cathodes of the single cells of the proton exchange membrane fuel cell stack.
11. The detecting method according to claim 9, wherein said voltage for hydrogen oxidation is from 0.3 to 0.9V.
12. The detecting method according to claim 10, wherein said applied positive voltage is 0.3-0.9V.
13. The detecting method according to any one of claims 9 to 10, characterized in that the volume concentration of hydrogen in the mixed gas is from 0.1 to 10%.
14. The detecting method according to any one of claims 9 to 10, wherein the inert gas is one or a mixture of two or more of nitrogen, helium and argon.
15. The detecting method according to claim 9, wherein said unit cell comprises a membrane electrode and an anode plate and a cathode plate disposed on both sides thereof; said membrane electrode comprising an electrolyte membrane and attached thereto The electrocatalyst; the electrocatalyst is a Pt-based or Pd-based catalyst.
16. A detecting device for detecting a fluid distribution consistency of a proton exchange membrane fuel cell stack according to any one of claims 1 to 15, characterized in that it comprises a mixed gas supply device, a direct current power source and a current detecting module;

    The mixed gas supply device is configured to supply a mixed gas to a anode inlet or a cathode inlet of a proton exchange membrane fuel cell stack; the DC power supply and current detecting module is configured to apply voltage to each of the cells of the proton exchange membrane fuel cell stack and simultaneously The current of each cell of the proton exchange membrane fuel cell stack is detected.
17. The detecting device according to claim 16, wherein the power source and current detecting module can be an integrated device, or can be independent power supply devices and current detecting devices.
18. The detecting device according to claim 16, wherein the power source and current detecting module is an electrochemical workstation or a programmable power source when the integrated device is an integrated device.
19. The detecting device according to claim 16, wherein when the power source and the current detecting module are mutually independent power source devices and current detecting devices, the power source device is one of a controllable switching power source and a power source module, and the current detecting device It is one of an ammeter, a Hall current sensor, and a current detecting module.

Images