WO2011032644A1

WO2011032644A1 - Reactant gas supply for fuel cells or electrolysis cells

Info

Publication number: WO2011032644A1
Application number: PCT/EP2010/005357
Authority: WO
Inventors: Pedro Nehter; Sune Danø; Harald Usterud; Henrik Franz Weineisen
Original assignee: Topsoe Fuel Cell A/S
Priority date: 2009-09-17
Filing date: 2010-09-01
Publication date: 2011-03-24

Abstract

A fuel cell, electrolysis cell or a fuel cell stack or an electrolysis cell stack comprising a plurality of planar fuel cells or electrolysis cells has alternating flow direction of the cathode gas, the anode gas or both leading to a mixture of un-used gas and partly used gas and equalization of the temperature profile of the fuel cell(s) or electrolysis cell(s).

Description

Title; Reactant Gas Supply for Fuel Cells or Electrolysis Cells

The present invention concerns supply of reactant gas for fuel cells or electrolysis cells, in particular Solid Oxide Fuel Cells or Solid Oxide Electrolysis Cells, and such fuel cell or electrolysis cell systems, since the advantages of the invention are particularly large for these high temperature cells. According to the invention, the reactant gas flow is oscillated and partially reacted and relative hot gas is mixed with un-reacted relative cold gas, which increases the efficiency of the cell system.

In the following, the invention is explained in relation to SOFC. The gas supply according to the invention can, however, also be used for other types of fuel cells such as Polymer Electrolyte fuel cells (PEM) or Direct Methanol Fuel Cells (DMFC) . Further the invention can also be used for electrolysis cells such as Solid Oxide Electrolysis Cells (SOEC) .

An SOFC comprises an oxygen-ion conducting electrolyte, a cathode where oxygen is reduced and an anode where hydrogen is oxidised. The overall reaction in an SOFC is that hydrogen and oxygen electrochemically react to produce electricity, heat and water. The operating temperature for an SOFC is in the range 650 to 1000°C, preferably 750 to 850°C. An SOFC delivers in normal operation a voltage of approximately 0.75V. To increase the total voltage output, the fuel cells are assembled in stacks in which the fuel cells are electrically connected via interconnector plates. In order to produce the required hydrogen, the anode normally possesses catalytic activity for the steam reforming of hydrocarbons, particularly natural gas, whereby hydrogen, carbon dioxide and carbon monoxide are generated. Steam reform- ing of methane, i.e. the main component of natural gas, can be described by the following equations :

CH₄ + H₂0 CO + 3H₂

CH₄ + C0₂ ±. 2CO + 2H₂

CO + H₂0 ±f. C0₂ + H₂

During operation an oxidant such as air is supplied to the solid oxide fuel cell in the cathode region. Fuel such as hydrogen is supplied in the anode region of the fuel cell. Al- ternatively, a hydrocarbon fuel such as methane is supplied in the anode region where it is converted to hydrogen and carbon oxides by the above reactions . Hydrogen passes through the porous anode and reacts at the anode/electrolyte interface with oxygen ions generated on the cathode side and con- ducted through the electrolyte. Oxygen ions are created in the cathode side as a result of the acceptance of electrons from the external circuit of the cell. interconnects serve to separate the anode and fuel sides of adjacent cell units and at the same time enable current conduction between anode and cathode. Interconnects are usually provided with a plurality of channels which forms a flow field, for the passage of fuel gas on one side of the interconnect and oxidant gas on the other side. The flow direction of the fuel gas is defined as the substantial direction from the fuel inlet portion to the fuel outlet portion of a cell unit. Likewise the flow direction of the oxidant gas, i.e. the cathode gas, is defined as the substantial direction from the cathode inlet portion to the cathode outlet portion of a cell unit. Due to the exothermicity of the reaction in the fuel cell, the outlet gases leave at higher temperature than their inlet temperature. When combined in an SOFC stack, a significant temperature gradient across the stack is generated. Such thermal gradients induce thermal stresses in the stack which are highly undesirable and they entail difference in current density and electrical resistance.

The losses of a fuel cell have to be removed as heat. In an SOFC, the heat is typically removed by a high amount of air. This gives benefits concerning a high oxygen partial pressure and less in-plane temperature gradients in the cell. However, it also causes many drawbacks with respect to the system efficiency, the heat exchanger size and the stack size. A solution to the thermal gradient problem is attempted in US 6830844, where the air (cathode side) flow path direction is reversed periodically. The solution is, however, suffering from poor gas mixing capability. A similar solution with similar drawbacks are offered in EP 1459543, DE 10334843, US 5935726 and US 6589678.

An oscillating flow direction is also suggested on the fuel side (anode) to minimize gas concentration gradients. In WO 2005104802 the fresh fuel and the reaction products are mixed by means of displacer pistons. Still, the mix can be improved, and the temperature gradient problem can be solved more effectively. Also EP 1447870 suggests oscilation of flow to reduce gradients, but the mix of the un-reacted and reacted gasses can be improved.

It is an object of the present invention to provide a reac- tant gas supply for a fuel cell, particularly a solid oxide fuel cell with improved thermal management across the cell.

It is a further object of the present invention to provide a reactant gas supply which minimizes or even omits the need for a heat exchanger to preheat the cathode inlet gas.

It is a further object of the present invention to provide a reactant gas supply for a solid oxide fuel cell which has efficient mixing of un-spent and partially spent reactant gas.

It is another object of the invention to provide a reactant gas supply for a solid oxide fuel cell which has reduced temperature gradients across the cell . Accordingly, it is an object of the invention to provide a reactant gas supply for a solid oxide fuel cell which can be used to adjust the shape and the location of the in-plane temperature profile of the fuel cell. It is yet a further object of the invention to provide a reactant gas supply for a solid oxide fuel cell which has reduced cathode air consumption and accordingly reduced energy loss to air blowers. A further object of the invention is to provide a reactant gas supply for a solid oxide fuel cell which provides an optimized hybrid between co-flow and counter-flow fuel cells. It is another object of the invention to provide a reactant gas supply for a solid oxide fuel cell with efficient cooling of the cell. It is a further object of the invention to provide a reactant gas supply for a solid oxide fuel cell with simple gas manifolding.

These and other objects are solved by the invention.

An alternating flow of the gas at the cathode can be used in combination with a mixing chamber or ejector to mix the cold and unused reactants with the hot and partly utilized reac- tants . The cold and unused gas can be preheated by the hot and partly utilized gas. The need for an expensive high temperature heat exchanger for the preheating of the air would be minimized or even become unnecessary. The benefits of using a mixing of reactants at the cathode side are higher compared to a similar mixing of reactants at the anode side. This is because the required compulsory preheating of the cathode gas can be 10 times higher than for the reformate gas .

The alternating flow can be used to equalize the temperature profile of the cell. The periodical change of movement of the reactants between a first direction and a second direction can be used to adjust the shape and the location of the in- plane temperature profile of the cell. If the oscillatory frequency is fast compared to the thermal timescales of the SOFC stack, then such periodic flow changes can be used to implement the mixed flow configurations such as co-flow and counter-flow without the need for an additional internal manifolding, which will otherwise reduce the size of the stack's active area. Conventional reactant gas supplies for a fuel cell comprise gas inlets on a first side of the fuel cell, gas outlets on a second side of the fuel cell, gas distribution channels on the inlet and the outlet side of the fuel cell and a flow field adapted to the fuel cell to distribute the gas across the entire or as much as the fuel cell active area as practically possible. To achieve the mentioned objects, the invention compared to conventional reactant gas supplies provides an oscillating member able to oscillate a flow stream, two gas mixing chambers and two reactant gas inlets - one mixing chamber and inlet on each side of the fuel cell. The oscillating member can be a piston or a membrane or any other member suited for oscillating a gas, known to the person skilled in the art. This gas supply system can be provided on the cathode gas side of the fuel cell or the anode gas side of the fuel cell, or even both sides of the fuel cell. However, the highest effect according to the objects of the invention is achieved when the supply system is applied to the cathode side of the fuel cell since, as mentioned, the volume flow of the cathode gas is usually larger than the anode gas volume flow.

Though the invention is explained in relation to a fuel cell, it is particularly advantageous to whole stacks of fuel cells where the gas distribution channels can be connected to sev- eral flow fields such that mixing chambers, inlets and the oscillating member apply to all the cells in the stack. The oscillating member moves the total gas volume contained in the fluid connected flow fields, mixing chambers, gas distribution channels and the void in the chamber above the oscillating member in alternating directions: A first direction from the first side of the fuel cell flow field across the flow field and towards the second side of the fuel cell flow field and further towards the gas outlet; and a second direction from the second side of the fuel cell flow field across the flow field and towards the first side of the fuel cell. A first and a second reactant gas inlet and mixing chamber are placed on each side of the fuel cell. When the gas flows through each mixing chamber, the gas fraction already within the supply system is mixed with fresh gas flowing in through each gas inlets. Therefore, as the oscillating member moves the gas volume in alternating directions, the net gas flow stream across the fuel cell is a mixture of a partly reacted gas fraction and an un-reacted gas fraction.

In an embodiment of the invention, the volume of the oscil- lating gas is larger than the volume of the corresponding flow field of the fuel cell. This means that all the gas in the reaction zone of the fuel cell is exchanged with the relative colder mixture of partly reacted gas and un-reacted gas fractions at the same frequency as the oscillation fre- quency.

Co-flow or counter-flow internally in fuel cell stacks as known in state of the art each has different characteristics and advantages. To a higher extent than . the co-flow stack, a counter-flow stack has its current output where it is relative hot which means relative low internal resistance (ASR- Area Specific Resistance) , while a co-flow stack has a higher cathode gas outlet temperature compared to the cathode gas inlet temperature (ΔΤ) and thus has the most effective cooling, but to a higher extent has the current output where it is relatively cold meaning a larger ASR.

According to the present invention, these different advantages can be combined by oscillating and mixing the flow in general throughout the stack and internally in the cells in the stack.

1. Oscillating reactant gas supply for at least one Solid Oxide Fuel Cell or Solid Oxide Electrolysis Cell comprising at least one

• anode

• electrolyte

• cathode

• interconnect comprising cathode and anode gas distribution flow fields,

the gas supply comprising on a first side of the fuel cell or electrolysis cell

• a first gas distribution channel

• a first gas mixing chamber

• a first gas inlet

• a gas oscillation member

and on a second side of the fuel cell or electrolysis cell

• a second gas distribution channel

• a second gas mixing chamber

• a second gas inlet,

wherein the gas oscillation member oscillates a reactant gas in a first and a second direction alternately, and reactant gas is supplied to the first and second gas distribution channels via the first and second gas inlet, whereby relatively cold and unused reactant gas is mixed with relatively hot and partially reacted reactant gas in the first and second gas mixing chambers on both sides of the at least one fuel cell or electrolysis cell prior to distribution of the gas through the interconnect flow fields to the fuel cell or electrolysis cell.

2. Oscillating reactant gas supply according to feature 1, wherein the oscillation member comprises a piston or a membrane pump .

3. Oscillating reactant gas supply according to any of the preceding features, wherein at least one mixing chamber comprises an ejector.

4. Oscillating reactant gas supply according to any of the preceding features, wherein the first and second gas inlets are connected to the first and the second mixing chambers.

5. Oscillating reactant gas supply according to any of the preceding features, wherein the volume of the oscillating gas is larger than the corresponding flow field volume of the fuel cell or electrolysis cell.

6. Oscillating reactant gas supply according to any of the preceding features, wherein the flow volume of the oscillat ing gas is in the range of 2 to 7 times larger than the cor responding flow field volume of the fuel cell or electrolysis cell, preferably in the range of 3 to 5 times larger than the corresponding flow field volume of the fuel cell o electrolysis cell. 7. Oscillating reactant gas supply according to any of the preceding features, wherein the reactant gas is supplied via the first gas inlet when the gas flows in the first direction and is supplied via the second gas inlet when the gas flows in the second direction.

8. Oscillating reactant gas supply according to any of the preceding features, wherein the oscillation frequency is 0.1 to 10 Hz.

9. Oscillating reactant gas supply according to any of the preceding features, wherein the cell is a fuel cell and the reactant gas is air, which is supplied to the cathode side of the at least one fuel cell via the first and second gas inlet, the distribution channel and the mixing chamber.

10. Oscillating reactant gas supply according to any of the features 1 to 8 , wherein the cell is a fuel cell and the reactant gas is a fuel which is supplied to the anode side of the at least one fuel cell via the first and second gas inlet, the distribution channel and the mixing chamber.

11. Oscillating reactant gas supply according to feature 10, further comprising a first and a second catalyst located in fluid connection to the first and the second gas distribution channels .

Fig. 1 is a cut principle drawing of oscillating reactant gas supply for a fuel cell comprising a piston and two mixing chambers , Fig. 2 is a cut principle drawing of oscillating reactant gas supply for a fuel cell comprising a piston and two ejectors,

Fig. 3 is a cut principle drawing of oscillating reactant gas supply for a fuel cell comprising a piston, two ejectors and two catalysts,

Fig. 4 is a principle drawing of a fuel cell with co-flow of the anode and the cathode gas,

Fig. 5 is a graph showing the temperature profile of the co- flow fuel cell of Fig. 4,

Fig. 6 is a principle drawing of a fuel cell with counter- flow,

Fig. 7 is a graph showing the temperature profile of the counter-flow fuel cell of Fig. 6, Fig. 8 is a principle drawing of a fuel cell with alternating co- and counter flow and mix of fresh cathode gas and reac- tants in a fuel cell according to the invention,

Fig. 9 is a graph showing the temperature profile of the al- ternating flow fuel cell of Fig. 8, and

Fig. 10 is a graph showing a comparison of the average power density of the fuel cells of Figs. 6, 8 and 10. Position Numbers :

100 . Solid Oxide Fuel Cell (SOFC) .

101 . Anode .

102 . Electrolyte.

103 . Cathode .

104 . Cathode flow field.

105 . Anode flow field

201 . First gas mixing chamber.

202 . Second gas mixing chamber.

203 . First gas distribution channel.

204 . Second gas distribution channel

205 . First gas inlet.

206 . Second gas inlet.

207 . Piston.

208 . Pump chamber.

301 . First ejector.

302 . Second ejector.

401 . First catalyst.

402 . Second catalyst.

501 . Heat exchanger .

In the following, three embodiments of the invention are described. These embodiments are not limiting the invention, but merely examples to show the advantages of the invention compared to the known art. For the sake of simplicity, the presented embodiments show the invention in connection to a single fuel cell. It is clear to the person skilled in the art that the shown principles can be used for a whole stack or even several stacks of connected fuel cells. Fig. 1 shows a Solid Oxide Fuel Cell (SOFC) 100 comprising an anode 101, an electrolyte 102 and a cathode 103. Both the anode and the cathode need reactant gases for the SOFC to operate, but as this embodiment of the invention concerns alter- nation of the cathode reactant gas flow direction, only the cathode flow field 104 is shown in Fig. 1. The cathode gas is supplied to the fuel cell via gas inlets 205, 206 and removed via a gas outlet (not shown) connected to the second gas distribution channel 204 by means of any common technique such as a blower, a compressor, a turbo charger (not shown) or any other technique well known to the person skilled in the art. Hence, the net gas flow has a direction from the inlets to the outlet. According to the invention, the gas flow oscillates such that the momentary gas flow direction through the flow field alternates in response to a frequency. At one moment the gas has a first flow direction and the next moment the gas has a second flow direction. The oscillating flow is induced by means of an oscillating member, which in the embodiment shown in Fig. 1 is a piston 207 in a pump chamber 208 connected to the first gas distribution channel 203. The piston is moving up and down in the chamber 208, where the downward movement creates an under pressure in the chamber and the upward move- ment creates an over pressure in the chamber relative to the connected channel system 203 and flow field 104. Thus, when the piston moves up, the cathode gas in the flow field moves in a first direction, and when the piston moves downwards the cathode gas in the flow field moves in a second direction. Un-reacted relatively cold cathode gas is continuously supplied to the system via inlets 205, 206 connected to the first and second mixing chamber 201, 202 which are placed on either side of the flow field. The described alternating flow direction of the cathode gas ensures therefore a mixing of this relatively cold un-reacted cathode gas with relatively hot and partly reacted cathode gas from the fuel cell . Thus the relatively cold un-reacted cathode gas is preheated by the relatively hot and partly reacted cathode gas before entering the fuel cell, and therefore a heat exchanger for the preheating of cathode gas is not necessary.

Fig. 2 shows an embodiment of the invention where the mixing chambers are replaced by ejectors 301 , 3 02 . The cathode gas in the system is flowing in alternating directions because of the pressure difference between the flow field 104 and the pump chamber 208 when the piston 207 is moving up and down. When flowing, the cathode gas is passing the converging- diverging zones of the ejectors where also the cathode gas inlets 205 , 206 are located. This zone leads to a conversion of the pressure energy of the relatively hot, partly reacted cathode gas in the system into a velocity energy creating a low pressure zone which draws in and entrains the relatively cold and un-reacted cathode gas from the inlets. After passing through the narrow throat section of the ejector, the mixed hot and cold gasses expand, and the velocity is reduced which results in recompressing the mixed gases by reconverting velocity energy into pressure energy.

In Fig. 3 the alternating gas flow principle is shown on the anode 101 side of the fuel cell . Relatively cold and unused fuel enters the ejectors 301 , 302 through first and second gas inlets 205 , 206 and is mixed with relatively hot and partly utilized fuel due to the movement in alternating flow- directions caused by the oscillation member 209 , 210 . Thus, the alternating flow direction principle on the anode side according to the invention is analogue to the principle on the cathode. But on the anode side, additionally the water and the heat of the partly utilized fuel can be used to adia- batic steam reforming by means of catalysts 401, 402 located on either side of the anode flow field 105.

To visualize some advantages of the invention, Figs. 4 to 10 show the temperature profiles for fuel cells having co-flow (Figs. 4 and 5), counter-flow (Figs. 6 and 7) and alternating flow with mixture of the un-used and partly used cathode gases according to the invention (Figs. 8 and 9), as well as a comparison of the average power density for the three situations (Fig. 10) . All three situations have comparable simulation parameters:

- fuel = hydrogen

- fuel utilization = 70%

- gas inlet temperature = 675°C

- maximum cell temperature = 830°C to 845°C

and the power density of Fig. 10 is shown for an average cell voltage of 0.8 V.

Both the co- flow and the counter- flow fuel cells shown in Figs. 4 and 6 need a heat-exchanger 501 to preheat the cath- ode gas, whereas the fuel cell according to the present invention can omit the he t-exchanger because of the mixture of un-used and partly used cathode gas in the mixing chambers 201, 202. According to Fig. 5, the fuel cell - where un-used anode and cathode gas enters in a first side - co- flows along the anode and cathode flow fields and exits the fuel cell in a sec- ond side, shows an unbalanced temperature profile with increasing temperature from the gas inlets to the outlets.

Even when the cathode gas counter-flows the anode gas as shown in Fig. 6, the temperature profile is still unbalanced according to Fig. 7. Now the temperature is lowest at the cathode gas inlet side of the fuel cell (viz. the anode gas outlet side) and increasing from said cathode gas Oinlet to the cathode gas outlet side (viz. the anode gas inlet side) . This is because the volume flow and thus the heat capacity of the cathode gas is far larger than the volume flow and heat capacity of the anode gas.

The temperature profile of the alternating flow fuel cell is more balanced. According to Fig. 9, the in- / out-let temperatures are rather equal to each other, the cell having the highest temperature near the center of the cell.

The resulting average power density can be seen in Fig. 10, which shows that the alternating flow fuel cell or fuel cell stack according to the invention with a rather balanced temperature profile has an average power density approximately 40% higher than the co-flow and the counter-flow cells.

Claims

• anode

• electrolyte

• cathode

• interconnect comprising cathode and anode gas distribution flow fields,

the gas supply comprising on a first side of the fuel cell or electrolysis cell

• a first gas distribution channel

• a first gas mixing chamber

• a first gas inlet

• a gas oscillation member

and on a second side of the fuel cell or electrolysis cell

• a second gas distribution channel

• a second gas mixing chamber

• a second gas inlet,

2. Oscillating reactant gas supply according to claim 1, wherein the oscillation member comprises a piston or a membrane pump .

3. Oscillating reactant gas supply according to any of the preceding claims, wherein at least one mixing chamber comprises an ejector.

4. Oscillating reactant gas supply according to any of the preceding claims, wherein the first and second gas inlets are connected to the first and the second mixing chambers .

5. Oscillating reactant gas supply according to any of the preceding claims, wherein the volume of the oscillating gas is larger than the corresponding flow field volume of the fuel cell or electrolysis cell.

6. Oscillating reactant gas supply according to any of the preceding claims, wherein the flow volume of the oscillating gas is in the range of 2 to 7 times larger than the corresponding flow field volume of the fuel cell or electrolysis cell, preferably in the range of 3 to 5 times larger than the corresponding flow field volume of the fuel cell or electrolysis cell.

7. Oscillating reactant gas supply according to any of the preceding claims, wherein the reactant gas is supplied via the first gas inlet when the gas flows in the first direction and is supplied via the second gas inlet when the gas flows in the second direction.

8. Oscillating reactant gas supply according to any of the preceding claims, wherein the oscillation frequency is 0.1 to 10 Hz.

9. Oscillating reactant gas supply according to any of the preceding claims, wherein the cell is a fuel cell and the reactant gas is air, which is supplied to the cathode side of the at least one fuel cell via the first and second gas inlet, the distribution channel and the mixing chamber.

10. Oscillating reactant gas supply according to any of the claims 1 to 8 , wherein the cell is a fuel cell and the reactant gas is a fuel which is supplied to the anode side of the at least one fuel cell via the first and second gas inlet, the distribution channel and the mixing chamber.

11. Oscillating reactant gas supply according to claim 10, further comprising a first and a second catalyst located in fluid connection to the first and the second gas distribu- tion channels.