WO2003081692A2

WO2003081692A2 - Flow field plate

Info

Publication number: WO2003081692A2
Application number: PCT/GB2003/001146
Authority: WO
Inventors: Mark Christopher Turpin; James Charles Boff; Brendan Michael Bilton
Original assignee: The Morgan Crucible Company Plc
Priority date: 2002-03-20
Filing date: 2003-03-14
Publication date: 2003-10-02
Also published as: GB2387263B; WO2003081692A3; GB2387263A; GB0206596D0

Abstract

The present invention provides a suspended electrolyte fuel cell flow field plate comprising on at least one face an assembly of channels comprising one or more gas delivery channels, and a plurality of gas diffusion channels connecting thereto.

Description

FLOW FIELD PLATE

This invention relates to fuel cells and is particularly, although not exclusively, applicable to suspended electrolyte fuel cells.

Fuel cells are devices in which a fuel and an oxidant combine in a controlled manner to produce electricity directly. By directly producing electricity without intermediate combustion and generation steps, the electrical efficiency of a fuel cell is higher than using the fuel in a traditional generator. This much is widely known. A fuel cell sounds simple and desirable but many man-years of work have been expended in recent years attempting to produce practical fuel cell systems.

Fuel cells are likely to become an important part of the so-called 'hydrogen economy'. One class of fuel cells in commercial production are those known as suspended electrolyte fuel cells, where a liquid electrolyte is suspended in an unreactive matrix between two electrodes. These fall into three main categories:

• the alkaline fuel cell;

• the phosphoric acid fuel cell; and

• the molten carbonate fuel cell.

Alkaline fuel cells utilise an aqueous solution of potassium hydroxide as an electrolyte, and operate well at room temperature. A wide range of catalysts to promote the reaction within the electrolyte is also available in comparison with other types of fuel cell. Such fuel cells fall into two categories: those with a static electrolyte and those with a mobile electrolyte. Static electrolyte systems involve a potassium hydroxide filled porous membrane placed between two electrodes. Hydrogen is supplied to the anode, and oxygen to the cathode. Hf^1" and O^2" ions combine in the electrolyte producing water which is expelled at the anode. One of the main drawbacks with this system is the production of carbonates in the electrolyte, which eventually blocks electrolyte pathways and electrode pores. This affects the long term stability of the fuel cell. In a mobile electrolyte system, the chemical reaction is the same, but instead of being suspended in a matrix, the electrolyte constantly circulates between two porous membranes. This is advantageous as not only are impurities such as carbonates removed from the system, but water is also expelled through the recirculation of the electrolyte. Ideally, the water is removed through a vaporisation process through porous electrodes into excess reactant gas streams.

There therefore exists a need to produce porous electrodes that effectively deliver reactant materials and remove waste products. A suitable material for this is carbon, allowing low cost electrode production, but a method of ensuring evaporation of waste and reactant delivery at constant pressures and with minimised leakage and parasitic losses needs to be found.

Phosphoric acid fuel cells utilise a phosphoric acid electrolyte suspended in a porous medium, generally a PTFE bonded-SiC matrix. Hydrogen is supplied to the anode, and oxygen in the form of air is supplied to the cathode. The mobile ionic species is let, which combines with electrons at the cathode to produce waste water and oxygen. The use of noble metal catalysts is necessary in phosphoric acid systems as the reaction kinetics are slower than in alkaline systems. Both electrodes are gas diffusion electrodes, comprising a gas side, a hydrophobic backing layer and a working layer. The working layer and the hydrophobic layer are separated by a PTFE bonded catalyst support and an electronic conductive layer of coarse porosity, the catalyst support comprising the catalyst and micro- and meso-pores. Typical electrode materials comprise carbon.

The electrodes used in phosphoric acid cells are formed using foils - a cathode foil, a matrix foil and an anode foil - which are porous, and only a fraction of a millimetre in thickness. The fuel cell also comprises a bipolar plate separator. This is used to connect the cathode from one cell to the anode of another in a fuel cell stack. The bipolar plate functions to guarantee the uniform distribution of reactant gases over the electrode surface, as well as the collection and transmission of current from one cell to the next. The bipolar plate may also be formed using the foil construction method.

The bipolar plate comprises a pattern of channels through which the reactant gases are distributed. These channels are generally arranged in a linear manner. One problem with such channels is the length of gas pathway the reactant gases have to travel and the problems associated with a drop in pressure from as gas flows from one side to the other of the bipolar plate. Therefore there exists a need to produce a bipolar plate that allows reactant gases to travel reduced pathways and provides a constant pressure across the surface of the plate.

Molten carbonate fuel cells utilise a carbonate electrolyte (for example mixtures of Li₂CO₃- K₂CO₃) suspended in a γ-LiA10₂ matrix. The operating temperature of such cells is typically around 500 to 700 °C. Fuel is supplied to the anode. This fuel may be hydrogen but may include carbon monoxide (as produced from a reformer) or may be a hydrocarbon since the fuel cell may act as its own reformer (see below). Oxygen and carbon dioxide (e.g. air) are supplied to the cathode. The mobile ionic species are CO ^" ions, which combine with electrons at the anode, producing waste water and carbon dioxide. The anode material is typically a Ni-Cr alloy, and the cathode material a lithiated NiO alloy.

In general, fuel cells require an external hydrogen reformer in order to function. However, the molten carbonate fuel cell has the potential to be used as its own internal reformer. Consequently, the performance of the fuel cell is dependent on the gas pressure within the fuel cell. Therefore there exists a need to accurately control such pressures to produce optimum performance, with the pressure controlling device being placed between cells in a fuel cell stack to maximise efficiency.

The inventors have realised that the operating difficulties of the three types of suspended electrolyte fuel cell described above may be solved by providing an efficient flow field design on a cell separator or bipolar plate. This would act to both distribute reactant gases to, and where necessary to remove waste from the electrodes.

The inventors have also realised that by looking to physiological systems (the lung) an improved flow field geometry may be realised. Such improved systems are likely to have lower parasitic losses due to their shorter gas flow pathways. They have also realised that such geometries are less likely to suffer from gas short-circuiting.

The present invention therefore provides a suspended electrolyte fuel cell flow field plate comprising on at least one face an assembly of channels comprising one or more gas delivery channels, and a plurality of finer gas diffusion channels having a width of less than 0.2mm connecting thereto.

The gas delivery channels may comprise one or more primary channels of a width greater than lmm, and a plurality of secondary gas delivery channels of a width less than lmm connecting thereto.

The gas diffusion channels may form a branched structure.

The gas diffusion channels may be of varying width, forming a branched structure of progressively diminishing channel width similar to the branching structure of blood vessels and air channels in the lung.

The invention is illustrated by way of non-limitative example in the following description with reference to the drawing in which:-

Fig. 1 shows schematically in part section a part of a flow field plate incorporating gas delivery channels and gas diffusion channels;

Fig. 2 shows schematically a partial plan view of a flow field plate incorporating gas delivery channels and gas diffusion channels;

Fig. 3 shows a part section of a branched flow field plate in accordance with the present invention.

In flow field plates the purpose behind the channels conventionally applied is to try to ensure a uniform supply of reactant material to the electrodes and to ensure prompt removal of reacted products. However the length of the passage material has to travel is high since a convoluted path is generally used.

Another system in which the aim is to supply reactant uniformly to a reactant surface and to remove reacted products is the lung. In the lung an arrangement of progressively finer channels is provided so that air has a short pathway to its reactant site in the lung, and carbon dioxide has a short pathway out again. By providing a network of progressively finer chaimels into the flow field pattern, reactant gases have a short pathway to their reactant sites.

The finest channels could simply discharge into wide gas removal channels or, as in the lung, a corresponding network of progressively wider channels could be provided out of the flow field plate. In the latter case, the two networks of progressively finer channels and progressively wider channels could be connected end-to-end or arranged as interdigitated networks with diffusion through a gas diffusion layer or through the electrode material providing connectivity. Connection end-to-end provides the advantage that a high pressure will be maintained through the channels, assisting in the removal of blockages.

Fig. 2 shows in a schematic plan a portion of a flow field pattern having broad primary gas delivery channels 4, which diverge into secondary gas delivery channels 3 which themselves diverge into gas diffusion channels 2. Gas diffusion channels 5 can also come off the primary gas delivery channels 4 if required. The primary and secondary gas delivery channels may each form a network of progressively finer channels as may the gas diffusion channels and the arrangement of the channels may resemble a fractal arrangement.

The primary gas delivery channels may have a width of greater than lmm, for example about 2mm. The depth of such a channel is limited only by the need to have sufficient strength in the flow field plate after forming the channel. The secondary gas delivery channels may have a width of less than lmm, for example 0.5mm and may be shallower than the primary gas delivery channels. The gas diffusion channels have a width of less than 0.2mm, for example about lOOμm and may be shallower still.

A branched flow field as shown in Fig 3, gas flows in a branching pattern 15 the pathway for reactant gas from the upstream side of droplet 13 to the downstream side of droplet 13 is long - effectively to the end of the flow field and back again. This means that the pressure upstream (B) of the droplet will be significantly higher than the pressure downstream (A), so providing a driving force for removal of water or other reactant products. The flow field plates may be made of any suitable material, although for alkaline fuel cells are preferably carbon. Any known means of providing a pattern in the surface of a porous material may be used, for example etching, pressing or stamping However, in the case of a carbon flow field plate, the process disclosed in WO01/04982, which is incorporated herein in its entirety as enabling the present invention. With this technique the plates may be formed from a carbon/resin composite or other non-porous electrically conductive material that does not react significantly with the reactants used.

It is found with this technique that the profiles of channels of different width vary due to the shadow cast by the mask. Fig. 1 shows a flow field plate 1 having a narrow channel 2 formed in its surface. Because of the shadowing effect of the resist used in fom ing the channel the channel is exposed to sandblast grit coming effectively only from directly above. This leads to a generally semicircular profile to the channel and to a shallow cutting of the channel.

For progressively larger channels (3 and 4) the resist casts less of a shadow allowing sandblasting grit from a progressively wider range of angles to strike the surface of the flow field plate, so allowing both deeper cutting of the surface and a progressively flatter bottom to the channel.

Accordingly, by applying a resist with different width channels to a plate and exposing the plate and resist to sandblasting with a fine grit, a pattern of channels of different widths and depths can be applied.

Applying such a pattern of channels of varying width and depth has advantages. In flow field plates the purpose behind the channels conventionally applied is to try to ensure a uniform supply of reactant material to the electrodes and to ensure prompt removal of reacted products. However the length of the passage material has to travel is high since a convoluted path is generally used. Using the flow field plate of the present invention results in a shortening of the gas pathway, and allows evaporation of waste and reactant delivery at constant pressures and with minimised leakage and parasitic losses. A suspended electrolyte fuel cell comprises a cathode, a suspended electrolyte, an anode and a flow field plate. A plurality of such cells comprising flow field plates may be stacked together to form a fuel cell stack to provide a maximum operating efficiency. The flow field may be provided on one or both sides of the flow field plate.

The invention is not limited to the embodiments described herein. Other embodiments within the scope of the claims will be apparent to a person skilled in the art.

Claims

1. A suspended electrolyte fuel cell flow field plate comprising on at least one face an assembly of channels comprising one or more gas delivery channels, and a plurality of finer gas diffusion channels having a width of less than 0.2mm connecting thereto.

2. A flow field plate as claimed in Claim 1 , in which the gas delivery channels comprise one or more primary channels of a width greater than lmm, and a plurality of secondary gas delivery channels of a width less than lmm connecting thereto.

3. A flow field plate as claimed in any of Claims 1 or 2, in which the gas diffusion channels form a branched structure.

4. A flow field plate as claimed in Claim 3 in which the gas diffusion channels are of varying width forming a branched structure of progressively diminishing channel width.

5. A flow field plate as claimed in any preceding claim comprising a first assembly of channels for gas delivery and a second assembly of channels for removal of reactant products.

6. A flow field plate as claimed in Claim 5, in which the first and second assemblies of channels are interdigitated.

7. A flow field plate as claimed in any preceding claim in which channels decrease in depth with diminishing width.

8. A fuel cell stack comprising a plurality of flow field plates as claimed in any preceding claim.