Gas diffusion substrate
View Patent ↗A gas diffusion substrate includes a non-woven network of carbon fibres, the carbon fibres are graphitised but the non-woven network has not been subjected to a graphitisation process. A mixture of graphitic particles and hydrophobic polymer is disposed within the network. The longest dimension of at least 90% of the graphitic particles is less than 100 μm. A process for manufacturing gas diffusion substrates includes depositing a slurry of graphitised carbon fibres onto a porous bed forming a wet fibre network, preparing a suspension of graphitic particles and hydrophobic polymer, applying onto, and pulling the suspension into, the network, and drying and firing the network. Another process includes mixing a first slurry of graphitic particles and hydrophobic polymer with a second slurry of graphitised carbon fibres and liquid forming a third slurry, depositing the third slurry onto a porous bed forming a fibre-containing layer, and drying and firing the layer.
1. A gas diffusion substrate comprising
a non-woven network of carbon fibres wherein the carbon fibres are graphitised but the non-woven network has not been subjected to a graphitisation process; and
a mixture of graphitic particles and hydrophobic polymer disposed within the network, wherein the longest dimension of at least 90% of the graphitic particles is between 20 and 0.1 μm.
2. A gas diffusion substrate according to claim 1 , wherein the non-woven network has a grammage of between 10-50 g/m 2 .
3. A gas diffusion substrate according to claim 1 , wherein the graphitic particles are small enough to be held in colloidal suspension.
4. A gas diffusion substrate according to claim 1 , wherein the weight ratio of graphitic particles to hydrophobic polymer is between 50:1 and 2:1.
5. A gas diffusion substrate according to claim 1 , wherein the weight ratio of graphitic particles to carbon fibres is between 1:2 and 10:1.
6. A gas diffusion substrate according to claim 1 , wherein the substrate has a gradient in the concentration of the graphitic particles and hydrophobic polymer across the thickness of the non-woven network.
7. A gas diffusion substrate according to claim 1 , wherein the graphitic particles and hydrophobic polymer are homogeneously disposed within the fibre network.
8. A gas diffusion substrate according to claim 1 , comprising a carbonised phenolic resin binder.
9. A gas diffusion substrate according to claim 1 , wherein a base layer of carbon black and hydrophobic polymer is present on at least one face of the substrate.
10. A gas diffusion electrode comprising a gas diffusion substrate according to claim 1 and an electrocatalyst layer.
11. A gas diffusion electrode according to claim 10 , wherein the substrate has a gradient in the concentration of the graphitic particles and hydrophobic polymer across the thickness of the non-woven network, wherein the electrocatalyst layer is adjacent to the face of the gas diffusion substrate and wherein there is a higher concentration of graphitic particles and hydrophobic polymer.
12. A membrane electrode assembly comprising a gas diffusion substrate according to claim 1 .
13. A membrane electrode assembly comprising a gas diffusion electrode according to claim 10 .
14. A process for preparing a gas diffusion comprising the steps of:
a) depositing a slurry of graphitised carbon fibres onto a porous bed to form a wet fibre network;
b) preparing a suspension of graphitic particles and hydrophobic polymer, wherein the longest dimension of at least 90% of the graphitic particles is between 20 and 0.1 μm;
c) applying the suspension onto the wet fibre network;
d) pulling the suspension into the wet fibre network; and
e) drying and firing the wet fibre network at a temperature that does not exceed 1000° C.;
wherein the gas diffusion substrate is according to claim 1 .
15. A process according to claim 14 , wherein in step (d) the suspension is pulled into the wet fibre network using a suction system.
16. A process according to claim 15 , wherein the suspension is applied to one face of the wet fibre network only, and the suction is applied to the other face of the wet fibre network.
17. A process for preparing a gas diffusion substrate comprising the steps of:
a) mixing graphitic particles, wherein the longest dimension of at least 90% of the graphitic particles is between 20 and 0.1 μm, with a suspension of hydrophobic polymer to form a first slurry;
b) mixing graphitised carbon fibres and a liquid to form a second slurry;
c) mixing the first slurry with the second slurry to form a third slurry;
d) depositing the third slurry onto a porous bed to form a fibre-containing layer; and
e) drying and firing the fibre-containing layer at a temperature that does not exceed 1000° C.;
wherein the gas diffusion substrate is according to claim 1 .
18. A process for preparing a gas diffusion substrate according to claim 14 , comprising a further step of applying a layer comprising carbon black and a hydrophobic polymer to at least one face of the fibre-containing layer.
19. A process for producing a gas diffusion electrode wherein a gas diffusion substrate is produced by a process according to claim 14 and an electrocatalyst layer is deposited onto the gas diffusion substrate.
20. The process of claim 17 wherein step (b) further comprises mixing a binder with the carbon fibres and the liquid.
21. A gas diffusion substrate comprising:
a non-woven network of carbon fibres wherein the carbon fibres are graphitised but the non-woven network has not been subjected to a graphitisation process; and
a mixture of graphitic particles and hydrophobic polymer disposed within the network, wherein the longest dimension of at least 90% of the graphitic particles is between 100 μm.
wherein the weight ratio of graphitic particles to hydrophobic polymer is between 50:1 and 2:1.
22. A gas diffusion substrate according to claim 21 , wherein the non-woven network has a grammage of between 10-50 g/m 2 .
23. A gas diffusion substrate according to claim 21 , wherein the graphitic particles are small enough to be held in colloidal suspension.
24. A gas diffusion substrate according to claim 21 , wherein the weight ratio of graphitic particles to carbon fibres is between 1:2 and 10:1.
25. A gas diffusion substrate according to claim 21 , wherein the substrate has a gradient in the concentration of the graphitic particles and hydrophobic polymer across the thickness of the non-woven network.
26. A gas diffusion substrate according to claim 21 , wherein the graphitic particles and hydrophobic polymer are homogeneously disposed within the fibre network.
27. A gas diffusion substrate according to claim 21 , comprising a carbonised phenolic resin binder.
28. A gas diffusion substrate according to claim 21 , wherein a base layer of carbon black and hydrophobic polymer is present on at least one face of the substrate.
29. A gas diffusion electrode comprising a gas diffusion substrate according to claim 22 and an electrocatalyst layer.
30. A gas diffusion electrode according to claim 29 , wherein the substrate has a gradient in the concentration of the graphitic particles and hydrophobic polymer across the thickness of the non-woven network, wherein the electrocatalyst layer is adjacent to the face of the gas diffusion substrate and wherein there is a higher concentration of graphitic particles and hydrophobic polymer.
31. A membrane electrode assembly comprising a gas diffusion substrate according to claim 21 .
32. A membrane electrode assembly comprising a gas diffusion electrode according to claim 29 .
33. A process for preparing a gas diffusion substrate comprising the steps of:
a) depositing a slurry of graphitised carbon fibres onto a porous bed to form a wet fibre network;
b) preparing a suspension of graphitic particles and hydrophobic polymer, wherein the longest dimension of at least 90% of the graphitic particles is less than 100 μm;
c) applying the suspension onto the wet fibre network;
d) pulling the suspension into the wet fibre network; and
e) drying and firing the network at a temperature that does not exceed 1000° C.;
wherein the gas diffusion substrate is according to claim 21 .
34. A process for preparing a gas diffusion substrate comprising the steps of:
a) mixing graphitic particles, wherein the longest dimension of at least 90% of the graphitic particles is between 100μm, with a suspension of hydrophobic polymer to form a first slurry;
b) mixing graphitised carbon fibres and a liquid to form a second slurry;
c) mixing the first slurry with the second slurry to form a third slurry;
d) depositing the third slurry onto a porous bed to form a fibre-containing layer; and
e) drying and firing the fibre-containing layer at a temperature that does not exceed 1000° C.;
wherein the gas diffusion substrate is according to claim 21 .