Noble metal catalyst layer, membrane electrode assembly, and method for producing noble metal catalyst layer
According to one embodiment, the noble metal catalyst layer includes first noble metal layer and a second noble metal layer formed on the first noble metal layer. The first noble metal layer includes a first noble metal element and has a porosity of 65 to 95 vol. %, a volume of pores having a diameter of 5 to 80 nm accounts for 50% or more of a volume of total pores in the first noble metal layer. The second noble metal layer includes a second noble metal element, and has an average thickness of 3 to 20 nm and a porosity of 50 vol. % or less.
1. A noble metal catalyst layer comprising:
a first noble metal layer which comprises a first noble metal element and a cavity having a diameter of 100 nm or more, the first noble metal layer excluding the cavity having a porosity of 65 to 95 vol. %, a volume of pores having a diameter of 5 to 80 nm accounting for 50% or more of a volume of the total pores excluding the cavity in the first noble metal layer; and
second noble metal layers which are formed on both sides of the first noble metal layer, comprise a second noble metal element and have an average thickness of 3 to 20 nm and a porosity of 50 vol. % or less,
wherein the noble metal catalyst layer has an overall porosity of 20 vol. % or more, and an amount of a porous part of the first noble metal layer and the second noble metal layers in the noble metal catalyst layer is 60 vol. % or more.
2. The noble metal catalyst layer according to claim 1 , wherein the porosity of the first noble metal layer is from 75 to 95 vol. %.
3. The noble metal catalyst layer according to claim 2 , wherein the porosity of the first noble metal layer is from 80 to 95 vol. %.
4. The noble metal catalyst layer according to claim 1 , wherein, in the first noble metal layer, the volume of pores having a diameter of 5 to 80 nm is 70% or more of the volume of the total pores in the first noble metal layer.
5. The noble metal catalyst layer according to claim 1 , wherein the average thickness of each of the second noble metal layer is from 3 to 15 nm.
6. The noble metal catalyst layer according to claim 5 , wherein the average thickness of each of the second noble metal layer is from 3 to 5 nm.
7. The noble metal catalyst layer according to claim 1 , wherein each of the second noble metal layers is formed of two or more layers.
8. The noble metal catalyst layer according to claim 1 , wherein, in the first noble metal layer, a volume of pores having a diameter of 30 nm or more accounting for 30% or more of the volume of the total pores in the first noble metal layer.
9. The noble metal catalyst layer according to claim 8 , wherein, in the first noble metal layer, the volume of pores having a diameter of 30 nm or more accounting for 40% or more of the volume of the total pores in the first noble metal layer.
10. The noble metal catalyst layer according to claim 9 , wherein, in the first noble metal layer, the volume of pores having a diameter of 30 nm or more accounting for 50% or more of the volume of the total pores in the first noble metal layer.
11. The noble metal catalyst layer according to claim 1 , wherein the overall porosity is 30 vol. % or more.
12. The noble metal catalyst layer according to claim 1 , wherein the noble metal catalyst layer comprises a noble metal element at a supporting density of 0.2 mg/cm 2 or less, the noble metal element comprising the first noble metal element and the second noble metal element.
13. The noble metal catalyst layer according to claim 12 , wherein the supporting density is 0.1 mg/cm 2 or less.
14. The noble metal catalyst layer according to claim 1 , wherein interval between the second noble metal layers is 500 nm or less.
15. The noble metal catalyst layer according to claim 14 , wherein the interval between the second noble metal layers is 400 nm or less.
16. The noble metal catalyst layer according to claim 1 , wherein 30 atomic % or more of the first noble metal element is platinum or iridium, and 30 atomic % or more of the second noble metal element is platinum or iridium.
17. The noble metal catalyst layer according to claim 1 , wherein the first noble metal element and the second noble metal element is platinum, and a specific surface area of platinum in the noble metal catalyst layer, which is determined from the amount of hydrogen desorption obtained by cyclic voltammetry, is 25 m 2 /g or more.
18. A membrane electrode assembly comprising:
a first electrode;
a second electrode; and
an electrolyte membrane sandwiched between the first electrode and the second electrode, at least one of the first electrode and the second electrode comprising a noble metal catalyst layer of claim 1 .