Electrodepositions
Example:
Electrodeposition of cobalt foam as the support for catalyst.
(a): Catalyst directly electrodeposited on carbon fibers.
(b): High magnification of image (a).
(c): Cobalt foam electrodeposited on carbon fibers.
(d): High magnification of image (c).
Remarks:
(a) & (b) Catalyst directly deposited on carbon fiber has a limited surface area, which is bad for catalyzing electrochemical reactions. (c) & (d) Cobalt foam (from cobalt salts) is electrodeposited on the carbon fibers as the supporter of the catalyst for better dispersion.
(e): Catalyst electrodeposited on Cobalt foam, i.e., (c)
(f): High magnification of image (e)
(g): Catalyst (2 times) electrodeposited on Cobalt foam
(h): High magnification of image (g)
The catalytic performances of the electrode are improved when catalysts are dispersed on cobalt foam/carbon fibers.
Background
(1) Types of electrodeposition-Depending on the applications, electrodeposition can either be used to (1) plate thin coherent metal layers on the conductive substrate (i.e., chrome plating for corrosion resistance) or (2) synthesize open-channel microporous structures (as a supporter of catalyst, etc.).
(2) The mechanisms of electrodeposition - Cations have solvation shells when dissolved in the electrolyte. For electrodeposition to occur, the hydration shell has to be removed as such the cations approach the surface of conductive substracts. The energy required to remove the hydration shell typically increases with the cation's charges (trivalent > divalent > monovalent). The applied overpotential is more negative than the equilibrium potential such that the metal cation can be chemically reduced and deposited on the substrate i.e., underpotential deposition.
(3) Electrodeposition Method-Electrodeposition can be either performed through chronopotentiometry (constant current) or chronoamperometry (constant voltage). It can be conducted in a 3-electrode system or a 2-electrode system under direct current. In industry, Cu and Cr are typically coated on another conductive substrate's surface.
(4) Components of electrodeposition- The cathode (negative electrode, where metal X plates), anode (positive electrode, often inert materials or X block materials), solution (X metal salts dissolved in DI water, e.g., copper sulfate or copper nitrate), and external power sources (e.g., potentiostat, a battery).
(5) Controlling the thickness- A slow electrodeposition rate generally produces a better-finished product. One can either control the electrodeposition time (at fixed current) or total coulomb (at fixed potential) to obtain a desired thickness. In the below example, the apparent thickness (Amount of loading mg/cm^2) of cobalt foam can be controlled via cycle number. 1 cycle (or a few cycles) seems to be acceptable while 10 cycles are not optimal (fewer pores).
(6) Electrodepositing on different substrates - Electrodeposition can be performed on different substrates (e.g., graphite paper, macrostructure Ni foam, and p-type heavily doped silicon); the pre-requisite is that they should be electrically conducting. However, the morphologies of cobalt foam may be varied slightly on different substrates.
