How and why does this Cu change into C (graphite)? Source: gutefrage.net

At the right side of the graphic, the Cu changes while discharging into C as graphite. How and why does this happen?

I thought it happens because of the dissolving of the Cu when the electrons stream from the Cu to the Al.
The electrode in here has two parts – the Cu and the graphite (C). The Cu won't dissolve. Only the graphite (C) will react. So there is no changing from Cu to C.