KOH solution is quite well. But I have to inform you that Co3O4 is not a supercapacitor material now. You can read this paper (Chem. Soc. Rev., 2016, 45, 5925--5950) if you want to know a brief reason.
KOH. I am agree with Q. Zhu... You will find non linear charge discharge curves and non- rectangular CV curves...which suggest it is not an ideal supercapacitor material.. instead it exhibits battery like behavior... You should go through prof. George Chen papers.