In solid oxide electrolysis cells (SOECs), the microstructure of the cathode (Ni-YSZ) has a great influence on the electrochemical performance of cells. In this study, novel hierarchical Ni-YSZ electrode substrates with open channel pores have been fabricated by freeze-drying tape casting (FTC). The microstructure characterization of the the Ni-YSZ electrode substrates, using SEM, mercury intrusion, 3D X-ray microscopy and gas permeability, shows that Ni-YSZ electrode substrates fabricated by FTC have large porosity and low tortuosity factor, which are beneficial for gas transport. The SOECs with the Ni-YSZ electrode substrates fabricated from FTC display excellent performance ( -0.95 A cm^-2 with 1.3 V at 750 ^oC) in the electrolysis mode, which is twice of those with the conventional sponge-like Ni-YSZ electrode substrate fabricated from dry-pressing. Furthermore, compared with the cell performance in the fuel cell mode, the advantages of the hierarchically porous Ni-YSZ electrode substrates are more obvious in the electrolysis mode, which may be due to the slower diffusion of H₂O and CO₂, compared with H₂.