Proton-conducting reversible solid oxide cells (PC-RSOCs) have attracted extensive attention due to their high efficiencies as energy conversion devices. Generally, the performance of the cell is affected to a certain extent by the microstructure of the electrodes, which is closely related to the gas diffusion and surface reaction processes. Herein, different contents of the carbon microspheres (CMSs) are used as the pore formers to control the microstructure of the hydrogen electrode. Experimental results reveal that the porosity, line shrinkage, and thermal expansion coefficient of the hydrogen electrode support simultaneously increase with the CMS content. The support with 30 wt% CMS presents high porosity (39.27 vol%) with uniform-size pores. Subsequently, the corresponding single cells were fabricated successfully, particularly, the cell with 30 wt% CMS exhibiting the best electrochemical performance in both fuel cell (0.46 W cm(-2) at 700 degrees C) and electrolysis cell (1.41 A cm(-2) at 1.3 V and 700 degrees C) operational modes. Further results demonstrated the highest performance was attributed primarily to the maximal three-phase boundary length, which mainly originates from the high porosity and unique microstructure of the hydrogen electrode. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.