Decreasing the size of an electrolyte used as a catalyst layer increased the platinum (Pt) utilization ratio by introducing the electrolyte into the primary pores, which are the spaces inside the agglomerated Pt-supported carbon black. An organic-inorganic hybrid material, a capping electrolyte, composed of autoclave-treated Nafion and surface-modified zirconium oxide with acetylacetone and aminosilane coupling agent, has a smaller Stokes diameter in solution than Nafion. Electrochemical measurements of a membrane electrode assembly (MEA) show that the Pt utilization ratio was improved when the capping electrolyte was used. Furthermore, a combination of the introduction of capping electrolyte into the primary pores and formation of the proton-transport pathway around the agglomerated particles improved the performance of the MEA in the low current density region, where the reaction area affects the performance, compared with the MEA made with a Nafion electrode. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.065302jes] All rights reserved.