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Nanomaterials are at the core of fuel cell electrodes, providing high‐area catalytic, proton, and electron conducting surfaces, traditionally on carbon black supports. Other carbons, e.g., carbon nanotubes (CNTs) and graphene are less prone to oxidation; however, their handling is not trivial due to health risks associated with their size. Assembling them into microscale structures without jeopardizing their performance is ideal, but there are mass transfer limitations as thickness increases. In this work, a soluble acicular calcium carbonate (aragonite) is used as a porogen to create connected porosity in microspheres. Increasing macroporosity has a considerable positive impact on the mass transfer process. The experimental manipulation of porosity of the microspheres is combined with pore network modeling to better understand how pore distribution throughout the whole microsphere can optimize platinum utilization decorated onto the CNTs. Oxygen reduction reaction (ORR) activity is compared with the prepared composite materials and a commercial Pt/C catalyst for 4 weeks. The composite materials exhibit a highly interconnected network resulting in a 3.4 times higher ORR activity (at 0.9 V vs reversible hydrogen electrode) than that of the nanoporous spheres with no macroporosity. [ABSTRACT FROM AUTHOR]