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This work aims to address the need for more durable electrocatalysts with lower precious metal content for proton exchange membrane fuel cells (PEMFCs), through the development of novel electrocatalyst materials and preparation routes. In this work, 'Nafion-Pt/C' electrocatalysts have been derived from ionomer-stabilised Pt nanoparticles synthesised via a novel, wet-chemical route that offers unprecedented control over the formation of the Pt-ionomer interface, with a view towards maximising the utilisation of the electrocatalyst. Nafion-Pt/C electrocatalysts have been characterised using ex-situ electrochemical techniques, and single-cell PEMFC testing to determine their activity and selectivity towards the oxygen reduction reaction (ORR), and to compare their utilisation and durability with commercially-available electrocatalysts. Nafion-Pt/C catalysts with agglomerated Pt particles exhibited a twofold improvement in durability vs. commercial catalysts, whilst offering similar ORR activities. Their enhanced durability was attributed to inhibition of Pt particle growth mechanisms by a passivating layer of Nafion introduced during the synthesis of Nafion-stabilised colloidal Pt. The second part of this work investigated methods for the synthesis of bimetallic nanoparticles consisting of an early transition-metal core (Ti) enclosed in a Pt shell, expected to offer higher intrinsic activity towards oxygen reduction than Pt alone, whilst being less prone to degradation than other alloys of Pt such as Pt-Ni, Pt-Co and Pt-Fe.