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Stable and active Pt electrocatalyst preparation by a modified citrate reduction method and a mechanism analysis of inorganic additives. • Pt with a clean surface were synthesized via a modified citrate reduction method. • NO 2 − ions possibly alter the reaction kinetics of the growth of Pt NPs. • The optimized Pt/CNT demonstrated high mass activity and moderate activity decay. • The maximum power density of the optimized Pt/CNT catalysts at 0.6 V is 744 mW cm−2. Ultra-small and monodispersed Pt nanoparticles (NPs) have been successfully synthesized in polymer electrolyte membrane fuel cells. The process normally involves the use of capping agents, organic species, templates, and substrates and is thus complex. Hence, obtaining Pt NPs with a clean surface is challenging. In this study, a method for preparing stable and highly dispersed Pt NPs with clean surfaces is proposed. The method involves the use of a modified Na 3 C 6 H 5 O 7 reduction process assisted by NaNO 3 stabilization. The specific complexations of NO 2 − ions possibly alter the reaction kinetics and lower the growth rate of Pt NPs by retarding the reduction reaction. The optimized Pt/carbon nanotube (CNT) catalysts exhibit high mass activity and moderate activity decay after 10,000 times of potential cycling compared with commercially available Pt/C catalysts. Then, membrane electrode assemblies based on the resultant catalysts are characterized. The cell performance of 744 mW cm−2 (maximum power density) is achieved after the optimized Pt/CNT catalysts are used in carbon black. [ABSTRACT FROM AUTHOR]