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Water electrolysis over a wide pH range offers promising technology for hydrogen production, which requires highly stable and active electrocatalysts for the hydrogen evolution reaction (HER). Herein, polyallylamine (PA)-functionalized ultrafine rhodium nanoparticles (Rhô€€€ uNPs) uniformly anchored on three-dimensional graphene aerogel (GA) nanocomposites (PA@Rhô€€€ uNPs/GA) are synthesized by using a facile surface adsorption-chemical reduction method at room temperature. Physical characterization shows that the PA@Rhô€€€ uNPs/GA nanocomposites have three-dimensionally porous architecture and that PA@Rhô€€€ uNPs with a 1.3 nm size are uniformly dispersed on the GA surface. Electrochemical measurements show that the molecular weight of PA affects the HER activity of PA@Rh- uNP nanocomposites. The molecular weight-optimized PA@Rh- uNPs/GA nanocomposites with 10% Rh mass show higher HER activity than the commercial 20% Pt/C electrocatalyst over a wide pH range, which originates from proton or water enrichment at the electrode/solution interface due to the specific function of - NH2 groups at PA molecules. [ABSTRACT FROM AUTHOR]