Here, we report a grain-boundary(GB)-rich Pt nanoparticle (NP) assembly material that catalyzes H2 oxidation in air at room temperature and exhibits low cross-sensitivity and negligible poisoning effect to common pollutant gases in the atmosphere (e.g., CO, NO2, SO2, H2S, and NH3) and various combustible organic gases and vapors (e.g., natural gas, alkanes, alkenes, alcohols, ketones, aldehydes, ethers, aromatic compounds, amine, and acetates). As illustrated in Fig. , the GB-rich Pt NP assembly was synthesized by performing water electrolysis in an unbuffered, high-concentration (~1.7 μM) colloidal Pt NP solution. The local high pH of ~14 at the cathode assists the removal of citrate capping agents from the Pt NP surface (fig. ). Gas bubbles generated during water electrolysis promote the random collisions of the destabilized Pt NPs after ligand removal, driving individual NPs to assemble into NP clusters, chains, networks, and eventually a macroscopic (sub-centimeter-sized) solid (Fig. , fig. ). Scanning transmission electron microscopic (STEM) images of Pt NPs and Pt NP assembly show no significant crystallite size change after the water-electrolysis-driven NP assembly (Pt NPs: 3.6 ± 0.5 nm and Pt NP assembly: 3.8 ± 0.7 nm, Fig. ). High-resolution TEM (HRTEM) and STEM images revealed that individual Pt NP building blocks in the Pt NP assembly were primarily connected by GBs. The representative GBs identified in the Pt NP assembly are Σ3 (111) twin boundaries, where two NPs share the low-energy {111} planes (e.g., NP1 and NP2 in Fig. ). We also observed Σ11 (113) GBs (fig. ) and highly-disordered GBs (e.g., the GBs between the {111} plane of NP4 and the {200} plane of NP5 in Fig. ). It is worth noting that an interface-free structure can also form when two NPs oriented attach along the {111} planes (e.g., NP3 and NP4 in Fig. ).