The carbothermal shock (CTS) method has attracted considerable attention in recent years because it enables the generation of finely controlled polyelemental alloy nanoparticles (NPs). However, fabricating high surface coverage of NPs with minimized exposure of the carbon substrate is essential for various electrochemical applications and has been a critical limitation in CTS method. Here, we developed a methodology for creating NPs with high surface coverage on a carbon substrate by maximizing defect sites of cellulose during CTS. Cu NPs with high surface coverage of ~85%, various single NPs and polyelemental alloy NPs were densely fabricated with high uniformity and dispersity. The synthesized Cu NPs on cellulose/carbon paper substrate were used in electrocatalytic CO2 reduction reaction showing selectivity to ethylene of ~49% and high stability for over 30 hours of reaction. Our cellulose-derived CTS method enables the greater availability of polyelemental NPs for a wide range of catalytic and electrochemical applications.