Carbonized Polymer Dots Assemble in Proton‐Conducting Channels to Enhance the Conductivity and Selectivity Simultaneously for High‐Performance Fuel Cells

Abstract Fabricating polymer electrolyte membranes (PEMs) simultaneously with high ion conductivity and selectivity has always been an ultimate goal in many membrane‐integrated systems for energy conversion and storage. Constructing broader ion‐conducting channels usually enables high‐efficient ion conductivity while often bringing increased crossover of other ions or molecules simultaneously, resulting in decreased selectivity. Here, the ultra‐small carbon dots (CDs) with the selective barriers are self‐assembled within proton‐conducting channels of PEMs through electrostatic interaction to enhance the proton conductivity and selectivity simultaneously. The functional CDs regulate the nanophase separation of PEMs and optimize the hydration proton network enabling higher‐efficient proton transport. Meanwhile, the CDs within proton‐conducting channels prevent fuel from permeating selectively due to their repelling and spatial hindrance against fuel molecules, resulting in highly enhanced selectivity. Benefiting from the improved conductivity and selectivity, the open‐circuit voltage and maximum power density of the direct methanol fuel cell (DMFC) equipped with the hybrid membranes raised by 23% and 93%, respectively. This work brings new insight to optimize polymer membranes for efficient and selective transport of ions or small molecules, solving the trade‐off of conductivity and selectivity.