Hybridization of a fuel cell with a high power energy source reduces the necessary fuel cell size, improves dynamic behaviour, alleviates degradation and allows the recovery of reversible energy, e.g. from magnetic brakes. Direct hybridization, i.e. the connection of a supercapacitor with one or two fuel cells on the unit cell level, is easy to implement, is energy self-managing, provides direct protection against load peaks and avoids power electronics which add weight and increase losses. This paper takes the direct hybridization concept one step further by integrating the supercapacitor into the fuel cell. This avoids redundant system and cell components and leads to a lightweight hybrid system. The supercapacitor electrode is integrated by functionalizing the gas diffusion layer (GDL) of the fuel cell, giving it an electrochemical double layer capacity. Possible synthesis routes for such a functionalization are developed based on physical theory of self-organization and established carbon surface modifications. A self-assembly of an ionomer skin around the carbon fibres of the GDL with a proton-enriched water phase in between leads to a structure with an added double layer capacity, a nanoscopic liquid water transport network and hydrophobic inter-fibre pores allowing unhindered oxygen transport. The dynamic electrode transport processes in the integrated hybrid were analyzed using porous electrode theory. Overall, the integrated hybrid behaves similarly to a direct, parallel hybrid. However, the parallel hybrid has two separate proton pathways, i.e. fuel cell membrane and supercapacitor separator, leading to an overall lower ohmic resistance. Because the pathways are separated, charge balancing oscillations between fuel cell and supercapacitor can occur, especially in asymmetric configurations with differing anode and cathode capacities. In the integrated hybrid, the proton conducting phases of supercapacitor and fuel cell are connected, which avoids such charge balancing and can lead to lower losses in the mid-frequency range. (C) 2019 Elsevier Ltd. All rights reserved.