Iron oxide (Fe 3 O 4 ) nanoparticles anchored over sulfonated graphene oxide (SGO) and Nafion/Fe 3 O 4 -SGO composites were fabricated and applied as potential proton exchange membranes in proton exchange membrane fuel cells (PEMFCs) operated at high temperature and low humidity. Fe 3 O 4 nanoparticles bridge SGO and Nafion through electrostatic interaction/hydrogen bonding and increased the intrinsic thermal and mechanical stabilities of Nafion/Fe 3 O 4 -SGO composite membranes. Nafion/Fe 3 O 4 -SGO composite membranes increased the compactness of ionic domains and enhanced the water absorption and proton conductivity while restricting hydrogen permeability across the membranes. The proton conductivity of Nafion/Fe 3 O 4 -SGO (3 wt%) composite membrane at 120 C under 20% relative humidity (RH) was 11.62 mS cm À1, which is 4.74 fold higher than that of a pristine recast Nafion membrane. PEMFC containing the Nafion/Fe 3 O 4 -SGO composite membrane delivered a peak power density of 258.82 mW cm À2 at a load current density of 640.73 mA cm À2 while operating at 120 C under 25% RH and ambient pressure. In contrast, under identical operating conditions, a peak power density of only 144.89 mW cm À2 was achieved with the pristine recast Nafion membrane at a load current density of 431.36 mA cm À2. Thus, Nafion/Fe 3 O 4 -SGO composite membranes can be used to address various critical problems associated with commercial Nafion membranes in PEMFC applications.

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reaction (OER), [4] which limited the stability of the catalyst and the DUFC performance.In general, OER occurs at the anodic surface under a higher electrode potential, and it is hypothesized that decreasing the anode UOR potential would effectively eliminate the OER. On the other hand, ORR suffers from sluggish reaction kinetics due to the large activation process involved in cleaving the double bonds in an O 2 molecule. [5] Despite the vast interest in this area, all reported DUFCs are based on precious metal ORR catalysts, including Pt/C, [1] Pd, [6] Pd/C@Ti, [7] and Ag/C, [2] which defeats the cost advantages of DUFCs. Furthermore, all of the efforts on DUFCs have been based on the use of two different catalysts for UOR and ORR. Recently, cobalt oxide (Co 3 O 4 )-based nanocatalysts have emerged as a potential bifunctional catalyst due to their ability to effectively eliminate the OER constraints and their preferable adsorption of O 2 molecules via the Pauling mode (Co … (O ads O)). [8] However, the UOR activity of Co 3 O 4 is hampered by the poisoning of the catalyst surface by the oxidation intermediates. [9] The peroxide species (HO 2− ) generated under an alkaline medium during the ORR deteriorates the catalytic stability of Co 3 O 4 . [10] These challenges could be overcome with a surface engineering of Co 3 O 4 nanostructures with specific UOR and ORR catalysts in the form of core-shell architectures.Furthermore, the conventional techniques used for the modification of DUFC electrodes with nanocatalysts involve tedious protocols such as substrate pretreatment, slurry preparation, tedious catalyst loading/coating, binder selection, blockage of active sites via binder, time consumption, high resistance, etc. [11] These difficulties could be circumvented if the nanocatalysts could be directly grown over the electrode materials. Accordingly, we present here a direct growth of core-shell nanostructures over carbon cloth (CC) fibers and address the challenges of Co 3 O 4 toward UOR and ORR electrokinetics via a decoration of Co 3 O 4 core with, respectively, NiO and MnO 2 shells. Results and DiscussionThe scanning electron microscopy (SEM) image of the bare CC (Figure 1a) clearly reveals the ordered woven structure with a smooth surface and interconnected 2D carbon fibers. From Figure 1b,c, it is clear that the CC nanofibers are homogeneously enveloped with the smooth surfaced and uniformly aligned Co 3 O 4 nanowires with sharp tips. The mean diameter and length of Co 3 O 4 nanowires are found to be 40 nm and 1.7 µm, respectively.Bifunctional cobalt oxide (Co 3 O 4 ) nanowire catalysts grown on carbon cloth (CC) fibers and their modification with nickel oxide (NiO) and manganese dioxide (MnO 2 ) to produce core-shell nanoarchitectures are explored as catalysts for urea oxidation reaction and oxygen reduction reaction in direct urea fuel cells (DUFC). Based on a systematic electrochemical characterization of the catalyst, the as-developed core-shell nanoarchitectures are optimized toward DUFC performance....