Long-range order, short-range disorder: Engineering one-dimensional flow channel arrays with hierarchically porous reaction interfaces for electrocatalytic reduction of oxygen

Abstract

Fuel cell performance depends heavily on electrochemical reaction kinetics and mass transport. Rapid kinetics requires high-activity catalysts while good mass transport demands efficient transport domains. Recent fundamental studies in these areas focus more on the improvement in catalyst activity, whereas the importance of mass transport should not be underestimated, which is the focal point of this work. Hierarchical pores are generally considered to be conducive to enhancing reaction kinetics and mass transport, but the degree of order of pore structures has a significant influence over the fuel cell performance. Herein, we introduce the concept of ‘long-range order, short-range disorder’ to design transport domains and reaction interfaces: dispensing with conventional microfabrication techniques, well-ordered, one-dimensional channel arrays are constructed using vascular bundles of natural material, capable of decreasing tortuosity, increasing porosity, optimizing reactant distribution and thus minimizing concentration losses; further, intrinsic biominerals and tissues of organisms existing on the inner and outer walls of channel arrays are harnessed to form hierarchically porous domains, that is, raising the roughness of reaction interfaces by increasing the degree of short-range disorder. The combination of one-dimensional flow channels (long-range order) and hierarchically porous reaction interfaces (short-range disorder), analogous to blood vessels composed of arteries/veins and capillaries, ensures the optimization of mass transport and catalytic kinetics, leading to an electrocatalytic oxygen reduction performance enhancement over a wide array of biomass-derived carbonaceous materials. Accordingly, this work provides inspiration for finding the right balance between mass transport and reaction kinetics from an order perspective.