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Hierarchical nanomaterials are highly suitable as electrocatalysts and electrocatalyst supports in electrochemical energy conversion devices. The intrinsic kinetics of an electrocatalyst are associated with the nanostructure of the active phase and the support, while the overall properties are also affected by the mesostructure. Therefore, both structures need to be controlled. A comparative state-of-the-art review of catalysts and supports is provided along with detailed synthesis methods. To further improve the design of these hierarchical nanomaterials, in-depth research on the effect of materials architecture on reaction and transport kinetics is necessary. Inspiration can be derived from nature, which is full of very effective hierarchical structures. Developing fundamental understanding of how desired properties of biological systems are related to their hierarchical architecture can guide the development of novel catalytic nanomaterials and nature-inspired electrochemical devices.

A lung-inspired approach is employed to overcome reactant homogeneity issues in polymer electrolyte fuel cells. The fractal geometry of the lung is used as the model to design flow-fields of different branching generations, resulting in uniform reactant distribution across the electrodes and minimum entropy production of the whole system. 3D printed, lung-inspired flow field based PEFCs with N = 4 generations outperform the conventional serpentine flow field designs at 50% and 75% RH, exhibiting a B20% and B30% increase in performance (at current densities higher than 0.8 A cm Broader contextFlow-field design is crucial to fuel cell performance, since non-uniform transport of species to and from the membrane-electrode assembly results in significant power losses. The long channels of conventional, serpentine flow fields cause large pressure drops between inlets and outlets, thus large parasitic energy losses and low fuel cell performance. This issue is exacerbated for small, portable fuel cells, where the power required for fluid transport should be minimal. To ensure uniform distribution of reactants across the electrode and a low pressure drop, we use a nature-inspired design that is rooted in thermodynamic and mechanical fundamentals, rather than biomimicry in a narrow sense. Inspiration is derived from the structure of the human lung, which ensures uniform gas distribution via an optimized fractal structure linking bronchi to alveoli, and realizing a remarkable combination of minimal entropy production, low pressure drop, and scale-invariant operation. Our 3D-printed, conducting flow-field plates maintain these unique characteristics of the lung, resulting in improved fuel cell performance over conventional serpentine flow-field based fuel cells. Uniformity in reactant distribution and minimal pressure drop are retained during scale-up, demonstrating the robustness of the proposed nature-inspired approach across length scales.