Truly hierarchical lung type porosity in carbon catalysts for oxygen reduction

Fuel cells offer a promising transportation solution, improving urban air quality and reducing CO₂ emission. Ideally, they run on renewable fuels (e.g. solar hydrogen) and emit only water. The cathode reaction in fuel cells – the oxygen reduction reaction (ORR) – is typically catalyzed by platinum, an expensive prone to poisoning. Nitrogen doped, highly porous carbons are an intriguing alternative.[i] They are cheap and durable, yet still inferior in catalytic activity to platinum.

A common challenge for high surface area carbons is limited mass transport through the tortuous porosity. Thus, many strive to achieve multi-modal (“hierarchical”) porosity, with macro- (>50 nm), meso- (2[ii] This arrangement provides both high surface area, and good flow through the material. However, this multi-modal porosity is still far from optimized flow. In nature, optimal flow solutions include lungs, veins, and river deltas.[iii] All such systems include a branching (dendritic) channel pattern.

We now report carbon catalysts with a lung-type internal porosity. We grew dendritic ZnO rods on various substrates and with various morphologies (trunks / trees). We used these as sacrificial hard templates to deposit N-doped carbon structure. The result is a carbon with porosity that closely follows the structure of the zinc oxide, enabling a careful tuning and manipulation of the (truly!) hierarchically porous structure. This approach introduces both interconnectivity and directionality to the pores in the material, leading to a boost in mass transport and ORR performance of these novel metal-free carbon catalysts.

[i] (a) R. Jasinski, Nature 201, 1964, 4925, 1212-13.
(b) X. Liu, L. Dai, Nature reviews materials, 2016, 1, 16064.

[ii] (a) D. Eisenberg, P. Prinsen, N.J. Gells, W. Stroek, N. Yan, B. Hua, J. Luo, G. Rothenberg, RSC Adv., 2016, 6, 80398-80407.
(b) T. K. Slot, D. Eisenberg, D. Noordenne, P. Jungbacker, G Rothenberg, Chem. Eur.J., 2016, 22,12307 –12311

[iii] A. Bejan, J. Heat Transfer, 2015, 137(6), 061003.