Site-selective reactions: Remodelling by diversity and design.

963 news & views the catalyst surface are believed to act as catalytically active centres binding O 2 and Li 2 O 2. Oxygen-vacancy ordering in the framework also contributes to the intrinsic high electronic conductivity of the pyrochlore. e porosity not only makes the surface atoms of catalysts more accessible to reactants but also favours mass transport for intermediate species such as superoxides. e porous structure with interconnected walls provides a facile electron conduction network, which is especially benecial to the redox reactions of insulating Li 2 O 2. e outstanding catalytic activity of the oxygen-defective pyrochlore can be seen from the high capacity and considerable stability under charging and discharging of the Li–air cells assembled using the new oxide. e capacity approaches the theoretical value (>10,000 mA h g –1), and it can be sustained over 25 cycles at a low discharge depth. e pyrochlore catalyst also aords higher discharge capacity than a standard carbon catalyst. Remarkably, it is much more ecient than carbon in catalysing the OER, resulting in better peroxide stripping and a lower charge potential (by ~0.6 and 0.4 V for the onset and peak potential, respectively). Compared with the ORR, the OER is a bigger challenge in a non-aqueous Li–air battery. e charge overpotential is signicantly greater than the discharge overpotential, leading to limited electrical energy eciency for a discharge–charge cycle 3. In this context, this active mesoporous pyrochlore represents impressive progress in developing ecient electrocatalysts — although full elucidation of the catalytic mechanism is still required. oughtful design of the structure of the electrodes in a Li–air system is certainly necessary given the involvement of such complex species (dissolved O 2 in electrolyte, soluble or solvated superoxide, and deposited solid Li 2 O 2) and processes (including absorption/desorption, solvation, redox reactions and decomposition). e combination of porosity and surface defects contributes greatly to the high activity of cathode electrocatalysts, as exemplied by this pyrochlore. Porosity is important because of the requirement for void space for ecient mass and charge transport, and also because of the need for a phase interface on which redox reactions can take place. In our opinion, desirable electrode architectures should incorporate hierarchical pores from micro-to meso-and macroscale to achieve eective gas diusion, liquid wetting and solid accommodation. Rechargeable Li–air batteries doubtless hold great promise to power our energy-thirsty society. eir practical application still requires long-term R&D work to tackle …