No abstract

The electrochemical carbon dioxide reduction reaction (CO2RR), which can produce value‐added chemical feedstocks, is a proton‐coupled‐electron process with sluggish kinetics. Thus, highly efficient, cheap catalysts are urgently required. Transition metal oxides such as CoOx, FeOx, and NiOx are low‐cost, low toxicity, and abundant materials for a wide range of electrochemical reactions, but are almost inert for CO2RR. Here, we report for the first time that nitrogen doped carbon nanotubes (N‐CNT) have a surprising activation effect on the activity and selectivity of transition metal‐oxide (MOx where M = Fe, Ni, and Co) nanoclusters for CO2RR. MOx supported on N‐CNT, MOx/N‐CNT, achieves a CO yield of 2.6–2.8 mmol cm−2 min−1 at an overpotential of −0.55 V, which is two orders of magnitude higher than MOx supported on acid treated CNTs (MOx/O‐CNT) and four times higher than pristine N‐CNT. The faraday efficiency for electrochemical CO2‐to‐CO conversion is as high as 90.3% at overpotential of 0.44 V. Both in‐situ XAS measurements and DFT calculations disclose that MOx nanoclusters can be hydrated in CO2 saturated KHCO3, and the N defects of N‐CNT effectively stabilize these metal hydroxyl species under carbon dioxide reduction reaction conditions, which can split the water molecules and provide local protons to inhibit the poisoning of active sites under carbon dioxide reduction reaction conditions.

Proton exchange membrane fuel cells (PEMFCs) are becoming a major part of a greener and more sustainable future. However, the costs of high-purity hydrogen and noble metal catalysts alongside the complexity of the PEMFC system severely hamper their commercialization. Operating PEMFCs at high temperatures (HT-PEMFCs, above 120 °C) brings several advantages, such as increased tolerance to contaminants, more affordable catalysts, and operations without liquid water, hence considerably simplifying the system. While recent progresses in proton exchange membranes for HT-PEMFCs have made this technology more viable, the HT-PEMFC viscous acid electrolyte lowers the active site utilization by unevenly diffusing into the catalyst layer while it acutely poisons the catalytic sites. In recent years, the synthesis of platinum group metal (PGM) and PGM-free catalysts with higher acid tolerance and phosphate-promoted oxygen reduction reaction, in conjunction with the design of catalyst layers with improved acid distribution and more triple-phase boundaries, has provided great opportunities for more efficient HT-PEMFCs. The progress in these two interconnected fields is reviewed here, with recommendations for the most promising routes worthy of further investigation. Using these approaches, the performance and durability of HT-PEMFCs will be significantly improved.

Metal fluoride−lithium batteries have been viewed as very promising candidates for next-generation rechargeable batteries with higher energy densities. However, the intrinsic insulating properties of metal fluoride cathode lead to the poor reaction kinetics and unsatisfactory electrochemical performance. Herein, a honeycombed CoF 2 @C nanocomposite with a high specific surface area up to 180.4 m 2 g −1 , in which the nanosized CoF 2 particles with size of 5−25 nm are evenly embedded in the honeycombed carbon framework, is prepared by the lowtemperature fluorination of honeycombed Co@C nanocomposite precursor. As expected, the as-produced CoF 2 @C nanocomposite can deliver a high-capacity utilization of 365 mAh g −1 and an average capacity retention of 81.9% over 300 cycles at a current density of 110 mA g −1 , as well as a reasonable capacity of 205 mAh g −1 at 1100 mA g −1 . Such excellent electrochemical performance is due to the unique configuration that achieves the nanoconfinement of conversion reaction in the metal fluoride cathode. To be specific, on the one hand, the honeycombed structure provides uniformly isolated nanospace to inhibit the volume expansion and product agglomeration in the conversion reaction. On the other hand, the excellent reaction kinetics is attributed to the threedimensional electron and ion conduction pathway, that is, the electrons are conducted through the honeycombed carbon walls, while Li + are transferred via the interconnected honeycomb channels, facilitating the high-capacity utilization.