Activation of Transition Metal(Fe,Co and Ni)-Oxide Nanoclusters by Nitrogen Defects in Carbon Nanotube for Selective CO_(2) Reduction Reaction

The electrochemical carbon dioxide reduction reaction(CO_(2)RR),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 CoO_(x),FeO_(x),and NiO_(x)are low-cost,low toxicity,and abundant materials for a wide range of electrochemical reactions,but are almost inert for CO_(2)RR.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(MO_(x)where M=Fe,Ni,and Co)nanoclusters for CO_(2)RR.MO_(x)supported on N-CNT,MO_(x)/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 MO_(x)supported on acid treated CNTs(MO_(x)/O-CNT)and four times higher than pristine N-CNT.The faraday efficiency for electrochemical CO_(2)-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 MO_(x)nanoclusters can be hydrated in CO_(2)saturated KHCO_(3),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.

Overcoming the Electrode Challenges of High‑Temperature Proton Exchange Membrane Fuel Cells

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℃)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 mem-branes 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.

Elucidating the electro-catalytic oxidation of hydrazine over carbon nanotube-based transition metal single atom catalysts

Elucidating the reaction mechanism of hydrazine oxidation reaction(HzOR)over carbon-based catalysts is highly propitious for the rational design of novel electrocatalysts for HzOR.In present work,isolated first-row transition metal atoms have been coordinated with N atoms on the graphite layers of carbon nanotubes via a M-N_(4)-C configuration(MSA/CNT,M=Fe,Co and Ni).The HzOR over the three single atom catalysts follows a predominant 4-electron reaction pathway to emit N_(2) and a negligible 1-electron pathway to emit trace of NH3,while their electrocatalytic activity for HzOR is dominated by the absorption energy of N2H4 on them.Furthermore,FeSA/CNT reverses the passivation effect on Fe/C and shows superior performance than CoSA/CNT and NiSA/CNT with a recorded high mass activity for HzOR due to the higher electronic charge of Fe over Co and Ni in the M-N_(4)-C configuration and the lowest absorption energy of N_(2)H_(4) on FeSA/CNT among the three MSA/CNT catalysts.