Design And Synthesis Of Carbon-dots Based Composites For Electrocatalytic Methanol Oxidation And Water Evaporation

The global energy crisis and the fresh water shortage are two of the most severe challenges in modern society,which have fueled the search for various efficient energy conversion methods and water purification technologies.Among them,direct methanol fuel cells(DMFCs),which use portable liquid methanol as fuel,are pollution-free,operated at low temperatures,and they have high energy density.Precious metal platinum is considered as an efficient electrocatalyst for methanol oxidation.However,their high cost and susceptibility against poisoning of the surface-adsorbed intermediates impede them for practical use.Therefore,reducing platinum content or developing alternative catalysts are feasible solutions.In addition,solar energy,as an abundant and sustainable energy source,is a potential natural source for boosting the evolution of renewable energy technology.The study of solar-driven interfacial water evaporation focuses on the heat confinement to generate steam.It has made significant progress in seawater desalination,waste treatment and power generation.However,most evaporators require expensive precursors and complex manufacturing process,which limits their commercial application.In this case,it is very challenging to develop low-cost,scalable and portable evaporators,which are still required to integrate the features of good solar absorption,efficient thermal management,and suitable structure for water transport and steam escape.Given the surface of carbon dots(CDs)are rich of functional groups,and its good light absorption capability,we design and synthesize carbon dots-based composites for energy storage and conversion devices.This provides new concepts for developing low-cost and highly active anode catalysts of DMFCs and highly efficient water evaporators.The corresponding performances and mechanisms for electrocatalytic methanol oxidation by Pt-CoOOH-CDs/C hybrid and the solar steam generation by polyvinyl alcohol/carbon dots composite microgel and the carbon fibers self-assembled by carbon dots were studied.The details are as follows:(1)The Pt-CoOOH-CDs/C catalyst with low Pt content(13 wt%)was synthesized by precipitate method followed with photochemical deposition.And its catalytic properties for electrocatalytic methanol oxidation reaction was studied.The catalyst exhibited high activity(856.3 mA mg-1Pt),favourable stability(3 hours)and good anti-poisoning ability for the intermediates.In the catalytic system,the Pt nanoparticles serve as the active centers to adsorb methanol molecules;CoOOH-CDs act as the co-catalyst to oxidize intermediate poisonous species to CO2 on neighbouring Pt sites and the cost-efficient black carbon enhances the conductivity for the system.The synergistic effects among the different components enhance the catalytic activity of Pt-CoOOH-CDs/C catalyst.(2)The heat-localized water evaporator was construsted by using polyvinyl alcohol microgel(PVA)as the skeleton and the reduced carbon dots(rCDs)as the light-absorbing substances.The xenon lamp was used as a sunlight simulator to test the water evaporation performance.The polyvinyl alcohol/reduced carbon dots composite microgels(PCM)based evaporator can achieve high evaporation rate of 1.58 kg m-2 h-1 under 0.5 kW m-2 irradiation.The water vapor was further condensed to obtain fresh water,and its ion concentrations met drinking water standards.The evaporator is combined with the forward osmosis process to build an "artificial tree",which realizes the vertical water transport against gravity and storage of solar energy in the form of potential energy.(3)To construct the highly efficient evaporator based on carbon dots.Carbon dots solution is obtained from electrolytic method.The solution is rapidly frozen by liquid nitrogen and followed by lyophilization,and it forms self-assembled carbon fiber,which is further calcined in a tubular furnace to receive the light absorber for the evaporator.The fiber structure is rich of channels,which is conducive to the absorption of sunlight,as well as water transport and steam escape.The evaporation rate of the evaporator reaches 2.07 kg m-2 h-1 under 1 kW m-2 irradiation.