Polyoxometalates-Mediated Functional Materials:Design,Synthesis And Application In Energy Conversion

Polyoxometalates(POMs)are a class of metal oxide clusters,which have many unique physicochemical properties,and show great application potential in the field of energy conversion.From the view of specific functional requirements of sustainable dye-sensitized solar cells(DSSCs),in this paper,the design and synthesis of new counter electrodes(CEs)catalytic materials and all-weather energy conversion functional materials have been carried out by employing POMs.Firstly,accrording to the requirements of catalysts for CEs,we designed and synthesized two kinds of POMs-based composite nanomaterials based on the redox reversibility,rich structure and elemental composition of POMs,which showed excellent catalytic activity and photovoltaic performance as CEs.Meanwhile,we also designed and synthesized two novel multimetal-based bifunctional composite nanomaterials by employing POMs as molecular preassembly platform,which can effectively replace platinum(Pt)as CEs and electrocatalytic hydrogen evolution active electrodes.Secondly,in order to resolve the intermittence of DSSCs,we have prepared a series of POMs-based films to realize the all-weather energy conversion,which lays the foundation for the construction of all-weather solar cells.The specific work is as follows:1.Five POMs and carbon nanotubes(CNTs)composites including PMo12/CNTs,PW12/CNTs,P2Mo18/CNTs,P2W18/CNTs and Co4PW9/CNTs have been prepared by an ultrasonic-driven strategy and used as CEs catalysts in DSSCs for the first time.In these composites,POMs nanocrystals are periodically deposited on the surface of the CNTs,forming a"necklace"-like structure that not only achieves periodic functionalization of the CNTs but also inhibits the aggregation and dissolution of POMs molecules during the electrochemical reaction.These five composite nanomaterials exhibit very high electrocatalytic activity as CEs.The DSSCs based on Co4PW9/CNTs CE show the highest photovoltaic performance,and the photoelectric conversion efficiency(PCE)is 7.6%.Compared with pure CNTs and traditional Pt CE,the PCE is increased by 32.4%and 15.3%,respectively.Moreover,PW12/CNTs,PMo12/CNTs,P2W18/CNTs and P2Mo18/CNTs CEs can also exhibit good electrocatalytic activity comparable to that of noble metal Pt CE.2.A series of heteroatom and nitrogen co-doped carbon-loaded heteroatom-modified tungsten carbide nanoparticles composites(XWXNC)were prepared by selecting(NH4)n[XW12O40](X=Co,Si,Ge,B,and P)and dicyandiamide as precursors and further used as CEs catalysts.The electrochemical results revealed that the PCE of the DSSCs based on Co WCo NC CE is higher than that of Pt-based DSSCs,and the PCE of XWXNC decreases in order of Co WCo NC,Si WSi NC,Ge WGe NC,BWBNC,PWPNC and WNC,displaying the opposite trend with the electronegativity of doped elements,which indicates that heteroatom doping has a positive effect on the catalytic activity of W2C,and the electronegativity of doped elements is the key affecting the catalytic activity.3.A multimetal-based nitrogen-doped carbon nanotube(Co Fe Ni Mo@NCNT)bifunctional nanocomposite was firstly fabricated by POMs-intercalated layered double hydroxide pyrolysis strategy,which can effectively replace Pt as CE and electrocatalytic hydrogen evolution active electrode.The structure and composition of the nanocomposite were adjusted by controlling the reaction conditions such as precursor ratio,annealing temperature and time.The PCE of the DSSCs based on the Co Fe Ni Mo@NCNT CE is comparable to the traditional Pt-based DSSCs.Moreover,the Co Fe Ni Mo@NCNT showed excellent electrocatalytic hydrogen evolution behavior with the overpotential of 209.9mV to drive a 10 m A cm^-2current density.The results revealed that the excellent bifunctional activity of Co Fe Ni Mo@NCNT is attributed to the synergistic effect between the multimetallic components and the in situ formed NCNT.4.A multi-site Ni Fe Co W@NC nanocomposite has been prepared based on anisotropic integration strategy using POMs and layered bimetallic hydroxides as precursors,which have shown excellent bifunctional catalytic activity.Under the atmosphere of dicyandiamide decomposition,Ni and Fe were reduced to form the active phase Fe Ni3 for CE,W was evolved into WC active for electrocatalytic water splitting due to the strong electron affinity between W and carbon.Co elements doped into the Fe Ni3 alloy phase during calcination and act as nanocatalysts for catalytic the growth of CNTs.The anisotropic integration makes Ni Fe Co W@NC exhibit higher catalytic activity than Co Fe Ni Mo@NCNT when used as CE and electrocatalytic hydrogen evolution catalyst.The PCE of DSSCs based on Ni Fe Co W@NC CE can reach 6.93%.The overpotential for electrocatalytic hydrogen evolution under acidic conditions is 127.8mV.5.A multifunctional POMs/ethyl cellulose film was prepared by a simple drop coating method using POMs as active materials,and the electrical output performance of the film under different weather conditions was systematically explored.The results show that the film can realize the simultaneous conversion from various low-value environmental energies to high-value electricity,and the complementary utilization of various environmental energies is beneficial to the further improvement of the output performance.This film achieves an average output power density of 17.7?W m^-2 when it is simultaneously driven by sunlight,wind and raindrops,which exhibits 4 and 70 times than individual wind and raindrops conditions.Furthermore,the film can achieve maximum output power density at very small external loads(about 10?),indicating the potential of this POMs-based film as a power output device.