Studies On Preparation,characterization And Catalytic Properties Of Tungsten-based Compounds

As energy shortage and environmental pollution are becoming increasingly serious,renewable energies need to be developed.Solar energy,as a relatively new type of energy,is considered one of the most exploitable renewable energy sources.As the third generation of solar cells,dye-sensitized solar cell?DSSC?have attracted considerable attention because of their unparalleled merits including wide range of sources,low cost,ease of manufacturing,environmentally friendly materials,and high theoretical photoelectric conversion efficiency.The counter electrode?CE?,as an important component of DSSCs,collects electrons from the external circuit and reduces I3-ions in the electrolyte solution.Pt is the preferred CE material in DSSC systems;however,the cost and scarcity of Pt have limited its broad application.Thus,it is imperative to seek low-cost,high catalytic activity,non-Pt CE materials.Tungsten?W?-based compounds as CEs show great application potential in DSSCs because of their adequate resource reserves,good ionic and electrical conductivity,excellent catalytic activity and selectivity of target products.In this study,W-based compounds were prepared via different methods and used as CE materials in DSSCs.The following experiments were carried out on the W-based DSSCs.?1?W-based compounds(W₁₈O₄₉,WO₃,and W₂N)electrocatalytic materials were synthesized via a simple chemical "one pot method" and the catalytic properties of W-based catalysts in DSSCs were systematically investigated.The results showed that as-prepared W-based compounds had good crystallinity,high purity and uniform size distribution.W-based compounds had excellent electrochemical properties and photovoltaic performance.As a parameter for characterizing the catalytic performance of CE catalysts,charge transfer resistance?Rct?values of W₂ N,WC,and WO₃ were 20.04?,22.18?,and 25.32?,respectively,and the corresponding DSSCs exhibited power conversion efficiencies?PCEs?of 5.97%,5.20%,and 4.69%,respectively.These values were similar to the Pt CE under the same conditions?10.81?,6.52%?.The non-stoichiometric W₁₈O₄₉ achieved the most excellent catalytic properties?Rct=2.52??because of the introduction of oxygen vacancies,and the corresponding DSSCs exhibited the optimal photovoltaic performance?6.69%?.Meanwhile,the electrochemical stability test results showed that W₁₈O₄₉,W₂ N,WC,and WO₃ could be stable and efficient catalytic reduction of I-/I3-.First-principle density functional theory calculations showed that the band gaps of W₁₈O₄₉,W₂ N,WC,and Pt were zero,and the band gap of WO₃ was 1.7eV,which further elucidated the intrinsic factors influencing the excellent electrocatalytic activity of W-based compounds.?2?The binary W-based compounds Ni_0.19WO₄ and CoWO₄ electrocatalytic materials were synthesized via a low-temperature hydrothermal method.The results showed that the as-prepared Ni_0.19WO₄ and CoWO₄ materials had the uniform composition and good crystallinity.Ni_0.19WO₄ and CoWO₄ CE materials showed the similar Rs?16.21? and 14.36??and Rct values?35.08? and 41.18??,clarifying Ni_0.19WO₄ and CoWO₄ had good catalytic properties for I-/I3-.The PCEs of the DSSC based on Ni_0.19WO₄ and CoWO₄ CEs achieved 4.71% and 4.55%,respectively,while the PCE of Pt-based DSSC was 6.27% under the same conditions.Meanwhile,electrochemical stability tests showed that Ni_0.19WO₄ and CoWO₄ exhibited good electrochemical stabilities.?3?The Fe₂WO₄ electrocatalytic material was synthesized using a homogeneous precipitation method.The results showed that as-perpared Fe₂WO₄ material had uniform particle size and high purity.The symmetrical cells composed of Fe₂WO₄ CE had Rs and Rct values of 16.1? and 27.42?,respectively,and the corresponds PCE of DSSC was 5.17%.Fe₂WO₄ CE using the synergistic effect of the bimetal exhibited catalytic properties comparable to that of the Pt electrode.At the same time,Fe₂WO₄ CE had the excellent electrochemical stability on the I-/I3-electrolyte.