Numerical Simulation On The Light Transport In The TiO₂ Porous Photoanode Of Photocatalytic Fuel Cells

Photocatalytic fuel cell（PFC）can use solar energy to directly convert the chemical energy contained in wastewater into electricity via the photo/electro/chemical reactions,which can realize the clean energy generation and wastewater treatment simultaneously.As a result,this new technology can potentially make tremendous contribution to the solar energy utilization and the energy recovery of wastewater.Past efforts have been mainly devoted to the development of the photoanode materials and structures to boost the performance of the PFC.In fact,the photoanode of the PFC has very complex micro/nano porous structure,in which the energy/species transport coupled with the photoelectrochemical reaction occurs.In this case,a deep understanding of the energy/species transport characteristics and its coupling mechanism with the photoelectrochemical reaction is critically important to design and optimize the photoanode structure and operating paramters so as to improve the PFC performance.However,just because of the complex micro/nano porous structure of the photoanode,it is rather difficult to directly achieve the energy/species transport characteristics via experimental approaches.Therefore,the numerical simulation method has become one of promising strategies to explore the energy/species transport coupled with the photoelectrochemical reactions.However,the light transport in existing numerical simulation works on the photoanode is usually described by a semi-empirical equation,which ignores the light scattering in the porous photoanode and thereby brings the uncertainty to the simulation results.To resolve this issue,this thesis targets at the TiO₂ porous photoanode,which is the most used in the PFCs.First of all,the radiative properties of the TiO₂ porous thin film were determined in this thesis.Based on these radiative properties and radiative transfer theory,a model describing the light transport in the TiO₂ porous thin film was developed,by which the light transport characteristics in the TiO₂ porous thin film could be obtained.Finally,based on the radiative properties and radiative transfer theory,the photoelectrochemical reaction coupled energy/species transport model for the TiO₂ porous photoanode was developed,in which the electron/hole/organics transport was also considered.With this model,the energy/species transport characteristics in the photoanode as well as its effect on the photoanode performance was achieved.Main outcomes of this thesis are summarized as follows.（1）Determination of the radiative properties of the TiO₂ porous thin filmsIn this thesis,the TiO₂ porous thin films with different loadings and the TiO₂/PMMA porous thin films with different porosities were prepared.The optical parameters and microstructures of these porous thin films were measured by UV-VIS and SEM,by which the hemispherical reflectance,the hemispherical transmittance and the thin film thickness could be determined.It was found that in the ultraviolet（UV）spectrum,the hemispherical reflectance of the porous thin film almost remained unchanged as the catalyst loading and porosity increased,while the hemispherical transmittance decreased with increasing the catalyst loading and porosity.In the visible-light range,the hemispherical reflectance of the porous thin film increased with increasing the catalyst loading and porosity,while the hemispherical transmittance decreased.Based on the experimental results,the absorption coefficients and the scattering coefficients of the TiO₂ and TiO₂/PMMA porous thin films were retrieved by the spectral element method.The results showed that the absorption coefficient sharply decreased with increasing the wavelength in the UV light region,and was almost unchanged in the visible-light range.For the scattering coefficient,it first increased and then dropped with increasing the wavelength.There existed a maximum scattering coefficient at the wavelength of about 378 nm.The scattering coefficient of the porous thin film remarkably increased with decreasing the TiO₂:PMMA mass ratio（increasing the porosity）,while slight increase of the absorption coefficient was observed.（2）Numerical simulation on the light transport in the TiO₂ porous thin filmBased on the radiative transfer theory,a model describing the light transport in the TiO₂ porous thin film was developed.Utilizing the determined radiative properties,the distribution of the local volumetric rate of energy absorption（LVREA）and photon absorption efficiency of the porous thin films could then be obtained.The results showed that the LVREA in both the TiO₂ and TiO₂/PMMA porous thin films decreased rapidly along the light irradiation.The effects of the film thickness,incident light intensity and porosity on the light transport characteristics were also studied.It was found that the variation of the LVREA showed the same trend in all the porous thin films.When the film thickness rised from 5μm to 10μm,the photon absorption efficiency increased but remained unchanged when the thickness was further increased to 30μm.It was also found that the LVREA of the TiO₂ and TiO₂/PMMA porous thin films increased significantly with increasing the incident light intensity,while the photon absorption efficiency kept unchanged.Given the incident light intensity,as the TiO₂:PMMA mass ratio decreased,the LVREA increased in the region near the thin film/substrate interface,while the photon absorption efficiency decreased instead.（3）Numerical simulation on the energy/species transfer coupled with the photoelectrochemical reactions in the TiO₂ porous photoanodeAn energy/speices transfer model describing the electron/hole/organics transport coupled with the photoelectrochemical reaction was developed based on the radiative transfer theory,by which the energy/species tranfer characteristics in the photoanode could be achieved.Besides,the effects of the thickness of the photoanode,the incident light intensity,the inlet methanol concentration and the porosity of the photoanode on the energy/species transfer and the photoelectrochemical performance were studied.It was found that as compared to the conventional models that employed the semi-empirical equation to decribse the light intensity distribution in the photoanode,the results achieved by the present model based on the radiative transfer theory were in better agreement with the experimental results,indicating the present model can more objectively describe the light transport in the photoanode.It was also found that when the thickness of the photoanode increased from 5μm to 10μm,the short circuit current density and the corresponding electron density increased,but remained unchanged as the thickness was further increased to 30μm.The methanol concentration decreased as the thickness increased.Increasing the incident light intensity and the inlet methanol concentration led to the increase of the short circuit current density and the corresponding electron density.Moreover,the methanol concentration in the photoanode decreased with increasing the incident light intensity,but increased with increasing the inlet methanol concentration.The numerical results also indicated that among the conventional TiO₂ photoanode and the TiO₂/PMMA photoanodes,the conventional TiO₂ photoanode yielded the largest short circuit current density.