title: Solar fuels production by photoelectrochemical CO2 reduction
Creators: name: Garcia Osorio, Dora
Publisher: name: University of Liverpool Repository
Year: 2024

description: Using solar energy to meet the world energy demand is one of the greatest challenges in modern society where a photoelectrochemical approach could aid with solar intermittency by storing the energy in chemical bonds. In this thesis, hybrid photocathodes were studied for producing solar fuels either H2 or to convert CO2 into syngas (CO:H2), which could be potentially used as a feedstock in the already installed chemical process. A hybrid photocathode uses a light absorbing material to harvest the sunlight and a catalyst to carry out the reduction reaction. In this work, we focused in the covalent immobilization of the molecular catalyst onto the electrode surface since it could improve the charge transfer at the interface. ZnTe and Sb2Se3 were explored, with focus on the latter since by the careful interface tailoring (mainly through the addition of CdS/TiO2) to improve charge extraction, significant progress was achieved. This work is summarized in chapter 2. Sb2Se3 has been mainly used with noble metals for H2 production as a photocathode, and to the best of our understanding, here it was first time coupled with molecular catalysts. To accomplish a hybrid photocathode for H2 production, NiP ([Ni(P2R′N2R″)2]2+ core (P2R′N2R″ = bis(1,5-R′-diphospha-3,7-R″-diazacyclooctane)) was chosen as model catalyst since it mimics the hydrogenase, ranks among the most active molecular catalyst and can be covalently immobilised on TiO2 by the phosphonic acid pendant arm in the outer coordination sphere. TiO2 was incorporated onto Sb2Se3/CdS as a protective layer and to provide an anchoring surface. The resultant Sb2Se3-NiP hybrid device gave rise to −1.3 mA cm–2 at 0 V vs RHE, representing the highest photocurrent for NiP decorated photocathodes. It was only possible accomplished by increasing the surface area through a TiO2 mesoporous scaffold (TiO2-meso) for increasing the catalyst loading from 7.08 ± 0.43 nmol cm–2 on the planar device to 45.76 ± 0.81 nmol cm–2. However, fast deactivation occurred in the minutes scale due to photoelectron accumulation which in turns triggers the failure of the TiO2 layer. This finding was supported when the Sb2Se3-NiP hybrid device was benchmarked with Pt as a photoelectron extractor and by transient photocurrent analysis, it was revealed that the TiO2-meso/Pt interface acted as a reservoir of photoelectron accumulation for the HER catalysis to occur on Pt suggesting that NiP was not able to consume all the photoelectrons Sb2Se3 generated. Ni and Co based molecular catalyst, which demonstrated high selectivity for CO production, were explored to build a hybrid photocathode for CO2 reduction. Ni cyclam in solution (Ni(cyc)2+, cyc =1,4,8,11-tetraazacyclotetradecane), and Ni(cycP) with cycP = [(1,4,8,11-tetraazacyclotetradecan-1-yl)methyl] phosphonic acid covalently anchored to the planar TiO2 were explored. Nevertheless, both approaches gave rise to low photocurrents ca. 5 uA cm-2 at -0.41 V vs RHE for Ni(cyc)2+ in solution, and ca. 20 uA cm-2 at -0.39 V vs RHE when immobilised. The complete CO2 hybrid photocathode using Ni-based molecular catalyst was not assembled due to the low Ni(cycP) content even when a TiO2-meso was used to increase the surface binding area (~14 nmol cm-2), this was attributed to the strong interaction with the Ni metal centre. CoPP (Co protoporphyrin IX) was covalently anchored to the TiO2-meso but found that the carboxylic acid anchoring group did not provide enough stability to be used in a CO2R hybrid photocathode. However, the light absorber architecture used for these devices did not have the CdS buffer layer which displayed the best PEC activity. Therefore, to assess the capabilities of Sb2Se3/CdS/TiO2 photocathode for CO2 reduction, a metal benchmark should be used like Au or Ag followed by the molecular catalyst for CO2 reduction.