In depth Understanding of Multilayered Photoelectrodes for Water Splitting Using Operando Electrochemical Methodologies

Abstract

Photoelectrochemical (PEC) water splitting has been considered as a potential technology to convert quasi-infinite solar energy into clean H2 chemical fuel. Over decades, a vast amount of research effort has focused on the design of multilayered photoelectrodes to achieve improved solar-to-H2 efficiency and long-term stability. However, due to the high complexity of this multilayer configuration, the traditional electrochemical characterization method involving current–voltage curves with a three-electrode setup is not able to ascertain an in-depth understanding of several fundamental issues in these systems, including properties of the buried junction, the electrode degradation mechanism and charge carrier processes under realistic photo-electrolysis conditions. This thesis details our work in utilizing operando electrochemical techniques to study the above-mentioned issues. Chapter 1 presents the basic knowledge and principle of PEC water splitting, semiconductors, and introduces different functional layers in a photoelectrode. The common electrochemical techniques to examine a photoelectrode were then discussed as well as their limitation in characterization. Then we used the dual-working-electrode (DWE) technique in Chapter 2 and 3, where a second working electrode (WE2) was introduced to probe in situ the surface potential of a pn+Si/TiO2/Pt photocathode for H2 generation. The traditional three-electrode setup–including a working electrode (WE1), a reference electrode and a counter electrode–can only provide the overall PEC performance, which is represented by a photocurrent density-back contact potential curve (J-V1 curve). With the help of the WE2, the photovoltaic performance of the buried pn+Si junction can be extracted, which is represented by a J-ΔV curve. By comparing the J-ΔV curves before and after a long-term photoelectrolysis test, the degradation mechanism of the photocathode could be revealed: for the pn+Si/TiO2/Pt photocathode, the J-ΔV curve showed a negligible change after the long-term test, implying that the pn+Si junction was stable, as expected; for the Cu2O/Ga2O3/TiO2/Pt photocathode, the J-ΔV curve showed an obvious loss in open circuit voltage (VOC), suggesting an instability of the Cu2O/Ga2O3 junction under operational conditions. Moreover, the DWE technique was applied to assess emerging materials (Sb2Se3/Sb2S3) and new methods such as tunable dipole layers (p-Si/Phosphonic acid (PA)/TiO2 photocathodes). The Sb2S3 formation during the post-sulfurization of Sb2Se3 produced enhanced photovoltage; the PA layer also increased the VOC generated by p-Si/TiO2 junction and exhibited good stability. In Chapter 4 and 5, photoelectrochemical impedance spectroscopy (PEIS) was applied to investigate charge-carrier processes in TiO2-protected photoanodes. For the np+-Si/TiO2/Ni photoanode, several basic processes were deconvoluted and studied in the form of equivalent resistances: (1) photo-excited electron-hole pair recombination inside the np+-Si junction (Rrec); (2) hole transport through the TiO2 (RTiO2); (3) water oxidation by holes at the Ni-based water oxidation catalyst (Rct). Moreover, we further elucidated the “hole leaky” property of TiO2. RTiO2 of both np+Si/TiO2/Ni and nSi/TiO2/Ni photoanodes, did not show a clear thickness dependence, suggesting that a conduction band mechanism was applicable. In addition, results from the DWE measurements strongly suggest that the “hole-leaky” property of TiO2 can be interpreted by charge transport in the conduction band, in spite of the presence of the defect states in TiO2. Last but not least, it was found that the RTiO2 behaved differently in the nSi/TiO2/Ni. RTiO2 increased as the photocurrent increases, which was interpreted by trapping in the defect states present at the interface of the nSi/TiO2. Therefore, a detailed picture of charge carriers process in both the np+Si/TiO2/Ni and nSi/TiO2/Ni photoanodes has been achieved. Chapter 6 concludes this comprehensive operando study of photoelectrodes realized by the DWE technique and PEIS. Several unclear issues are then laid out, and suggestions for future research are put forward.