Design and Fabrication of Chiral Photoelectrodes for the Generation of Hydrogen through Photoelectrocatalytic Water Splitting

Abstract

The prevailing energy challenges, compounded by depletion of non-renewable resources and the effects of climate change, have resulted in the global community investing in renewable and sustainable energy. Green hydrogen stands out as a favourite candidate owing to its higher energy density and a benign combustion by-product (water), which can be further split to produce more hydrogen, thereby creating a hydrogen circular economy. The principle of the Chirality Induced Spin Selectivity (CISS) effect, which states that electron spin control is introduced in chiral photocatalysts, brings new prospects to photoelectrocatalytic water splitting. In this work, the synthesis and application of Iron (MIL 53) as well as Zinc (Zn (bdc)(bpcda)) based Metal Organic Frameworks (MOFs) and their chiral derivatives suitable for hydrogen generation through photoelectrocatalytic water splitting in the visible range is discussed. Characterization using: Nuclear Magnetic Resonance (NMR), Fourier Transform Infrared Spectroscopy (FTIR), Powder X-Ray Diffraction Spectroscopy (PXRD), Thermogravimetric Analysis (TGA), Scanning Electron Microscopy coupled with Energy Dispersive Spectroscopy (SEM-EDS), Field Emission Scanning Electron Microscopy (FESEM) and UV-vis Spectroscopy has been carried out to confirm the: completion of reaction in synthesis, functional groups, phases, thermal stability, surface morphology, elemental analysis as well as the photocatalytic activity of the MOFs. The parent MOFs, were doped using R- (-) and S- (+) camphor sulphonic acid as well as L-Cysteine. The synthesized MOFs have been found to have better absorbances in the visible range, making them suitable for utilising the available radiation within the solar spectrum. The photoelectrocatalytic activities of the synthesized MOFs, have been evaluated using electrochemical techniques including: Cyclic voltammetry, Linear sweep voltammetry and Chronoamperometry under illumination with an AM 1.5 G Solar simulator. The chiral MOFs demonstrate higher current densities under illumination. Results revealed production of greater current densities in chiral doped MOFs with the iron-based MOFs producing larger current densities than the zinc-based MOFs. Mixed-metal MOFs were synthesized by incorporation of nickel and cobalt into the MOF framework, as post synthesis modification of Zn-MOF, a tandem device incorporating α-Fe2O3 nanoparticles was also fabricated, this was meant to address the low current densities observed in the zinc-based MOFs. Remarkable improvement in the current densities has been observed in the mixed-metal MOFs as well as the tandem device. Evaluation of the effect of the chiral MOFs on the overpotential of photoelectrodes was done using Tafel plot analysis, it has been observed that the iron-based MOFs have lower overpotentials compared to the zinc-based MOFs. The introduction of the α-Fe2O3 nanoparticles further reduces the overpotential. This can be attributed to the ferromagnetic nature of the iron that is present in MIL 53 and α-Fe2O3. has been attributed to the Chirality Induced Spin Selectivity (CISS) effect whereby chiral and/or ferromagnetic photoelectrodes have the ability to control the spin of transmitted electrons. When the electrons are spin filtered, production of triplet oxygen is enhanced, thereby minimizing the production of hydrogen peroxide and resulting in improved hydrogen yield. From the results of this study, it has been observed that the introduction of chiral moieties in the metal organic framework greatly improves the performance of the photoelectrochemical cell by minimizing production of hydrogen peroxide. These results indicate the importance of controlling the spin of electrons in the efficiency of artificial photosynthesis in producing green hydrogen.The improved water splitting capabilities observed for ferromagnetic and chiral MOFs