Development of hematite and cupric oxide photoelectrodes for water splitting tandem cells

Resumen

Since the industrial revolution, the energy consumption is continuously increasing due to the economic growth. This increase is linked with a rise in the consumption of fossil fuels (main source of energy until now), and thus, aggravating environmental issues such as climate change due to the emission of greenhouse gases i.e. CO2. Taking into consideration that the average power consumption is expected to reach values up to 36 TW in 2050 (now, 15 TW), the urgent need for investigating energy sources capable of both supplying such power demand and being environmentally friendly is undeniable. In spite of the increase in the use of renewable energies to produce electricity, reaching a 25 % in 2017, the use of fossil fuels will still be a necessity during the next decades due to the lack of infrastructure and efficient technologies in the context of renewable energies. Within the different types of renewable energy sources, solar energy holds by far the largest potential capacity. Each year, the amount of solar energy reaching our planet is about 105 TW, which implies that, even if only a small fraction of the solar energy could be harvested (up to 3 %), it should be enough to satisfy the predicted energy demand in 2050. Therefore, the possibility of harnessing solar energy is of great interest. In this respect, artificial photosynthesis is a promising technology not only to harvest solar energy, but also as a means of storage by producing energy-rich chemical fuels such as H2, CH4 or CH3OH. Among these environmentally friendly fuels, hydrogen presents several advantages compared to methane or methanol, including its high energy density, and its carbon-free combustion. It is important to highlight that the main raw material for hydrogen generation by photoelectrolysis is water, abundant and affordable. The work of Fujishima and Honda in 1972 was the first photoelectrochemical device for splitting water into O2 and H2. In that case, the device was comprised of a TiO2 photoanode for the generation of oxygen and a Pt counter electrode at which the hydrogen evolution took place: H2O(l)→H2(g) +1/2 O2(g) ΔG0(298 K)= 237 kJ/mol After having passed almost 50 years and despite of the efforts of the scientific community, direct solar water splitting is still under intense research since no competitive system has been found yet. One of the most promising devices for solar water splitting are photoelectrochemical tandem cells. The main components of these devices are semiconductor light absorbing photoelectrodes. The principal characteristics that semiconductor materials should have to be suitable as photoelectrodes are: · Visible light absorption · High chemical stability (both in the dark and under illumination) · Favorable band edge positions for oxidation/reduction of water · Efficient charge transport · Low overpotentials for oxidation/reduction of water · Low cost The spectral region in which the semiconductor material absorbs light is determined by its band gap. A minimum band gap of 1.9 eV is required in order to overcome the thermodynamic and kinetic barriers associated to photoelectrochemical water splitting. Additionally, below 400 nm the intensity of sunlight drastically decreases, for instance, the optimum value of the band gap for solar water splitting with one material should be located somewhere between 1.9 and 3.1 eV, which corresponds to the visible range of the solar spectrum. Among the characteristics that semiconductors should meet in order to be employed as photoelectrodes, stability plays an important role because it limits the usefulness of many photoactive materials. Since most non-oxide semiconductors are unstable, oxide semiconductors are commonly used. Only a few semiconductors fulfill the band edge position requirements, and, unfortunately, they have too wide band gaps or they are unstable in aqueous solutions. For the time being, a single semiconductor material that meets all these requirements has not been found yet. Instead, a tandem cell combining semiconductors with complementary light absorption presents several advantages, including an increase in the efficiency of the process. Additionally, in this way the choice of materials is simplified because the requirements are less strict. These devices are composed of a photocathode, where the solar hydrogen is generated, and a photoanode where the photooxidation of water occurs. Within all the possible candidates for water splitting, hematite has been extensively studied as a photoanode because of its abundancy, high stability and favorable band gap. However, hematite presents several drawbacks (high recombination rate, low carrier mobility and low charge carrier efficiency) that have to be addressed in order to use it in a practical device. On the other hand, cupric oxide is postulated to be a promising candidate as a photocathode due to its narrow band gap, low toxicity and low cost. Unfortunately, cupric oxide presents a major drawback related to its instability against photocorrosion in aqueous solutions. For all the reasons mentioned above, the present PhD has the following objectives: 1. Synthesize oxide semiconductor electrodes specifically hematite and cupric oxide by chemical bath deposition and electrodeposition respectively, for their application in photoelectrochemical devices. 2. Characterize the materials synthesized by means of spectroscopy, microscopy and diffraction techniques such as UV-vis spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy and X-ray diffraction. 3. Photoelectrochemically characterize the materials synthesized in order to quantify both their photoactivity in respects to hydrogen/oxygen generation and their stability. For these purposes, techniques such as cyclic voltammetry, linear sweep voltammetry, chronopotentiometry, chronoamperometry and impedance measurements were used. 4. Study different strategies to improve charge transfer, limit recombination and increase efficiency for oxygen evolution in hematite by means of doping, surface modification or morphological change. 5. Study different strategies to overcome the instability against photocorrosion, and thus, to improve the efficiency of solar hydrogen generation in cupric oxide electrodes by forming a protective layer or by passivating the active sites for copper reduction. 6. Develop a photoelectrochemical tandem cell using hematite and cupric oxide as photoelectodes and study the incorporation of a polymer electrolyte membrane.