Development of photoelectrochemical cells for solar hydrogen production using layer-by-layer deposition and in situ nuclear magnetic resonance spectroscopy

Since the start of the industrial revolution, access to large amounts of energy has driven human development. Decades of energy exploration have led to the use of coal, petroleum, natural gas and uranium. Today, climate change and environmental concerns are steering scientific research in the direction of renewable energy sources. More than half of the newly installed power capacity worldwide consists of hydroelectric, solar, wind and geothermal power. In this work a strategy is investigated to complement renewable power with renewable fuels. This strategy is solar water splitting.

In solar water splitting, solar energy is used to split the water molecule into its constituents, hydrogen and oxygen. Hydrogen may be used as a reducing agent to obtain other products or can be directly used as a fuel to once again generate water. In this work, a two-compartment reactor is built with an ion exchange membrane separator. On either side of the membrane, a catalyst-coated carbon electrode is pressed. One electrode is coated with titanium dioxide semiconductor and faces a window to receive sunlight. Photons which exceed the band gap energy of the semiconductor excite electrons to the conduction band, leaving a hole in the valence band. The holes are at a sufficiently positive potential to oxidise water and produce molecular oxygen and protons. The oxygen gas is evacuated and the protons are transported through the ion exchange membrane to the second electrode. The conduction band electrons are transmitted to this electrode through an external circuit. The second electrode is coated with platinum catalyst, which performs the hydrogen evolution reaction using the proton and electron products from the titanium dioxide electrode. The hydrogen gas is thus produced in a separate compartment. Crossover of hydrogen and oxygen is prevented by the membrane separator.

Such a photoelectrochemical reactor simultaneously manages the transport and reaction of photons, electrons, ions and molecules. There are many possible reactor designs which all follow similar principles. In Chapter 1, an account is given of the current stage of development of photoelectrochemical reactors. Their operation, recent progress and forthcoming directions are discussed. Over the last decade, increasing attention was given to integrated, autonomous cells. Their design has evolved from typical two- or three-electrode laboratory setups towards two-compartment sealed reactors with well-defined dimensions that require no external bias.

In this work the monolithic, porous assembly of membrane and electrodes allows for free choice of operating conditions. An unavoidable requirement of electrochemical cells is the presence of a conducting pathway between anode and cathode. In practice a liquid electrolyte with high ionic strength is commonly used. In the monolithic assembly, the ion exchange membrane serves as solid electrolyte with charged sulfonic groups that are fixed to the polymer backbone. A flow of pure liquid water can be used, which contains no dissolved salts. Gas phase operation is possible, without requiring any liquid at all. Depending on the conditions used, the current response of the cell under illumination changes. In Chapter 2, the chronoamperometric cell signature is analysed by varying the operating conditions. Observed effects are attributed to capacitive charging of the semiconductor, build-up and side reactions of hydrogen and oxygen products, membrane dehydration and a proton concentration gradient.

From this investigation it is clear that photoelectrochemical cells are complex systems that require a fine-tuned interplay of all the active components. In Chapter 3, layer-by-layer deposition is explored as a strategy to assemble these components. Layer-by-layer deposition is a versatile technique with a high degree of control over the type and amount of deposited material. Titanium dioxide nanoparticles are embedded in a polymer matrix to fabricate electrodes with low mass loading and high specific activity. The same type of film is also used for photocatalytic degradation of pollutants.

In Chapter 4, the original reactor is revisited to explore a new concept: gas phase operation using outside air as the source of water. The reactor is installed on a rooftop to give the first ever demonstration of hydrogen production using solely natural sunlight and outside air. For this demonstration electrodes are fabricated using atomic layer deposition on carbon nanotubes to obtain thin film architectures which are inert towards molecular oxygen present in air. The operation in air opens completely new possibilities for solar hydrogen production. Contrary to liquid-based devices, gas bubbles, frost, membrane poisoning nor corrosion cause problems. Moreover, only limited peripherals are needed and no source of clean water. In Chapter 5, the state of the art of photoelectrochemical water splitting and photovoltaic-driven electrolysis is discussed, along with the potential of air-based water splitting. A model is built which shows that air contains about ten times more water than is required to run such devices.

In Chapter 6 of this thesis, the potential role of solar fuels in a global energy landscape is investigated. More than half of our energy consumption requires fuels and a large portion of this could be supplied by air-based water splitting. Solar fuel production is usually envisioned as large-scale plants in deserts, using high-maintenance equipment and transporting the fuel to end users by pipelines or trucks. In this work, a second type of solar fuel production is added to the portfolio: smaller scale autonomous devices which readily produce hydrogen from the sun and the air that surround us.