Stable and efficient photoelectrodes for solar fuels production

Carlos Ros Figueras

Morante Lleonart, Joan Ramon Andreu Arbella, Teresa

Hydrogen production through photoelectrochemistry of water allows a direct conversion of solar energy into hydrogen and oxygen, storing the energy in chemical bonds, a promising way to solve the problem of photovoltaic energy storage. For a large-scale implementation of this technology, efficient, stable and scalable photoelectrodes are necessary. In this doctoral thesis, work has been done from improving the efficiency of metal oxide photoelectrodes to stabilizing highly efficient photovoltaic materials by introducing protective, transparent, conductive and catalytic layers. Efforts have been concentrated on doing so using abundant and low-cost materials and manufacturing techniques. Titanium dioxide (TiO2) photoanodes stable in alkaline electrolytes and anodic applied potentials have been achieved, but their photocurrent conversion remains low. This is due to an excessive TiO2 bandgap, which causes only a small part of the visible spectrum to be absorbed, and also to a low mobility of electrons and holes. The efficiency has been increased by microstructuring the substrate and nanostructuring the layer into nanorods, as well as modifying the electronic structure with a reducing treatment in H2, improving the potential drop inside the nanorods. From here, the proposed strategy has changed towards stabilizing small bandgap semiconductors such as those used by the photovoltaic industry. Silicon photoelectrodes have been protected by TiO2 layers grown by atomic layer deposition and it has been discovered that the temperature of the deposit plays a key role in both the conductivity and stability of the layer, thanks to its polycrystallinity. We have also converted copper-indium-gallium-selenium (CIGS) solar cells and the alternative made with abundant materials copper-zinc-tin-sulfur/selenium (CZTS/Se) into photoelectrodes by protecting them with TiO2 layers. The bandwidth of these materials can be modified by changing the sulfur/selenium ratio, which makes them very interesting for designing tandem photoelectrochemical devices. Protective layers grown via atomic layer deposition in anodic polarizations and alkaline electrolytes have also been studied. By varying the TiO2 deposition temperature, we have observed a clear change in conductivity, correlated with the crystallization of the material, where preferential conductivity paths are observed. In stability measurements, a reduction in photocurrent is observed, attributed to the applied oxidative potentials that facilitate the introduction of hydroxyls into the layer, reducing its n-type semiconductor properties. In contrast, the protective nickel oxide (NiO) layers present a conductivity that increases due to lower deposition temperatures, which we attribute to an increase in Ni2+ vacancies, responsible for the p-type semiconductor behavior of the material. More than 1000 hours of stability have been achieved if voltammetry is periodically applied, which avoids the deactivation of the photoanodes, a fact that we attribute to chemical modifications of the surface in highly oxidizing conditions. These results have allowed the field of photoelectrochemistry to advance both in our institute and worldwide, and point to protective layers as a key piece to achieve photoelectrodes capable of producing hydrogen at medium-term prices. This thesis has resulted in 9 publications, 1 patent and 19 contributions to conferences.

The results of the thesis are part of a significant leap in productivity that the field of photoelectrochemistry has made during the last decade and that brings it closer to the market. The results have included modern and complex techniques but with industrial application today such as atomic layer deposition (ALD) and advanced second and third generation photovoltaic materials such as thin-film chalcogenides (CIGS), IBC silicon or kesterites (CZTS/Se) which are part of the group of materials that are beginning to find market gaps to be industrialized during the last decade, proposing an alternative to monocrystalline silicon. These results obtained are applicable not only to produce green hydrogen (not from breaking down hydrocarbons) through pure water, but also to study the challenges of using seawater as a source of protons, to recharge redox-flow photobatteries such as vanadium ones or to photo-reduce CO2 again into hydrocarbons such as methane or ethanol. The potential impact of these solar fuel production technologies on the paradigm of energy generation, storage and consumption would be enormous if they were to be implemented on a large scale in the fight against climate change and pollution in cities. Implementation of the patented technology on a laboratory scale. This can be demonstrated with the industrial and patent projects carried out during the execution of the thesis, with which companies such as Repsol, since 2014 and within the framework of the “LUXHOR” project, of more than 1M€, have jointly bet with IREC on the feasibility of applying the technology as described in the thesis and especially in the patent made within the framework of the doctoral thesis (“SUBSTRATE-ELECTRODE (SE) INTERFACE ILLUMINATED PHOTOELECTRODES AND PHOTOELECTROCHEMICAL CELLS”, Application Number EP15382658.1, Date of filling: 23-12-2015). A 600 cm2 demonstrator (composed of 4 adapted commercial PV cells) on the roof of the IREC building has served to validate the results and advance larger-scale projects. Demonstrator installed at IREC. Today, the results regarding hydrogen and biogas production are being validated in a relevant environment since 2019 within the framework of the SUN2HY project led by Repsol and Enagás within a consortium that also includes IREC, the University Institute of Electrochemistry of the University of Alicante and the Hydrogen Foundation of Aragon, as well as the company Magrana (expert in innovative solutions) and with funding from the CDTI (Centro para el Desarrollo Tecnológico Industrial) and the European Union through the European Regional Development Fund (ERDF). The Sun2Hy project, which will conclude in its first phase with a full-scale demonstrator in the pre-commercial phase (TRL-6) will be installed at the Repsol Technology Lab facilities in Móstoles. This first phase lasted two years (2019-2020). The intention of the two energy companies, once this phase is complete, is to take the necessary steps to bring the technology to a commercial and competitive status. At the Spanish level, it is the first green hydrogen generation project (solar hydrogen via photoelectrochemistry (PEC)), the strategy undertaken in the thesis and which should allow the hydrogen produced to be more economical than the general one using the strategy of using commercial photovoltaics and coupling it to already commercial electrocatalysts (PV+EC). It aims to validate that the technology is robust, develop full-scale modules based on the technology and build a plant of more than 10m2. The next step will be a real study of commercialization of the knowledge obtained within the framework of this project. A step that could also be closer to the thesis could be to continue advancing the challenges found in the thesis and testing other advanced materials in demonstrators, which so far has only been done with IBC silicon, and comparing the PV-EC vs PEC results thanks to the role of the protective layers in facilitating a monolithic implementation and lower electrical transport losses.

Hydrogen Production; Photoelectrochemistry of Water; Solar Energy Conversion; Hydrogen; Oxygen; Energy Storage; Photovoltaic Energy Storage; Large-Scale Implementation; Efficient Photoelectrodes; Stable Photoelectrodes; Scalable Photoelectrodes; Metal Oxide Photoelectrodes; Highly Efficient Photovoltaic Materials; Protective Layers; Transparent Layers; Conductive Layers; Catalytic Layers; Abundant Materials; Low-Cost Materials; Manufacturing Techniques; Titanium Dioxide (TiO2) Photoanodes; Alkaline Electrolytes; Anodic Applied Potentials; Photocurrent Conversion; TiO2 Bandgap; Visible Spectrum Absorption; Electron Mobility; Hole Mobility; Microstructuring Substrate; Nanostructuring Layer; Nanorods; Electronic Structure Modification; Reducing Treatment in H2; Potential Drop; Small Bandgap Semiconductors; Photovoltaic Industry; Silicon Photoelectrodes; Atomic Layer Deposition; Deposit Temperature; Conductivity; Stability; Polycrystallinity; Copper-Indium-Gallium-Selenium (CIGS) Solar Cells; Copper-Zinc-Tin-Sulfur/Selenium (CZTS/Se); Photoelectrodes; Sulfur/Selenium Ratio; Tandem Photoelectrochemical Devices; Anodic Polarizations; TiO2 Deposition Temperature; Crystallization; Preferential Conductivity Paths; Photocurrent Reduction; Oxidative Potentials; Hydroxyl Introduction; N-type Semiconductor Properties; Protective Nickel Oxide (NiO) Layers; Ni2+ Vacancies; P-type Semiconductor Behavior; Stability Measurements; Voltammetry; Photoanode Deactivation; Chemical Modifications of Surface; Highly Oxidizing Conditions; Publications; Patent; Conference Contributions.