Investigations into Nanostructured Materials for Water Splitting and Direct Solar Energy Harvesting

Filip Mateusz Podjaski
EPFL, 2019
EPFL thesis

Abstract

This thesis deals with different aspects of renewable energy transformation and storage concepts and connects them. Although renewable energy installations are growing worldwide, their use is limited by storage technologies. As such, water splitting for hydrogen production and battery technologies are research areas with high potential impact. Especially earth abundant material solutions and integrated technologies that combine solar energy absorption and transformation operando such as photoelectrochemical cells or solar batteries are highly promising concepts. The technical feasibility of large scale electrochemical renewable fuel generation is most limited by overpotential losses at catalysts, their often low abundance and limited stability. For this purpose, we studied delafossite oxides, making use of an intrinsically expanded noble metal sublattice. We observed an operando increasing activity for the materials with the lowest stability in acidic media, outperforming even bulk Pt in the interesting low overpotential region. Investigations into the growth of a corrosion induced cap layers revealed substrate induced strain effects, modifying the ability of Pd to catalyze the hydrogen evolution reaction. The kinetically facilitated and stabilized phase transition to Î²-PdHx appears to be induced by these strain effects, opening up an alternative materials perspective for HER and an interesting pathway to modify material properties and phase transition barriers. Second, we studied the photocatalytic activity origins of covalent organic frameworks (COFs). This family of crystalline porous polymers enables a bottom-up design of periodic organic structures for engineering opto-electronic properties on the molecular level. An application is photocatalysis. Here, the activity description often lacks knowledge about intrinsic optoelectronic properties, which consequently rely on calculations. This work is one of the first in the field addressing these issues and actually measuring photocurrents, which are related to photocatalytic activities. Besides, these currents were used to adapt band position determination theories, which were used for classical semiconductors, to COFs for the first time. Third, we investigated and explained the direct solar energy storage properties of new, 2D carbon nitrides (CNx), namely cyanamide (NCN-)-functionalized poly(heptazine imide) polymers (NCN-PHI). CNx are organic based semiconductors showing promise in photocatalytic applications. After having shown that photocatalytic hydrogen evolution could be triggered with significant time delays, we revealed the storage process of this material and have shown that it is suitable for new types of direct solar batteries, where the absorber and the storage medium are combined in the very same material. Besides this conceptual novelty, we reveal many interesting material features, such as light induced conductivity increase, the compatibility with different earth abundant ions and the interesting use as a (solar) battery electrode in aqueous media, where the material allows for larger potential windows of water based batteries, affecting energy densities positively.

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Filip Mateusz Podjaski
EPFL, 2019
EPFL thesis

Abstract

Official source

https://infoscience.epfl.ch/record/264974?ln=en

About this result

Related concepts (26)

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Related publications (89)

Related publications

Nanostructured Electrocatalysts for Water Splitting Reactions

Lucas-Alexandre Stern

At the dawn of the 21st century, mankind is facing an important energy challenge. Future energy demand can only be met by large scale exploitation of renewable energy resources. Solar energy is abundant, with a sufficient capacity for the growing energy demand. However, it is necessary to implement renewable energy storage means. Hydrogen is considered as the energy carrier of the future. An energy economy based on hydrogen is viable only if hydrogen is produced from sustainable means. Water electrolysis is the most promising technique to produce hydrogen directly from water. Yet, the efficiency is limited and electrocatalysts are required to drive both the hydrogen evolution reaction and oxygen evolution reaction. Scarce and expensive materials are currently used to drive these reactions. It is, thus, imperative that efficient Earth-abundant catalysts are developed. Nanostructuring enhances the performance of inexpensive materials for water splitting and several catalysts were studied for this goal. Chapter 2 describes the application of nanostructuring technique to the archetypical catalysts that are nickel oxides for oxygen evolution. The prepared nanoparticles show superior activity towards oxygen evolution than that of their bulk counterpart. The ultrafine size of nanoparticles synthesized allowed the exposure of a higher number of active sites. The activity for oxygen evolution was, thus, significantly enhanced. We also detailed that short conditioning of the electrode improved the performance of the evaluated materials. In chapter 3 we carefully studied nanoparticles and nanowires of nickel phosphide. The catalysts is known to be really active for hydrogen evolution. Our group suspected that this material was, also, a potential oxygen evolution catalyst. We proved, for the first time, that nickel phosphide is a remarkable oxygen evolution catalyst. Under alkaline conditions, used for oxygen evolution, we observed an in-situ formation of a core-shell heterostructure, a typical nanostructure architecture. The surface oxidation of this material allowed high oxygen evolution capabilities. We concluded that careful oxidation of nanostructured materials, as observed for nickel phosphide, is essential for the preparation of future outstanding oxygen evolution catalysts. Chapter 4 details the fabrication of direct solar-to-fuel electrode using cobalt phosphide as hydrogen evolving catalyst. Instead of using the electrical energy provided by solar energy, solar irradiation on the developed assembly allows direct hydrogen production. The careful design of the light-sensitive electrode (photocathode) is detailed. Simple incorporation of cobalt phosphide by means of photodeposition alleviates the cost of the electrode fabrication. The assembled electrode is active for hydrogen evolution under visible light irradiation. In the chapter 5 we sought to fabricate a 3-dimensional hydrogen evolving catalyst. This nanostructure is based on polymer brushes on a flat conducting substrate. Once the brushes are grown on the substrate, a molybdenum sulfide catalyst is then incorporated by simple soaking technique. We were able to tune the loading of the catalyst and the corresponding activity by changing the polymer properties. The height of the polymer was modified as well as its packing density. The resulting activity proved superior to similar approaches and to previous reports on carefully engineered molybdenum sulfide catalysts.

EPFL2017

Cu-based p-type Oxide Photocathodes for Solar Hydrogen Production

Adriana Paracchino

The actualization of a hydrogen economy requires cost-effective and environmentally benign solutions to hydrogen production. Chemical energy in the form of hydrogen is more interesting than electricity to satisfy our ever-increasing energy demand because it can be stored more easily. Photoelectrochemical (PEC) water splitting is a carbon-neutral process which converts solar energy into chemical energy (hydrogen) using a light-absorbing semiconducting material to generate the necessary photovoltage to split water into hydrogen and oxygen. So far, record efficiencies in solar-to-hydrogen conversion have been achieved using p-type semiconductors, but only with very expensive materials. However, the spread of this technology relies on the development of semiconductor electrodes made of cheap and abundant elements that are stable in water for a reasonable long time. The semiconductors investigated in this thesis are p-type copper based oxides. Hydrogen production is measured by the photocurrent of water reduction at the semiconductor surface. In the first chapter a roundup of the basics of photoelectrochemical hydrogen generation is presented. In Chapter 2, ternary oxides presenting the delafossite crystal structure are explored, a class of materials that is expected to be stable in water under reducing conditions and that has never been studied for PEC photocathodes. The main achievements of this thesis are presented in Chapters 3-6. It is shown that CU2O, a semiconductor notoriously unstable in aqueous solutions yet a very efficient light absorber, can be stabilized against photocathodic degradation and thus used for water reduction. A preliminary optimization study is carried out to understand the synthetic parameters that control the photoactivity of cuprous oxide obtained by electrodeposition. The deposition current density, temperature and bath pH are changed to find the conditions where photoactivity is optimized. They most photoactive films are then characterized in terms of their optical and electrical properties, which are relevant to the photon-to-electron conversion efficiency (Chapter 3). The surface protection technique consists in the growth of ultrathin oxide layers (about 10-20 nm) by atomic layer deposition (ALD) on the CU2O electrodes (Chapter 4). This powerful technique enables a uniform and continuous coverage of a substrate and a control of the thickness at the atomic scale. Therefore, the passivation technique developed in this thesis can be applied to CU2O morphologies much more complex than those employed in this study and most probably to other semiconductors as well. Photocurrents up to 7.6 mA cm-2 are achieved using CU2O electrodes protected with amorphous Al-doped ZnO and TiCh. and decorated with a Pt catalyst. This photocurrent equals half of the maximum photocurrent possible for C112O and is the highest photocurrent ever measured for an oxide-based photoelectrode for PEC water splitting under AM 1.5 illumination. Further improvements to extend the performance over longer timescales and improve the overall photocathode stability are presented in Chapter 5. The concept of surface passivation against reductive decomposition of a semiconductor is proved for water splitting and important progresses are made in optimizing and understanding the key parameters for preserving the photoactivity of the material under operation. Particularly, the studies presented in Chapter 5 focus on the protective layers, exploring the relationship between the electrical properties of the ALD layers and their chemical stability in different electrolytes and after different temperature treatments. Although some decay in the hydrogen evolution over time is still observed, the last results on the protected electrodes show conversion efficiencies of 60% of the initial value after 10 hours of testing at the standard potential of hydrogen evolution, while for the unprotected electrodes the efficiency drops to 0% after a few minutes.

EPFL2012

A sustainable route to store the energy provided by the Sun, is to directly convert sunlight into molecular hydrogen using a semiconductor performing water photolysis. Hematite (α-Fe2O3) is promising for this application due to its ample abundance, chemical stability and significant light absorption (with a band gap of 2.0 - 2.2 eV). Despite these advantageous properties, several drawbacks restrain the utilization of iron oxide for photoelectrochemical water splitting. The first limitation, namely the conduction band edge lower than the water reduction potential, can be straightforwardly overcome by adding a second solar system in tandem, which can absorb a complementary part of the solar spectrum and bring the electron at an energetic level higher than the hydrogen evolution potential. The second drawback arises from the disaccord between the short charge carrier diffusion length and the large light penetration depth. It is therefore necessary to control the hematite morphology on a length scale similar to the hole transport length. To further enhance the photoelectrochemical performance, a new concept for water splitting is introduced in this thesis. The host-guest approach consists in decoupling the different tasks of the photoanode (on one side light harvesting and water oxidation center, on the other side electron conduction to the substrate) by depositing a thin layer of hematite onto a mesoporous host (WO3 in this study). This concept has been demonstrated to increase the photocurrent by ca. 20% due to enhanced quantum efficiencies at long wavelengths. This demonstration has been nonetheless limited by the iron oxide thin films overall efficiency. Thin films photoactivity is then investigated by two means: first by controlling their nucleation on a modified substrate and secondly by incorporating plasmonic nanoparticles aimed to localize absorption in the thin film. The formation of a SiOx buffer layer on the substrate prior to deposition of hematite by Fe(acac)3 spray is shown to modify the film formation mode and its physical properties. These films exhibit photoactivity from an optical thickness of 12.5 nm (as compared to 25 nm without underlayer). The study of hematite photoanodes with gold nanoparticles, embedded or deposited on its surface, establish that charge transfer from metal nanoparticles is occurring only at overlapping wavelengths between the plasmonic resonance and the semiconductor absorption. Nevertheless, photoelectrochemical performances are reduced because of high recombination rate at the metal/semiconductor interface. Finally the third limitation, i.e. the large overpotential required to observe the onset of water splitting photocurrent, is tackled in the last part. The onset potential of photocurrent is decreased by a very thin coating of Al2O3 (0.1 - 2 nm), deposited by ALD, on the nanostructured photoanode. The subsequent application of the Co2+ catalyst further reduces the overpotential and results in a record photocurrent at 0.9 VRHE of over 0.4 mA cm-2. This investigation clearly distinguishes two causes for this energy loss: surface traps and slow oxidation kinetics. The charge accumulation and the Fermi level pinning, observed at low bias potential and assigned to these surface states were further rationalized in an investigation on photocurrent and photovoltage transients.

EPFL2011