Highly active oxide photocathode for photoelectrochemical water reduction

A clean and efficient way to overcome the limited supply of fossil fuels and the greenhouse effect is the production of hydrogen fuel from sunlight and water through the semiconductor/water junction of a photoelectrochemical cell, where energy collection and water electrolysis are combined into a single semiconductor electrode. We present a highly active photocathode for solar H-2 production, consisting of electrodeposited cuprous oxide, which was protected against photocathodic decomposition in water by nanolayers of Al-doped zinc oxide and titanium oxide and activated for hydrogen evolution with electrodeposited Pt nanoparticles. The roles of the different surface protection components were investigated, and in the best case electrodes showed photocurrents of up to -7.6 mA cm(-2) at a potential of 0V versus the reversible hydrogen electrode at mild pH. The electrodes remained active after 1 h of testing, cuprous oxide was found to be stable during the water reduction reaction and the Faradaic efficiency was estimated to be close to 100%.

Coupling of electrocatalysts to photoabsorbers for solar fuels production

Carlos Gilberto Morales Guio

The direct transformation of sunlight into fuels and chemicals has attracted considerable attention for the potential impact on the storage of solar energies at large scales and the reduction of CO2 emissions. More than 80% of our current global energy consumption comes from the burning of non-renewable fossil fuels which produces enormous quantities of CO2 and contributes to global warming. We believe today that the key to reduce our dependence on exhaustible natural resources and limit the emission of greenhouse gases is the transition to CO2-free renewable energy sources. However, harvesting of energy from renewable sources (i.e. solar, wind, tidal) is intermittent and the deployment of devices for their transformation into useful forms of energy (i.e. electricity, chemicals, fuels, biomass) in a global scale is conditioned upon the improvement in efficiency of the energy storage mechanisms. A promising approach to store energy is the use of sunlight to drive the production of hydrogen fuel from water. Hydrogen produced from water and sunlight can be transformed back to electricity on demand or be used to generate mechanical energy in cars to power the transportation industry. The oxidation of hydrogen generates useable energy and at the same time exhaust clean, carbon-free water as the only product. Therefore, the production of hydrogen using a renewable energy source in an efficient and inexpensive device has the potential to discourage the continued use of fossil fuels and improve our environment. The viability of any mechanism for the storage of sunlight as fuel will depend on the reduction of energy penalties during the transformation process. Electrocatalysts are advanced materials that bring into one active site electrons and molecules in the appropriate orientation to lower activation energy barriers, accelerate rate of transformations and improve overall efficiencies for chemical reactions. The best catalysts for water splitting are precious metals such as Pt, Ir and Ru, which show superior rates of reaction at low overpotentials. However, these noble metals are too expensive and scarce for large scale applications. This thesis summarizes our recent research in the preparation, characterization, and application of Earth-abundant electrocatalysts for solar water splitting. Although in the last decade a great number of efficient and inexpensive electrocatalysts have been reported, methods for the deposition of these materials on the surface of photoabsorbers are rarely reported. This work shows various simple and scalable techniques for the deposition of hydrogen and oxygen evolving electrocatalysts on the surface of photocathodes and photoanodes, respectively. Surface-protected photocahodes were activated for solar hydrogen production in alkaline conditions with MoS2+x, Ni-Mo and Mo2C. A novel method for the deposition of an optically transparent amorphous iron nickel oxide oxygen evolution electrocatalyst is also reported. The catalyst was deposited on both thin film and high-aspect ratio nanostructured hematite photoanodes. The low catalyst loading combined with its high activity at low overpotential results in significant improvement on the onset potential for photoelectrochemical water oxidation. This transparent catalyst further enables the preparation of a stable hematite/perovskite solar cell tandem device, which performs unassisted water splitting.

EPFL2016

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

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