Kinetic Assessment and Numerical Modeling of Photocatalytic Water Splitting toward Efficient Solar Hydrogen Production

Photocatalytic water splitting has been studied extensively as a promising technology for scalable and cost-efficient hydrogen production using solar energy. Although overall water splitting has been achieved under visible light irradiation, significant progress in reaction efficiencies at longer wavelengths is needed to enable practical solar energy conversion. This article reports recent advancements in kinetic investigation and numerical modeling of photocatalytic water splitting to understand problems with existing photocatalysts and develop strategies to boost the reaction efficiency. Kinetic investigation based on cocatalyst loading, light intensity, hydrogen/deuterium isotopes, and reaction temperature suggests that the majority of photoexcited carriers recombine before contributing to the water splitting reaction on the surface in most cases, and that improvement in semiconducting properties of photocatalysts by decreasing defect and donor concentrations is beneficial to boost photocatalytic performance. It is demonstrated that an appropriate material design to suppress generation of donor species actually improves photocatalytic activity. Numerical modeling of steady state carrier concentrations suggests that an asymmetric built-in potential in photocatalyst particles enhances charge separation and thus extends the lifetimes of photoexcited carriers. On the basis of the recent findings, we suggest strategies to improve the reaction efficiency of photocatalytic water splitting.

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

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

Light-active Metal-Organic Frameworks for Photocatalytic Hydrogen Evolution.

Stavroula Kampouri

The general scope of this thesis lies in the application of MOFs in photocatalysis. MOFs demonstrate inherent properties â such as high porosity, tunable optoelectronic and catalytic properties, which render them promising candidates for photocatalysis. The main research activity relative to this thesis aims at the advancement of MOF-based photocatalytic systems in terms of activity, cost and sustainability. This was achieved by employing different strategies that can lead to the favorable modification of the three major photocatalytic steps of light absorption, charge separation-migration and catalysis, which govern the overall performance of a photocatalytic system. More specifically, Chapter 2 describes the impact of different co-catalysts on the photocatalytic activity of a MOF-based system, based on the well-known MIL-125-NH2. Variation of the co-catalysts can significantly influence the two major photocatalytic steps of charge separation and the catalytic reaction. All the metal oxide and phosphide co-catalysts investigated are found to significantly improve the activity of MIL-125-NH2, with the system using Ni2P nanoparticles (NPs) exhibiting a high H2 evolution rate (1230 ÎŒmol h-1 g-1) and an apparent quantum yield of 6.6% at 450 nm, which is comparable to the state of the art. Comparison of Ni2P with Pt showed that the noble-metal-free Ni2P/MIL-125-NH2 system significantly outperforms Pt/MIL-125-NH2. These results are attributed to the enhanced electronic interactions between MIL-125-NH2 and Ni2P, and prove that earth abundant co-catalysts can challenge the commonly used noble metals. The low cost and high efficiency of Ni2P/MIL-125-NH2 prompted me to focus on another component used in photocatalytic systems, which is the electron donor. Chapter 3 shows that variation of the electron donors highly influences the activity of Ni2P/MIL-125-NH2, with triethylamine significantly boosting the H2 evolution rate. However, the utilization of electron donors is another factor hindering the industrialization of such systems, since these substances can be toxic and expensive. Inspired by this challenge, we then replaced the electron donor with rhodamine B (RhB) â a simulant organic pollutant â envisioning dual-functional photocatalysis for simultaneous H2 generation and organic pollutant degradation. This research project revealed the first example of a MOF-based dual-functional photocatalytic system able to generate H2 in a high rate and degrade RhB under visible-light. Chapter 4 describes a strategy for enhancing the photocatalytic performance of MOF-based systems by favourably altering the photocatalytic steps of light harvesting and charge separation. This was achieved by developing MOF/MOF heterojunctions. The combination of MIL-125-NH2 with MIL-167 â a Ti-based MOF with complementary light absorption properties â leads to the formation of a type II heterojunction MIL-167/MIL-125-NH2, with enhanced optoelectronic properties. MIL-167/MIL-125-NH2 significantly outperforms its single components MIL-167 and MIL-125-NH2, in terms of photocatalytic H2 production. This strategy contributes to the discovery of novel MOF-based photocatalytic systems that can harvest the solar energy and exhibit high catalytic activities in the absence of co-catalysts.

EPFL2020