Energy harvested directly from sunlight offers a desirable approach toward fulfilling, with minimal environmental impact, the need for clean energy. Solar energy is a decentralized and inexhaustible natural resource, with the magnitude of the available solar power striking the earth’s surface at any one instant equal to 130 million 500 MW power plants.1 However, several important goals need to be met to fully utilize solar energy for the global energy demand. First, the means for solar energy conversion, storage, and distribution should be environmentally benign, i.e. protecting ecosystems instead of steadily weakening them. The next important goal is to provide a stable, constant energy flux. Due to the daily and seasonal variability in renewable energy sources such as sunlight, energy harvested from the sun needs to be efficiently converted into chemical fuel that can be stored, transported, and used upon demand. The biggest challenge is whether or not these goals can be met in a costeffective way on the terawatt scale.2

http://pubs.acs.org/doi/pdf/10.1021/cr1002326

Solar Water Splitting Cells

Ultrathin two-dimensional nanosheets of layered transition metal dichalcogenides (TMDs) are fundamentally and technologically intriguing. In contrast to the graphene sheet, they are chemically versatile. Mono- or few-layered TMDs - obtained either through exfoliation of bulk materials or bottom-up syntheses - are direct-gap semiconductors whose bandgap energy, as well as carrier type (n- or p-type), varies between compounds depending on their composition, structure and dimensionality. In this Review, we describe how the tunable electronic structure of TMDs makes them attractive for a variety of applications. They have been investigated as chemically active electrocatalysts for hydrogen evolution and hydrosulfurization, as well as electrically active materials in opto-electronics. Their morphologies and properties are also useful for energy storage applications such as electrodes for Li-ion batteries and supercapacitors.

http://nanotubes.rutgers.edu/PDFs/nchem_TMDs_2013.pdf

The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets

BACKGROUND With a rising global population, increasing energy demands, and impending climate change, major concerns have been raised over the security of our energy future. Developing sustainable, fossil-free pathways to produce fuels and chemicals of global importance could play a major role in reducing carbon dioxide emissions while providing the feedstocks needed to make the products we use on a daily basis. One prospective goal is to develop electrochemical conversion processes that can convert molecules in the atmosphere (e.g., water, carbon dioxide, and nitrogen) into higher-value products (e.g., hydrogen, hydrocarbons, oxygenates, and ammonia) by coupling to renewable energy. Electrocatalysts play a key role in these energy conversion technologies because they increase the rate, efficiency, and selectivity of the chemical transformations involved. Today’s electrocatalysts, however, are inadequate. The grand challenge is to develop advanced electrocatalysts with the enhanced performance needed to enable widespread penetration of clean energy technologies. ADVANCES Over the past decade, substantial progress has been made in understanding several key electrochemical transformations, particularly those that involve water, hydrogen, and oxygen. The combination of theoretical and experimental studies working in concert has proven to be a successful strategy in this respect, yielding a framework to understand catalytic trends that can ultimately provide rational guidance toward the development of improved catalysts. Catalyst design strategies that aim to increase the number of active sites and/or increase the intrinsic activity of each active site have been successfully developed. The field of hydrogen evolution, for example, has seen important breakthroughs over the years in the development of highly active non–precious metal catalysts in acid. Notable advancements have also been made in the design of oxygen reduction and evolution catalysts, although there remains substantial room for improvement. The combination of theory and experiment elucidates the remaining challenges in developing further improved catalysts, often involving scaling relations among reactive intermediates. This understanding serves as an initial platform to design strategies to circumvent technical obstacles, opening up opportunities and approaches to develop higher-performance electrocatalysts for a wide range of reactions. OUTLOOK A systematic framework of combining theory and experiment in electrocatalysis helps to uncover broader governing principles that can be used to understand a wide variety of electrochemical transformations. These principles can be applied to other emerging and promising clean energy reactions, including hydrogen peroxide production, carbon dioxide reduction, and nitrogen reduction, among others. Although current paradigms for catalyst development have been helpful to date, a number of challenges need to be successfully addressed in order to achieve major breakthroughs. One important frontier, for example, is the development of both experimental and computational methods that can rapidly elucidate reaction mechanisms on broad classes of materials and in a wide range of operating conditions (e.g., pH, solvent, electrolyte). Such efforts would build on current frameworks for understanding catalysis to provide the deeper insights needed to fine-tune catalyst properties in an optimal manner. The long-term goal is to continue improving the activity and selectivity of these catalysts in order to realize the prospects of using renewable energy to provide the fuels and chemicals that we need for a sustainable energy future.

/pdf/combining-theory-and-experiment-in-electrocatalysis-insights-3l4yblici7.pdf

Combining theory and experiment in electrocatalysis: Insights into materials design

The identification of the active sites in heterogeneous catalysis requires a combination of surface sensitive methods and reactivity studies. We determined the active site for hydrogen evolution, a reaction catalyzed by precious metals, on nanoparticulate molybdenum disulfide (MoS2) by atomically resolving the surface of this catalyst before measuring electrochemical activity in solution. By preparing MoS2 nanoparticles of different sizes, we systematically varied the distribution of surface sites on MoS2 nanoparticles on Au(111), which we quantified with scanning tunneling microscopy. Electrocatalytic activity measurements for hydrogen evolution correlate linearly with the number of edge sites on the MoS2 catalyst.

http://science.sciencemag.org/content/sci/317/5834/100.full.pdf

Identification of active edge sites for electrochemical H2 evolution from MoS2 nanocatalysts.

Advanced materials for electrocatalytic and photoelectrochemical water splitting are central to the area of renewable energy. In this work, we developed a selective solvothermal synthesis of MoS2 nanoparticles on reduced graphene oxide (RGO) sheets suspended in solution. The resulting MoS2/RGO hybrid material possessed nanoscopic few-layer MoS2 structures with an abundance of exposed edges stacked onto graphene, in strong contrast to large aggregated MoS2 particles grown freely in solution without GO. The MoS2/RGO hybrid exhibited superior electrocatalytic activity in the hydrogen evolution reaction (HER) relative to other MoS2 catalysts. A Tafel slope of ∼41 mV/decade was measured for MoS2 catalysts in the HER for the first time; this exceeds by far the activity of previous MoS2 catalysts and results from the abundance of catalytic edge sites on the MoS2 nanoparticles and the excellent electrical coupling to the underlying graphene network. The ∼41 mV/decade Tafel slope suggested the Volmer–Heyrovsky mec...

MoS2 Nanoparticles Grown on Graphene: An Advanced Catalyst for the Hydrogen Evolution Reaction

The electrochemical hydrogen evolution reaction is catalyzed most effectively by the Pt group metals. As H2 is considered as a future energy carrier, the need for these catalysts will increase and alternatives to the scarce and expensive Pt group catalysts will be needed. We analyze the ability of different metal surfaces and of the enzymes nitrogenase and hydrogenase to catalyze the hydrogen evolution reaction and find a necessary criterion for high catalytic activity. The necessary criterion is that the binding free energy of atomic hydrogen to the catalyst is close to zero. The criterion enables us to search for new catalysts, and inspired by the nitrogenase active site, we find that MoS2 nanoparticles supported on graphite are a promising catalyst. They catalyze electrochemical hydrogen evolution at a moderate overpotential of 0.1−0.2 V.

Biomimetic Hydrogen Evolution: MoS2 Nanoparticles as Catalyst for Hydrogen Evolution

Combined spectroscopy and microscopy of supported MoS2 nanoparticles

The title of this thesis is Correlating Structure and Reactivity: MoS2 Nanoparticles for Hydrogen Evolution. In the work, MoS2 nanoparticles have been imaged with UHV STM on different substrates to relate their structure with their activity for hydrogen evolution measured at ambient conditions. The MoS2 morphology on etch pitted highly ordered pyrolytic graphite was imaged showing 3-4nm wide, 1-1.5nm high particles of MoS2 on the surface. The binding of the particles to the substrate was very weak and increased annealing temperatures during preparation of the particles and the introduction of sputter defects were chosen as a strategy to improve the conditions for resolving the particles. During these experiments, the Besenbacher group achieved the first resolved imaged of hexagonal MoS2 nanoparticles on HOPG and further studies on this system was therefore stopped. The attention was then turned to studying MoS2 on Au(111). On the Au(111) surface MoS2 nanoparticles were synthesized using annealing temperatures of 400◦C and 550◦C and characterized by XPS and STM verifying that that both preparation procedure yielded primarily triangular or truncated triangular MoS2 nanoparticles. The morphology as a function of coverage was investigated and for the single layered morphologies, the activity for hydrogen evolution was measured in an electrochemical cell at ambient conditions. The activity was shown to scale with the edge length of the particles, identifying the active site for the reaction to the particle edge. Finally, the activity of the active site was shown to be in the high range, above that of the common metals.

/pdf/correlating-structure-and-reactivity-mos2-nanoparticles-for-52wxcizy90.pdf

Correlating Structure and Reactivity: MoS2 Nanoparticles for Hydrogen Evolution

It has recently been proposed that the activity of HER electrocatalysts is governed by a simple descriptor: the free energy of hydrogen adsorption (∆GH). The activity follows the Sabatier principle with high catalytic activity at ∆GH≈0 [1]. The predictor can be calculated by Density Functional Theory (DFT), which opens new possibilities in terms of finding alternative HER catalysts [2]. Another system where the free energy of hydrogen adsorption ∆GH is close to zero is nanoparticulate MoS2. The active sites of MoS2 are solid state analogs to the active sites on hydrogen evolving enzymes like hydrogenase. Preliminary results showed that hydrogen in fact was evolved on graphite supported nanoparticulate MoS2 at a low overpotential [3].

/pdf/identification-and-quantification-of-the-active-sites-for-4q6endyuez.pdf

Kristina Pilt Jørgensen

Papers

Identification of active edge sites for electrochemical H2 evolution from MoS2 nanocatalysts.

Biomimetic Hydrogen Evolution: MoS2 Nanoparticles as Catalyst for Hydrogen Evolution

Combined spectroscopy and microscopy of supported MoS2 nanoparticles

Correlating Structure and Reactivity: MoS2 Nanoparticles for Hydrogen Evolution

Identification and Quantification of the Active Sites for Hydrogen Evolution on MoS2 Nanoparticles.