Nanostructured Materials for Next-Generation Energy Storage and Conversion

With the foreseeable depletion of fossil fuels and their significant contribution to greenhouse gas emissions, the development of an alternative energy source has become an urgent research field. Among renewable energy resources, solar energy is the largest exploitable resource by far. In view of the intermittency of sunlight, if solar energy is to be a major energy source, it must be converted and stored. An especially attractive approach is to store solar-converted energy in the form of chemical bonds, i.e., by solar-driven water splitting. This chapter will give a brief introduction to the fundamental principles of semiconductor-based photoelectrochemical water splitting into hydrogen and oxygen. The semiconductor photocatalysts for photoelectrochemical water splitting are introduced in details. Strategies to optimize solar to hydrogen conversion efficiencies by optimization of light harvesting semiconductors, surface catalysis, and devices design will also be described.

Hydrogen activation is a very important industrial process for hydrogenation reactions and ammonia production. The hydrogen splitting and hydride transfer process can be classified as homolytic and heterolytic cleavage of molecular hydrogen on mono- and multinuclear transition metal centers. Hydrogenase enzymes have inspired researchers in the field of organometallic chemistry to develop small molecule structural models of active sites and thus to mimic the biological system to activate molecular hydrogen. Multinuclear cluster complexes, including those containing heavy main group metals, can bind hydrogen molecule under mild conditions in a reversible fashion. This chapter aims at providing introductory review to cover various types of transition metal complexes that can split molecular hydrogen. The interaction between hydrogen molecule and metal centers, which determines the distance between two hydrogen atoms, will affect hydrogen splitting. The mechanism of such interactions will be discussed in details. Hydrogenation reactions catalyzed by transition metal complexes or heterogeneous nanocatalysts derived from metal cluster complexes will also be introduced.

The increased requirement of a clean and efficient green energy source promotes the development of hydrogen-based economy. In order to lower the cost of manufacturing, the future development of the next generation of hydrogen separation membrane is necessary. This chapter summarized the hydrogen separation membrane technology, the membrane separation mechanism, the polymer material selection and membrane structure design, current industrial market and separation modules, as well as the future development of the next generation hydrogen separation membrane.

Hydrogen is considered as one of the promising alternative fuels to replace oil, but its storage remains to be a significant challenge. The main hydrogen storage technologies can be broadly classified as physical, chemical, and hybrid methods. The physical methods rely on compression and liquefaction of hydrogen, and currently compressed hydrogen storage is the most mature technology that is commercially available. The chemical methods utilize materials to store hydrogen, and hydrogen can be extracted by reversible (on-board regenerable) or irreversible (off-board regenerable) chemical reactions depending on the type of material. The hybrid methods take advantage of both physical and chemical storage methods. The most prominent hybrid method is the cryo-adsorption hydrogen storage which utilizes physisorption-based porous materials. In this chapter, all of the main hydrogen storage technologies are discussed in detail along with their limitations and advantages.

In the past two decades, metal-organic frameworks (MOFs), constructed with coordination bonds between organic linkers and inorganic metal clusters, have become a burgeoning field of research and a great potential candidate for hydrogen storage due to their exceptional high porosity, high crystallinity, uniform yet tunable pore size and pore shape, great structural diversity, and various kinds of hydrogen occupation sites. Here, some technical elements are introduced in tailoring MOFs as hydrogen storage resins, including syntax, synthesis, fabrication, evaluation, and benchmark testing. As way of example, MOFs constructed by carboxylate, azolate or mixed linkers, are discussed in the context of hydrogen storage. Last but not least, the postsynthetic modifications on MOF materials to increase the hydrogen storage capacities will be carefully illustrated.

Porous carbon-based materials are promising candidates as adsorbents to increase the gravimetric and volumetric uptake of hydrogen at cryogenic temperatures and moderate pressures. In most cases, this uptake increases linearly with surface area, but strategies to increase uptake beyond that predicted by this “chahine rule,” to increase surface area, and to otherwise improve these materials are discussed.

Gas storage by using porous materials has been a hot research topic in recent years. In this review, we highlight advances in porous organic polymers for their hydrogen storage applications.

This chapter discusses about metal hydride technologies for on-board reversible hydrogen storage applications. The metal hydrides such as intermetallic alloys and solid solutions have interstitial vacancies where atomic hydrogen is absorbed via an exothermic reaction; however, by endothermic path, the metal hydride desorbs the hydrogen reversibly at ambient to moderate temperatures. In any case, the hydrogen storage capacity of interstitial metal hydrides is rather low (<2 wt%) due to limitation in the crystal structure and unit cell volume. In order to increase the hydrogen storage densities, transition metal assisted Mg-based hydrides and other nontransition metal complex hydrides have been reviewed as part of exploratory studies which have been aligned with the US Department of Energy 2020 technical targets. A number of useful characterization techniques (X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, thermo gravimetric analysis, differential scanning calorimetry, Fourier transform infrared spectroscopy) and hydrogen storage property measurements (kinetics, pressure-composition isotherms, thermal programmed desorption, gas chromatography-mass spectrometry) have been employed for the investigation of some candidate materials.

The most significant parameters such as pore size and binding energy need to be quantified to design advanced absorbents and improve their hydrogen uptake. The in situ and ex situ examination of hydrogen storage materials is helpful to provide the information with hydrogen favorable sites. The most common techniques to probe hydrogen molecules are:

In this chapter, several examples demonstrated how the hydrogen molecules store in the host materials. Knowing which structural characteristics contribute to hydrogen uptake became the design guidance because the effective moieties can be added by elegant synthetic methods. Scientists can introduce strong functional groups, modify the weak interacting parts, or tailor the structural geometry in the candidate materials based on the analysis results of material–H₂ interactions. Ligand elongation, interpenetration, impregnation, mixed-ligand, as well as introduction of open metal sites and charged frameworks were proposed to enhance H₂ uptake in the storage materials.

At present, the economy is dominated by carbon (coal, gasoline, petroleum) for generation of electricity and transport. These sections account for almost 56% of greenhouse emissions and contribute towards global warming. This realization that carbon dioxide leads to general increase in global temperatures was released from 1966 to the present day. Current awareness among the members of the general public policy makers and industrial captains has started a dialogue on transitioning from predominately carbon economy to hydrogen or electron (batteries) economy. This can be realized initially in the transport sector (26% of CO₂ emissions). The key problem is generation of sustainable hydrogen with zero or lower CO₂ emissions than current practice, which is geared towards industrial processes. A log fold increase in hydrogen production would be required from a diverse pool both fossil and renewable sources. Examples discussed include hydrogen production from biomass (glucose economy) through steam reformation (shown in Eq. 10.1), partial oxidation of hydrocarbon (Eq. 10.2), pyrolysis (Eq. 10.3), microbial (Eq. 10.4), and electrolysis (Eqs. 10.5a, 10.5b, and 10.5c). The above processes could generate sufficient hydrogen to meet the needs of the transport sector, with hydrogen used as a fuel. Examples of hydrogen usage in this manner include: fuel cell powered automobiles either as hybrid (fuel cell – lithium ion batteries), plug-ins (Li-ion battery or hydrogen fuel cell vehicle with on-board storage. The function of the fuel cells such as proton exchange membrane fuel cells is to convert chemical energy to electrical energy spontaneously during electrochemical reactions (Eqs. 10.6a and 10.6b): While no single technology appears to be superior in all aspects, steam reformation and biomass gasification offer practical approaches to generate sufficient hydrogen to meet transportation needs in the near term. The steam reformation of natural gas coupled with CO₂ capture can offer a sustainable method, while gasification of biomass using supercritical chromatograph with catalyst offers another approach to generate sustainable hydrogen. In the mid-term, the electrolysis of water using off-peak grid electricity offers a pathway to generate hydrogen with negligible greenhouse emission. In the long term, the biogeneration of hydrogen and splitting of water using photo electrolysis or thermochemical pyrolysis are feasible avenues. The development of these technologies also needs policy makers to create a favorable legislature environment such as tax incentives for end-user or product generator and wider disseminations on the need to transition away from carbon, particularly for nations that do not have a native supply of coal, or petroleum.