Hydrogen is among the most abundant elements on Earth, and its energy density by mass is significantly greater than other chemical fuels. Additionally, the only byproduct of a simple hydrogen fuel cell is pure water. Amidst concern over the environmental impacts of ongoing fossil fuel usage and associated carbon dioxide emissions, these benefits are of critical interest to the energy and transportation sectors, and thus, hydrogen fuel cells have been the focus of widespread research for decades. However, hydrogen fuel is a gas under ambient conditions, making it difficult to store and transport efficiently. The hydrogen either must be contained and moved in large volumes or kept under high pressure to reduce the volume. Both methods have drawbacks: huge containers of gas are generally unwieldy, while pressurized hydrogen carries high expense and safety hazards.
Under the recently instituted Catalyst Design for Decarbonization Center (CD4DC), researchers intend to increase the feasibility of using hydrogen as fuel by disguising it as more familiar carbon-based fuels called liquid organic hydrogen carriers (LOHCs). LOHCs can serve as chemical “shuttles” for hydrogen atoms by bonding with hydrogen through controlled chemical reactions and later releasing it into fuel cells. Because LOHCs store hydrogen as part of a chemical liquid, they are significantly easier to use as a fuel than pure hydrogen.
Despite their advantages, it may not be immediately clear why LOHCs can aid in emissions reduction. After all, they are still carbon containing fuels. To understand the environmental benefits of LOHCs, we must first consider the wider industrial implications of a wide-scale adoption. Existing petroleum infrastructure is suited to constantly receive material from underground reservoirs. When extracted, this fossil carbon is suddenly introduced to the high-flux global carbon cycle. As long as it is somewhere above ground—dissolved in the ocean, part of the ecosystem, or hidden in consumer products—this carbon always has the potential to enter the atmosphere as CO2. Underlying the vision of LOHCs is the realization of a system with the ability to transform biological or waste carbon into LOHCs so that new fossil carbon need not be harvested. Moreover, the depletion of LOHCs for their hydrogen does not render them useless, but merely returns them to their prior chemical state, ready to be enriched with hydrogen again. In this way, LOHCs are a fully recyclable fuel. By replacing fossil fuels, LOHCs can diminish the continued accumulation of atmospheric carbon and usher in a so-called cyclic carbon economy in which waste itself is a commodity.
A question remains concerning the maintained reliance on chemical fuels in any form. Battery-powered electric vehicles are commonplace these days, so it seems that there is no need for an alternative. For all their promise, batteries have the drawback of low mass energy density. Put concisely, they’re too heavy. Although they perform well in small devices and personal vehicles, batteries are problematic for freight and flight in which weight is a critical component of transportation cost. Since liquid fuels far outrank batteries in terms of weight, LOHCs are a promising solution that can be adopted with greater convenience.
LOHCs can also be a solution to green energy intermittence. Zero-emission energy options like solar and wind often depend on unpredictable environmental circumstances to generate power. To get the most out of these technologies, we can adopt systems that work alongside them to store the excess energy produced while they are active and keep up with grid demands while they are not. With the right catalysts, LOHCs can be regenerated by electrochemical methods. This means that, with little more than the input of electricity, a properly equipped refinery can generate useful fuel, essentially storing the electricity in a stable and distributable form. And because an infrastructure for the storage and distribution of liquid fuels already exists, LOHCs would not require nearly as much new construction as a vast electric vehicle fleet. In summary, chemical energy is a reliable form of long-term storage that integrates well with the existing energy infrastructure.
The implementation of LOHC refineries requires the right catalysts to function economically, and the right catalysts are currently lacking. These materials must be capable of two things: first, synthesizing the base molecules for LOHCs, and second, driving the reversible hydrogen exchange reactions required to use them in fuel cells. Although the optimal catalysts for each occupation can be expected to look different, the research taking place under the CD4DC focuses on developing metal-organic frameworks (MOFs) to serve as high-performance catalysts for both purposes.
First, catalysts optimal for hydrogen transfer reactions—the addition or removal of hydrogen to or from larger molecules—are needed. Researchers within CD4DC aim to employ metal sulfides for this purpose, which are anticipated to be favorable in this role due to their regular presence in naturally occurring enzymes involved in such reactions. Metal-organic frameworks, defined by their molecular-scale porosity and periodicity, are primed as effective platforms for these catalysts. They have a large amount of surface area accessible to chemicals, and much of that surface area can be constructed into well-defined active sites for catalysis. MOFs also tend to contain well-defined metallic clusters, and because small metal-sulfide clusters are typically favored as catalysts, MOFs are an especially attractive prospect.
Following this theme is the prospect of developing MOFs that perform well in electrochemical environments. Thermally driven hydrogenation incorporates molecular hydrogen, which is most often derived from methane, the core component of natural gas. If LOHCs are to be true to their identity as low-carbon fuels, they should be generated by a carbon-light process. Electrocatalysis is a preferred choice because, in addition to its compatibility with green electricity sources, it can source hydrogen from water instead of methane, taking natural gas out of the (chemical) equation.
MOFs by nature have a high potential for modular design, and with that comes a library of possible chemistries and structures so vast that it would be impossible for synthetic chemists to chart and test them all in the lab. Many aspects of metal sulfide chemistry are also underexplored, which can create challenges for chemists interested in applying them to new techniques. A significant group of CD4DC’s contributors is dedicated to solving these problems with computational chemistry. By simulating the behavior of metal-organic frameworks and catalysts under desirable conditions, computational chemists can suggest a direction for their colleagues to pursue or otherwise give theory-based support to existing results.
Ultimately, researchers at CD4DC will combine theoretical and experimental chemistry to develop and optimize catalytic systems for the production of LOHCs. The research and insight generated here may serve as a solid foundation for the implementation of hydrogen fuels, contributing to the eventual decarbonization of global energy systems.
