Introduction
To make an alkane from an alkene, a carboxylic acid from a nitrile, or a ketone from an alcohol, a chemist will most likely consider hydrogen a key ingredient. What a chemist might not consider, for these and many other cornerstone reactions, is from where that hydrogen is sourced.
Any concern raised about our hydrogen supply might seem like a moot point. Hydrogen is, after all, the most abundant element in the universe. But while hydrogen is vital in processes that support everything from global food production to fibers in clothing, the supply and production methods aren’t often topics of conversation. As a key building block for ammonia, hydrogen is an important ingredient in fertilizers that support global food production. Meanwhile, ammonia is the second largest chemical product produced worldwide. In addition, many biological products like vegetable oils are often hydrogenated to improve their oxidative stability. Polymers, another huge segment of the chemical industry, often rely on methanol produced using hydrogen as a reagent. To provide some scale for the magnitude at which chemical industry relies on hydrogen, the plastics market alone (a subcategory of the broader polymers market) is projected to be worth $654.38 billion by 2020.
Despite much talk about a pollution-free “hydrogen economy” for hydrogen as an energy carrier, the majority of hydrogen – up to 95% - is produced for industrial use in chemical upgrading. The process of steam –methane reforming uses methane derived from natural gas. The energy-intensive reaction operates under far-from mild conditions, requiring 1000-degree Celsius steam under up to 362 psi pressure in the presence of a catalyst. The products, in addition to hydrogen, include carbon monoxide and carbon dioxide. Although natural gas is not absolutely essential for hydrogen production it’s an inexpensive and well-established process for doing so.
Research and Innovation
Most often, hydrogen production is talked about in terms of its viability as a fuel. Although that’s not the focus of this article, the use of hydrogen in transportation has been the impetus for many recently developed technologies that could enable sustainable hydrogen production for large-scale use. While the amount of GHG emissions generated from producing hydrogen via natural gas are less than half compared to conventional gasoline vehicles, it is still a limited technology since employing this process often requires a continued dependence on non-renewable petroleum feedstocks. In short, as Dr. James Jackson, a professor of chemistry at Michigan State University, made clear in a presentation for the Energy Biosciences Institute in 2011, “Hydrogen has to be understood as a petrochemical. If you buy commercial hydrogen today, it’s made by reforming of methane, and it’s essentially always downstream from a fossil source.” Or, in other words, there is work to do whether the context is a hydrogen fuel cell car or a chemical production facility.
Although hydrogen production for energy and for chemical manufacturing are quite different, it’s easy to imagine current work on hydrogen fuel cells or energy storage becoming the basis for technology at an industrial scale. While there is a great deal of research toward producing renewable hydgrogen, there are challenges for all of the current methods.
For example, hydrogen can be produced via electrolysis, but the energy input is higher than that which hydrogen returns. At the National Renewable Energy Laboratory (NREL), a wide array of more sustainable water splitting methods are being investigated. Photoelectrochemical (PEC) water splitting is just one method of harvesting sunlight to produce hydrogen from water, reaching conversion efficiencies around 12%. These systems employ semiconducting materials which, unfortunately, often require materials with undesirable supply chain risks like gallium, indium or platinum.
Alternatives to using critical materials could include metal-organic frameworks made using more abundant materials, eventually achieving similar efficiency. Water can also be split by microbes that produce oxygen and hydrogen from sunlight. Hydrogen photoproduction is, however, currently limited by an organism’s ability to withstand the unusually high oxygen levels that are generated.
Similarly, microbes carefully selected for their genetics – for example, microbes that show both potential for direct conversion of hemicellulose to hydrogen and don’t produce unwanted byproducts – are being employed for their potential in hydrogen production via fermentation with low feedstock costs.
Biochemical technology has the potential to take the process one step further. For some chemicals it’s possible to skip the hydrogen production step in the first place, directly synthesizing the desired product. For example, researchers at the Tokyo Institute of Technology are working to engineer nitrogen-fixing cyanobacteria for the production of ammonia.
Of course, not all current research in the field is bio-based. High-flux solar furnaces can sustainably reach the high temperatures needed to drive thermochemical hydrogen-generating reactions. At NREL, biomass gasification is driven by concentrated solar energy. This process can produce bio-oil as well as up to 83% of the hydrogen potentially available in the biomass. Again, a drawback of this process is that, currently, critical materials are used to achieve the catalytic reformation.
But sustainable synthetic molecular catalysts are simultaneously being explored for sustainable hydrogen production. Again utilizing solar energy, carbon quantum dots have been employed in mild conditions for photocatalytic hydrogen generation. Catalysis could provide a twist on the conventional methane steam-reforming – a candidate for more sustainable hydrogen production using bio-derived feedstocks. Although researchers at National Taiwan University of Science and Technology achieved a relatively low efficiency hydrogen conversion from a copper-nickel catalyst and bio-based ethanol, it could be what’s needed to break the trail for more efficient - yet sustainable – catalysts for hydrogen production.
Looking Ahead
It remains to be seen whether or not recently developed hydrogen production methods, like the few discussed here, hold potential for use on an industrial level. The question may become: what technology is the most practical for large-scale hydrogen production? As fuel cells gain acceptance in cars and hydrogen is increasingly generated using renewable energy, perhaps hydrogen infrastructure and understanding will grow to pique industrial interest.
Achieving sustainable hydrogen production is likely to be a very slow transitional process, but already the U.S. Department of Energy Fuel Cell Technologies Office anticipates a hydrogen economy where production is diversified, utilizing natural gas augmented with “renewable, nuclear, coal (with carbon capture and storage), and other low-carbon, domestic energy resources.” With ever-increasing capabilities for sustainable hydrogen production, chemical manufacturing that relies little on non-renewable feedstocks may indeed become a thing of the past.
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