Green hydrogen has been receiving a lot of interest, especially in the past few years. While hydrogen is already considered a “zero-emissions” fuel, one nonpolluting way to produce this gas is through electrolysis, which requires water, a large electrolyzer, and a lot of electricity. When this electricity comes from renewable energy sources like solar or wind energy, the resulting hydrogen is considered to be “green” or “renewable” hydrogen.
Green hydrogen has a variety of uses, ranging from powering fuel-cell vehicles and replacing industrial hydrogen to its use in energy storage or as a combustion fuel.
Figure based on information from Greentech Media
In recent years we have witnessed a renewable energy boom through improvements in technology and the execution of proactive policies. The reduction in the cost of renewable electricity has also contributed to green hydrogen gaining a lot of traction. Many countries, such as Australia, Canada, China, France, Germany, Japan, Norway, South Korea, the United Kingdom, and the United States, are exploring the adoption of a green hydrogen economy and potentially benefiting from having the “first mover” advantage.
Green hydrogen challenges:
As promising and practical as green hydrogen technology appears to be, there are a number of challenges that stand in the way of its effective large-scale deployment.
From a conventional standpoint, water electrolysis is expensive. It requires rare catalysts such as platinum and iridium, which are hard to source and are therefore high priced. These catalysts also have the undesirable tendency to degrade or dissolve with time. Additionally, the high pressurizing energy associated with volume reduction, safety concerns over hydrogen’s flammability, and material compatibility (hydrogen embrittlement) are burdens on any transportation and storage infrastructure. The requirement of substantial quantities of pure water that is suitable to be fed to the electrolyzer is also a growing concern.
Fortunately, the scientific community has been working on all these challenges, with very promising results.
Addressing the high cost of green hydrogen generation:
The electrolysis of water to produce hydrogen and oxygen at the cathode and anode, respectively, occurs through two main reactions: the hydrogen evolution reaction (HER), and the oxygen evolution reaction (OER). Studies have shown that an acidic environment could be advantageous for the electrolytic process as compared to an alkaline environment in the presence of efficient and stable catalysts. In the case of the HER, these catalysts are inexpensive, non-noble metals. However, the major bottleneck lies in the OER, which usually requires noble-metal catalysts that are hard to source and extremely expensive.
Researchers at the Monash School of Chemistry have identified a catalytic system that is intrinsically stable and “self-healing.” The dissolved metal could end up getting redeposited at the electrode during the electrolytic process. These catalysts (a combination of cobalt, iron, and lead oxides) are made from elements that are nonprecious and thus inexpensive.
This research has demonstrated the increased stability and efficiency of the proposed catalyst. It is therefore potentially scalable in the production of industrial green hydrogen, addressing one of the major hindrances of this technology.
The HER and OER usually require the use of different catalysts due to the marked difference in their respective catalytic mechanisms. However, another study performed by researchers at the University of New South Wales addressed these challenges by developing a robust bifunctional catalyst consisting of a heterogenous interface of nickel and iron oxide that could be used in water electrolysis conducted in an alkaline environment. Their examination used density functional theory simulations as well as a combination of physical and electrochemical characterization techniques. It demonstrated that this catalyst shows high activity and electrochemical stability for both the HER and OER. Having similar active sites for both these reactions reduces the possibility for electrode degradation due to depolarization and the presence of reversed current brought about by the intermittency of renewable power — a major factor to be considered in green hydrogen production.
Addressing the issue of water scarcity:
A research group at the University of Newcastle led by Professor Behdad Moghtaderi has come up with a technique of harnessing water from the air using solar energy through a “Hydro Harvester” and then using this water in the electrolytic production of green hydrogen. This ensures that the huge amounts of pure water required for generating green hydrogen don’t come from the existing water economy, which is also facing a threat in many countries. A pilot system of this plant has been set up at the Newcastle Institute for Energy and Resources compound within the university.
Addressing the issue of hydrogen transportation:
The research group headed by Professor Moghtaderi has also come up with a technique to convert hydrogen to green methane and use it as a carrier to facilitate easy transport. The transported green methane may be used as is or converted back to hydrogen at the point of use, depending on the final application.
Transportation by green methane is more beneficial compared to that of green hydrogen due to the safety considerations, material compatibility, and presence of existing transport infrastructure.
Addressing the issue of green hydrogen storage:
A diverse team from 10 different universities located in Austria, Russia, the United Kingdom, Turkey, and Slovakia has proposed an innovative method for storing hydrogen. It involves the electrochemical hydrogenation in iron-nickel-based metallic glass. For their analysis, the team used a combination of chronoamperometry followed by cyclic voltammetry. Metallic glasses are amorphous, which allows them to absorb gases better than the usual crystalline metal options, such as palladium, that are used. Due to the absence of crystalline defects, they tend to be corrosion resistant. These materials are also potentially cheap and abundantly available.
Figure from Journal of Power Sources
The team concluded that this process yields a hydrogen-to-metal ratio that is an order of magnitude larger than any previous study. While it appears that this could work well with small-scale storage applications such as replacing the use of lithium ion batteries, it has a definite potential for scalability.
A study performed by Zhijie Chen and colleagues published in the Science Journal addresses the possibility of safe hydrogen storage in vehicles. While hydrogen vehicles currently store hydrogen at 700 bar — a pressure that is high and unsafe — the US Department of Energy came up with a tentative safety limit metric for storing hydrogen onboard vehicles at 100 bar. Through this project, the researchers have designed a metal-organic framework with trialuminum nodes that optimize both gravimetric and volumetric storage and meet the DOE target requirements for hydrogen storage. The development of new adsorbent materials is an important strategy that is broadening the scope of green hydrogen usage.
The use of metal hydrides in the storage of hydrogen has also been studied deeply over the past decade. Notable among these is the study conducted by a research group in the International Energy Agency investigating hydrogen-based energy storage, where diverse compounds have been identified to be scaled up in the near future and tested for large-scale use in hydrogen storage applications.
The following table provides a summary of investigations cited in this article:
The way ahead:
There is a definite consensus that, through upcoming research developments and financial support from the world’s energy giants, large-scale green hydrogen production is on the cusp of a major boom. In fact, hydrogen is no longer considered a fuel of the future, but rather, a fuel of the present!
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