Green hydrogen and the global energy economy
The search for sustainable energy has been in progress for many decades but fossil fuels still account for more than 80% of current energy consumption. In a world that is urgently seeking viable sustainable energy sources or energy carriers, hydrogen is an attractive option. Hydrogen is a potentially superior energy carrier since it has a greater energy density (142kJg-1) compared with fossil fuels (10–50 kJg-1) with no carbon emissions. The vital integration of renewable hydrogen production, storage, and utilization into the global energy system is known as the Green Hydrogen Economy (GHE).
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Despite being emission-free at the point of use, hydrogen is only as green as the methods used to produce it. The primary mode of hydrogen production is currently fossil fuels; 96% of hydrogen is made using natural gas, oil, or coal. Most of this hydrogen is used in ammonia production which is key in the production of fertilizers and food. One of the challenges with green hydrogen is its low ambient temperature density making it very hard to store safely and efficiently. The rapid development of renewable energy technologies for both large-scale production and convenient storage of green hydrogen for key applications, particularly electricity generation and propulsion, is likely to be a critical part of achieving net zero carbon emissions in the coming decades.
Current and developing methods for green hydrogen production, storage, and utilization
A common form of green hydrogen production is electrolysis of water using electrolyzers. These cells use substantial amounts of energy and involve the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode (Figure 1). The hydrogen produced can be stored and then oxidized to liberate energy and water. The main water hydrolysis cell is an alkaline electrolyzer and other important types include polymer electrolyte membrane (PEM) and solid oxide electrolyzers (SOE). Optimizing these electrolyzers is the focus of much ongoing research.
Performing electrolysis at high temperatures (e.g. SOEs at 1000oC) can reduce power consumption by 40% and photocatalytic water splitting can reduce or eliminate the need to apply an electrochemical potential. Photocatalysts and electrocatalysts are currently in development to further improve electrolyzer efficiency.
Improved storage technologies are needed before green hydrogen can be safely and efficiently contained. Among the two key storage approaches (Figure 2), physical methods involve hydrogen compression into high-strength tanks that can withstand 350-750 bar pressures. Other methods use liquified hydrogen, using extreme cooling or cryo-compression which requires high energy to reach the required storage density.
Chemical/material-based methods can be divided into two categories:
- Physisorption, which depends on the surface area of various materials including activated carbon, carbon nanotubes, graphene and metal-organic frameworks. These materials require lower storage pressures but have lower binding energies which must be overcome using cryogenic temperatures.
- Chemisorption, which involves hydrogen being chemically bonded to the storage medium. Examples of storage materials include metal hydrides, Hydrogen storage alloys and liquid organic hydrogen carriers. These require dehydrogenation catalysts to liberate the hydrogen in use.
Utilization of green hydrogen often involves fuel cells that convert chemical energy into electricity and have a diverse range of transport, residential, commercial applications, among others. If continuously supplied with hydrogen, fuel cells can provide an indefinite supply of power, but in general, durability is an issue for some types of fuel cells more than others. Hydrogen fuel cells reverse water electrolysis to produce electric current; they usually have a semipermeable membrane between a porous cathode and anode (Figure 3).
The different types of hydrogen fuel cells, either in current use or development are:
- Alkaline fuel cells (AFC) are the first type of fuel cells that have been widely used (e.g., in the space program, vehicles and stationary power sources) - they use a concentrated alkaline solution such as potassium hydroxide (KOH), or anion exchange membrane and can operate below 100oC - uses various catalysts but sensitive to contaminants.
- Proton exchange membrane fuel cells (PEMFC) – use an acidic membrane (usually a perfluorosulfonic acid ionomer - Nafion - Dupont) – it is a low weight cell but is susceptible to CO poisoning.
- Phosphoric acid fuel cells (PAFC) are known to be the first generation of commercial modern fuel cells. They operate at high temperatures (170-210oC) and use a highly concentrated form of phosphoric acid (H3PO4) as its electrolyte with the porous carbon electrodes containing the platinum catalyst. They are also less sensitive to CO, but must be heated to run.
- Solid oxide fuel cells (SOFC) are the most well recognized example of the devices that use the electrochemical reaction. The reaction takes place in the electrodes with the electrolyte and the difference in concentration of oxygen between fuel i.e., hydrogen or hydrocarbon and air/oxygen allows the diffusion of oxygen ions through the electrolyte at higher temperatures, permitting enough mobility of the ions. They are mostly used in stationary power sources and have no need for precious metal catalysts however, they have a high operating temperature and possess durability problems.
Publication and research trends in green hydrogen technologies
The CAS Content Collection™ is the largest human-curated collection of published scientific knowledge, suitable for quantitative analysis of global scientific publications against variables such as time, research area, formulation, application, and chemical composition. A new white paper reports a literature analysis of academic and patent literature published from 2011-2021 on green hydrogen production, hydrogen storage, and hydrogen-based power fuel cells. This aimed to achieve an understanding of green hydrogen research trends, the general progress of each field, and classes of materials and concepts driving innovation.
The 2011-2021 literature analysis retrieved 107,293 journal articles and 79,193 patents. Most green hydrogen publications were from China, Japan, the U.S., the Republic of Korea, and Germany. The most prolific and fastest-growing publisher of green hydrogen journal articles and patents together was China (Figure 4 and 5), probably due to a national drive to achieve carbon neutrality by 2060. Japan had the second highest number of publications, with the highest number of patents (Figure 4). The US had the third highest number of total publications (Figure 4) but the rate of journal publication began to decline in 2013 (Figure 5).
A breakdown of the literature analysis shows that green hydrogen production journals and patents increased markedly throughout the decade (Figure 6). The hydrogen storage journal numbers fluctuated and generally decreased whilst patent numbers steadily increased through the decade (a brief peak in 2012-13 coincided with the world’s first production hydrogen cell vehicle). Hydrogen fuel cell journal numbers were generally static whilst patents initially declined up to 2016, but then increased until 2021. The number of journals and patents in these three areas published by the top six countries is given in Table 1.
Table 1. Journal articles and patents on green hydrogen economy by top-producing countries/regions from 2011-2021
|Country/Region||Green Hydrogen Production||Hydrogen Storage||Hydrogen Fuel Cells|
|Republic of Korea||2,475||218||420||377||3,635||7,707|
An analysis of the six top patent assignees on green hydrogen economy in each of the three research areas is given in Table 2. These are almost exclusively East Asian (Japan and South Korea) automotive and electronics companies with the inclusion of one German corporation, Bosch. This again emphasizes the predominance of Asian commercial interests in the development of green hydrogen technologies whilst activity in the US and Europe is at a lower level.
Table 2. Top patent assignees on green hydrogen economy in each research area from 2011-2021 (Multinational companies are combined under individual names)
|Assignee||Number of Patents|
|Green Hydrogen Production||Hydrogen Storage||Hydrogen Fuel Cells|
Notable publication trends of distinct substances used in green hydrogen research (Figure 7) were mainly green hydrogen production catalyst materials, particularly general inorganics and oxides alongside a growing interest in polymers. However, there has been a more recent tailing-off suggesting that this field is approaching commercial maturity.
The most prevalent elements in green hydrogen production catalysts were carbonaceous materials, transition metal oxides and sulfides. There was also interest in critical metals including cobalt, nickel and platinum, and the d8 transition metals which are typical of HER catalysts (Figure 8).
Hydrogen storage material publication trends showed a general decline; the key materials identified were mainly alloys, general inorganics, organic/inorganic small molecules, and hydrides (Figure 7). Publications also continued to feature elements and oxides representing carbonaceous sorbents, dehydrogenation catalysts and modifiers. Carbon was a prevalent element as activated carbon, graphene, in MOFs, or polymers. Other important storage elements were magnesium (in metal hydrides, and storage alloys), lithium, sodium, aluminum, and transition metals (Figure 8).
Research trends in hydrogen fuel cell materials identified a general decrease in publications since 2013 (Figure 7) but there was continued interest in oxides, organic/inorganic small molecules, polymers and alloys. The key substances in hydrogen fuel cells included alloys of cobalt and nanoalloys/nanoparticles to reduce the amount of valuable platinum used as a catalyst. Research is also striving to increase the durability of fuel cells using nanostructures with non-noble metals. Other materials of interest were carbon, nickel, oxides (e.g., Ceria [CeO2]), titania (TiO2) nickel monoxide (NiO) and yttrium sesquioxide (Y2O3) (Figure 8).
Will green hydrogen play a key role in the energy mix needed to save the planet?
The CAS literature search, producing over 107,000 relevant articles and 79,000 patents, provides some very useful insights into recent and ongoing intensive research and development into green hydrogen production, storage, and utilization in fuel cells. The trends in publications suggest that hydrogen storage and fuel cells are becoming technologically mature whereas green hydrogen production is still at an exploratory stage. Green hydrogen economy research appears to be concentrated in China and Japan and patents are being filed mainly by the Japanese and South Korean automotive and electronics sectors. This may be of concern to Western nations which have committed to zero carbon emissions in the coming decades.
To further advance the green hydrogen economy, it will be vital to create the necessary global infrastructure for the production, delivery, and safe use of hydrogen for various industrial, domestic, transport, and other applications. It will also be necessary to increase public awareness and acceptance of the technology. Numerous challenges remain in the green hydrogen space, but the literature search has revealed considerable progress and ongoing effort, which suggests that hydrogen is likely to be a game-changing part of the world’s sustainable energy mix in the coming decades.