Introduction

Energy storage is a crucial intermediary between supply and demand in the energy economy. It is essential for maximizing efficiency of electricity production, reducing curtailment of intermittent renewables, and providing power when unavailable in real-time. The 2020 Annual Energy Outlook (AEO) report from the United States Department of Energy's (DOE) Energy Information Administration (EIA) projects the nation will double to triple its electricity generation capacity from intermittent renewable sources, such as solar and wind, between 2019 and 2050.1 Wood Mackenzie and the U.S. Energy Storage Association have projected the U.S. energy storage market will grow from a $0.7 billion in 2019 to a $6.8 billion market in 2025.2 The increase in installed intermittent renewable electricity generating capacity is being driven by rapidly decreasing cost of renewably generated electricity and the initiative to decarbonize the electricity sector. Unlike the petroleum and gas energy sector, electricity sector generation and use are typically balanced without a storage buffer. As the installed capacity of intermittent sources like wind and solar becomes a greater percentage of the total, the ability to store excess capacity and supplement times when renewable supply is low becomes ever more critical. As illustrated in Fig. 1, potential solutions for storing surplus energy include superconducting magnets, supercapacitors, flywheels, pumped hydroelectric storage, compressed air energy storage, and batteries.3 Each of these energy storage technologies poses different advantages and challenges, with no single technology capable of fulfilling all application needs.4

Figure 1.
figure 1

Overview of the energy storage technologies landscape.

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In addition to these energy storage options, chemical energy storage is also of interest. Hydrogen not only serves as a vital feedstock for critical industrial processes (e.g., the Haber–Bosch process for ammonia production) but is also a versatile energy storage medium that can be produced from a wide variety of sources, including fossil fuels, nuclear power, and renewables. There is growing recognition of its potential for significant integration into the modernized electrical grid.5 While much of today's hydrogen is produced through the steam reforming of methane (SMR), renewable sources coupled to advanced water-splitting technologies to produce hydrogen play an increasingly important role as society shifts toward low-carbon options and the cost of renewable energy continues to decrease. Solar, wind, geothermal, and other renewable energy-based technologies have advanced considerably in recent decades, reaching a key milestone when renewables surpassed conventional coal-powered electricity generation in the U.S. for the first time in April 2019.6 As renewable energy generation increases to unprecedented levels, hydrogen is an attractive means of storing energy harnessed from these often intermittent sources. Hydrogen-based energy storage can enable grid stability and resiliency, while providing a pathway for distributed generation, to bring reliable energy through the creation of micro-grids in remote locations where wind or solar power are sparse or unreliable. Despite hydrogen being a zero-carbon fuel, an ideal energy storage medium, and vital industrial feedstock, the appeal of a hydrogen-based energy economy has seen a cyclical pattern of optimism, followed by waning activity and interest.7

Though hydrogen has the highest specific energy of any fuel, as illustrated in Fig. 2,8 the low energy density poses a significant challenge for safe and cost-effective hydrogen storage. A primary role of the DOE Office of Energy Efficiency and Renewable Energy's Hydrogen and Fuel Cell Technologies Office (HFTO) is to enable innovations through funding research and development, and to oversee collaborative, united efforts by convening diverse stakeholders to close the gap on R&D shortcomings. These efforts aim to advance the technology such that large-scale integration of hydrogen into the energy sector can occur.

Figure 2.
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Energy density and specific energy of various fuels. Adapted from Ref. 8.

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To address the need for advanced energy storage technologies, DOE has been increasing resources, funding, and public engagement activity in this area. In 2018, Congress passed the DOE Research and Innovation Act,9 and as part of this codification, the DOE's Research and Technology Investment Committee (RTIC) launched the Energy Storage Grand Challenge in 2020.10 This cross-cutting effort aims to accelerate energy storage technologies by targeting R&D, technology transfer, policy and valuation, manufacturing and supply chain, and workforce development. The Challenge builds on the $158 million Advanced Energy Storage Initiative, a collaboration plan introduced in the 2020 fiscal year budget, and aligns with the fundamental H2@Scale vision of HFTO. The H2@Scale concept describes the multi-faceted pathway toward hydrogen integration into the current energy system through large-scale production, delivery, and storage, across sectors and applications. This H2@Scale vision serves as a guide for the planning, deployment, and evaluation of activities and R&D focus areas of HFTO to accomplish the DOE mission of a sustainable and resilient U.S. energy economy.11

Hydrogen for energy storage applications

Power-to-gas (PTG) technology converts surplus or intermittent energy into hydrogen, typically through water electrolysis. An advantage of PTG over traditional electrical energy storage technologies such as batteries, is that the converted excess energy does not necessarily have to be put back into the grid, but can also be transitioned to other higher value uses, such as transportation fuel or industrial applications.12 Hydrogen is relevant to stationary energy storage, where it can be used for primary power in off-grid locations, as well as serving as a reliable source of backup power for applications that cannot risk even short power outages, such as data and telecommunications centers.

Hydrogen offers flexibility that cannot be realized with many energy storage options. A key advantage of hydrogen as an energy storage medium is the ability to decouple power conversion from energy storage. This feature allows for the independent sizing of the power conversion devices (e.g., electrolyzer and fuel cell or turbine) from the energy storage reservoir. Hydrogen is especially advantageous for long-term storage of large amounts of energy – one metric ton of hydrogen contains 33 MWh of chemical energy – where only the storage portion of the system needs to be increased to store more energy. Additionally, hydrogen allows for spatial separation between the point of generation and the point of use in that hydrogen can be transported with relative ease between locations, whereas most energy storage technologies cannot.

Hydrogen storage technologies

While hydrogen has high specific energy (by unit mass), its low energy density (by unit volume) is a challenge for compact, economical, and safe energy-dense storage. It can be stored in various ways that pose advantages and disadvantages when both cost and performance, which depend on application requirements, are considered. For most applications, no current storage technologies are considered universally ideal; thus, HFTO has supported a comprehensive technology development portfolio for the past two decades to advance hydrogen storage technologies. As depicted in Fig. 3, hydrogen storage methods can be generally categorized into physical storage, where elemental hydrogen is stored, or materials-based storage where hydrogen is bound within other materials.

Figure 3.
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Overview of physical and materials-based hydrogen storage technologies.

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Traditional hydrogen storage

Liquid hydrogen is currently the most mature and conventional commercial method for storing and transporting bulk quantities of hydrogen, with a density of 71 kg/m. Hydrogen liquefaction is energy-intensive, requiring about 8–12 kWh/kg, (roughly one-third the energy content of the stored hydrogen) for liquefiers in operation today, with advanced designs projected to require around 7.0 kWh/kg.13 Large quantities of liquid hydrogen are typically stored in highly insulated spherical vessels that minimize surface-to-volume ratio to reduce heat transfer and the evaporation and venting, or boil-off loss. Liquid hydrogen is valuable as a distribution medium in applications with frequent usage and short duration, rather than long-term storage due to these boil-off losses. Liquid hydrogen is used most extensively in rocketry applications, with the National Aeronautics and Space Administration (NASA) currently maintaining the largest vessels with capacities of up to 270 metric tons.14 In a new venture, HySTRA aims to supply liquid hydrogen produced in Australia that is transported via ship to Japan. The initial prototype transport ship is designed with a 1250 m tank volume,15 thus a capacity of less than 90 metric tons of hydrogen. Estimates for potential exports of Australian-produced hydrogen range from a low of 26.5 kilotonnes in 2025 to a high of 3180 kilotonnes in 2040.16 To reach these projected quantities, significantly larger transport and stationary liquid hydrogen storage vessels will be required that will likely need advanced designs and insulation technologies.

The other conventional hydrogen storage method is as a compressed gas. The density of hydrogen gas is significantly lower than liquid hydrogen, ranging from 11.5 to 50.5 kg/m for pressures from 150 to 1000 bar at 15 °C.17 Small quantities of merchant hydrogen are typically stored in Type I all metal steel high-pressure cylinders at pressures in the 150–200 bar range. For bulk transport of compressed hydrogen, Type I superjumbo tube trailers, with payload capacities of only a few hundred kilograms, typically operate at maximum pressures around 200 bar.18 For use as a vehicular fuel, hydrogen is typically stored onboard in either metal- or polymer-lined carbon fiber composite overwrapped pressure vessels (COPVs), Type III and Type IV, respectively, at pressures ranging from 350 to 700 bar. To service the refueling needs for hydrogen vehicles, a relatively recent development is COPVs for bulk transport and stationary storage. These COPV systems may use long, horizontally or vertically mounted COPVs with typical maximum operating pressures in the 250–500 bar range and can offer payload capacities up to approximately one metric ton of hydrogen.19–21 While COPVs are lighter weight compared with all metal pressure vessels, they are relatively expensive, driven primarily by the cost of high-tensile strength carbon fiber.

As hydrogen storage capacities grow, the preferred gaseous storage options quickly revert to underground and geologic technologies. Underground storage in geologic caverns, depleted oil and gas reservoirs, and aquifers has been considered for large-scale and long-term storage that can mitigate supply disruptions or changing seasonal demand.22 The current technology development status for underground hydrogen storage is still relatively immature, with salt caverns being the most advanced. In a recent study, Argonne National Laboratory conducted a literature review of bulk gaseous storage technologies and identified several feasible options, which were further narrowed to three options: two geologic (salt and lined hard rock caverns) and one “geographically agnostic” (buried underground pipes), for a more in-depth bottoms-up comparison analysis. The analysis found that the underground pipes, operating at pressures of up to 100 bar, were more economical for capacities of up to about 20 metric tons. For greater capacities, the two geological options were more economical, with salt caverns being the lowest-cost option.23 Due to the geographic dependence of these geologic features, however, availability and projected cost can vary significantly.24

Materials-based hydrogen storage

The challenges associated with gaseous and liquid hydrogen storage have elevated the profile of materials-based technologies as potential alternatives.25 In 2016, HFTO initiated the Hydrogen Materials Advanced Research Consortium (HyMARC) with the objective of addressing the scientific gaps blocking the advancement of materials-based storage development, and to accelerate progress by focusing on foundational research from an applied perspective.26 The benefit of materials-based storage is that significantly higher density of hydrogen can potentially be realized when it is either physi- or chemisorbed to other elements.24 As an illustrative example, the hydrogen density of water at ambient temperature and pressure is about 111.8 kg/m, whereas hydrogen is 40 kg/m as a 700 bar compressed gas and 71 kg/m as a liquid at −253 °C. To realize the potential of materials-based storage, however, materials must exhibit thermodynamics and kinetic rates for hydrogen uptake and release that are reasonable for the application. Additional factors, such as mass of the host material [i.e., the mass percentage (wt.%) of hydrogen in the hydrogenated material], need to be considered. Materials-based storage technologies can be generally categorized as metal hydrides (MH) (encompassing binary, intermetallic, and complex hydrides), adsorbents, and chemical carriers.

Hydrogen carriers

In late 2019, HFTO held a workshop on hydrogen carriers which brought together over 70 technical experts representing industry, universities, and national labs, and from North America, Europe, and Asia.27 Though various distinctions exist in terminology, the consensus of the participants was for keeping a broad definition of what constitutes a “hydrogen carrier” and letting the specific application requirements narrow the scope. Today, HFTO and the leading hydrogen carrier R&D community define hydrogen carriers as hydrogen-rich liquid or solid-phase materials from which hydrogen can be liberated on-demand. Ideal hydrogen carriers have relatively high hydrogen densities at low pressure and near-ambient temperature. Due to the expansive definition of hydrogen carriers, it encompasses the classes of hydrogen storage materials: chemical carriers, MH, and adsorbents. The following discussion will focus on chemical carriers, such as organic hydrocarbon liquids and inorganic materials like ammonia and ammonia borane,28 followed by MH and adsorbents.

A major advantage of liquid-phase hydrogen carriers is the ability to potentially use existing petrochemical infrastructure for production, storage, and delivery, e.g., storage tanks and tanker trucks, but additional considerations, such as toxicity and material compatibility, must be factored into their value proposition. Lifecycle cost and emission implications need to be assessed and considered in determining the feasibility of specific hydrogen carriers. Hydrogen carriers can be broadly classified as one-way and two-way carriers. In contrast to one-way carriers, two-way carriers are materials whose byproducts are typically returned for processing for reuse or disposal after release of hydrogen.

Examples of one-way carriers include methanol, with a hydrogen content of 12.6 wt.% and 99 g/l, and ammonia with a hydrogen content of 17.6 wt.% and 109 g/l at 10 bar and 15 °C. With concomitant release of a byproduct gas, e.g., CO2 and N2, separation and purification of the hydrogen is likely to be required for many use applications. In principle one-way carriers can be regenerated from their decomposition products, e.g., ammonia is indeed produced from H2 and N2 in the Haber–Bosch process, and CO2 reduction to form hydrocarbon molecules is an active area of R&D throughout the world.29 The use of materials like methanol and formic acid as hydrogen carriers will result in local release of CO2 unless carbon capture is incorporated into the decomposition system. Depending on the feedstock used for their production, however, these can be either net-zero or even net negative carbon. For example, the use of atmospheric CO2 and renewable energy for production would result in net-zero carbon, and coupling the decomposition with carbon capture would result in net negative carbon emissions.

Two-way carriers, in contrast, result in formation of a byproduct that is returned for regeneration or disposal after hydrogen is extracted. Ideal two-way carriers are efficiently and economically rehydrogenated for reuse. Historically, hydrogen storage in organic compounds was not considered viable, as it was assumed that reversible, low-temperature hydrogen release was not feasible, but with the development of catalytic processes, this area has gained traction.30 An example is toluene,31,32 which can be hydrogenated to form methylcyclohexane to store hydrogen and then subsequently dehydrogenated to liberate it. Other liquid organic hydrogen carriers (LOHC) that have been significantly investigated include N-ethylcarbazole33 and dibenzyltoluene.34 Formate salts are another example of potential two-way carriers via catalytic formate/bicarbonate interconversion, as solids or as aqueous solutions that can be used as a liquid-phase carrier that can release and uptake hydrogen at moderate pressures and temperatures.

Certain hydrogen carriers can offer additional advantages in addition to relatively dense hydrogen storage and transport at near-ambient temperature and pressure conditions. Formic acid has a relatively low hydrogen capacity at 4.4 wt.%, and 53 g/l, but the thermodynamics of the decomposition reaction are favorable, such that hydrogen pressures up to 1200 bar can be directly produced on release with relative ease.35 Direct generation of high-pressure hydrogen can be a significant benefit in reducing costs of hydrogen infrastructure since a large portion of the cost and energy is for the compression equipment and processes. Hydrogen carriers such as aqueous solutions of formic acid may provide an alternative approach for transport and delivery to fueling stations, where high pressures are needed to refuel fuel cell vehicles at 700 bar.36

In recent years, HFTO has supported techno-economic analysis at Argonne National Laboratory to establish a baseline comparison between conventional compressed and liquid hydrogen delivery with several of these hydrogen carriers. Preliminary results have shown that methanol, a one-way carrier produced directly through the reaction between methane and steam, can be cost-competitive with compressed hydrogen delivery even when the carrier is transported several thousand kilometers by rail. The preliminary analysis also indicates that ammonia and toluene/methylcyclohexane have a cost premium for reasons such as more energy-intensive production and higher costs of transport.37 Additionally efforts were initiated within HyMARC to investigate hydrogen carrier materials with an emphasis on additional potential benefits.38

Metal hydrides

MH have been widely explored as hydrogen storage materials for several decades.39–42 The hydrogen mass percentage is typically low for intermetallic hydrides (1–2 wt.%), but can exceed 10% for complex hydrides (e.g., Mg(BH4)2 is 14.8 wt.%). Low hydrogen mass percentage is perceived as a shortcoming for onboard vehicles, but may not be disadvantageous for stationary applications. Furthermore, applications such as materials handling equipment (e.g., forklifts) that require ballast for counterbalancing can actually benefit from the added weight to stabilize the unit when lifting rated loads.43

Current R&D needs for MH materials depend on the class of MH material as well as the application for their use. While numerous prototype hydrogen storage devices have been demonstrated over the years, system-level engineering for MH materials is a primary need for advancement to meet operational performance requirements at acceptable cost and safety levels. Extensive R&D has been carried out on intermetallic hydrides, especially in light of their commercial use in batteries, and as a class of materials, they possess relatively rapid sorption rates and acceptable thermodynamics for near-ambient operating conditions. The primary R&D need for this class of MH materials for energy storage applications is cost reduction while improving tolerance to gaseous impurities found in hydrogen gas such as O2, H2O, and CO, and durability under extensive sorption cycling. Complex hydrides represents a much broader class of materials that render generalizations less accurate; in general, key areas of R&D needed include improving sorption rates under more moderate temperature and pressure conditions. Additionally, the ability for most complex hydrides to undergo repeated sorption cycling needs to be significantly improved.

Adsorbents

Adsorbents are high specific surface area porous materials that include metal-organic frameworks (MOFs), carbonaceous materials (e.g., activated carbon, nanostructures, and aerogels), and porous polymers. The adsorption relies on weak van der Waals forces between molecular hydrogen and the surface(s) of the highly porous material. Due to the relatively low strength of this physisorption interaction between molecular hydrogen and carbonaceous materials, low temperatures are usually necessary to reach significant adsorbed hydrogen density. While most adsorbents to date have not exhibited significantly higher, system-level hydrogen density compared with 700 bar compressed gas, their potential advantage is that the maximum adsorbed hydrogen density is typically achieved at pressures below 100 bar. This could significantly reduce the cost and energy penalties that arise with high-pressure compression.

Current R&D for adsorbent materials primarily focuses on increasing the adsorption energy, so that significant adsorbed hydrogen density can be achieved nearer to ambient temperature, and increasing the hydrogen density by volume.44 No validated pathway has been shown for carbonaceous materials to sufficiently increase the adsorption energy to have significant hydrogen adsorption near-ambient temperature. The molecular tunability of MOFs, however, elicits several pathways for rational design of structures with specific strong binding sites for hydrogen, such as coordinatively unsaturated metal centers, linkers with polarizable groups, or organic moieties that can be metalated.45 To increase adsorbed hydrogen density, efforts have focused on optimizing pore dimensions and total pore volume of porous materials, aided by computational efforts that can screen databases of model MOF structures.46–48 Research has also been carried out on more efficient packing of adsorbent materials within the storage vessel to minimize free space. These efforts have included engineering crystal geometrics, investigating compaction methods, and developing near-crystal-density monoliths.

Future outlook

The future outlook for using hydrogen as an energy storage medium is very promising. While interest in hydrogen use in the energy sector has experienced boom and bust cycles for several decades, the current increase in interest appears to be more sustainable. Several factors and observations support this view.

First is the success of commercial products. In 2019, the global market of fuel cell shipments exceeded 1 GW of capacity.49 As of July of 2020, in the U.S. alone, there were more than 8500 hydrogen fuel cell electric vehicles on the streets, primarily in California,50 with the number of vehicles increasing at a steady rate. This is coupled with a growing infrastructure of public hydrogen refueling stations, and a goal to have at least 200 stations open in California by 2025.51 Additionally, there were over 32,000 hydrogen fuel cell-powered material handling equipment vehicles,52 e.g., forklifts, operating in warehouses and factories. Most of these units were sold without government subsidies, as they have demonstrably provided value over alternatively powered equipment.

Second, the support and interest in hydrogen is coming from multiple fronts and being driven by corporations as well as government. At the 2017 World Economic Forum in Davos, Switzerland, a group of 13 global industry leaders, including companies such as BMW Group, Daimler, Honda, Hyundai Motor, Toyota, Royal Dutch Shell, Total, Air Liquide, and The Linde Group, announced the formation of the CEO-led Hydrogen Council “to voice a united vision and long-term ambition for hydrogen to foster the energy transition.” The Hydrogen Council emerged as a coalition of energy, transportation, and industry companies that aims to accelerate investment into hydrogen and fuel cell technologies, and to encourage stakeholders to support this initiative through policy and other structures.53 The founding members represented over €1 trillion in revenue and more than 1.7 million employees around the world.54 By early 2020, the Council announced before the group's third anniversary that the membership roster was up to 81 companies. The member companies collectively represent over €18.7 trillion and around 6 million employees globally.

For well over a decade, the DOE has worked to help promote the safe use of hydrogen and to facilitate and coordinate the development of relevant codes, standards, and regulations for the use of hydrogen and fuel cell technologies in non-industrial applications. The lack of accepted codes and standards, and inconsistent codes and standards between national and regional jurisdictions, have been seen as major barriers to the deployment and use of hydrogen and fuel cell technologies. Coordination has included numerous codes and standards developers, such as the Society of Automotive Engineers, National Fire Protection Association, CSA Group, International Code Council, and the International Organization for Standardization, as well as with national and regional regulatory bodies. In 2019, the DOE collaborated with the American Institute of Chemical Engineers to launch the Center for Hydrogen Safety (CHS). As further evidence of the growing interest in the use and deployment of hydrogen and fuel cell technologies, within 1 year of launch, CHS membership grew to 43 organizations.55

The European Commission issued on 8 July 2020, their document “A hydrogen strategy for a climate neutral Europe,”56 which states that large-scale deployment of hydrogen technologies “at a fast pace” will be key for the EU to achieve its goals of reducing greenhouse gas emissions by at least 50% by 2030. As part of the strategy, it proposes installation of at least 40 GW of renewably powered electrolyzers to produce 10 million metric tons green hydrogen by 2030. Electrolyzer investments in this timeframe are expected to be in the range of €24–42 billion. Another €65 billion is expected to be needed for hydrogen transport, distribution, storage, and fueling stations.

With strong domestic energy production growth and modest energy consumption growth, the U.S. is expected to remain a net energy exporter through 2050;57 innovative technologies for energy storage to balance supply and demand will be essential for maximizing this capability to establish a cleaner and more efficient energy economy, not only domestically but also globally. R&D efforts surrounding hydrogen technologies by DOE have been complemented by flourishing global partnerships. The International Partnership for Hydrogen and Fuel Cells in the Economy (IPHE) includes 20 member nations that organize, evaluate, and coordinate multinational research, development, and deployment programs that advance the introduction of hydrogen and fuel cell technologies on a global scale. The formation of IPHE was facilitated by the U.S. DOE and the U.S. Department of Transportation (DOT) in 2003 to foster international cooperation on hydrogen and fuel cell R&D, common codes and standards, and information-sharing on infrastructure development.58

Finally, a growing public interest in hydrogen and fuel cells, which HFTO supports through educational workshops such as its H2IQ public webinars, suggest that the American public increasingly views hydrogen as a valid part of the modern economy. This combination of R&D progress and unexplored possibility, partnership among industry and government collectives, and increasing public appetite for a clean and efficient energy economy will continue to drive the advancement of hydrogen-based energy storage technology.

In order for hydrogen to realize its potential as an energy carrier and storage medium, continued development, performance improvements, and cost reduction are needed. While certain electrolyzer technologies are now commercially available, reductions in system capital costs and improvements in efficiencies are needed to bring down the cost of electrolytic hydrogen to a level competitive with hydrogen produced by SMR, However, for hydrogen to be truly viable for many of the energy storage, power generation, and industrial applications, significant advances in conventional and alternative storage technologies will be needed. Most impactful to full integration would be technology developments that enable increased hydrogen density, reduce and mitigate hydrogen losses during cryogenic processes, and that are geographically agnostic for underground storage. Cost reductions for conventional technologies such as liquid and compressed storage are needed, as is validation of the viability of geologic formations other than salt dome caverns for storage. Materials-based storage technologies have potential for both long-term storage and transport of hydrogen, but further development of the materials and system engineering are needed. Additionally, consistent and harmonized codes, standards, and regulations relevant to the use of hydrogen as a transportation fuel and energy storage medium would aid in accelerating and growth and deployment of hydrogen technologies across application sectors and geographic areas. Many of the relevant technologies are currently at a technology readiness level for demonstration, and with further development they are expected to be able to compete directly on a cost and performance level.