In This Issue
Summer Bridge on Engineering the Energy Transition
June 26, 2023 Volume 53 Issue 2
This issue explores the energy transition needed to address the mounting threats of climate change. The articles are an excellent resource to help inform meaningful decisions and steps for energy-related contributions to reduce carbon emissions.

Critical Materials for Low-Carbon Technologies in US Markets

Wednesday, June 7, 2023

Author: Alexander H. King

Wind energy and EVs have demonstrated the value of wise materials choices and point the way forward for other clean energy technologies.

The deployment of any new technology at large scale burdens the supply of the materials from which it is made, and the availability (or lack thereof) of those materials can, in turn, impact the deployment of the technology. Here, I describe the impact of materials availability on the progress of recently emerging technologies, and identify materials that may be needed in a growing clean energy economy. Based on anticipated needs and recent case studies, I offer approaches to the problem that may be effective in reducing the threat of material unavailability as a barrier to clean energy deployment.


Technologies that grow to large scales cause changes in demand for the materials from which they are made. Thus in the 19th century the advent of railroads in the United States spurred the development of a robust steel industry, and in the 20th century the growth of aviation relied on efficient production of aluminum alloys. Today, new and evolving technologies require an ever greater variety of elements (King 2019), and even small-scale emerging technologies can stress the supplies of materials that are produced in small quantities.

Rapid growth in the demand for materials may outstrip the capacity for developing new sources, which can take two decades or more (Ali et al. 2017). Emerging clean energy technologies have already impacted the supplies of several materials and can be expected to affect more of them as the world’s energy portfolio becomes cleaner and more diverse.

Constrained supplies of materials also affect the adoption of new technologies, as illustrated in the following examination of the complex interactions between materials supplies and clean energy technology adoption. I describe the widely used definition of critical materials, explain factors that influence their criticality, and consider some of the materials that may become critical for specific energy-related technologies.

Recent Impacts of Materials Supply Challenges on the Energy Sector

Wind Energy

Since the mid-2000s, wind has been the second--fastest-growing source of energy for generating electricity in the United States, behind natural gas (EIA 2022).

Several generator technologies can be used in wind turbine systems, but they generally fall into two categories: direct-drive generators that turn at the same rate as the turbine blades and require powerful permanent magnets; or electromagnetic induction generators that require higher rotation rates, which are achieved by coupling them to the turbine blades through gearboxes.

Direct-drive systems are more efficient, quieter, and avoid the risk of gearbox failures that are the most common cause of downtime for induction generator systems (Faulstich et al. 2011). However, the magnets required for direct-drive generators are made from neodymium-iron-boron, with neodymium partly substituted by other rare earth elements (REEs) in many cases.

King Figure 1.gifAs wind energy started to emerge, China commanded a large and growing share of global REE mine production (figure 1), and it announced export restrictions in 2005. With questionable supplies of the REEs needed for the direct-drive technology, land-based wind turbines in Europe and North America almost exclusively used induction generator systems, with resulting impacts on efficiency, site selection, and reliability (King 2020). China’s dominance of rare earth mining diminished from 2010 to 2020, and although it has recently regained some share of mining and still processes much of the ore extracted elsewhere, there is cautious optimism that a robust global supply chain will eventually emerge.

When the wind energy focus shifted from onshore to offshore installations, the technology was able to move to direct-drive systems that avoid the gearbox maintenance and repair issues that are so much more challenging at greater heights and in marine environments. Concerns about REE supply were allayed by the emergence of new sources and intense R&D efforts on magnet materials and generator designs that reduced the REE quantities required (particularly the heavy rare earths dysprosium and terbium), so the new offshore turbines can use direct-drive technology.

In this case, technology choices arising from supply concerns initially compromised the decarbonization impact of a clean energy technology, but materials supply and performance challenges are continuously being overcome, allowing for ever greater effectiveness.


The mid-2010s saw an unanticipated collapse of the market for fluorescent lighting as it was rapidly overtaken by LEDs (Navigant Consulting Inc. 2012, 2014). LEDs represent a significant improvement in terms of energy efficiency, but the twin drivers for this revolution were the rising cost of producing fluorescent lamps (because of their reliance on the REEs europium and terbium) and the falling cost of LEDs (driven by improvements in the technology). The price per lumen for LEDs dropped below that of fluorescent lamps in 2013, and the “rare earth crisis” of that time, coupled with conventional free market forces, helped to accelerate this step in the clean energy revolution.

The wind and LED stories are linked by their needs for the same materials. Until 2013 fluorescent lamps were the largest global consumer of both europium and terbium; demand for these elements in lighting applications has slowed since then. Rare earth magnets used in large motors and generators are based on the compound Nd2Fe14B, and the inclusion of up to a few percent of dysprosium improves the performance of these magnets, especially at elevated temperature. Terbium has the same effect as dysprosium and has been used in magnets at increasing levels since the decline of its demand for fluorescent lighting. The reduction in demand for terbium in lighting thus helps to alleviate the shortage of dysprosium for magnets (King 2020).

Electric Vehicles

The revolution in electric vehicles started in 2008 with the introduction of Tesla’s first commercial vehicle, the Roadster. At the time, 98 percent of global REE mining was in China, which was threatening ever more stringent export restrictions on the materials needed for high-strength permanent magnets.

Choice of energy storage system depends on several factors, with material cost and availability offsetting performance issues such as power density.

Tesla began with a distinctive marketing strategy. The Roadster was produced in small numbers and it was expensive; target customers were not particularly price-sensitive and the car competed against luxury sportscars with internal combustion engines—its key distinguishing features from a marketing perspective were acceleration and handling. Tesla continued to compete in the same market niche when it introduced the Model S and Model X.

All of Tesla’s first three cars used induction technology for their tractor motors, avoiding the need for permanent magnets and concerns about rare earth supplies. Induction motors can produce greater torque and acceleration than permanent magnet (PM) motors, playing well into the market niche at which they were aimed. There are, however, a few downsides: induction motors are more complicated than PM motors, require more complex control software, are more failure-prone, and convert stored energy to mechanical work less efficiently.

Tesla’s marketing strategy shifted toward the mass market with the introduction of the Model 3 in 2017 and the Model Y in 2020. Manufacturers’ concerns about rare earth supplies had somewhat abated by this time, the target consumer was more focused on range (and hence efficiency) than acceleration, and the long-range versions of the new cars featured one PM motor with rare earth magnets (to drive the rear wheels) and one induction motor (for the front wheels). In 2023 Tesla announced that it had developed a new PM motor that uses no rare earth elements, potentially removing concerns about future REE supplies for EVs.

As other manufacturers have entered the EV market, they have made a variety of choices for their traction motors, reflecting different values placed on acceleration, range, reliability, and supply risks. Greater efficiency generally comes from PM motors, but performance, market penetration, and rapid deployment remain critical interests amid efforts to combat rising global temperatures and may be better served by attending to other concerns.

Technology choices are also important in the selection of onboard energy storage systems for EVs. Most of the attention is on lithium-ion batteries, which currently offer the greatest range, but several varieties of these contain varying quantities of cobalt, iron, and nickel in addition to lithium (Marom et al. 2011); nickel-metal-hydride remains a lower-cost, lower-performance option in some markets. Fuel cell systems based on hydrogen or natural gas add further options and may gain market share if battery materials face supply challenges. The choice of an energy storage system depends on several factors, with material cost and availability offsetting performance issues such as power density.

The Takeaway from These Cases

The cases of power generation, lighting, and electric vehicles show that questionable supplies of essential materials have mostly negative impacts on both the adoption of new technologies and the efficacy of the technologies’ first generations. Rapid deployment of clean energy systems may be of greater benefit to the environment than pursuing the greatest possible motor efficiency or energy storage capacity using more ideal materials or technologies (Lesk et al. 2022). Efficiency can be expected to improve as the systems evolve.

What Are Critical Materials?

The concept of a critical mineral stems from a National Research Council report (NRC 2008), and the definition has also been applied to critical materials, which are distinct in certain respects from critical minerals. While critical minerals may be the ores from which critical materials are derived, they tend to be defined in terms of overall demand for the downstream materials across all their uses. Critical materials, on the other hand, are defined in the context of their applications, and a material may be critical for some of its uses but not for others. If a material is critical for a niche application that consumes only a small fraction of the global output, the minerals from which it is derived may not be classified as critical.

A critical material (or mineral) meets two conditions:

  1. it is essential for a particular technology, corporation, business sector, or regional economy; and
  2. it has some degree of supply chain fragility within a timescale of relevance.

Materials may be considered essential for a variety of reasons but usually rank highly if they embody specific properties such as catalytic activity, density, electrical or thermal conductivity (or lack thereof), magnetism, neutronics, photonics, mechanical strength, or combinations of these properties, any of which may be significant in particular clean energy technologies. The assessment of essentiality is typically only semi-quantitative and is based on factors that include measurable performance indicators and substitutability, which is usually more a matter of “expert” opinion.

Supply chain fragility is also assessed semi-quantitatively and depends on factors such as the capacity of the global supply chain to adjust to meet anticipated demands and its vulnerability to natural and other threats. Materials that depend on single global sources tend to have higher fragility scores than those available from a variety of sources; for those with only a single source, the potential for them to be cut off is a significant consideration. Materials that are coproduced with other materials are considered vulnerable (Nassar et al. 2015). Finally, coproduction tends to reduce the effectiveness of the supply-demand dynamic for less-produced or lower--valued materials, making them less responsive to traditional market forces and increasing their supply chain fragility.

King Figure 2.gifSeveral studies have produced rankings of material criticality that are typically summarized in plots of the form shown in figure 2. The definitions of “essentiality” and “supply chain fragility” used in these studies, and the weighting of the different components considered, vary depending on the geographical region, industrial sector or product, and the concerns, preferences, or biases of those performing the studies (Schrijvers et al. 2020). Notwithstanding those variabilities, some consensus emerges based on specific applications of a material: Criticality studies of materials required for decarbonization or the development of clean energy technologies globally (APS 2011), in the United States (DOE 2010), and in Europe (Moss et al. 2013) agreed that rare earth elements are highly critical because of (i) their essential roles in making catalysts, light-emitting devices, and strong permanent magnets and (ii) their supply chain vulnerabilities associated with coproduction and China’s dominance of their extraction and processing.

Many of the materials required for decarbonization of the energy sector fall into categories where short- or long-term shortages can be expected.

But rare earths are not the only materials that are critical for the transition to clean energy technologies. Lists of critical materials published over the past 15 years include some that focus on clean energy or decarbonization technologies and others that focus on regional economies. A consistent theme is the rise over a relatively short period in the number of materials identified as critical. The first list published by the US government addressed the needs for clean energy technologies and identified just six critical materials (DOE 2010); the most recent (USGS 2023) lists 50 across all sectors of the US economy, of which 37 relate to clean energy technologies.

Plots of the form shown in figure 2 have become popular and have certain uses, but they are not necessarily the best way to analyze the criticality of any particular material. They do not, for example, provide rigorous risk assessments associated with reliance on a potentially critical material (Gloeser et al. 2015), and they are most commonly retrospective, addressing historical supply and demand data, or relying on relatively simple projections if they address future needs. They also tend to focus on reducing materials criticality either by increasing supplies (addressing the horizontal axis) or inventing alternative materials (addressing the vertical axis) although these are not the only options, as explained in the next section.

How Criticality Emerges

When supply shortfalls are threatened, prices rise. This may stimulate increased production, but the process is less straightforward than one would hope.

When the demand for a material grows in response to a growing industry, existing sources may be able to increase their output to meet some of the demand in the short term but the capacity to do this can be quite limited. On the other hand, nonessential uses of the material are displaced, freeing up supplies for more critical uses, as seen in the case of REEs in fluorescent lighting. Meanwhile, the process of identifying and commissioning new sources can take as long as 20 years, risking the loss of a new technology’s window of opportunity. And in some cases, the necessary mineral sources may fail to increase production if the material in question is not the primary revenue generator for its source mineral (Nassar et al. 2015), or the necessary geological resources may simply not exist.

Many of the materials required for decarbonization of the energy sector fall into categories where short- or long-term shortages can be expected. The materials that cause the greatest concern are those for which

  1. the interplay between consumption, production, and price is disrupted so increased demand does not result in increasing supply. This applies particularly to materials that are minor byproducts of others and where there is geopolitical interference in the supply chain: both conditions apply to the rare earths.
  2. there is a large time lag between increased demand and increased supply, so the time responses of supply and demand are out of phase with the shifting needs, with the result that the supplies do not emerge within the time window for adoption of a particular technology. This applies mostly to materials for which new sources must be found and/or developed to meet a growth in demand: it has repeatedly impacted supplies of cobalt over the last 50 years and will probably apply to beryllium if plasma fusion emerges as a viable energy source.

How Criticality Evolves

When a material becomes critical, efforts are made to increase supplies (from either primary sources or recycling) or reduce need by identifying substitute materials. Both of these approaches, however, usually take too long to meet the needs of emerging technologies. Manufacturers frequently find other ways to work around needs for materials for which there are doubtful supplies, and solutions have been highly nuanced, focusing on improvements rather than revolutions in supply or materials use, and acceptable compromises on performance.

Technologies evolve most rapidly when they are new. Thus, for example, traction motors and batteries are undergoing rapid development as the market for EVs grows. In some cases research and development efforts are aimed at improving performance and in some they are driven by reducing reliance on critical materials, but every advance involves changes to the palette of materials—and changes the criticality landscape. Technology evolution also impacts recycling, as older devices that reach the end of their life after a few years of service may not match the compositions of current technologies.

Critical Materials in Emerging and Growing Clean Energy Technologies

Creating lists of critical materials has become a cottage industry since 2010 and there has been a steady growth in the number of materials considered critical over that time. USGS now publishes a list of minerals critical for the United States at least every three years. More than two-thirds of the 50 minerals on its most recent list (USGS 2023) are used in current or emerging energy production, storage, or utilization technologies.

King Table 1.gif

Table 1 shows 37 energy-related materials from the USGS (2023) list and the clean energy technologies that they might impact. The table also identifies 17 additional materials, not currently listed by USGS, that could impact emerging clean energy technologies in the coming decades.

In almost all cases, Earth’s crust contains sufficient mineral resources to provide the materials required to produce enough clean electrical power to meet immediate climate goals. But it is not clear that the necessary extraction rates can be achieved, environmental impacts from mining can be controlled, or ethical workforce practices can be assured (Wang et al. 2023).

As seen in the cases of wind, lighting, and EVs, multiple factors that either elevate or alleviate criticality can vary independently over time and occasionally converge to cause tipping points that precipitate supply shortfalls. This particularly impacts emerging technologies.

Efforts directed toward improving supplies or inventing alternatives may reduce the risks for some critical materials, but these strategies cannot respond with sufficient speed to avert supply chain failures once they occur. It is important to conduct proper risk analyses (rather than criticality analyses) for all the materials needed for a new technology and prepare appropriate supply chain strategies for them.

The best choice of material for an emerging technology (along with the design choices that follow from it) may not be the one that produces the best performance if it also causes unacceptable supply chain risks. Wind energy and EVs have demonstrated the value of wise materials choices and acceptable performance compromises and they point the way forward for other clean energy technologies.


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About the Author:Alex King is professor emeritus of materials science and engineering, Iowa State University of Science & Technology.