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Anthropogenic greenhouse gas emission rates increased by more than 80% from 1970 to 20101, and emissions from the transport sector increased at a faster rate than any other energy end-use sector2. In 2010, transportation was responsible for 23% of total energy-related CO2 emissions2, with total energy consumption reaching 27% of the total end-use energy, of which about half was consumed by light-duty vehicles2. There is currently an estimated one billion light-duty vehicles worldwide, and as a result of increasing standards of living and economic activity, this number is expected to double by 20353, with obvious repercussions for energy security, climate change and urban air quality.

Vehicles with electric powertrains are seen as attractive alternatives to conventional internal combustion engine vehicles2, and many governments have introduced policies promoting market uptake of EVs4,5. With the increasing market for EVs, most major automobile manufacturers now have one or more EVs in their production line. The significant drop in the cost of LIBs over the past decade will further accelerate the adoption of EVs6.

When combined with clean energy sources, EVs can offer a range of advantages over conventional vehicles, such as reduced greenhouse gas emissions and local air pollution7,8 and improved energy efficiency9. However, a shift in drivetrain technology to LIBs and PEMFCs leads to changes in supply chains, introducing more environmentally intensive materials and production processes in exchange for potentially lower operating emissions10. To understand the environmental implications arising from transport electrification therefore requires a systems perspective, such as that provided by life-cycle assessment (LCA). LCA offers a way to quantify environmental impacts associated with the production, use and waste handling of goods and services11 (Box 1).

Due to the unique electrical and mechanical properties only attainable at the nanoscale, nanostructured materials developed for LIBs and PEMFCs may significantly improve their performance. Nanomaterials can notably offer advantages over bulk-structured materials through reduced diffusion lengths of ions and electrons, and in some cases, through changes in the phase diagram resulting in changes in the reaction mechanism. However, the synthesis of nanomaterials may be more energy demanding12 than that of their bulk counterparts, which in turn may have significant bearings on the life-cycle environmental impact of EVs13, particularly with respect to greenhouse gas emissions. For EVs to offer environmental benefits over conventional vehicles, any technical improvements introduced by nanomaterials must result in environmental advantages outweighing the potentially increased production impacts.

In this Analysis, we investigate how nanomaterials can contribute to more environmentally sustainable electromobility and compare different candidates for development in this direction. For the purpose of this study, the term EVs includes vehicles with a fully electric drivetrain using LIBs or PEMFCs. First, we briefly review the LCA literature of EVs to identify potential trade-offs and sources of environmental impacts of the current state of EV technology. This serves to identify areas in which the development of novel materials may bring about the greatest improvements from a systems perspective. These identified challenges are grouped into three life-cycle attributes that we use to evaluate and compare different nanotechnological developments for environmentally sustainable batteries and fuel cells for electric transport. Finally, we distil the overarching evaluations and provide insights into the contribution of nanotechnologies for more environmentally sustainable mobility.

Life-cycle assessment of EVs

Several academic studies have assessed the environmental impact of EVs7,10,14,15,16,17,18,19,20,21,22. Studies assessing EVs and relevant components have assumed LIBs for battery electric vehicles22,23,24,25 and PEMFCs for fuel cell vehicles10,15,20,21. Compared with conventional vehicles, a larger share of EVs' life cycle impacts occur in the material processing and vehicle production phase, notably because of their reliance on relatively scarce materials and on production processes with high energy requirements10,14,15,19. Consequently, studies have found up to 40–90% higher greenhouse gas production-phase emissions for EVs compared with conventional vehicles. Whether or not EVs can compensate for their higher up-front environmental impact depends on the emission intensity of electricity sources and hydrogen for charging LIBs and fuelling PEMFCs, respectively. A life-cycle perspective is therefore required when evaluating their environmental performance7,10,14,19.

Studies assessing impact categories beyond climate change find that EVs can offer substantial improvement during its use phase, such as reductions in photochemical smog and fossil resource depletion8,19. However, EVs can also have a negative impact in other categories (for example, human toxicity, freshwater ecotoxicity, metal depletion), mostly arising from material extraction in the production chain14,19,20,26.

Due to the relatively high environmental impacts associated with the production of LIBs and PEMFCs, the lifetime expectancy and the recyclability of these energy devices are key parameters in determining their life-cycle environmental performance. Several studies have pointed to challenges with PEMFC durability due to degradation in the membrane and catalyst layer during long-term operation27,28,29. Battery EVs, on the other hand, generally suffer from limited driving ranges, and while larger batteries allow for longer driving ranges, they also cause more production-phase impacts and add weight to the vehicle, thereby increasing electricity consumption during EV operation30.

As many excellent reviews already cover the contribution of nanomaterials to overcoming technological and commercialization challenges of LIBs and PEMFCs31,32,33,34,35,36, this Analysis rather screens the environmental effects arising from the use of nanomaterials in these devices. For example, while the battery literature indicates that increasing volumetric energy density is an important factor for LIB adoption in battery EVs due to the limited space available37,38,39,40, the LCA literature rather focuses on the need for higher gravimetric energy density to avoid the additional material production and use-phase energy consumption associated with the transport of heavier batteries7,18,19,23,25.

Life-cycle approach for environmental screening

LCAs strive to guide product development by quantifying all environmental impacts associated with each product, but such a comprehensive assessment is typically limited by data quality and quantity. Multiple simplified, or streamlined, LCA methods have been proposed as a first iteration toward complete LCAs41,42,43 to provide life-cycle guidance as early as possible in product design, that is, before the design is decided and improvement options restricted. In contrast to full LCAs, there is no standard method to guide the performance of these scoping approaches. In this Analysis, we develop a framework that draws elements from streamlined LCA methods, the qualitative environmentally responsible product matrix scoping approach41,42,43 and key principles of green chemistry44,45(see Supplementary Figs S1 and S2). These elements are adapted, combined and updated to address the parameters that both can be influenced by nanotechnological research and determine the environmental impacts of EVs. The development of the framework is made all the more pertinent by the fast pace of nanotechnology research, the great diversity of competing nanomaterials and their differing technological readiness levels, which ranges from laboratory-scale proof of concept to commercialization.

The framework used here appraises nanomaterial candidates with respect to three life-cycle attributes: environmental intensity of materials, material and weight efficiency, and energy efficiency, which are described in detail below and illustrated in Fig. 1 and Supplementary Fig S3. Together, these life-cycle attributes cover all life-cycle phases of the material: production, use and end of life. To guide action, we distinguish between intrinsic parameters that are attributed to the material itself, and value-chain parameters that are characteristic not of the material but of the activities involved in its production. The evaluation of materials is adapted to the special nature of electromobility. Section 3 in the Supplementary Information describes the criteria and basis of comparison and provides further details in Supplementary Tables S1–S5 and Supplementary Figs S4–S9.

Figure 1: Early life-cycle environmental screening of lithium-ion batteries and proton exchange membrane hydrogen fuel cells for electric vehicles.
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Solid lines denote intrinsic aspects of the material itself. Dotted lines denote properties that are attributes of the value-chain aspects or embodied activities related to the material's production. Red lines denote production aspects, black lines use-phase aspects and the blue line end-of-life aspects.

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Environmental intensity of materials. The environmental intensity of a material describes the extent to which producing and using a given mass of a given material causes damages to the life-cycle areas of protection: human health, ecosystems and resource availability (Box 1). For example, energy-intensive extraction or production processes can result in high greenhouse gas emissions, which in turn can lead to damages to human health and ecosystems. This life-cycle attribute is highly relevant since, on the one hand, LCA studies on EVs find that materials used in LIBs and PEMFCs have environmentally intensive extraction and refining processes10,14,15,19, while on the other hand, nanotechnological developments are likely to alter the materials used in LIB and PEMFC productions. Some materials can themselves cause damages through exposure risks and hazards. The use of non-renewable materials can exacerbate resource scarcity, while material extraction and processing activities throughout the production chain result in embodied damage to human health and damage to ecosystems. Reducing the particle size from bulk material to the nanoscale can change both the material properties (for example, increased reactivity) and lead to differing environmental intensity (for example, damage to human health).

Material and weight efficiency. The material efficiency characteristic is a metric of the functionality that a material can achieve per unit of mass. Because the environmental aspects of materials as described in the previous section scale directly with the amount of material used, we should strive to attain the same functionality with less material. Given the relatively high environmental impacts associated with material processing in the production of LIBs22,24,46 and PEMFCs10,15,21 for EVs, optimizing the utilization of the materials in these devices is important. Increasing gravimetric energy density in LIBs or power density increases the material efficiency as less material can be used for the same energetic output. Improvements in material lifetime and stability allow for devices that last longer and in turn can reduce the need for replacement, thereby avoiding the use of additional materials. Energy density, power density, and lifetime and stability of nanomaterials were compared with the performance of a commercial 'baseline' material. Reducing material losses during synthesis and increasing the recyclability both improve material efficiency by minimizing waste. The use of nanomaterials in LIBs and PEMFCs may affect the material efficiency (for example, change in energy or power density) due to large surface areas, but it may also result in unwanted side reactions (for example, influence lifetime and stability). Material efficiency considerations such as energy and power density allow for lighter batteries and PEMFCs; these lightweighting effects also provide side benefits in the form of gains in energy efficiency.

Energy efficiency. Energy efficiency is a measure of how much functionality a given energy input can provide; here we consider energy losses during operation and energy use in the synthesis of nanomaterials. Depending on the energy sources used for producing electricity or hydrogen, the energy losses in LIBs and PEMFCs during operation can contribute to a substantial share of the device's life-cycle greenhouse gas emissions and other environmental impacts9,19,24,25. Here, we consider the device efficiency to measure how well nanomaterials enable the device to transform and deliver energy. LCA studies find that energy consumption in the value chains of LIBs can also contribute significantly to their greenhouse gas emissions and production impact24,25,46. Energy of nanosynthesis measures how energy efficient the manufacturing processes of nano-enabled LIB and PEMFC materials are. While using nanomaterials instead of bulk materials may improve the device efficiency due to increased reactivity, the differing methods to synthesize these nanomaterials require varying amounts of energy. As energy is often produced from carbon-intensive sources, energy use often translates to greenhouse gas emissions.

In the following sections, qualitative and semi-quantitative comparisons will be performed in terms of the three life-cycle attributes for various nanomaterials. Figures 2, 3, 4, 5 use colour coding to illustrate the perceived relative strengths of different nanostructure materials with respect to the above life-cycle attributes. Green denotes relative strength, red relative weakness, yellow intermediate characteristics and white a lack of data. Nanostructures are given by circles, whereas the paler background indicates the characteristics of the material in bulk form. Absence of a circle indicates a lack of data relevant to nanostructures. The grey background denotes the 'baseline' material. Although many of these life-cycle attributes pertain to the device as a whole (for example, energy density, power density and lifetime), we will consider the materials in isolation for greater ease of analysis. Thus, a cathode with high specific capacity and operating voltage will be described as a 'high energy density cathode'38 because its combination with an appropriate anode allows for a high energy density LIB.

Figure 2: Anode materials for lithium-ion batteries.
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Nanoarchitectured materials are given by a circle. Background colours reflect characteristics of bulk materials. Green denotes relative strength, red relative weakness, yellow intermediate characteristics and white no data. Absence of circle indicates no data for the nanomaterial. The grey background denotes the 'baseline' material. LTO, lithium titanium oxide. See Supplementary Information for the sources of the data in this figure.

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Figure 3: Cathode materials for lithium-ion batteries.
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Circles and colour coding are as defined in Fig. 2. NCA, lithium nickel cobalt aluminium oxide; NMC, lithium nickel manganese cobalt oxide; LCO, lithium cobalt oxide; LMR, lithium/manganese rich transition metal oxide; LFP, lithium iron phosphate; LVP, lithium vanadyl phosphate; LMO, lithium manganese oxide. See Supplementary Information for the sources of the data in this figure.

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Figure 4: Cathode catalyst materials for polymer electrolyte membrane fuel cells.
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Circles and colour coding are as defined in Fig. 2. PGM, platinum group metals; *, material on non-carbon black support. See Supplementary Information for the sources of the data in this figure.

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Figure 5: Catalyst support materials for polymer electrolyte membrane fuel cells.
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Circles and colour coding are as defined in Fig. 2. See Supplementary Information for the sources of the data in this figure.

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Nanotechnologies in battery developments

Battery cells are composed of several key components: anode, cathode, separator, electrolyte and current collectors. However, their energy density and environmental footprint are mainly determined by the properties of the electrode materials39. We therefore focus on the environmental performance of different nanostructured anode and cathode materials.

Anode materials. The use of pure lithium anodes is precluded in rechargeable LIBs with liquid electrolytes because of the formation of lithium dendrites on charging, which short the cell, leading to thermal runaway and fires36. Due to this increased reactivity and the associated safety issues, pure lithium anodes in nanoform are, so far, unsuitable for LIBs. Most current LIBs rely on the intercalation of lithium ions in anodes predominantly composed of graphite47,48,49. More recently, the use of nanosized lithium titanium oxide spinel (Li4Ti5O12, LTO) has also been adopted. In addition to these commercial anode materials, multiple alloys and conversion anode materials are currently under research. Figure 2 presents the material life-cycle attributes of reviewed anode nanomaterials, as well as graphite, which is considered to be the 'baseline' anode material.

Graphite is an abundant material47, and its extraction or synthesis has relatively low environmental impact50,51. Today, it also requires little energy during its production22 and allows for batteries with good cyclability47 and high energy efficiency52,53. The main weakness of this chemistry from a sustainability standpoint relates to its low material efficiency; its limited energy density leads to heavier, larger batteries54.

Alternative carbon nanostructures with higher theoretical energy densities are under investigation34, but neither carbon nanotubes nor graphene have been found to be technically feasible because they have too many side reactions55. Carbon nanotubes and graphene also exhibit more environmentally intensive50,51 profiles and, like other carbon nanostructures, their handling requires more precaution56 than graphite57. The current carbon nanotube synthesis routes are energy intensive58,59,60. Even when potential economies of scale are taken into account, energy requirements for the synthesis of carbon nanotubes through chemical vapour deposition, arc discharge or laser-assisted methods remain significant61, which in turn result in high greenhouse gas emissions62. Furthermore, carbon nanotube anodes have lower charge–discharge energy efficiencies34,52. Increasing evidence points to toxicity effects of carbon nanotubes similar to those of asbestos fibres63,64, which may affect production and end-of-life processing and recycling of the batteries65.

LTO is obtained from relatively abundant resources47,66 and has moderate production impacts50,51. It intercalates lithium in a safer manner than carbon because it is 1.5 V away from lithium metal deposition33, but must be nanostructured to reach acceptable power densities due to of its low conductivity34. Contrary to carbon nanotubes, LTO can be synthesized with moderate amounts of energy and low reagent losses, especially if a hydrothermal synthesis route is used13,67. The resulting nanostructured anodic material offers high cycling energy efficiency47,68, increased safety34, high power density69 and extended lifetimes52. Although LTO is already used in small commercially available EVs70, the 1.5 V operating potential of LTO leads to inherently low energy densities33, which reduces its material and weight efficiency and thus its environmental desirability for EVs. LTO nanoparticles also pose a high exposure risk71. The positive properties of LTO, however, potentially make it an environmentally sustainable candidate for static and high power applications.

Even more abundant than carbon47, silicon presents the highest theoretical capacity to store lithium of all studied anode materials52, potentially allowing for high energy density anodes. Refining silicon to metallurgical grade for use in the chemical industry causes moderate damage to human health and ecosystems50,51. Regarding electrochemical performances, bulk silicon anodes suffer from poor power density72 and extreme volume changes (up to 320%)73 that lead to rapid structural degradation of the electrode33, resulting in poor lifetime. The material must therefore be nanostructured to ensure that voids can buffer such swelling34,74. Silicon nanoparticles in carbon-based nanocomposites and silicon nanowires have been shown to improve electrochemical performance and lifetime with cycle life of 1,000–2,000 cycles73,75. Nanostructured silicon anodes thus open the possibility for high material efficiency in the LIB life cycle, particularly with respect to lifetime76 and energy and power density73,77. However, handling silicon nanoparticles in carbon nanostructures56,78 and silicon nanowires79 requires some precaution. The most popular technique used to grow silicon nanowires is chemical vapour deposition75, which has moderate to high energy requirements60,75. As a result, the synthesis of nanostructured silicon may result in high greenhouse gas emissions72. Furthermore, during the use phase, silicon anodes also suffer from higher voltage hysteresis47 and thereby lower cycling energy efficiencies than graphite or LTO.

Tin and germanium can also reversibly alloy lithium. Nanostructured tin-based anodes cycle with a higher Coulombic efficiency than silicon47, and germanium-based anodes allow for exceptional power densities34. However, given the greater scarcity47,66 of these metals and the environmental impacts of their extraction and refining50,51, their life-cycle environmental sustainability performance remains unremarkable57,80,81. Tin may nonetheless prove attractive because of its superior performance when combined with other elements, such as abundant and low-impact iron (for example, Sn2Fe nanoparticles)82,83,84.

Many nanostructured transition metal oxides can enter in a conversion reaction with lithium, which in principle offers more options as potential anode materials. Among these, iron oxides, such as haematite (α-Fe2O3) and magnetite (Fe3O4)34, are by far the most abundant47,66 and the least environmentally intensive50,51,85,86, in contrast to more scarce elements47,66 such as chromium, molybdenum, ruthenium and cobalt87,88,89. Green synthesis routes for iron oxide nanoparticles should lead to relatively lean use of reagents and energy54,90. Though high specific capacities have been demonstrated54,90, their relatively high voltages during delithiation34 substantially reduces the overall cell voltage and consequently, energy and power density. High voltage hysteresis68,91 makes all these issues worse and also leads to low cycling energy efficiencies, typically less than 60%. Such low energy efficiencies constitute a major handicap for an otherwise environmentally attractive material.

Cathode materials. The energy density of LIBs is largely determined by the cathode, as its practically achievable energy is greatly inferior to that of the anode92,93. There are two broad categories of cathode materials: intercalation and conversion. Intercalation materials are the most widely investigated and are already used as bulk materials in commercial LIBs47. Of the conversion-type cathode materials, none have reached commercialization47,94. Figure 3 presents the material life-cycle attributes of reviewed cathode nanomaterials. LiNi0.8Co0.15Al0.05O2 (NCA) is considered to be the 'baseline' cathode material.

By far the most commonly used cathodes today are the layered oxides, such as LiCoO2 (LCO). Due to the use of the relatively scarce cobalt47,66, commercially available LCO causes moderate direct exposure risks86,95 and embodied damages to human health and ecosystems50,51. In addition, cobalt's high cost has led a drive to replace most of it in many applications96, resulting in the adoption of materials with lower cobalt content such as LiNi1/3Mn1/3Co1/3O2 (NMC) and NCA. The popular NMC and NCA pose exposure risks and hazards because they, as with many nickel-containing compounds, are suspected of being human carcinogens57,97,98,99. Their high energy and power densities have nevertheless made them attractive as bulk materials, and these materials are already used in EVs47. As nanostructures, however, the decomposition of the electrolyte and formation of surface films result in unsatisfactory lifetime for EV applications. Even though these layered oxides are not used in nanoform, alternative materials must have equal or superior energy density while demonstrating better lifetime and stability than bulk NMC and NCA to displace them from the EV market.

A promising layered oxide is the lithium/manganese-rich material (LMR)100, often written as Li2MnO3·nLiMO2 (where M is Mn, Co, Ni, and so on). LMR contains more than one lithium atom per transition metal and has more manganese than other metals. Here, we focus on 0.5Li2MnO3·0.5LiNi1/3Mn1/3Co1/3O2. Due to its higher content of manganese relative to NMC, LMR is slightly less environmentally intensive than NMC50,51,98,101. Furthermore, LMR also has a high voltage and specific capacity that allows for a significant increase in energy density over current commercially available cathode materials102. Despite these advantages, poor rate capability103 results in low power density, while thermal safety issues37 and voltage fade104 result in poor lifetime and stability, all of which complicate its commercial introduction for EVs.

Lithium iron phosphate (LFP) is found in nature as the mineral triphylite105 and has low exposure risks and hazards86. Furthermore, environmental impacts associated with its production value chain are lower than most other cathode materials16,50,51. As a bulk material, LFP has moderate electric potential47, outstanding thermal stability52 and excellent cycling performance106, but its two-phase reaction mechanism, with low ion diffusion rate and very low electronic conductivity107, makes it difficult to reach capacities close to the theoretical limit52. However, research found that in nanoparticle form, the material could produce stable cycling much closer to its theoretical capacity because the phase diagram is changed and the reaction proceeds via a metastable single-phase mechanism37. This development increased the material's energy52 and power33 densities, but its energy density remained inferior to that of other commercially available cathode materials such as NMC47,48. The lower energy density47 and the claimed lower charge–discharge energy efficiency of LFP106 can result in higher electricity use per kilometre driven compared with other cathode materials, which in turn would lead to higher indirect greenhouse gas emissions in the use phase. LFP can be produced through several nanosynthesis methods108, which particularly influences the energy use, and consequently greenhouse gas emissions, associated with its production. The superior electrochemical and safety properties of nano-LFP have spurred interest in finding other phosphates that might have much higher energy densities. One approach is to use materials that can incorporate up to two lithium ions. One such material is VOPO4, which must be nanosized and carbon coated to be operative109, but has the advantage of being made of relatively abundant materials47,66. This material forms Li2VOPO4 (LVP) on discharge and has a capacity of 305 Ah kg−1 compared with the 170 Ah kg−1 of LFP. However, the lifetime and stability are inadequate for EV use and much work is still needed to make LVP commercially viable.

Spinel LiMn2O4 (LMO) is made of abundant manganese47,66, is relatively safe to handle86,110 and has relatively low damages associated with its production50,51. Nanosized spinel LMO has been synthesized in various morphologies. Studies have found increased power densities47, and although increased energy densities have also been obtained107, these are not as high as those of bulk NMC and NCA47,106. In the case of LMO, nanoparticles tend to increase the undesirable dissolution of manganese to the electrolyte32,107, leading to lifetime issues. Porous nanorods, however, have been found to have remarkable lifetime111.

As one of very few viable options to the intercalation materials, the conversion material sulfur has received intense interest in the past decade due to exceptionally high theoretical energy density112,113,114,115. Supply of sulfur is unlikely to become an issue as it is an abundant element in the Earth's crust47. In batteries, the insulating nature of sulfur results in poor power density and creates large internal resistance and polarization of the battery116, resulting in poor device efficiency. Furthermore, volume expansion (∼80%) and dissolution of intermediate reaction products (polysulfides) in the electrolyte result in poor lifetime47,113. The most promising approach to mitigate poor conductivity and lifetime is the encapsulation of sulfur within conductive additives to form sulfur–carbon and sulfur–polymer nanocomposites47,117. Sulfur–carbon nanocomposites pose higher exposure risks and hazards56,118 than sulfur nanocomposites with polymers such as polyacrylonitrile, polyvinylpyrrolidone, polydimethylsiloxane118,119 and polyaniline118,120. Even if the issue of lifetime is overcome, the sulfur cathode must be paired with a lithium metal or a lightweight lithiated anode for high energy density47,114,121,122. In contrast, lithium sulfide (Li2S) can be paired with lithium-free anodes, which avoids safety concerns and short lifetime122. Although the Li2S cathode has a high theoretical capacity, it is both electronically and ionically insulating47, which has led to various efforts using conductive additives, such as metals and carbon114. Earlier studies tended to focus on Li2S–metal composites, but the inherent disadvantages of Li2S–metal composites have prompted extensive interest in the development and use of Li2S–carbon composites in the past five years116. Due to a high content of lithium and carbon nanostructures, care should be taken when handling nanostructured Li2S–carbon composites56,86. Studies have reported different nanostructures, synthesis methods and carbon content in Li2S–carbon nanocomposites. This can lead to significant differences in material losses and energy use, which in turn influence greenhouse gas emissions and damages to human health and ecosystems. Further improvement on lifetime is required for Li2S cathode materials to replace the layered oxides on the EV market.

Recycling of LIBs. There are several competing industrial LIB recycling processes123. LIB recycling is typically a combination of two or more of the following processes: mechanical separation, pyrometallurgical treatment and hydrometallurgical treatment. The various industrial recycling pathways offer different yields depending on the recycling route and electrode materials. As the metal value in batteries is mainly driven by prices of cobalt and nickel metals, current recycling processes still focus on the recovery of these metals97,124,125. Other transition metals, such as copper and iron, are also typically recovered in the current industrial LIB recycling processes. In only a few recycling routes are aluminium, lithium and manganese recovered97,123,125. According to relevant literature97,123,126 and two European recycling companies (Accurec, personal communication, 2016; Umicore, personal communication, 2015), phosphate and graphite are normally not recycled in current industrial processes. Nanostructured LFP is currently recycled successfully (Accurec, personal communication, 2016), which suggests that nanostructuring electrode materials does not affect recycling yields compared with bulk materials. During recycling, however, nanomaterials may become airborne, which can pose exposure risks and hazards to workers127.

Nanotechnologies in fuel cell developments

While there are multiple fuel cell types, we focus here on PEMFCs, which demonstrate the most potential within the transport sector10,15,21. High cost, durability and lifetime challenges are all barriers to the mainstream adoption of fuel cell EVs27; in contrast to battery EVs, commercial sale of fuel cell EVs has only very recently become reality (for example, Honda Clarity and Toyota Mirai). In contrast to LIBs, the 'baseline' materials are already in nanoform; rather, we review here alternative nanostructures and nanomaterials that have the potential to replace current state-of-the-art materials. These advances in nanotechnology have shown promising opportunities to improve the technical and environmental performance of PEMFCs in EVs and thus encourage their widespread commercial adoption.

Figures 4 and 5 summarize the life-cycle attributes of some of the most promising nanostructured materials for cathode catalyst and catalyst support, respectively. Although the electrocatalyst often refers to the catalyst and support together (Pt/C), they are considered as two components independent of each other in this study. Electrolyte membranes, being a bulk material, are discussed in section 4 of the Supplementary Information while nanotechnological improvements to these bulk materials are discussed in the text.

Cathode catalysts. The oxygen reduction reaction occurring at the cathode is enabled by the cathode catalyst; a well-performing catalyst is therefore a determinant of the device's overall power output. At present, both PEMFC anodes and cathodes rely on platinum catalysts supported on high surface area carbon (Pt/C), which are costly, scarce47 and have extremely high environmental implications from platinum extraction50,51. In terms of efficient use of this high-impact, non-renewable material, the cathode is the key technological bottleneck as the oxygen reduction reaction occurs five to six orders of magnitude slower than the hydrogen oxidation reaction occurring at the anode28, thus greatly limiting the cell power density. Furthermore, the pure platinum catalysts suffer from poisoning from impurities in the hydrogen fuel as well as dissolution and agglomeration, which can drastically shorten the fuel cell lifetime29,128,129. A shorter lifetime demands more frequent replacement of PEMFC stacks in EVs, and may ultimately require more platinum extraction per kilometre driven.

Current research therefore focuses on reducing or eliminating platinum use in the catalyst130,131,132. Several solutions are being explored, including the use of ultra-low platinum loading, platinum alloys and platinum-free catalysts to reduce material costs while maintaining or improving catalytic activity over current Pt/C catalysts. Compared with the commercial Pt/C catalyst, most of these platinum-containing alternatives yield enhanced durability and demonstrate similar or superior oxygen reduction reaction catalytic ability.

Alternative platinum nanomorphologies and nanostructured platinum alloys can maintain or even increase the catalytic activity relative to conventional Pt/C catalysts. Increasing the specific catalytic activity allows for a reduction in the amount of platinum used, thus improving material efficiency over the conventional catalyst. In addition to the various nanomorphologies, research using different assembly methods, such as electrospraying, improve catalytic activity by influencing the hierarchical structure of the electrode133,134. Similarly, platinum alloys with nickel135,136,137, cobalt136,138 and copper139,140 have also demonstrated good performance while decreasing platinum use.

While platinum reduction is a desirable goal for PEMFC development, the complete elimination of platinum use in PEMFCs would be an even greater improvement of the material environmental impacts66,141. Non-precious metal catalysts using more abundant metals such as iron have been tested, but present severely depressed technical performance and stability in acidic operating conditions142. Other metal catalysts based on niobium, tantalum and zirconium have improved lifetime over Pt/C, but do not meet power density expectations, and are more scarce47,66 and environmentally intensive to produce than iron50,51, although they still represent an improvement over platinum. Metal-free catalysts using functionalized carbon nanostructures, particularly nitrogen-doped carbon nanotubes and graphene materials, are promising candidates for platinum-free catalysts that capitalize on abundant precursor materials, though they require further research to improve the energy efficiency of their synthesis and to provide adequate catalytic ability in acidic environments143,144,145. A clear trend, however, is that platinum-free catalysts continue to struggle in catalytic activity and lifetime compared with low-platinum and platinum-alloy catalysts143.

In addition to the morphological and material nature of the catalyst, the hierarchical organization of the nanostructured materials in the device also affects catalyst performance. While such organization may increase material efficiency by increasing catalytic activity, it may also present consequential side issues such as water flooding, which in turn cancels out or exceeds the gains in performance, or causes unstable cell performance146.

If the goal is to reduce the amount of platinum used in fuel cell EVs to the amount used in the catalytic converters of conventional internal combustion engine vehicles, the device lifetime must be accounted for. Since fuel cell EVs currently have a shorter lifetime than conventional vehicles, the amount of platinum required to drive an equal distance increases, that is, several fuel cell stacks will be required. Furthermore, the growing light-duty vehicle market represents an unsustainable demand for further platinum extraction into the future. Rather, focus should be placed on robust, low- or non-platinum catalysts with long lifetime.

Cathode catalyst supports. Effective support materials enhance catalytic catalyst utilization and thus increase material efficiency by allowing for smaller quantities of catalyst while maintaining similar levels of catalytic activity. A catalyst support would ideally maximize the catalyst surface area available for reactions and maintain high electric conductivity for high energy efficiency. Supports made of carbon black, our 'baseline' material, are currently used in commercial PEMFC catalysts. These supports are vulnerable to corrosion, which causes catalyst sintering and decreases the amount of conductive material in the electrode, thereby decreasing power density and PEMFC lifetime29,147. Carbon black-based support materials also suffer from deep micropores that physically block reagent access to the catalyst and thus decrease catalyst efficiency148.

Nanostructured materials can provide the characteristics needed for an effective catalyst support, including a high surface area with a mesoporous structure that does not inhibit catalytic activity149. Catalyst support materials must also be sufficiently electrically conductive to reduce internal resistance, thereby enhancing charge transport within the cell, and be stable at higher temperatures and in the acidic environment of a PEMFC.

Carbon nanostructures and titanium dioxide are two promising catalyst support materials that demonstrate improved technical performance. In their bulk form, these materials have low environmental intensity50,51. The synthesis methods for the nanomorphologies, however, may potentially have high energy demand60, and thereby be detrimental to the overall climate change performance of the manufacturing process. The graphitized carbon-based nanomaterials have enhanced durability under fuel cell operating conditions29, which improves the climate change performance of the PEMFC over the lifetime as a counterpoint for the increased synthesis energy. Doping the carbon with heteroatoms, such as nitrogen, phosphorus or sulfur, functionalizes the otherwise inert carbon to allow catalyst deposition148. In some cases, functionalization, such as with nitrogen-doped carbon nanotubes, also allows the otherwise catalytically inert carbon supports to become catalytically active, thereby increasing power density of the PEMFC150. Some carbon–polymer nanocomposites have shown improved material efficiency via power density, but in some cases this is in exchange for reduced lifetime.

Carbon-free, transition metal oxide-based supports such as titanium dioxide in mesoporous or nanofibre morphologies, while relatively robust, have not yet achieved the same performance level as the baseline carbon black catalyst support. Composite titanium dioxide catalyst supports may also be more sensitive to scarcity47,66 and material production impacts50,51 than carbon-based supports, as are supports of niobium- and ruthenium oxide-doped titanium dioxide.

Electrolyte membrane. The PEMFC membrane, with its high cost151, poor durability29 and intolerance to fuel impurities152, represents another obstacle to the widespread commercialization of transport PEMFCs. The current commercial standard, Nafion, is a perfluorinated membrane that performs poorly in temperatures beyond 80 °C and in low-humidity environments, and is not stable with impure feed gases153,154. An ideal membrane for transport PEMFCs must therefore have satisfactory performance and stability at these conditions. Research has been directed towards more robust membranes, which would allow for thinner membranes that represent an improvement in material efficiency (less membrane material used) and device efficiency (for example, superior ion exchange/proton conductivity performances). While membrane polymers conduct protons at the nanoscale, the membrane material itself does not constitute a nanomaterial. A brief review of the main membrane polymer groups may be found in Section 4 of the Supplementary Information. Nanotechnology offers several options for improving these bulk membranes. Such options include the use of nanofillers to enhance the membrane, or the use of nanosynthesis methods to provide a superior hierarchical structure to the membrane.

One attractive strategy of generating an optimum balance between ion conduction and physicochemical stability in electrolyte membranes is to create a 'microphase separated' morphology in polymers made of highly ordered ion-nanochannels and a hydrophobic phase. An example is the fabrication of ion-conductive polymer nanofibres. These demonstrate distinctive electrochemical, physicochemical and thermal properties owing to their high specific surface area and polymer orientation along the nanofibre direction155,156. The use of a reinforcing, mechanically strong nanofibre morphology can minimize in-plane swelling changes during wet(on)/dry(off) fuel cell operation and thus extend the device lifetime157. Some success has been achieved with a dual electrospun composite of poly(phenyl sulfone) and Nafion158, where the poly(phenyl sulfone) polymer provides mechanical stability to the Nafion membrane, thus improving lifetime while maintaining device efficiency (cell power output). Similarly, improved proton conductivity, leading to increased power density, was achieved with electrospun acid-doped polybenzimidazole in a sulfonated polymer matrix compared with a similar composite membrane without nanofibre morphology156.

In one type of composite membrane, a polymer membrane matrix may have embedded nanostructures of inorganic materials to improve membrane characteristics. Such materials may be metal oxides or synthetic clays to improve mechanical stability159 and water uptake, or nanocarbons or nanofibres to provide ionic channels and thus improve device efficiency of the PEMFC. Heteropolyacids such as phosphotungstic acid are used as fillers to improve proton conductivity (device efficiency), but decrease mechanical stability and therefore have a shorter lifetime. Phosphotungstic acid also has significant exposure risks160. However, while hygroscopic particles are intended to increase the device efficiency by improving proton conductivity via increased water retention, these particles decrease device efficiency by diluting the concentration of the proton-conducting ionomer when made of material less conductive than that the ionomer membrane161,162,163,164. Nanofillers may also increase the mechanical strength of the polymer, as in the case of zwitterionic structured SiO2 in polybenzimidazole159,165. In addition, the heterogeneous hybrid membranes also experience phase separation due to differing water uptake and thermal expansion coefficients of the nanofillers and the polymer matrix, causing stresses and strains in the membrane and thereby shortening the lifetime and decreasing material efficiency166.

Hierarchical ordering in these nanocomposites is also a promising strategy to improve membrane performance; in particular, the alignment of one-dimensional (nanotubes, nanofibres or nanorods) and two-dimensional nanomaterials (nanoflakes, nanosheets or nanoplates) in the membrane have a two-fold benefit. In the direction parallel to the membrane, proton conductivity is improved, while across the membrane, mechanical properties, chemical stability and fuel permeability characteristics are improved. Graphene oxide167 and electrospun156,158 nanofibres are particularly emphasized due to the creation of long-range ordered ionic nanochannels for proton conduction and excellent physicochemical stability.

Recycling of PEMFCs. In terms of both cost and environmental intensity, platinum catalyst and fluorinated membranes are of greatest interest for recycling and recovery processes. The most common platinum recovery approaches include selective chlorination or gas phase volatilization, hydrometallurgical and pyrometallurgical processes168. Selective chlorination or gas phase volatilization, however, require carbon monoxide and chlorine gases or aggressive solvents such as aqua regia or cyanide. Many of these compounds pose considerable risks to workers169,170,171. Many hydrometallurgical approaches also require high operating temperatures and pressures, making them energy-intensive processes. Pyrometallurgical processes for PEMFCs containing fluorinated membranes such as Nafion would result in the emission of highly toxic hydrogen fluoride172,173. The Pt/C catalyst can also be recovered using a chemical recovery process after carbon-based supports are incinerated172,173. In general, alloying and non-combustible elements consisting of 10% or less of the total recoverable materials will not detrimentally affect recoverability or reusability of precious metal catalysts (Umicore, personal communication, 2015).

Mechanical separation of membranes from the catalyst layers is difficult, as these components are generally hot-pressed together172. Re-use of the membrane is also unlikely as performance drops in fuel cells are usually caused by membrane degradation or failure due to dehydration and pin-holing, which makes recycling a more likely end-of-life fate for membranes172. Nafion membranes are generally recovered using chemical extraction172,173,174, after which a new membrane may be re-cast, although possibly with some loss of quality174. As for the catalyst, it is unknown whether the adoption of novel multi-element catalysts and alternative catalyst support materials in PEMFCs will affect the yield or quality of recovered precious metals given the current PEMFC recycling techniques.

The road ahead

Nanomaterials are opening a broad range of opportunities to improve the technical and life-cycle environmental performance of EVs. Identifying the alternative material candidates with the most promising opportunities for enhancing overall environmental performance of LIBs and PEMFCs in EVs at an early stage is therefore important. To this end, we performed an early stage life-cycle environmental screening of the material candidates and mapped their potential strengths and weaknesses with respect to key life-cycle attributes (Figs 2–5). We found that no single nanomaterial seems poised to outcompete its rivals in terms of all reviewed sustainability criteria for any of the reviewed LIB and PEMFC materials. Rather, the current research frontier presents multiple promising candidates for continued development, each subject to non-trivial environmental trade-offs that should be addressed.

To maximize climate change mitigation benefits offered by EVs, we must improve both the electrochemical and environmental performance of LIBs and PEMFCs. Nanomaterials show great promise in providing the necessary technical breakthrough in these devices, but their ability to be a part of the mitigation solution for transport-related greenhouse gas emissions depends on several life-cycle attributes spanning extraction, refinement, synthesis, operational performance, durability and recyclability. As such, the next generation of LIBs and PEMFCs should ideally be based on abundant resources that can be extracted and refined with low energy consumption and environmental impacts. It should be resource and material efficient, achieved through improvements in synthesis yields, lightweighting, durability and, ultimately, recyclability. Finally, it should be energy efficient, both in the production and use phase. In practice though, we are likely to have to make some trade-offs.

Our analysis of the current situation clearly outlines the challenge: the materials with the best potential environmental profiles during the material extraction and production phase (less environmentally intensive materials, lower nanosynthesis energy use and facile synthesis) often present environmental disadvantages during their use phase (lower energy efficiency, heavier battery or shorter lifetimes), and vice versa.

Meeting this challenge will require concerted efforts and a new focus within the nanotechnology community. Throughout this Analysis, we found that publications on novel nanomaterials rarely explicitly communicate synthesis yields, solvent use and energy consumption during production. These are all are key parameters that significantly influence the environmental performance and that can largely be improved through the choice of alternative synthesis protocols and foreseeable economies of scale. Increased, systematic and consistent reporting of these attributes would remove a very avoidable source of uncertainty. Improved flow of information would be of mutual benefit to both the LCA and nanotechnology communities; through joint efforts, both communities would be able to direct research efforts towards the materials and synthesis protocols with the best environmental sustainability potential. An extension of the above aspect is the current lack of data regarding potential toxic effects, which unfortunately remains a challenge for nearly all of the investigated nanomaterials. Similarly, we also found little literature on how the physicochemical properties of novel nanomaterials affect existing recycling and disposal processes. Addressing these issues would allow us to efficiently manoeuvre towards the most environmentally superior options. As more detailed and consistent information becomes available, one can move from screening studies to detailed LCAs to refine our understanding and ultimately make the right design trade-offs that optimize LIB and PEMFC nanomaterials for EV usage towards mitigating climate change.

This will require a cross-disciplinary collaboration between material scientists and LCA practitioners to reap — and maximize — the benefits offered by simultaneously incorporating nanotechnology, nanotoxicology, eco-design and green chemistry considerations. If we succeed, nanotechnology can be a key contributor to climate change mitigation in the transport sector.