Spiers Memorial Lecture: CO 2 utilization: why, why now, and how?

This introduction to the Faraday Discussion on carbon dioxide utilization (CDU) provides a framework to lay out the need for CDU, the opportunities, boundary conditions, potential pitfalls, and critical needs to advance the required technologies in the time needed. CDU as a mainstream climate-relevant solution is gaining rapid traction as measured by the increase in the number of related publications, the investment activity, and the political action taken in various countries.


Introduction
The need for carbon dioxide utilization Decisive, rapid, and large-scale action is needed to address the escalating negative impacts of climate change.Massive efforts are required to deploy technologies, policies, and business models that quickly reduce the further release of carbon dioxide (CO 2 ) into the atmosphere at large scales. 1 Taking the post-2050 view for humanity, there will be processes where the formation and possible release of CO 2 cannot be avoided, such as some heavy industry (steel, cement) and aviation. 2lso, there will always be products whose creation requires carbon, such as many chemicals, polymers, and more.These products will eventually end up as CO 2 at the end of their life.Therefore, solutions are needed to handle the unavoidable CO 2 emissions and satisfy the continued need for carbon without tapping into fossil sources that would inevitably increase the atmosphere's carbon content.
Beyond that, it is also clear that the legacy of 200 years of industrial growth includes more than a trillion tons of excess CO 2 in the atmosphere.Signicant amounts of this CO 2 must be removed to keep average atmospheric temperatures no more than 2 C above pre-industrial levels. 1,3merging technologies exist that can capture CO 2 from factories, power plants, and from the air. 4These technologies will have to be scaled and deployed quickly to handle the gigaton-scale CO 2 removal projected in Fig. 1.Scenarios like the ones shown are abundant in the literature, and they all share the conclusion that climate stabilization requires large-scale CO 2 removal.[7][8] The opportunities Vast amounts of CO 2 have to be disposed of permanently in underground caverns, aquifers, and spent oil and gas elds at a high nancial cost. 9However, there is an additional pathway to handle some of the captured CO 2 .Captured CO 2 can be used directly to create products that require a carbon feedstock, and through this, CO 2 is repurposed without adding new carbon to the utilization cycle and the atmosphere. 10This also has the advantage of avoiding excessive land use if all carbon were to come from biological sources since that land may be needed to produce food and preserve natural habitats.In addition, since commercial products can be made from CO 2 utilization, the outcomes will generate revenue projected to eventually exceed the cost of production. 11or long-term climate stability, CO 2 emissions have to decrease rapidly, 12 and in the long-run, the global carbon cycle has to be balanced to a net-zero scenario.
Handling the inevitable emissions from some industries and consumer activities will require a combination of storage and utilization, 13 as schematically shown in Fig. 1 Projections for annual global CO 2 emissions that would have to be achieved to limit the average atmospheric temperature increase to 1.5 C over pre-industrial levels 3 (reproduced with permission from IPCC).

Faraday Discussions Paper
Fig. 2. For economic reasons alone, the utilization capabilities must be maximized, clearly identifying the reasons for CO 2 utilization (CDU).A 2016 study described a roadmap to a trillion-dollar market, highlighting the opportunities and documenting that at the time, less than 200 entities were engaged in commercial endeavors for CO 2 utilization. 11,16That number has since increased incrementally, and some technologies have now achieved a sufficient maturity level to be deployed to markets, such as the production of chemicals, CO 2 -cured concrete, and aggregates.The potential for CO 2 utilization, economically and environmentally, is demonstrated.

CDU gaining rapid traction
It is paramount to accelerate the transition from research laboratories to market introduction.However, as recently stated by an industry representative, "It seems to me that there is such a canyon between what the academic world considers complete and what the industry considers emergingit is the old Valley of Death." 17 Measured on a technology readiness level (TRL) scale, university research readiness rarely exceeds levels 3-4, i.e., research at the level of proven feasibility and perhaps beginning technology development.Conversely, industrial interest, and that of many investors, begins at TRL 6 or higher, i.e., with pilot-sized, demonstrated technology ready to transition to full-scale engineering.This gap, the Valley of Death, is oen a barrier that causes long delays in advancing inventions towards market introduction or literally leads to the death of a technology and termination of further pursuits.This valley is especially pronounced in developments that require complex and expensive hardware installation to advance the TRL.Shared incubators and collaborative partnerships can help to Fig. 3 The Web of Science publication count based on the keyword "Carbon Dioxide Utilization" shows the typical fast increase in publications in an emerging field.

Paper
Faraday Discussions overcome the technological challenges. 18,19In addition, public policy plays a vital role in supporting the transition to higher TRL. 20apidly developing and deploying CO 2 utilization technologies in a responsible manner requires an accelerated process to translate inventions from research laboratories to the market.Connecting and coordinating communities in academia, business, and government will be essential to address technical, nancial, and societal questions in a responsible and impactful manner. 21The needs and opportunities align at this time, and CDU is expected to advance rapidly, even though many details have to be considered along the way.

Boundary conditions
The following questions need to be addressed to assist with decisions about CDU deployment.
What to make from CO 2 ?How much CO 2 is permanently removed, or new release is avoided?What is the source of CO 2 ?Do we have enough zero-carbon energy for CDU? Do we have enough zero-carbon hydrogen for CDU?How and when will CDU become nancially competitive?What policies exist or are needed to deploy CDU? Are businesses prepared for CDU?What is the public sentiment towards CDU?As a note, enhanced oil and gas recovery with CO 2 can be considered a form of CO 2 utilization and has been in use for decades already. 22The use of CO 2 as a cooling or cutting uid can equally be regarded as a signicant use case with potential climate benets through avoided emissions. 23However, neither of these use categories will be addressed in this paper.

CO 2 product categories
Deciding the 'what', 'how', and 'where' of CDU is a multi-factorial process.Decisions will depend on location as well as on technology readiness, supply chain considerations, policies, and more.Each CDU project decision will have unique considerations, but all must fundamentally meet the following two criteria.First, the CO 2 -based product must have a better environmental footprint than the incumbent products, in particular a lower carbon footprint.Second, the economics of making and selling the product must be competitive with those of the incumbent product to engage the industry with the adaptation of the CO 2based product pathway.
Carbon is a central element in human life, and carbon plays a direct or indirect role in most existing products.When holistically considering CO 2 utilization as a climate mitigation tool, it is useful to group opportunities into product categories and examine these in terms of their environmental and economic potential.Table 1 presents a list of product categories that include those that are directly made from fossil carbon sources, such as fuels and chemicals, and products that aren't directly linked to fossil carbon sources, such as construction materials like concrete, aggregates, and related inorganic materials.This table shows that, regardless of the current relationship with fossil carbon sources, a wide range of industries can leverage CDU.Some of the largest opportunities for CO 2 utilization are in making construction materials, e.g., concrete and aggregate materials.This is especially interesting since these materials have the ability to keep CO 2 out of the atmosphere indenitely.For the other product categories, CO 2 mostly becomes a circular source of carbon that is currently taken from fossil sources.

Market opportunities
Successful market introduction of CDU products will depend on their competitive advantages over the incumbent products, be those of technical, nancial, or regulatory nature.In many cases, the CDU product will be indistinguishable from the incumbent product, and therefore, market introduction will be primarily driven by cost and policy requirements.5][26] However, such mandates may not lead to a pure cost advantage in the absence of indenite policy support.Ultimately, if the economics do not work for a particular product, it will fail in the market, no matter the size of the environmental advantage. 27Fig. 4 shows that the magnitudes and ranges of potential revenue and CO 2 use are signicant.This points to a favorable combination of economic prospects and signicant environmental benets to address climate change.
In this context, the overview in Table 2 points towards challenges with market introduction.Drop-in replacements are materials that are identical to the incumbents, and therefore they require either a lower cost or policy-supported procurement requirements for widespread market success.It appears unlikely that markets of that magnitude would be enabled by consumers choosing to pay a green-premium for the replacement product based on environmental concerns.The advantage of drop-in products, though, is that their functions and performances are known.
CO 2 -based products, e.g., CO 2 -cured concrete materials, may be different from the incumbents and would therefore be considered new products, seeking to displace existing products.If such replacement products have superior properties not available in the incumbents, that competitive advantage can help to offset concerns about costs.On the other hand, regulatory requirements such as building codes and standards might complicate market introduction.
Exemplary products and technologies are briey presented to illustrate the growing breadth and depth of the CDU eld.While certainly considered an emerging eld, the amount of literature available is already considerable, and so is the number of products that are being explored.This paper does not attempt to provide a comprehensive review of the entire eld.

Construction materials
The highest climate impact will be achieved with products that permanently remove the largest amount of CO 2 and avoid the net release of large amounts of new CO 2 into the atmosphere.Construction materials including cementitious materials, concrete, and aggregates play a critical role in this category because of their potential to permanently bind CO 2 and their signicant presence in global economies.Cement production alone accounts for 5-8% of global CO 2 emissions, Fig. 4 Projection of the economic and environmental potential for CDU products, based on a previous market assessment. 11,16ble 2 CO 2 -based products may just be substitutes or become alternatives to incumbent products

Fuels and chemicals
Introducing CO 2 as a source for carbon, in principle, opens up the entire value chain of organic chemistry. 10,58-63CO 2 as a building block becomes much more interesting to industry. 64echnologies to produce fuels and chemicals from CO 2 span a wide range of pathways that are oen collectively called Power-to-X technologies. 65These can be based on syngas-based synthesis with Fischer-Tropsch processes, 66 bio-organisms, [67][68][69][70] and so-called articial photosynthesis processes. 71,72hese can help overcome some of the complexity of fuel synthesis from CO 2 by integrating the harvesting of sunlight, splitting water and CO 2 in articial photosynthesis devices, labs-on-a-chip, to directly produce fuels such as methanol, methane, or hydrogen (Fig. 6). 724][75][76][77][78][79][80][81][82] Particular challenges in this context are associated with the amount of energy and thus the cost required to transform CO 2 into hydrocarbons.Careful consideration needs to be given to use Fig. 5 Self-healing properties of highly ductile carbonated Engineered Cementitious Composite (ECC) 52 (reproduced with permission by the publisher).
cases where alternative energy carriers are not an option. 59,83The availability of cost-competitive green or blue hydrogen (if combined with carbon capture) is a key barrier to the growth of many Power-to-X technologies, and this requires equal attention as do the energy and CO 2 footprint factors. 84,85roadly, there is a rapidly growing number of photo-and electrochemical pathways to produce chemicals from CO 2 .7][88] The integration of multiple reaction steps into one system, including CO 2 capture, is of particular interest. 61,89,90gineered materials Fundamentally, the conversion of CO 2 into carbon black and derived solid carbon products opens extensive market opportunities.2][93][94][95] Higher-value products such as carbon nanotubes 96,97 or diamonds 98 can also be made from CO 2 .

Polymers
Building on the analog in nature, CO 2 has been investigated intensely as a building block for polymers. 99,100High-volume materials, such as polyurethanes, can be made with substantial fractions of CO 2 101 and thus CO 2 as a raw material opens pathways to green chemistry for polymer production. 102Introducing CO 2 into polymer synthesis had already been described in the 1960's. 103The production of polyurethane for foam manufacturing can serve a wide range of industries (Fig. 7). 104The versatility of CO 2 as a renewable feedstock for polymers is considered an important element of the circular carbon economy. 105od and agriculture Perhaps the most prominent example of CO 2 use in this category is the production of urea, a key chemical for the fertilizer industry that can make use of the CO 2 released during the production of NH 3 , the second key ingredient in the synthesis of urea. 106The use of CO 2 to fumigate greenhouses to promote plant growth is established, though only recently have efforts started to use CO 2 captured directly from the air instead of deliberately burning fossil fuels to generate the necessary CO 2 .[109]

A new paradigm of energy use
Most methods of CO 2 utilization are enormously energy-intensive.Therefore, CDU's success will depend on the widespread availability of carbon-free energy to meet demand sufficiently.Undoubtedly, the energy requirements are substantial.A scenario for what is called 'emergency CO 2 direct air capture' projects the need to use 9-14% of electricity and 53-83% of natural gas available by 2075 just to capture the CO 2 that needs to be removed to stabilize the climate. 5Related to this, a study examining the electricity needs to operate the entire production of key chemicals from CO 2 projects the need to double the current world production of electricity. 59It is thus imperative to develop processes that minimize energy needs, but it is equally important to consider a new paradigm for energy conversion and use.While it will always be important to use any resource as efficiently as possible, in terms of energy input to CO 2 utilization (storage), the more important factor is avoiding adding new CO 2 to the atmosphere.In the transition to such a scenario, it is clear that at present times, the available energy mix will render most CO 2 utilization projects not helpful in mitigating the carbon problem or making it even worse.On the other hand, assuming future energy mixes, scenario planning will identify the full potential of new technologies. 59,110hus, the path to carbon-free energy conversion and the development and deployment of CDU will have to go hand-in-hand.

Policy and other incentives
Undeniably, CDU is a portfolio of expensive technologies, but these technologies will become more affordable following the typical economies of scale.2][113] Policies, oen local, regional, or national, will lead to a fragmented landscape where incentives at one location will promote CDU while elsewhere CDU will fail. 114An example is the production of ethanol from industrial waste gases which would be counted as a renewable fuel in the European Fig. 7 Polyurethane foam made from polycarbonates that were synthesized with CO 2 . 104Reproduced with permission by the publisher).
Union due to the Renewable Energy Directive II (REDII).In contrast, in the United States, renewable fuels have to be bio-based, and therefore this CDU ethanol production would not receive benets.On the other hand, legislation is being discussed in some US states that would create procurement requirements for low embodied carbon concrete (more below).These examples show that opportunities for CDU products can be region-and product-specic.
CDU is appealing as it provides a way to a new, truly circular carbon economy, coupled with new employment opportunities. 115,116Additionally, the United Nations Sustainable Development Goals are a suitable existing framework to assess the societal implications of CDU. 117Support for launching new industries in the form of procurement incentives or requirements has helped in the past, e.g., renewable portfolio standards for the solar energy eld.Developing state procurement policies like the Low Embodied Carbon Concrete Leadership Act (LECCLA), which is currently being considered in the New York State and New Jersey legislatures, and Buy Clean California are examples that could help drive demand and build capacity for CDU products.Carbon offset markets will eventually look for opportunities to acquire offsets, and therefore, CDU offers business options to create suitable offsets for sale.Demand and supply forces for sources and uses of CO 2 118,119 will then be expanded by these additional drivers.The rapidly growing number of corporate commitments towards carbon neutrality will increase CDU deployments since offsets alone are not sustainable.As more demand for offsets is created, availability will decrease, costs will rise, and sooner rather than later, offset opportunities from agricultural or forestry efforts as well as selling credits that are not used will be used up.
In support of developing solutions, examples include the Shopify Sustainability Fund and Stripe's Climate program which allocate and generate support to create a carbon removal market; the Carbon Capture XPrize, funded by Elon Musk; Microso's commitment to offset all their emissions retroactively to when the company was founded and then go beyond to become carbon negative; Breakthrough Energy Ventures; the Oil & Gas Climate Initiative; Amazon's Climate Pledge, and in addition Jeff Bezos's personal pledge; Unilever's commitment to stop using fossil carbon sources by 2030; similar commitments by Covestro; and so forth.Similarly, for power companies and cement plants, achieving net-zero emissions gets more and more difficult for the last 10's of percent of their carbon emissions.CDU can offer an opportunity to reach the ambitious, yet necessary, goals.
Life cycle assessments and techno-economic assessments CO 2 utilization, in contrast to CO 2 storage, has the advantage that the resulting products can, fundamentally, achieve nancial protability and sustainability with far less, if any, subsidies.The cost of CO 2 itself is a key cost factor for many CDU products, 120 but these costs vary widely across product categories 11 and based on the source of CO 2 . 9espite the urgency of developing and deploying CO 2 utilization, it is paramount that CDU practitioners comprehensively assess the impact and opportunities of new technologies to ensure environmental and economic gains will be achieved.Complete life cycle assessments (LCA) and techno-economic assessments (TEA) are essential in guiding research, development, and deployment.As an example, while CO 2 curing of concrete should provide a sink for CO 2 , the actual outcome strongly depends on the design and execution of the processes used, and in the worst case, the CO 2 footprint is higher than for traditionally made concrete. 42The production of methanol is another prominent example that illustrates the importance of assessing technologies in a context that allows comparison across production methods, available energy mixes, and other factors (Fig. 8). 88Concepts such as a carbon return on investment to evaluate CDU technology's benets will be guided by LCA. 121he entire value chain must be optimized as a system for the best environmental impactso that breakthrough technologies can be brought closer to market readiness and environmentally costly mistakes will be avoided.One is reminded of what Sir Harry Ricardo said in 1922 about direct-injection engines in that "working with a stratied charge . is possible and the high efficiency theoretically obtainable from it can be approached.The worst feature about it is that, if not just right, it may be very wrong; a small change in form or dimension may upset the whole system." 122ransparent, complete, and uniform assessments for the life cycle emissions of product production and use have to be available for rigorous and impartial evaluation for the continuation of research work, scale-up work, investment in emerging companies, and certication for possible tax consideration.
Assessments, both TEA and LCA, rarely provide a unique and straightforward answer; rather, several factors need to be considered to draw a conclusion and make decisions about continuation of research, scale-up, deployment, policy support, etc. 123,124 ISO standards 14040 and 14044 are available for LCA in general.Applying these standards for CDU, though, le key assumptions and procedures to the user's discretion.As a result, LCA results were difficult to compare against each other.][127][128][129] Building on these and other efforts, further work is underway to harmonize guidance for CDU assessments 130 and provide guidance for decision makers. 131,132g. 8 The assessed CO 2 footprint of methanol production from CO 2 strongly depends on the technologies used and the details of the assessment methods. 88(Reproduced with permission by the publisher).
Information about the environmental and economic benets and risks would ideally be available early in the research and development process.Unfortunately, the assessment of low TRL efforts suffers from considerable uncertainty in the data or even lack of required data.Therefore, LCA and TEA of low TRL processes require particular attention to produce meaningful outcomes. 133,134qually challenging is the joint interpretation of LCA and TEA if conducted separately, possibly not using shared data and/or functional units for the analysis.Therefore, combined or integrated LCA and TEA should be preferred to achieve outcomes that allow accurate interpretation and meaningful decisions for action. 135,136hile the technical, economic, and environmental feasibility of CDU product manufacturing and use is essential for deploying related technologies, social acceptance and social justice factors are critically important.Deployment of CDU technologies will, literally, be visible with large-scale installations, debates about the best use of resources, concerns about environmental factors, and more.[139][140][141][142][143][144] Digitalization Implementing a carbon economy built around CO 2 as a feedstock requires building up entirely new supply and value chains.This new carbon economy can be structured around current technological infrastructure to improve efficiency and real-time communication.In designing and operating new industrial processes, digital tools can optimize additional factors, including globally minimized CO 2 emissions, social equity, and global resources management.None of that will work without the fullest deployment of digitalization tools across the CO 2 utilization eld.Continuing enhancements and implementation of digitalization will be a technological enabler to optimize systems for minimal CO 2 emissions.The key is that reaching net-zero emissions requires fully considering any action's impact on the system response.Fully integrated digital systems will allow one to transparently track emissions, to record certication of carbon credits, carbon dividends, tax obligations, and other nancial instruments essential to managing ambitious environmental goals.
Digital tools are already being used to react to the worst impacts of climate change. 145Big data and articial intelligence are being used to respond to res, oods, and other climate disasters.They have shown their potential to save lives and minimize economic losses.However, these tools must be used for more than adaptation; climate regeneration industries must embrace digital tools in the toolkit needed to ght climate change.Among other means, articial intelligence and digitalization tools must be brought to a level of performance and reliability to accurately predict the system-level impact of proposed climate action in all aspects: climate-related, nancial or societal.

Capture of carbon dioxide
Above, many reasons and opportunities for the fast and large-scale growth of CDU were given.However, a key barrier to faster deployment of CDU is the cost of CO 2 itself.The nancial and energy cost of capture technology, both in terms of installations and operation, are still too high. 7,9,146,147Depending on the source of the CO 2 , the associated carbon footprint of capture and post-processing will vary substantially. 148][151][152][153][154][155] The present amount of captured CO 2 is on the order of 77 million tons a year, and only a small fraction of that is used in CDU processes.This is a far cry from where the activity level needs to be, based on climate models and business opportunities, as shown in Fig. 9. 156

Conclusions
The conditions for further developing, scaling, and deploying CO 2 utilization (CDU) are such that it is clear that these technologies are needed now.These technologies must rapidly be brought to market, though many questions remain.
Fundamentally, climate model predictions consistently show that CO 2 must be removed from the atmosphere at rates of 10's of gigatons per year.Even once the current excess level of CO 2 in the atmosphere has been brought back down to tolerable levels, there is a continuing need for CO 2 capture, utilization, and storage to handle emissions that cannot be avoided.Simultaneously, there will be a continued need for carbon-based products and these could be made from CO 2 .Thus, utilizing CO 2 to create economically sustainable products becomes both a source for essential products and a process to achieve and maintain a net-zero carbon economy.Its contribution is through the permanent sequestration of CO 2 into rock-like materials, such as concrete, and as an alternative source of carbon for products traditionally made from fossil carbon.Thereby, CDU avoids the addition of new CO 2 to the atmosphere and contributes substantially to a circular carbon economy.
The climate scenarios predicted by integrated assessment models show the urgency at which action is needed, prompting some to call for war-like efforts. 5At this time, CDU has the potential to become an additional 'wedge' in efforts towards a net-zero carbon economy 158 and advances are being made. 159g. 9 The current CO 2 capture capacity is far less than what IPCC projections and IEA state is required for a net-zero carbon economy. 157It is less than 1% of the amount that could be used for CDU products. 11,27uch of the work described above is enabled by and developed in parallel with the rapid increase in available carbon-free energy and growing industry that can produce and deliver hydrogen in an environmentally friendly manner.In this context, the use of energy has to follow a new paradigm, one where it is still necessary to push for the highest possible efficiencies and lowest overall energy use.However, the need to avoid net CO 2 addition to the atmosphere supersedes some of these needs.
The case has been made for CDU based on environmental, technological, and economic grounds.Avoiding a climate-related disaster requires decisive, fast, and large-scale action.Some solutions are available and new technologies are emerging, while work on additional efforts needs to intensify. 160The prospects of economic growth can be a powerful enabler.The timing might be perfect now since a growing number of current and emerging business leaders are beginning to see management as a calling in service to society. 161But much work is still needed to create public awareness and political support for CDU.

Conflicts of interest
There are no conicts to declare.

Fig. 3
Fig.3highlights how carbon dioxide utilization has found attention as an emerging eld, measured by the number of annual publications indexed under "Carbon Dioxide Utilization" by the Web of Science.The increase in research publications during the second decade of the 21 st century indicates the interest in CDU from funding agencies and the Academic Community.CDU is now being positioned towards market introduction, as observed previously in other technology elds.14,15A 2016 study described a roadmap to a trillion-dollar market, highlighting the opportunities and documenting that at the time, less than 200 entities were engaged in commercial endeavors for CO 2 utilization.11,16That number has since increased incrementally, and some technologies have now achieved a sufficient maturity level to be deployed to markets, such as the production of chemicals, CO 2 -cured concrete, and aggregates.The potential for CO 2 utilization, economically and environmentally, is demonstrated.It is paramount to accelerate the transition from research laboratories to market introduction.However, as recently stated by an industry representative, "It seems to me that there is such a canyon between what the academic world considers complete and what the industry considers emergingit is the old Valley of Death."17Measured on a technology readiness level (TRL) scale, university research readiness rarely exceeds levels 3-4, i.e., research at the level of proven feasibility and perhaps beginning technology development.Conversely, industrial interest, and that of many investors, begins at TRL 6 or higher, i.e., with pilot-sized, demonstrated technology ready to transition to full-scale engineering.This gap, the Valley of Death, is oen a barrier that causes long delays in advancing inventions towards market introduction or literally leads to the death of a technology and termination of further pursuits.This valley is especially pronounced in developments that require complex and expensive hardware installation to advance the TRL.Shared incubators and collaborative partnerships can help to

Fig. 6
Fig.6Schematic illustration of a solar refinery system for converting CO 2 and wastewater into chemicals and fuels72 (reproduced with permission from Energy in Frontiers).