The technological and economic prospects for CO2 utilization and removal

Authors:

Cameron Hepburn, Ella Adlen, John Beddington, Emily A. Carter, Sabine Fuss, Niall Mac Dowell, Jan C. Minx, Pete Smith & Charlotte K. Williams

Abstract:

Carbon dioxide utilisation – making valuable products from CO2 – is a potential way to lower the net costs of reducing emissions or of removing carbon dioxide from the atmosphere.  We review the economic and technological prospects of ten such pathways to assess scale and cost prospects in 2050. Using CO2 in chemicals, fuels, and via microalgae to make products, comprise ‘cycling’ pathways: they might reduce carbon dioxide emissions by displacing fossil fuel derived CO2 but they have limited potential for carbon dioxide removal.  Some CO2 chemicals, such as CO2 based polymers, are profitable in the present day, but CO2 based fuels are high up on the cost curve. ‘Closed’ pathways such as those involving construction materials can both utilise and remove carbon dioxide for the long term. They might be low cost, and the end-markets are large, but they have high regulatory barriers to scale.  Land-based CO2 utilisation pathways such as soil carbon sequestration, afforestation/reforestation and biochar can increase agricultural output and remove carbon dioxide. They can be characterised as ‘open’ pathways wherein the CO2 can return to the atmosphere easily, and they are relatively low cost. Using a process of structured estimation and an expert opinion survey, our assessment suggests that each of the ten pathways could scale to over 0.5 Gt carbon dioxide utilisation annually, although barriers remain substantial and resource constraints prevent the simultaneous deployment of all pathways.  Uncertainty over scaling means that there is a wide range of potential outcomes for 2050. Notwithstanding the many caveats, the potential scale of utilisation could be considerable. Much of this potential CO2 utilisation – notably in ‘closed’ and ‘open’ pathways – may be economically viable without dramatic shifts in prices. The specific assumptions of the low scenario imply an upper bound of over 1.5 Gt CO2 yr-1 at well under $100/t CO2u. For policymakers interested in climate change, these figures demonstrate the theoretical potential for correctly designed policies to incentivise the displacement of fossil fuels or the removal of CO2 from the atmosphere.  

 

 

Fixing the Climate? How Geoengineering Threatens to Undermine the SDGs and Climate Justice

Author: Linda Schneider

Abstract:

In the early 21st century, the world faces multiple existential challenges and global crises – among them is the climate crisis, but equally dramatic – and intimately related – are the escalating loss of biodiversity and natural ecosystems, growing social inequality, human rights violations, poverty and hunger, concentration of wealth, power and control, and authoritarian state tendencies. 

The international community adopted the Sustainable Development Goals (SDGs) in 2015, which aim to, among other goals, end poverty, hunger, reduce inequality, protect life on land and achieve gender equality, peace and justice. The framework of climate justice, promoted by grassroots social movements around the world, highlights the crucial role of global environmental and social justice in addressing the climate crisis – rather than treating it as a purely technical problem.

Both the SDGs at the international-institutional level, and the call for climate justice from the grassroots and social movement level, serve as a framework for addressing the interrelated crises of the 21st century in an integrated fashion. 

In the wake of the Paris Agreement, however, and with global emissions still on the rise, one set of alleged responses to the climate crisis is pushing to the front in international climate policy discussions. 

Geoengineering—large-scale technological interventions in the Earth’s natural processes and ecosystems are being promoted to counteract or delay some of the symptoms of climate change.

In the present paper I argue that geoengineering – both large-scale Carbon Dioxide Removal and Solar Radiation Management, the two main categories of climate geoengineering – are fundamentally at odds with the SDGs and with the strive for climate justice. In fact, they threaten to undermine the achievement of SDGs and climate justice for three broad reasons that I develop in the present article.

First, geoengineering is bound to exacerbate many of the other socio-ecological and socio-economic global crises we are facing. I show this for several of the proposed geoengineering technologies on land, in the oceans and in the atmosphere and how they would detrimentally affect the SDGs and the strive for climate justice. 

Furthermore, I elaborate on how geoengineering would deepen societal dependence on large-scale technological systems and on technocratic elites controlling them. One of the effects of such technological dominance in trying to solve global problems is that alternative and more holistic forms of knowledge, expertise, and practices that allow for complexity, diversity and resilience are excluded. By the same token, concerns over ecological and social risks and rights violations are pushed aside. Such one-dimensional large-scale technological fixes for the climate crisis run counter the aim of achieving global ecological and social justice and redressing past unjustness.

Finally, rather than overcoming the economic and political power structures that have caused the climate crisis in the first place, they create new spaces for profit and power for new and old economic elites and thereby serve to uphold the current status quo. As such, it is fundamentally incompatible with the notion of climate justice and will make it impossible to achieve the SDGs.

Southern Ocean Iron Fertilization: An Argument Against Commercialization but for Continued Research Amidst Lingering Uncertainty

Author: Tyler Rohr

Affiliations:

Current:

United States Department of Energy, Water Power Technologies Office, Washington D.C., USA

Past (but at time of original writing):

Massachusetts Institute of Technology, Department of Earth and Planetary Sciences, Cambridge, Massachusetts, USA 

Woods Hole Oceanographic Institution, Department of Marine Chemistry and Geochemistry, Woods Hole, Massachusetts, USA 

Abstract:

With substantive global action on climate change mitigation sputtering, some have begun to look at geoengineering as a possible alternative. Geoengineering is any deliberate, large-scale, manipulation of natural processes to affect the climate system, ostensibly to curb the effects of global warming. One such strategy is Southern Ocean Iron Fertilization, which seeks to remove carbon dioxide from the atmosphere by fertilizing the growth of marine phytoplankton in the ocean. Phytoplankton are microscopic plants that, like land-based plants, convert carbon dioxide into oxygen through photosynthesis. Together, marine phytoplankton produce half the oxygen we breathe. Further, when they die, they can sink out of the surface ocean and trap carbon in the deep ocean for hundreds of years. The goal of Southern Ocean Iron Fertilization is to increase this oceanic drawdown of carbon dioxide by stimulating marine phytoplankton growth by adding iron, an important nutrient, to large parts of the ocean where there is currently not enough iron to support growth. The notion of simply dumping dissolved iron into the ocean is seductively cheap compared to other forms of climate action, but it is not yet clear that fertilizing the Southern Ocean could actually create a sustainable carbon sink. A comprehensive review of the literature reveals that iron fertilization will almost certainly increase phytoplankton growth in the surface ocean, but it is much less clear how much of that carbon will sink out of the surface ocean and stay there versus how much will quickly end up back in the atmosphere. Given the lingering scientific uncertainty, it would be ill-advised to commercialize iron fertilization under emerging carbon offset markets. Offset markets allow companies or individuals to buy and sell activity that offsets their own emissions, either by sequestering carbon or reducing emissions elsewhere. Compliance offset markets, such as many proposed cap-and-trade frameworks, are carefully regulated to ensure activity is legitimate and appropriately compensated; however, voluntary offset markets, powered predominately by a sense of social responsibility, are not necessarily well regulated. Commercializing Southern Ocean Iron Fertilization on compliance offset markets is unlikely, and unadvisable, due to concerns over the strategy’s fundamental feasibility, the potential for unpredictable side-effects, and the inability to establish reliable baselines or accurately measure the full effects of fertilization, making it impossible to provide fair and consistent compensation. Never-the-less, recent history shows that fertilization activity on unregulated voluntary offset markets motivated by a the promise of an easy fix can, and will continue to, emerge. Continued research is needed to constrain the public perception and clarify the reality of an iron bullet.

 

Figure Caption: Idealized schematic of carbon cycling and the biological  in a natural High Nutrient Low Chlorophyll Region (HNLC) and an iron fertilized HLNC. White arrows represent carbon transport. The addition of iron may dramatically increase surface biomass but only a small fraction of that is additional sequestered in the deep ocean or the sea floor.

Link: http://www.sciencepolicyjournal.org/uploads/5/4/3/4/5434385/rohr_jspg_v15.pdf

Decision making in contexts of deep uncertainty – an alternative approach for long-term climate policy.

Authors: Mark Workmana,b, Kate Dooleyc,Guy Lomaxd, James Maltbye, Geoff Darchf,

  1. Visiting Researcher, Energy Futures Lab, Imperial College London, South Kensington, London SW7 2AZ, UK
  2. Corresponding Author available on mark.workman07@imperial.ac.uk 
  3. University of Melbourne, School of Geography, Parkville, Melbourne 3010, Australia
  4. The Nature Conservancy, 26-28 Ely Place, London, EC1N 6TD
  5. Defence Scientific Technology Laboratory (DSTL), Portsdown West Fareham PO17 6AD
  6. Anglian Water, Block C – Western House, Peterborough Business Park, Lynch Wood, Peterborough PE2 6FZ

Key Words: 

International Climate Policy | Carbon Dioxide Removal Technologies | Integrated Assessment Modelling | Predict then Act | Robust Decision Making | Pluralistic Approaches | Diversity in value-sets

Highlights: 

  • Integrated Assessment Models (IAMs) have been used to inform international climate policy development.
  • There is a strong tendency to view IAMs as providing objective analysis, however, they are developed within a very small, narrow community and can heavily distort decision making processes.
  • The largescale (Billions of tonnes pa) reliance of IAM scenarios on Carbon Dioxide Removal (CDR) is one such distortion; this is problematic for a number of reasons.
  • Most worrying, dependence on CDR is being baked into international emissions targets without a public debate.
  • This is likely to lead to polarisation as a function of the trade-offs and side-effects of large-scale CDR deployment; this is likely to hinder progress on CDR and alternative mitigation strategies.
  • Approaches to allow diversity in value-sets in climate policy making is essential – Robust Decision Making Approaches is provided as an exemplar to accommodate this.

Abstract

Scientists have produced a significant body of analysis detailing how we might mitigate so as to avoid dangerous levels of global warming. The dominant analytical tools are integrated assessment models (IAMs), which represent the world’s energy, agricultural and land systems over a time period spanning from the present to (most commonly) the end of the 21st century. As noted in recent high-profile commentaries, these models do not consider – or at least have not been used to explore – how unexpected disruptions could impact either positively or negatively on carbon policy efforts.  Most importantly they are constructed by a narrow elite with limited engagement with the publics and heavily distort decision making in climate policy development.  As the world struggles with increased complexity and uncertainty, there is the need to introduce the process of systematically imagining alternative, sustainable futures that should be conducted in a more democratic and inclusive manner. As distrust in institutions grows and societies become more polarised, there is the need to explore alternative and emerging methods that are being used to bring communities together to debate and create collective visions for the futures they desire. 

One of the most high profile distortions in climate policy development has been that the majority of global emissions scenarios compatible with holding global warming to less than 2°C depend on the large-scale use of bioenergy with carbon capture and storage (BECCS) of up to 15 billion tons pa – to compensate for an overshoot of atmospheric CO2 concentrations. Recent critiques have highlighted the ethical and environmental risks of this strategy – The scale of carbon removal deployment would rival the worlds’ largest industries and sectors such as Oil and Gas and Agriculture.  At present less than a few thousand tonnes of carbon dioxide are removed annually.

In this paper, we critically examine both the use of BECCS in mitigation scenarios and the decision-making philosophy underlying the use of integrated assessment modelling to inform climate policy. We identify a number of features of integrated assessment models that favour selection of BECCS over alternative strategies. However, we argue that the deeper issue lies in the tendency to view model outputs as objective science, capable of defining “optimal” goals and strategies for which climate policy should strive, rather than as exploratory tools within a broader policy development process. Effectively, a model-centric decision-making philosophy is highly sensitive to uncertainties in model assumptions and future trends, and tends to favour solutions that perform well within a narrow modelled framework at the expense of exploring a wide and diverse mix of strategies and values.

Drawing on the principles of Robust Decision Making, we articulate the need for an alternative approach that explicitly embraces uncertainty, multiple values and diversity among stakeholders and viewpoints, and in which modelling exists in an iterative exchange with policy development rather than separate from it. Such an approach would provide more relevant and robust information to near-term policy-making, and enable appropriate goals for climate policy in a given context being agreed and defined by dialogue between multiple stakeholders rather than ex ante by a single community.

 

Estimating economic and environmental benefits of urban trees in desert regions

Authors: Rima J. Isaifan1, Richard W. Baldauf2

1Division of Sustainable Development (DSD), Hamad Bin Khalifa University (HBKU) / Qatar Foundation (QF), Education City, P.O. Box 5825, Doha, Qatar.

2Office of Research and Development, United States Environmental Protection Agency, The United States

Full Citation: Rima J. Isaifan, Richard W. Baldauf, Estimating economic and environmental benefits of urban trees in desert regions, Frontiers in Ecology and Evolution, in press, doi: 10.3389/fevo.2020.00016

Abstract: 

Air pollution in urban areas has drawn significant attention from researchers, urban planners and policy makers due to the rapid increase in industrial, transportation and construction activities and its link with adverse human health effects. Several mitigation measures have been proposed recently including planting trees for abatement of air pollution. Vegetation can remove particulate matter (PM) and gaseous pollutants by deposition and absorption through leaves and branches.  Trees also convert carbon dioxide into oxygen through the process of photosynthesis. In urban areas, vegetation has also been reported to have significant positive impacts on raising property values, intercepting storm water runoff and improving air quality. Moreover, trees provide various ecosystem services in urban contexts such as the regulation of temperature by providing shading and thermal comfort. 

Despite these economic and environmental benefits, few studies have considered the efficiency of different urban tree species to reduce air pollution in desert cities experiencing high PM concentrations due to anthropogenic and natural sources along with providing other positive ecosystem services. In general, the ability of trees to improve air quality varies because of different plant characteristics. The effect of leaf surface structure, for instance, has been shown to have a major influence on PM capture and removal.

In this article, we estimated the economic and environmental benefits of three tree species typical for desert regions: Acacia tortilis, Ziziphus spina-christi and Phoenix dactylifera. The benefits varied by species with Acacia tortilis having the highest overall benefits, mostly because of its large leaf surface area and canopy shape. Mature trees tended to be more beneficial than smaller, young trees for improving environmental conditions. The location of trees had minimal impact on the overall economic value when considering regional benefits, although locations close to air pollution sources may provide additional air quality benefits. This assessment provides urban planners, foresters, and developers in desert regions with information needed to make informed decisions on the economic and environmental benefits of urban tree planting.

Annual CO2 reduction by trees in urban desert areas. Note: “M” indicates mature trees with trunk diameter of 45 in. or greater, “Y” indicates young tress with trunk size of less than 10 in, “P Dact” indicates Phoenix Dactylifera [the article].

 

From Zero to Hero?: Why Integrated Assessment Modeling of Negative Emissions Technologies Is Hard and How We Can Do Better

Authors: Jay Fuhrman1, Haewon McJeon2, Scott C. Doney3, William Shobe4 and Andres F. Clarens1*

1Department of Engineering Systems and Environment, University of Virginia, Charlottesville, VA, United States

2Joint Global Change Research Institute, University of Maryland and Pacific Northwest National Laboratory, College Park, MD, United States

3Department of Environmental Sciences, University of Virginia, Charlottesville, VA, United States

4Batten School of Leadership and Public Policy, University of Virginia, Charlottesville, VA, United States

Full citation: Fuhrman, Jay, Haewon McJeon, Scott C. Doney, William Shobe, and Andres F. Clarens. “From Zero to Hero?: Why Integrated Assessment Modeling of Negative Emissions Technologies Is Hard and How We Can Do Better.” Frontiers in Climate 1 (2019): 11. https://doi.org/10.3389/fclim.2019.00011.

Abstract: Integrated Assessment Models (IAMs) of the Earth’s economic and climate system increasingly rely the presumed future ability to achieve negative emissions in order to limit global warming to “well below 2 C” by 2100.  The scales at which these models project so-called “negative emissions technologies” (NETs) to be deployed rivals that of our current (positive) emissions.  There are a number of ways in which we could in theory remove CO2 from the atmosphere, all of which have their own set of potential synergies and tradeoffs with other goals for sustainable development. Yet the vast majority of scenarios assume the availability of just two NETs: bioenergy with carbon capture and storage, and afforestation. Just as the impacts of climate change itself will fall disproportionately on the developing world, IAM results suggest that so too, would the impacts of removing already-emitted CO2 from the atmosphere using these methods. In all regions of the world but especially Asia, Latin America, and Africa, enormous amounts of land would need to be converted to bioenergy crop cultivation or managed forest, with profound implications for food security and biodiversity.  Only a few recent studies have incorporated direct air capture, a fully engineered process previously thought too expensive to be viable but now receiving increasing attention. Other NETs (e.g., coastal wetlands restoration, accelerated weathering) have largely been excluded from IAM scenarios because they lack obvious connections with existing economic sectors. Our analysis finds that more complete treatment of NETs by IAMs could highlight substantial opportunities for more limited, sustainable deployment now, provided the appropriate policy incentives. But modeling results suggesting large-scale future deployment of NETs should be communicated to and interpreted by policymakers and other stakeholders as warnings of the potential impacts of the NETs themselves, rather than prescriptive licenses to delay taking action and attempt to reverse the damage later.

Read the full article in Frontiers in Climate here. 

 

 

Jacobson & Sanchez (2019). Opportunities for Carbon Dioxide Removal Within the United States Department of Agriculture

Authors: Rory Jacobson1, Daniel L. Sanchez2

1Yale School of Forestry and Environmental Studies, New Haven, Connecticut, United States

2Department of Environmental Science, Policy, and Management, University of California, Berkeley, Berkeley, California, United States

Full citation: Jacobson, R., & Sanchez, D. L. (2019). Opportunities for carbon dioxide removal within the United States Department of Agriculture. Frontiers in Climate1, 2.

Abstract: Over the next century, atmospheric carbon dioxide removal (CDR) will be necessary to limit the most significant impacts of catastrophic climate change. Nonetheless, many CDR technologies and land management strategies currently remain at the research and development stage, with minimal capacity for commercialization and deployment. Moreover, farming, ranching, and forestry communities in the United States sit at the front lines of climate change impacts and responses. Fortunately, some of the most inexpensive and promising opportunities for CDR deployment exist within the ability of managed ecosystems, agricultural soils, and forest biomass and products to sequester carbon dioxide. In particular, terrestrial atmospheric carbon dioxide removal (CDR) can reduce the most extreme impacts of climate change while increasing resilience to extreme weather. 

Currently, many CDR technologies and land management strategies are still at the research and development (R&D) phase, and lack sufficient federal support to reach widespread deployment. Here, we summarize the United States Department of Agriculture’s (USDA) previous and ongoing efforts to incorporate CDR relevant R&D within their research, education, and economics mission, as well as opportunities to refocus and expand existing programs. Potential future actions to expand CDR R&D capabilities include: 1) the establishment of a new extramural research agency and a new intramural technology commercialization program within USDA, 2) improved coordination between the Foundation for Food and Agriculture (FFAR) and USDA, 3) improved intra-agency and inter-agency coordination, and 4) congressional action to establish and fund new CDR programs within USDA. Finally, we provide an assessment of how changes in funding through legislative actions could support USDA’s leadership on applied RD&D for land-use CDR.  

We find that new agencies and programs, such as an Advanced Research Projects Agency for Agriculture and an Agriculture “I-Corps” technology incubator and entrepreneurial training program, could provide breakthrough advances for land-use CDR at relatively low costs. Additionally, we find that many USDA agencies and offices are well suited and equipped to incorporate the National Academies of Sciences, Engineering, and Medicine’s (NASEM) recommended research programs for carbon sequestration in managed and agricultural lands. In order to implement many of these recommended programs, congressional action will be necessary in order to increase funding to certain agencies or provide explicit direction through appropriations report language. Since all existing USDA programs and agencies are not necessarily well equipped to carry out research on specific land-use CDR research topics, in some cases new programs will also need to be developed in order to execute the recommended research projects. Still, some land-use CDR RD&D activities will require external collaboration or partnerships with existing federal departments or external foundations in order to optimize the effectiveness of USDA research. Specifically, we recommend that for certain projects, USDA should partner and/or more effectively coordinate and collaborate with the Department of Energy and the Foundation for Food and Agriculture.

Nonetheless, we argue that many USDA offices and agencies are well positioned to pursue many land-use CDR R&D projects identified and detailed by NASEM as crucial to scaling and commercializing CDR technologies and land management strategies. Although we find many existing agencies will require some expansion, modification, or increased support, it is evident that USDA will be crucial to, and play a major role in, mitigating emissions from the agricultural sector and increasing carbon sequestration across the United States.  

Read Jacobson and Sanchez’s complete paper in Frontiers in Climate.

Low & Schäfer (2019). Is bio-energy carbon capture and storage (BECCS) feasible? The contested authority of integrated assessment modeling

Authors: Sean Low and Stefan Schäfer

Full Citation: Low, S., & Schäfer, S. (2020). Is bio-energy carbon capture and storage (BECCS) feasible? The contested authority of integrated assessment modeling. Energy Research & Social Science60, 101326.

Abstract: How are novel energy, technology, and land-use systems strategies for limiting climate change judged to be ‘feasible’? Controversy has arisen around the research community behind the integrated assessment modeling (IAM) scenarios used in IPCC Assessment Reports. This regards the role played by bioenergy carbon capture and storage (BECCS), an unproven component in projected energy systems that allows IAMs to achieve ambitious temperature targets since adopted by the Paris Agreement. Our study inquires after the lodestone of that controversy: how the ‘feasibility’ of BECCS is understood, calculated, and communicated. Why does this matter? ‘Feasibility’ is an amorphous concept contested by the creators, critics, and users of IAM scenarios, and in those contestations, we witness how the expertise and authority held by the IAM community is being challenged. We find that judgements of BECCS’s feasibility are proxies for wider worldviews, tied up with different understandings of the freedom of scientific inquiry in policy-driven assessments, the proper relationship between science and policy, and the shape of appropriate science communication. In turn, these understandings underpin different ideas for the reform of IAM assessments, with repercussions for how future mitigation strategies will be mapped—and perhaps, executed.

Read Low and Schäfer’s full paper in Energy Research & Social Science.

Galina et al. (2019). Evolution of Carbon Capture and Storage by Mineral Carbonation: Data Analysis and Relevance of the Theme.

Authors: Author links open overlay panelNatália Ribeiro Galina, Gretta L.A.F. Arce, & Ivonete Ávila.

Full citation: Galina, N. R., Arce, G. L., & Ávila, I. (2019). Evolution of carbon capture and storage by mineral carbonation: Data analysis and relevance of the theme. Minerals Engineering142, 105879.

Abstract: Given the increased concentration of atmospheric carbon dioxide in the atmosphere due to the use of fossil fuels, the development of technologies to reduce emissions and negative effects of CO2 concentration is of crucial importance. Among all CCSU methods, mineral carbonation is a promising technology in which CO2 is transformed into a thermodynamically and environmentally stable carbonate. Thus, a recent analysis of the evolution of carbon capture research by mineral carbonation has been performed by the Laboratory of Combustion and Carbon Capture (LC3) at São Paulo State University (UNESP/BRAZIL). An article entitled “Evolution of carbon capture and storage by mineral carbonation: Data analysis and relevance of the theme” presents data on the evolution of mineral carbonation research over the last 15 years by analyzing the number of publications and citations concerning mineral carbonation research. This paper aimed to identify the main advances and technological challenges of CO2 capture by mineral carbonation, in which a critical analysis of 117 articles published between 2002 and 2018 has been performed. The results demonstrate the evolution of scientific interest and continued relevance of the theme in recent years by academic and industrial sectors. The analysis results reveal that the development of carbon capture technologies is vital to reduce CO2 emissions, once mineral carbonation technology is a promising route to reduce CO2 footprint of industrial processes in a near future. It was observed that the number of publications and citations have increased proportionally in the latest 7 years. The article also presented an analysis of the number of publications and lines of research according to sector. From the analysis of publications in different countries, it was possible to observe the most promising raw materials and carbonation routes in each region under analysis. In order to identify the most relevant research on mineral carbonation, the articles analyzed have been classified according to the number of citations and the 10 most cited articles were analyzed. Finally, the article presents the main technical and economic challenges to be overcome in order to enable the implementation of mineral carbonation processes on an industrial scale, as those that increase the efficiency of converting CO2 into carbonates and reduce its overall process budget.

Read Galina et al.’s complete paper in Minerals Engineering.

Lin, M. et al. (2019). Clearing Up the ‘Dark Side’ of Artificial Leaves.

Authors: Meng Lin, Lihao Han, Meenesh R. Singh, Chengxiang Xiang.

Full citation: Lin, M., Han, L., Singh, M. R., & Xiang, C. (2019). An Experimental-and Simulation-Based Evaluation on the CO2 Utilization Efficiency in Aqueous-based Electrochemical CO2 Reduction Reactors with Ion-Selective Membranes. ACS Applied Energy Materials.

Abstract: While artificial leaves hold promise as a way to take carbon dioxide — a potent greenhouse gas — out of the atmosphere, there is a “dark side to artificial leaves that has gone overlooked for more than a decade,” according to Meenesh Singh, assistant professor of chemical engineering in the University of Illinois at Chicago College of Engineering.

Artificial leaves work by converting carbon dioxide to fuel and water to oxygen using energy from the sun. The two processes take place separately and simultaneously on either side of a photovoltaic cell: the oxygen is produced on the “positive” side of the cell and fuel is produced on the “negative” side.

Singh, who is the corresponding author of a new paper in ACS Applied Energy Materials, says that current artificial leaves are wildly inefficient. They wind up converting only 15% of the carbon dioxide they take in into fuel and release 85% of it, along with oxygen gas, back to the atmosphere.

“The artificial leaves we have today aren’t really ready to fulfill their promise as carbon capture solutions because they don’t capture all that much carbon dioxide, and in fact, release the majority of the carbon dioxide gas they take in from the oxygen-evolving ‘positive’ side,” Singh said.

The reason artificial leaves release so much carbon dioxide back to the atmosphere has to do with where the carbon dioxide goes in the photoelectrochemical cell.

Graphic depicting the mechanics of an artificial leaf.
Artificial leaf with membrane that reduces the release of carbon dioxide back into the atmosphere. Credit: Aditya Prajapati and Meng Lin

When carbon dioxide enters the cell, it travels through the cell’s electrolyte. In the electrolyte, the dissolved carbon dioxide turns into bicarbonate anions, which travel across the membrane to the “positive” side of the cell, where oxygen is produced. This side of the cell tends to be very acidic due to splitting of water into oxygen gas and protons. When the bicarbonate anions interact with the acidic electrolyte at the anodic side of the cell, carbon dioxide is produced and released with oxygen gas.

Singh noted that a similar phenomenon of carbon dioxide release occurring in the artificial leaf can be seen in the kitchen when baking soda (bicarbonate solution) is mixed with vinegar (acidic solution) to release a fizz of carbon dioxide bubbles.

To solve this problem, Singh, in collaboration with Caltech researchers Meng Lin, Lihao Han and Chengxiang Xiang, devised a system that uses a bipolar membrane that prevents the bicarbonate anions from reaching the “positive” side of the leaf while neutralizing the proton produced.

The membrane placed in between the two sides of the photoelectrochemical cell keeps the carbon dioxide away from the acidic side of the leaf, preventing its escape back into the atmosphere. Artificial leaves using this specialized membrane turned 60% to 70% of the carbon dioxide they took in into fuel.

“Our finding represents another step in making artificial leaves a reality by increasing utilization of carbon dioxide,” Singh said.

Earlier this year, Singh and colleagues published a paper in ACS Sustainable Chemistry & Engineering, where they proposed a solution to another problem with artificial leaves: current models use pressurized carbon dioxide from tanks, not the atmosphere.

He proposed another specialized membrane that would allow the leaves to capture carbon dioxide directly from the atmosphere. Singh explains that this idea, together with the findings reported in this current publication on using more of the carbon dioxide captures, should help make artificial leaf technology fully implementable.

Read Meng et al.’s complete paper in ACS Applied Energy Materials