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Chem Catalysis
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Deriving value from CO2: From catalyst design to industrial implementation

      In this activity article, Rafaella Buonsanti (tenure track at EPFL) and Stafford W. Sheehan (chief technology officer at Air Company) discuss research objectives and key challenges for scaling up electrochemical and thermochemical CO2 utilization.

      S.W.S. on transitioning systems and catalysts for CO2 valorization from proof of concept to commercial use

      In August 2021, the Intergovernmental Panel on Climate Change (IPCC) reported that global CO2 and greenhouse-gas-mitigation goals have not been met. Their report stated for the first time that, due to this failure, CO2 removal has become essential to limit global warming to reasonable levels.
      IPCC
      Technology development in this field is, thus, unique compared to many others in history, because the problem it aims to solve, climate change, is time sensitive and immediate. This urgency requires rapid technology implementation from fields with high fundamental research activity but low industrial maturity, specifically carbon capture, utilization, and sequestration (CCUS). Because fossil fuels are low cost and widely available, massive scale must be rapidly achieved for eco-compatible emissions-mitigation technologies. These technologies must also derive suitable value from CO2 to be economically competitive.
      Chemically upgrading CO2 with renewable energy, using H2O as a proton source, has potential for low feedstock costs and massive scale. Earth’s photosynthetic history has also shown that processes such as these, which emit only O2, are compatible with global geochemistry. However, natural photosynthesis alone cannot match the rate at which anthropogenic CO2 emissions are currently being generated, necessitating technological systems that accomplish CO2 fixation with higher rate and efficiency. Like natural systems, thermochemical or electrochemical CO2-reduction technologies must have robust catalysts and processes that provide optimal reaction conditions. Although laboratory research has defined several catalysts and reactions for electrochemical and thermochemical CO2 reduction, very few of these proof-of-concept systems have made traction toward deployment at scale. This is because there are several complex problems that are encountered when transitioning technology from the laboratory. Understanding some of the most common challenges associated with CO2-conversion catalysis can, therefore, help determine goals and set expectations for technology deployment along the timescales required by the IPCC.
      Like other industries where a byproduct or waste material is being chemically upgraded to generate additional value, CO2 valorization requires optimized utilization to economically compete. This necessitates a careful balance between catalyst productivity (which must remain high to activate relatively inert CO2), product selectivity, and carbon efficiency to minimize life-cycle CO2 emissions. In both electrochemical and thermochemical CO2-utilization systems, catalytic conditions that result in increased areal productivity, such as increased current densities or gas-flow rates, can give rise to side reactions that decrease selectivity for the desired product. If side products are utilized, that gives more operational flexibility to increase production rate and energy efficiency but can otherwise pose separation challenges. Different technological approaches and end markets warrant different approaches to optimize for throughput, energy efficiency, carbon efficiency, and/or selectivity. For example, at Air Company, process development to convert CO2 to alcohols (methanol, ethanol, and upgraded fuels) is driven by carbon efficiency and selectivity toward liquid products to ensure that all CO2 is utilized, and no carbon is vented, which are critical parameters in life-cycle analyses.
      • Sarp S.
      • Gonzalez Hernandez S.
      • Chen C.
      • Sheehan S.W.
      Alcohol Production from Carbon Dioxide: Methanol as a Fuel and Chemical Feedstock.
      Scaling up does change the conditions that catalysts require to achieve these parameters, and it is no surprise that the local chemical environment around a material can drastically change its catalytic activity, stability, and even the nature of the active species. Therefore, the importance of evaluating catalysts under industrially relevant conditions for even fundamental comparative studies cannot be stressed enough.
      • De Gregorio G.L.
      • Burdyny T.
      • Loiudice A.
      • Iyengar P.
      • Smith W.A.
      • Buonsanti R.
      Facet-Dependent Selectivity of Cu Catalysts in Electrochemical CO2 Reduction at Commercially Viable Current Densities.
      For CO2-electrolysis systems, operating at high current density under continuous flow conditions in a zero-gap electrolyzer, at relevant temperature and pressure, is a good start. Predicting and mitigating new stresses that will be introduced during scale up could continue this critical research direction. Further methods must be developed to test under system-level conditions able to mitigate buildup of compounds that block or inhibit catalysis, such as carbonate or biofilms.
      Long-term deactivation mechanisms, such as leaching of catalyst metals or changes to gas-diffusion electrodes that alter the local catalytic environment, are less studied because of focus on comparatively short-term (<200 h) testing in the field. For this reason, systems that appear to be sufficiently stable in the laboratory rarely maintain their robustness in practice. Although useful for initial material screening and generating research output in a competitive field, the focus on short-term performance might be hindering, rather than accelerating, deployment of CO2-conversion technologies in an industry where stability is crucial and should be improved.
      Measuring degradation over time trials of 1,000 h has been proposed as a key performance indicator for polymer electrolyte membrane (PEM) electrolysis for H2 generation,
      • Siegmund D.
      • Metz S.
      • Peinecke V.
      • Warner T.E.
      • Cremers C.
      • Greve A.
      • Smolinka T.
      • Segets D.
      • Apfel U.
      Crossing the Valley of Death: From Fundamental to Applied Research in Electrolysis.
      which is a comparable technology at a later stage of commercialization than most CO2 utilization technologies are. When reaching these timescales, system-level deactivation pathways and side reactions become more prominent. This suggests standards for scalability and third-party verification could be implemented, which have aided the photovoltaics research community for decades.
      • Osterwald C.R.
      • McMahon T.J.
      History of Accelerated and Qualification Testing of Terrestrial Photovoltaic Modules: A Literature Review.
      Photovoltaics experienced gradual efficiency improvements and cost reduction from a time when it was only economic on satellites in space, to the point of widespread terrestrial use, which would be a good path for CCUS to follow.
      In comparison with other applied fields in chemistry, commodity chemical-process development has occurred evolutionarily rather than revolutionarily, with iterative optimization and understanding to drive down cost rather than development of specific revolutionary compounds (as in pharmaceuticals, for example). Following that logic, it is unsurprising that despite several new materials for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) reported, there has not yet been a breakthrough catalyst that has revolutionized the electrolysis industry, and commercial PEM systems still use fundamentally the same IrO2 and Pt OER and HER catalysts that they have for decades.
      • Siegmund D.
      • Metz S.
      • Peinecke V.
      • Warner T.E.
      • Cremers C.
      • Greve A.
      • Smolinka T.
      • Segets D.
      • Apfel U.
      Crossing the Valley of Death: From Fundamental to Applied Research in Electrolysis.
      Although improving catalyst efficiency (overpotential) is a focus of research in the field, it is not a substantial pain point for commercialization if the legacy materials are both efficient and durable. Catalysts with novel properties that drive system-level efficiencies, on the other hand, have historically driven further commercial use.
      There are two chemical-production processes that utilize syngas on large scales with CO2-derived equivalents: the Fischer-Tropsch process and methanol production.
      • Sarp S.
      • Gonzalez Hernandez S.
      • Chen C.
      • Sheehan S.W.
      Alcohol Production from Carbon Dioxide: Methanol as a Fuel and Chemical Feedstock.
      ,

      Schuetzle, R., and Schuetzle, D. (2015). Catalyst and process for the production of diesel fuel from natural gas, natural gas liquids, or other gaseous feedstocks. U.S. Patent No. 9,090,831 B2.

      Highlighting the advantages of using Earth-abundant materials, typical catalysts for these processes contain only first-row transition-metal elements; for Fischer-Tropsch, this would be cobalt impregnated on aluminum oxide, and for methanol, a coprecipitated solid mixture of copper oxide, zinc oxide, and aluminum oxide. Changes in relative concentration of components, surface properties, particle dispersity, and synthetic procedures enable changes in activity, product selectivity, production rate, and process robustness. From these relatively minor alterations in activity and selectivity, new system-level efficiencies can be derived (Figure 1). For example, improved catalyst selectivity can remove costly separations
      • Greenblatt J.B.
      • Miller D.J.
      • Ager J.W.
      • Houle F.A.
      • Sharp I.D.
      The technical and energetic challenges of separating (photo) electrochemical carbon dioxide reduction products.
      or refining steps,

      Schuetzle, R., and Schuetzle, D. (2015). Catalyst and process for the production of diesel fuel from natural gas, natural gas liquids, or other gaseous feedstocks. U.S. Patent No. 9,090,831 B2.

      and improvements in per-pass conversion decrease the cost of immediate downstream separation of product from reactant gases (or electrolyte in CO2 electroreduction). New commercial technologies in these fields focus on process efficiencies that drive significant reductions in production cost and accelerate implementation of CO2-valorization technology.
      Figure thumbnail gr1
      Figure 1Example of improved catalysts driving system-level process efficiencies
      (A) High-level process flow diagram for a typical thermochemical CO2-to-liquids system employing H2O electrolysis.
      (B) A diagram of an equivalent system with a more selective and impurity-resistant catalyst-enabling process simplification with fewer major components. Components colored in green also have reduced thermal-energy requirements in the improved system, further lowering operational costs.

      R.B. responds: Advocating for capital investments in new technologies from industry giants

      Indeed, CCUS technologies will play a big role toward putting the entire world on a path to zero emissions. Implementing these technologies on large scales must be regarded as an urgent need for our society. The developmental stage of CO2-utilization technologies is not as advanced as that of CO2 capture and storage, yet accelerating it is essential to move toward a carbon-neutral and, ultimately, carbon-negative global cycle.
      As mentioned above, thermal and electrochemical conversion emerge as interesting strategies. Thermal CO2 conversion requires high temperatures and hydrogen, which must eventually come from water electrolysis. Earth-abundant and more efficient catalysts are still required. Nevertheless, this technology has been already deployed at industrial scale. CO2-to-methanol conversion using renewable electricity is a focus for rapid industrialization by several organizations, for example, Carbon Recycling International in Iceland powers a heterogeneous catalytic methanol process with geothermal energy. On the other hand, the electrochemical reduction of CO2 holds promise to convert CO2 while also storing energy from renewables in chemical bonds in a single step, which makes it ultimately more attractive. However, a huge amount of fundamental research is still needed to demonstrate its viability. For example, the discovery of catalysts that are more active, selective, and stable is an important topic in academic research, especially as it regards to transforming CO2 into highly energy-dense long-chain hydrocarbons and alcohols.
      In the meantime, catalysts that can convert CO2 to CO with high selectivity and decent stability exist. Thus, one question is, why hasn’t electrochemical CO2 reduction been implemented on large scales, apart from the efforts of a handful of small startups? Why don’t industry giants in the chemicals, petrochemicals, and energy sectors, together with the public sector, increase capital investments and later-stage development in this and other CO2-utilization technologies rather than waiting for startups and academic laboratories to find a solution to all these technical problems? Only working together will enable faster progress towards a greener and more sustainable future.

      Declaration of interests

      S.W.S. has several patents on CO2-conversion technologies and is an employee and shareholder of Air Company, which has a financial interest in these patents.

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