One promising strategy for mitigating the environmental effects of CO2 emissions is to convert CO2 via electrochemical reduction into high value-added molecules such as methanol and ethylene. These commercially valuable products could help ensure the economic viability of deploying carbon capture and conversion technologies to offset CO2 emissions, while simultaneously reducing our reliance on new fossil sources to produce fuel and petrochemicals. To improve the efficiency and economic viability of CO2 electrolysis, however, various components of the electrolyzer still need to be optimized, including the catalysts. Recent work by CIFAR fellows in the Bio-inspired Solar Energy program demonstrates some of the latest advances in this exciting arena, and points to progress that still needs to be made in order to develop commercially viable CO2 electrolytic systems.
Why this matters
In order for CO2 electrolysis to truly make an impact on emissions reduction and carbon-neutral/negative fuel, electrolytic systems must work at an industrially relevant scale. Part of the challenge is to design electrocatalysts that can efficiently carry out the reduction reaction with high selectivity (such that the desired product is preferentially formed, rather than competing products such as H2) and high current density (and thus high reaction rate). Additionally, many traditional electrocatalysts are based on precious metals such as Au, Ag or Pt, which increase the cost of CO2 reduction reactions (CO2RR). The work summarized in this brief explores various ways of optimizing CO2RR catalysts, and uses a variety of in situ and in operando characterization techniques to better understand the structure of catalysts as well as the chemical reactions happening at the catalytic surface. These insights can inform the better design and tuning of catalysts to help CO2 electrolysis achieve commercial viability.
- To improve the efficiency, selectivity and stability of CO2RR electrocatalysts, researchers have been experimenting with a variety of methods to tune the structure and chemistry of these catalysts. In a recent review article published in Science Advances, Haotian Wang and colleagues presented an overview of some of these approaches as applied to Cu-based catalysts. These strategies include: adjusting the ratio of Cu to other metals (e.g., Au, Ag, Pt and Pd) and their atomic arrangement in alloy materials; changing the oxidation state of Cu catalysts to create positively-charged sites that are resistant to reduction; using Cu nanocrystals of different sizes and shapes (and thus the amount of reactive “edge sites”), or Cu meshes of different pore sizes and depths to control the retention of reactants and intermediates; adding small molecules such as amino acids to stabilize reaction intermediates; and using defects in Cu crystal structure, such as vacancies and heteroatom dopants, to modulate reaction selectivity. Further research will be needed to increase the stability of Cu catalysts in aqueous reaction conditions.
A better understanding of the thermodynamics of the reaction that occurs at the site of electrocatalysis is crucial for the rational design of catalysts. In a preprint in ChemRxiv, Yogesh Surendranath and colleagues report on experiments employing a technique known as in-situ surface-enhanced infrared absorption spectroscopy (SEIRAS) to investigate the adsorption of CO — an important intermediate and product of CO2RR — on electrodes with Au or Cu catalysts under electrochemical conditions. The experiment shows how it is more energetically favourable for CO to stay close to a Cu-electrode surface than a Au-electrode, which in the former case allows the CO to be further reduced to hydrocarbons (such as CH4) and oxygenates. Importantly, the results demonstrate the importance of taking into consideration the interactions with reaction intermediates, solvent and electrolyte ions when designing CO2RR electrocatalysts.
- Rather than trying to optimize catalysts based on precious metals, an alternative approach is to use inexpensive molecular catalysts — macrocyclic organic molecules coordinated to earth-abundant transition metals. In an article in Science, Curtis Berlinguette and colleagues report on using cobalt phthalocyanine (CoPc) as a catalyst for CO2 to CO electroreduction. Using a flow reactor with a gas diffusion electrode, the researchers were able to achieve >98% selectivity and a Faradaic efficiency of >95% at a current density of up to 150 mA/cm2. Additionally, in another study reported in Nature Communications, Berlinguette and some of the same researchers showed that by adding one trimethyl ammonium and three tert-butyl groups to the CoPc molecule, this modified catalyst is able to carry out selective and efficient reaction across a pH range from 4 to 14. In particular, in basic conditions, the system produces CO at 94% selectivity at a current density of 165 mA/cm2. The electrolyzer’s performance remained stable over more than 10 hr, and using a variety of X-ray absorption spectroscopy techniques such as X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), the researchers demonstrated that the catalysts retained chemical and structural integrity over this period. Together, these two studies demonstrate the potential for using molecular catalysts for CO2RR at commercially-relevant scale, and that the performance of these catalysts can be rationally tuned with chemical modifications to their molecular structures.
- In contrast to molecular catalysts immobilized on inert electrodes, another approach to integrating catalysts and electrodes that could allow for higher catalyst loading, and thus higher current density, is to use molecular catalysts as building blocks for extended, porous, crystalline structures called reticular frameworks, such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs). In an article in the Journal of the American Chemical Society, Peidong Yang and his colleagues report a MOF catalyst based on CoPc catechol subunits (named MOF-1992∙[Fe]3), which displays electron transport properties throughout the framework and has highly accessible catalytic sites. The researchers used single crystal X-ray diffraction analysis to obtain the crystal structure of MOF-1992∙[Fe]3, and a variety of techniques including Mössbauer spectroscopy, inductively coupled plasma atomic emission spectroscopy (ICP-AES) and XPS to elucidate its chemical formula. In electrochemical tests, a MOF-1992 cathode catalyzed CO production at a partial current density of 16.5 mA/cm2, and retained structural integrity for at least 6 hrs. This represents among the best reported performance yet for CO2 electrocatalysts based on reticular frameworks, and points at a possible direction for further optimization.
- Some researchers are taking inspiration from naturally occurring catalysts — biological enzymes — in order to further optimize CO2 electrocatalysts. In a preprint in ChemRxiv, Christopher Chang and colleagues take such a bio-inspired approach to design a new tetrapyridyl-Fe catalyst that functions stably in aqueous conditions. Building on previous work in devising electrocatalysts with a multidentate polypyridine scaffold that provides an electron reservoir and stabilizes a transition metal active site, the researchers adopted certain elements from the carbon monoxide dehydrogenase enzyme, which carries out the reductive CO2 fixation reaction in addition to CO oxidation. Specifically, by adding a Lewis acidic moiety (in this case, an ethylamine group) to the second coordination sphere of the reactive Fe in 6-(1,1-di(pyridin-2-yl)ethyl)-2,2’-bipyridine, the authors created an electrocatalyst that selectively produces CO from CO2 with a Faradaic efficiency of 81%, and remains stable after 12 hr of electrolysis.
- Building on the work above, researchers in the Bio-inspired Solar Energy program have identified a number of key questions or research directions that may lead to better catalysts for CO2RR:
○ Continue to optimize the performance of metallic catalysts by modulating their composition and structure;
○ Better characterize the competition between reaction intermediates, products, solvent and electrolyte in order to design catalysts that are optimized for the desired reaction products;
○ Test catalysts in flow cells under electrochemical conditions to get a more relevant picture of their reactivity and stability;
○ Optimize molecular catalysts through rational modification of their chemical structures, including by taking cues from biological enzymes; and,
○ Explore more ways of integrating molecular catalysts into electrodes so as to improve their efficiency, including incorporation into reticular frameworks or the conjugation and electronic coupling to graphite electrodes.
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Ion Selective Membranes in CO2 Electrolysis (event brief)
Making fuel from water and CO2: The Importance of stabilizing highly reactive nickel catalysts in neutral pH solution (research brief)
Solar conversion of CO2 to CO using Earth-abundant electrocatalysts prepared by atomic layer modification of CuO (research brief)