Electrified CO₂ valorization in emerging nanotechnologies: a technical analysis of gas feedstock purity and nanomaterials in electrocatalytic and bio-electrocatalytic CO₂ conversion

Abstract

Engineered nanomaterials that catalyze the transformation of waste carbon dioxide (CO₂) into value-added products are crucial to mitigate climate change and enable a new circular carbon economy. Gas separations are expected to be a major cost barrier to CO₂ conversion scalability, but the importance of feedstock purity is yet to be carefully evaluated in emerging nanotechnologies under environmentally relevant conditions. Here we assessed the performance of state-of-the-art electrocatalytic and bio-electrocatalytic CO₂ reduction nanomaterials under a range of influent CO₂ concentrations using data from recent publications. We quantitatively compared the activity of various electrocatalysts and discussed interactions at the nano-bio interface. Through this perspective, we developed initial life-cycle assessments and technoeconomic analyses for the integration of CO₂ conversion nanotechnologies with natural and engineered systems. Altogether this evaluation can inform innovative nanomaterial design and delivers useful insights towards a sustainable future without waste or pollution.

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Environmental significance

Breakthroughs in nanomaterial design are needed to minimize wastes, reuse materials, and promote a new circular economy. Though electrocatalytic and bio-electrocatalytic approaches have shown promise for CO₂ upgrading, their performance has largely been demonstrated with high purity gas feedstock that is often not practical or environmentally relevant. As such, new insights are needed to integrate waste CO₂ streams with state-of-the-art nanotechnologies. For the first time, we analyzed the importance of gas purity on nanomaterial activity and design. We discuss potential mechanisms for electrocatalyst inactivation, rate limiting steps at the nano-bio interface, and identify challenges in employing nanomaterials in industrial applications. Altogether, this perspective aims to illuminate new interdisciplinary opportunities to advance carbon circularity and environmental science through next-generation nanoscale processes.

1. Introduction

Rapid decarbonization is imperative to avoid the severe economic and environmental implications of global climate change. Despite the existence of technically feasible CO₂ capture and sequestration methods, the widespread adoption of these technologies has been constrained by the low intrinsic value of CO₂ molecules. To overcome these challenges, current research initiatives have focused on the development of new reactive CO₂ conversion nanotechnologies that can boost the value proposition of carbon cycling by combining cheap and abundant renewable electrons with waste CO₂ to generate value-added products [Fig. 1]. Recently, engineered nanomaterials have been leveraged in diverse CO₂ conversion applications with electrocatalytic and bio-electrocatalytic processes being amongst the most promising. For instance, the electrochemical reduction of CO₂ using copper nanoparticles can produce useful molecules such as CO, HCOOH, C₂H₄, and C₂H₅OH at high production rates.¹ As such, many studies have focused on developing new nanomaterials for the targeted production of fuels and chemicals from CO₂ and have shown excellent titers and efficiencies.² In particular, CO has been an attractive product as it is an essential building block for industrial chemical manufacturing and can be produced with high selectivity.^3,4 Alongside electrocatalytic approaches, researchers have also been developing new bio-electrocatalytic CO₂ conversion processes such as microbial electrosynthesis (MES) that uses whole-cell bacteria supported on conductive nanomaterials to generate valuable products from waste CO₂ and renewable energy.⁵ Recently, several MES studies have shown exceptional selectivity for energy rich molecules such as acetate.⁶ For example, researchers have shown that Sporomusa ovata can achieve exceptional columbic efficiencies (∼90%) for acetate biosynthesis when grown on 3D-graphene functionalized carbon felt electrodes.⁷

Fig. 1 Overview of emerging CO₂ capture and conversion pathways. Proposed process flow for electrified CO₂ conversion with selection of waste gas feedstock, omission or inclusion of gas separation device, and choice of CO₂ conversion nanotechnology.

Though promising, the performance and value proposition of these new CO₂ conversion nanotechnologies are expected to be inherently tied to the source of waste CO₂ feedstock. This source can span the range from the near-infinite but dilute natural atmospheric supply (∼415 ppm) to more concentrated but location specific streams released from ammonia synthesis plants or biorefineries that can be up to 99% (v/v) CO₂ ⁸ [Fig. 2].

Fig. 2 Estimated CO₂ separation costs across typical industrial point source and non-point source emission concentrations. Expected CO₂ separation costs ($/tonne CO₂) versus typical waste CO₂ concentrations % (v/v). Emission sources listed in legend where ethanol = ethanol fermentation, ammonia = ammonia synthesis plant, NG wells = natural gas well, coal flue = coal fired power plant flue gas, atmosphere = ambient air, gas flue = natural gas-powered plant flue gas. Interpolation as dotted line. Data obtained from recent publication ref. 8.

At large, CO₂ separation costs are expected to be directly related to the purity of the waste gas stream, with more dilute CO₂ sources requiring higher energy inputs and contact areas that directly translate to increased costs.⁹ As a result, the most concentrated CO₂ streams will require the least processing and can theoretically offer the best value proposition for reactive CO₂ conversion. For example, bioethanol plant effluent (∼99% (v/v) CO₂) could potentially be purified at costs as low as $20–25 per tonne CO₂ while more dilute sources like power plant flue gases (10–30% (v/v) CO₂) may cost almost two to four times as much ($40–100 per tonne), and exceptionally dilute sources like air (∼420 ppm) may cost almost ten times as much ($300–1000 per tonne, Fig. 2). As the capital and operational expenditures for CO₂ separation technologies can be substantial, an ideal scenario might be to avoid or limit gas feed separation in emerging CO₂ valorization processes. In-line with this idea, some have begun to test dilute sources of CO₂ such as flue gas in electrochemical and bio-electrochemical CO₂ conversion nanotechnologies but the impact of gas purity on nanomaterial performance and design has yet to be considered across studies. In this perspective, we conducted a state-of-the-art technical analysis to evaluate the impact of gas purity on CO₂ transformation in emerging electrified CO₂ conversion processes. We identify suitable nanomaterials for dilute gas conditions, analyze CO₂ conversion mechanisms at the nano-bio interface, and discuss opportunities to improve the economic viability of employing these technologies in large-scale industrial applications. Through this wide-ranging evaluation, we provide comparative life-cycle assessment and technoeconomic analysis calculations for CO₂ conversion and illuminate bottlenecks in process design and operation, guiding future nanomaterial research and development.

2. Impact of gas purity on CO₂ conversion.

2.1 Electrocatalytic CO₂ conversion nanomaterials

In electrocatalytic CO₂ conversion, an external voltage is applied across a set of electrodes, causing electrons and protons to be liberated in an oxidation reaction occurring at the anode electrode for subsequent use in the reduction of CO₂ at the cathode electrode. Typically, redox active nanomaterials are deposited on the surface of the cathode to lower activation energies and mediate the transfer of electrons to CO₂ to produce an extensive portfolio of products. Recently, a wide range of metals,¹⁰ 2D materials,¹¹ alloys,¹² metal oxides,¹³ single atoms,¹⁴ and hetero atom-doped carbon materials¹⁵ have been used as active catalysts for the CO₂ reduction reaction (CO₂-RR) [Fig. 3]. The reduction of CO₂ to its simplest product, CO, is currently thought to occur through the following reaction steps as indicated by recent density functional theory (DFT) calculations:¹⁶ CO₂ adsorption (i); the formation of COOH_abs (ii, eqn (1)); the formation of CO_abs (iii, eqn (2)) and CO desorption from the catalyst surface (iv).

CO_2(abs) + H⁺ + e⁻ → COOH_(abs) (1)

COOH_(abs) + e⁻ → CO_abs + OH⁻ (2)

Notably, nanoparticles of noble metals such as silver have been shown to generate CO with excellent Faradaic efficiencies (FEs) approaching unity.¹⁷ Moreover, nitrogen-doped carbon materials including N-doped mesoporous carbon,¹⁸ graphene quantum dots,¹⁹ and nanofibers²⁰ have demonstrated excellent selectivity for the CO₂-RR, owing to the high electronegativity of N-atoms that can increase the electron density of CO₂ reduction sites. In particular, single-atom decorated N-doped (Mi–N–C) materials have highly dense active sites dispersed at the atomic level that can achieve exceptional CO₂-RR rates and selectivity.²¹ Likewise, single atom catalysts supported on carbon black have also achieved high FEs for CO up to 98.9% with low overpotentials.²² Though promising, the bulk of these catalysts have exclusively been tested under high purity CO₂ gas streams (i.e. 99% (v/v)) and the performance of the select nanomaterials that have been tested under dilute CO₂ gas blends are yet to be compared across studies. Data was compiled from various publications that have tested dilute CO₂ blends as a feedstock for electrochemical CO₂ conversion to elucidate the state-of-the-art under these operating conditions [Fig. 4].

Fig. 3 Overview of state-of-the-art nanomaterials employed for electrocatalytic CO₂ reduction using dilute CO₂ gas feedstock. Transmission electron microscopy image of porous nitrogen doped carbon material from wheat flour substrate, reproduced from ref. 23 with permission of Elsevier publishers, Copyright 2017 (A). Imine-nitrogen doped carbon nanotubes used for the electrocatalytic reduction of flue gases, reproduced from ref. 24 with permission of Wiley publishers, Copyright 2021 (B). Fabrication method for single-atom decorated N-doped carbon catalysts based on multivariate metal–organic frameworks, reproduced from ref. 21 with permission of Wiley publishers, Copyright 2020 (C). Synthesis of bimetallic copper–palladium nanoalloys, reproduced from ref. 25 with permission of American Chemical Society publishers, Copyright 2018 (D). Depiction of CO₂ reduction pathway to CO using single-atom decorated N-doped carbon catalysts, reproduced from ref. 21with permission of Wiley publishers, Copyright 2020 (E).

Fig. 4 Summary of reported performance metrics for abiotic electrocatalytic CO₂ conversion nanomaterials across various feedstock purity. Reported partial current density for CO formation (J_CO) vs. influent CO₂ concentration by percent volume, balanced with inert gas (i.e. argon). (A) Faradic efficiencies for CO formation (FE_CO) vs. influent CO₂ concentration used by percent volume, balanced with inert gas. (B) Detailed performance impact on current densities (C) and faradaic efficiencies (D) using different electrocatalysts and reactor designs at various influent CO₂ concentrations. Relative performance drop under dilute gas conditions listed as percentage above data bars. Electrocatalyst material abbreviations defined as follows: Co-Typ-C = cobalt terpyridine on carbon, Ag El = silver nanoparticles in electrolyzer, NCNT-NH₃-800 = imine-nitrogen-doped carbon nanotubes prepared under NH₃ at 800 °C, Ni–N–C = single atom nickel on nitrogen doped carbon. Data adapted from recent publications ref. 21, 23–25 and 34–36.

Rates of CO₂ conversion seem to be heavily reliant on the composition of the influent gas feedstock [Fig. 4]. For instance, when utilizing a high purity CO₂ blend of 99% (v/v), an Ag nanoparticle catalyst achieved a partial current density of 52 mA cm⁻² compared to just 28 mA cm⁻² when using a much more dilute CO₂ blend of 10% (v/v) [Fig. 4A]. This aligns with expected behavior, as a higher reactant concentration in the feed should lead to a higher reaction rate, and thus a higher partial current density for CO formation. Notably, the severity of the performance decline under dilute gases seems to be strongly correlated with the reactor architecture used for electrolysis. While using a flow cell fitted with gas diffusion electrodes (GDEs), the decrease in reported current densities between operating with 99% (v/v) CO₂ and 20% (v/v) CO₂ was only 25%. In comparison, a nearly 80% drop was reported when using an H-cell reactor under similar conditions [Fig. 4C]. This is likely because the triple phase boundary layer offered by GDEs allows for CO₂ to be better dispersed and utilized across the electrode surface.²⁶ In addition, CO₂ is sparingly soluble in water, presenting kinetic challenges in H-cell reactors that rely on dissolved CO₂.²⁷ Similar to reactor architecture, the intrinsic properties of the electrode nanomaterials also seem to play a large role in the current densities observed for CO formation. For instance, Ni–N–C catalysts were reported to experience around a 90% drop in current density when operating in 15% (v/v) CO₂ rather than 99% (v/v) CO₂, while in a separate study comparable NCNT-NH₃ catalysts experienced just a ∼60% drop in current density across similar conditions [Fig. 4B]. Though these materials are alike in composition, the disparity in their catalytic activity under dilute CO₂ conditions may stem from differences in their nano-morphology, availability of active sites, relative binding affinities for CO₂-RR intermediates, and microenvironments surrounding surface N-atoms on the carbon matrix. This highlights the importance of careful nanomaterial design in achieving high rates of CO₂ conversion.

Interestingly, the FEs (i.e. selectivity) of the electrocatalysts seem to be far less sensitive to the composition of the feed gas. For example, similar FEs were reported for each catalyst material when operating with 99, 20, and 15% (v/v) CO₂ as the average slope of the line of best fit across all studies was <1%. This implies that for every 1% increase in CO₂ concentration in the feed there was a near negligible 0.1% increase in average FEs for CO production with a given catalyst material [Fig. 4B]. This data is promising as FEs seem to be highly dependent on the nanomaterial properties rather than the CO₂ concentration of the feed gas. This is further illustrated by the observation that the drop in FEs across all catalysts were between 5% and 16% when operating in dilute CO₂ blends of 15% and 20% (v/v) rather than 99% (v/v) [Fig. 4D]. Altogether these relationships suggest that a high purity CO₂ feedstock may not be an absolute prerequisite for attaining high CO₂ conversion efficiencies.

Though this data is encouraging, several challenges of operating with dilute CO₂ gas streams should be considered. These include the competitive hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) that can impact CO₂-RR selectivity as well as toxic impurities in the feed gas that can cause catalyst poisoning and deactivation. As noted in several works, low concentrations of CO₂ in the gas feed may enable greater selectivity for the HER that occurs at similar redox potentials.²¹ For instance, Kim et al. (2021) recently noted that H₂ evolution sharply increased when operating an Ag catalyst in 0.1 atm CO₂ rather than a higher purity gas feed containing 1 atm CO₂.²⁸ Mechanistically, this is likely because the lack of surface-adsorbed CO₂ under dilute CO₂ feedstock enables active sites to be used for H₂ production rather than the CO₂-RR. Future studies may be able to address this issue by fine tuning the active sites on the nanomaterials to have a higher binding energy for *H adsorption, making the Volmer step of the HER (eqn (3)) more difficult to initiate.²⁹ Likewise, permselective coatings may be used at the cathode to repel HER reactants, while allowing the passage of CO₂ to the electrode surface.

H⁺ + e⁻ → H_(ads) (3)

Similar to the HER, industrial CO₂ emissions such as flue gas often contain non-innocent reactants (e.g. O₂, SO₂, or NO_X) that can divert electrons away from the CO₂-RR or cause electrode poisoning. For instance, transition metal nanocatalysts on N-doped carbon nanomaterials can readily catalyze the ORR, which may lower CO₂-RR selectivity.^30,31 In-line with this, Shi et al. (2021) noted the appearance of ORR redox peaks when conducting linear sweep voltammetry experiments in simulated flue gas (15% (v/v) CO₂, 77% (v/v) N₂ and 8% (v/v) O₂) using NCNT-NH₃ catalysts.²⁴ As such, future operations may seek to remove O₂ from feed gases prior to CO₂ conversion. Alternatively, catalysts may be redesigned to resist ORR reaction steps such as OH⁻ adsorption³² but this could also destabilize CO₂-RR intermediates if not done carefully. Distinct from the ORR, Komatsu et al. (1994) also found that SO₂ can reduce CO₂-RR efficiencies when using copper-solid polymer electrolyte-based electrodes with a feed gas of 60% (v/v) CO₂ and 170 ppm SO₂.³³ The authors attributed this to corrosion of the catalyst surface, however, the reaction mechanisms for the performance drop were not further investigated. As such, additional studies are needed to systematically test the impact of common non-innocent reactants (e.g. O₂, SO₂, NO_X) on electrode poisoning as the majority of previous works have simply blended CO₂ with chemically inert gases. Overall, these experiments could provide researchers with improved understanding of catalyst deactivation modes to help future studies fine-tune the nano-morphology and surface-active sites of nanomaterials to avoid competitive reduction reactions and mitigate catalyst poisoning.

2.2 Bio-electrocatalytic CO₂ conversion nanomaterials

Microbial electrosynthesis (MES) is a bio-electrocatalytic approach to convert CO₂ and renewable electricity into value-added fuels and products. Distinct from abiotic electrochemical CO₂ reduction, a typical MES process uses chemolithoautotrophic microorganisms as the living biocatalysts to convert CO₂ into organic compounds at a solid electrode. The cathode serves as the reducing power for the microbial metabolism, providing electrons through either direct electron transfer (DET) and/or mediated electron transfer via H₂ or alternative soluble redox shuttles.^5,37 In a seminal work by Nevin et al. (2010), the researchers showed that homoacetogenic bacteria like Sporomusa ovata can produce extracellular multi-carbon products from CO₂ and water using electrons derived from the cathode.³⁸ Moreover, microbes such as Geobacter and Clostridium sp. have been theorized to conduct DET using conductive nanowires or c-type cytochromes to obtain electrons from the cathode.^39,40 In addition, H₂ produced via abiotic water splitting can readily be metabolized into organics at the cathode using CO₂ as the carbon source via metabolisms such as the Wood–Ljungdhal pathway (WLP).⁴¹ Researchers have also suggested that microbes can self-produce extracellular enzymes⁴² or deposit endogenous metal nanoparticles⁴³ onto electrodes that can catalyze the in situ production of H₂ from water, alluding to yet another route of electron transfer.

To date, an extensive portfolio of products (e.g. C2–C6 carboxylic acids, alcohols) has been demonstrated using MES, however, acetate remains the most common end-product and presents one of the most feasible options for full-scale deployment.⁴⁴ This is because the production of acetate through the WLP is one of the most energy efficient carbon fixation mechanisms as it avoids the majority of ATP-consuming reactions by combining endergonic reactions with other non-ATP consuming exergonic reactions.⁴⁵ At large, incorporating microbes as active CO₂ conversion catalysts offers several advantages over abiotic processes including 1) highly selective synthesis of large chain organics, 2) robust operational stability, 3) exceptional process flexibility across various pH, temperatures, and pressures, and 4) low operational costs owing to the self-replicating nature of microbes that avoids the need for expensive catalyst replacement (e.g. precious metals).

Contrary to these key benefits, MES processes have routinely been challenged by low production rates due to the limited conductivity and surface area of the electrodes used to culture microbes.⁴⁶ Over the last decade, researchers have tested a variety of carbonaceous, metallic, and composite carbon-metallic nanomaterials to accelerate catalytic activity at the electrode-microbial interface.⁴⁷ In general, MES cathodes must possess low CO₂ mass transfer resistance as well as high surface area, chemical stability, and biocompatibility for optimal performance. Recently, highly conductive metals such as iron,⁴⁸ nickel,⁴⁹ rubidium,⁵⁰ platinum,⁵¹ gold,⁵² and molybdenum,⁵³ have been tested as MES cathodes but have generally shown poor corrosion resistance in microbial media-electrolytes, leading to metal leaching into the surrounding solution that can inhibit microbial growth and metabolic activities.⁶ Similar challenges have also been a concern when using metal nanomaterials (e.g. nickel nanowires,⁵⁴ Fe₂O₃ ⁵⁵) deposited on carbon electrodes as MES cathodes.⁵⁶ As such, carbonaceous materials have been the most popular class of MES cathode materials owing to their high specific surface areas, excellent chemical stability, and low-cost. These include 2D planar structures such as carbon cloths⁵⁷ as well as porous 3D materials such as carbon foam,⁵⁸ felts,⁵⁹ and brushes.⁶⁰ Moreover, some have recently begun to coat carbon substrates (2D or 3D) with additional carbon nanomaterials such as graphene⁷ or nanotubes⁶¹ that can lead to higher surface areas for microbial attachment and improved electrical conductivities. For instance, cathodes made of multiwalled carbon nanotubes deposited onto reticulated vitreous carbon achieved some of the highest current densities (200 A m⁻²) and acetate production rates (1330 g m⁻² per day) reported to date.⁶²

Though innovations in nanomaterial design have led to significant improvements in MES production rates and scalability, the added cost of feed gas separation may drag the overall value proposition of MES technologies. Fig. 5 summarizes the performance of various MES studies that have tested dilute CO₂ feedstock as a carbon source. Distinct from abiotic catalysts, living biocatalysts can maintain similar CO₂ conversion rates and selectivity across a wide range of influent CO₂ purity [Fig. 5]. High coulombic efficiencies (CE) for acetate production were reported across the various CO₂ blends examined [Fig. 5A]. For instance, when using reticulated vitreous carbon nanotube (RVC-NT) cathodes the reported drop in CE was only ∼2% when operating in 30% (v/v) CO₂ rather than the ideal 99% (v/v) CO₂ feedstock. Likewise, carbon/graphite cathodes enabled CEs of around 80–100% regardless of CO₂ purity. This partly may be because microbes can scavenge even small quantities of CO₂ from their environments,^63,64 leading to similar metabolic efficiencies regardless of the initial amount of CO₂ that is available to them beyond a certain threshold. In addition, many of these studies used enriched cultures in combination with chemical additions (i.e. 2-bromoethanesulfonic acid) to prevent methanogenesis, which may have further enhanced CEs for acetate production via acetogenesis. Analogous to CEs, comparable current densities and production rates were also observed across the range of influent gas blends used for MES [Fig. 5B and D]. For example, RVC-NT electrodes achieved similar current densities around 35 A m⁻², regardless if the feed gas contained 30% or 99% (v/v) CO₂ [Fig. 5B]. In most cases, corresponding trends were also observed for the concomitant acetate production rates. For instance, similar acetate synthesis rates (ca. 11–16 g m⁻² per day) were observed using gas diffusion (GD) electrodes with CO₂ feedstock concentrations of 20 or 80% (v/v) CO₂ [Fig. 5D]. Taken together, this indicates that CO₂ availability may not be a primary limitation in microbial electron consumption or productivity in many MES studies.

Fig. 5 Summary of reported microbial electrosynthesis performance metrics using various CO₂ gas blends and nanomaterials. Coulombic efficiency for acetate production (A) average cathodic current density (B) applied cathodic voltage versus reversible hydrogen electrode (RHE) (C) and maximum acetate production rates observed (D) versus influent CO₂ feed purity % (v/v). Cathode material acronyms: carbon/graphite (C/G), reticulated vitreous carbon nanotubes (RVC-NT), gas-diffusion (GD) electrodes. Data adapted from recent MES publications.^{62,67,73–76}

Interestingly, increasing the applied cathode voltage does not seem to lead to higher production rates under dilute CO₂ gas blends. For example, under 10% (v/v) CO₂ a carbon felt cathode operated at −0.82 V vs. RHE attained one of the lowest production rates of 19 g m⁻² d⁻¹ while a graphite plate cathode using less than half of the applied voltage (−0.39 V vs. RHE) achieved a production rate nearly three times as high (60 g m⁻² d⁻¹, Fig. 5C). Conventionally it would be expected that the surplus of reducing power offered at higher voltages would enable more rapid production of organics. This would especially be the case in operating potentials beyond the HER. However, it is likely that the limited surface area available to the bacteria and the slow rates of extracellular electron transfer at the electrode surface were major bottlenecks to CO₂ conversion rather than a lack of sufficient reducing equivalents. This is evident as the RVC-NT cathodes that have a very high surface area (3902 m² m⁻³ electrode) for microbial attachment achieved 2–10 times higher average current densities than alternative cathode materials [Fig. 5B].

Though mechanisms can be difficult to discern due to a lack of standardized reporting metrics, reactor culturing techniques, and microbial communities, the data that is available so far points to the promising ability of MES processes to achieve consistent CO₂ conversion rates, electron exchange, and product selectivity, irrespective of gas feed purity. Still, several important factors must be accounted for when operating with dilute waste gases including the impact of non-innocent gaseous constituents as well as possible reactions that can compete with the CO₂-RR. In many industrial waste gases like coal power plant flue gas, O₂ can be a notable constituent ranging from 5–14% (v/v).⁶⁵ MES primarily relies on anaerobic bacteria to conduct CO₂ conversion, many of which are highly sensitive to oxygen exposure and can become inactive even at low O₂ pressures.⁶⁶ As such, the presence of O₂ in the gas feed could significantly reduce MES CO₂ conversion rates and productivities, especially when operating with pure cultures that are strict anaerobes. A potential solution could be to operate MES reactors with mixed cultures of bacteria that can leverage synergistic metabolisms for enhanced process stability. For instance, Roy et al. (2021) found that a genus of microaerophilic bacteria, Sulfurospirillum, can scavenge trace amounts of O₂ and help maintain anaerobic conditions in MES reactors.⁶⁷ However, this species can also oxidize acetate using O₂ as an electron acceptor potentially reducing CO₂-RR rates and titers. Still, if alternative organics such as aliphatic or aromatic hydrocarbons are available in the waste gas feed, bacteria such as Pseudomonas may be able to use these compounds as electron donors with the concurrent reduction of O₂ as noted by researchers when operating an MES reactor with brewery waste gas.⁶⁷ Nevertheless, more studies on this topic are needed as hydrocarbons and other volatile organic compounds could potentially be toxic to acetogenic bacteria used for CO₂ conversion. Moreover, similar microbial dynamics may impact the removal of other non-innocent compounds like SO₂ and NO_X that are routinely found in industrial waste gases like coal power plant flue gas.⁶⁸ For instance, sulfate reducing bacteria (SRB) such as Desulfovibrio desulfuricans⁶⁹ could potentially be used to remove SO₂ from MES biocathodes. However, these microbes would likely need to be supplied additional carbon and energy sources as to not impact the overall CO₂-RR rates and efficiencies towards desired products. Likewise, denitrifying bacteria could be used to remove NO_x from the MES cathode through microbial denitrification (i.e. NO₃ → NO₂ → NO → N₂O → N₂) but this would require strict anaerobic conditions, presenting significant compatibility issues with waste gas streams that contain high levels of oxygen.⁷⁰ Nevertheless, some have reported the existence of aerobic denitrifying bacteria such as Pseudomonas aeruginosa that may be useful for NOx removal from waste gases that contain oxygen.⁷¹

As engineering an optimal MES microbiome may be a complex task, a more elegant approach to improve MES compatibility with waste gas blends may be to develop new porous 3D electrode nanomaterials. As found with carbon foam, the inner pores of these structures can allow microbes to colonize areas separated from the bulk aqueous phase solution.⁷² This could potentially shield microbes from non-innocent gas constituents that are transported from the dissolved bulk liquid to the electrode surface and mitigate competitive reaction pathways like the ORR. Nevertheless, these shielding effects may also decrease CO₂ diffusion to the microbes and the overall rate of CO₂ conversion. As such, new nanomaterials should be carefully designed to provide both improved microbial protection and adequate CO₂ delivery for product formation.

3. Expected costs and environmental impacts

As electrified CO₂ conversion technologies continue to mature, the cost and quality of gas feedstock will likely play a key role in the economic feasibility and sustainability of these decarbonization approaches. At a minimum, selling costs should be on-par with current fossil fuel production methods to be deemed competitive without considerable tax incentives. In the absence of new investments into CO₂ pipelines to transport purified CO₂ gas streams to distributed conversion sites, CO₂ upgrading processes will likely need to overcome substantial economic hurdles accompanying location specific separation techniques. This could ultimately lead to a trade-off between CO₂ separation costs and electricity costs for CO₂ conversion.

To better understand these relationships, production costs of various feed gas scenarios were estimated using performance metrics from recent literature [Fig. 6]. When analyzing electrocatalytic CO₂ conversion processes, representative performance values were selected for Ag nanoparticle electrodes³⁵ as these electrocatalysts show excellent potential for scale-up owing to their high current densities and CO₂-RR selectivity. The total production costs were taken as the sum of the estimated energy costs required to produce a kg of CO₂-derived product and the accompanying separation costs to provide the associated mass of initial CO₂ feedstock [calcs. in ESI† 1.0]. Notably, capital costs for the electrolyzer and separation units were considered outside the scope of this preliminary analysis. Initial calculations were based on the scenario that a waste gas stream containing 20% (v/v) CO₂ could be used without pretreatment for CO₂ conversion [20% CO₂ scenario, Fig. 6]. This scenario is feasible as coal-fired power plants, industrial cement plants, and steel mills can frequently emit flue gases containing 15–30% (v/v) CO₂.^77,78 As such, the estimated 324 coal-fired power plants in the U.S. could potentially provide around 1200 MMT of waste CO₂ feedstock per year for CO₂ conversion.⁸ In a second scenario, a high purity gas stream of 99% (v/v) CO₂ was presumed to be available for direct use without upstream separation [99% CO₂ scenario, Fig. 6]. This could be practical in CO₂ conversion sites located near industrial bioethanol fermentation or ammonia synthesis plants that typically emit concentrated gas streams containing 95–99% (v/v) CO₂.⁸ Finally, a third scenario was considered where a waste gas stream containing 20% (v/v) CO₂ might be available from an industrial point source (e.g. cement plant) and upgraded on-site using a monoethanolamine (MEA) CO₂ separation unit to produce a purified 99% (v/v) CO₂ feedstock for subsequent conversion [20% CO₂ + MEA, Fig. 6]. Accordingly, the integration of CO₂ capture followed by electrified CO₂ upgrading has long been envisioned as the status quo for CO₂ conversion devices.

Fig. 6 Estimated production costs and carbon footprint of electrified CO₂ conversion under different waste gas feedstock scenarios. Production costs ($ per kg CO) (A) and carbon footprint (kg CO₂ eq. kg⁻¹ CO formed) for electrochemical CO₂ conversion to CO (B). Production costs ($ per kg acetate) (C) and carbon footprint (kg CO₂ eq. kg⁻¹ acetate formed) for bio-electrocatalytic CO₂ conversion to acetate (D). Feedstock scenarios defined as raw 20% (v/v) CO₂ purity feed without separation (20% CO₂), raw 99% (v/v) CO₂ purity feed without separation (99% CO₂), and raw 20% CO₂ (v/v) upgraded to 99% (v/v) CO₂ using standard monoethanolamine (MEA) CO₂ separation. For bio-electrocatalytic studies, 30% (v/v) CO₂ was used for dilute gas scenarios. Power source acronyms defined in Table S3.†

While analyzing each scenario, energy costs were expected to be a major barrier to scalability and will likely be highly dependent on source, location, availability, and existing governmental policies/incentives.⁷⁹ As such, approximate costs of CO₂ conversion were calculated using levelized costs of energy (LCOE) from a recent U.S. Energy Information Administration (EIA) report.⁸⁰ As expected, production costs were highly sensitive to the type of energy source used [Fig. 6]. For instance, if converting a 20% (v/v) CO₂ feedstock to CO with a fossil fuel energy source like coal, estimated production costs could be as high as $0.66 per kg CO compared to just $0.23 per kg CO when using a renewable energy source like solar photovoltaics (PV). Though price differences could be further bolstered by additional tax incentives, this highlights the tremendous opportunity to leverage low-cost renewable energy for CO₂ conversion. Notably, operating with a highly concentrated CO₂ source (i.e. 99% (v/v)) without upstream separation may offer even lower production costs ranging from $0.14–0.23 per kg CO [Fig. 6]. Still, these prices are 2–3 times higher than current fossil fuel-based production methods,⁸¹ emphasizing the need to develop innovative nanomaterials that can lower energy demands (i.e. overpotentials) of electrochemical CO₂ conversion. Correspondingly, the 20% CO₂ (v/v) + MEA scenario yielded the highest CO production costs ranging between $0.44–1.63 per kg CO owing to expensive CO₂ separations. This suggests that purifying the 20% (v/v) CO₂ waste gas stream to a 99% (v/v) CO₂ feedstock would yield an 58% increase in CO production costs but only a 28% increase in CO production rates [Fig. 4 and 6]. Altogether, this underscores the potential benefits of avoiding expensive CO₂ separation processes and operating electrified CO₂ conversion devices with raw waste gas blends, even with existing nanomaterials.

Similar to electrocatalytic processes, acetate production costs were estimated for a pilot scale bio-electrocatalytic CO₂ conversion plant. Performance metrics were based on RVC-NT cathodes,⁶² which have shown amongst the highest acetate production rates of any MES study to date and have outstanding potential for scale-up [Fig. 6]. Due to the availability of published data, this analysis was structured around a 30% (v/v) CO₂ stream rather than the 20% (v/v) CO₂ feed used for the electrocatalytic scenarios. As expected, similar trends in production costs were found with bio-electrocatalytic processes as with the abiotic processes across different energy sources. However, the production costs of the bio-electrocatalytic devices seemed to be far less impacted by gas feed composition, owing to their relatively stable performance in dilute gas streams. For instance, the average acetate production costs using on-shore wind energy were around $0.26 per kg acetate when operating with either raw 30% (v/v) or 99% (v/v) CO₂ [Fig. 6]. In contrast, the price gap was predicted to be around 34% different when operating abiotic processes under similar conditions. At large, this showcases the potential advantages of employing biocatalysts that are far less sensitive to gas feed composition for CO₂ conversion. Nevertheless, the average production costs ($0.86 per kg acetate) may be around 52% higher than current industrial acetate production methods⁸² if using an MEA separation unit for gas pretreatment [30% CO₂ + MEA]. This scenario might be the case in operations that seek to maximize productivities and process stability. As such, new cathode nanomaterials are needed to improve production rates and lower energy demands in waste gas streams in order for bio-electrocatalytic processes to be competitive with existing methods at an industrial scale.

Complementing production cost estimates, the projected carbon footprint for each feedstock scenario was calculated for both electrocatalytic and bio-electrocatalytic CO₂ conversion methods [Fig. 6B and D]. Briefly, the carbon intensity of each energy source and the emission rates for a typical MEA separation unit were estimated using a current Intergovernmental Panel on Climate Change (IPCC) Report on Carbon Dioxide Capture and Storage⁸³ [calcs. in ESI† 2.0]. Overall, these calculations accounted for emissions from both the energy used for CO₂ conversion and power required for initial CO₂ feedstock separation while other emissions were considered outside the scope of this analysis. As expected, the carbon footprint of each process was highly dependent on the energy source used for CO₂ conversion with the most sustainable scenarios being those where renewable energy could easily be accessed. For instance, using a raw 20% (v/v) CO₂ blend for electrocatalytic CO production yielded a carbon footprint of 4.7 kg CO₂ eq. per kg CO when using coal-derived electricity versus just 0.27 kg CO₂ e per kg CO when using electricity generated from solar PV. Notably, the addition of a CO₂ separation unit significantly increased the estimated carbon footprint for both CO₂ conversion approaches. As an example, if using coal-derived electricity with an integrated MEA separation unit, the carbon emissions could be as high as 8.64 kg CO₂ eq. per kg CO, marking a 59% increase in emissions compared to the scenario where the separation process was omitted [i.e. 20% CO₂ scenario, Fig. 6]. Likewise, integration of the MEA separation unit with the bio-electrocatalytic process yielded a nearly 65% increase in emissions compared to the scenario without feedstock pretreatment. Importantly, the case where a high purity 99% (v/v) CO₂ blend could be used directly offered some of the lowest carbon footprints available. For example, using the raw 99% (v/v) CO₂ blend for acetate production with solar PV delivered an estimated carbon footprint of just 0.26 kg CO₂ eq. per kg acetate which is much lower than current petrochemical based production methods (∼1 kg CO₂ eq. per kg acetate).⁸⁴ As such, the most competitive approach in terms of cost and environmental sustainability would be to use a raw 99% (v/v) CO₂ blend without feedstock separation combined with a renewable energy source like solar PV or onshore wind. Still, a scenario where renewable energy is readily available alongside a high purity CO₂ emission source may be rare and prove difficult to utilize at meaningful production scales. Accordingly, the widespread adoption of emerging carbon cycling technologies will demand new nanomaterials that cannot only improve present energy efficiencies and production rates under ideal feedstock scenarios but also enable stable CO₂ conversion using a wide range of industrial waste gas emissions.

4. Future outlook for CO₂ conversion nanomaterials

This technical review and analysis underscores the outstanding potential of waste CO₂ valorization as an economic and sustainable pathway to reduce carbon emissions. By evaluating recently published data, unique challenges faced by respective electrocatalytic and bio-electrocatalytic processes were brought to light that warrant the synthesis of innovative nanomaterials. In general, both electrocatalytic and bio-electrocatalytic methods show excellent potential for converting CO₂ into useful fuels and chemicals. However, these technologies have typically shown complementary limitations as electrocatalytic devices can generally reduce CO₂ into smaller C1 molecules (e.g. CO, formic acid) at high rates and efficiencies but have struggled to produce larger compounds (>C2) with high selectivity.⁸ Meanwhile, bio-electrocatalytic approaches can produce a wide range of larger molecules (e.g. alcohols, olefins) from CO₂ with high selectivity but often suffer from slow production rates.⁸⁵ As such, a tradeoff currently exists between the high rates of electron transfer that can be achieved electrochemically and the high product selectivity that can be achieved using microbial catalysts.

Through this study, the rates of electrocatalytic CO₂-RR showed a considerable dependence on feedstock purity that was correlated with reactor architecture and electrocatalyst design. Though high selectivity can still be achieved under dilute CO₂ feedstock, new engineered nanomaterials that can attain low overpotentials and high turnover frequencies are needed to realize scalable CO₂ conversion under low CO₂ pressures. Other challenges include the presence of non-innocent compounds and particles that can lower CO₂-RR efficiencies or cause catalyst poisoning. Key approaches to address these challenges may include the development of new catalyst materials with surface active sites that are highly selective for the CO₂-RR but resistant to side reactions such as the HER or ORR. Regardless of CO₂ feedstock composition, these are highly desirable attributes and should be a strong focus of future nanomaterial studies. In addition, more work is needed to address issues stemming from catalyst poisoning by common constituents of waste gas feedstock that can be chemically active including NO_X and SO_X. Accordingly, the mechanisms for catalyst poisoning should be systematically studied with these constituents (and others) on various CO₂-RR nanomaterials to guide future catalyst design for improved waste gas compatibility. This iterative process could be accelerated using new machine learning approaches that combine DFT models with material science libraries to predict catalyst designs with desirable properties.⁸⁶ Once optimal catalysts have been identified, approaches such as permselective coatings may be applied overtop of the CO₂-RR catalysts to block impurities while allowing efficient transfer of reactants (i.e. CO₂) to the electrode interface. Similar methods have already been applied in seawater electrolysis using MnO_x protective layers over IrO_x catalysts.⁸⁷ Furthermore, the surface charge or electronic state of the CO₂-RR catalysts can be adjusted to repel non-innocent compounds by adding co-catalysts or polymers directly to the catalyst inks.⁸⁸ This strategy has proven effective when adding imidazolium polymers during CO₂-RR catalyst preparation to mitigate the HER.²⁷

Aside from abiotic methods, bio-electrocatalytic CO₂ conversion processes seem to be able to achieve stable current densities and CO₂-RR selectivity across a wide range of feed gas purity. This performance may even translate to atmospheric CO₂ concentrations but further studies are needed to evaluate this scenario. In addition, new nanomaterials are required to improve the rates of the CO₂-RR that are currently limited by the usable surface area and electrical conductivity of the biocathodes. Additional work is also needed to assess the impact of non-innocent reactants such as oxygen that may be present in waste feed gases at significant concentrations. As microbiome engineering may be an important strategy to improve MES stability under waste gases, the identity and ecological role of individual microbial species needs to be further elucidated under these conditions. Specifically, parameters such as waste gas tolerance, CO₂ fixation pathway, and desired gene expression profile could be fed into machine learning algorithms trained on biological databases to help identify suitable microbial strains and communities for waste CO₂ valorization.⁸⁹ In combination with this approach, the development of new porous 3D electrodes that can potentially shelter microbes from harmful gases may also be useful in improving production efficiencies. These 3D porous electrodes could be used in conjunction with permselective coatings that block non-innocent compounds from entering the inner pores while allowing efficient CO₂ mass transport to the biocatalyst surfaces. Similarly, innovative cytoprotective layers that can be deposited onto the surface of individual microorganisms may also be a valuable tool to protect biocatalysts from waste gas constituents. This approach has been effective in protecting bacteria from reactive oxygen species in photocatalytic CO₂ conversion processes⁹⁰ and could easily be adapted to MES. Looking ahead, there has been an increasing focus on developing reactive CO₂ capture (RCC) processes that aim to directly convert captured CO₂ into products without an intermediate step where CO₂ is released from the initial CO₂ capture unit.⁹¹ Though still in its infancy, RCC technologies could potentially provide new opportunities for electrified waste gas CO₂ conversion. Nevertheless, gas separations are expected to be a major economic and environmental burden to CO₂ conversion processes, across many of the most promising conversion technologies. As such, the rapid development of new highly stable and active engineered nanomaterials will be crucial in achieving scalable waste CO₂ conversion as part of a future net-zero economy.

Author contributions

J. Jack was the primary author of this work and contributed to the overall conceptualization. S. Bolzman and A. Weber conducted the initial literature review and preliminary TEA/LCA calculations with supervision from authors J. Jack and S. McCord.

Conflicts of interest

The authors declare no competing financial interests associated with the writing of this article.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3en00912b

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