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Wed Oct 30, 2019, 07:39 PM

Faradaic electro-swing reactive adsorption for CO2 capture.

The paper I'll discuss in this post is this one: Faradaic electro-swing reactive adsorption for CO2 capture (Sahag Voskian and T. Alan Hatton Energy Environ. Sci., 2019, Advance Article Accessed 10/30/19).

I came across reference to this paper, out of MIT, in the scientific popular press, actually in several places, and since the quality of journalism describing science is often quite bad, decided to access the original paper.

The paper is, happily open sourced, and anyone can read it. I'll excerpt in offer the graphics in any case.

I've spent a lot of time reading about separations of carbon dioxide from various matrices and I will say that this one is somewhat unique, an electrochemical approach. Since the separation of carbon dioxide, a low energy gas, from dilute matrices requires overcoming entropy, this process is not energy neutral by any means; it costs energy, but it may be more efficient. It does not seem operative at air concentrations of CO2, requiring concentrations of 0.6% as compared to 0.041% in air (as of this writing).

The system uses a rather well known organic redox system, dibenzoquinones/dibenzoyhydroquinone in a very creative way

From the introductory text:

With the alarming increase in the atmospheric concentration of carbon dioxide (CO2) and its implications for global climate pattern developments,1,2 mitigation of climate change through curtailment of anthropogenic CO2 emissions has been one of the most urgent socioeconomic and scientific problems in the global arena over the last decade.3 To this end, a number of technologies have been developed for the large-scale capture of CO2 from combustion and other industrial processes to produce high-purity CO2 streams for storage or valorization.4 The most mature of these technologies are solvent scrubbing, mainly amine scrubbing,5 and oxyfuel combustion,6 which target high CO2 concentration streams (>10%). These approaches have a large footprint and, when retrofitted to a process, can require major modifications to the plant. Consequently, there has been a major effort to develop new materials and processes for high efficiency CO2-capture, including sorbents for pressure and temperature swing adsorption systems,7 and membranes for selective transport of the CO2.8 Furthermore, many potential applications of carbon capture require compact devices due to space limitations, such as in the direct capture of CO2 from tailpipe exhausts on board mobile sources, in which there is growing interest given the large contribution of transportation exhaust to greenhouse gas emissions (33.5% of U.S. CO2 emissions in 2016).9

In addition to the capture of CO2 from direct combustion processes, there is a need to remove CO2 from enclosed spaces for ventilation purposes in buildings and car cabins, or for cabin environmental control systems on board spacecraft and submarines, where the maximum allowed CO2 concentration in habitable spaces is 5000 ppm (or 0.5%).10 The first of such systems was developed by Winnick et al., for the electrochemical capture of CO2 in spacecraft cabins using molten carbonates.11 However, the low concentration of CO2 in such applications poses a challenge, mainly due to the low driving forces for mass transfer and the large quantities of other species present in air in addition to CO2.12 Thus, carbon capture is a multi-scale problem, where the CO2-rich streams to be treated vary greatly in volume, concentration and composition, and different criteria need to be fulfilled to ensure optimal processing depending on whether sources are industrial or small-scale (e.g., power plants or oil and gas heaters), concentrated or dilute (exhausts from combustion or air in confined spaces), and clean or contaminated with other pollutants.

Many of the CO2-capture chemical processes that involve a capture agent such as amines or solid sorbents require temperature and/or pressure swings to release the captured CO2 and regenerate the agents for further capture. These swings result in inefficiencies due to energy wasted in heating solvents and sorbents, pressurizing feed gas, or drawing a vacuum for desorption. Electrochemical systems can minimize such parasitic energy losses as they can be operated at near isothermal conditions, with significantly higher efficiencies than their thermal-swing (TSA) and pressure-swing (PSA) adsorption counterparts.13 One mode of electrochemical capture of CO2 is through the use of a redox-active carrier.

Electrochemically mediated selective transport of chemical species was first reported by Ward et al.,14 where a redox-active carrier (ferrous ion) was used to transport nitric oxide across a membrane. Since then, a number of systems have been developed for transporting chemical species by redox-active carriers that are activated at one electrode, to bind with the target species, and deactivated at the opposite electrode, to release the target and regenerate the carrier.15,16 Systems that have been proposed for the concentration of CO2 through this approach have been based on a number of different carrier molecules, such as quinones,1720 4,4′-bipyridine,21 and thiolates.22,23 Quinones are of particular interest to this work for their superior electrochemical performance, serving as redox-active carriers for CO2 in electrochemically mediated separation processes. DuBois et al. demonstrated this possibility, and studied the thermodynamics of an electrochemical CO2 pumping system that utilizes quinones.18 More work followed, where Scovazzo et al. demonstrated the electrochemical separation of CO2 from <1% concentration gas mixtures using 2,6-di-tert-butyl-1,4-benzoquinone as a carrier in ionic liquid (IL) and organic solvent electrolytes media,19 while Gurkan et al. screened a number of ILs to serve as suitable electrolytes for quinone carriers in an electrochemically mediated selective transport system for CO2.20 All of these systems, however, require the transport of the electrolyte and the dissolved carrier molecules between two electrodes in an electrochemical cell for capture and release of CO2. This limits their implementation in a number of applications where the requirement for flow systems and pumping, and the large footprint, are problematic.

The quinones are oxidized and reduced by a porous matrix carbon nanotube (CNT) supported ferrocene polymer.

This nice graphic cartoon shows the systems operation.

The caption:

Fig. 1 Schematic of a single electro-swing adsorption electrochemical cell with porous electrodes and electrolyte separators. The outer electrodes, coated with poly-1,4-anthraquinone composite, can capture CO2 on application of a reducing potential via carboxylation of quinone, and release the CO2 on reversal of the polarity. The inner polyvinylferrocene-containing electrode serves as an electron source and sink for the quinone reduction and oxidation, respectively.

This system is designed to treat flue gases, but may be adapted to other types of systems. Recently I've been thinking quite a bit about carbon dioxide as a working fluid for Brayton type devices, and in particular have been focusing attention on a cycle with which I was not familiar until recently, the Allam cycle.

It is a closed cycle, where the combustion gas is also the working fluid.

The Allam cycle is designed primarily for use with dangerous natural gas, but I would imagine that it could also be adapted to other systems, notably those derived from biomass.

During the Allam cycle, portions of the carbon dioxide working fluid are removed from the system, commonly described as being for the purpose of "storage," but coupled with nuclear primary energy, could be utilized for the purpose of making materials, for example carbon nanotubes impregnated with, um, ferrocene polymers, and millions of other similar products.

Anyway, from the paper, some SEM images of the system:

Fig. 2 (a) SEM micrograph of the cathode non-woven carbon mat coated with P14AQCNT, with details of coated and uncoated areas. (b, c and f) SEM micrographs of increasing magnification of carbon fibers coated with P14AQCNT. (d) SEM micrograph of the uncoated carbon fibers. (e) TEM of PAQCNT showing the amorphous polyanthraquinone decorating the MWCNT, a result of the π π interaction.

The "π π" here is reference to the interaction between the aromatic rings of the benzoquinones and those of the carbon nanotubes.

Cyclic voltamograms of the reduction system:

Fig. 3 Superimposed CVs of PVFcCNT ( ) and P14AQCNT ( ) under N2 and ( ) under CO2 in [Bmim][TF2N], at 20 mV s−1, vs. Fc, at T ∼ 21 C. The two potential windows are shown; ΔV1 under CO2 and ΔV2 under N2.

(It is worth noting that increasingly more electrochemical reduction systems for carbon dioxide are known.)

More SEM images:

Fig. 4 (a) SEM micrograph of the anode non-woven carbon mat coated with PVFcCNT with details of coated and uncoated areas. (b and c) SEM micrographs of carbon fibers coated with PVFcCNT, the squares indicates the region which is magnified in the next micrograph. (d) SEM micrograph of the magnified polymer-coated CNTs from a different area on the electrode.

Nice photographs of the experimental apparatus:

Fig. 5 (a) Custom-made sealed chamber for closed system experiments with pressure transducer to monitor the changes in pressure as CO2 is adsorbed and desorbed upon cycling of the cell potential. The internal of the sealed chamber (b) with and (c) without the insulating cup. (d) Layers of the electrochemical cell assembled in the sealed chamber.

They obviously have a nice machine shop at MIT.

The system shows nice stability over a large number of cycles:

Fig. 6 (a) Changes in the number of moles of CO2 captured upon charging and discharge of the electrochemical cell over 10 cycles, normalized by the moles of quinone on the electrode ( ). The CO2 captured from and released to the chamber tracks the charge applied to the electrochemical cell, normalized by the area of the cell ( ). (b) The CO2 captured under different feed concentrations. (c) Capacity of cell over 7000 cycles. In a different set of experiments using a larger cell and cavity, (d) shows the effect of varying charging potential for a 1000 s capture and (e) shows the effect of varying the capture duration at −1.8 V capture potential. These experiments were conducted at T ∼ 21 C.

A cartoon of the configuration of the system:

Fig. 7 (a) Schematic illustration of the parallel passage electrochemical cell contactor. The blue region indicates the saturated zone and the development of the mass transfer zone. (b) Photograph of a flow bed with a stack of the electrochemical cells.

Breakthrough at various concentrations:

Fig. 8 (a) Breakthrough profiles obtained at four inlet concentrations. (b) Same breakthrough profiles in (a) normalized by the inlet concentrations. (c) Breakthrough profile obtained from a large system operating at ∼10% inlet concentration. (d) Breakthrough profiles obtained from five replicate runs of a smaller system operating at ∼0.8% inlet concentration. These experiments were conducted at T ∼ 21 C.

A chemical schematic of the process:

Scheme 1 (a) Two single-electron reduction waves of anthraquinone in the absence of electrophiles. (b) One two-electron reduction wave of anthraquinone in the presence of CO2.

The electrochemical reaction scheme:

Scheme 2 Reaction steps of the double carboxylation of quinones (a) in high and (b) low CO2 fluxes towards the anthraquinone electrode. E represents an electrochemical reaction step. C represents a chemical reaction step.

A cartoon of the electrochemical cell configuration:

Fig. 9 Cross-section of the electrochemical cell used in the simulations.

A graphic of charge and discharge of the system:

Fig. 10 Simulation of charging the electrochemical cell at different CO2 concentrations at constant current. (a) Potential difference of the cell. The change in concentration of quinone with charge is shown at (b) 0%, (c) 2% and (d) 5% CO2.

More on breakthrough (the physical saturation of the system):

Fig. 11 (a) Breakthrough profiles from simulation at 50% CO2 and charging potential of 1.7 V; inset: the concentration of CO2 in the channel and the concentration of unbound quinonic species in the cathode with bed volume. (b) Breakthrough profiles at multiple capture potentials at 50% inlet concentration. (c) Breakthrough profiles from simulation at multiple inlet concentrations at a charging potential of 1.3 V. (d) Normalized breakthrough profiles of the capture experiments in (c). (e) The current across the electrochemical cells during the capture experiments in (b). (f) The current across the electrochemical cells during the capture experiments in (c).

More electrochemical schematics:

Scheme 3 The reduction of anthraquinone at a potential higher than its second reduction potential with a limited flux of CO2.

An important graphic showing the energy penalties associated with carbon capture using this device:

Fig. 12 Fraction of CO2 released from the PAQCNT electrode with release voltage. At a constant capture cell voltage of 1.3 V, less of the bed is recovered with increasing release cell voltage, but the energy per mole of CO2 captured and released also decreases.

There are several types of "swing" approaches to gas separations commonly used, "temperature swing" - a simple well known example is to use a metal hydroxide, calcium hydroxide ("slaked lime" is often considered where the carbon dioxide is captured at low temperatures and the lime regenerated at very high temperatures - "pressure swing absorption" which relies on the differential diffusion of gases into porous beads, and this system, "electroswing absorption".

They are compared in this graphic:

Fig. 13 Comparison of temperature- (TSA), pressure- (PSA) and electro- (ESA) swing operations showing the impact of sorption isotherms on total working capacity.

I note that electricity is always produced at a thermodynamic loss, and thus electricity can, and often is, thermodynamically questionable. Irrespective of popular opinion to the contrary, electricity is not "green" or "clean."

However, there are circumstances where it can be utilized as a thermodynamic enhancer, specifically at very high temperatures, where it is a side product of another process. For example, the thermochemical splitting of water (or carbon dioxide) can be driven at high temperatures with far greater thermodynamic efficiency than via electrochemical approaches, most famously used for water electrolysis. Since the hydrogen and oxygen in the thermochemical water case, and carbon monoxide and oxygen in the carbon dioxide thermochemical case, will ultimately be brought to ambient temperatures, a temperature gradient is necessary, and as such can be utilized to drive turbines (Brayton), boil water (Rankine) or both, raising the efficiency.

Such temperatures are only economically, thermodynamically and environmentally viable with nuclear energy.

Cool idea; cool paper.

Have a nice day tomorrow.

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Reply Faradaic electro-swing reactive adsorption for CO2 capture. (Original post)
NNadir Oct 30 OP
erronis Oct 30 #1

Response to NNadir (Original post)

Wed Oct 30, 2019, 08:12 PM

1. Thanks for an interesting article. I'm still absorbing all the bits and pieces...

However "Such temperatures are only economically, thermodynamically and environmentally viable with nuclear energy" (Your opinion, right?)

This seems like a personal value judgement. Not one that I disagree with, but maybe subject to discussion.

I worked for US/French nuclear fission reactor companies for several years in the 80s and 90s. Unless significant advances have been made in waste disposal recently, I still question the "environmentally viable" clause.

As an aside, if fusion-based generators become a reality, how will this change these equations?

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