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Assessment of Artificial Photosynthetic Systems for Integrated Carbon Capture and Conversion Aditya Prajapati, and Meenesh R. Singh ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04969 • Publication Date (Web): 05 Feb 2019 Downloaded from http://pubs.acs.org on February 11, 2019

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ACS Sustainable Chemistry & Engineering

Assessment of Artificial Photosynthetic Systems for Integrated Carbon Capture and Conversion Aditya Prajapati and Meenesh R. Singh*

Department of Chemical Engineering, The University of Illinois at Chicago, Chicago, IL 60607

Corresponding Author: Prof. Meenesh R. Singh Assistant Professor Department of Chemical Engineering 810 S. Clinton St. The University of Illinois at Chicago Chicago, IL 60607 Tel: (312) 996-3424 Email: [email protected]

Keywords: carbon capture and conversion, artificial photosynthesis, photoelectrochemical cell, CO2 reduction, STF efficiency 1 ACS Paragon Plus Environment

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Abstract Sustainable and continuous operation of artificial photosynthetic (AP) system requires a constant supply of CO2 captured from the dilute sources such as the flue gas and the air to make fuels and chemicals. Although the architecture of AP systems resembles that of the natural leaves, they lack an important component like stomata to capture CO2 directly from the dilute sources. Here we design and evaluate the solar-to-fuel (STF) efficiency of the integrated AP system that captures CO2 directly from the air/flue gas and converts it to fuels using sunlight. The thermodynamic limit to the STF efficiency of such integrated AP system range from 34% - 40% for various products such as CO, HCOOH, CH4, CH3OH, C2H4, and C2H5OH using ideal multijunction light absorbers and reversible carbon capture process. The performance limits of real, integrated AP systems are obtained here for two different integration schemes such as integrated cascade systems and fully integrated systems that use technology-ready materials and components. The fully integrated AP systems can be > 66% more efficient than the integrated cascade systems as they do not need additional energy for compression, separation, and recycling of CO2. While the integrated cascade systems show highest STF efficiency with the adsorption-based carbon capture process, the fully integrated AP systems are only compatible with the membrane-based carbon capture process. We also show that the synthesis of higher-electron products such as CH4, CH3OH, C2H4, and C2H5OH can be more favorable for the robust operation of integrated AP system. A design of the fully integrated AP system is proposed that uses moisture-gradient across the anion-exchange membrane to capture CO2 from the air, which is then converted directly to fuels using water, and sunlight. Such a fully integrated AP system can produce ~0.4 ton/day of CO at a cost of ~$185/ton and STF efficiency of ~14% while reducing the CO2 level of the surrounding air by 10% at steadystate operation. The fully integrated AP systems are modular, scalable, and ~14 times more efficient than natural leaves.

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Introduction Integrated artificial photosynthetic (AP) systems consisting of photovoltaic or photoelectrochemical cells in direct contact with the mildly acidic1-2, near-neutral3-5, and mildly alkaline6-8 pH electrolyte, have been developed to convert pure CO2 gas dissolved in the electrolyte into fuels and chemicals using sunlight. Since the efficiency of such AP systems reduces drastically with depleting partial pressure of CO2,9-11 its continuous operation requires a constant supply of concentrated CO2 from the diluted source streams such as flue gas or air using a suitable carbon capture system. The overall solar-to-fuel (STF) efficiency of AP systems, therefore, depends on the performance of the carbon capture and conversion systems and their integration. Carbon capture and conversion processes can be coupled as either an integrated cascade system or a singlepass, fully integrated system. When carbon capture and conversion processes are operating independently in an integrated cascade system, as shown in Fig 1A and 1B, pure CO2 supplied from the carbon capture system is diluted due to partial conversion of CO2 and formation of gaseous products in the AP system, which requires downstream purification for recycling of CO2.12 Whereas the fully integrated carbon capture and conversion systems (Fig. 1C) is a single-pass process that works on the principles of reactive mass transport and does not require any downstream purification or recycling of CO2. The reactive mass transport can improve not only the efficiency of CO2 capture but also the STF efficiency of the fully integrated AP system. Unlike the integrated cascade system where carbon capture and conversion processes can operate at different capacities, the fully integrated systems can be limited by the mass transport of CO2 in the gas phase. The concept of integrated carbon capture and conversion is central to process intensification13-14 where overall efficiency of the process can be enhanced by converting CO2 immediately after the capture. Stucki et al.15 have initially proposed an integrated carbon capture and conversion process where CO2 from the air is absorbed in KOH solution to produce K2CO3 which after electrodialysis regenerates KOH and a mixture of H2 and CO2. While KOH is recycled back to absorb CO2 from the air, the post-electrolysis mixture of CO2 and H2 is catalytically converted to CH3OH.15 Another cascade configuration of an integrated carbon capture and conversion process showed almost 99.7% utilization of CO2 to produce synthesis gas.16 Such integrated cascade systems not only require recycling of unreacted species but also lead to higher energy consumption. Conversely, the fully integrated systems can couple CO2 capture and conversion processes into a single reactor such that nearly 100% utilization of CO2 can be achieved in a single-pass. Kim et al.17 have developed such a fully integrated system where CO2 is captured using CaO and the resulting CaCO3 reacts with CH4 over supported Ni catalyst to yield synthesis gas and regenerated CaO. Recently, a solar-driven, integrated system was developed utilizing a moisture-swing process to capture CO2 from the air, which was fed continuously to the membrane carbonation photobioreactor for CO2 conversion.18 However, the AP system with integrated carbon capture process that can regulate the uptake of CO2 from the dilute streams to produce chemicals or fuels using sunlight has not been designed yet. The architecture of such a fully integrated AP system resembles natural leaf which has stomata to capture CO2 from the air and photosystems to harvest 3 ACS Paragon Plus Environment


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sunlight for the conversion of CO2 to carbohydrates. A large-scale implementation of such a fully integrated AP system can have a profound environmental impact as it can drawdown the concentration of CO2 in atmospheric layer by ~130 ppm at a steady-state operation.19 The fully integrated AP system consists of six different components - i) electrocatalyst for oxygen evolution reaction (OER), ii) electrocatalyst for CO2 reduction reaction (CO2RR), iii) electrolyte, iv) membrane for product separation, v) light absorber and vi) carbon capture unit. Various electrocatalysts have been investigated and used in AP systems for the conversion of CO2 to oxygenates such as CO, HCOOH, C2H2O4, CH3OH, and C2H5OH; and hydrocarbons such as CH4, C2H4, and C2H6.20-21 The CO2-saturated, near-neutral pH electrolyte and the anion-exchange membrane have been used in such efficient AP systems.22 The optimal band-gaps and junctions of light absorbers can be employed to yield maximum photocurrent density and photovoltage to balance polarization losses from electrocatalyst, membrane, and electrolyte.23 When carbon capture unit is integrated with the AP system, the light absorber must also supplement the power required to capture CO2, which in turn, provides the necessary CO2 flux to support the photocurrent density. The carbon capture processes24 that can be integrated with the carbon conversion process are i) cryogenic distillation, ii) absorption, iii) adsorption, and iv) membrane separation. Cryogenic distillation can effectively capture CO2 but is a very energy intensive process. Liquid absorbents,25 solid adsorbents,26 and membranes27 are often used to absorb CO2 as they require lesser energy as compared to cryogenic distillation. However, releasing CO2 after absorption can also be energy intensive. CO2 can be desorbed from the solid adsorbent by varying either temperature or pressure, and from liquid absorbent by varying temperature or by electrodialysis.28-29 More recently quaternary ammonium resins were used to selectively capture CO2 from the air using a moistureswing adsorption.30 These carbon capture sorbents can be integrated with the AP system such that CO2 is directly supplied to the cathode for electrocatalytic reduction. We describe herein performance characteristics of an integrated AP system coupled with different carbon capture processes such as liquid absorption followed by temperature swing desorption or electrodialysis, solid adsorption followed by temperature swing desorption, and membrane separation followed by moisture swing desorption, for two different CO2 sources – air, and flue gas. The theoretical limits of STF efficiencies were obtained for three configurations of integrated AP systems – i) ideal AP system integrated with ideal carbon capture unit, where the integrated system consists of a reversible carbon capture process, an adiabatic electrocatalytic reaction, and an intrinsically ideal light absorber, ii) integrated cascade system where ideal AP system is integrated with the actual carbon capture process, and iii) fully integrated AP system consisting of state-of-the-art and efficient components such as InGaP/GaAs/Ge triple-junction light absorber, IrO2 anode, Ag cathode, 0.1 M KHCO3 electrolyte, and membrane-based carbon capture unit. The STF efficiencies of these integrated systems were calculated with respect to the primary products of CO2RR such as H2, CO, HCOOH, CH3OH, CH4, C2H5OH, and C2H4.31


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Strategies to alleviate the mass transfer limitations in the fully integrated AP systems are also discussed.

Figure 1: Configurations of integrated carbon capture and conversion processes. (A, B) integrated cascade systems coupling absorption- or adsorption-based carbon capture with the AP system 5 ACS Paragon Plus Environment


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where unreacted CO2 is separated from the product stream and recycled back to the carbon capture unit, (C) fully integrated system consisting of a hollow-fiber carbon capture membrane in contact with the catholyte that supply CO2 to the cathode to produce fuels and chemicals. The objectives of this study were to calculate the thermodynamic, and realistic STF efficiencies for integrated capture and conversion of CO2 to fuels and chemicals; to determine optimal band-gaps for alternative light-absorber configurations required to achieve efficient CO2 capture and conversion; to identify carbon capture process compatible with the AP system and to develop strategies for direct capture of CO2 from air and its conversion to fuels and chemicals in a fully integrated system. The remaining of this article is organized as follows. Theoretical Methods section describes the mathematical expressions used to determine the Shockley-Queisser (SQ) limits of multi-junction light absorbers, the characteristics of electrochemical polarization curve inclusive of energy losses due to carbon capture, and the properties of the light absorber, catalysts, electrolyte, membrane, and carbon capture unit used to define the STF efficiency. The Results and Discussion section presents STF efficiencies of integrated AP system coupled with different carbon capture processes, the effect of the energy of carbon capture, CO2 concentration, and the configuration of light absorber on the STF efficiency, and analysis of a fully integrated AP system. The Conclusions and Perspectives section presents conclusions and strategies to implement the fully integrated AP system for direct conversion of CO2 from the air or the flue gas to fuels and chemicals. Theoretical Methods This section discusses theoretical methods for calculation of current density-voltage (JV) characteristics for various components of the integrated AP system such as light absorber, electrocatalysts, electrolyte, membrane separator, and carbon capture unit. These JV characteristics are then used to calculate STF efficiencies of integrated systems.

JV Characteristics of a Multi-junction Light Absorber The ideal, intrinsic light absorbers with up to ten junctions of optimal band-gaps are considered to evaluate performance limits of an ideal, integrated AP system. The JV characteristic of each junction in the light absorber is obtained from a detailed balance of photons32 involving thermal generation of carriers, electron-hole recombination and total absorption of photons of energy higher than the band-gap of the junction. The extrinsic losses due to light reflection, contact shadowing, series resistance, an inefficient collection of electrons and holes, non-radiative recombination, and temperature rise are ignored in this analysis as they can be minimized by appropriate material selection and design.33 The JV characteristics of an ideal, multijunction light absorber obtained by adding bias across each junction for a terrestrial air mass (AM) 1.5 spectrum at 1 sun is given as-23. 6 ACS Paragon Plus Environment

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V (J ) 

kT e

n

 J sc , i  J

 ln  i 1



J 0, i

  1 

(2)

where V (unit: V) is the total potential generated across the light absorber, e (unit: C) is the electronic charge, k (unit: J K-1) is the Boltzmann constant, T (unit: K) is the temperature, n is the number of junctions, J (unit: mA.cm-2)is the photocurrent density, Jsc,i (unit: mA.cm-2) is the shortcircuit current density corresponding to AM 1.5 spectrum at 1 sun, and J0,i (unit: mA.cm-2) is the saturation current density of the ith junction. Due to current matching between the junctions of the light absorber, the lowest short-circuit current among all the junctions determines the short-circuit current density of the light absorber. The details regarding the calculation of saturation current density from SQ theory is given in the Supporting Information. To assess the maximum realistic STF efficiency, Spectrolab’s34 28% efficient InGaP/GaAs/Ge triple-junction light absorber with a fill factor of 0.756 is utilized in the integrated AP system. The measured JV characteristics under 1 sun are shown in Fig. S2 of the Supporting Information.

JV Characteristics of Electrocatalysts, Electrolyte, and Membrane The JV characteristics (also referred to as polarization curve) of the electrochemical CO2 reduction system depends on the i) kinetic overpotential of the electrocatalysts,35 ii) conductivity and transference number of electrolyte and membrane,31, 36-37 iii) operating conditions such as CO2 flowrate, partial pressure, and temperature, 22 and iv) physical dimensions of the components.22, 38 The cell potential is a sum of the equilibrium potentials E 0 and kinetic overpotentials  for the OER and CO2RR, the solution losses solution in the electrolyte and membrane, and the Nernstian losses Nernstian at the electrodes,39-41 and is given by 0 0  ECO  OER  J   CO2 RR  J   solution  J   Nernestian  J  V  J   EOER 2 RR

(3)

The polarization curve was computed according to the method given in Singh et al.22 for the most commonly employed cell configuration31, 42-44 – Ag nanofoam cathode45, IrO2 anode, 0.1 M KHCO3 electrolyte, 5 mm spacing between membrane and electrode, 0.1 mm thickness of boundary layer, and 20 sccm CO2 flowrate at standard temperature and pressure (STP). The operation of an ideal electrochemical cell can be considered in two different ideal states – equilibrium or adiabatic. An electrochemical cell under equilibrium does not produce a net current and therefore cannot yield a non-zero STF efficiency for any configuration of integrated AP system. However, the electrochemical cell operating at adiabatic (or isentropic) condition can produce a net positive current at potentials higher than the equilibrium potential. The operating cell potential under adiabatic condition is known as thermoneutral potential,23, 46 defined as



UH 

H 0 , nF

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(4)

where F (unit: C mol-1) is the Faraday constant, U H (unit: V) is the thermoneutral potential, H 0 (unit: J mol-1) is the standard enthalpy change of reaction per mole of product, and n is the moles of electrons transferred per mole of product. Table S1 in the Supporting Information shows the thermoneutral potentials for various products - H2, CO, HCOOH, CH3OH, CH4, C2H4, and C2H5OH based on their lower heating and higher heating values. Since the thermoneutral potential is independent of the current density, the JV characteristic of an adiabatic electrochemical cell is a vertical line,

V  J   UH

(5)

JV Characteristics of Carbon Capture Unit The ideal carbon capture unit is modeled as a reversible process that concentrates CO2 from the dilute sources such as the air (~0.04% CO2) and the flue gas (~15% CO2) to a relatively pure gas (>90% CO2). Assuming ideal mixtures at STP conditions, the free energy change of concentrating CO2 from the air and the flue gas is 19.4 kJ mol-1 and 4.7 kJ mol-1, respectively. The calculation for the free energy change of ideal carbon capture process is given in section S4 of the Supporting Information. For integrated systems where the captured CO2 is reduced to an n-electron product, the energy of carbon capture  EC  can be represented as carbon capture potentialUC 

EC nF

(6)

Table 1 shows the reported values of energy consumption for various carbon capture processes using two different CO2 sources – air, and flue gas. It can be seen that the energy consumption for real processes is at least an order of magnitude higher than the ideal process. The energy of capturing CO2 from the air is at least two-fold higher than that from the flue gas. The reported values of energy consumption in Table 1 are independent of the rate of CO2 capture and depends only on the concentration of CO2 in the source and concentrated streams. For example, electrodialysis of seawater creates stable pH gradients for the release of CO2, and therefore the measured JV characteristics29 of an electrodialysis-based carbon capture process is almost a vertical line with a few outliers (see the Supporting Information). Similarly, the rate of CO2 capture using a moisture-swing membrane is independent of the energy of carbon capture. A discussion on the mechanism of carbon capture and the energy requirement of such a process is given in the Supporting Information. The integrated AP systems operating at current densities < 100 A m-2 require CO2 flux < 51.8  mol m-2 s-1, which is within the carbon uptake range- 10-100  mol m2

s-1 of most carbon capture processes.28 Therefore, the JV characteristics of carbon capture processes can be approximated by a current-independent carbon capture potential, 8 ACS Paragon Plus Environment

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V  J   UC

(7)

Table 1: List of carbon capture processes showing methods of CO2 sorption and release, energy consumption and carbon capture potential for concentrating CO2 from the air and the flue gas. The values of carbon capture potential are based on the direct synthesis of 2-electron products such as CO and HCOOH. CO2 Source

Air

CO2 Sorption

CO2 Release

Absorbent/ Adsorbent

Ideal

Ideal

Liquid Absorption

Electrodialysis

Ideal Mixture K2CO3 Seawater NaOH CaO MOFs Amine-based Resin Ideal Mixture Monoethanolamine

Solid Adsorption Membrane

Flue gas

Heating

Ideal Liquid Absorption Solid Adsorption Membrane

Heating MoistureSwing Ideal Heating

Energy Requirement (kJ/mol) 20 1284.32 242 679 10600 90

Carbon Capture Potential (V) 0.10 6.7 1.3 3.5 54.9 0.46

190.94

0.99

4.31

0.02

181

0.94

48

Ref.

15 29 47 28 25 30

Heating

K2CO3

44.72

0.23

49

MoistureSwing

Amine-based Resin

46

0.24

*

* The values of energy requirement and carbon capture potential are estimated assuming logarithmic dependence of energy on the concentration of CO2

Calculation of STF Efficiency The integrated AP systems utilize solar photons to generate the necessary electrical work for the synthesis of fuels. This electrical work can be expressed as the composite JV characteristic, which is the sum of the individual JV characteristics for CO2 capture, species transport and photo/electrocatalytic reactions. For example, the composite JV characteristic of an ideal system is given by the sum of thermoneutral potential U H  and carbon capture potential U C  for ideal mixtures; and for a real system is given by the sum of JV characteristics in equations (3) and (7). In the case of mass transfer limitations of CO2 in the gas phase, the Nernstian losses can be significantly larger as compared to other potential losses resulting in a limiting current density,22 which can be approximated as J lim  nFk g cCO2

(8) 9

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where cCO2 (unit: mol lit-1) is the concentration of CO2 in the gas phase and k g (unit: m s-1)is the mass transfer coefficient which can be estimated using an empirical correlation in terms of Sherwood number provided in the Supporting Information. The operating potential Vop  and current density  J op  of integrated AP systems can be then obtained from the intersection of JV characteristic of a light absorber (2) and the composite JV characteristic representing carbon capture, species transport and photo/electrocatalytic reactions.50 The STF efficiency STF is defined as the ratio of power generated as a fuel from the CO2RR to the incident solar power and is given by

STF 

J op   F , p U H , p  p

PS

(9)

where  F , p is the Faradaic efficiency of a product p , which is a function of the operating potential Vop ; and PS is the average power of solar insolation per unit area. We noted that there were other

definitions of the STF efficiency that have been proposed that use the equilibrium potential of the fuels instead of the thermoneutral potential based on the lower heating value of the fuel.51 Such descriptions of STF efficiency are useful for the fuels (such as hydrogen) which can be used in fuel cells. However, the definition of the STF efficiency based on the thermoneutral potential is better suited for other chemicals and fuels.23 Simulation of Species Transport in Fully Integrated AP System The mathematical model for the electrochemical cell previously developed by Singh et al.22 was used to calculate polarization losses, species distribution, and STF efficiency in the fully integrated AP system. A 2-D model of fully integrated AP system was developed where the width of the photoelectrode was set to 10 mm, the width of the membrane was 1 mm, the height of the photoelectrode was 100 µm, and the height of the electrolyte was 5 mm which also ensures the mean ionic path to be similar to 1-D cell described in section JV Characteristics of Electrocatalysts, Electrolyte, and Membrane .22 The mathematical model described in section S7 of the Supporting Information was solved using COMSOL Multiphysics to obtain species distribution, polarization losses, and current density.


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Results and Discussion The thermodynamic and achievable limits of STF efficiencies for various integrated AP systems are discussed next. The optimal configurations of light absorbers and conditions of carbon capture process for attaining maximum STF efficiency are also examined in this section.

Thermodynamic Limits of STF Efficiency for Ideal, Integrated AP Systems The ideal, integrated AP system can be realized for an intrinsically ideal light absorber powering adiabatic electrochemical reactions coupled with a reversible carbon capture process. The maximum operating current density for this system can be obtained by optimizing band-gaps of the light absorber at the operating potential that is equal to the sum of thermoneutral potential and reversible carbon capture potential. The thermodynamic limit of the STF efficiency can be calculated from Eqn. (9) using the maximum operating current density and the thermoneutral potential of the products. Fig. 2 shows the thermodynamic limits of STF efficiencies for direct conversion of CO2 from the air and H2O to H2, CO, HCOOH, CH3OH, CH4, C2H4, and C2H5OH using a single-, double-, triple-, or quadruple-junction light absorber. The optimal values of bandgaps corresponding to the thermodynamic STF efficiencies in Fig. 2 are provided in Fig. S3 of the Supporting Information. The increase in STF efficiency from single-to double-junction light absorber is due to increase in absorption of the solar spectrum, whereas the decrease in the STF efficiency with increasing number of junctions greater than 2 is due to increasing mismatch between photocurrent from each junction. Fig. S1 shows that the maximum photocurrent density is constant for the range of thermoneutral potentials from 1.1 V to 1.5 V and it decreases with increasing number of junctions from 2 to 10. Therefore, the variation between the STF efficiency of different products for junctions > 2 is due to differences in their thermoneutral potentials. The maximum STF efficiencies for different products lie between 34.6 and 40.1% when CO2 is captured directly from air, which are achieved using a double-junction light absorber. The thermodynamic limits of STF efficiencies for a double-junction light absorber and CO2 captured from the flue gas are - 39.1% for H2, 41.6% for CO, 35.8% for HCOOH, 36.3% for CH3OH, 34.7% CH4, 37.2% for C2H4, and 34.7% C2H5OH. Since the ideal carbon capture potential for the flue gas is ~ 0.1 V, the thermodynamic STF efficiencies for the integrated AP system is not significantly different than that for AP systems without carbon capture process.23



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Figure 2: Thermodynamic limits of STF efficiencies for direct conversion of CO2 from the air and H2O to produce H2, CO, HCOOH, CH3OH, CH4, C2H4, and C2H5OH using a single-, double-, triple-, or quadruple-junction light absorber.

Effect of the Energy of Carbon Capture on the STF Efficiency for Integrated Cascade System To assess the effect of the energy of carbon capture on the STF efficiency of the integrated cascade system, the carbon capture potential for each product is varied while operating the electrochemical reaction and the light absorber at the ideal conditions. Fig. 3 shows that the thermodynamic limit of STF efficiency for integrated cascade system decreases with increasing the energy of carbon capture from 0 to 375 kJ mol-1 of CO2 for single-, double-, and triple-junction light absorbers. The number of electrons available for CO2 conversion decreases with increasing the energy of carbon capture process, which in turn, reduces the STF efficiency of the integrated AP system. The rate of change of STF efficiency for lower-electron products (H2, CO, and HCOOH) is higher as compared to the STF efficiency for higher-electron products because the energy requirement per electron (the carbon capture potential) decreases with increasing number of electrons per mole of product (see Eqn. (6)). Therefore, the synthesis of higher-electron products 12 ACS Paragon Plus Environment

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such as CH3OH, CH4, C2H5OH or C2H4 yields higher STF efficiency in the integrated cascade system using either single- or double-junction light absorber. However, the synthesis of lowerelectron products such as H2, CO or HCOOH can be more favorable when triple- or higher-junction light absorbers are used (see Fig. 3C), in which case the thermoneutral potentials governs the variation in the STF efficiency of products as the maximum photocurrent density is constant. Fig. S12 of the Supporting Information shows the variation in the STF efficiency of CO and CH3OH as a function of the number of junctions in the light absorber, which confirms that the higherelectron products yield higher STF efficiency up to double-junction and thereafter the lowerelectron products provide highest STF efficiency.



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Figure 3: Maximum STF efficiency of H2, CO, HCOOH, CH3OH, CH4, C2H5OH, and C2H4 versus the energy of carbon capture for an integrated cascade system consisting of a non-ideal carbon capture process, an adiabatic electrochemical reaction, an intrinsically ideal (A) single-junction, (B) double-junction, or (C) triple-junction light absorbers. The energy of carbon capture using liquid absorbent (ABS), solid adsorbent (ADS), and membrane (MEM) are marked as black, red, 14 ACS Paragon Plus Environment

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and blue vertical lines, respectively, for CO2 sources such as flue gas (dashed vertical lines) and the air (solid vertical lines). The vertical lines in Fig. 3 shows the energy of carbon capture using three different sorbents – liquid absorbent (ABS), solid adsorbent (ADS) and membrane (MEM) utilizing two different CO2 sources – air (solid) and flue gas (dashed), which are given in Table 1. The six state-of-theart processes considered here are- (i) ABS (solid line) representing absorption of CO2 from the air using seawater followed by its release using electrodialysis, (ii) ABS (dashed line) representing absorption of CO2 from the flue gas using monoethanolamine followed by heating to release CO2, (iii) ADS (solid line) representing adsorption of CO2 from the air using MOFs followed by heating to release CO2, (iv) ADS (dashed line) representing adsorption of CO2 from the flue gas using K2CO3 followed by heating to release CO2, and (v, vi) MEM representing absorption of CO2 from either the air (solid line) or the flue gas (dashed line) using ion-exchange resin followed by moisture-swing desorption to release CO2. The physics and mechanisms of these carbon capture processes are discussed in the Supporting Information. Fig. 3 shows that the STF efficiency of an integrated cascade system increases in the order ABS < MEM < ADS, which suggests that solid adsorbents and ion-exchange membranes are better sorbents than liquid absorbents for the integrated cascade system. However, the sensitivity of the STF efficiency on the choice of sorbents decreases with increasing number of junctions and becomes negligible for quadruple- or higherjunction light absorbers (see Fig. S10 of the Supporting Information). Fig. 3 also shows that the STF efficiency is higher for the integrated processes utilizing CO2 directly from the flue gas than the air, which is due to the difference in the energy of carbon capture and not a consequence of CO2 mass transfer limitations. The integrated cascade systems can be designed to eliminate mass transfer limitations by operating carbon capture and conversion systems independently at different capacities to produce fuels. However, the energy requirement for secondary processes such as separating CO2 from the product gases, pumping, and compressing CO2 for recycling in the cascade system can be as high as 123 kJ mol-1,48 which will add on to the total energy of carbon capture of various processes in Fig. 3 and thereby reducing significantly the STF efficiency of the integrated cascade system. Whereas the fully integrated system does not require CO2 recycling, separation or compression as it captures CO2 as per the consumption at the cathode of the AP system.

Effect of CO2 Mass Transfer on the STF Efficiency of a Fully Integrated Process The fully integrated processes as shown in Fig. 1C can be energy efficient as they do not require separation, recycling, and compression of CO2. However, due to the synergistic operation of carbon capture and conversion processes, any limitations to the mass transfer of CO2 from the dilute sources such as the flue gas or the air can severely affect the STF efficiency of the fully integrated system. Of the three different types of sorbents used for CO2 capture, anion-exchange membrane is the most suitable sorbent for the fully integrated AP system as it can selectively capture CO2 at a higher flux while maintaining the optimal electrolyte composition near the 15 ACS Paragon Plus Environment


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cathode for photo/electrochemical conversion of CO2. Solid and liquid sorbents require separate regeneration to release CO2 and therefore they cannot be integrated seamlessly with the AP system.

Figure 4: Effect of CO2 concentration in the feed stream and the energy of carbon capture on the STF efficiency of CO production in a fully integrated AP system comprising of (A) an ideal 16 ACS Paragon Plus Environment

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double-junction, (B) an ideal triple-junction, and (C) 28% efficient InGaP/GaAs/Ge triple-junction light absorber, Ag nanofoam cathode, IrO2 anode, 0.1 M KHCO3 electrolyte and anion-exchange resin for carbon capture. The STF efficiencies in (A) and (B) were obtained for optimal band-gaps of ideal double- and triple-junction light absorbers, respectively. There are three different regions in each panel where STF efficiency is limited by i) the mass transfer of CO2 in the gas phase for lower CO2 concentrations, referred to as a mass transfer limited region, ii) the higher energy requirement of a carbon capture process, referred to as a carbon capture limited region, and iii) the absorption of sunlight by a light absorber, referred to as light absorption limited region. Fig. 4 shows the effect of CO2 concentration of the source and the energy of carbon capture on the maximum STF efficiency of CO formation in a fully integrated system (schematic shown in Fig. 1C), which consists of an intrinsically ideal double- or triple-junction light absorber coated with IrO2 anode, and a nanofoam Ag cathode, and immersed in 0.1M KHCO3 electrolyte in contact with an ion-exchange resin for CO2 capture. The maximum STF efficiency in Fig. 4A and 4B were obtained for optimal band-gaps of double- and triple-junction light absorber, respectively. The STF efficiency in Fig. 4C corresponds to a fully integrated system with InGaP/GaAs/Ge triplejunction light absorber. The STF efficiency in Fig. 4 increases with increasing CO2 concentration up to a maximum of 18%, which is an effect of decreasing Nernstian losses coupled with increasing limiting current density.22 Therefore, the STF efficiencies are mass transfer limited for CO2 concentrations < 40% and light absorption limited for higher concentrations of CO2. Fig. 4 also shows that the STF efficiency decreases with increasing energy of carbon capture, which is due to ineffective absorption of light at higher photovoltages. Increasing the number of junctions from double (Fig. 4A) to triple (Fig. 4B) increases the STF efficiency in the region of higher carbon capture energy. The STF efficiency is therefore limited by the carbon capture process for the energies > 170 kJ mol-1 of CO2. Fig. 4 also shows that the STF efficiency of fully integrated systems is severely affected by the concentration of CO2 in the source. For example, the maximum STF efficiency of fully integrated systems is 0.01% and 3.83% when the concentrations of CO2 in the air is 0.04% and the flue gas is 15%, respectively, which are very low for the large-scale implementation of AP systems.52-53 The strategies to alleviate mass transfer limitations of CO2 from dilute sources and thereby increase the STF efficiency of fully integrated AP system are discussed next.

Evaluation of a Fully Integrated AP System for Direct Capture and Conversion of CO2 from the Air or the Flue Gas to Fuels The rate of CO2 mass transfer in the gas phase can be increased by increasing the interfacial area of the membrane contactors such as hollow fiber membrane. The modules of hollow fiber membranes have been employed to effectively capture CO2 from the flue gas54-55 and to supply CO2 to the photo/microbial reactors.18, 56 A similar hollow fiber module made of anion-exchange resin when immersed in an aqueous electrolyte can continuously capture CO2 from the air or the 17 ACS Paragon Plus Environment


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flue gas by developing moisture gradient across the membrane. This mechanism is similar to the moisture-swing process30 where the dry-side of membrane absorbs CO2 and the wet-side of the membrane releases CO2 into the electrolyte (for example, see the Supporting Information for additional details). The fully integrated AP system shown in Fig. 1C can have such hollow fiber module in direct contact with the catholyte to capture CO2 from the dry air or the dry flue gas of relative humidity < 0.4. Fig. 5A and 5B show the effect the specific interfacial area1 of the hollow fiber membrane on the STF efficiency of the fully integrated AP system capturing CO2 from the air and producing CO. The fully integrated system consists of InGaP/GaAs/Ge triple-junction light absorber, IrO2 anode, nanofoam Ag cathode and pH 6.8, 0.1 M K+ electrolyte. Fig. 5A shows that increasing the specific interfacial area of the hollow fiber membrane increases the limiting current density, which in turn, increases the operating current density of the integrated AP system. Fig. 5B shows that the specific interfacial area > 30 m2/ m2 is required to support photo-limited current density of 12.65 mA cm-2, which entails to supplying air at the flowrates > 75.4 sccm per unit geometric area of the device to reduce the inlet concentration of CO2 by 10%. Processing of air at volumetric flowrates > 75.4 sccm/m2 will create a local depletion of CO2, whose effect on the atmosphere on a global scale has been discussed elsewhere19. Fig. 5C shows the distribution of CO2 concentration and flux in the fully integrated system utilizing CO2 directly from the air using a hollow fiber membrane of specific interfacial area 9.5 cm2/cm2 to produce CO at the current density of 9.2 mA cm-2. The moisture gradient across the hollow fiber membrane, located at the bottom edge of catholyte compartment, facilitates the capture of CO2 from the air and its transport to the electrolyte as bicarbonate ions. The boundary layer near the electrode was set to 50 μm, which is within the reported range of boundary layer for bubbles evolving at ~9 mA cm-2. 57 The effective concentration of CO2 in the electrolyte near the hollow fiber membrane is 22 mM, which depends on the relative humidity of the air, the operating current density, and the specific interfacial area (see the Supporting Information). The operating current density of the fully integrated AP system is close to the limiting current density, as the average concentration of CO2 at the cathode ~6 mM. Fig. 5D shows that the current density decreases, and pH increases as the distance from the edge to the center of the cathode increases.

Specific interfacial area of the hollow fiber membrane contactor is defined as the ratio of the interfacial area of the membrane per unit geometric area of the contactor. 1


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Figure 5:(A) Intersection of JV characteristics for InGaAs/GaP/Ge triple-junction light absorber with the composite JV characteristics representing electrochemical load for producing CO directly from CO2 in the air using hollow fiber membrane module of varying specific interfacial area (SA). The composite JV characteristic is obtained for a cell containing Ag nanofoam cathode, an IrO2 anode, 0.1M KHCO3, and hollow fiber module of anion-exchange resin. (B) Effect of the specific interfacial area on the STF efficiency of a fully integrated AP system defined for Fig. 5(A), (C) Distribution of CO2 in a fully integrated AP system operating at average photocurrent density of 9.2 mA cm-2 using hollow fiber membrane with SA = 9.5, and (D) Variation in the photocurrent density, and pH from the edge to the center of the cathode

Techno-economic Analysis Besides the evaluation of the STF efficiency, a detailed assessment of the material and operating costs is required for the feasibility of implementing integrated AP systems. A short economic analysis is presented here to estimate the cost per ton of CO produced from various integrated AP systems. The total cost of production includes: 1) the cost of materials for the construction of photoelectrochemical cells (PEC) such as OER catalyst, CO2RR catalyst, anionexchange membrane, and chassis. The costs of electrolyte and the labor for assembling the components of PEC are neglected, which are relatively lower than the cost of other materials. 2) 19 ACS Paragon Plus Environment


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the cost of capturing CO2, which was obtained from the techno-economic study reported elsewhere.25, 58-59 3) the costs of pumping and compression of CO2, and separation of CO. The list of all costs and associated process parameters considered in this analysis is provided in section S8 of the Supporting Information. Figure 6 shows the total cost including CO2 capture (black bar) and reduction (red bar) of producing 1 ton of CO using integrated cascade systems such as ABS (sorbent: MEA, source: flue gas), ADS (sorbent: K2CO3, source: flue gas), ABS (sorbent: seawater, process: electrodialysis), and MEM (sorbent: amine-based resin, source: air); and fully integrated system utilizing MEM process. Although the cost of CO2 capture for ABS is lower than ADS using the flue gas, the cost of CO2 reduction is higher with ABS due to its higher carbon capture capacity requiring a higher number of PEC units and therefore the total cost of CO production is almost the same ~ $400/ton for these two techniques. However, the cost of producing CO using CO2 captured from electrodialysis is much higher $1394/ton which is mostly due to the cost of membranes used in ~7000 electrodialysis units required to capture CO2 from the seawater. The cost of CO production is lowest for MEM among the integrated cascade systems, as the cost of operation of the moisture-swing process to capture CO2 and the cost of its conversion for this process are much lower than ABS and ADS. Except for ABS (with electrodialysis), the estimated costs of producing CO using ABS, ADS, and MEM are lower than the market price of CO $660/ton which is due to various factors not considered in this analysis such as land area, maintenance, inflation, interest, and return on investment. The fully integrated AP system shows the lower cost for CO production as compared to the integrated cascade system with MEM, as it circumvents the need for the transportation of CO2 from the carbon capture unit to the PEC units. The cost of producing 0.38 ton/day of CO was $185.24/ton using an integrated AP system spanned over a land area of ~122 m2.


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Figure 6: Economic analysis of the integrated AP systems. The unshaded data points represent cascaded integrated AP systems and the shaded data point (only for membrane) represents a fully integrated AP system.

Conclusions and Perspectives A detailed assessment of the STF efficiency of integrated AP systems for direct capture and conversion of CO2 from the air or the flue gas to fuels and chemicals has been carried out using technology-ready materials and processes. Integration of carbon capture processes with AP systems is essential for the sustained, continuous, and efficient conversion of sunlight, CO2 and H2O to chemicals. The thermodynamic limits of STF efficiency were less sensitive to the concentration of CO2 in the feed stream as compared to the number of junctions in the light absorber. For instance, STF efficiency of CO increased marginally from 40.1% for the air to 41.6% for the flue gas, whereas it decreased significantly from 40.1% for double-junction light absorber to 33.8% for triple-junction light absorber for air as the CO2 source. Since the ideal, thermodynamic STF efficiency provides the ultimate performance limits that are hard to achieve, a more realistic and achievable performance limits were obtained using existing state-of-the-art 21 ACS Paragon Plus Environment


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components in the integrated AP systems. The STF efficiency was dependent strongly on the energy of carbon capture for systems operating with single- or double-junction light absorbers. The STF efficiency was also sensitive to the selectivity of CO2R reaction, such that the synthesis of higher-electron products such as CH3OH, CH4, C2H5OH, and C2H4 yielded higher efficiency as compared to lower-electron products. In general, the integrated cascade systems utilizing either an adsorbent (ADS) or anion-exchange membrane (MEM) were more energy efficient than the integrated cascade systems with the absorbent (ABS)-based carbon capture process. Although the integrated cascade processes were not limited by the mass transfer of CO2, the additional energy requirement of > 123 kJ mol-1 for operating secondary processes such as pumping, compression, and separation reduced the STF efficiency of lower-electron products by almost 40%. This loss of STF efficiency in integrated cascade system can be minimized by the seamless integration of carbon capture and conversion processes in a fully integrated AP system. For a fully integrated AP system, the anion-exchange membrane was a better sorbent than the solid adsorbent and liquid absorbent as it does not require a separate regeneration for continuous operation. Such an anion-exchange resin is robust and has been previously applied for selective capture of CO2 from the air using a moisture-swing process.30 The proposed construction of a fully integrated AP system involves such anion-exchange membrane that continuously captures CO2 from the dry air and supply to the catholyte by developing a moisture gradient across the membrane. This architecture is bio-inspired and resembles closely with that of natural leaf, where light absorbers are the artificial photosystems and the anion-exchange membrane is the artificial stomata. The mass transport of CO2 from the air to the device can limit the performance of a fully integrated AP system. The maximum STF efficiency of fully integrated systems were 0.01% and 3.83% when the concentrations of CO2 in the air was 0.04% and the flue gas was 15%, respectively. The STF efficiency of the fully integrated AP system was increased by using the hollow fiber membranes contactors with the higher specific interfacial area (> 20 cm2/cm2). To access the feasibility of such system, a full-scale simulation has been performed using measured properties of the state-of-the-art components such as InGaAs/GaP/Ge triple-junction light absorber, nanofoam Ag cathode, an IrO2 anode, 0.1 M KHCO3 electrolyte, and quaternary ammonium ion resin for carbon capture from the air. The hollow fiber membrane with a specific interfacial area of 9.5 cm2/cm2 was shown to capture CO2 from the air to support the operating current density of 9.2 mA cm-2 to produce CO at STF efficiency of ~14%. The design of such an efficient, fully integrated AP system can also be configured to produce CH3OH, C2H5OH or HCOOH directly from the air, water, and sunlight using an appropriate catalyst. Finally, a short techno-economic analysis was performed to assess the feasibility of implementing such an integrated AP system at a large scale. The fully integrated system was more cost-effective than the integrated cascade processes to produce CO at a scale of ~0.4 ton/day, as it bypasses the need for the secondary processes such as pumping, and compression required in cascade systems. The cost of CO produced was almost the same for the AP system integrated with 22 ACS Paragon Plus Environment

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the absorption- and adsorption-based carbon capture processes. The integration of AP system with electrodializer was most cost-ineffective as the current technology is not efficient to create large, stable pH gradients required for carbon capture. The fully integrated system with the amine-based resin for carbon capture coupled with a PEC was the most cost-effective system producing ~0.4 ton of CO per day at a cost of ~ $185/ ton. Although the fully integrated AP system for CO2 capture and conversion is a promising technology, there are several technical challenges that need to be addressed to implement such a system at a larger scale. The primary technical challenge is to develop an anion-exchange membrane that can transport HCO3- on the dry side of the membrane to sustain higher moisture gradient. The other challenges are i) develop earth abundant, low overpotential CO2R catalyst for synthesis of higher-electron products, ii) develop stable liquidjunction semiconductors to sustain pH gradients, and iii) develop effective techniques to separate CO2R products, and utilize and manage CO2.



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Associated Content Supporting Information Additional information on SQ limits analysis and optimal band-gaps for multi-junction light absorbers; calculation of thermoneutral potentials, description of CO2 capture processes, STF efficiency of ideal AP systems, and the effect of mass transfer on total current density; additional information of the COMSOL model of a fully integrated system, and economic analysis of the integrated AP systems. Author Information Corresponding Author *Email: [email protected] ORCID Meenesh R. Singh: 0000-0002-3638-8866 Notes The authors declare no competing financial interest. Acknowledgements This material is based on the work performed in the Materials and Systems Engineering Laboratory at the University of Illinois at Chicago. AP acknowledges financial support from the University of Illinois at Chicago’s Provost Graduate Research Award.

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A blueprint of a fully integrated artificial photosynthetic system capturing CO2 from the air or the flue gas and converting it directly to fuels and chemicals


Assessment of Artificial Photosynthetic Systems for ... - ACS Publications

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