Analysis of pyridinium catalyzed electrochemical and ...nopr.niscair.res.in/bitstream/123456789/14664/1... · Keywords: Electrochemical reduction, Photoelectrochemical reduction,

Indian Journal of Chemistry Vol. 51A, Sept-Oct 2012, pp. 1284-1297

Analysis of pyridinium catalyzed electrochemical and photoelectrochemical reduction of CO2: Chemistry and economic impact

Kate A Keets a, Emily Barton Colea, Amanda J Morrisc, Narayanappa Sivasankara, Kyle Teameya,

Prasad S Lakkarajub, c, * & Andrew B Bocarslyc, * aLiquid Light Inc, Monmouth Junction, New Jersey, USA

bDepartment of Chemistry, Georgian Court University, New Jersey, USA cDepartment of Chemistry, Princeton University, Princeton, New Jersey, USA

Email: [email protected] (ABB)/ [email protected] (PSL)

Received 9 June 2012; revised and accepted 22 June 2012

This review highlights the recent work related to the electrochemical and photoelectrochemical conversion of carbon dioxide into methanol and formic acid. Information related to the structural, mechanistic and kinetic aspects of the pyridinium and imidazolium catalyzed reduction processes is presented. The economic impact of these processes is delineated by calculating the cost per billion gallons of gasoline equivalence for methanol by three methods viz., electrochemical conversion, photoelectrochemical conversion, combined electrochemical conversion and thermal conversion. Our initial analysis shows that the last method may be the most economically feasible method under the existing technologies.

Keywords: Electrochemical reduction, Photoelectrochemical reduction, Carbon dioxide reduction, Pyridinium catalysed reactions, Imidazolium catalysed reactions

Conversion of carbon dioxide (CO2) into fuels and organic precursors addresses three problems of global significance.1-5 Firstly, it provides a path for the utilization of CO2 as chemical feedstock and helps mitigate global warming. Secondly, the fuels such as methanol and ethanol generated from CO2 reduction can address the increasing global demand for energy sources. Thirdly, the conversion of CO2 into smaller organic molecules containing carbonyl functional groups generates highly needed precursors for organic synthesis.

One of the earliest reports of the reduction of CO2 dates back to 1870 when researchers synthesized formic acid from aqueous bicarbonate. Enzymatic6,7 and photoelectron reductions8,9 of carbon dioxide have received significant attention. Achievements in the field of photochemistry in the 1970s sparked interest in the photoelectrochemical reduction of CO2. Electrochemical10-12 and photoelectrochemical conversion of CO2 provides interesting challenges on many fronts. How do we design a suitable catalyst for the electrochemical or photoelectrochemical conversion of CO2? What are the kinetic, mechanistic and structural parameters and species of significance? How can we control the product distributions? What are the issues associated with scaling up the

laboratory methodologies? These are some of the aspects that we have addressed in the recent past.

Pyridinium Catalyzed Electroreduction at Pt and

Pd Cathodes We first reported the pyridinium-catalyzed

electroreduction of carbon dioxide to methanol in 1994 at a hydrogenated Pd electrode.13 It was found that at a modest overpotential of ~200 mV, carbon dioxide was reduced to methanol with a faradaic yield of ~30 %. The electrochemistry of the pyridinium radical had been previously reported however for the catalytic production of hydrogen.14 In acidic media, pyridine is protonated to give the pyridinium cation. This species can be reduced by 1e- to give the pyridinium radical (Eq. 1) which is coupled to the catalytic generation of hydrogen (Eq. 2).

N

H

.

N

H

+e-

-e-

(Eo, α, ket) …(1)

N

H

.

N

H

.

N

2 + H2+

kHy

…(2)

KEETS et al.: PYRIDINIUM CATALYZED REDUCTION OF CARBON DIOXIDE

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We reported on the mechanistic pathway for the reduction of carbon dioxide to methanol in 2010.15,16 It was postulated that the same pyridinium radical, shown above in Eqs (1) and (2), is responsible for at least four of the six proton-electron charge transfers involved in methanol production. In general, the rate determining step17 was found to be an inner-sphere reaction between the pyridinium radical and carbon dioxide following Eqs (3) and (4) to generate the surface bound hydroxyl-formyl radical.

N

H

. + CO2

N

C

.

O O(H+) …(3)

N

C

O

O+

(H+)N

C

.

O O(H+) …(4)

In order to understand the complete mechanistic picture, a range of electroanalytical and theoretical studies were conducted. For instance, Fig. 1 shows the digital simulations of cyclic voltammograms for the reduction of pyridinium to the radical in a CO2-purged electrolyte. Using digital simulation software, important kinetic parameters were determined to yield mechanistic insight, such as the rate constant for the reaction of the pyridinium radical with carbon dioxide. Similarly, we performed a series of molecular orbital calculations also to gain insight

into the full mechanistic picture.16 Figure 2 shows the bonding interaction between the nitrogen of pyridine and the carbon of carbon dioxide, following Eq. (3). A predicted N-C bond length of 139 pm and an O-C-O bond angle of 124º indicated a change in hybridization for the carbon of carbon dioxide from sp to nearly sp

2.

Since formic acid and formaldehyde were also found to be produced in the system, we assumed that these products were intermediates on route to methanol. To complete the mechanistic picture, similar electroanalytical analysis for kinetic information and theoretical modeling was carried out.16 Table 1 shows the free energies of reactions from Gaussian calculations for the complete reduction of carbon dioxide to methanol through the intermediates of formic acid and formaldehyde, the 2e- and 4e- reduced species, respectively. Overall, the complete reduction of carbon dioxide to methanol was found to be energetically favorable.

Using all information from electrochemical simulations and molecular orbital calculations, we proposed a complete mechanistic picture for the formation of methanol16 shown in Scheme 1. A combination of surface-based, heterogeneous reactions and homogenous, pyridinium-catalyzed reactions were found to be the most plausible. The light grey regions showed possible reaction branching pathways that might occur; however, the highlighted path was determined to be the main reaction pathway using a platinum electrode.

Using the kinetic parameters obtained from digitally fitting experimental cyclic voltammograms to specific reaction mechanisms, a steady state approximation was made considering the rate limiting reaction of the first electron-proton transfer to carbon

Fig. 1 — Comparison of experimental (solid) and simulated (dash) CV’s for the reduction of pyridinium in CO2-saturated aqueous solution at pH 5.3 with 0.5 M KCl as supporting electrolyte. Scan rates of 1, 5, 10, 50, and 100 mV/s are shown. [Reproduced from Ref. 16 with permission from American Chemical Society, Washington DC, USA].

Fig. 2 — Highest occupied molecular orbital (HOMO) of pyridinium-CO2 radical adduct. The HOMO shows a π bonding interaction between the nitrogen of pyridine and the carbon of CO2. [Reproduced from Ref. 16 with permission from American Chemical Society, Washington DC, USA].

INDIAN J CHEM, SEC A, SEPT-OCT 2012

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Table 1 — Free energies of reactions of interest in the reduction of CO2 to methanol calculated using free energies obtained for the

B3LYP/6-31G(D,P) basis set†

Reaction ∆G (kcal mol-1)

N

H

. + CO2

N

C

.

O O(H+)

7.46

N

C

.

O O(H+)

N

H

.+

N

2 + C

O

OHH

–50.2

C

O

OHHN

H

. + + H2O

N

C

.

O H

0.292

N

H

.

N

2 ++C

O

H HN

C

.

O H

–38.4

N

H

. +C

O

H H N

C

.

HO H

H

1.29

N

H

.

N

2 + C

H

H

H

OH

N

C

.

HO H

H

+

–62.5

†Reproduced from Ref. 16 with permission from American Chemical Society, Washington DC, USA.

dioxide to yield the hydroxyl-formyl radical. Once this species was generated, it was found that the reaction could proceed either to formic acid (an observed reaction product) or to the formyl radical (an intermediate on route to methanol formation). Examining the catalysts pyridine and 4-tert-butyl pyridine (Table 2), a comparison of the rates of production of formic acid and methanol were made using the kinetic parameters determined from digital CV simulations. It was found that the calculated faradaic yields for formic acid and methanol very closely matched the experimental results. This gave good support for the plausibility of the proposed mechanistic pathway.

Kinetics studies – Pyridinium catalysis

In order to improve the catalytic yields of the carbon dioxide electrochemical reduction processes, it is imperative that a basic understanding of the mechanistic aspects is pursued.17 Initially the reduction of pyridinium was studied under Ar and CO2 atmospheres in a pH adjusted (typically 5.3) solution (Fig. 3).

The enhancement in current from Ar to CO2

atmosphere is associated with increased electron transfer from the electrode surface due to the presence of carbon dioxide. Nicholson and Shain diagnostics indicate that the scan-rate dependent peak-to-peak separation is due to an underlying electrocatalytic mechanism.

Overall proposed mechanism for the pyridinium-catalyzed reduction of CO2 to the various products of formic acid, formaldehyde and methanol [Reproduced from Ref. 16 with permission from American Chemical Society, Washington DC, USA]

Scheme 1


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Fig. 3 — Cyclic voltammograms of 10 mM pyridine in aqueous solutions of 0.5 M KCl at pH of 5.3 under Ar atmosphere (─) and under CO2 atmosphere, 0.0333 M [CO2(aq)] (---). Both scans were recorded at 5 mV/s). [Reproduced from Ref. 17 with permission from Wiley-VCH, Weinheim, Germany].

Further, we have studied the current dependence on the concentration of the pyridinium ion. The current was found to increase with the concentration of pyridinium linearly, until about 6 mM and appears to level off at higher concentrations (Fig. 4).

The electron transfer process at the electrode surface appears to be first order with respect to the pyridinium concentration in the 1-8 mM range. Above this concentration (8-32 mM), the additional current appears to be primarily due to the production of hydrogen gas and is unrelated to an increase in pyridinium concentration.

The electron transfer rate at the electrode surface has also been investigated as a function of the dissolved CO2 concentration. Such studies were done in a high pressure electrochemical cell wherein the concentration of CO2 gas was increased up to 100 psi. The variation of the maximum cathodic peak current with respect to CO2 pressure was linear (Fig. 5)

Fig. 4 — Cyclic voltammograms collected at 5 mV s-1 for pyridinium concentrations of 2.24 (black), 3.32 (red), 3.65 (green), 5.24 (blue), 6.25 (cyan), 13.84 (magenta) and 22.75 mM (orange) in a 0.5 M aqueous solution of KCl under a CO2 atmosphere (0.033 M [CO2 (aq)]). The inset shows the dependence of the differential cathodic peak current (measured as the difference between the cathodic peak current of the cyclic voltammogram of pyridinium, pH-adjusted and under an argon atmosphere, to the peak current for pyridinium reduction in the presence of CO2). The concentration of pyridinium was corrected for the solution pH. [Reproduced from Ref. 17 with permission from Wiley-VCH, Weinheim, Germany].

indicating that the electron transfer rate is first order with respect to the dissolved CO2. The collective information from Figs 4 and 5 led us to conclude that the first electron transfer is a first order reaction both with respect to CO2 and pyridinium cation. Therefore, we proposed that formation of a pyridinium radical-CO2 complex, a carbamate species, is the crucial first step in the electrochemical reduction process (Eq. 3).

The activation energy for the first electron transfer was measured by analyzing the cyclic voltammogram cathodic peak currents as a function of temperature. The variable temperature experiment was done in the

Table 2 — Comparison of the rates of production of formic acid and methanol (through formaldehyde) from the two competing electron transfer reactions of kCOOH (to formic Acid) and kFA (to formyl radical)†

Electrocatalyst [•COOH] (mol cm-3)

kCOOH

(cm s-1) kFA

(cm s-1) HCOOH rate (mol cm-2 s-1)

[•CHO] rate (mol cm-2 s-1)

Calculated Faradaic yield range HCOOH (%),

CH3OH (%)

Pyridinium 4×10-8 2×10-4

±1×10-4 1.0×10-3

±0.2×10-3 3.6×10-12

±1.8×10-12 4.0×10-11

±0.8×10-11 1.5 – 4.5, 18 – 28

4-tert- Butyl pyridinium

9.5×10-9 1.6×10-4

±0.7×10-4 3.3×10-3

±0.9×10-3 1.5×10-12

±0.6×10-12 3.1×10-11

±0.9×10-11 < 1,

13 – 23

†Reproduced from Ref. 16 with permission from American Chemical Society, Washington DC, USA.


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Fig. 5 — Cyclic voltammograms collected at 5 mV s-1 for 10 mM pyridine in a 0.5 M aqueous solution of KCl under 14.1 (solid), 28.2 (dash), 42.3 (dot), 56.4 (dash dot), 70.5 (dash dot dot) and 84.6 psi (short dash) of CO2. The inset shows the dependence of the maximum cathodic peak current on the concentration of CO2. The concentration of CO2 was determined by using the appropriate Henry’s law constant (0.03 mol L-1 atm-1). [Reproduced from Ref. 17 with permission from Wiley-VCH, Weinheim, Germany].

range 2–45 ºC and the cathodic peak current was assumed to be directly proportional to the rate of electron transfer. An Arrhenius rate plot was developed by plotting ln(current) versus 1/T and from the slope of the resulting straight line (Fig. 6), the activation barrier for the first electron transfer was deduced to be 69±10 kJ mol–1.

It should be noted that we are dealing with an endoergic process here, and an activation barrier of 69 kJ mol–1 translates to a reduction potential of –0.72 V. This matches spectacularly with the reduction potential of –0.72 V, which is typically employed in our experiments with the pyridinium species. Considering that the thermodynamic reduction potential for the first electron transfer is about –0.52 V, an overpotential of 0.2 V is typically employed in these experiments. Such overpotential is considered low; however, we are in the process of finding other catalysts that would lower the overpotential further.

Mechanistic studies – Imidazole catalysis18

Investigations into electrocatalysis by aromatic compounds that contain sp

2 hybridized nitrogen atom(s) was extended to five membered π electron rich heterocyclic compounds like imidazole. These may bind carbon dioxide more strongly than pyridine due to the fact that the six π electrons are distributed over five ring atoms. These investigations were carried out over illuminated iron pyrite semiconductor

Fig. 6 — Cyclic voltammograms collected at 5 mV s-1 for 10 mM pyridine in a 0.5 M aqueous solution of KCl under CO2 atmosphere at 2.5 (black), 10 (red), 23 (green), 35 (blue) and 45 °C (cyan). The inset shows a logarithmic inverse temperature dependence of the maximum cathodic peak current. The peak current shown in the inset was corrected for dissolved CO2 concentration. [Reproduced from Ref. 17 with permission from Wiley-VCH, Weinheim, Germany].

Fig. 7 — Scan rate dependence of the reduction of pyridine at an illuminated pyrite electrode under a CO2 atmosphere. [Inset: The linear dependence of cathodic peak current with the square root of the scan rate from 5–1000 mV/s indicates a diffusion limited electrochemical reaction.

electrodes in order to find an electrochemical system that makes use of solar energy as well as earth abundant catalyst materials.

Additionally, the dependence of cathodic peak current with the concentration of imidazole levels off after 0.01 M indicates that the surface electron transfer is essentially diffusion controlled. This aspect is also reflected in the scan rate dependence of the cathodic peak current (Fig. 7). The mechanism of the imidazole based catalysts


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was investigated by analyzing a series of imidazole derivatives by cyclic voltammetry. For example, 2-methyl imidazolium does not show any catalytic activity for CO2 reduction in contrast toimidazolium. This led us to conclude that the C2 proton is essential for imidazole catalysis and hence, an alternative mechanism involving C2 hydrogen is proposed. The proposed mechanism, Eqs (5 a-c), proceeds through the C2 based carbene intermediate.

It is significant to note that the two distinct mechanisms proposed by us—one pyridinium catalyzed and the other imidazolium catalysed—have the catalytic intermediate attacking the carbon of the CO2.

Photoelectrochemical Reduction of CO2 at Semiconductor Electrodes

Semiconductors are desirable as electrode materials for the reduction of CO2 because of their ability to convert solar energy into electricity. In a p-type semiconductor liquid junction, absorption of band gap or higher energy photons results in the promotion of electrons from the valence band to the conduction band. Electrons move to the solution junction while holes move into the bulk of the material. These electrons are now able to perform interfacial reduction reactions on solution species which possess a redox potential that matches the conduction band edge potential of the semiconductor. CO2 could then be converted into a useful product without the release of additional greenhouse gases. Small band gap semiconductor materials are of particular interest because of their significant overlap of the solar spectrum. However, for these systems to be energy efficient, they must operate at an underpotential.

In 2008 we reported that the pyridinium chemistry for the production of methanol at a Pd or Pt electrode could be exported to a p-type semiconductor. We found that very high faradaic yields for methanol (nearly 100 %) could be achieved using a p-GaP electrode at an underpotential to the thermodynamic potential for the reduction of carbon dioxide to methanol9 of ~300 mV. This was the first report of actually using light energy to drive the reduction of carbon dioxide. Without pyridine, the system was not stable, showing a drop-off in current within one hour of reaction. The addition of the homogenous pyridine catalyst led to a stable system as shown in Fig. 8.

Fig. 8 — (left) Time response for potentiostatic reduction of CO2 at -0.4 V vs SCE at pH 5.2 both (_____) without pyridine and (_____) with pyridine. (right) I-V curve at illuminated p-GaP (Hg0Xe lamp 200 W), acetate buffer containing 10 mM pyridine maintained at pH 5.2 under Ar (red) and under CO2 (green). [Reproduced from Ref. 15 with permission from American Chemical Society, Washington DC, USA].


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p-GaP has an indirect band gap of 2.24 eV, which means that it should utilize light of wavelengths shorter than ~550 nm. A photocurrent was observed at wavelengths lower than ~530 nm with a sharp rise in photocurrent observed at 440 nm, indicative of the lowest energy direct band gap (2.8 eV). In order to examine the detailed photoresponse of the system, two wavelengths were selected, 465 nm (indirect transition) and 365 nm (direct transition), for photoelectrochemical evaluation for the production of methanol.

Table 3 lists the quantum yields for the

photogeneration of electrons (Φe–) and the quantum

efficiencies (ΦMeOH) for production of methanol at the chosen wavelengths of 365 nm and 465 nm. Quantum efficiency is defined by Eq. (6),

Quantum efficiency (ΦMeOH) = (molsMeOH× 6)/mols incident photons …(6)

and measures the net yield of the six electron process for the reduction of CO2 to methanol. The highest

values of Φe– and ΦMeOH were found to be 71 % and 44 %, respectively at –0.5 V vs SCE and under 365 nm illumination. At an underpotential of > 300 mV, a faradaic yield for methanol production of 96 %, was observed, approaching 100 %. To the best of our knowledge, this is the highest reported faradaic yield and quantum efficiencies for methanol generation, highlighting the conversion of light energy into storable chemical energy.

Although p-GaP is highly effective at reducing CO2 to methanol using only light energy, a large portion of the solar spectrum is wasted due to the

inherently large band gap of the material (2.24 eV indirect and 2.8 eV direct). With a band gap as large as 2.24 eV, the expected utilization of light includes wavelengths shorter than ~550 nm. As a result, we have attempted to apply the pyridinium electrocatalyst system to a small band gap p-GaAs semiconducting electrode. p-GaAs has several advantages over p-GaP, which makes it an attractive material for the photoelectrochemical reduction of CO2. First, it is a small band gap semiconductor with a band gap of 1.42 eV resulting in absorption of wavelengths greater than 874 nm, which consists of a large majority of the solar spectrum. Furthermore, the lowest energy electronic transition is a direct transition giving the material a large absorption coefficient. More photons per unit length are absorbed with a smaller amount of material, within the space charge layer. The charge carriers generated will have a higher probability of reaching the surface of the semiconductor before they can recombine, hence the material can be deposited as a thin film. Unfortunately, GaAs and other small band gap materials are more susceptible to photocorrosion and typically require stabilizing agents to enhance current-voltage characteristics and stabilize the surface.19,20 It has been found that coating the surface of GaAs with a thin metal layer, such as Ru21,22,23 or Pt,24 can significantly enhance the semiconductors performance.

I-V curves of p-GaAs in a 10 mM pyridine solution containing 0.5 M KCl as supporting electrolyte at a pH of 5.2 are presented in Fig. 9(a). In the dark, no current is passed until very negative potentials. Under the illumination of a 75 W Xe arc light source

Table 3 — Optical conversion of CO2 to methanol†

465 nm

E(V)a Underpot.a

(mV) J

(mA/cm2) Faradaic eff.

CH3OH ξ (%)

Quantum yield Φe- (%)

Quantum eff. CH3OHb

Φ MeOH (%) OCEcη

(%)

-0.70 - 1.1 56 (8.3)d (4.6)d (1.3)d

-0.60 - 1.0 51 (5.1)d (2.6)d (1.3)d

-0.50 20 0.46 78 3.4 2.65 1.05

-0.40 120 0.33 83 2.3 1.9 1.03

-0.30 220 0.27 90 1.6 1.35 0.84

365 nm

-0.50 20 0.92 62 71 44 10.9

-0.40 120 0.48 89 38 34 8.9

-0.30 220 0.28 92 16 15 5.8

-0.25 270 0.21 96 12 11.5 4.65

-0.20 320 0.21 96 13 12.5 4.8 aAll potentials referenced versus SCE. Underpotentials stated are versus the standard potential of -0.52 V for the reduction of CO2 to methanol at pH 5.2. bAs defined by Eq. 7 to be (mols methanol × 6)/mols photons. c As defined to be (chemical power out – electrical power in)/light power in. dThese values were obtained at an overpotential and thus external electrical power was also used. †Reproduced from Ref. 15 with permission from American Chemical Society, Washington DC, USA.


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in a solution purged with Ar, photocurrent onset is observed at –1.0 V vs SCE. Purging of the solution with CO2 enhances the cathodic peak current. This enhancement of current can be attributed to a catalytic interaction between CO2 and pyridinium.13 This is 500 mV above the standard potential for CO2 reduction of –0.52 V vs SCE at the system pH of 5.2. Moreover, the photocurrent of the cell drops to a miniscule value within a few hours. Electrodeposition of a thin film of platinum greatly enhanced the I-V characteristics of the semiconductor electrode, with the onset potential approaching –0.5 V vs SCE under white light illumination (Fig. 9b). The current stability was also significantly enhanced, remaining steady for ~24 hours or more.25

Platinum was an obvious choice for a surface treatment since it has been extensively studied as an electrode in the pyridinium system by Bocarsly et al.9,10 Deposition of a thin layer of platinum onto the surface of p-GaAs resulted in an increase of current intensity, as well as a positive shift of the photocurrent onset potential. As seen in Fig. 9, the photocurrent onset is shifted to –0.55 V vs SCE in the presence of Ar or CO2 at a pH of 5.2. Furthermore, the maximum current density is increased from –0.3 mA/cm2 to –1.7 mA/cm2 for etched and platinized p-GaAs, respectively. Although the potential of the platinized electrode is significantly more positive than etched p-GaAs, it is still more negative than the standard thermodynamic potential of CO2 reduction (–0.52 V vs SCE at a pH of 5.2).

More negative potentials are required to produce sustainable current on p-GaAs than on p-GaP. Products were observed after electrolysis of pyridinium

containing solutions at potentials of –0.8 V vs SCE. This corresponds to an overpotential of 280 mV from the standard reduction potential of –0.52 V vs SCE for pyridinium. Previous reports required potentials greater than –1.2 V vs SCE for the reduction of CO2 to formic acid or methanol. The system reported here is capable of producing higher order products with low overpotential. However, current densities are as low as 20 µA/cm2. Stabilizing the surface to cathodic photocorrosion could improve current densities and product yields.

Solar Fuels at Large Scale

Though researchers have debated the merits of the various pathways for making solar fuels for decades, few articles are available that directly compare them, and no articles were found that explore the differences quantitatively. In order to better understand the technical and economic requirements for a large-scale system of artificial photosynthesis, a generalized theoretical model was developed. The three approaches compared are electrochemical (EC), photoelectrochemical (PEC), and a hybrid process incorporating electrochemical and thermochemical aspects (EC-TC). In order to facilitate suitable comparisons, each process was modeled to make one billion gallons of gasoline equivalent (gge) fuel. In this case, all were assumed to be making methanol, because there are no available options for selectively making a multi-carbon alcohol from CO2 using a one-reactor thermochemical process. One billion gge represents less than 1 % of US consumption given that the annual requirements exceed 131 billion gallons.26

Fig. 9 — (a) I-V curve of illuminated clean p-GaAs (Xe 75W lamp) in a solution containing 10 mM pyridine and 0.5 M KCl. [(1, black) dark; (2, red) solution purged with Ar and pH maintained at 5.2; (3, blue) solution purged with CO2 and pH 5.2. (b) I-V curve of illuminated (1, blue) Pt/p-GaAs versus clean (2, red) p-GaAs (Xe 75W lamp), and, (3, black) dark in 10 mM pyridine 0.5 M KCl purged with CO2 at a pH of 5.2.


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General assumptions

A full list of the assumptions common to the processes modeled is provided in Table 4. In general terms, assumptions were made using the best information available in the literature and from the experts in the field. Though the primary energy source for all processes is assumed to be solar, some additional energy from non-solar sources is assumed for aspects of the processes such as product separation. For the purpose of simplifying the analysis, the conversion yields of both CO2 and water were assumed to be 100 % for all processes. Issues associated with obtaining and/or moving water to locations lacking adequate resources were not taken into account in this analysis, but would be an important part of future models. The cost of carbon dioxide is assumed to be $50 per metric ton, which is near the mid-range of estimates from the Intergovernmental Panel on Climate Change (IPCC).27 Solar electricity is assumed to be $50 per MWh, which assumes about 5–10 years of additional improvement beyond today’s prices.28 The solar insolation chosen was 5 kWh/(m2*day) for flat plate solar collectors without tracking devices. Insolation data available from the National Renewable Energy Laboratory (NREL) indicates that this level of insolation is available over a large region of the southern and western United States (Fig. 10). Common assumptions were also made for product separation. While assumptions are made for maintenance costs, no additional capital replacement costs are made for any of the systems. All systems assume 100 % selectivity for methanol.

The common assumptions for items such as fixed costs, maintenance, separations, etc., preclude high accuracy estimates of costs of production necessary for directly comparing the technologies against existing sources of liquid fuels. However, they provide a means of controlling difficult to estimate parameters in order to allow a relative comparison between the technologies. The sum effect is a model that allows the technologies to be compared and the cost sensitivity of technical parameters to be evaluated. PEC system assumptions

The PEC system is assumed to have a solar energy to fuel efficiency of 5 %, which matches the highest recently reported system.34 The cost of the system is based on low-end costs for photovoltaic systems on a per m2 basis and is set at $100 per m2 (Ref. 35). The assumed price per m2 is very aggressive and essentially indicates that the photovoltaic material accounts for all of the system cost. More realistic prices, given the current situation, could be an order of magnitude greater. Degradation has always been a major problem for PEC systems and few have ever been demonstrated to operate for more than a few days.36 Typical photovoltaic devices degrade at a rate of 0.5 % per year, so that is the optimistic rate assumed in this case.37 Like the system cost, this is an aggressive assumption given most PEC systems degrade to zero within hours or days. For the PEC system, it is assumed that a pipeline will be necessary for CO2 delivery because land costs will necessitate the use

Table 4 — Assumptions common to the systems

Fuel production Methanol production

∆Gf Methanol CO2 utilized Water utilized Solar insolation CO2costa

Water costb

Land leasec

1,000,000,000 gge/yr 5,874,129 T/yr 6.02 MWh/T 8,047,557 T/yr 2,616,572,093 gal/yr 5 kWh/(m2*day) $50 per T $0.004 per gal $1,000 per acre

Capital for separationd

Separation energye

Natural gas price Fixed costsf

PV efficiency Transmission lossesg PV area multiplierh Annual maintenancei

$88,691,304n 8.19 MMBtu / T $4.00 per MMBtu $25,000,000 per yr 20 % 7 % 2 3 % of capital costs

aMid-range estimate for capture from power plants. bAssumes $1,233 per acre-foot ($1 per m3). Typical costs in the U.S. are much lower. cBased on a range from $15 to $3,000 typical for use of public land in the United States.29 dEstimate based on high-end cost of $16.50/(gal*day) provided by Sulzer. eEstimate based on high-end requirements for the ethanol industry.30 fEstimate based on 250 personnel at a fully-loaded cost of $100,000 per person. gU.S. Energy Information Agency data.31 hAssume that not all land can be used for PV and there will be gaps for roads, space between panels, etc. iBased on available data from the chlor-alkali industry.32


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of public lands that are not likely to be located near CO2 sources. IPCC data indicate that a 24-inch (0.6 m) diameter is necessary for the quantity of CO2 required annually to make one billion gge. The estimated capital cost for a pipeline of this size is $500,000 per km with an operating cost of $1.50 per metric ton per 250 km of pipeline.38 A total of 250 km of pipeline was assumed for the model.

Fluid pumping and product separation (i.e. distillation) are assumed to be powered by electricity and natural gas, respectively, and these are assumed to be purchased from a third party. The energy requirement for pumping is based on the estimated requirement for a typical electrochemical process. However, this could be underestimated since thousands of kilometers of piping will be required for a large scale PEC system.

The model assumes only methanol is made in the PEC system; it does not take into consideration the

formation of any other product, including hydrogen gas. Though this is possible, it is generally unlikely that hydrogen is not formed in a system utilizing water as a solvent and reactant.15 The presence of hydrogen bubbles in the system could create efficiency problems which are beyond the scope of this simplified model. Likewise, the heating of the catholyte by sunlight is not integrated into the model and can likewise cause problems such as accelerated cell degradation and CO2 solubility limitations, while also providing a thermal energy source for product separation.

EC-TC system assumptions

EC-TC systems are assumed to be co-located with industrial emitters of CO2 and to receive solar energy via the grid. The electrochemical portion produces hydrogen via water electrolysis while the thermochemical portion reacts CO2 and H2 to make methanol. The hydrogen evolution portion is

Fig. 10—NREL insolation map of the United States for flat plate solar collectors. [Taken from Ref. 33].


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assumed to be a PEM-type electrolyzer, which allows relatively high rates of reaction and efficiency. Specific values are 75 % efficiency (LHV) and a current density of 1,000 mA/cm2. Parasitic losses from power electronics and pumping are estimated at 5 % and 7 % of total plant energy requirements, respectively, and are based on chlor-alkali numbers.39 Electrolyzer capital costs of $2,000 per m2 electrode area are assumed to be inclusive of the balance of system requirements and are relatively low because of the very large scale of the plant(s). Thermochemical capital costs are assumed at $150 per metric ton of name-plate capacity per year. The thermochemical portion of the system is assumed to require no additional thermal energy because the reaction between CO2 and H2 to make methanol is moderately exothermic. Energy for product separation is assumed to come from natural gas. The separation process is assumed to be about 50 % less energy intensive than that for PEC and EC since methanol will be produced at a roughly 1:1 molar ratio with water and waste heat will be available from the methanol evolution process. EC system assumptions

Like the EC-TC system, EC systems are assumed to be co-located with industrial emitters, thus mitigating pipeline costs. Efficiency assumptions are based on best results reported in the literature, though no system to date has combined the energy efficiency and current density assumed here. Specific metrics are cell potential of 2 V, current efficiency of 80 %, and current density of 300 mA/cm2. Waste H2 made as a byproduct of the process is assumed to be reclaimed for use as a thermal energy source in product

separation, which reduces natural gas requirements. The capital cost assumption of $2,000 per m2 electrode area is the same as that for EC-TC and is based on a PEM-type system. Losses due to pumping and power electronics are assumed to be the same as for the EC-TC system. Comparative study

As show in Table 5, all the three systems have relatively high costs for fuel production. While PEC and EC are essentially tied at $4.82 and $4.87 per gge, respectively, EC-TC has the lowest costs at $3.70 per gge. EC-TC has the smallest requirement for land used by solar power systems, while EC has the lowest capital costs and lowest additional emissions. Of the three systems evaluated, EC-TC incorporates technology generally available today, making it a near-term possibility. However, it is likely that EC-TC has limited room for additional improvements in technical performance as water electrolysis and conversion of CO2 and H2 to methanol are relatively mature technologies.

The relatively low capital cost of EC is a key advantage for an emerging technology. In addition, if the approximately 40 % energy efficiency assumed for the EC system can be improved to a level on par with EC-TC and the current density can also be equivalent to that for a PEM hydrogen system, EC costs per gallon drop 10 % below those for EC-TC. There also remains the possibility of selective production of multi-carbon alcohols, such as propanol or butanol, with better fuel characteristics than methanol. Though technology for electrolyzers can be borrowed from existing industries, advances in catalysis are still necessary for a breakthrough EC system.

Table 5 — Relative costs for making one billion gge annually by EC, PEC and EC-TC methods

EC PEC EC/TC

$ (gge) $4.87 $4.82 $3.70

OpEx (gge) $4.73 $4.06 $3.47

Electricity cost (% OpEx) 88.5 % 72.5 % 81.4 %

CO2 cost (% OpEx) 8.5 % 9.9 % 11.6 %

Water cost (% OpEx) 0.2 % 0.2 % 0.3 %

Land cost (% OpEx) 0.0 % 1.7 % 0.0 %

Natural gas cost (% OpEx) 0.9 % 4.7 % 2.8 %

Capital exp (gge) $0.15 $0.75 $0.23

Total capital exp. $2.9 Billion $14.4 Billion $4.5 Billion

Land for solar power system 180 km2 409 km2 (283 PEC, 126 PV) 121 km2

Net process CO2 emissions -7,817,592 T/yr -7,037,581 T/yr -7,542,569 T/yr


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Table 6 — Sensitivity analysis of the studied technologies to changes in input costsa

EC PEC EC/TC

Electricity (MWh) 0.084 0.059 0.057

Natural gas (MMBtu) 0.011 0.048 0.024

CO2 (T) 0.008 0.008 0.008

Water (m3) 0.010 0.010 0.010 aUnits are change in gge cost per dollar change in the input price. For instance, a $1/MWh change in the price of electricity changes the $/gge cost for EC by $0.084.

Both EC and EC-TC have an additional advantage over PEC in that they can have flexibility in their power source. While PEC requires sunlight and can only operate during daylight hours, EC and EC-TC can utilize other clean energy sources to operate 24 hours per day. For instance, an EC system located in the Pacific Northwest might utilize solar energy during the day, wind energy at night and/or hydropower at various times. Flexibility of this sort can allow significant improvements in cost competitiveness and ability to utilize the lowest cost sources of clean electricity available at any given time.

As shown in Table 6, the processes are sensitive to the cost of electricity, but less sensitive to changes in the cost of CO2, water, and natural gas. The relative importance of these inputs stands to reason, given that a ton of methanol requires only 1.37 tons of CO2 at 100 % conversion, while it needs 6.02 MWh/T at 100 % electrical efficiency and will experience significant parasitic electric loads from power conversion, pumping, etc. The impact of a $1 increase in the cost of CO2 is a $0.008 increase in gge for all technologies. The impact of a $1 increase in the cost of natural gas varies by technology and is most significant for PEC. As demonstrated in Fig. 11, EC is the most sensitive to electricity costs, followed by EC-TC, and then PEC. The relatively high sensitivity of EC to electricity costs is driven by an assumption of less than 100 % current efficiency and lower system efficiency than EC-TC as measured by cell potential.

The results for PEC are interesting in that they indicate that this process could work economically with an efficiency >5 %, low system costs, and high long-term stability. With relatively high operating expenses resulting from energy use in pumping and product separation, solar electricity could be utilized in order to replace operating expenses with capital expenses. However, this would cause an already very

capital intensive system to become unfeasible unless solar costs fall well below $1 per watt-power installed. While both degradation rate and system costs per unit area have critical impacts on PEC capital costs, the system is particularly sensitive to efficiency, which impacts both operating expenses (i.e. land, pumping) and capital expenses (i.e. total system size). The overall effect is demonstrated in Fig. 12 below and strongly suggests that efficiencies >10 % are desirable.

Conclusions

Although the model presented here is relatively simple, the results give a clear delineation of relative resource requirements for the different technologies, particularly in terms of land-area requirement. The model also highlights often overlooked aspects of the technologies, such as the need to either transport mass (CO2, water, fuel) or

Fig. 11 — Sensitivity of the processes to changes in the cost of electricity. [1, EC; 2, PEC; 3, EC-TC].

Fig. 12 — Sensitivity of PEC system to solar to fuel efficiency. [The chart includes efficiency within the PEC reactor only].


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electricity, the massive engineering tasks associated with very large scale PEC systems, and, the product separation and pumping requirements for all of the technologies at scale. The model suggests that EC-TC is the easiest technology to scale today given the relatively low technology risk, while EC is promising in the medium term. PEC will require large breakthroughs in materials science that enable improved conversion efficiency and much improved stability over time.

The model created for this analysis is a good start and could use significant improvements. For instance, major unit operations could be technically modeled for each process and then used to create a full project finance model for each. Improved assessment of capital replacement costs could be of particular importance and help guide requirements for the durability of specific components such as electrodes, membranes, etc. These improvements would provide greater fidelity on the relative merits of the technologies and a better roadmap for the technical improvements required to make practical systems. These efforts are currently in progress.

Pyridinium catalyzed reduction of CO2 to methanol appears to proceed through six one electron reductions. We are the first to report photoelectrochemical reduction of CO2 into methanol at potentials wherein light incident on the electrode is converted into chemical energy. The rate determining step appears to be the formation of the first intermediate formed by the addition of CO2 to pyridinium radical. It is not clear if such a reaction proceeds through a concerted solution phase (homogeneous) step or the surface plays a significant role. Our data show great dependence of the product distribution on both the electrode surface and the catalyst. We are in the process of delineating with greater clarity the dependence of the product distribution with the electrode surface and identifying key intermediates through optical and magnetic resonance spectroscopy techniques. Increasing the cathode current density (100-300 mA/cm2) and finding suitable catalysts for carbon-carbon bond formation are envisioned to be issues of critical significance for the future growth in the electrochemical reduction of carbon dioxide.

Acknowledgement This study is in part based upon work supported by

the Air Force Office of Scientific Research, USA, under AFOSR Award No. FA9550-10-1-0157.

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