General technoeconomic analysis for electrochemical coproduction coupling carbon dioxide reduction with organic oxidation

Electrochemical processes coupling carbon dioxide reduction reactions with organic oxidation reactions are promising techniques for producing clean chemicals and utilizing renewable energy. However, assessments of the economics of the coupling technology remain questionable due to diverse product combinations and significant process design variability. Here, we report a technoeconomic analysis of electrochemical carbon dioxide reduction reaction–organic oxidation reaction coproduction via conceptual process design and thereby propose potential economic combinations. We first develop a fully automated process synthesis framework to guide process simulations, which are then employed to predict the levelized costs of chemicals. We then identify the global sensitivity of current density, Faraday efficiency, and overpotential across 295 electrochemical coproduction processes to both understand and predict the levelized costs of chemicals at various technology levels. The analysis highlights the promise that coupling the carbon dioxide reduction reaction with the value-added organic oxidation reaction can secure significant economic feasibility.

1. Introduction and conclusion: I miss two very important points in the discussion. A) For parallel processes, divided cells have to be used and electrolysis conditions have to be found, which suit both half-reactions (solvent, salt, pH, temperature ...). The process development is therefore more challenging and the cost significantly higher. B) Naturally, there is a difference in market scale for the products from the anodic and cathodic half-reaction. The bigger the difference, the less attractive is the coproduction.
This manuscript developed a process synthesis framework and performed extensive technoeconomic analysis for a wide range of electrochemical CO2RR and OOR technologies and processes. The results provide a good reference for screening the economically viable CO2RR-OOR coproduction processes and will be of interest to the research community in the electrochemical conversion of CO2. The overall language, figures, tables, ets. in the manuscript are adequate. It can be accepted for publication after addressing the below points:

(Reviewer's Comment)
The author should be careful to select the market prices for chemicals. For example, the bulk price for FDCA should not be as high as $32-580/kg. Ref [117] may mislead its market price because of very small quantity sales. FDCA's bulk market price is expected to be competitive to its alternative (e.g., terephthalic acid, ~$1.5/kg).
(Authors' Response) Thank you for your valuable comment. We strongly agree that the FDCA market price in Ref [168] (originally Ref [117]) can be overestimated. Data in Ref [168] is based on export between India and other counties, which contains small quantity to large capacity bulk FDCA. As far as we know, the industrial scale FDCA sales information is rarely available. Thus, we referred the Indian export data ranged from $32/kg to $580/kg.
In order to respond possibilities that the FDCA market price get cheaper than its alternatives such as terephthalic acid mentioned by reviewers, we performed additional economic analysis. We monitored the LCC over market price ratio via changing FDCA market price as $0.1-10/kg and identified minimum market price of FDCA that secures economic feasibility, regardless CO 2 RR products. The result indicates that economic feasibility will be maintained for all cathode products until the market price of FDCA is reduced to $4.25/kg at the base case and $1.3/kg at the optimal case. On the other hand, none of the CO 2 RR-FDCA coproduction secures economic feasibility as the market price of FDCA is reduce below $2.60/kg at the base case and $0.63/kg at the optimal case. We add this results in the manuscript and provide the related figure in Supplementary Fig. 7.
Changes made: • In page 19 of manuscript Original sentence: "Notably, although FDCA has a very low LCC-to-market price ratio due to its high current market price ($32 -580 kg -1 ), economic feasibility will be maintained until the market price of FDCA is reduced to ~$2 kg -1 , regardless of the CO 2 RR products." Revision: Notably, although FDCA has a very low LCC-to-market price ratio due to its high current market price ($32 -580 kg -1 ), economic feasibility will be maintained until the market price of FDCA is reduced to $4.25 kg -1 at the base case and $1.3 kg -1 at the optimal case, regardless of the CO 2 RR products ( Supplementary  Fig. 7).  Table 8). When the market price of FDCA is over $4.25 kg -1 at base case and $1.30 kg -1 at optimal case, CO 2 RR-OOR process can secure the economic feasibility, regardless of the CO 2 RR products.

(Reviewer's Comment)
It's good to see downstream separation and recovery are considered, but the simplified separation system with shortcut models may not reflect the real difficulties and costs for separations. Can the authors comment on this point and it would be good to see some sensitivity analysis regarding the key parameters in the separation systems.
(Authors' Response) Thank you for your important comment on the possibility of modelactual plant mismatch. We agree that the shortcut models can underestimate the separation cost and cannot reflect various real difficulties such as flooding, cracking, pipeline blockage, and etc. Although we designed product-specific separation processes according to the product properties, our process design, sizing, and costing still contain high degree of uncertainty. According to your comment, we additionally performed sensitivity analysis on the electrolyzer and separation systems for every of CO 2 RR-OOR electrochemical coproduction processes. As a result, we identify the electrolyzer and the extraction process have large influence on LCC and should be improved for more accurate LCC calculation. However, the shortcut models can be sufficient for the screening stage conceptual design 1 and the sensitivity (<10) of the optimal case is not so significant.
In the case of real difficulties, our approach is based on "conceptual design", which is usually performed when the technology is at low level of technology readiness level (TRL). Most of electrochemical CO 2 RR-OOR coproduction technologies are in the relatively low TRL as compared with commercial chemical processes, thus we believe that conceptual design is appropriate to conduct extensive comparative technoeconomic analysis for screening processes. The detail designs based on rigorous process models can be an alternative approach, however, they are not generally adequate for screening process alternatives because of expensive computation, large numbers of parameter, and requirement of experimental data. According to your comment we include methods and results in main manuscript and related figure in Supplementary Fig. 6.
Changes made: • In page 18-19 of manuscript Revision: We also performed sensitivity analysis regarding the equipment cost of the electrolyzer and separation systems for every CO 2 RR-OOR electrochemical coproduction processes because our shortcut models may have uncertainly for the real plant application ( Supplementary Fig. 6). The flash, distillation, PSA, compressor, and heat exchanger have low impact on LCC sensitivity (<10%) in most cases. The extraction has slightly higher sensitivity but with an average sensitivity of 10%. However, the sensitivity of the electrolyzer can be as high as 100% depending on CO2RR-OOR combination. Interestingly, as the conditions such as FE, current density, overpotential, and electricity cost become lower, the LCC becomes robust to the equipment cost change ( Supplementary Fig. 6b). Altogether, the more precise electrolyzer and extraction model are expected to improve the accuracy of LCC, but the current shortcut models can be sufficient for the early stage screening process.  Table 9). The definition of sensitivity for a unit i is given by, where ratio equipment represents equipment cost changed ratio compared to the equipment cost evaluated by the proposed shortcut electrolyzer and separation models. For example, the meaning of sensitivity PSA equals to 0.5 is that if equipment cost of PSA increases by 100%, then LCC increases by 50%.

Responses to the Comments of the Reviewer 2
In the present work, the possibilities and limitations for coupling the cathodic CO2 reduction with the anodic oxidation of organic compounds is explored from the techno-economic point of view. Both half-reaction types are currently studied independently by large scientific communities and chemical companies, whereas a successful experimental coupling of the two reaction types in a single electrochemical cell has so far rarely been achieved. I absolutely agree with the authors that this technology would be a significant advance in sustainable chemicals production, and that the opportunities re. energy efficiency and production cost are immense.
With respect to the (electro)chemistry, the following points need to be addressed:

(Reviewer's Comment) Introduction and conclusion: I miss two very important points in the discussion. A) For parallel processes, divided cells have to be used and electrolysis
conditions have to be found, which suit both half-reactions (solvent, salt, pH, temperature ...). The process development is therefore more challenging and the cost significantly higher. B) Naturally, there is a difference in market scale for the products from the anodic and cathodic half-reaction. The bigger the difference, the less attractive is the coproduction.
(Authors' Response) Thank you for the reviewer's important comment and authors completely agree with that the parallel processes can be challenging and more expensive. In order to respond your comment, we include Supplementary Table 1 indicating typical operating conditions for cathodic and anodic products. As shown in the table, numbers of CO 2 RR-OOR combination can have significantly different operating conditions, and they eventually result higher electrolyzer cost. Therefore, we carried out a sensitivity analysis demonstrating effect of equipment cost change (i.e., flash, distillation, extraction, PSA, compressors, and heat exchangers) on LCC ( Supplementary Fig. 6). Please note that the definition of sensitivity for a unit i is given by, where ratio equipment represents equipment cost changed ratio compared to the equipment cost evaluated by the proposed shortcut electrolyzer and separation models. For example, the meaning of sensitivity PSA equals to 0.5 is that if equipment cost of PSA increases by 100%, then LCC increases by 50%. The result shows that the sensitivity of the electrolyzer can be as high as 100% depending on CO 2 RR-OOR combination. Interestingly, as the conditions such as FE, current density, overpotential, and electricity cost become lower, the LCC becomes robust to the equipment cost change (Supplementary Fig. 6b). The maximum sensitivity of the electrolyzer at the optimal case II with 95% confidence is about 10% and it can be interpreted as LCC only changes by 10% although the electroylzer cost is doubled.
We also strongly agree that the electrochemical coproduction becomes less attractive if a significant differences exist in product market sizes. Thus, we revise the conclusion and indicate that the difference in the market size between cathode and anode products make the electrochemical coproduction less attractive. In addition, we include market sizes of the chemicals investigated in this study for providing related information to readerships. (Supplementary Table 8) Changes made: • In page 6 of manuscript Original sentence: "The most well-known, industrially established example is the chlor-alkali process, wherein chlorine and sodium hydroxide are produced at the anode and cathode, respectively. 14 " Revision: "The most well-known, industrially established example is the chlor-alkali process, wherein chlorine and sodium hydroxide are produced at the anode and cathode, respectively. 14 Interestingly, small-scale CO2RR-OOR process demonstration regarding the oxidative condensation through molecular electrocatalysts belongs to a parallel type. 15 The parallel paired electrolysis can be very challenging if significant differences exist between half reaction operating conditions (i.e., solvent, pH, temperature, etc.) and the different operating conditions may cause expensive electrolyzer design and fabrication cost. We summarize operating conditions of both cathodic and anodic products in Supplementary • In page 6 of manuscript Original sentence: "Additionally, GSA was not performed with certain optimal design variables, such as recycle ratio and operating pressure." Revision: "Additionally, GSA was not performed with certain optimal design variables, such as recycle ratio and operating pressure. It is worth to note that the difference in the market size between cathode and anode product make the electrochemical coproduction less attractive, thus their market size also take into account for choosing coproduction pair products."

(Reviewer's Comment) l23
: Cement production is another excellent point source of highly concentrated CO2. Assuming that at some point in the future, all fossil fuels are replaced by renewables, cement production would constitute the largest source of concentrated CO2. It should therefore also be mentioned here.
(Authors' Response) Thank you for your valuable comment. We completely agree that cement production is an important CO 2 point source and should not be ignored. Thus, we mentioned cement production in the manuscript according to IEA report 2 Changes made: • In page 19 of manuscript (Authors' Response) Thank you for your comment and we apologize for awkward definition of electrochemical coproduction. For more clear explanation of the electrochemical coproduction that we intended to describe in this manuscript, we modified original sentence as below.
Changes made: • In page 6 of manuscript Original sentence: "To run an electrolysis cell, two half-reactions, oxidation and reduction, should be paired to create a complete redox reaction. Such coupling of reduction and oxidation reactions is defined as electrochemical coproduction." Revision: "To run an electrolysis cell, two half-reactions, oxidation and reduction, should be paired to create a complete reaction. Herein, we define the electrochemical coproduction as a paired electrolysis that both cathodic CO 2 RR and anodic OOR produce chemicals with market values." (Authors' Response) We appreciate and agree with the reviewer's comment that the general cell voltage range of HER-OOR coupling should consider wide range of organic oxidation reactions.

(Reviewer's Comment
Changes made: • In page 3 of supplementary information Original sentence: "Recently, the electrochemical oxidation of organic chemicals has emerged as an effective reaction for coupling with the HER; such coupling significantly reduces the overall cell voltage to approximately 1.44 V, which is 200-300 mV lower than that of HER-OER coupling." Revision: "Recently, the electrochemical oxidation of organic chemicals has emerged as an effective reaction for coupling with the HER; such coupling significantly reduces the overall cell voltage to within a range of approximately 1.5 V, which is 100-300 mV lower than that of HER-OER coupling."

(Reviewer's Comment)
The authors may be interested in the following paper: J. Am. Chem. Soc. 2016, 138, 46, 15110-15113. It contains a small-scale experimental example of a paired CO2RR-OOR process and highlights some of the experimental challenges of the approach. This work should definitely be cited.
(Authors' Response) Thank you for your comment. We totally agree with this is a very important and interesting paper which contains a good example of a paired CO 2 RR-OOR process and highlights some challenges. We cited the paper in the manuscript Ref [15] that introduce the concept of a paired CO 2 RR-OOR process.
Changes made: • In page 6 of manuscript Original sentence: "Parallel paired electrolysis features the simultaneous occurrence of two unrelated half-reactions in a divided cell. The most well-known, industrially established example is the chlor-alkali process, wherein chlorine and sodium hydroxide are produced at the anode and cathode, respectively. 14 " Revision: "Parallel paired electrolysis features the simultaneous occurrence of two unrelated half-reactions in a divided cell. The most well-known, industrially established example is the chlor-alkali process, wherein chlorine and sodium hydroxide are produced at the anode and cathode, respectively. 14 Interestingly, smallscale CO 2 RR-OOR process demonstration regarding the oxidative condensation through molecular electrocatalysts belongs to a parallel type. 15 "