# Molecular Catalyst Enables CO2 Electroreduction at 650 mA/cm2 CO Partial Current Density

Chengyu Liu, Dorian Joulié, Junming Shao, Bishnubasu Giri, Medhanie

Gebremedhin Gebru, Umit Isci, Marc Robert

► To cite this version:

Chengyu Liu, Dorian Joulié, Junming Shao, Bishnubasu Giri, Medhanie Gebremedhin Gebru, et al. Molecular Catalyst Enables CO2 Electroreduction at 650 mA/cm2 CO Partial Current Density. ACS Energy Letters, 2025, pp.4957-4961. 10.1021/acsenergylett.5c02397. hal-05267245

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Submitted on 18 Sep 2025

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# Molecular Catalyst Enables CO₂ Electroreduction at 650 mA/cm² CO Partial Current Density

Chengyu Liu,ª Dorian Joulié,ª Junming Shao,ª Bishnubasu Giri,ª Medhanie Gebremedhin Gebru,ª Umit Isciᵇ and Marc Robertª,ᶜ,\*

## Abstract

The electrochemical reduction of CO₂ to CO using renewable electricity offers a compelling pathway for greenhouse gas recycling. The two-electron, two-proton process is particularly attractive due to its operational simplicity and scalability, with copper- and silver-based nanomaterials being the most widely studied catalysts as the field approaches industrial maturity. However, achieving the necessary efficiency and stability for practical application remains a significant challenge. Recently, molecular catalysts immobilized on conductive surfaces with carbon-based inks have emerged as highly tunable hybrid systems capable of remarkable selectivity. In this work, we report that a straightforward cobalt phthalocyanine complex, simply modified with a single trimethylammonium group, delivers outstanding CO₂-to-CO conversion rates and selectivity-reaching a Faradaic efficiency of 93% at a total current density of 700 mA/cm² (jCO 650 mA/cm²) at neutral pH. Notably, CO selectivity above 90% was sustained for over 42 hours at 150 mA/cm², illustrating the potential of simply designed molecular catalysts for large scale applications.

The need to transition to a carbon-neutral economy has spurred intensive research into novel strategies for converting CO₂ into fuels using renewable electricity. Among these, the electrochemical reduction of CO₂ to CO (eCO₂RR) stands out for its compatibility with ambient aqueous conditions and its scalability potential. Over the past fifty years, eCO₂RR to CO has been extensively explored, with molecular catalysts playing a central role in both mechanistic studies and catalyst innovation.¹⁻³ Despite this progress, achieving high current densities with robust efficiency and stability-key requirements for industrial-scale application-remains a major hurdle when using molecular catalysts. One might even wonder whether such a challenge can be met using molecular catalysts. However, such catalysts offer unique advantages due to their precisely defined active sites and adjustable ligand environments, which enable enhanced efficiency and selectivity in CO₂ reduction. The development of flow cell technology, particularly those incorporating gas diffusion electrodes (GDEs), has further addressed the challenges posed by CO₂'s limited solubility and mass transport in traditional H-cell configurations.⁴ Flow cells facilitate efficient CO₂ reduction at high current densities, and recent breakthroughs have demonstrated the viability of molecular catalysts within these systems. Berlinguette and Robert demonstrated rapid and selective CO₂ conversion using Fe and Co molecular catalysts in flow cells (Figure 1b,c).⁵,⁶ It has also been shown recently that introducing electrostatic features to molecular catalysts

can enhance CO₂ binding at the metal center, thereby boosting CO production rates and suppressing proton reduction.⁷

The role of electrolyte cations in steering CO₂ reduction outcomes has also been studied extensively in heterogeneous catalysis,⁸ and also in homogeneous catalysis.⁹ Cations not only locally buffer the pH near the electrode but also influence surface charge density and the electric field, all of which affect the reaction's selectivity and efficiency.¹⁰ Specifically, alkali metal cations have been observed to impact CO₂-to-CO conversion activity in the order Li⁺ < Na⁺ < K⁺ < Cs⁺.¹¹,¹² Cs⁺ could help stabilizing the CO₂ reduction intermediate and making a higher local pH.¹³ A recent study revealed that hydronium ions (H₃O⁺) play a key role in proton transfer during eCO₂RR, and that elevated K⁺ concentrations work synergistically to accelerate CO production by stabilizing the *COOH intermediate*.¹⁴

Moreover, incorporating cationic groups in the second coordination sphere of molecular catalysts has enabled eCO₂RR to CO at fast rate and high selectivity,¹⁵ further allowing to reach industrially relevant current densities (up to 239 mA/cm²) with high selectivity (95%) when using GDEs.⁷,¹⁶ However in these cases the catalyst is not stable for long term electrolysis. Interestingly, Sato et al. has made significant progress by introducing potassium salt with molecular catalyst immobilized on carbon support. It shows nearly 7 days stability with > 95% selectivity for CO production in flow cell at 100 mA/cm² and in KOH electrolyte.¹⁷ That the potassium cations may both exert stabilizing effects on CO₂ binding and reduction and favor

<footnote>ª Sorbonne Université, CNRS, Institut Parisien de Chimie Moléculaire, IPCM, F-75005 Paris, France.</footnote>

<footnote>ᵇ Marmara University, Faculty of Technology, Department of Metallurgical & Materials Engineering, 34722 Istanbul, Türkiye.</footnote>

<footnote>ᶜ Institut Universitaire de France (IUF), F-75005 Paris, France.</footnote>

<footnote>\* corresponding author: marc.robert@sorbonne-universite.fr</footnote>
selectivity through local field effect is likely, still the underlying mechanisms are not yet fully understood. Recent advances have involved immobilizing various molecular catalysts on a range of support materials to form GDEs, yielding high efficiencies in flow cell setups (see Figure 2d). Nevertheless, despite this progress, most systems still fall short of meeting industrial targets-namely, CO partial current densities exceeding 500 mA/cm² and robust stability at high currents (j > 100 mA/cm²). In this study, we employed a straightforward cobalt phthalocyanine molecular catalyst (CoPc2, Figure 1a), which incorporates a single cationic quaternary trimethylammonium group in the second coordination sphere. This feature enables outstanding CO₂-to-CO conversion rates and selectivity in a flow cell system. Additionally, by harnessing the effects of electrolyte cations, we achieved notable enhancements in both selectivity and stability. The optimized cell reached a CO partial current density of 650 mA/cm² and demonstrated 42 hours of stable operation at 150 mA/cm², maintaining a selectivity above 90%. These results represent a significant advance toward the practical use of molecular catalysts for high current density, large-scale CO₂ electroreduction.

The electrocatalytic reduction of CO₂ was carried out in a three-compartment flow cell operated in a "flow-through" configuration (see Figures 1b, 1c, and S1). In this setup, CO₂ gas is directed through the gas diffusion electrode (GDE, Figure S2), which enhances mass transfer to the catalyst layer and helps to mitigate flooding issues.¹⁸ The

GDE served as the cathode and was prepared by drop-casting a catalyst ink—either non-substituted Co phthalocyanine (CoPc) or CoPc2—immobilized on carbon black powder or multiwall carbon nanotubes (MWCNTs). Detailed preparation procedures can be found in the Supporting Information. A Pt-Ti electrode was used as the anode, and a 0.5 M electrolyte solution (NaHCO₃, KHCO₃ or CsHCO₃) was circulated at a flow rate of 10 mL/min. The cathode and anode compartments were separated by an anion exchange membrane (AEM). Gaseous products were collected with the catholyte flow and subsequently quantified by gas chromatography following gas-liquid separation. Constant current electrolysis (CCE) experiments were performed at various current densities. At moderate current densities (j = 125 mA/cm²) under optimized conditions, both CoPc and CoPc2 yielded similar CO Faradaic efficiencies (FEₐ₀), each approaching 99%. However, at higher current densities (> 250 mA/cm²), CoPc2 exhibited superior selectivity (Figure 1d), maintaining a FEₐ₀ of 98% compared to 91% for CoPc. This suggests that the cationic group in CoPc2 may stabilize the CO₂-bound intermediate via electrostatic interactions and/or create a localized electric field that promotes selectivity.⁷,¹⁶ Further optimization of CoPc2 loading and evaluation across different current densities (Figure S3) revealed that at 150 mA/cm², FEₐ₀ was largely independent of catalyst loading in the range of 0.05 to 0.40 mg/cm². However, at higher current densities of 300 and 500 mA/cm², a loading of 0.15 mg/cm² delivered the best performance, with differences between loadings becoming more pronounced as current density increased.

<figure><img src="image_2.png" /><figcaption>**Figure 1.** (a) Chemical structures of CoPc and CoPc2 used in this study. (b) Scheme of the three compartment flow cell used in this study (c) Graphical representation of flow-through system (d) Faradaic efficiency of CO during CCE using CoPc (0.7 mg/cm² loading) or CoPc2 (0.15 mg/cm² loading) immobilized at carbon black (CB, 3 mg/cm²) as catalyst with 0.5 M NaHCO₃ as electrolyte, current densities ranging from 125 to 500 mA/cm².</figcaption></figure>
This study shows that a CoPc2 loading of 0.15 mg/cm² offers an optimal balance between providing sufficient active sites and minimizing aggregation, which is crucial for efficient electrolysis at high current densities.

We further examined the influence of cations on our system at the optimized catalyst loading by conducting 30-minute constant current electrolysis (CCE) at 500 mA/cm² using 0.5 M solutions of CsHCO₃, KHCO₃, and NaHCO₃ as electrolytes. The Faradaic efficiency for CO (FEₐ) remained above 90% with CsHCO₃ but dropped markedly to 60% and 25% with KHCO₃ and NaHCO₃, respectively (Figure 2a). These findings highlight that Cs⁺ significantly enhances the stability of the molecular flow cell. Crucially, this improvement arises from the synergistic interaction between Cs⁺ and CoPc2, since negligible CO formation occurred with CsHCO₃ electrolyte in the absence of CoPc2 (Figure S4). Control experiments under Ar atmosphere confirmed that Cs⁺ does not promote CO generation from HCO₃⁻ alone (Figure S4). Similar stability benefits were observed using CoPc as the catalyst: while initial FEₐ values (~90%) were comparable for NaHCO₃ and CsHCO₃,

after 30 minutes FEₐ dropped to ~30% with NaHCO₃ but remained at 80% with CsHCO₃ (Figure S5a). This improvement likely stems from reduced bicarbonate crystallization at the gas diffusion electrode (GDE) when using CsHCO₃, which has about three times greater solubility than NaHCO₃ at 20°C.¹² Crystallization can block active sites (Figure S5b), favoring hydrogen evolution over CO₂ reduction and raising hydrogen production.

After optimizing catalyst loading and electrolyte, the CO₂-to-CO conversion rate further increased, achieving a total current density of 700 mA/cm² with 93% FEₐ within the initial 5 minutes of electrolysis (Figure 2b), maintaining FEₐ above 90% for over 10 minutes-though sustaining this remains challenging. Achieving a CO partial current density of 651 mA/cm² marks a significant advancement for molecular catalysts in high-current electrolysis (Figure S6), illustrating that simple modification of the ligand can create a highly favorable micro-environment for boosting the reaction rate, and paving the way for their industrial application.

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>**Figure 2.** (a) Faradaic efficiency of CO during CCE at 500 mA/cm² using CoPc2 (0.15 mg/cm²) @ CB (3.0 mg/cm²) as catalyst with 0.5 M electrolytes. (b) Faradaic efficiency of CO during CCE using CoPc2 (0.15 mg/cm²) @ CB (3 mg/cm²) as catalyst with 0.5 M CsHCO₃ as electrolyte at current densities ranging from 200 to 700 mA/cm² (at the highest current, the cell voltage was 4.5 V) (c) Faradaic efficiency of long-term CCE using CoPc2 (0.15 mg/cm²) @ CB (3.0 mg/cm²) as catalyst with 0.5 M CsHCO₃ electrolyte at 150 mA/cm² current density. Conditions: 10 mL/min flow of electrolyte, 20 sccm flow of humidified CO₂, room temperature. (d) Recent reports on the CO partial current density and the Faradaic efficiency of CO of electrochemical CO₂-to-CO conversion using Pc-type molecular catalysts. jₐ were taken or calculated from the respective literature references.</figcaption></figure>
We next evaluated the system’s stability, initially employing multiwalled carbon nanotubes (MWCNTs)-a commonly used carbon support-under optimized conditions. With a catalyst loading of 0.15 mg/cm² CoPc2 on 3.0 mg/cm² MWCNT and 0.5 M CsHCO₃ as electrolyte, the system maintained stability for over 48 hours at a current density of 100 mA/cm², with FEₐCONSistently above 80% (Figure S7). The catalytic current remained steady throughout electrolysis. X-ray photoelectron spectroscopy (XPS) analysis showed comparable Co 2p and N 1s binding energies before and after catalysis, indicating minimal changes to the gas diffusion electrode (GDE) (Figure S8). Likewise, scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) mapping revealed negligible structural or compositional differences in the GDE post-electrolysis (Figures S9-S10).

Subsequently, carbon black was tested as the support material under the same optimized conditions. This configuration delivered efficient performance, sustaining FE<sub>CO</sub> above 90% for over 42 hours at 150 mA/cm² (Figure 2c). The catalytic current was stable during electrolysis, with SEM and EDX analyses before and after suggesting minimal alterations in the GDE (Figures S11-S12). These findings highlight the excellent long-term stability and durability of CoPc2-loaded GDEs on both MWCNT and carbon black supports, underscoring the promise of molecular catalysts for stable CO₂-to-CO electroreduction.

In conclusion, we have developed a straightforward approach for optimizing a molecular-based flow cell catalytic system that yields substantial enhancements in both selectivity and stability, as well as increased eCO₂RR to CO conversion rates. By introducing electrostatic effects at the molecular catalytic site-via a positively charged substituent-and harnessing the influence of electrolyte cations, we have demonstrated

the potential to achieve industrially relevant CO partial current densities (> 650 mA/cm²) with high selectivity (> 90%) and extended stability (> 42 hours at 150 mA/cm²). Notably, this long-term stability appears to be limited not by catalyst degradation, but by cell design constraints such as carbonate salt precipitation and persistent cathode flooding. Our findings show that even a simple molecular system can surpass CO partial current densities of 500 mA/cm², offering a promising strategy for advancing molecular flow cell technologies.

## AUTHOR CONTRIBUTIONS

The project was conceived by M.R. C.L, D.J., J.S., B.G., M.G.G., U.I. C.L., D.J., J.S. and B.G. performed experiments. Results analysis, draft preparation and editing were performed by all authors.

## CONFLICTS OF INTEREST

There are no conflicts to declare.

## SUPPORTING INFORMATION

Material and Instrumentation, Quantification of the CO₂RR products, Figures (catalytic performances, time dependent gas chromatograms, XPS data and SEM images).

## ACKNOWLEDGEMENTS

This research is supported by the European Union grant (Project: 101118129 - PHOTOSINT - HORIZON-CL5-2022-D3-02). This work has also benefited from French State aid managed by the Agence Nationale de la Recherche under France 2030 plan (ANR-22-PESP-0010 Projet ciblé "POWERCO2") within the PEPR project SPLEEN. J.S. is grateful to the China Scholarship Council for PhD fellowship. Partial financial support to M.R. from the Institut Universitaire de France (IUF) is thanked.

## References

(1) Birdja, Y. Y.; Pérez-Gallent, E.; Figueiredo, M. C.; Göttle, A. J.; Calle-Vallejo, F.; Koper, M. T. M. Advances and Challenges in Understanding the Electrocatalytic Conversion of Carbon Dioxide to Fuels. Nat. Energy 2019, 4 (9), 732–745.

(2) Xin, Y.; Musgrave III, C. B.; Su, J.; Li, J.; Xiong, P.; Meng-Jung Li, M.; Song, Y.; Gu, Q.; Zhang, Q.; Liu, Y.; Guo, W.; Cheng, L.; Tan, X.; Jiang, Q.; Xia, C.; Zhong Tang, B.; Goddard III, W. A.; Ye, R. Subtle Modifications in Interface Configurations of Iron/Cobalt Phthalocyanine-Based Electrocatalysts Determine Molecular CO₂ Reduction Activities. Angew. Chem. 2025, 137 (8), e202420286.

(3) Ren, S.; Lees, E. W.; Hunt, C.; Jewlal, A.; Kim, Y.; Zhang, Z.; Mowbray, B. A. W.; Fink, A. G.; Melo, L.; Grant, E. R.; Berlinguette, C. P. Catalyst Aggregation Matters for Immobilized Molecular CO₂RR Electrocatalysts. J. Am. Chem. Soc. 2023, 145 (8), 4414–4420.

(4) O'Brien, C. P.; Miao, R. K.; Shayesteh Zeraati, A.; Lee, G.; Sargent, E. H.; Sinton, D. CO₂ Electrolyzers. Chem. Rev. 2024, 124 (7), 3648–3693.
(5) Ren, S.; Joulié, D.; Salvatore, D.; Torbensen, K.; Wang, M.; Robert, M.; Berlinguette, C. P. Molecular Electrocatalysts Can Mediate Fast, Selective CO₂ Reduction in a Flow Cell. *Science* **2019**, *365* (6451), 367–369.

(6) Torbensen, K.; Joulié, D.; Ren, S.; Wang, M.; Salvatore, D.; Berlinguette, C. P.; Robert, M. Molecular Catalysts Boost the Rate of Electrolytic CO₂ Reduction. *ACS Energy Lett.* **2020**, *5* (5), 1512–1518.

(7) Wang, M.; Torbensen, K.; Salvatore, D.; Ren, S.; Joulié, D.; Dumoulin, F.; Mendoza, D.; Lassalle-Kaiser, B.; Işçi, U.; Berlinguette, C. P.; Robert, M. CO₂ Electrochemical Catalytic Reduction with a Highly Active Cobalt Phthalocyanine. *Nat. Commun.* **2019**, *10* (1), 3602.

(8) Ovalle, V. J.; Hsu, Y.-S.; Agrawal, N.; Janik, M. J.; Waegele, M. M. Correlating Hydration Free Energy and Specific Adsorption of Alkali Metal Cations during CO₂ Electroreduction on Au. *Nat. Catal.* **2022**, *5* (7), 624–632.

(9) Hammouche, M.; Lexa, D.; Momenteau, M.; Saveant, J. M. Chemical Catalysis of Electrochemical Reactions. Homogeneous Catalysis of the Electrochemical Reduction of Carbon Dioxide by Iron(“0”) Porphyrins. Role of the Addition of Magnesium Cations. *J. Am. Chem. Soc.* **1991**, *113* (22), 8455–8466.

(10) Hicks, M. H.; Nie, W.; Boehme, A. E.; Atwater, H. A.; Agapie, T.; Peters, J. C. Electrochemical CO₂ Reduction in Acidic Electrolytes: Spectroscopic Evidence for Local pH Gradients. *J. Am. Chem. Soc.* **2024**, *146* (36), 25282–25289.

(11) Marcandalli, G.; Monteiro, M. C. O.; Goyal, A.; Koper, M. T. M. Electrolyte Effects on CO₂ Electrochemical Reduction to CO. *Acc. Chem. Res.* **2022**, *55* (14), 1900–1911.

(12) Sassenburg, M.; Kelly, M.; Subramanian, S.; Smith, W. A.; Burdyny, T. Zero-Gap Electrochemical CO₂ Reduction Cells: Challenges and Operational Strategies for Prevention of Salt Precipitation. *ACS Energy Lett.* **2023**, *8* (1), 321–331.

(13) Siritanaratkul, B.; Khan, M. D.; Yu, E. H.; Cowan, A. J. Alkali Metal Cations Enhance CO₂ Reduction by a Co Molecular Complex in a Bipolar Membrane Electrolyzer. *Philos. Trans. R. Soc. Math. Phys. Eng. Sci.* **2024**, *382* (2282), 20230268.

(14) Yun, H.; Yoo, S.; Son, J.; Kim, J. H.; Wu, J.; Jiang, K.; Shin, H.; Hwang, Y. J. Strong Cation Concentration Effect of Ni–N–C Electrocatalysts in Accelerating Acidic CO₂ Reduction Reaction. *Chem* **2025**, *0* (0).

(15) Azcarate, I.; Costentin, C.; Robert, M.; Savéant, J.-M. Through-Space Charge Interaction Substituent Effects in Molecular Catalysis Leading to the Design of the Most Efficient Catalyst of CO₂-to-CO Electrochemical Conversion. *J. Am. Chem. Soc.* **2016**, *138* (51), 16639–16644.

(16) Su, J.; Zhang, J.-J.; Chen, J.; Song, Y.; Huang, L.; Zhu, M.; Yakobson, B. I.; Tang, B. Z.; Ye, R. Building a Stable Cationic Molecule/Electrode Interface for Highly Efficient and Durable CO₂ Reduction at an Industrially Relevant Current. *Energy Environ. Sci.* **2021**, *14* (1), 483–492.

(17) Sato, S.; Sekizawa, K.; Shirai, S.; Sakamoto, N.; Morikawa, T. Enhanced Performance of Molecular Electrocatalysts for CO₂ Reduction in a Flow Cell Following K⁺ Addition. *Sci. Adv.* **2023**, *9* (45), eadh9986.

(18) de Sousa, L.; Benes, N. E.; Mul, G. Evaluating the Effects of Membranes, Cell Designs, and Flow Configurations on the Performance of Cu-GDEs in Converting CO₂ to CO. *ACS EST Eng.** **2022**, *2* (11), 2034–2042.
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