# Molecular Electrochemical Catalysis of CO-to-Formaldehyde Conversion with a Cobalt Complex

Ajeet Singh, Afridi Zamader, Reza Khakpour, Kari Laasonen, Michael Busch, Marc Robert

► To cite this version:

Ajeet Singh, Afridi Zamader, Reza Khakpour, Kari Laasonen, Michael Busch, et al. Molecular Electrochemical Catalysis of CO-to-Formaldehyde Conversion with a Cobalt Complex. Journal of the American Chemical Society, 2024, 146 (32), pp.22129-22133. 10.1021/jacs.4c06878. http://dx.doi.org/10.1021/jacs.4c06878

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Submitted on 29 Apr 2025

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# Molecular electrochemical catalysis of CO-to-formaldehyde conversion with a Cobalt complex

Ajeet Singh,† Afridi Zamader,† Reza Khakpour,§ Kari Laasonen,§ Michael Busch,||,# Marc Robert†,‡*

**Keywords:** *Co phthalocyanine, molecular catalysis, CO electroreduction, formaldehyde, methanol.*

**ABSTRACT:** Formox, a highly energy-intensive process, currently serves as the primary source of formaldehyde (HCHO), a crucial and steadily growing chemical demand. The alternative electrochemical production of HCHO from C1 carbon sources such as CO₂ and CO is still in its early stages, with even the few identified cases lacking mechanistic rationalization. In this study, we demonstrate that cobalt phthalocyanine (CoPc) immobilized on multi-walled carbon nanotubes (MW-CNTs) constitutes an excellent electrocatalytic system for producing HCHO with productivity through the direct reduction of CO—the two-electron reduction product of CO₂. By carefully adjusting both the pH and the applied potential, we identified conditions that enable the production of HCHO with a partial current density of 0.64 mA cm⁻² (17.5% Faradaic efficiency) and a total 61.2% Faradaic efficiency for the liquid products (formaldehyde and methanol). A reduction mechanism is proposed.

Cobalt phthalocyanine (CoPc) has undergone thorough investigation as a molecular catalyst for the conversion of carbon dioxide (CO₂) to carbon monoxide (CO) through a 2e⁻/2H⁺ reduction, with pioneering studies already performed about half a century ago.¹,² The introduction of this catalyst family adsorbed or grafted onto conductive carbon-based support, such as carbon nanotubes (CNTs), exhibits the potential to attain near-perfect selectivity in generating CO from CO₂ with industry-relevant current densities in aqueous environments.³⁻⁶ Further catalyzing CO₂ reduction beyond 2e⁻ products using molecular catalysts has long been a significant challenge, although there is no intrinsic impeding of such reactivity.⁷ One possible strategy for CO₂ conversion to higher-order C1 products, such as formaldehyde (HCHO; 4e⁻, 4H⁺ reduced) and methanol (CH₃OH; 6e⁻, 6H⁺ reduced) is to develop a cascade process, where CO₂ initially converts to CO, followed by subsequent 2e⁻, 2H⁺ reduction leading to HCHO, and further 2e⁻, 2H⁺ reduction resulting in CH₃OH.⁸,⁹ Using such a strategy, we, followed by others, have recently demonstrated that CoPc can indeed convert CO₂ to CO and further to CH₃OH, with traces of HCHO being formed, at least in basic conditions. In these processes,

CO bound CoPc appears as a key intermediate for further reduction.⁹,¹⁰ Once formed however, CO tends to easily de-coordinate from the cobalt center. Moreover, in the presence of both CO₂ and CO, cobalt preferentially binds CO₂, as recently demonstrated by Liu, Robert et al⁹ and quantified by McCrory et al. (there is a factor 3 higher binding constant for CO₂ as compared to CO).¹¹ As a consequence, CO₂ catalysis with CoPc mainly terminates at the CO stage. Also higher local concentration of CO₂ (with a solubility in water of ~33 mM at 25 °C) as compared to CO (solubility in water of ~1 mM at 25 °C) further favors CO production.¹¹,¹² Otherwise, the reduction of CO₂ to CO requires a lower overpotential compared to the reduction of CO to CH₃OH.¹³ Overall, CO₂RR predominantly leads to the formation of CO as the main product (with Faradaic efficiency (FE) typically in the range of 90% or above) and CH₃OH as a side product with FE below or in the range of ca. 10%.¹⁰,¹⁴

Different strategies may be employed to enhance the yield of CH₃OH from COₓ (i.e., CO₂ or CO) using CoPc as catalyst. It should be first noted that, like with other electrocatalytic materials, pH and electrolyte conditions (in particular the nature of the cation) may play a significant role.

<footnote>† Université Paris Cité, CNRS, Laboratoire d'Electrochimie Moléculaire (LEM), F-75013, Paris, France</footnote>

<footnote>§ Department of Chemistry and Material Science, School of Chemical Engineering, Aalto University, 02150 Espoo, Finland</footnote>

<footnote>|| Division of Materials Science, Department of Engineering Sciences and Mathematics, Luleå University of Technology, 971 87 Luleå, Sweden</footnote>

<footnote># Wallenberg Initiative Materials Science for Sustainability (WISE), Luleå University of Technology, 971 87 Luleå, Sweden</footnote>

<footnote>‡ Institut Universitaire de France (IUF), F-75005, Paris, France</footnote>
cant role for CH₃OH production.¹⁰ Preventing catalyst aggregation through dispersion on the carbon support is also important to have significant reactivity.¹⁵,¹⁶,¹⁷ One important strategy is of course to increase the local concentration of CO at active sites, either simply upon using CO itself as a reactant or upon tailoring the microenvironment at active sites so that CO can accumulate as an intermediate. Recently, introducing molecular strain at CoPc metal center was generated through adsorption of the complex to single wall carbon nanotube of 1 nm diameter size and it led to an increased interaction between CO and CO and consequently to higher CH₃OH yield.¹⁸ Faradaic efficiency and partial current density of CH₃OH (jCH₃OH) at such materials have reached high metrics ranging from 30% to 65% and 20-90 mA cm⁻² respectively, in MEA (Membrane Electrode Assembly) cells, the higher activities reported to date.⁹,¹⁸

Obtaining HCHO as a product remains challenging, since it may be at best an intermediate species towards CH₃OH. In preliminary studies, low amount of HCHO (ca. 5% FE) was produced from CO electroreduction in basic conditions (pH 13, 0.1 M KOH) with CoPc as catalyst.⁸,¹⁰ That HCHO is properly detected and quantified poses hurdles, as techniques like high pressure liquid chromatography (HPLC) coupled with a Ultraviolet (UV) detector can identify the product but is not suited for isotope labeling experiments-a crucial step for determining the origin of the formed HCHO.¹⁰ While proton nuclear magnetic resonance (¹H NMR) could be utilized, HCHO often exhibits a minute peak at 9.66 ppm or reacts with water leading to its hydrated form H₂C(OH)₂, displaying a peak at 4.91 ppm which is close to that of water. To overcome this issue, we recently reported a method for detecting and quantifying HCHO upon reacting with sodium bisulfite (NaHSO₃).¹⁹ Detection of the adduct and isotope labeling study unambiguously identified HCHO as a product, indicating that some HCHO can decoordinate from the active site and diffuses out from the catalytic film.⁸ Kinetic studies pointed to a scenario in which once formed at Co, HCHO would further be reduced to CH₃OH with 2 electrons and 2 protons or desorb from the metal and diffuse out into the solution where it can be trapped. This scenario is summarized in Figure 1, where partial reduction of CO to HCHO is followed by a branching between formation of solvated HCHO and further reduction to the alcohol (see the DFT calculation section for details). Overall, formation of HCHO hinges on the possibility of de-coordination of HCHO from the CoPc-HCHO intermediate.⁹ As a general remark, it could be noted that both molecular and heterogeneous electrocatalysts hardly produce HCHO due to low activation barriers for the conversion from HCHO to CH₃OH.²⁰ Otherwise, in the case of CoPc as catalyst, no or little HCHO could be detected at near-neutral pH. Despite these moderately encouraging results, HCHO is an important chemical precursor for the industry, and being able to get high amounts of HCHO from COₓ (CO₂, CO) as reactants is worth pursuing. Obtaining significant amounts of HCHO is also important for coupling reac-

tions with nitrogen species (e.g. NO₃⁻, NO₂⁻) to make compounds such as urea or alkyl amines.

From our previous studies, we speculated that both the pH and the potential may influence HCHO production and may lead to various ratio with CH₃OH. Notably, higher pH and weakly negative potentials may favor formaldehyde at the expense of CH₃OH. We have performed extensive electrochemical studies with CoPc adsorbed at MWCNT (multi-walled carbon nanotubes). We discovered that HCHO production is highly influenced by the solution basicity, pH 12 being an optimum value, and is also dependent on the applied electrode potential, with a potential window within which the ratio between HCHO and CH₃OH can reach 7.5. Complementary DFT computations of the reaction pathways added insights and further support the interpretation of the experimental results.

*Electrochemical studies.* We started our investigation using our CoPc-functionalized electrodes. A mixture of CoPc catalyst (1 mg) and MWCNTs (10 mg) were suspended and sonicated for 30 min in 10 mL of ethanol. Subsequently, 1% (v/v) Nafion© (100 µL) was added followed by another 30 min sonication, and stirring for 16 h at room temperature (T = 25 °C).

Then 100 µL (50 µL × 2 times) of the prepared ink was drop-casted onto a 1 cm² of H₁₅C₁₃ carbon paper and dried for 16 h at room temperature (see SI CoPc/MWCNTs film preparation

<caption>Figure 1. General mechanism for cascade electrochemical catalysis of CO₂ reduction into CO, and then to formaldehyde and methanol with molecular cobalt phthalocyanine. M is the doubly reduced, protonated Co phthalocyanine (CoPc).⁹ See text for details.</caption>

for details), leading to CoPc loading of 17.5 nmol cm⁻². The cyclic voltammetry of CoPc in presence of argon and CO showed two one-electron reduction waves at E<sub>red1</sub> = 0.31 V<sub>RHE</sub> and E<sub>red2</sub> = -0.38 V<sub>RHE</sub>, followed by a more negative catalytic wave (Figure S1), showing that the Co complex needs to be doubly reduced before binding CO (or CO₂), as previously shown.⁹,²¹ We varied the pH between 7 and 13 and performed 30 min controlled potential electrolysis

<figure><img src="image_2.png" /></figure>
(CPE) at a constant potential of -0.548 V vs. RHE ($V_{RHE}$) and $T = 10$ °C (Table 1, Figure S2). While only trace of HCHO was detected in neutral basic conditions ($pH \le 9$), it started to be significantly produced at pH above 10, reaching 16.5% FE at pH 12 with a partial current density of 0.21 mA cm⁻² (Figure S2). Hydrogen (major) and $CH_3OH$ (minor, FE below 10%) were obtained as complementary products. These findings support the idea that CoPc-HCHO de-coordination is possibly favored at higher pH values (see the DFT calculation section for further discussion).⁸ We studied in parallel the effect of the temperature, upon conducting CPEs from 25°C to 2°C. As expected, since CO solubility increases with decreasing temperature, a slight increase for FE(HCHO) was observed, showing best results between 10 °C and 2°C (SI, Figure S3). For practical reasons, we continued all our experiments at 10 °C.

<table><caption>Table 1. Effect of pH on Faradaic efficiency for HCHO production upon 30 min CPE of a CO saturated solution at a potential $E = -0.548$ V vs. RHE, $T = 10$ °C.</caption><thead><tr><th>pH</th><th>j<br>(mA cm<sup>-2</sup>)</th><th>Charge<br>(C)</th><th>FE<sub>HCHO</sub></th><th>j<sub>HCHO</sub><br>(mA cm<sup>-2</sup>)</th></tr></thead><tbody><tr><td>13</td><td>1.61</td><td>2.89</td><td>8.9</td><td>0.14</td></tr><tr><td>12.5</td><td>1.15</td><td>2.89</td><td>13.6</td><td>0.16</td></tr><tr><td>12</td><td>1.27</td><td>2.28</td><td>16.5</td><td>0.21</td></tr><tr><td>11</td><td>0.88</td><td>1.59</td><td>10.3</td><td>0.09</td></tr><tr><td>10</td><td>0.84</td><td>1.52</td><td>3.1</td><td>0.03</td></tr></tbody></table>

We then varied the electrode potential (pH 12, $T = 10$ °C) in a potential window ranging from -0.44 $V_{RHE}$ and -0.99 $V_{RHE}$, since CO is activated by the catalyst within this range (Figure 2). It can be seen, for potential values lower than -0.7 $V_{RHE}$, that hydrogen became the dominant product, while at potential more positive than -0.5 $V_{RHE}$, barely any CO reduction products were obtained. Optimized yield for HCHO production corresponded to $E = -0.624 V_{RHE}$, leading to a high FE for HCHO (20%), along with significant $CH_3OH$ formation ($FE_{CH_3OH} = 35.3\%$). Maximum partial current density for HCHO was found at $E = -0.650 V_{RHE}$ with a $j_{HCHO}$ of 0.64 mA cm⁻² ($FE_{HCHO} = 17.5\%$), corresponding to a turnover number ($TON_{HCHO}$) of 349 and a mean rate (turnover frequency- $TOF_{HCHO}$) of 0.2 s⁻¹ (Figure 2 and Figure S4). The Cannizaro reaction, that involves the base-induced disproportionation of two molecules of formaldehyde into methanol and formate (Figure S5), was negligible under these conditions of potential, pH and time scale of experiments since the FE for formate remained in all investigated cases lower than 1 %.²² A bell-shaped distribution of FEs was observed for both HCHO and $CH_3OH$, illustrating a significant impact of the potential on the formation of these products. As initially postulated, $FE_{HCHO}$ remained almost independent of the potential value (from -0.55 to -0.65 $V_{RHE}$). At the most

<figure><img src="image_3.png" /><figcaption>Figure 2. Electrode potential effect on Faradaic efficiencies for $H_2$ (light orange), HCHO (green) and $CH_3OH$ (light purple) during 30 min CPE of a CO saturated solution at pH = 12 ($T = 10$ °C) and $j_{HCHO}$.</figcaption></figure>

positive value of the investigated potential range, FE ratio between HCHO and $CH_3OH$ reached a value of 7.5, corresponding to a total 88% chemical selectivity for HCHO among liquid products (HCHO, $CH_3OH$), a weakly negative applied potential (-0.548 $V_{RHE}$) being thus an important factor to favor the HCHO production. This demonstrates that it is possible to generate formaldehyde with limited production of $CH_3OH$. It is likely due to the need of two additional electrons to generate $CH_3OH$ as compared to HCHO. The maximum current density for HCHO obtained at $E = -0.65 V_{RHE}$ corresponds to the best compromise in terms of potential to activate CO at a sufficiently fast rate with still not an overwhelming $CH_3OH$ production.

Going to more cathodic potential values ($E < -0.650 V_{RHE}$) hydrogen evolution becomes favored over CO reduction with $FE_{H_2}$ reaching nearly 90% at $E = -1 V_{RHE}$. At lower pH (pH = 10), $FE_{HCHO}$ and $j_{HCHO}$ were smaller and at -0.542 $V_{RHE}$, the ratio of HCHO to $CH_3OH$ was reduced from 7.5 to 0.75 (Figure S6), a decrease of 1 order magnitude as compared to pH 12, indicating a less efficient de-coordination of the bound CoPc-HCHO intermediate. Kinetic studies (30 min electrolysis) showed a linear increase of HCHO concentration at both pH, with faster production of HCHO at pH 12 as compared to pH 10 (Figure 3; see Figure S8 for linear fitting parameters. We also checked that only traces of HCHO was present more acidic conditions (from pH 7 to 9), even at -0.650 $V_{RHE}$ (Figure S9). Interestingly, increase of the electrolyte cation ($K^+$) concentration from 0.3 to 1.0 M showed little variation for HCHO production, still with a slightly better yield at large concentration (> 0.5 M) (Figure S10). Changing the cation from $K^+$ to the larger $Cs^+$ with otherwise same electrolysis conditions did not bring noticeable change in HCHO quantities (Figure S11).
<figure><img src="image_4.png" /><figcaption>Figure 3. HCHO production vs. time (30 min CPE) at pH 10 (black square) and pH 12 (red dots) ($E = -0.650$ V<sub>RHE</sub>, $T = 10$ °C).</figcaption></figure>

*Insights from DFT calculations.* The underlying mechanism resulting in high HCHO selectivity was further rationalized using some DFT calculations. According to our speciation analysis (Figure S12), the catalyst is mostly present as formal Co("o")Pc²⁻. Ligand protonation has a pKa of 9 with a mean absolute error of ~1.5 pKa units.²³ Thus, the complex is present both in its protonated and unprotonated form under HCHO formation conditions which correspond to a high pH > 10 and a potential of the order of ~0.6 V vs. RHE.

The most likely reaction mechanism identified in our computations is shown in Figure 1. The reaction is initialized by the adsorption of CO which is slightly endergonic by 0.16 eV. This reaction is subject to a moderate barrier of 0.34 eV (unprotonated complex) which increases to 0.52 eV once the ligand gets protonated (Figure 4a). Thus, protonation kinetically stabilizes the CO intermediate and therefore plays an important role in shifting the selectivity towards post-CO products. CO adsorption is followed by the double hydrogenation of Co-CO at the carbon atom. Both hydrogenation steps are thermodynamically feasible under experimental conditions for the protonated and unprotonated complex. However, the first reduction step is subject to a moderate to high barrier of 0.60 eV for the unprotonated complex. This barrier is reduced to 0.49 eV upon ligand protonation. An identical trend is also observed for the second protonation step which is subject to a moderate barrier of 0.38 eV (unprotonated complex) while it is barrierless for the protonated complex. Thus, ligand protonation renders post-CO product formation more favorable. Reactivity toward both HCHO and CH₃OH formation then proceeds through a rearrangement whereby the HCHO goes from Co-C bonded adduct to a weak Co-O bonded adduct (Figure 4a; reaction (IV)). This reaction is again more favorable for the protonated complex. i.e., the reaction is roughly thermoneutral (ΔG = 0.04 eV) and subject to a moderate barrier of 0.42 eV. With the unprotonated complex, this reaction becomes thermodynamically unfavora-

ble (ΔG = 0.33 eV) and subject to a rather high barrier of 0.61 eV. Intermediate 5 (Figure 4) formed after the rearrangement is the bifurcation point between HCHO release (Figure 4b, reaction (V)) and CH₃OH formation through hydrogenation (Figure 4b; reaction (VI)). Hydrogenation and thus CH₃OH formation is barrierless for the unprotonated complex and accounts for ~0.73 eV (Potential: 0.73 V vs. RHE). Therefore, it is strongly preferred for unprotonated Co("o")Pc²⁻. However, this reaction is rather slow owing to the very low proton concentration. Naturally, also other species (such as the phosphate buffer or bicarbonate) could contribute as proton donors but studying their role is beyond the scope of the present work. For the protonated complex, this reaction is also thermodynamically highly favorable but subject to a moderate barrier of 0.42 eV. This together with the fact that formaldehyde release is independent from the proton concentration seems sufficient to shift the selectivity towards release of HCHO from the protonated complex. Its release is slightly exergonic by ~0.22 eV to ~0.34 eV and almost barrier less for both forms of the catalyst. The lack of any appreciable barrier is the result of the weak interaction of HCHO to the complex as indicated by a long Co-O bond length (2.5 Å). CH₃OH release through insertion of hydrogen into the Co-O bond is, with a reaction energy of ~0.26 eV (unprotonated) and ~0.46 eV (protonated), thermodynamically highly favorable and barrierless for both protonated and unprotonated Co("o")Pc²⁻ (Figure 4b, reaction (VII)).

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>
Figure 4. CO to formaldehyde and methanol formation energy profiles for Co("o")Pe<sup>2-</sup> and Co("o")Pe<sup>H</sup> active species; red arrows represent chemical reactions; a water cluster contains of two H<sub>2</sub>O and one H<sub>3</sub>O<sup>+</sup> was used for transition state calculations. Values for electrochemical steps are indicated by bold fonts and by normal fonts for chemical steps. The energy values in the energy profile (connected lines) represent the thermodynamic energies of chemical and electrochemical reactions and the values below the long arrow correspond to the activation barriers for the respective reactions above. All energies are based on Gibbs Free Energies.

Our experimental studies and computational insights converge to the conclusion that the selectivity towards HCHO formation is the result of catalyst protonation which favors post-CO product formation and HCHO release, and of a limited proton supply at high pH. Control of the electrode potential allowed to reach a 7.5 ratio between formaldehyde and methanol quantities at optimized pH 12, along with a high rate for HCHO formation (0.64 mA cm<sup>-2</sup>). That formaldehyde can be produced in good yields opens up large perspectives for further engaging it in a variety of coupling reactions and chemical processes. Such processes will be soon reported.

## ASSOCIATED CONTENT

**Supporting Information.** Experimental details on the preparation of catalytic electrodes; electrolysis experiments and analytical procedures (GC, NMR) for detecting and quantifying reduction products; additional cyclic voltammetry data (Figure S1) and electrolysis experiments (Figures S2-S11); details on computational methods (Figure S12) and cartesian coordinates of calculated structures.

## AUTHOR INFORMATION

### Corresponding Author

robert@u-paris.fr

### Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

### Funding Sources

This work has benefited from French State aid managed by the Agence Nationale de la Recherche under France 2030 plan (ANR-22-PESP-0010 Projet ciblé "POWERCO<sub>2</sub>") within the PEPR project SPLEEN. K.L. and R.K. gratefully acknowledge the support received from the Jane and Aatos Erkko Foundation and Finland CSC-IT Center for generous grants of computer time. M.B. acknowledges financial support by the Wallenberg Initiative Materials Science for Sustainability (WISE) funded by the Knut and Alice Wallenberg Foundation.

## ACKNOWLEDGMENT

Partial financial support to M.R. from the Institut Universitaire de France (IUF) is warmly thanked.

## ABBREVIATIONS

CO, Carbon Monoxide; CO<sub>2</sub>RR, CO<sub>2</sub> reduction reaction; CoPc, Co phthalocyanine; CPE, Constant Potential Electrolysis; FE, Faradaic efficiency

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<figure><img src="image_7.png" /></figure>