Efficient electrolyzer for CO2 splitting in neutral waterusing earth-abundant materialsArnaud Tatina, Clément Commingesb, Boniface Kokohb, Cyrille Costentina,1, Marc Roberta,1,and Jean-Michel Savéanta,1

aLaboratoire d’Electrochimie Moléculaire, Unité Mixte de Recherche Université - CNRS No 7591, Université Paris Diderot, Sorbonne Paris Cité, 75205 ParisCedex 13, France; and bUnité Mixte de Recherche Université - CNRS No 7285, Université de Poitiers, 86022 Poitiers Cedex, France

Contributed by Jean-Michel Savéant, March 25, 2016 (sent for review February 16, 2016; reviewed by Allen J. Bard and Marc Koper)

Low-cost, efficient CO2-to-CO+O2 electrochemical splitting is a keystep for liquid-fuel production for renewable energy storage anduse of CO2 as a feedstock for chemicals. Heterogeneous catalystsfor cathodic CO2-to-CO associated with an O2-evolving anodic re-action in high-energy-efficiency cells are not yet available. An ironporphyrin immobilized into a conductive Nafion/carbon powderlayer is a stable cathode producing CO in pH neutral water with90% faradaic efficiency. It is coupled with a water oxidation phos-phate cobalt oxide anode in a home-made electrolyzer by meansof a Nafion membrane. Current densities of approximately 1 mA/cm2

over 30-h electrolysis are achieved at a 2.5-V cell voltage, splittingCO2 and H2O into CO and O2 with a 50% energy efficiency. Remark-ably, CO2 reduction outweighs the concurrent water reduction. Thesetup does not prevent high-efficiency proton transport through theNafion membrane separator: The ohmic drop loss is only 0.1 V andthe pH remains stable. These results demonstrate the possibility toset up an efficient, low-voltage, electrochemical cell that convertsCO2 into CO and O2 by associating a cathodic-supported molecularcatalyst based on an abundant transition metal with a cheap, easy-to-prepare anodic catalyst oxidizing water into O2.

CO2-to-CO conversion | carbon dioxide electrolyzer | electrochemistry |molecular catalysis | solar fuels

The production of carbon-based fuels or chemicals using themost abundant carbon source (CO2) requires designing effi-

cient, cheap, selective, and sustainable processes able to convertCO2 into useful products (1–7). Carbon monoxide production isan important step to fuels because it can be used as a feedstock inthe Fischer–Tropsch process. Compared with water splitting,electrochemical reduction of CO2 into CO is a greater challenge.This is particularly true when aiming to carry out this reactionselectively in friendly conditions, namely at neutral pHs, ambienttemperature, and with abundant and cheap materials as catalystsas opposed to solid-state high-temperature electrolyzers (8).We recently discovered that substitution of the four paraphenyl

hydrogens of iron tetraphenylporphyrin by trimethylammoniogroups provides a water-soluble iron porphyrin (WSCAT) able tocatalyze selectively the electrochemical conversion of CO2 intoCO in neutral water in homogeneous conditions (9). The nextchallenge was to efficiently immobilize this molecular catalystonto the cathode and to set up an integrated electrochemical cellable to split CO2 and H2O into CO and O2 according to

CO2 + 2 H+ + 2  e� ⇄CO+H2O�E0CO2=CO =�0.11 V

�,

H2O⇄ 1/ 2 O2 + 2 H+ + 2  e��E0O2=H2O = 1.23 V

�,

CO2 ⇄CO+ 1/ 2 O2�ΔG0 = 2.68 eV

�,

[potentials referred to the standard hydrogen electrode (SHE)].Immobilization of the catalyst was achieved by preparation of

a suspension containing Nafion, WSCAT, and carbon powder

(Materials and Methods) (10). This solution was then sprayed ontoa carbon support (glassy carbon electrode for cyclic voltammetryexperiments and carbon felt or carbon Toray for electrolysis) andair-dried. Interactions between the positively charged catalyst andthe negatively charged functionalities of the ionic polymer securerobust integration of the catalyst into the coated film. This wasattested to by the absence of UV-vis signal corresponding to aniron porphyrin in a water solution in which the electrode wasimmersed for a few hours. The catalytic film consists of a thincoating of the electrode surface as confirmed by scanning electronmicroscopy (SEM) (Fig. 1).Conductive and catalytic properties for CO2 reduction of the

prepared electrode were characterized by cyclic voltammetrycarried out in quasi-neutral water with no added buffer, thuspreventing acid reduction (Fig. 2). Under argon, almost no ca-pacitive current and no faradaic current were observed in theabsence of carbon powder in the film, whereas the electrodeexhibited a large capacitive current when carbon powder wasincorporated in the coated film.This indicates that carbon powder renders the film conducting

and thus makes addressable the molecular catalyst contained inthe film. This is further confirmed by the observation of faradaicwaves when started from the FeIII complex. Although not beingwell defined, these waves may be assigned to the FeIII/II, FeII/I

redox couples. The slight increase of the current observed at morenegative potentials (approximately −1.15 V vs. SHE) presumablyreflects some catalysis of water reduction. In the presence of CO2,a large increase of the current is seen, confirming the catalyticactivity of WSCAT immobilized in the Nafion film toward CO2

Significance

Electrochemical CO2-to-CO conversion is one important optionfor storing intermittent, renewable electricity into chemicalbonds so as to produce fuels and to use CO2 as a feedstock forchemicals. The setup of an electrolyzer, associating cheap andabundant materials able to split CO2 into CO and O2, in envi-ronmentally friendly conditions (neutral pH, ambient temper-ature) with a high selectivity and stability, and a 50% energyconversion efficiency is reported. The results open the way tosolar energy driving of the CO2 /CO + 1/2 O2 splitting by asso-ciating the electrochemical cell with a light-to-electricity conver-sion device, and more generally with surplus electricity fromrenewable intermittent sources.

Author contributions: C. Costentin, M.R., and J.-M.S. designed research; A.T., C. Comminges,and B.K. performed research; A.T., C. Comminges, B.K., C. Costentin, M.R., and J.-M.S. analyzeddata; and C. Costentin, M.R., and J.-M.S. wrote the paper.

Reviewers: A.J.B., The University of Texas at Austin; and M.K., Leiden University.

The authors declare no conflict of interest.1To whom correspondence may be addressed. Email: cyrille@univ-paris-diderot.fr.,robert@univ-paris-diderot.fr, or saveant@univ-paris-diderot.fr.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1604628113/-/DCSupplemental.

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reduction. The onset of the catalytic wave is approximately −0.8 Vvs. SHE corresponding to an overpotential of

η=E0CO2=CO � ðRT ln 10=FÞ× pH= 390 mV.

An electrochemical membrane-separated cell was then as-sembled to couple the CO2 reduction half-reaction to wateroxidation. Although a significant overpotential is usually asso-ciated with the latter reaction, important progress has been re-cently made leading to a cheap and abundant material, easy toprepare, well characterized, and efficient at neutral pH, namelyelectrodeposited phosphate cobalt oxide (CoPi), which could beused as water oxidation catalyst on the anodic side (6). A thin(50 mC/cm2, i.e., approximately 330 nm) CoPi film was thusdeposited on a small stainless steel gauze (Fig. 1), with, as anodicelectrolyte, a phosphate buffer (0.4 M, pH 7.3) ensuring a min-imal anodic overpotential. The cell was constructed from flaskscontaining the electrolytes with a proton-permeable Nafionmembrane and both electrodes clamped between the flasks(Fig. 3).A reference electrode was also introduced in the cathodic

compartment so that both the cell voltage and the cathode po-tential electrode could be measured during electrolysis.The electrochemical characteristics of the cell were obtained

by applying a potential scan to the cathode while simultaneouslyrecording cell voltage, hence leading to the potential of both thecathode and the anode including ohmic drop vs. the referenceelectrode and the current (Fig. 4A). At pH 7.3, the onset of thecurrent (at 1 mA/cm2) corresponds to a cell voltage of 2.3 V with

ηcathodic =nE0CO2=CO � ðRT ln 10=FÞ× pH

o� Ecathodic = 320 mV

and

ηanodic +Ri=Ucell � Ecathodic �nE0O2=H2O

� ðRT ln 10=FÞ× pHo

= 630 mV.

An independent measurement indicated a 10-Ω cell resistance,amounting to an ohmic drop contribution of 50 mV to theoverpotential.A first series of electrolysis was performed at controlled

cathodic potential (Ecathodic = −0.86 V vs. SHE) for severalhours, with an expansion vessel over the flasks to assessproduct selectivity and evaluate faradaic efficiency. The av-eraged current density was 0.7 mA/cm2, the cell voltage 2.4 V(corresponding to 1.05-V overpotential), and gas chroma-tography analysis of gas in the headspace above solutionleads to faradaic efficiencies of 90% for CO along with 10%for H2 in the cathodic compartment and 99% for O2 in theanodic compartment. It is worth noting that no hydrocarbonswere detected in the cathodic compartment headspace. Ionicchromatography analysis of the cathodic electrolyte showsonly traces of formate and oxalate. These results clearly in-dicate a high selectivity and a high faradaic efficiency of thecell for CO2 and H2O conversion to CO and O2, respectively.A second series of electrolysis was performed at a controlledcathodic potential (Ecathodic = −0.96 V vs. SHE) for 30 hunder CO2 flow to evaluate cell stability. A linear increase ofthe charge passed vs. time was observed, corresponding to anaveraged 1 mA/cm2 current density (Fig. 4C); the cell voltageremained stable at a value of 2.5 V (Fig. 4D). Analysis of thegas mixture produced in the cathodic chamber over thecourse of the electrolysis showed an excellent stability ofthe cell selectivity (Fig. 4E). Note that the same experimentcarried out with iron tetraphenylporphyrin instead ofWSCAT led to a rapid decrease of the current to zero anda poor CO selectivity. The cell energy efficiency (EE) at1 mA/cm2 was evaluated to be 50% from the product of thefaradaic yield for CO production (FYCO = 0.9) and the ratioof the thermodynamics of the reaction over cell voltage:EE = FYCO × {E0

O2/H2O − E0CO2/CO}/Ucell (Fig. 4F). From the

overpotential measured at the cathode (ηcathodic = 330 mV)and at the anode (ηanodic = 720 mV, ohmic drop being

Fig. 1. SEM images of the Nafion/carbon powder/WSCAT (Left) and CoPi(Right) films. (A and B) Electrode structures. (C and D) Catalytic films onthe supporting material (polymer-wrapped carbon microfiber and stain-less steel microwire, respectively). (E and F ) High-magnification images ofthe films.

Fig. 2. Cyclic voltammetry (pH 6.7). Nafion/WSCAT film deposited on a3-mm-diameter glassy carbon electrode, v = 0.1 Vs−1, pH = 6.7. Light gray, filmwithout carbon powder under argon. Blue, film plus carbon powder underargon. Red, film plus carbon powder under 1 atm of CO2.

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approximately 100 mV), the cathodic energy efficiency wasevaluated as

EEcathodic =FYCO ×nE0O2=H2O � E0

CO2=CO

o.nE0O2=H2O � E0

CO2=CO + ηcathodic

o= 72%,

and the anodic energy efficiency as

EEanodic =FYCO ×nE0O2=H2O � E0

CO2=CO

o.nE0O2=H2O � E0

CO2=CO + ηanodic

o= 63%.

Further improvements of the overall cell energy efficiencyconcern both the anodic and cathodic catalytic films, and will

allow running electrolysis at higher current densities. However,the results described here demonstrate that supported molecularcatalyst based on the cheapest transition metal can be used toefficiently split CO2 to CO and O2 in neutral water and the per-formance of the low-cost, simply designed cell provides a robustbasis to couple this electrolysis with a photovoltaic device produc-ing electricity from solar energy (11–13), opening an avenue toCO2-based solar fuels.

Materials and MethodsCO2 splitting was performed in a two-compartment electrochemical cell wherecathodic electrolyte was a CO2-saturated 0.1 M KCl + 0.5 M KHCO3 aqueous

Fig. 4. Cell characterization and electrolysis at pH 7.3. (A) Current densityas function of the electrode potential (vs. SHE) for both the cathode andthe anode. (B) Entire cell overpotential as function of current density(dotted line corresponds to ohmic drop corrected data). (C) Charge trans-ferred as function of time for an electrolysis at controlled cathodic potential,Ecathodic = −0.96 V vs. SHE (the dotted line corresponds to the partial chargefor CO production). (D) Cell voltage as function of time for an electrolysis atcontrolled cathodic potential, Ecathodic = −0.96 V vs. SHE. (E) Product se-lectivity as function of time for an electrolysis at controlled cathodic po-tential, Ecathodic = −0.96 V vs. SHE. H2 (o) and CO (red •). (F) Cell energyefficiency as function of time for an electrolysis at controlled cathodicpotential, Ecathodic = −0.96 V vs. SHE.

Fig. 3. Proton exchange membrane electrolysis cell configuration. Cathodichalf-reaction is CO2 reduction to CO on a carbon electrode coated with aNafion/carbon powder/WSCAT film and anodic half-reaction is oxidation ofH2O to O2 on a stainless steel gauze with a CoPi electrodeposited film.

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solution at pH 7.3 and anodic electrolyte was a 0.4 M potassium phosphatebuffer at pH 7.3 degassed under argon. A Nafion NRE-212 membrane enabledproton transfer between aforesaid compartments. Cathodes were manufac-tured by spraying a suspension of WSCAT catalyst, conductive carbon powder,and Nafion solution in 2-propanol onto Toray carbon paper. CoPi anodes wereprepared by electrodeposition of a thin cobalt film on a stainless steel mesh ina 0.1 M potassium phosphate solution at pH 7 containing 0.5 mM Co2+. Theheadspace of the cathodic chamber was continuously purged with CO2 andperiodic manual injections in a gas chromatograph gave CO2 reduction

products selectivity. The anodic chamber was originally filled with electrolyte;gas evolution was gauged over time and oxidation products were investigatedby gas chromatography after electrolysis had run to completion. Experimentaldetails concerning the synthesis of WSCAT and full complementary experi-mental details can be found in the SI Appendix.

ACKNOWLEDGMENTS. Partial financial support from the Société d’Accélérationdu Transfert de Technologie (S.A.T.T.) IDF Innov (Project 054) is gratefullyacknowledged.

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