Efficient electrolyzer for CO 2 splitting in neutral water using earth-abundant materials
Arnaud 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 Paris Cedex 13, France; andbUnité 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+O2electrochemical splitting is a key
step for liquid-fuel production for renewable energy storage and use of CO2as a feedstock for chemicals. Heterogeneous catalysts for cathodic CO2-to-CO associated with an O2-evolving anodic re- action in high-energy-efficiency cells are not yet available. An iron porphyrin immobilized into a conductive Nafion/carbon powder layer is a stable cathode producing CO in pH neutral water with 90% faradaic efficiency. It is coupled with a water oxidation phos- phate cobalt oxide anode in a home-made electrolyzer by means of a Nafion membrane. Current densities of approximately 1 mA/cm2 over 30-h electrolysis are achieved at a 2.5-V cell voltage, splitting CO2and H2O into CO and O2with a 50% energy efficiency. Remark- ably, CO2reduction outweighs the concurrent water reduction. The setup does not prevent high-efficiency proton transport through the Nafion membrane separator: The ohmic drop loss is only 0.1 V and the pH remains stable. These results demonstrate the possibility to set up an efficient, low-voltage, electrochemical cell that converts CO2into CO and O2by associating a cathodic-supported molecular catalyst based on an abundant transition metal with a cheap, easy- to-prepare anodic catalyst oxidizing water into O2.
CO2-to-CO conversion
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carbon dioxide electrolyzer|
electrochemistry|
molecular catalysis
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solar fuelsT
he production of carbon-based fuels or chemicals using the most abundant carbon source (CO2) requires designing effi- cient, cheap, selective, and sustainable processes able to convert CO2into useful products (1–7). Carbon monoxide production is an important step to fuels because it can be used as a feedstock in the Fischer–Tropsch process. Compared with water splitting, electrochemical reduction of CO2into CO is a greater challenge.This is particularly true when aiming to carry out this reaction selectively in friendly conditions, namely at neutral pHs, ambient temperature, and with abundant and cheap materials as catalysts as opposed to solid-state high-temperature electrolyzers (8).
We recently discovered that substitution of the four paraphenyl hydrogens of iron tetraphenylporphyrin by trimethylammonio groups provides a water-soluble iron porphyrin (WSCAT) able to catalyze selectively the electrochemical conversion of CO2 into CO in neutral water in homogeneous conditions (9). The next challenge was to efficiently immobilize this molecular catalyst onto the cathode and to set up an integrated electrochemical cell able to split CO2and H2O into CO and O2according 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 onto a carbon support (glassy carbon electrode for cyclic voltammetry experiments and carbon felt or carbon Toray for electrolysis) and air-dried. Interactions between the positively charged catalyst and the negatively charged functionalities of the ionic polymer secure robust integration of the catalyst into the coated film. This was attested to by the absence of UV-vis signal corresponding to an iron porphyrin in a water solution in which the electrode was immersed for a few hours. The catalytic film consists of a thin coating of the electrode surface as confirmed by scanning electron microscopy (SEM) (Fig. 1).
Conductive and catalytic properties for CO2reduction of the prepared electrode were characterized by cyclic voltammetry carried out in quasi-neutral water with no added buffer, thus preventing acid reduction (Fig. 2). Under argon, almost no ca- pacitive current and no faradaic current were observed in the absence of carbon powder in the film, whereas the electrode exhibited a large capacitive current when carbon powder was incorporated in the coated film.
This indicates that carbon powder renders the film conducting and thus makes addressable the molecular catalyst contained in the film. This is further confirmed by the observation of faradaic waves when started from the FeIIIcomplex. Although not being well defined, these waves may be assigned to the FeIII/II, FeII/I redox couples. The slight increase of the current observed at more negative potentials (approximately−1.15 V vs. SHE) presumably reflects some catalysis of water reduction. In the presence of CO2, a large increase of the current is seen, confirming the catalytic activity of WSCAT immobilized in the Nafion film toward CO2
Significance
Electrochemical CO2-to-CO conversion is one important option for storing intermittent, renewable electricity into chemical bonds so as to produce fuels and to use CO2as a feedstock for chemicals. The setup of an electrolyzer, associating cheap and abundant materials able to split CO2into CO and O2, in envi- ronmentally friendly conditions (neutral pH, ambient temper- ature) with a high selectivity and stability, and a 50% energy conversion efficiency is reported. The results open the way to solar energy driving of the CO2/CO+1/2 O2splitting by asso- ciating the electrochemical cell with a light-to-electricity conver- sion device, and more generally with surplus electricity from renewable 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. analyzed data; 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 atwww.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 V vs. 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 water oxidation. 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 to prepare, well characterized, and efficient at neutral pH, namely electrodeposited phosphate cobalt oxide (CoPi), which could be used as water oxidation catalyst on the anodic side (6). A thin (50 mC/cm2, i.e., approximately 330 nm) CoPi film was thus deposited on a small stainless steel gauze (Fig. 1), with, as anodic electrolyte, a phosphate buffer (0.4 M, pH 7.3) ensuring a min- imal anodic overpotential. The cell was constructed from flasks containing the electrolytes with a proton-permeable Nafion membrane 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 simultaneously recording cell voltage, hence leading to the potential of both the cathode and the anode including ohmic drop vs. the reference electrode and the current (Fig. 4A). At pH 7.3, the onset of the current (at 1 mA/cm2) corresponds to a cell voltage of 2.3 V with
ηcathodic=n
E0CO2=CO ðRT ln 10=FÞ×pH
oEcathodic=320 mV
and
ηanodic+Ri=UcellEcathodicn
E0O2=H2O ðRT ln 10=FÞ×pH o
=630 mV.
An independent measurement indicated a 10-Ωcell resistance, amounting to an ohmic drop contribution of 50 mV to the overpotential.
A first series of electrolysis was performed at controlled cathodic potential (Ecathodic=−0.86 V vs. SHE) for several hours, with an expansion vessel over the flasks to assess product 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 solution leads to faradaic efficiencies of 90% for CO along with 10%
for H2in the cathodic compartment and 99% for O2in the anodic compartment. It is worth noting that no hydrocarbons were detected in the cathodic compartment headspace. Ionic chromatography analysis of the cathodic electrolyte shows only traces of formate and oxalate. These results clearly in- dicate a high selectivity and a high faradaic efficiency of the cell for CO2and H2O conversion to CO and O2, respectively.
A second series of electrolysis was performed at a controlled cathodic potential (Ecathodic = −0.96 V vs. SHE) for 30 h under CO2flow to evaluate cell stability. A linear increase of the charge passed vs. time was observed, corresponding to an averaged 1 mA/cm2current density (Fig. 4C); the cell voltage remained stable at a value of 2.5 V (Fig. 4D). Analysis of the gas mixture produced in the cathodic chamber over the course of the electrolysis showed an excellent stability of the cell selectivity (Fig. 4E). Note that the same experiment carried out with iron tetraphenylporphyrin instead of WSCAT led to a rapid decrease of the current to zero and a poor CO selectivity. The cell energy efficiency (EE) at 1 mA/cm2was evaluated to be 50% from the product of the faradaic yield for CO production (FYCO=0.9) and the ratio of the thermodynamics of the reaction over cell voltage:
EE=FYCO×{E0O2/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. (AandB) Electrode structures. (CandD) Catalytic films on the supporting material (polymer-wrapped carbon microfiber and stain- less steel microwire, respectively). (EandF) High-magnification images of the films.
Fig. 2. Cyclic voltammetry (pH 6.7). Nafion/WSCAT film deposited on a 3-mm-diameter glassy carbon electrode,v=0.1 Vs−1, pH=6.7.Light gray, film without carbon powder under argon.Blue, film plus carbon powder under argon.Red, film plus carbon powder under 1 atm of CO2.
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approximately 100 mV), the cathodic energy efficiency was evaluated as
EEcathodic=FYCO×n
E0O2=H2OE0CO2=CO
o.
n
E0O2=H2OE0CO2=CO+ηcathodico
=72%, and the anodic energy efficiency as
EEanodic=FYCO×n
E0O2=H2OE0CO2=CO
o.
n
E0O2=H2OE0CO2=CO+ηanodico
=63%.
Further improvements of the overall cell energy efficiency concern both the anodic and cathodic catalytic films, and will
allow running electrolysis at higher current densities. However, the results described here demonstrate that supported molecular catalyst based on the cheapest transition metal can be used to efficiently split CO2to CO and O2in neutral water and the per- formance of the low-cost, simply designed cell provides a robust basis to couple this electrolysis with a photovoltaic device produc- ing electricity from solar energy (11–13), opening an avenue to CO2-based solar fuels.
Materials and Methods
CO2splitting was performed in a two-compartment electrochemical cell where cathodic electrolyte was a CO2-saturated 0.1 M KCl+0.5 M KHCO3aqueous
Fig. 4. Cell characterization and electrolysis at pH 7.3. (A) Current density as function of the electrode potential (vs. SHE) for both the cathode and the 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 charge for CO production). (D) Cell voltage as function of time for an electrolysis at controlled 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 energy efficiency as function of time for an electrolysis at controlled cathodic potential,Ecathodic=−0.96 V vs. SHE.
Fig. 3. Proton exchange membrane electrolysis cell configuration. Cathodic half-reaction is CO2reduction to CO on a carbon electrode coated with a Nafion/carbon powder/WSCAT film and anodic half-reaction is oxidation of H2O to O2on a stainless steel gauze with a CoPielectrodeposited film.
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solution at pH 7.3 and anodic electrolyte was a 0.4 M potassium phosphate buffer at pH 7.3 degassed under argon. A Nafion NRE-212 membrane enabled proton 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. CoPianodes were prepared by electrodeposition of a thin cobalt film on a stainless steel mesh in a 0.1 M potassium phosphate solution at pH 7 containing 0.5 mM Co2+. The headspace of the cathodic chamber was continuously purged with CO2and periodic 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 investigated by gas chromatography after electrolysis had run to completion. Experimental details concerning the synthesis of WSCAT and full complementary experi- mental details can be found in theSI Appendix.
ACKNOWLEDGMENTS.Partial financial support from the Société d’Accélération du Transfert de Technologie (S.A.T.T.) IDF Innov (Project 054) is gratefully acknowledged.
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