Electrochemical reduction of CO2 on platinum electrodes in acid solutions

Electrochimica Acta, 1963, Vol. 8, pp. 857 to 865. Pergamon Press Ltd. Printed in Northern Ireland

ELECTROCHEMICAL R E D U C T I O N OF CO2 ON PLATINUM ELECTRODES IN ACID SOLUTIONS J. GINER WiUgoos Phys. Chem. Lab. Pratt and Whitney Aircraft, Division of United Aircraft Corporation, East Hartford, Connecticut, U.S.A. A~traet--CO~ reacts in acid solutions with chemisorbed hydrogen on platinum in the potential range of Eh = 0-250 mV to form a chemisorption product of reduced COs (perhaps CO). The rate of reduction on bright platinum at room temperature is very slow, taking more than 2 min at Eh = 0 for the total formation of a monolayer of reduced COs. This rate increases greatly with temperature. On platinized platinum at 90°C the hydrogen reacts extremely rapidly in the potential range En = 0-250 mV to reduce CO~. The oxidation of the chemisorbed product to COs appears as a current peak in the "fast" i(E)sweep curve ("COs-peak"). This oxidation is highly irreversible. With bright platinum an activation energy of 22 kcal/g mol has been measured. Rrsumr--CO~ rragit en solution acide avec l'hydrog~ne chimisorb6 sur platine, dans l'intervalle de tension E~ = 0-250 mV, pour former un produit de rrduction de CO2 (peut-rtre CO) lui-mrme chimisorbr. La vitesse de rrduction sur Pt poli est faible h la temprrature ordinaire (plus de 2 minutes ~, Eh = 0 pour une couche monomolrculaire de CO2 rrduit). Elle est par contre grande ~. 90°C. sur platine platinr, dans tout l'intervalle E h - 0-250 inV. L'oxydation anodique en CO2 du corps chimisorb6 se manifeste par un pic de la courbe "rapide" (i, E); elle est hautement irrrversible et drnote une 6nergie d'activation de 22 kcal/mol sur platine poli. Zusammenfassung--CO2 reagiert in saurer Lrsung im Potentialbereich Eh = 0-250 mV mit Wasserstoff, der an Platin chemisorbiert ist. Es bildet sich ein chemisorbiertes Reduktionsprodukt de CO2 (mrglicherweise CO). An blankem Platin verlfiuft die Reduktion bei Zimmertemperatur sehr langsam. Bei Eh = 0 dauert es mehr als 2 Min, bis sich eine monomolekulare Schicht yon reduziertem CO2 ausgebildet hat. Die Reduktionsgeschwindigkeit nimmt bei steigender Temperatur rasch zu. An platiniertem Platin reduziert der Wasserstoff das CO2 bei 90 °im Potentialbereich Eh = 0-250 mV ausserordentlich rasch. Die Oxidation des chemisorbierten Produktes der CO2-Reduktion fiussert sich in der Form einer Stromspitze in der kurzzeitigen i(E)-Kurve ("CO2-Spitze"). Diese Oxidation ist stark irreversibel. An blankem Platin wurde eine Aktivierungsenergie yon 22 kcal gemessen. INTRODUCTION IT IS g e n e r a l l y a s s u m e d t h a t CO2 is e l e c t r o c h e m i c a l l y inert, so t h a t o n l y u n d e r v e r y e x t r e m e c o n d i t i o n s o f p o t e n t i a l m a y it be oxidized (to p e r c a r b o n i c acid) o r r e d u c e d . O u r e x p e r i m e n t s s h o w t h a t in acid s o l u t i o n s even at a p o t e n t i a l Eh = 0 - 3 0 0 m V , CO2 reacts w i t h c h e m i s o r b e d h y d r o g e n (electrochemically f o r m e d ) to give a c h e m i s o r b e d species. EXPERIMENTS T h e r e d u c t i o n o f C O S has b e e n o b s e r v e d b o t h for s m o o t h a n d p l a t i n i z e d p l a t i n u m electrodes, w i t h l i n e a r p o t e n t i a l - s w e e p t e c h n i q u e s a n d w i t h g a l v a n o s t a t i c pulses. F o r all e x p e r i m e n t s a j a c k e t e d all-glass cell, t h e r m o s t a t e d to 0" 1°C w i t h a p o l a r i z e d h y d r o g e n - e v o l v i n g electrode as reference electrode I was used. T h e electrolyte was a l w a y s 2 N H 2 S O 4. C o m m e r c i a l p u r e C O s a n d research g r a d e C O s ( M a t h e s o n g u a r a n t e e d p u r i t y o f 99-99 per cent) were used w i t h o u t a n a p p r e c i a b l e difference * Manuscript received 19 March 1963. 857

858

J. GINER.

between them. As working electrode, a 1 cm × 1 cm platinum sheet, welded to a Pt wire and sealed in a glass tube was used, either stationary or rotated as specified with each experiment. The potential-sweep curves were obtained with a Wenking potentiostat. When working with bright platinum a triangular-wave was supplied to the potentiostat from an electronic fast linear function generator (10 mV/s to 106 mV/s). The current/potential curve was then recorded on a Tektronix oscilloscope. In the case of platinized platinum the linear potential sweep was applied to the potentiostat by using a slow mechanical linear function generator (5 mV/min to 200 mV/min). In this case an x-y recorder was used to record i vs. E. All potentials are referred to the reversible hydrogen electrode in the same solution (En).

Bright Pt electrode Galvanostatic experiments. A P t electrode was immersed in HzSO4 stirred with inert gas (N 2, He) and polarized cathodically with constant current; if the direction

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FIG. 1. E(t)i curves for bright Pt/1 M H~:SO4 at 25"C; i -- 0.1 mA/cm 2. Effect of CO2: (a) He; (b) CO2.

of the constant current was changed from cathodic to anodic a curve such as curve (a) of Fig. 1 was obtained. The potential of the electrode increases almost linearly with time; during this time (A) the cathodically formed chemisorbed hydrogen is oxidized. After all the hydrogen has been oxidized, the potential increases much more rapidly (B) while the double layer is charged. Later, at a potential E ---- 850 mV, oxygen deposits on the electrode. If the current is reversed before reaching this

Electrochemical reduction of COz on platinum electrodes in acid solutions

859

potential, a symmetrical course of the potential is observed. During B' the double layer is discharged, during A' hydrogen is deposited on the electrode (see for instance2). Substituting the inert gas by CO 2 a curve such as curve (b) in Fig. 1 was found. Segment A becomes substantially smaller and, after a shorter Segment B, a new potential step appears before the potential of the oxygen deposition (Eh ---- 850-900 mV) is reached. After reversing the potential a cathodic curve as seen in curve (b) of Fig. 1 was obtained, indicating that no oxygen is deposited during the step X, neither has a product been formed which can be reduced before one reaches the hydrogen deposition potential. Step X is apparently caused by the oxidation of the chemisorbed product formed by interaction between P t - - H and CO 2. The remaining length of A plus the length of X on curve (b) of Fig. 1 is approximately equal to the length A'. It will be seen in the following experiments that: (a) The potential at which step X appears is a function of the current and temperature, indicating that the oxidation of this chemisorbed product is a very irreversible process. (b) The length of X (the amount of chemisorbed reduced CO 2 and also the rate of cathodic reduction of CO2) is a function of the time the electrode is prepolarized cathodically before the current is reversed to anodic, and also of the temperature. Rate of the cathodic reduction of CO 2. The following experiments were made to measure the rate of reaction between chemisorbed hydrogen and CO2. The Pt electrode was always prepolarized anodically for 15 sec with 1.0 mA/cm 2 (to deplete impurities from the surface by an oxide layer). Afterwards the polarity was reversed. In doing this the surface oxide layer is reduced and the chemisorbed layer of hydrogen is formed. iO00

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FIG. 2. E(t)~ curves for bright Pt/1 M H~SO~, CO2: i ~ mA/cm ~. Cathodic polarization time: (a) 0 s ; (b) 2 0 s ; (c) 8 0 s ; (d) 120s.

As soon as the constant potential of hydrogen evolution was reached a certain time was allowed; then the polarity of the current reversed. This was repeated for different temperatures. Fig. 2 shows examples of curves obtained at different temperatures after different cathodic times. It can be seen that with increasing cathodic time segment A decreases and segment X increases. In Fig. 3 the remaining length of

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FIG. 3. Bright Pt/1 M H2SO4, CO2. Decrease of hydrogen adsorption as function of cathodic polarization time at 1 mA/cm2: (a) 25°C; (b) 45°C; (c) 65°C; (d) 90°C.

segment A, representing the charge (mC/cm ~) equivalent to the residual adsorbed hydrogen as a function of the cathodic charge time, has been plotted for different temperatures. From the curve, the rate of reduction of CO S for different temperatures can be derived.

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FIG. 4. E(t)~curves on bright Pt/1 M H2SO4, CO2. Effect of current and temperature CO2-potential step": (a) 25°C; (b) 45°C (c) 65°C; (d) 90°C.

Effect of temperature and

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oxidation potential of product X to CO2.

The electrode was polarized anodically for 1 min followed by 2 min cathodically with 0-1 m A / c m ~. After a short period of time at ()pen circuit the anodic charging curve was taken with a given current. This experiment was repeated at four different temperatures. Fig. 4 shows the curves obtained under these conditions. From these curves an activation energy of 22 kcal/g tool at Eh = 600 mV can be estimated.

Electrochemical reduction of CO= on platinum electrodes in acid solutions

861

Potential-sweep curves. Curve (a) of Fig. 5 shows a typical potential/current curve obtained with fast sweep for a Pt-electrode in I M H2SO 4 under Ncstirring. The curve was obtained periodically, i.e. the potential oscillated linearly and continuously between E h = 0 and 850 mV. When the potential is changed from 850 to 300 mV, a cathodic current due to charging of the double layer of the electrode is seen. After reaching 300 mV and further decreasing the potential, two current peaks are observed due to the hydrogen deposition on the electrode to form chemisorbed hydrogen. Later, at Eh = 0 mV hydrogen begins to be evolved. If now the potential sweep is reversed before hydrogen is evolved, two anodic peaks are found at the same potential as the cathodic peaks. Afterwards, from 300 to 850 mV a current to charge the double layer is obtained again (this is well knowna). 0.5

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Fxo. 5. ((fiE)))sweep on bright Pt/1 M H2SO4 at 90~'C. Sweep rate 150 mV/s. Effectof CO2: (a) N2; (b) CO2: (c) CO2. single sweep after 2 rain at Eh = 0 mV.

Curve (a) of Fig. 5 was obtained with an electrode activated anodically shortly before taking the picture, by impressing a periodic linear wave of 0 to 1500 mV and back for some seconds. This allows P t - - O formation and reduction. Curve (b) of Fig. 5 was obtained under same conditions as (a) but with CO2 instead of N 2. It can be seen that the second anodic peak decreases and a clear but small broad peak appears at 625 inV. Also in this measurement no cathodic current is observed corresponding to the CO2 peaks (compare with curve (b) of Fig. 1). Curves (a) and (b) were made with 150 mV/sec sweeps. At ten times faster sweeps, no

862

J. GINER

difference between N s and CO 2 was observed, indicating that CO s reacts very slowly. This was confirmed using non-periodic sweeps. Curve (c) of Fig. 5 was obtained by using a non-periodical potential sweep of 150 mV/sec. For 2 min the electrode was polarized cathodically with 1 mA/cm s. Later, the linear wave of about 150 mV/sec was applied in the anodic direction and back. The anodic COs peak at 650 mV is very well defined. From curve (c) it can be seen that the CO s peak really is two peaks. This point was not clear from the ga!vanostatic measurements, although indications do exist (see Fig. 4). Effect of temperature. Fig. 6 shows potential-sweep curves with constant cathodic

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time for 25°C and 90°C, obtained as described for curve (c) of Fig. 5. Comparison shows a smaller peak displaced to higher potential at low temperatures; this agrees with the galvanostatic experiments described above.

Platinized platinum Potential-sweep curve. The following experiments were made with electrodes of very high roughness platinum black. The real surface was estimated from the measured hydrogen charge at Eh = 0; surfaces requiring ca. 600 mC/cm 2 were obtained, i.e. with a roughness factor of about 2000. The use of such high surfaces has the advantage that very small "real" current densities (i.e. current per unit of real area) can be used, without a significant amount of impurities diffusing to the electrode, since: the surface of the diffusion layer is in first approximation equal to the geometric surface, and this is small. Also, potential sweeps 1000 times slower than used with bri~ght platinum can be employed to obtain the same effect. This simplifies the technique. Curve (a) of Fig. 7 shows a typical potential-sweep for a Pt electrode in 2 N H2SO 4 acid under an inert (N2) atmosphere. The same qualitative description applies to this curve as to curve (a) of Fig. 5. Now if the inert gas (N s or He) is replaced by CO 2, the same two cathodic peaks are found when sweeping the potential downwards, although they occur at slightly

Electrochemical reduction of COs on platinum electrodes in acid solutions

863

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FIo. 7. i(E) sweep curves on platinized platinum 1 M H2SO4 at 90°C. Sweep rate 50 mV/min. Effect of CO2: (a) N2; (b) COs (c) COs, potential reversed at E = 200 mV; (d) COs, potential maintained at Eh -= 200 mV for 5 rain.

more positive potentials than with N2, especially the more positive peak, indicating that under this condition, depositing H reacts immediately with CO 2. When this sweep is reversed in the positive direction the two anodic peaks at 100 and 200 mV are not found; instead a single peak at 400 mV is found. The area under this peak is equal to the sum of the two hydrogen peaks (curve (b), Fig. 7). If the stirring is completely stopped before the anodic potential sweep is recorded, gas evolution at the electrode is observed when the maximum of the current is reached, indicating that the chemisorbed product formed by reduction of CO 2 by chemisorbed hydrogen is oxidized back to CO2, which is evolved. Similarly, a CO~ peak is found if the potential is reversed at an intermediate potential (such as 200 mV) (curve (c) of Fig. 7), but the anodic CO2 peak is smaller, corresponding to the amount of deposited hydrogen. If the potential is kept at 200 mV for 5 rain, then an amount corresponding to the hydrogen deposited at this potential is formed (curve d, Fig. 7). If the potential is reversed before hydrogen is deposited (potentials higher than 250 mV) no anodic peak is obtained, not even if hydrogen is added to the CO 2 gas. Change from stirring with a rotated electrode to only gas stirring has no perceptible influence on CO2 reduction, indicating that the rate of reduction is controlled by

864

J. G1NER 500

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the surface reaction and not by bulk diffusion. This also tells against the probability that impurities in the CO2 are the cause of the effect. In experiments with 10% CO2-90 % Nz, a peak of nearly 100 per cent of the total adsorption peak obtained with pure CO2 was found after initial activation of 2 min at 1800 mV and 2 min at -- 100 mV, with a following potential sweep of + 2 0 mV/min,

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FIG. 9. i(E)-sweep curves on platinized platinum/1 M HzSO4, CO2. Sweep rate = 50 mV/min. Effect of temperature: (a) 25°C; (b) 50°C; (c) 75°C; (d) 90°C.

Electrochemical reduction of COs on platinum electrodes in acid solutions

865

from Eh = 0 V. This indicates that the CO~ reduction was not limited by concentration. All these experiments were made at 90°C. Effect of sweep rate. Figure 8 shows the effect of sweep rate on the anodic CO x peak. Before the anodic sweep was impressed, the electrode was polarized for 10 min at EI~ = 15 mV. The displacement of the peaks to more positive potentials with increasing sweep rate (i.e. with increasing current according to equation 1) indicates the irreversibility o f the oxidation process. It is apparent from Fig. 8 that the charge required to oxidize the chemisorbed c o m p o u n d is independent of the sweep rate. Effect of temperature. Figure 9 shows the effect of the temperature on the potential and the form o f the peak. Lower temperatures produce a slightly smaller peak displaced to higher potentials, in accordance with the experiments on bright platinum. CONCLUSION It has been shown that CO 2 reacts with chemisorbed hydrogen in the potential range o f E h = 0-250 mV. The rate o f reduction o f CO 2 has been measured as a function o f temperature. The oxidation of the p r o d u c t is very irreversible, since it is strongly influenced by temperature and current density. N o conclusions can be drawn at the present m o m e n t about the nature of the "reduced CO2" which is chemisorbed on the platinum-electrode. Tentatively either C O or a radical such as

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Acknowledgements--The author wishes to thank Dr. J. G. TSCHINKELfor valuable discussions during the writing of this paper and to Mrs. M. Barr and Mr. B. Tycz for the careful execution of the experiments. REFERENCES 1. J. GINER,J. Electrochem. Soc. To be published. 2. J. GINER,Z. Elektrochem. 63, 386 (1959). 3. F. WILL and C. A. KNORR,Z. Elektrochem. 62, 378 (1960).