Voltammetry of carbon dioxide

J. Electroanal. Chem., 148 (1983) 17-24 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

VOLTAMMETRY

17

OF CARBON DIOXIDE

P A R T I. A G E N E R A L S U R V E Y O F V O L T A M M E T R Y A T D I F F E R E N T ELECTRODE MATERIALS IN DIFFERENT SOLVENTS

BRIAN R. EGGINS and JOANNE McNEILL School of Physical Science, Ulster Polytechnic, Newtownabbey, Co Antrim BT37 OQB (Northern Ireland)

(Received 18th March 1982; in revised form 4th November 1982)

ABSTRACT We have studied voltammetric reduction waves for carbon dioxide in water, dimethylsulphoxide, acetonitrile and propylene carbonate using electrodes of glassy carbon, mercury, platinum, gold and lead as applicable. The apparent number of electrons involved was determined from the current function and in some cases by chronoamperometric analysis of the diffusion controlled tail of the wave. Results were compared with those from other workers.

INTRODUCTION C o n s i d e r i n g the c u r r e n t interest in the e l e c t r o c h e m i s t r y of c a r b o n d i o x i d e [1], relatively few p o l a r o g r a p h i c or v o l t a m m e t r i c studies have been m a d e . P o l a r o g r a p h i c studies i n c l u d e those o f J o r d a n a n d S m i t h [2] in a q u e o u s solution. S a w y e r a n d c o - w o r k e r s [3,4] have m a d e v o l t a m m e t r i c a n d c h r o n o p o t e n t i o m e t r i c studies in d i m e t h y l s u l p h o x i d e . Sav6ant a n d co-workers r e p o r t cyclic v o l t a m m e t r y in d i m e t h y l f o r m a m i d e . G r e e n e r [6] has d e s c r i b e d some v o l t a m m e t r i c studies in a q u e o u s solutions a n d in p r o p y l e n e c a r b o n a t e . This p a p e r presents a survey of v o l t a m m e t r i c results in different solvents using v a r i o u s electrodes. EXPERIMENTAL V o l t a m m o g r a m s were r e c o r d e d using a C h e m i c a l Electronics P o t e n t i o s t a t 703 A, a H e w l e t t P a c k a r d w a v e f o r m g e n e r a t o r 3310 B a n d an A d v a n c e H R 2000 X - Y recorder. W o r k i n g electrodes were B e c k m a n n disc inlay electrodes of glassy c a r b o n , p l a t i n u m , gold, m e r c u r y p l a t e d p l a t i n u m o r lead. A s a t u r a t e d c a l o m e l reference e l e c t r o d e was used in aqueous solutions a n d a s i l v e r / s i l v e r b r o m i d e reference e l e c t r o d e in a p r o t i c solvents. S e c o n d a r y electrodes consisted of p l a t i n u m foil in 0022-0728/83/$03.00

© 1983 Elsevier Sequoia S.A.

18

aqueous solutions and a mercury pool in non-aqueous solutions. Supporting electrolytes consisted mainly of 0.1 mol dm -3 tetraethylammonium bromide though in one case the chloride was used. Carbon dioxide was bubbled through the appropriate solvent for 2 h. Contrary to expectations (and assumptions made by Greener [6]) this did not produce a saturated solution. The actual concentrations were determined by addition of excess standard 0.02 mol d m - 3 barium hydroxide solution and back titration with standard 0.02 mol d m - 3 oxalic acid using phenolphthalein as indicator [7]. RESULTS A N D DISCUSSION

Voltammetric waves were observed in all solvents with one or more of the electrodes used. A selection of these are shown in Figs. 1-4. A summary of our results is shown in Table 1 compared with data from other workers. Analysis of the characteristics of the waves was made in two ways. From the peak current f u n c t i o n (ip//Ac~)1/2) the apparent number of electrons may be determined from the Delahay equation:

ip/Acv 1/2 =

2.98 ×

105(an)l/2nD I/2

and

0.0477/(an)

Ep - Ep/2 =

i/mA

vs SCE i

I

-1"0

-1"2

-1"4

-I"G

-1"8

-2"0

-2"2

-2"4

Fig. 1. Cyclic voltammogram of carbon dioxide (1 × 10 -S mol cm - 3 ) in aqueous tetramethylammonium bromide (0.1 tool dm - 3 ) on glassy carbon at v = 0.0675 V s-1.

19 4"5.

4"0.

i/rnA

3-5. 3"0.

;b5.

2- O.

1'5. 1-0. 0"5. E

-114

-1'6

-1"8

-2:0

-212

Iv

-2"4

vs. SCE

-216

-218

Fig. 2. Cyclic voltammogram of carbon dioxide (1 x 10 - s mol ern - 3 ) in dimethylsulphoxide containing tetramethylammoniurn bromide (0.1 mol dm - 3 ) on platinum at v = 0.0675 V s-1.

The value of the diffusion coefficient for carbon dioxide has been determined [3] in dimethylsulphoxide on gold as 1.5 × 10 -6 cm 2 s-1. There must however be a misprint as we have reworked their chronopotentiometric data and obtain 6.15 + 0.21 × 10 -6 cm 2 s-1. From this value and the viscosities of other solvents diffusion coefficients for carbon dioxide in them can be obtained using the Stokes-Einstein equation. Bewick et al. [8] use the value 1 × 10 -5 cm 2 s -1 for carbon dioxide in aqueous media, in line with our value of 1.07 × 10 -5 cm2 s -l. The other method was to apply the Cottrell equation to the diffusion controlled tail of each peak [9]:

i(t-

t o ) 1/2 =

nFAcDI/2//~r 1/2

(1)

where t - t o is the true time required to fit this equation, t is the measured time and t o the unknown effective zero time. Time is actually measured on the potential axis where E = vt. If eqn. (1) is squared and rearranged one obtains

1/i 2 = ¢rt( FAc)E ( nED ) -~rto/( FAc)2 ( nED )

(2)

20

1"2

i/mA

1"0

0"8.

0.0.

0'4.

0"2.

i

-1.1

~

-1.3

i

-1-5

-'----I

-1.7

I

-1.9

!

-2"1

l

-2-3

t

-2.5

Fig. 3. Cyclic voltammogram of carbon dioxide ( 4 x 10 -5 tool cm -3) in dimethylsulphoxide containing tetramethylammonium bromide (0.1 mol dm -3) on mercury plated platinum at v ~ 0.0755 V s-1.

i l mA 2"0,

1"5.

1"0,

0"5.

E I V vs. Ag/AgBr -1'4

-1J6

-1"8

-2'-0

-2"2

-2~4

-2"6

-2~8

-3"0

Fig. 4. Cy¢lcic voltammogram of carbon dioxide (4 × 10 -5 mol d m - 3 ) in acetonitrile containing tetramethylammonium bromide (0.1 tool dm - 3 ) on mercury plated platinum at v = 0.0718 V s - ~.

21 TABLE 1 Peak potential for CO 2 reduction in various solvents at various electrodes, from voltammetric studies unless otherwise indicated a.b Electrode

Ep/V(vs. SCE) Solvent

Hg Vb

H20(pH 6-8)

MeCN

DMSO

PC

DMF

- 2.16 a [2] -2.58 b [6] - 2.06 [6]

- 2.76 c

- 2.40 [3] -2.25 c

- 2.59 [6] -2.76 ~ - 2.61 [6] - 2.75 ~

-2.21 [51

2.70 c - 2.32 c

mu

Pt C Zn Cd Sb Sn In Cu(H8)

- 2.18 ~

-

-2.15 [31 - 2.48 c - 2.45 c

- 2.58 c - 2.65 c

-2.21 c - 1.5 b [10] - 1.6 [6] - 1.72 [6] - 1.60 b [6] - 1.12 b [10] - 2.02 [6]

a Polarographic El~ 2. b From polarisation curves. c Present work. E q u a t i o n (2) is l i n e a r s o a p l o t o f 1 / i 2 v e r s u s t g i v e s n2D f r o m t h e s l o p e a n d a n i n t e r c e p t l e a d i n g t o t o f r o m w h i c h E 0 = vt o m a y b e f o u n d . T h e s e d a t a w e r e a n a l y s e d b y a l i n e a r l e a s t s q u a r e s c o m p u t e r p r o g r a m . I n all c a s e s the correlation coefficient was 0.99 or better. The data from these analyses are s h o w n i n T a b l e 2.

Aqueous solutions A voltammetric wave was obtained at -2.21 V on glassy carbon in unbuffered tetramethylammonium b r o m i d e ( F i g . 1). O t h e r w o r k e r s o b t a i n a r a n g e o f v a l u e s depending on the electrode material, that on In at -1.12 V being particularly i n t e r e s t i n g ( T a b l e 1). O n e w a v e o b t a i n e d w h i c h i n d i c a t e d nap p = 2, w h i c h w o u l d c o r r e s p o n d t o t h e n o r m a l l y e x p e c t e d f o r m i c a c i d f o r m a t i o n i n u n b u f f e r e d n e u t r a l s o l u t i o n s [11 ].

Dimethylsulphoxide O u r r e s u l t s a r e i n g e n e r a l a g r e e m e n t w i t h t h o s e r e p o r t e d b y S a w y e r et al. [3,4]. They observed only one-electron reduction on gold corresponding to disproportiona-

22

"

o o ,,~.

o

~.~

~

~',

i.~

I

t I I I

< I

"~

0

r.

pII

u

II

~ ~ o o o

I

~

-

~ x

~

" o "

~ ~

'

'

'

'

'

~×××× T

I

N

,.4 II

il

x

II

•~ d d <~

II d d d d

~ d d d d

T T

m

~s

23

tion of the initially formed radical anion, to give CO and CO 2 - . Both our peak current function (ip/ACv 1/2) and it 1/2 analysis gives n~pp = 1 for gold and for mercury. Roberts and Sawyer [3] found n~pp for mercury varied with CO 2 concentration approaching 2 at low concentrations but decreasing towards 1.5 at 0.04 mol d m - 3 using chronopotentiometry. However, Haynes and Sawyer's [4] coulometry experiments showed that at - 2 . 3 V n~pp = 1, but at - 2 . 5 V nap p = 2. Coulometry on gold gave n~pp = 1 at both - 2 . 3 V and - 2 . 5 V. For platinum our results gave napp --- 2 for both methods. Sawyer did not study the reduction on platinum. The most likely competing process which could result in n~o p ~ 2 is protonation of the CO~ by residual water, followed by a further electron transfer to give formate. However, Haynes and Sawyer [4] suggest an intriguing alternative giving carbon dioxide dianion: CO 2 + 2 e - - ~ CO 2-

A cetonitrile The results were qualitatively the same as for DMSO except that the n a p p values for platinum were slightly low. The value for lead w a s n a p p = 1. No other workers have yet reported studies on CO 2 in this solvent.

Propylene carbonate The waves in the solvent were rather drawn out often not showing a clear peak (probably the solvent needed further purification). Hence only ip/Acv I/2 data were obtained and the nap p values are very approximate. Mercury shows a higher value than before, but gold, platinum and lead are similar. They agree with Bewick's [8] proposed ece mechanism giving oxalate with n = 1. Greener [6] reports a reversible cycle voltammogram at - 2 . 6 0 V with a peak separation of 32 mV suggesting a two-electron process. CONCLUSION

While our results are in general agreement with those of other workers, the range of solvents and electrode materials has been extended and there is clearly scope for more voltammetric studies coupled with controlled potential electrolyses. Further results for aqueous solutions of different pH values using variable sweep rates will be reported in Part II. Such studies help to throw light on the obviously complex processes involved in the electrolysis of carbon dioxide [1 ].

24 ACKNOWLEDGEMENT S u p p o r t for this w o r k f r o m the R o y a l Society of C h e m i s t r y i s g r a t e f u l l y acknowledged. REFERENCES 1 2 3 4 5 6 7 8 9 10 11

E.M. Bennett, B.R. Eggins, J. McNeill and E.A. McMullan, Anal. Proc., 17 (1980) 356. J.J. Jordan and P.T. Smith, Proc. Chem. Soc., (1960) 246. J.L. Roberts and D.T. Sawyer, J. Electroanal. Chem., 9 (1965) 1. L.V. Haynes and D.T. Sawyer, Anal. Chem., 39 (1967) 332. E. Lamy, L. Nadjo and J.M. Sav6ant, J. Electroanal. Chem., 78 (1977) 403. E.P. Greener, Ph.D. Thesis, Southampton University, 1970. G.H. Ayres, Quantitative Chemical Analysis, Harper and Row, New York, 1969, p. 607. A. Aylmer-Kdly, A. Bewick, P. Cantril and A. Tuxford, Faraday Discuss., 56 (1974) 96. B.R. Eggins and N.H. Smith, Anal. Chem., 51 (1979) 2282. K. Ito, T. Murata and S. Ikedo, Nagoya Kogyo Daigaku Gakuho, 27 (1975) 209. K.S. Udupa, G.S. Subramanian and H.V.K. Udupa, Electrochim. Acta, 16 (1971) 1593.