Electrochemical synthesis of methane

A method is described for electrochemically reducing carbon dioxide to form methane by electrolyzing an aqueous solution containing carbon dioxide utilizing a cathode which comprises ruthenium. If desired, solar energy can be utilized to provide the potential for the electrolyzing. In such an instance, solar energy is, in essence, stored as chemical energy which can later be recovered from the methane.

DESCRIPTION 
1. Technical Field 
The invention relates to a method for electrochemically reducing carbon 
dioxide to form methane utilizing a ruthenium cathode. 
2. Background 
The prior art does not set forth an efficient method for the direct 
electrochemical synthesis of methane in an aqueous electrochemical 
process. Indeed, when carbon dioxide has been electrochemically reduced in 
an aqueous solution the products have usually been carbon monoxide and 
formaldehyde. Under certain conditions and using certain particular 
electrodes it has been possible to convert carbon dioxide to methanol by 
electrolysis of an aqueous carbon dioxide solution. D. Canfield and K. W. 
Frese, Jr., Electrochem. Soc. 130, 1772 (1983) and K. W. Frese, Jr., and 
D. Canfield, J. Electrochem. Soc. 131, 2518 (1984). 
DISCLOSURE OF INVENTION 
In accordance with the present invention, a method is set forth for 
electrochemically reducing carbon dioxide to form methane. The method 
comprises electrolyzing an aqueous solution containing carbon dioxide and 
utilizing a cathode which comprises ruthenium to produce the methane. 
Utilizing a ruthenium cathode in accordance with the present invention 
allows the production of methane by the aqueous electrolysis of a carbon 
dioxide solution at reasonably high faradaic efficiency. In this manner, 
electrical energy can be converted into chemical energy, effectively 
storing the chemical energy in the methane fuel. Methane fuel can later be 
burned to recapture the energy. The energy for the electrochemical 
reaction can be provided by solar cells or the like. 
BEST MODE FOR CARRYING OUT THE INVENTION 
The standard electrode potential for the electrochemical half-cell: 
EQU CO.sub.2 +8H.sup.+ +8e.sup.- .revreaction.CH.sub.4 +2H.sub.2 O.sub.(1) ( 1) 
is only +0.17 V(SHE). Under standard conditions it is therefore expected 
that reaction (1) should occur at potentials close to that for the 
hydrogen evolution reaction. 
In accordance with the present invention it had been found that when 
ruthenium is utilized as the cathode and when the pH of the aqueous 
solution falls within a range from about 1 to about 7 methane is 
electrochemically produced from carbon dioxide. Generally, the aqueous 
solution is substantially saturated with, and will be maintained 
substantially saturated with, carbon dioxide throughout the electrolysis. 
Preferably, the solution is at a temperature which falls within a range 
from about 10.degree. C. to about 100.degree. C., more preferably from 
about 50.degree. C. to about 80.degree. C. Electrolyzing is generally 
carried out at a potential which falls within a range from about -0.25 V 
to about -1.0 V versus SCE and is preferably carried out at a potential 
which falls in a range from about -0.3 V to about -0.7 V.

The invention will be better understood by reference to the experimental 
data which follows: 
EXPERIMENTAL 
Both electroplated and teflon-supported Ru electrodes (geometrical area, 
1-3 cm.sup.2) were used for CO.sub.2 electrolysis. The former were formed 
on spectroscopic pure carbon rods using a plating bath consisting of 
0.0084M Ru(NO)Cl.sub.3 and 0.4M reagent H.sub.2 SO.sub.4. The latter type 
of electrode was fabricated by pressing a mixture of 4-5 gm of Ru sponge 
(99.99%, Mattey-Bishop) with 5-12 weight % Halon TFE resin, type 6-80 onto 
a Cu mesh. The resulting pellets were contacted by a Cu wire and sealed in 
a glass tube with epoxy resin (Epoxy Patch). A 5 minute dip in 
concentrated HCl was used to clean the Teflon bonded Ru surfaces. This 
cleaning procedure is not essential for producing methane. 
Electrolytes were prepared from reagent grade Na.sub.2 SO.sub.4 or H.sub.2 
SO.sub.4 and purified, distilled H.sub.2 O (milligard filtered). A Pine 
RDE-3 potentiostat was used for controlled potential electrolysis in a 
closed system consisting of a 1.1 l CO.sub.2 reservoir, a teflon 
circulation pump, and an electrolysis cell. The anode and cathode 
compartments were separated by a Na.sub.2 SO.sub.4 /agar bridge. 
Current-time plots were obtained and manipulated by a laboratory 
microcomputer. 
Product Analysis 
Formaldehyde was determined by the chromotropic acid colorimetric method. 
Plots of absorbance versus concentration were linear from 10.sup.-4 to 
10.sup.-6 M. 
Methanol was detected by flame ionization gas chromatography (FID) using a 
column of Porapak N. To obtain good separation from H.sub.2 O and 
reproducibility in the results, the CH.sub.3 OH/H.sub.2 O vapor was 
analyzed in equilibrium with the CH.sub.3 OH/Na.sub.2 SO.sub.4 
electrolyte. Peak area versus CH.sub.3 OH concentration in the liquid was 
linear from 10.sup.-4 to 5.times.10.sup.-2 M. Methane was also analyzed by 
FID using Porapak R and Q. Varying amounts up to 10.sup.-7 M were found in 
the CO.sub.2 reactant gas. CH.sub.4 was detectable at .ltoreq.10.sup.-8 M 
in a calibration of CH.sub.4 /N.sub.2 mixtures. 
Results 
The faradaic efficiencies for CH.sub.4, CO, and CH.sub.3 OH are given in 
Table 1. These results were obtained with the electroplated electrodes in 
CO.sub.2 -saturated 0.2M Na.sub.2 SO.sub.4 or 0.1M H.sub.2 SO.sub.4 
electrolytes. The average current obtained by dividing the integral charge 
(Column 5) by the total elapsed time is given in Column 3. The apparent 
increase in CH.sub.4 yield with increasing temperature is not surprising 
in view of the expected kinetic complications for an 8-electron reduction. 
The efficiency for CO was always 1 to 5% with the exception of one datum. 
Rather high yields of CH.sub.3 OH were also found as shown in the last 
column of Table 1. A portion of the CO and CH.sub.3 OH may be due to 
CO.sub.2 reduction by localized cathodic and anodic reactions at the Ru 
electrode (see below). Reaction [1] in 0.5M H.sub.2 SO.sub.4 at -0.15 to 
-0.3 V(SCE) (55.degree.-60.degree. C.) have given faradaic efficiencies of 
0.4%. Perhaps too high a coverage with H.sub.ad is obtained in such 
strongly acidic solutions. 
It was necessary to show that CH.sub.4 could be produced in the absence of 
the carbon substrate because of the potential reactivity of carbon atoms 
adsorbed on the Ru surface. For this reason, teflon-supported Ru sponge 
electrodes were utilized. The results of two trials in 0.1N H.sub.2 
SO.sub.4 at 46.degree. C. were faradaic decimal efficiencies of 0.059 and 
0.098 for CH.sub.4. The electrode potential was -0.37 to -0.39 V(SCE) and 
the average current was 0.3 to 0.6 mA. Clearly the carbon substrate is not 
essential to the formation of CH.sub.4. 
It was also shown that the Ru surface is active in CO.sub.2 reduction under 
open circuit conditions in 0.2M Na.sub.2 SO.sub.4, pH 4-5. The data in 
Table 2 reveal that CO and small amounts of CH.sub.3 OH are produced by a 
localized cell reaction. The time of these open circuit experiments was 
equivalent to the 15-20 coulomb runs in Table 1. Note the similar pH 
change accompanying the CO.sub.2 reduction. Importantly, no CH.sub.4, was 
detected in these trials. 
TABLE 1 
______________________________________ 
FARADAIC EFFICIENCIES AS DECIMAL FOR CH.sub.4, CO, 
AND CH.sub.3 OH ON ELECTROPLATED RUTHENIUM 
ELECTRODES IN CO.sub.2 --SATURATED ELECTRODES 
pH T &lt;i&gt; Q 
range .degree.C. 
mA V(SCE) coul F.sub.CH.sbsb.4 
F.sub.CO 
F.sub.CH.sbsb.3.sub.OH 
______________________________________ 
0.2 M Na.sub.2 SO.sub.4 
4.2-6.8 
46 0.300 -0.65 98.5 0.046 
0.025 
0.029 
4.2-4.8 
50 1.6 -0.60 3.9 0.086 
0.042 
-- 
.sup. 4.2-5.5.sup.a 
55 0.243 -0.56 15.4 0.045 
0.048 
0.094 
.sup. 3.5-5.5.sup.b 
60 0.387 -0.54 27.2 0.11 0.012 
0.42 
4.2-6.8 
61 0.313 -0.55 19.8 0.30 0.45 0.25 
4.2-5.9 
67 0.270 -0.57 19.0 0.24 0.03 0.15 
1.4.sup.c 
46 0.500 -0.35 114.0 0.088 
0.024 
trace 
______________________________________ 
.sup.a Also contained 0.1 M H.sub.3 BO.sub.3 to slow pH increase 
.sup.b 0.1 N H.sub.2 SO.sub.4 added to lower pH range 
.sup.c In 0.1 N H.sub.2 SO.sub.4 
TABLE 2 
______________________________________ 
MOLARITY OF CO AND CH.sub.3 OH PRODUCED 
UNDER OPEN-CIRCUIT CONDITIONS WITH TEFLON- 
SUPPORTED Ru ELECTRODES IN CO.sub.2 -- 
SATURATED 0.2 M Na.sub.2 SO.sub.4.sup.a 
T Time 
.degree.C. 
hrs. pH.sub.initial 
pH.sub.final 
[CH.sub.4 ].sub.g 
[CO].sub.g 
[CH.sub.3 OH].sub.l 
______________________________________ 
21 18 4.2 5.2 ND 6.0 .times. 10.sup.-6 
ND 
21 20 4.2 5.0 ND 6.1 .times. 10.sup.-6 
&lt;10.sup.-4 
46 16.5 4.2 5.2 ND 1.3 .times. 10.sup.-6 
1 .times. 10.sup.-4 
.sup. 68.sup.b 
18.6 4.2 5.6 ND 2.3 .times. 10.sup.-6 
5 .times. 10.sup.-4 
______________________________________ 
.sup.a Cell vapor space, 1.3 liter; electrolyte volume, 0.025 l 
.sup.b Unknown small concentration of formaldehyde detected 
ND = none detected 
It will be noted that faradaic efficiencies for production of methane 
reached as a high a value as 30% (Table 1) which is particularly 
surprising since the prior art provides no electrodes which will produce 
more than trace quantities of methane by the electrolysis of carbon 
dioxide in an aqueous solution. It should also be noted that at higher 
temperatures the production of methane is more favored. 
INDUSTRIAL APPLICABILITY 
According to the present invention, methane may be formed by the 
electrochemical reduction of carbon dioxide. The methane formed can later 
be used as a fuel to produce power. 
While the invention has been described with respect to certain specific 
embodiments thereof it will be understood that many variations are 
possible within the scope and spirit of the invention as defined by the 
appended claims.