Electrocatalytic Reduction of CO2 to CO and CH4 by Co–N–C Catalyst and Ni co-catalyst with PEM Reactor

Utilization of renewable energy has been proposed for solution of the Global Warming. Electroreduction of CO2 into valuable chemicals using renewable electricity is a promising technology. Electroreduction of CO2 to CO and CH4 was studied using the polymer-electrolyte-membrane (PEM) gas cell. The electrocatalyst prepared by partial pyrolysis of Co-4,4’-dimethyl-2,2’-bipyridine supported on KEJENBLACK at 673 K (Co-dmbpy/KB(673K)) has been found for selective reduction of CO2 to CO. Screening of co-catalysts to promote formation of CH4 during the CO2 reduction by the Co-dmbpy/KB(673K) cathode was conducted in this work. An effective Ni/KB co-catalyst prepared by H2 reduction at 673 K was found. Suitable preparation conditions and methods of the cathode and effects of cathode potentials on the CO2 reduction to CO and CH4 were studied. The reaction path for the formation of CH4 in the CO2 reduction was studied and the successive reduction of CO2 to CO on the Co-dmbpy/KB(673K) catalyst and CO to CH4 on the Ni/ KB(673K) co-catalyst was determined.


Introduction
Inhibition of global warming is essential problem for sustainable society and human life. Emission of CO 2 is major factor for the global warming and must be reduced. Utilization of renewable energy instead of fossil fuels has been proposed for the reduction of the CO 2 emission. In this regard, electrochemical reduction of CO 2 into valuable chemicals such as carbon monoxide (CO) and methane using renewable electricity is a promising technology. [1][2][3][4] Kato has proposed application of the Active Carbon Recycle Energy System (ACRES) to iron making process for reduction of CO 2 emission (iACRES). The iACRES uses recycling CO for reduction of iron oxides. 5) CH 4 is also useful reductant for reduction of iron oxides.
Electroreduction of CO 2 has mainly examined in aqueous carbonate solutions dissolving CO 2 . In the case of the liquid phase reduction, the solubility of CO 2 limits a maximum reduction rate of CO 2 . On the other hand, electroreduction in the gas phase using a polymer electrolyte membrane (PEM) reactor has an advantage for a higher reduction rate of CO 2 . Gaseous CO 2 is directly reduced on a gas-diffusion cathode. [6][7][8][9][10] Our research group has reported electro-synthesis of chemicals using the PEM reactor using a Nafion-H membrane. [11][12][13] The Nafion-H membrane has strong acidity. Almost previous studies for CO 2 reduction were conducted in weakly acidic solution dissolving CO 2 ; therefore, it was no guarantee that the electrocatalysts reported functioned in the PEM reactor. Because reaction mechanisms for the CO 2 reduction should change and the dissolution of electrocatalysts should be serious under highly acidic conditions. Recently, we reported the selective reduction of CO 2 to CO by the Co-N-C electrocatalyst in the PEM reactor. [14][15][16] In the previous work, the Co-N-C electrocatalyst was prepared by partial pyrolysis of Co(dmbpy) 3 (dmbpy = 4,4'-dimethyl-2,2'-bipyridine) complex supported on carbon materials (c.a. KETJENBLACK) in inert gas (He) at 673 K, as abbreviated Co-dmbpy/KB(673K). Electrocatalytic activity of the Co-dmbpy/KB(673K) cathode for the CO 2 reduction strongly depended on reaction conditions; however, the major product was always CO and byproduct was H 2 . The sum selectivity of CO and H 2 , corresponding to the current efficiency, was > 99.5%. However, a trace amount of CH 4 formation ( < 0.5% current efficiency) was sometimes observed. CH 4 can be utilized as a reductant for the Iron making; therefore, direct formation of CH 4 by electroreduction of CO 2 would be attractive topic. Purposes of this study is to increase the formation rate of CH 4 during the electroreduction of CO 2 by modification of the Co-dmbpy/ KB(673K) electrocatalyst by addition of co-catalysts and to study reaction scheme for the CH 4 formation.

Preparation of Co-N-C Electrocatalyst and coelectrocatalysts
The standard preparation procedure of the Co-N-C electrocatalyst using the impregnation method and the heat-treatment in inert gas has been already reported. 15,16) A brief procedure is as following, KETJENBLACK ECP (KB) powder is added to ethanol solutions of dmbpy (4,4'-dimethyl-2,2'-bipyridine) and Co(NO 3  and KB powder by using a conventional impregnation method and reduction with H 2 at 773 K, as abbreviated metal/KB(773K). Loadings of metals were 10.0 wt%.

MEA Fabrication
A membrane electrode assembly (MEA) was prepared by a hot-press method. A Nafion membrane (Nafion 117) was pressed between the cathode and anode at 413 K at 59 MPa and then immersed in deionized water for 5 min. The formed MEA was set in a gas-electrolysis cell. Figure 1 shows the gas-electrolysis cell for CO 2 reduction. [14][15][16] Pure H 2 gas (20 mL min − 1 ) and pure CO 2 gas (10 mL min − 1 ) were introduced into the anode and cathode compartments, respectively. In addition, to humidify the Nafion membrane, a 0.5 mL of deionized water was introduced in the cell and H 2 bubbled through deionized water. Potentiostatic electrolysis was performed using an electrochemical measurement system (Hokuto Denko Co. HZ-5000) at 273 K. Potentials were indicated basis on SHE. Constant potentials from − 0.50 to − 0.75 V (SHE) were applied to the cathode. Products were analyzed by using two gas-chromatograph equipments (H 2 analysis: a TCD detector, an activated-carbon column with Ar; CO analysis: a molecular-sieves-5A column with He).

Electrochemical Reduction of CO 2
In electroreduction of CO 2 at cathode, formation of CO and CH 4 ... (2) A conversion of CO 2 , formation rates of CO, CH 4

Effect of co-electrocatalyst Addition on CH 4 Formation
The Co-dmbpy/KB(673K) electrocatalyst was active for the reduction of CO 2 to CO and a trace amount of CH 4 was produced.
over the Codmbpy/KB(673K)-GDL cathode prepared by the coating method. 15) Effects of co-catalysis of metal (10 wt%) supported on KB (metal: Mn, Fe, Co, Ni, Cu, Zn, Ce Ag, Au, Pb) were studied for electroreduction of CO 2 by the Codmbpy/KB(673K) cathode prepared by the mixing method at − 0.70 V (SHE) in Table 1. A 9 mg of metal/KB(773K) co-catalyst was mixed into a 9 mg of the Co-dmbpy/ KB(673K) electrocatalyst and a 30 mg of VGCF power, which was prepared to a cathode by the hot-press method as described in the experimental section. Cathodes made from various co-catalysts and the Co-dmbpy/KB(673K) electro- catalyst were applied for CO 2 reduction. Products were CO 2 , H 2 and CH 4 for all case. A sum of CEs was almost 100% within experimental errors. A cathode prepared by addition of KB powder to the Co-dmbpy/KB(673K) electrocatalyst was tested as a reference in run1. Though a formation of H 2 was enhanced and Conv and FR(CO) values were decreased a little by addition of KB, the major reduction of CO 2 to CO and the minor reduction to CH 4 were conformed as mentioned above. Among the additives tested in the Table 1, significant enhancing effects on FR(CH 4 ) or CE(CH 4 ) were observed for co-catalysts of Ni/KB(773K) in run 5, Zn/KB(773K) in run 7 and Au/KB(773K) in run 10. On the Ni/KB(773K) and Zn/KB(773K) co-catalysts, the both FR(CH 4 ) and CE(CH 4 ) increased with holding the electrocatalytic activity of the CO formation of the reference cathode (run 1). On the Au/KB(773K) co-catalyst, the FR(CH 4 ) significantly increased but FR(CO) decrease a little and the FR(H 2 ) drastically increased. Therefore, the small increase in CE(CH 4 ) and the large decrease in CE(CO) were observed for the Au/KB co-catalyst. Other additives were not effective for the CO 2 reduction. In addition, the Co/KB(773K) additive in run 4 enhanced the FR(H 2 ) and suppressed the FR(CO). This indicated that Co 0 was not the active site for the CO 2 reduction on the Co-dmbpy/ KB(673K) cathode.

Effects of Cathode Potentials on CO and CH 4 Formations
As described above, the Ni/KB(773K), Zn/KB(773K) and Au/KB(773K) co-catalysts accelerated the formation of CH 4 through the electroreduction of CO 2 at − 0.70 V(SHE). Effects of cathode potentials on the CO 2 reduction at the Co-dmbpy/KB(673K) + Ni/KB(773K), Zn/KB(773K) or Au/KB(773K) cathodes were studied in Fig. 2(a)  with decreasing in the potentials on the same exponential curve, whereas the FRs(CH 4 ) of the cathodes increased on different exponential curves, as shown in Fig. 2(b). An onset potential around − 0.4 V for the formation of CH 4 at the Au/KB-modified cathode was higher than that around − 0.5 V at other cathodes. Fairly high FRs(CH 4 ) were observed around − 0.7 V; however the electroreduction of CO 2 could not be conducted by an explosive formation of H 2 under − 0.75 V. The FR(CH 4 ) on the Ni/KB(773K)modified cathode increased with decreasing in potentials and a high FR(CH 4 ) of 3.9 μmol h − 1 cm − 2 was obtained at − 0.8 V(SHE). In the case of the Zn/KB(773K)-modified cathode, the electroreduction could not be done by the explosive formation of H 2 . A low FR(CH 4 ) of 0.34 μmol h − 1 cm − 2 on the KB(773K)-modified cathode was observed at − 0.8 V(SHE). As described above, the FRs(CO) were very similar among the four cathodes but CEs(CO) were different. Especially, the CE(CO) at the Au/KB-modified cathode was very low to compare with that at other cathodes. This low CE(CO) was due to fast formation of H 2 on the Au/KB-modified cathode. Therefore, CEs(CH 4 ) on the Au/KB(773K)-modified cathode were very low though the FRs(CH 4 ) were fairly high. On the other hand, the Ni/ KB(773K)-modified cathode showed a fairly high CE(CH 4 ) of 2.2% at − 0.80 V.
From the screening test of co-catalysts to the Co-dmbpy/ KB(673K) electrocatalyst, the Ni/KB(773K) co-catalyst showed a good ability to promote formation of CH 4 during the CO 2 electroreduction because of the higher FR(CH 4 ) and CE(CH 4 ) with keeping high FR(CO)and CE(CO) at − 0.80 V.

Improvement of the Co-dmbpy/KB(673K) + Ni/KB
Cathode on CH 4 Formation In order to improve electrocatalytic activity of the cathode, effects of reduction temperature of Ni/KB co-catalyst were studied in run 2, 12, 13 of Table 2. The Ni/KB cocatalyst used in Fig. 1 and Table 1 was reduced with H 2 at 773 K, as abbreviated Ni/KB(773K). The Ni/KB(773K) material was characterized by XRD analysis and formation of Ni 0 crystal as 17.1 nm crystal size was determined from XRD pattern and the Scherrer's equation. To expect enhancing co-catalysis of Ni 0 by reduction of the crystal size, the reduction temperature with H 2 was decreased from 773 K to 673 and 573 K and crystal sizes of 12.1 and 6.7 nm were obtained, respectively. Electrocatalytic reduction activity for CO 2 on the Co-dmbpy/KB(673K) + Ni/KB(673K) and Ni/KB(573K) cathodes at − 0.70 V(SHE) were indicated in runs 12 and 13, respectively. The i d values largely increased but the reduction rate of CO 2 did not drastically change. The FR(CH 4 ) was enhanced from 0.67 to 1.23 μmol h − 1 cm − 2 , 2 times, by mixing the Ni/KB(673K) co-catalyst and 1.5 times by mixing Ni/KB(573K) co-catalyst though their CE(CO) and CE(CH 4 ) were low. The Ni/KB(673K) co-catalyst was suitable for the CH 4 formation.
In run 14, Nafion solutions were painted on the surface of the Co-dmbpy/KB(673K) + Ni/KB(673K) cathode because an increase in an area of electrochemical active site and an improvement in electrocatalysis of the cathode were expected. Though the large increase in the i d value corresponded to a large increase in FR(H 2 ), the FR(CH 4 ) increased from 1.23 to 4.17 μmol h − 1 cm − 2 and the CE(CH 4 ) increased from 0.86 to 1.74% by the Nafion painting. In run 15, the catalyst ink of Co-dmbpy/KB(673K) and Ni/KB(673K) was coated on the GDL-25BC gas-diffusionelectrode as described in the experimental section. Remarkable increases in FR(CH 4 ) of 11.69 μmol h − 1 cm − 2 and in CE(CH 4 ) of 2.25% were observed against the results in run   Ni/KB(673K) would be expected for the formation of higher hydrocarbons.

Reaction Path for CH 4 Formation on the Codmbpy/KB(673K) + Ni/KB(673K) Cathode
As described so far, the electroreduction of CO 2 to CO and CH 4 proceeded on the Co-dmbpy/KB(673K) + Ni/ KB(673K) cathode. In order to know reaction path for CH 4 formation, (i) electroreduction of CO on the Co-dmbpy/ KB(673K) cathode, (ii) electroreduction of CO 2 on the Ni/ KB(673K) cathode, and (iii) electroreduction of CO on the Ni/KB(673K) cathode were conducted at − 0.70 V(SHE) and 273 K. It was revealed that (i) the electroreduction of CO to CH 4 did not proceed on the Co-dmbpy/KB(673K) cathode, (ii) the electroreduction of CO 2 to CO and CH 4 did not proceed on the Ni/KB(673K) cathode, and (iii) the electroreduction of CO to CH 4 proceeded on the Ni/KB(673K) cathode. In addition, when a gas mixture of CO (0.5 atm) and H 2 (0.5 atm) was introduced over the Ni/KB(673K) at 273 K, formation of CH 4 was not observed. In other words, the hydrogenation of CO with H 2 to CH 4 does not proceed by the Ni/KB(673K) catalyst. Figure 3 shows effects of potentials at the Ni/KB(673K) cathode on the electroreductions of CO 2 and CO, (a) i d and (b) FR(CH 4 ), CE(CH 4 ). As mentioned above, the electroreduction of CO 2 to CO did not proceed on the Ni/KB(673K) cathode from − 0.30 to − 0.80 V though higher i d values corresponding to H 2 formation were observed. However, very low formation of CH 4 was observed at lower potentials from − 0.70 to 0.80 V as shown in Fig. 3(b). On the other hand, the electroreduction of CO to CH 4 proceeded on the Ni/KB(673K) cathode from − 0.60 to − 0.80 V. The i d values in CO were suppressed to compare with that in CO 2 . The CEs(CH 4 ) were not so good and below 6% though 1atm of CO, high concentration of CO, was flowed.
The above results suggested that CO 2 was reduced to CO on the Co-dmbpy/KB(673K) (Eq. (1)) and this CO was successively reduced to CH 4 on the Ni/KB(673K) (Eq. (5)

Conclusion
Effects of co-catalysts on the reduction of CO 2 by the Co-dmbpy/KB(673K) cathode were studied. Various cocatalysts were screened and the enhancing effects of Ni/ KB(773K), Au/KB(773K) and Zn/KB(773K) co-catalysts on the formation of CH 4 during the CO 2 electroreduction were found. The Ni/KB(773K) co-catalyst was most effective for the formation of CH 4 among the three co-catalysts. Effects of reduction temperatures of the Ni/KB precursor with H 2 , of the mixing and coating methods for cathode preparation, and of cathode potentials on the CO 2 reduction to CO and CH 4 were clarified and the most suitable cathode was the GDL electrode coated the 10wt%Ni/KB(673K) cocatalyst and 1wt%Co-dmbpy/KB(673K) catalyst with and Nafion solutions. Although the maximum CE(CH 4 ) on the suitable cathode was only 2.15% at − 0.70 V, the cathode showed the unique electrocatalysis for the formations of C 2 H 6 with 0.08% CE, C 2 H 4 with 0.01%, C 3 H 8 with 0.02%. Reaction paths for the formations of CO and CH 4 on the Co-dmbpy/KB(673K) + Ni/KB(673K) cathode were studied and the successive reduction scheme of CO 2 to CO on the Co-dmbpy/KB(673K) catalyst and CO to CH 4 on the Ni/ KB(673K) co-catalyst was clarified.