Abstract:
An electrochemical electrode comprising a tin-based catalyst, method of making, and method of use are provided. Catalyst particles are prepared which comprise tin deposits of about 0.1 nm to 10 nm deposited onto carbon support. Preparing an ink comprising the catalyst particles and a binder enable an electrode to be prepared comprising the catalyst particles bound to an electrode substrate. The electrode may then be used in an apparatus and process to reduce carbon dioxide to products such as formate and formic acid at Faradaic Efficiencies up to 95 percent.

Description:
FIELD 
       [0001]    This description relates generally to electrodes for electrochemical reactions, particularly to cathodic electrodes comprising tin-based catalyst, and more particularly to such electrodes for the electrochemical reduction of carbon dioxide to formate and formic acid. Also included are methods of making and using such electrodes. 
       BACKGROUND 
       [0002]    Electrochemical reactions involve the use of an electric current to effect chemical reactions. In a typical arrangement, an anode immersed in an anolyte and a cathode immersed in a catholyte are separated by a membrane, generally an ion exchange membrane. An oxidation reaction takes place at the anode and a reduction reaction takes place at the cathode. For example, with the electrochemical reduction of carbon dioxide, carbon dioxide is reduced to formate and formic acid at the cathode and oxygen is evolved from water at the anode. In some applications, carbon dioxide is introduced into a catholyte compartment through a porous cathode which porous cathode comprises a cathodic catalyst adapted to reduce carbon dioxide to formate and formic acid. 
         [0003]    Cathodic and anodic catalysts play important roles in determining what reactions occur at the cathode and at the anode, respectively, as well as the electrolytic efficiency of such reactions. Efficiency is typically measured as Faradaic Efficiency (FE) and is the efficiency with which electrons (charge) are transferred. That is, the percent of the total current that passes through the electrochemical cell that is used to produce the desired product (e.g., formate). In addition, it is desirable to achieve high current density in stable, long-term use. Tin-based cathodic catalysts, which generally exist as tin, tin oxide (II) (SnO), and tin oxide (IV) (SnO 2 ) (collectively, SnO X ), have been shown to give good results in the selective electrochemical reduction of carbon dioxide to formate and formic acid. In addition to selectivity to formate and formic acid, desirable characteristics of such tin-based cathodic catalysts include high surface area of the catalyst on the cathode, physical and mechanical stability, high FE, and high and stable current density during operation. Finally, while catalyst activity can be unpredictable and is highly dependent upon structure, morphology, and electrolysis conditions, other post-transition metals such as lead and indium may also be candidates. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    Electrochemical electrodes comprising tin-based catalysts, method of making, and method of use are disclosed. Particularly, the electrodes comprise tin-based nano-scale deposits on carbon support to produce Sn—C particles and such Sn—C particles are applied to a support substrate using a binder. In use, the resulting electrodes provide stable high current density and high FE or selectivity over time. 
         [0005]    The catalysts are prepared by depositing tin onto a carbon support such as carbon black [e.g., Vulcan® XC72 (Cabot Corp., Boston, Mass.)] using a solution of tin (IV) chloride (SnCl 2 ), ethylene glycol, carbon black, and water. After recovery, the resultant Sn—C catalyst particles are combined with a binder such as a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer (e.g., Nafion®, The Chemours Co., Wilmington, Del.) or compounds comprising tetrafluoroethylene groups [e.g., polytetrafluoroethylene (PTFE)] and applied to a support substrate such as carbon fiber paper (e.g., TORAY® Carbon Paper 120, Toray Industries, Inc., Tokyo Japan). The finished electrodes are used in an electrolytic process to reduce carbon dioxide to formate and formic acid. 
         [0006]    In one exemplary embodiment, an apparatus is provided for the electrochemical reduction of carbon dioxide to formate. The apparatus includes an anolyte compartment, the anolyte compartment at least partially defined by an anode and a membrane, a catholyte compartment, the catholyte compartment at least partially defined by the membrane and a cathode, and a gas compartment, the gas compartment in fluid communication with the cathode. The cathode comprises a porous substrate and a catalytic coating at least partially covering the substrate. The catalytic coating is applied to the substrate using a process comprising the steps of (a) mixing catalyst powder, the catalyst powder comprising tin-carbon particles, the tin-carbon particles comprising 0.1 nm to 10 nm tin deposits on carbon support, an alcohol-based solvent, and polymeric binder, to form a catalyst ink, (b) applying the ink to the substrate, and (c) drying the ink to the substrate. 
         [0007]    In a further exemplary embodiment, the tin deposits comprise greater than 22 weight percent of the catalyst powder. 
         [0008]    In a further exemplary embodiment, the substrate is carbon fiber paper, the catalyst loading is between 0.1 mg/cm 2  and 10 mg/cm 2 , and the binder content is between 0.1 to 5 weight percent of the catalyst loading. 
         [0009]    In a further exemplary embodiment, utilizing the apparatus for the electrochemical reduction of carbon dioxide to formate, a process comprises the steps of feeding an appropriate anolyte into the anolyte compartment, (b) feeding an appropriate catholyte into the catholyte compartment, (c) feeding carbon dioxide into the gas compartment, through the cathode, and into the cathode compartment, and (d) impressing a voltage between the anode and the cathode. In a further exemplary embodiment, the impressed voltage is sufficient to provide a V CATHODE  range between −1.5 and −2.0 V SCE . In a further exemplary embodiment, the Faradaic Efficiency of carbon dioxide to formate is about 55 to 95 percent over a period of over 100 hours. 
         [0010]    In another exemplary embodiment, a process is provided comprising the steps of (a) preparing catalyst powder by (i) mixing, proportionally, 0.84 g tin chloride (IV), 200 ml ethylene glycol, 2 ml deionized water, and 0.20 g carbon support, (ii) refluxing the mixture of Step (a)(i) at between 170 deg. C. and 200 deg. C. for about three hours, (iii) cooling the refluxed material, and (iv) filtering and drying the cooled, refluxed material to obtain the catalyst powder, (b) preparing a catalyst ink by mixing, proportionally, 0.02 g catalyst powder, 100 ml isopropyl alcohol, and between 0.02 mg and 1 mg sulfonated tetrafluoroethylene-based fluoropolymer-copolymer binder, (c) spraying a portion of catalyst ink uniformly onto a porous substrate, (d) allowing the catalyst ink sprayed onto the porous substrate to dry, and (e) repeating Steps (c) and (d) until a desired coating weight is achieved. 
         [0011]    In a further exemplary embodiment, an apparatus is provided comprising an anolyte compartment, which anolyte compartment is at least partially defined by an anode and a membrane, a catholyte compartment, which catholyte compartment is at least partially defined by the membrane and a cathode, and a gas compartment, the gas compartment in fluid communication with the cathode. The cathode is prepared by a process comprising the steps of (a) preparing catalyst powder by (i) mixing, proportionally, 0.84 g tin chloride (IV), 200 ml ethylene glycol, 2 ml deionized water, and 0.20 g carbon support, (ii) refluxing the mixture of Step (a)(i) at between 170 deg. C. and 200 deg. C. for about three hours, (iii) cooling the refluxed material, and (iv) filtering and drying the cooled, refluxed material to obtain the catalyst powder, (b) preparing a catalyst ink by mixing, proportionally, 0.02 g catalyst powder, 100 ml isopropyl alcohol, and between 0.02 mg and 1 mg sulfonated tetrafluoroethylene-based fluoropolymer-copolymer binder, (c) spraying a portion of catalyst ink uniformly onto a porous substrate, (d) allowing the catalyst ink sprayed onto the porous substrate to dry, and (e) repeating Steps (c) and (d) until a desired coating weight is achieved. 
         [0012]    In another exemplary embodiment, a process is provided comprising the steps of (a) preparing a catalyst powder by (i) mixing, proportionally, tin chloride (IV) and carbon support in a ratio of about 4.5 g/g and sufficient ethylene glycol and water to maintain a dilute slurry, (ii) refluxing the mixture of Step (a)(i) at between 170 deg. C. and 200 deg. C. for about three hours, (iii) cooling the refluxed material, and (iv) filtering and drying the cooled, refluxed material to obtain the catalyst powder (b) preparing a catalyst ink by mixing, proportionally, catalyst powder and isopropyl alcohol in a ratio of about 0.02 g/100 ml to 1 mg with a suitable binder, which binder is proportional to the isopropyl alcohol in a ratio of about 0.02 mg to 1 mg per 100 ml isopropyl alcohol, for about 30 minutes, (c) spraying a portion of the catalyst ink uniformly onto a porous substrate, (d) allowing the catalyst ink sprayed onto the porous substrate to dry; and (e) repeating Steps (c) and (d) until a desired coating weight is achieved. 
         [0013]    In another exemplary embodiment, an apparatus is provided which apparatus includes an anolyte compartment, the anolyte compartment at least partially defined by an anode and a membrane, a catholyte compartment, the catholyte compartment at least partially defined by the membrane and a cathode, and a gas compartment, the gas compartment in fluid communication with the cathode. The cathode is prepared by a process comprising the steps of (a) preparing a catalyst powder by (i) mixing, proportionally, tin chloride (IV) and carbon support in a ratio of about 4.5 g/g and sufficient ethylene glycol and water to maintain a dilute slurry, (ii) refluxing the mixture of Step (a)(i) at between 170 deg. C. and 200 deg. C. for about three hours, (iii) cooling the refluxed material, and (iv) filtering and drying the cooled, refluxed material to obtain the catalyst powder (b) preparing a catalyst ink by mixing, proportionally, catalyst powder and isopropyl alcohol in a ratio of about 0.02 g/100 ml to 1 mg with a suitable binder, which binder is proportional to the isopropyl alcohol in a ratio of about 0.02 mg to 1 mg per 100 ml isopropyl alcohol, for about 30 minutes, (c) spraying a portion of the catalyst ink uniformly onto a porous substrate, (d) allowing the catalyst ink sprayed onto the porous substrate to dry; and (e) repeating Steps (c) and (d) until a desired coating weight is achieved. 
         [0014]    In a further exemplary embodiment, utilizing the apparatus described just above, a process comprises (a) feeding an appropriate anolyte into the anolyte compartment, (b) feeding an appropriate catholyte into the catholyte compartment, (c) feeding carbon dioxide into the gas compartment, through the cathode, and into the cathode compartment, and (d) impressing a voltage between the anode and the cathode. In a further exemplary embodiment, the impressed voltage is sufficient to provide a V CATHODE  range between −1.5 and −2.0 V SCE . In a further exemplar embodiment, the Faradic Efficiency of carbon dioxide to formate is about 55 to 95 over a period of over 100 hours. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL FIGURES 
         [0015]    The invention will be more readily understood by reference to the accompanying figures. The figures are incorporated in, and constitute a part of, this specification, illustrate several embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention. For purposes of illustration, drawings may not be to scale. 
           [0016]      FIG. 1  is a graphical representation of Current Density i (mA/cm 2 ) v. Cell Potential (−V) for Sn—C-1 particles. 
           [0017]      FIG. 2  is a graphical representation of FE v. Cell Potential (−V) for Sn—C-1 particles. 
           [0018]      FIG. 3  is a schematic diagram of an experimental test apparatus. 
           [0019]      FIGS. 4-9  are transmission electron microscopy (TEM) images of Sn—C-2 particles from Sn—C Experiment 2. 
           [0020]      FIGS. 10 and 11  are TEM images of Sn—C-3 particles from Sn—C Experiment 3. 
           [0021]      FIG. 12  is a graphical representation of Current Density i (mA/cm 2 ) v. V ANODE  (V SCE ) for two runs each of Test Protocol 1 and Test Protocol 2, each protocol using Sn—C-3. 
           [0022]      FIG. 13  is a graphical representation of Current Density (mA/cm 2 ) v. IR (V or V SCE ) for two runs each of Test Protocol 1 and Test Protocol 2. 
           [0023]      FIG. 14  is a graphical representation of I TOTAL  (mA/cm 2 ), FE (%), and V CATHODE  v. Time (t) (hrs.) for Electrode Experiment S1. 
           [0024]      FIGS. 15-18  are TEM images of unused and used Sn—C-3 particles from Electrode Experiment S1. 
           [0025]      FIG. 19  is a graphical representation of i TOTAL  (mA/cm 2 ), FE (%), and V CATHODE  v. Time (t) (hrs.) for Electrode Experiment S2. 
           [0026]      FIGS. 20-23  are TEM images of unused and used Sn—C-3 particles from Electrode Experiment S2. 
           [0027]      FIG. 24  is a graphical representation of Current Density (mA/cm 2 ) and Formate Product Selectivity (%) v. Time (t) (days) for Electrode Experiment S3. 
           [0028]      FIGS. 25-28  are TEM images of unused and used Sn—C-3 particles from Electrode Experiment S3. 
       
    
    
       [0029]    In describing the various embodiments of the invention, specific terminology will be resorted to for the sake of clarity. However, it is not intended that the invention be limited to the specific terms so selected and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. 
       DETAILED DESCRIPTION 
     Preparation of Sn—C Catalyst Particles 
       [0030]    Catalyst particles of nano-sized tin deposited on carbon powders (Sn—C) were prepared using the procedure shown in Table 1 below. 
         [0000]    
       
         
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 (1) mix predetermined amounts of deionized water and ethylene glycol; 
               
               
                 (2) add predetermined amounts of tin chloride (IV) and ethylene glycol 
               
               
                 and ultrasonicate (e.g., ten minutes); 
               
               
                 (3) add a predetermined amount of carbon black and ultrasonicate 
               
               
                 (e.g., 30 minutes); 
               
               
                 (4) reflux the mixture at a predetermined temperature (e.g., 196 deg. C.) 
               
               
                 for a predetermined time (e.g., three hours) while stirring; 
               
               
                 (5) allow the mixture to cool to room temperature; 
               
               
                 (6) vacuum filter the solution through a 0.2 micron (μm) membrane; 
               
               
                 (7) immerse the membrane with the cake into 50 ml isopropyl alcohol 
               
               
                 (IPA) and ultrasonicate (e.g., ten minutes) until all black powder is 
               
               
                 removed; 
               
               
                 (8) heat black powder/IPA mixture to evaporate the IPA; 
               
               
                 (9) scrape out resulting black powder; 
               
               
                 10) calculate final weight percent SnO X   
               
               
                 {[(final weight Sn-C catalyst powder) − (initial weight carbon black 
               
               
                 powder)]/final weight Sn-C catalyst powder} * 100 
               
               
                   
               
             
          
         
       
     
         [0031]    In the above procedure, the ethylene glycol acts as a solvent for the tin chloride and also as a refluxing agent. Importantly, however, ethylene glycol also acts as a reducing agent for tin. The tin used in the procedure is tin chloride (IV), but ethylene glycol reduces the tin during the refluxing process so that by the end of the process, ethylene glycol reduces the tin to tin (II) (SnO). 
         [0032]    The carbon black used was Vulcan® XC72 (Cabot Corp., Boston, Mass.) having a nominal particle size of 1 micron. Typical surface area is about 250 m 2 /gram. Other suitable carbon-based support materials include carbon nanowires and carbon nanotubes. 
       Sn—C Experiment 1 (Sn—C-1) 
       [0033]    Using the procedure shown in Table 1, Sn—C particles were prepared using the ingredients and recipe shown in Table 2 below. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Compound 
                 Amount 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Tin Chloride (IV) 
                 5.00 
                 g 
               
               
                   
                 Ethylene Glycol 
                 400 
                 ml 
               
               
                   
                 Deionized Water 
                 15 
                 ml 
               
               
                   
                 Carbon Black 
                 2.00 
                 g 
               
               
                   
                   
               
             
          
         
       
     
         [0034]    The mixture was refluxed at 170 deg. C. for four hours, cooled, and filtered to obtain the Sn—C particles (Sn—C-1). Electrodes were prepared using a modification of the electrode preparation protocol outlined herein below in Table 6. The significant difference in the modified electrode preparation protocol was that no binder was used to adhere the Sn—C-1 particles to the electrode substrate [i.e., carbon fiber paper (CFP)]. This allowed measurement of the performance of the catalyst particles themselves in terms of FE (%) as a function of applied cathode voltage without the possible complex effects due to the presence of a binder. While not wishing to be bound by any particular theory, it is believed that without a binder to adhere the Sn—C-1 particles to the CFP substrate, the current density [i (mA/cm 2 )] may decrease continuously with Sn—C-1 particles being washed away in the cell. For whatever current is noted, however, the FE (%) can be calculated. Thus, the FE (%) so obtained may be compared with that observed where solid Sn or tin-electroplated CFP electrodes are tested (i.e., 70-90% FE). 
         [0035]    As shown in  FIG. 1 , at total cell voltages between 3.35 and 3.95 (−V), the resulting catalyst exhibited low current densities. While not wishing to be bound by any particular theory, it is believed that in the absence of a binder to hold the particles to the CFP substrate, the particles did not adhere well. However, as shown in  FIG. 2 , at the chosen cell voltage range, FE is well-maintained. 
       Sn—C Experiment 2 (Sn—C-2) 
       [0036]    Using the procedure shown in Table 1, Sn—C particles were prepared using the ingredients and recipe shown in Table 3 below. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Compound 
                 Amount 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Tin Chloride (IV) 
                 0.84 
                 g 
               
               
                   
                 Ethylene Glycol 
                 200 
                 ml 
               
               
                   
                 Deionized Water 
                 2 
                 ml 1   
               
               
                   
                 Carbon Black 
                 0.60 
                 g 
               
               
                   
                   
               
               
                   
                   1 1% of ethylene glycol volume 
               
             
          
         
       
     
         [0037]    The mixture was refluxed at 196 deg. C. for three hours, cooled, and filtered to obtain the Sn—C-2 particles (Sn—C-2). Transmission electron microscopy (TEM) images for the resulting Sn—C-2 particles are shown in  FIGS. 4-9 . Tin-based crystallites of about 5 nm are distributed on the carbon support. The percent tin-based material, comprising SnO X , was about 22 weight percent. 
       Sn—C Experiment 3 (Sn—C-3) 
       [0038]    Using the procedure shown in Table 1, Sn—C particles were prepared using the ingredients recipe shown in Table 4 below. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Compound 
                 Amount 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Tin Chloride (IV) 
                 0.84 
                 g 
               
               
                   
                 Ethylene Glycol 
                 200 
                 ml 
               
               
                   
                 Deionized Water 
                 2 
                 ml 
               
               
                   
                 Carbon Black 
                 0.20 
                 g 
               
               
                   
                   
               
             
          
         
       
     
         [0039]    The mixture was refluxed at 196 deg. C. for three hours, cooled, and filtered to obtain Sn—C particles (Sn—C-3). Using less carbon, by proportion, than Sn—C Experiment 2 (Sn—C-2), the percent tin-based material was about 30 weight percent. TEM images for the resulting Sn—C-3 particles are shown in  FIGS. 10-11 . Tin-based crystallites of about 5 nm are distributed on the carbon support. 
       Sn—C Experiment 4 (Sn—C-4) 
       [0040]    Using the procedure shown in Table 1 and the recipe shown in Table 4, an effort was made to increase the size of the tin-based nano deposits on the carbon support to the range of 10 nm. Such size variation is dependent upon such factors as the time of hydrothermal processing, the water:ethylene glycol ratio, and the concentration of tin chloride. The procedure was the same as that for Experiment 3, except that the time of hydrothermal processing was six hours, instead of three. The results, shown below in Table 5, were disappointing, however, as the resulting catalytic electrodes exhibited very low FE (i.e., 33 to 60 percent). 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                   
                   
                 Current 
                 Formate 
                   
               
               
                 Time 
                 Cell Potential 
                 Density 
                 Current Density 
                 FE 
               
               
                 (hrs) 
                 (−V) 
                 [i (mA/cm 2 )] 
                 [i (mA/cm 2 )] 
                 (%) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0 
                 3.75 
                 106.0 
                 33.4 
                 31.6 
               
               
                 1 
                 3.75 
                 100.5 
                 59.5 
                 59.2 
               
               
                 3 
                 3.75 
                 9805 
                 51.6 
                 52.4 
               
               
                 24.5 
                 3.75 
                 92.0 
                 40.2 
                 43.7 
               
               
                   
               
             
          
         
       
     
       Electrode Preparation 
       [0041]    Electrodes were prepared based upon Sn—C-3 particles prepared according to Sn—C Experiment 3 above. A porous and electrically conductive electrode substrate was used which comprised carbon fiber paper (CFP). The CFP used in these experiments was TORAY® Carbon Paper 120 which had a thickness of about 350 microns and a porosity of about 80 percent. The CFP microstructure consists of carbon fibers about 7-10 microns in diameter and are held together with polytetrafluoroethylene (PTFE) binder. A catalyst ink comprising Sn—C-3 particles was prepared using a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer (e.g., Nafion®). As supplied, the Nafion® binder was a 5 percent w/w solution in aliphatic alcohols, principally isopropyl, and water, from Sigma Aldrich. This solution was further diluted with water to form a solution with a binder concentration of about 2 mg/ml. Requisite amounts of this diluted solution was used to produce electrodes with a binder concentration between 0.03 percent w/w and 5 percent w/w of catalyst loading. The electrodes were prepared according to the procedure shown in Table 6 below. 
         [0000]    
       
         
               
             
           
               
                 TABLE 6 
               
               
                   
               
             
             
               
                 (1) prepare a catalyst ink mix by ultrasonicating predetermined amounts of 
               
               
                 catalyst powder (e.g., 0.02 g) prepared as above, isopropyl alcohol 
               
               
                 (e.g., 100 ml), and a requisite amount of a solution containing sulfonated 
               
               
                 tetrafluoroethylene-based fluoropolymer-copolymer binder (e.g., 
               
               
                 Nafion ®) to provide the desired binder loading for a period of time 
               
               
                 (e.g., 30 minutes); 
               
               
                 (2) prepare an electrode substrate from a section of carbon fiber paper 
               
               
                 (CFP); 
               
               
                 (3) spray catalyst ink uniformly onto the CFP until just wet and 
               
               
                 immediately place sprayed electrode onto hot plate until dry (e.g., few 
               
               
                 seconds) 1 ; 
               
               
                 (4) repeat (3) until desired coating weight is achieved; 
               
               
                 (5) calculate coating weight per unit area: 
               
               
                 (coating weight)/(geometric area of electrode) 
               
               
                   
               
               
                   1 Attempts to apply large amounts of ink in one coating may result in Sn—C particle “clumps” which inhibit catalytic effectiveness. 
               
             
          
         
       
     
         [0042]    Best results are obtained if thick coatings as well as low catalysts loadings are avoided. Thicker coatings may crack and fall away from the substrate surface, particularly due to physical abrasion caused by flow-through carbon dioxide. Low loading, on the other hand, may not be sufficient to cover the entire substrate surface which a resulting loss of efficiency. Experimental catalyst loadings ranged from 0.45-4.5 mg/cm 2  with varying binder concentrations of 0.03-5 weight percent. Catalyst loadings are calculated as shown in Table 6. While not wishing to be bound by any particular theory, it is believed that excess binder produces an undesirable overload of the binder onto the Sn—C particles which effectively blocks the surface area of the Sn—C particles available for catalyst activity. Too little binder may reduce the ability of the Sn—C particles to adhere to the CFP substrate surface. 
         [0043]    To study the operational effects of the various electrodes as cathodes, it was necessary to obtain measurements of the cathode voltage. Since the cathode voltage cannot be directly applied, continuous, single-pass experiments were conducted where applied cell voltage was monitored and controlled such that the resulting cathode voltage was kept as close a possible to a predetermined value using the following formula: 
         [0000]        V   APPLIED-TOTAL-CELL   =V   CATHODE +IR+ V   ANODE ,  (1)
 
         [0000]    where V ANODE =anode voltage at the prevailing current density, i (mA/cm 2 ) and IR=cell internal resistance or ohmic potential drop in the solution at the prevailing current density, i (mA/cm 2 ). Experiments were performed to determine IR and V ANODE  so that V CATHODE  could be calculated from Eq. (1). 
         [0044]    A general schematic of the apparatus used is shown in  FIG. 3 . A three-compartment electrochemical reactor  350  is shown in which a three-compartment container  12  encloses an anolyte compartment  18 , an anode  14 , and, during operation, anolyte  24  contained within the anolyte compartment  18 ; a membrane  22 ; a catholyte compartment  20 , a porous cathode  16  as described herein, and, during operation, catholyte compartment mixture  26  contained within the catholyte compartment  20 ; and a gas compartment  28 , the gas compartment  28  containing, during operation, CO 2  gas  30 . The membrane  22  separates the anolyte compartment  18  and the catholyte compartment  20  and the porous cathode  16  separates the catholyte compartment from the gas compartment  28 . Also during operation, an anolyte feed  36  introduces anolyte  24  into the anolyte compartment  18 , an anolyte withdrawal  44  removes anolyte  24  as well as other anode reaction products, a catholyte feed  34  introduces catholyte  25  into the catholyte compartment  20 , a catholyte compartment mixture withdrawal  42  removes catholyte compartment mixture  26 , and a CO 2  gas feed  32  introduces CO 2  gas  30  into the gas compartment  28 . During operation, the CO 2  gas  30  in the gas compartment  28 , under a pressure differential across the porous cathode  16 , is distributed (indicated by arrows  40  and flows through the porous cathode  16  and into the catholyte mixture  26 . 
         [0045]    To obtain IR and V ANODE , two experiments were performed with reference electrode probes extended into the cathode and anode chambers. The potential difference between the probes on either side of a membrane (e.g., Nafion®) was measured and the potential difference between the cathode and the probe in the respective chamber was also measured. An Ag—AgCl reference electrode was inserted from the side into the cathode chamber and a Cu—CuSO 4  reference electrode was inserted into the anode chamber. 
         [0046]    With reference to  FIG. 3 , Test Protocol 1 was performed using a Sn-electroplated CFP cathode, 2M KCl catholyte, and 0.5M H 2 SO 4 +0.5M K 2 SO 4  anolyte. Test Protocol 2 was performed using a nano-Sn particle-based cathode (1.8 mg/cm 2 ); the catholyte and anolyte were identical to Test Protocol 1. The anode comprised a Ti substrate coated with a IrO 2 . Two runs were made of each set of test conditions. 
         [0047]    For the two test protocols of two runs each,  FIG. 12  shows Current Density i (mA/cm 2 ) v. V ANODE  (V SCE ).  FIG. 13  shows Current Density i (mA/cm 2 ) v. IR (V or V SCE ) [ohmic loss (V or V SCE )]. After the tests, since V APPLIED-TOTAL-CELL  was measured directly, V CATHODE  was obtained using Eq. (1). Although shown in  FIGS. 12 and 13 , Test Protocol 2 results were not used in the calculations as there was a large loss in current densities. While not wishing to be bound by an particular theory, this large loss in current densities may have been caused by degradation of the particle-coated cathode, perhaps due, in part, to mechanical abrasion from the reference electrode. 
         [0048]    Post-experiment reference probe measurements versus SCE in saturated KCl are shown in Table 7 below, which indicates very little degradation over time. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 7 
               
               
                   
                   
               
               
                   
                 Probe Type 
                 Before (mV) 
                 After (mV) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Ag—AgCl 
                 −42 
                 −28 
               
               
                   
                 Cu—CuSO 4   
                 72 
                 50 
               
               
                   
                   
               
             
          
         
       
     
         [0049]    Table 8 shows the results of a series of experiments performed with Sn—C-3 cathode electrodes made according to Experiment 3 above. Catalyst loading was 1.8 mg/cm 2  at 30 weight percent SnO X  on CFP substrate. In all experiments, the catholyte was 2M KCl at 9-11 ml/min. saturated with carbon dioxide. The anolyte was H 2 SO 4 +K 2 SO 4  at 55-65 ml/min. The carbon dioxide flowrate was about 100 ml/min. The anode comprised a mixed metal oxide anodic catalyst (IrO 2 ) on a Ti substrate. 
         [0000]    
       
         
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 8 
               
             
             
               
                   
                   
               
               
                   
                 Sn-C-3 Experiment 
               
             
          
           
               
                 Parameter 
                 Units 
                 S1 
                 S2 
                 S3 
               
               
                   
               
             
          
           
               
                 Run Time 
                 hours 
                 120 
                 200 
                 288 
               
               
                 V CATHODE   
                 (min-max) V SCE   
                 −1.77 to 
                 −1.63 to 
                 −1.66 
               
               
                 Range 
                   
                 −1.89 
                 −1.72 
               
               
                 V CATHODE   
                 V SCE   
                 1.85 
                 1.7 
                 1.66 
               
               
                 (expected) 
               
               
                 i TOTAL  (t = 0) 
                 mA/cm 2   
                 200 
                 140 
                 127 
               
               
                 i TOTAL  (t = max) 
                 mA/cm 2   
                 170 
                 101 
                 114 
               
               
                 Decrease in 
                 {[(i TOTAL  (t = 0) − 
                 15 
                 40 
                 10 
               
               
                 Current (%) 
                 i TOTAL   
               
               
                   
                 (t = max)]/i TOTAL   
               
               
                   
                 (t = 0)} * 100 
               
               
                 FE (%) (t = 0) 
                   
                 80-87 
                 67-77 
                 70 
               
               
                 FE (%) 
                   
                 68 
                 63 
                 70 
               
               
                 (t = max) 
               
               
                 Catalyst Loading 
                 mg/cm 2   
                 1.8 
                 1.8 
                 1.8 
               
               
                 Binder Content 
                 weight percent of 
                 0.1 
                 0.5 
                 1 
               
               
                   
                 catalyst loading 
               
               
                   
               
             
          
         
       
     
       Sn—C-3 Electrode Experiment S1 
       [0050]    As shown in Table 7, V CATHODE  is in the range of −1.77 to −1.89 V SCE  with the majority of the experiment at V CATHODE &gt;−1.8 V SCE .  FIG. 14  shows i TOTAL  (mA/cm 2 ), FE (%), and V CATHODE  v. Time (hrs.). One could conclude, from  FIG. 14  that, with time, the cathode voltage (V CATHODE ) must be increased to maintain the same current density (i TOTAL ). Notably, at about t=115 hours, V CATHODE  is about the same as V CATHODE  at t=20, but there is about a 15 percent loss in i TOTAL , and an 18.5 percent loss in FE. A more abrupt loss in current density (i TOTAL ) occurs at about t=120 hours. A comparison of associated TEM images shown in  FIGS. 15-18  shows a possible loss of Sn—C-3 particles with time from the CFP support surface. While not wishing to be bound by any particular theory, physical abrasion could cause this loss, which may be reduced by adjusting binder content. On the other hand, such loss could be due to the higher applied V CATHODE . (Compare, Sn—C-3 Electrode Experiments S2 and S3, using lower V CATHODE .)  FIG. 14  also indicates a sharp rise in V CATHODE  as well as i TOTAL  from t=0 up to about t=20 hours, possible due, in part, to some preconditioning effects. 
       Sn—C-3 Electrode Experiment S2 
       [0051]    Turning now to  FIG. 19 , V CATHODE  is in the range of −1.65 to −1.72 V SCE  with the majority of the experiment at V CATHODE ≦−1.70 V SCE . As seen in  FIG. 8 , a large drop in total current density, i TOTAL , of about 25-40 percent is observed between t=25 hours and t=100 hours and then remains constant from about t=100 hours to about t=200 hours. While not wishing to be bound by any particular theory, the fluctuations in V CATHODE  and FE in the last 100 hours may have been caused by poor adhesion of the particles to the CFP substrate. 
         [0052]    Turning now to related  FIGS. 20-23 , the TEM images show no loss in Sn—C particle density for used versus unused. Thus, applying V CATHODE  of about −1.7 V SCE  for about 200 hours did not appear to lead to a loss of Sn—C particles from the CFP support. While not wishing to be bound by any particular theory, the overall loss of total current density, i TOTAL  may be attributable to the loss of Sn—C particles due to lack of adhering which may be corrected with improved binder content. (Compare, Electrode Experiment S3.) 
       Sn—C-3 Electrode Experiment S3 
       [0053]    In Sn—C-3 Electrode Experiment S3, the binder content was increased to 1 percent in an effort to reduce Sn—C particle loss from the CFP substrate. (Compare, Sn—C-3 Electrode Experiment S2.) In addition, cathode potential, V CATHODE  is held at −1.66 V SCE  to avoid Sn—C particle loss. (Compare Sn—C-3 Electrode Experiment 51,  FIGS. 14 and 15-18 .) The results of Sn—C-3 Electrode Experiment S3 are shown in  FIGS. 24 and 25-28 . Loss in total current, i TOTAL  over 240 hours was 10 percent and FE remained fairly constant at about 70 percent. Looking at  FIGS. 25-28 , there seems to be no apparent loss in total Sn—C-3 particles. (Compare, Sn—C-3 Electrode Experiment 51,  FIGS. 14 and 15-18  at higher V CATHODE. ) Still looking at  FIGS. 25-28 , there does appear to be some agglomeration and possible breaking up of Sn—C-3 particles. Total Sn—C-3 loss seems negligible, however, thus allowing for the use of such electrodes for longer times at lower applied V CATHODE . 
         [0054]    While certain embodiments of the present invention have been disclosed in detail, it is to be understood that various modifications may be adopted without departing from the spirit of the invention or scope of the following claims.