After CO2 Electrochemical reduction on 6 samples, i obtained both gaseous and liquid products from GC-MS and NMR respectively. i quantified the liquid products by integrating each peak. for example I quantified the formate by dividing the peak area by 1 (since only 1 proton is resposible for that peak) and then the peak area of DMSO(internal standard) of known concentration was divided by 6 (since 6 protons are responsible for that peak). Then i divided the area per nuclei of the Formate by area per nuclei of the DMSO to get the relative area. with my standard curve of formate, i calculated the concentration from the equation of the standard curve obtained from Microsoft excel.
using FE=C x n x F/Q x100 where C is concentration, n number of electrons transferred for the formation of a product, Q total charge obtained by integrating all the currents i and the corresponding time t. I calculated the FE of both the gaseous and liquid products ensuring that all the units cancel out. However summing up all the FE on each catalyst, i noticed that on some catalysts the TOTAL FE WAS HIGHER THAN 100% (103% up to140%), while others in which less number of liquid products were obtained had TOTAL FE way less than 100% (30% to 55%). I find it difficult to account for the remaining FE. I detected H2,CO,CH4,HCOO- and CH3OH on the first 3 samples and only H2,CO,CH4 and HCOO- were detected on the remaining 3 which give lesser than 100% Total FE.
Please must my Total FE be approx.100%, is there mistake in my liquid product quantification?
I look forward to your contribution to my challenges.
Thank you.
You may have numerous sources of errors.
First, the measured FE is commonly less 100%. One of the reason is a leak of air into your reactor. O2 reduction increases the overall current. The measurements of gaseous products is not easy. Gases are in equilibrium between the solution and the headspace. A part of your electrical energy is converted to heat.
Thank you so much sir @ Yurii V Geletin. I appreciate your contribution.
Did you see changes in the current response, probably a decreased response indicates that another process occurs. When you stop the reduction?
Thank You Guillermo Ahumada I did observe a slight gradual decrease in the the Current after 2.5hrs. A Friend recently suggested I use FE= (Cp x Vcell x n x F) /Qtotal x 100. for the efficiency calculation for my liquid products, where n is number of transferred e- for the particular product formation, F is Faraday Constant, Q is the total charge Vcell is the volume of the electrolyte and C is the concentration of the liquid product obtained using the formula: Cp =(Np/Ni.s ) X Ci.s , Where the Np is the integrated area per protons from the nmr peaks of the product, Ni.s is the integrated area per protons from the nmr peaks of the internal standard while Ci.s is the concentration of the internal standard. I used this method and observed all the Total FE less than 100% (70% - 28%, including H2). Because our GC-MS doesn't detect C2 products. I'm suspecting the C2 products are being formed because the HER is decreasing with decreasing CO2R, considering the potential i am using. However, i still doubt if the lack of C2H4 and probably C2H6 could account for that huge lost especially for the sample with lowest Total FE of 28%. Other factors could be playing a role as earlier suggested by Yurii V Geletii .
Furthermore, because i am using standard calibration curve for the concentration Cp Calculation, i multiplied the concentration i obtained from the calibration curve by Vcell just as in the FE Equation above and obtained Similar result as the direct method discussed above.Although there were some noticeable variations in the 2 results for some samples, others are showed negligible differences. Do you think this approach is okay?
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