Electricity-to-chemical efficiency, namely, Faradaic efficiency, was evaluated for CO2/steam electrolysis using Faraday’s law. To realize this, gas chromatography was conducted to analyze the SOEC cathode downstream gas compositions in various conditions.
2.6.1 Gas chromatography (GC)
Gas chromatography is a technique to separate chemicals in a complex sample. Fig. 2.17 shows the schematic diagram of a typical gas chromatograph, which consists of an injection port, a flowing mobile phase known as carrier gas, a column containing the stationary phase inside, a column oven, a detector, and a data recording system. The carrier gas serves to carry the molecules of sample through separation column, and is generally inert gases such as helium, argon, or nitrogen. The separation column is usually placed in a thermostat-controlled oven due to the fact that the sample mixture partitioning process is dependent on temperature. The detector and recorder located at the end of column to output the separated results electronically.
Fig. 2.17Schematic graph of a typical gas chromatograph[22]
In a gas chromatograph, fundamentally, a sample mixture is injected and vaporized, if a liquid, at the entrance of column, then the sample mixture is carried through the column by a carrier gas. Adsorption and partitioning of sample mixture occur in the column, and components in the sample mixture exit from the end of column at different time
Chapter 2: Methods and techniques
their interaction with the column filling, i.e. the stationary phase. Factors influencing the retention time also include the flow rate of carrier gas, column length and temperature.
In our experiment, an Agilent 3000 Micro gas chromatograph (instrument series number: US10713003), which was equipped with an injection port, was employed to analyze the cathode outlet gas compositions. At a certain external load, the cathode downstream gas was connected to GC. Ar and He were respectively used as carrier gas for module A and
B in the operation of instrument. Both modules used 1.0μl backflush injector for sample
injection, with Mol-sieve 5A and PLOTU, adopted as columns in module A and B respectively. Different gases were detected by thermal conductivity detectors (TCD) and distinguished according to their fingerprint retention time in the chromatography. The concentrations, i.e. the volume percentages of each gas in the cathode outlet gas mixture were proportional to the GC peak areas, which were calibrated regularly using mass flow controllers.
2.6.2 Calculation of electrolysis efficiency
Faradaic efficiency was calculated to assess the CO2/H2O electrolysis efficiency. At a
certain current, the Faradaic efficiency η for, e.g. CO yield thus could be described as: η= ୡ୲୳ୟ୪େ ୮୰୭ୢ୳ୡ୲୧୭୬ ୰ୟ୲ୣୢୣ୲ୣ୰୫ ୧୬ୣୢ ୠ୷ୋେ ୟ୬ୟ୪୷ୱ୧ୱ
୲୦ୣ୭୰ୣ୲୧ୡୟ୪େ ୷୧ୣ୪ୢ ୡୟ୪ୡ୳୪ୟ୲ୣୢ ୟୡୡ୭୰ୢ୧୬୲୭ ୟ୰ୟୢୟ୷′ୱ୪ୟ୵
The actual CO production rate was calculated from the total flow rate and the variation in volume percentage of CO in the condition between a certain load and OCV, based on GC peak analysis. The theoretical CO yield was calculated using the current at the loading potential, according to Faraday’s law. The electric efficiency, i.e. Faradaic efficiency of CO production was evaluated in different conditions, for instance, various CO2/CO ratios, operation temperatures, and cathode materials. In some cases, 5% H2/Ar was introduced to study its impact on the CO2electrochemical reduction process as well as on the Faradaic efficiency of CO2electrolysis.
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