A Novel Photoelectrochemical Dems Thin Layer Flow Cell for the Online Product Analysis in C-1 Molecule Oxidation and Water Splitting
Authors: Jusys Zenonas , Reichert Robert , Behm Rolf Jürgen 
Published: February 2020

Light-driven electrocatalysis (photoelectrocatalysis) and related photoelectrochemical reactions have attracted increasing attention, after the pioneering work by Fujishima and Honda in the early 1970s [1], as a promising way for the production of sustainable fuels. The detailed understanding of these reactions, on a molecular scale, was often hampered, however, by the lack of chemical product information. One possibility of obtaining such kind of product information would involve a combination of photoelectrochemistry and online mass spectrometry (DEMS). A previous attempt performed in a stagnant electrolyte [2] indicated, however, significant mass transport limitations.
In this contribution we present a novel rectangular thin-layer photoelectrochemical flow cell, which allows for simultaneous photoelectrochemical measurements and mass-spectrometric detection of gaseous products [3]. Its performance is demonstrated for two applications, the photoelectrochemical O2 evolution (water splitting) on WO3 electrodes in the presence of organic molecules (mostly C1 molecules) serving as sacrificial agent, and the photoelectrochemical water splitting (H2 evolution) on Au/TiO2 model catalysts.
For these measurements, the electrolyte outlet of the photoelectrochemical thin layer flow cell was connected via a capillary to a separate thin layer flow cell unit, equipped with a porous Teflon membrane, which served as interface to the mass spectrometer for the online analysis of the gaseous products formed at the photo-electrode. A 200 W power Hg(Xe) arc light source (LOT-QD) equipped with a home-made water filter, a neutral density filter, an UV irradiation transmitting filter, and a focusing lens served as light source. A quadrupole mass spectrometer (Pfeiffer Vacuum, QMS 422) was used for online monitoring of the gaseous products, and potential control was achieved by a AFRDE 5 (Pine Instruments) potentiostat.
In the first reaction, the reaction products CO2 and O2 were monitored and quantified online [4]. The current efficiencies for CO2 (CCE) and oxygen (OCE) formation in the photoelectrochemical oxidation of formic acid, formaldehyde and methanol were determined as a function of bias potential over a tungsten oxide photoanode [4]. This was fabricated as circular thin metal oxide film electrodeposited on a conductive fluorine-doped tin oxide covered glass slice, and illuminated via a quartz window in the photo-electrochemical flow cell, which was positioned directly in front of the photoanode.
In the photoelectrochemical oxidation of water on the WO3 electrode in pure supporting electrolyte (0.5 M H2SO4), the OCE is only around 10% at low bias potentials, while at higher bias values O2 evolution contributes less than 50% to the overall photocurrent [4]. The rest is mainly due to hydrogen peroxide formation [4]. In the presence of formic acid the OCE drops to ca. 10%, and with formaldehyde it becomes negligible. Hence, C1 molecule oxidation and water oxidation compete for holes in the photoassisted reaction. The CCE for formic acid oxidation as the most simple reactant is ca. 90%, only the rest of the photocurrent results from water oxidation. For formaldehyde and methanol oxidation the CCE values reach at maximum only ca. 12 and 2%, respectively [4]. In combination with their low OCE values this indicate that their incomplete oxidation prevails [5].
Second, the photocatalytic hydrogen formation over photo-deposited Au nanoparticles on TiO2 was studied as a function of Au loading and illumination intensity [6]. H2 formation on Au/TiO2 photocatalysts under open-circuit conditions (measurable H2 evolution rates could only be detected in the presence of methanol) shows that even a small loading of gold particles (0.17 wt.%) increases the Au mass normalized photocatalytic H2 evolution rate compared to bare TiO2 catalyst. With increasing Au particle loading up to 9.4 wt.%, however, the Au mass normalized H2 formation rate gradually decreases [6]. Furthermore, with increasing light intensity, the H2 formation rate on a 0.17 wt.% Au/TiO2 catalyst gradually increases, in parallel to a simultaneous shift of the open-circuit potential to more negative values [6].
Further developments of the photoelectrochemical flow cell, aiming at simultaneous in situ IR spectroscopy in an ATR configuration, will be discussed.
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