Titanium Oxycarbide as Platinum-Free Electrocatalyst for Ethanol Oxidation

The compound material titanium oxycarbide (TiOC) is found to be an effective electrocatalyst for the electrochemical oxidation of ethanol to CO2. The complete course of this reaction is one of the main challenges in direct ethanol fuel cells (DEFCs). While TiOC has previously been investigated as catalyst support material only, in this study we show that TiOC alone is able to oxidize ethanol to acetaldehyde without the need of expensive noble metal catalysts like Pt. It is suggested that this behavior is attributed to the presence of both undercoordinated sites, which allow ethanol to adsorb, and oxygenated sites, which facilitate the activation of water. This is a milestone in DEFC research and development and opens up innovative possibilities for the design of catalyst materials for intermediate temperature fuel cells.

for 30 minutes.All the electrolytes were prepared using high purity 18 MΩ deionized (DI) water (Milli-Q, 18.2 MΩ, Millipore, Merck).Differential Electrochemical Mass Spectrometry (DEMS): DEMS measurements were performed using a Hiden HPR-40 mass spectrometer in an online configuration with the electrochemical cell.In this configuration, the inlet of the capillary was covered by a Teflon membrane (Gore-Tex, 75 µm thick, 50% porosity, 0.02 mm pore diameter) and was placed through a hole on the back side of the GC. 5 mL of the powder ink were deposited on the GC and the membrane.The GC (equipped with the capillary, the membrane and the powder ink) was used as the working electrode (WE) in a hanging meniscus configuration. 2 The applied electron energy for ionization of all species was 70 eV and the used emission current was 500 mA.The secondary electron multiplier (SEM) energy was set to 850 V for detection of all species.Current transient measurements were recorded at 0.60, 0.70, 0.80, 0.85 and 0.90 V for 900 s each.

Transmission Electron Microscopy (TEM):
For all TEM analyses, an FEI Tecnai F20 S-TWIN highresolution analytical transmission electron microscope operated at 200 kV was used.An EDAX Apollo XLT2 silicon-drifted detector was employed to collect the energy-dispersive X-ray (EDX) spectra of the Ti K-, O K-and C K-edges.All samples were supported on holey carbon films without further treatment.

Subtractively Normalized Interfacial Fourier Transform Infrared Spectroscopy (SNIFTIRS):
The SNIFTIRS studies were carried out using a VERTEX 70v spectrometer (Bruker) with an additional external chamber (XSA, Bruker), equipped with a liquid N2 cooled mercury cadmium telluride (MCT) photodetector.A thin-layer cell configuration was employed to minimize the contribution of the electrolyte solution.The IR beam entered the spectro-electrochemical cell through a linear polarizer (Edmund Optics) and with the use of a movable gold-coated mirror.After passing a CaF2 hemisphere constituting the bottom of the homemade spectro-electrochemical cell, the IR beam was reflected from the surface of the WE that was pressed against the hemisphere and entered the detector with the use of a second gold-coated mirror.The cell was equipped with the TiOC thin film WE, the above-mentioned carbon rod CE and the Hg/Hg2SO4 (0.1M H2SO4) RE.The strong absorption of the thin electrolyte film was accounted for by subtracting and normalizing the single spectra recorded at a specific applied potential (ES) with the single spectrum at a specific reference potential (ER = 0.05 VSHE), at which no reaction should occur, according to equation 1: eq. 1.
with R(ES) being the reflectance sample single spectra and R(ER) being the reflectance reference single spectra.After normalization, each upward or downward facing band corresponds to consumed/disappeared or formed/accumulated species at the electrode surface, respectively.SNIFTIR     formation takes place at potentials as early as 0.6 VSHE (middle panel), whereas significant deviation of the signal related to CO2 formation from the background (bottom panel) is only well observed at 0.9 V.

Results and Discussion
At these high anodic potentials, oxidation of the TiOC takes place.This strongly indicates that the TiOC catalysts oxidizes EtOH towards acetaldehyde, however the further oxidation seems to be inhibited as only small amounts of CO2 (close to the detection limit of the instrument, compare Figure 2, bottom panel) are formed.After the electrochemical measurements, an XPS analysis was conducted to investigate the surface chemistry of the thin film electrode.In both the Ti 2p and C 1s regions, distinctive features associated with TiC are detected.In the Ti 2p region, aside from the contribution of Ti 4+ (i.e.TiO2), there are observable contributions from further reduced (suboxide) Ti species (Ti 3+ and Ti 2+ ).This strongly points to the existence of substoichiometric TiO2-x and to the presence of oxygen vacancies at the electrode surface.
Furthermore, the presence of TiC after the electrochemical experiments provides compelling evidence that the material remains stable under operation conditions.This aligns seamlessly with findings in a previous publication (reference 1 in the main manuscript).The Raman spectrum of the TiOC powder reveals noteworthy characteristics.Specifically, the Eg(1) peak exhibits a maximum around 151 cm -1 , showing a noticeable shift towards higher wavenumbers when compared to pure TiO2 (anatase), where the peak is typically around 145 cm -1 . 5The Eg(2) peak at approximately 202 cm -1 shows a slight blue shift towards higher wavenumbers . 5The peak maxima of the B1g(1), B1g(2)+A1g, and B1g(3) modes are slightly lower in comparison to pure TiO2 (anatase). 5at is especially intriguing is the resemblance of the Raman spectrum to the spectra of TiO2-x anatase, as previously presented in ref. 6 .This earlier work emphasized the significance of oxygen vacancies in relation to charge storage in sodium-ion batteries.This correspondence strongly suggests the presence of oxygen vacancies in the TiOC powder, which is well in line with the XPS data as depicted in Figure S5.The D and G bands are related to carbon.

Figure
FigureS1shows blank potentiodynamic DEMS measurements (CV and the corresponding mass spectrometric cyclo-voltammograms (MSCVs)) of the TiOC powder ink electrodes in 0.5 M H2SO4 without EtOH.Apart from the CO2 evolution starting at potentials lager than 0.9 VSHE, 1 visible in the ionic DEMS signal at a mass to charge (m/z) ratio of 44 and 22 (CO2 ++ ), respectively (FigureS1 b,c), no product formation could be observed.3

Figure S2 :
Figure S2: Exemplary onset determination for the DEMS signals with a m/z ratio of 29.

Figure S3 :
Figure S3: a) Blank CV measurement of a TiOC powder ink between 0.03 and 1.1 V, followed by b) three CO stripping sweeps at 2 mV s -1 , recorded in 0.5 M H2SO4.

Figure S4 :
Figure S4: (a) Current transients and corresponding mass spectrometric ionic current signals for (b) m/z = 29 and (c) m/z = 22 of TiOC powder ink recorded in 0.5 M H2SO4 and 0.1 M EtOH.For further evaluation of the CO2 conversion efficiency at TiOC, and to exclude any kinetic issues due to potential cycling, current transients of the EOR are recorded after stepping the potential from an initial value of 0.05 V to the anodic potentials of interest that ranged from 0.6 VSHE to 0.9 VSHE.The corresponding DEMS signals for m/z ratio of 29 and 22 are simultaneously recorded and shown for the relevant potentials in Figure S4.From this one can directly observe that significant acetaldehyde

Figure S5 :
Figure S5: High-resolution XP spectra of the a) Ti 2p and b) C 1s region, respectively.Spectra of an aged TiOC electrode after SNIFTIRS measurements (Figure 3, main text) are recorded.Fitting of the Ti 2p region was performed according to reference 4 .

Figure S6 :
Figure S6: Raman spectra of the TiOC powder catalyst.The vertical red lines mark the wavenumbers of the peak maxima with their assignments.