Time Evolution Characterization of Atmospheric-Pressure Plasma Jet (APPJ)-Synthesized Pt-SnO x Catalysts

: We characterize the time evolution ( ≤ 120 s) of atmospheric-pressure plasma jet (APPJ)-synthesized Pt-SnO x catalysts. A mixture precursor solution consisting of chloroplatinic acid and tin(II) chloride is spin-coated on ﬂuorine-doped tin oxide (FTO) glass substrates, following which APPJ is used for converting the spin-coated precursors. X-ray photoelectron spectroscopy (XPS) indicates the conversion of a large portion of metallic Pt and a small portion of metallic Sn (most Sn is in oxidation states) from the precursors with 120 s APPJ processing. The dye-sensitized solar cell (DSSC) efﬁciency with APPJ-synthesized Pt-SnO x CEs is improved greatly with only 5 s of APPJ processing. Electrochemical impedance spectroscopy (EIS) and Tafel experiments conﬁrm the catalytic activities of Pt-SnO x catalysts. The DSSC performance can be improved with a short APPJ processing time, suggesting that a DC-pulse nitrogen APPJ can be an efﬁcient tool for rapidly synthesizing catalytic Pt-SnO x counter electrodes (CEs) for DSSCs.


Introduction
Atmospheric-pressure plasma (APP) technology is operated without using a vacuum chamber and associated pumping system. It is therefore considered a cost-effective manufacturing tool. Recent developments have resolved stability and arcing problems, making APP technology promising for industrial applications. Traditional APP sources include transferred arc, corona discharge, dielectric barrier discharge, and atmospheric pressure plasma jet (APPJs) [1,2]. APPs with various heavy particle temperatures and charge densities can be produced by using different excitation methods and electrode configuration designs. The synergy between the reactive plasma species and heat can promote rapid chemical reactions during material processing [3][4][5][6]. APPs have been used for processing various types of materials, such as carbon nanotubes [3,7,8] and reduced graphene oxides [9][10][11]. Applications of APPs for surface cleaning or modification [12][13][14], deposition of metal oxides [15,16], and syntheses of metal compounds from liquid precursors [6,17,18] have been extensively investigated.

Preparation of Pt-SnO x CEs
25-mM chloroplatinic acid (H 2 PtCl 6 ) (purity: 99.95%, Uniregion Biotech, Taipei, Taiwan) and 25-mM tin(II) chloride (SnCl 2 ) isopropanol solutions were separately stirred for 24 h. These two solutions were mixed with the same volume ratios and were stirred using a magnetic stirrer (PC-420D, Corning Inc., Corning, NY, USA)for another 24 h. Next, 60 µL of the mixture precursor was spin-coated onto fluorine-doped tin oxide (FTO) substrates with an area of 1.5 cm × 1.5 cm at a speed of 1000 rpm for 15 s. The spin-coated precursors were then processed by a nitrogen APPJ for 5, 15, 30, 60, and 120 s. Figure 1a shows the APPJ setup. The operation parameters are as follows: nitrogen flow of 46 standard liter per minute (slm), power supply voltage of 275 V, and ON/OFF duty cycle of 7/33 µs. The temperature evolution of the substrates, shown in Figure 1b, was measured using a K-type thermocouple (OMEGA Engineering, Norwalk, CT, USA). The temperature rapidly increased tõ 510 • C, and it dramatically decreased after the APPJ was turned off. Because our process is conducted at~510 • C, we use FTO glass substrates (Sigma-Aldrich, St. Louis, MO, USA) which can tolerate a higher processing temperature.  [6,17,18] have been extensively investigated. Metals and metal oxides are common catalysts [19][20][21][22][23][24]. APPs also have been used for syntheses and post-treatments of catalysts [25]. In 1991, Grätzel et al. reported a great breakthrough of DSSCs [26], and since then, dye-sensitized solar cells (DSSCs) have been extensively investigated. A conventional DSSC consists of a dye-adsorbed photoanode, an electrolyte, and a counter electrode (CE). A catalytic CE is used for reducing triiodide into iodine in the electrolyte. Generally, Pt is the most commonly used CE material in DSSC, owing to its high catalytic activity and stability [27]. Various alternative CE materials such as carbon-based materials, metal oxides or chalcogenides, and alloys or intermetallics have been studied extensively [3,5,[28][29][30][31][32][33][34][35][36]. Composites containing Pt and Sn have been used as electrocatalysts for CEs of DSSCs [36,37], methanol or ethanol oxidation [38][39][40][41][42][43][44], aqueous phase oxidation [45], and gas sensing [46]. The addition of metal oxides has been reported to improve the catalytic activity [40,47]. Pt:SnO2 electrocatalytic films were used as CEs of DSSCs [48]. Dao et al. fabricated DSSCs with a PtSn alloy supported by reduced graphene oxides via dry plasma reduction [36]. In the present study, Pt-SnOx composites were synthesized by mixing chloroplatinic acid and tin(II) chloride that were processed using a DC-pulse nitrogen APPJ. X-ray photoelectron spectroscopy (XPS) results showed that the majority of Sn was in the oxidation state. The DSSC efficiency can be improved rapidly through 5 s APPJ processing of the chloroplatinic acid and tin(II) chloride mixture precursor; no metallic Pt was converted within such a short processing time. This suggests the catalytic effect of oxidized Pt and Sn compounds. A DSSC with a 120 s APPJ-processed Pt-SnOx CE shows efficiency comparable to that of a cell with a furnace-processed Pt CE.

Preparation of Pt-SnOx CEs
25-mM chloroplatinic acid (H2PtCl6) (purity: 99.95%, Uniregion Biotech, Taipei, Taiwan) and 25-mM tin(II) chloride (SnCl2) isopropanol solutions were separately stirred for 24 h. These two solutions were mixed with the same volume ratios and were stirred using a magnetic stirrer (PC-420D, Corning Inc., Corning, NY, USA)for another 24 h. Next, 60 μL of the mixture precursor was spin-coated onto fluorine-doped tin oxide (FTO) substrates with an area of 1.5 cm × 1.5 cm at a speed of 1000 rpm for 15 s. The spin-coated precursors were then processed by a nitrogen APPJ for 5, 15, 30, 60, and 120 s. Figure 1a shows the APPJ setup. The operation parameters are as follows: nitrogen flow of 46 standard liter per minute (slm), power supply voltage of 275 V, and ON/OFF duty cycle of 7/33 μs. The temperature evolution of the substrates, shown in Figure 1b, was measured using a K-type thermocouple (OMEGA Engineering, Norwalk, CT, USA). The temperature rapidly increased to ~510 °C, and it dramatically decreased after the APPJ was turned off. Because our process is conducted at ~510 °C, we use FTO glass substrates (Sigma-Aldrich, St. Louis, MO, USA) which can tolerate a higher processing temperature.

Preparation of TiO 2 Photoanode and Assembly of DSSCs
The photoanode consists of a TiO 2 compact layer and a TiO 2 nanoporous layer for dye adsorption. First, a 0.23-M titanium isopropoxide solution (Fluka, St. Louis, MO, USA) was spin-coated on a FTO substrate and then baked at 200 • C for 10 min to form a TiO 2 compact layer to prevent electron recombination. Then, 1.6 g of TiO 2 nanoparticles (diameter:~21 nm), 8 mL of ethanol, 6.49 g of terpineol (anhydrous, #86480, Fluka, St. Louis, MO, USA), 4.5 g of 10 wt % ethyl cellulose ethanolic solution (5-15 mPa·s, #46070, Fluka, St. Louis, MO, USA), and 3.5 g of 10 wt % ethyl cellulose ethanolic solution (30-50 mPa·s, #46080, Fluka, St. Louis, MO, USA) were mixed together. Next, a 0.4 g mixture containing TiO 2 was mixed with 500 µL of ethanol and stirred using a magnetic stirrer for 24 h. The mixed solution was baked at 53 • C until its weight became 0.175 g, thus completing the preparation of the TiO 2 pastes. The TiO 2 pastes were screen-printed onto the TiO 2 compact layer coated FTO substrate with a printed area of 0.5 cm × 0.5 cm. The screen-printed pastes were calcined at 510 • C for 15 min in a conventional furnace to form the TiO 2 photoanode. Next, the TiO 2 photoanode was immersed in a 0.3-mM N719 solution, which is mixed with acetonitrile and tertbutyl alcohol in a 1:1 volume ratio for 24 h. This completed the preparation of the dye-anchored nanoporous TiO 2 photoanodes.
The Pt-SnO x CEs and dye-anchored TiO 2 photoanodes were assembled with a 25-µm-thick spacer to form sandwich-structure DSSCs. Then, a commercial electrolyte (E-Solar EL 200, Everlight Chemical Industrial Co., Taipei, Taiwan) was injected into the solar cells.
Counterpart DSSC with furnace-processed Pt CE was fabricated for comparison. In this case, 60 µL of 25-mM H 2 PtCl 6 isopropanol solution was spin-coated on the FTO substrate and calcined at 400 • C for 15 min using a tube furnace. The assembly procedure of DSSC with furnace-processed Pt CE is the same as that of DSSC with APPJ-processed Pt-SnO x CE.

Characterization of Materials and DSSCs
During the APPJ reduction processes, a spectrometer (USB4000, Ocean Optics, Largo, FL, USA) was used for monitoring the plasma optical emission spectra (OES). Pt-SnO x nanoparticles were inspected using a scanning electron microscope (SEM, JSM-7800F Prime, JEOL, Tokyo, Japan) with an energy-dispersive spectroscopy (EDS) attachment. To investigate the chemical configuration of Pt-SnO x , XPS (Thermo K-Alpha, VGS, Waltham, MA, USA was used for analyzing the binding status. The C1s core level was centered at 284.6 eV to calibrate the binding energy scale. XPSPEAK 4.1 software (was used for fitting binding energy positions. XPS samples were prepared with Corning glass substrates instead of FTO glass ones to avoid the interference of Sn signals emitted from FTO substrates. To examine the electrochemical catalytic activities of Pt-SnO x CEs, electrochemical impedance spectroscopy (EIS) and Tafel measurements were performed using an electrochemical workstation (PGSTAT204, Metrohm Autolab, Herisau, Switzerland). EIS measurements were performed with a sinusoidal amplitude of 10 mV with frequencies of 0.1-10 5 Hz, and the data were fitted using Z-view 3.1 software. Tafel curves were recorded from −0.6 V to 0.6 V at a scan rate of 50 mV/s. Both measurements were performed on a symmetrical cell with two equal Pt-SnO x CEs. A solar simulator (WXS-155S-L2, WACOM, Saitama, Japan) with an AM 1.5 filter equipped with an electrometer (Keithley 2440, Tektronix, Beaverton, OR, USA) was used for measuring the photocurrent-voltage characteristics of the DSSCs. Figure 2a shows the plasma OES evolution during APPJ processing of the mixed H 2 PtCl 6 /SnCl 2 precursor. NO γ , NO β , N 2 1st positive, and N 2 2nd positive emissions were observed clearly during 120 s APPJ processes. Figure 2b shows the plasma spectra when processing H 2 PtCl 6 , SnCl 2 , and mixed H 2 PtCl 6 /SnCl 2 precursors on the FTO substrates. The NO γ system (A 2 Σ + -X 2 Π) is located at wavelengths lower than 280 nm. The NO β system (B 2 Π-X 2 Π) is located from around 260 to 500 nm, and it partially overlaps the NO γ system. The other emissions at 357, 385, and 389 nm were attributed to the N 2 2nd positive system (C 3 Π u -B 3 Π g ); these overlap with the NO β system. The N 2 1st positive system (B 3 Π g -A 3 Σ u + ) was located at wavelengths higher than 530 nm. attributed to the N2 2nd positive system (C 3 Πu-B 3 Πg); these overlap with the NOβ system. The N2 1st positive system (B 3 Πg-A 3 Σu + ) was located at wavelengths higher than 530 nm.   Figure 3a-e shows the SEM images of Pt-SnO x nanoparticles converted from mixed H 2 PtCl 6 /SnCl 2 precursors on the FTO glass substrates using various APPJ processing times. The nanoparticle size and morphology remained similar for APPJ processing times of 5-120 s. Figure 3f shows EDS results for the 120 s and APPJ-processed sample. Pt and Sn signals indicate the presence of two elements in the nanoparticles. Both of Sn and O signals could result from the nanoparticles and the FTO substrates. To identify the chemical states of Pt-SnOx compounds, Figure 4a,b shows the XPS spectra of Pt4f and Sn3d for samples. The Pt4f spectrum can be deconvoluted into three components including Pt, Pt 2+ , and Pt 4+ . The metallic peaks of Pt are located at 71.30 and 74.65 eV, Pt(II) components are located at 72.70 and 76.50 eV, and Pt(IV) components are located at 73.80 and 77.15 eV [49,50]. In Figure 4a, the major peaks belong to Pt 2+ and Pt 4+ for as-deposited and 5 and 15 s APPJ-processed samples. These results indicate that most of the H2PtCl6/SnCl2 precursor was not converted to metallic Pt by APPJ processing for less than 15 s. As the APPJ processing time increases, increased To identify the chemical states of Pt-SnO x compounds, Figure 4a,b shows the XPS spectra of Pt4f and Sn3d for samples. The Pt4f spectrum can be deconvoluted into three components including Pt, Pt 2+ , and Pt 4+ . The metallic peaks of Pt are located at 71.30 and 74.65 eV, Pt(II) components are located at 72.70 and 76.50 eV, and Pt(IV) components are located at 73.80 and 77.15 eV [49,50]. In Figure 4a, the major peaks belong to Pt 2+ and Pt 4+ for as-deposited and 5 and 15 s APPJ-processed samples. These results indicate that most of the H 2 PtCl 6 /SnCl 2 precursor was not converted to metallic Pt by APPJ processing for less than 15 s. As the APPJ processing time increases, increased conversion of precursors into metallic Pt was clearly observed. The Pt 2+ signal is noted as the oxidation state of Pt, and it could indicate PtO [51,52] or Pt(OH) 2 [53]. The presence of Pt oxidation states, due to the interaction with the Pt-support, is attributed to an electronic effect or oxygen absorption from air [54,55]. Figure 4b shows the oxidation state of Sn3d under various APPJ processing times. The binding energy of Sn3d can be deconvoluted into two categories: one at 485.8 and 494.2 eV for the zero-valent state of Sn, and the other at 487.3 and 495.7 eV for Sn(II/IV) components [56]. The major peak is attributed to the oxidation state of Sn for up to 120 s, and the percentage of metallic Sn increased only slightly increased with the APPJ processing time. Sn(II) and Sn(IV) species are difficult to distinguish from XPS measurements because of the small difference between their binding energies [57,58]. Tables 1 and 2 show the percentages of Pt and Sn species, respectively. The Pt-support interaction may influence charge transfer from Pt to oxygen species on the surface and improve the electrochemical catalytic abilities and catalyst stability [47]. To identify the chemical states of Pt-SnOx compounds, Figure 4a,b shows the XPS spectra of Pt4f and Sn3d for samples. The Pt4f spectrum can be deconvoluted into three components including Pt, Pt 2+ , and Pt 4+ . The metallic peaks of Pt are located at 71.30 and 74.65 eV, Pt(II) components are located at 72.70 and 76.50 eV, and Pt(IV) components are located at 73.80 and 77.15 eV [49,50]. In Figure 4a, the major peaks belong to Pt 2+ and Pt 4+ for as-deposited and 5 and 15 s APPJ-processed samples. These results indicate that most of the H2PtCl6/SnCl2 precursor was not converted to metallic Pt by APPJ processing for less than 15 s. As the APPJ processing time increases, increased conversion of precursors into metallic Pt was clearly observed. The Pt 2+ signal is noted as the oxidation state of Pt, and it could indicate PtO [51,52] or Pt(OH)2 [53]. The presence of Pt oxidation states, due to the interaction with the Pt-support, is attributed to an electronic effect or oxygen absorption from air [54,55]. Figure 4b shows the oxidation state of Sn3d under various APPJ processing times. The binding energy of Sn3d can be deconvoluted into two categories: one at 485.8 and 494.2 eV for the zero-valent state of Sn, and the other at 487.3 and 495.7 eV for Sn(II/IV) components [56]. The major peak is attributed to the oxidation state of Sn for up to 120 s, and the percentage of metallic Sn increased only slightly increased with the APPJ processing time. Sn(II) and Sn(IV) species are difficult to distinguish from XPS measurements because of the small difference between their binding energies [57,58]. Tables 1 and 2 show the percentages of Pt and Sn species, respectively. The Pt-support interaction may influence charge transfer from Pt to oxygen species on the surface and improve the electrochemical catalytic abilities and catalyst stability [47].      Figure 5a,b shows the EIS Nyquist and Bode phase plots to evaluate the catalytic activities of APPJ-processed Pt-SnO x CEs. The inset of Figure 5a shows the equivalent circuit for Nyquist curve fitting [59]. The series resistance (R s ) and charge-transfer resistance (R ct ) can be described as the resistance of substrates and the catalytic effect of the electrode-reducing triiodide ions, respectively. R s can be obtained from the high-frequency intercept on the real axis and R ct , from the radius of the real semi-circle [60]. Table 3 shows the EIS parameters including R s , R ct , and constant phase element (CPE1) [29]. A higher catalytic effect and lower charge-transfer resistance would improve the DSSC performance. For all cases, R s of Pt-SnO x CEs remained similar. R ct generally decreased (i.e., semi-circle became smaller) as the APPJ processing time increased, indicating that APPJ processing can enhance the catalytic activity. R ct was comparable for APPJ processing times of 60 s (4.72 Ω) and 120 s (4.69 Ω). Lower R ct results in a higher electrocatalytic activity at the interface between the CEs and the electrolytes [61]. CPE1, which represents the interfacial capacitance between the electrode and the electrolyte, is also a good indicator of the surface activity of CEs [62][63][64]. The 120 s APPJ-processed CEs had a higher CPE1-T (105.5 µF/cm 2 ), indicating larger surface reaction between the CE and the electrolyte. Bode phase plots show the electron lifetime for recombination in devices; the electron lifetime is expressed as τ e = 1/(2πf peak ), where f peak is the frequency of the highest peak. Shorter electron lifetime indicates faster charge transfer at the interface between the CE and the electrolyte [64,65]. In Figure 5b, the trend of the electron lifetime follows the EIS results. The 5 s APPJ-processed CE has the smallest peak frequency, indicating the largest electron lifetime with slower charge transfer. Furthermore, electron lifetimes are comparable in 60 s and 120 s APPJ-processed CEs, and this is consistent with the results for R ct .

Results and Discussion
To further clarify the catalytic activities of Pt-SnO x CEs, Tafel polarization experiments were conducted and the results are shown in Figure 6. The exchange current density (J 0 ) was measured by the intercept of the Y-axis (zero voltage) from the tangential line of the curve [66,67]. The 120 s APPJ-processed CEs had a large J 0 , indicating better electrocatalyic activity and lower charge-transfer resistance at the interface of the CE and the electrolyte. Table 3 shows that J 0 increases with the APPJ processing time. APPJ processes enhanced the triiodide reduction reaction [60]. The exchange current density is also proportional to R ct obtained from the EIS measurement. It can be described as J 0 = RT/nFR ct , where R is a gas constant; T is temperature; n is the number of electrons involved in the redox reaction; and F is the Faraday's constant [68]. EIS and Tafel measurements both indicate that APPJ-processed Pt-SnO x elecrodes show suitable catalytic performance for use as the CEs of DSSCs. J 0 increases with the APPJ processing time, indicating that APPJ processing can enhance the catalyst activity of Pt-SnO x . APPJ-processed CEs had a higher CPE1-T (105.5 μF/cm 2 ), indicating larger surface reaction between the CE and the electrolyte. Bode phase plots show the electron lifetime for recombination in devices; the electron lifetime is expressed as τe = 1/(2πfpeak), where fpeak is the frequency of the highest peak. Shorter electron lifetime indicates faster charge transfer at the interface between the CE and the electrolyte [64,65]. In Figure 5b, the trend of the electron lifetime follows the EIS results. The 5 s APPJ-processed CE has the smallest peak frequency, indicating the largest electron lifetime with slower charge transfer. Furthermore, electron lifetimes are comparable in 60 s and 120 s APPJ-processed CEs, and this is consistent with the results for Rct.    To further clarify the catalytic activities of Pt-SnOx CEs, Tafel polarization experiments were conducted and the results are shown in Figure 6. The exchange current density (J0) was measured by the intercept of the Y-axis (zero voltage) from the tangential line of the curve [66,67]. The 120 s APPJ-processed CEs had a large J0, indicating better electrocatalyic activity and lower charge-transfer resistance at the interface of the CE and the electrolyte. Table 3 shows that J0 increases with the APPJ processing time. APPJ processes enhanced the triiodide reduction reaction [60]. The exchange current density is also proportional to Rct obtained from the EIS measurement. It can be described as J0 = RT/nFRct, where R is a gas constant; T is temperature; n is the number of electrons involved in the redox reaction; and F is the Faraday's constant [68]. EIS and Tafel measurements both indicate that APPJ-processed Pt-SnOx elecrodes show suitable catalytic performance for use as the CEs of DSSCs. J0 increases with the APPJ processing time, indicating that APPJ processing can enhance the catalyst activity of Pt-SnOx.       Table 4 shows the photovoltaic parameters, including the open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF), and efficiency (EFF) with their standard deviations. The power conversion efficiencies (PCEs) of DSSCs with 5 s and 15 s APPJ-processed Pt-SnOx CEs are 3.87 ± 0.58% and 3.86 ± 0.28%, respectively, indicating that APPJ processing for a short duration can improve the DSSC performance. XPS results show that almost no metallic Pt was converted with 5 s and 15 s APPJ processing, indicating the catalytic effect of oxidized Pt and Sn compound CEs in DSSCs, and this agrees with previous reported findings [30,32]. As the APPJ treatment time increases, the PCE of DSSCs with 30 s, 60 s, and 120 s APPJ-processed CEs reaches 4.01 ± 0.34%, 4.20 ± 0.41%, and 4.46 ± 0.29%, respectively. The performance of DSSC with a 120 s APPJ-processed Pt-SnOx CE was comparable to that with a conventional furnace-processed Pt CE (4.42 ± 0.26%). Figure 8 shows the statistics of the DSSC parameters. APPJ processing gradually increased the FFs and PCEs of DSSCs, consistent with the results obtained from EIS and Tafel measurement. The improved FF and efficiency with APPJ processing time could result from the better conversion of metallic Pt from the precursor solution.   Table 4 shows the photovoltaic parameters, including the open-circuit voltage (V oc ), short-circuit current (J sc ), fill factor (FF), and efficiency (EFF) with their standard deviations. The power conversion efficiencies (PCEs) of DSSCs with 5 s and 15 s APPJ-processed Pt-SnO x CEs are 3.87 ± 0.58% and 3.86 ± 0.28%, respectively, indicating that APPJ processing for a short duration can improve the DSSC performance. XPS results show that almost no metallic Pt was converted with 5 s and 15 s APPJ processing, indicating the catalytic effect of oxidized Pt and Sn compound CEs in DSSCs, and this agrees with previous reported findings [30,32]. As the APPJ treatment time increases, the PCE of DSSCs with 30 s, 60 s, and 120 s APPJ-processed CEs reaches 4.01 ± 0.34%, 4.20 ± 0.41%, and 4.46 ± 0.29%, respectively. The performance of DSSC with a 120 s APPJ-processed Pt-SnO x CE was comparable to that with a conventional furnace-processed Pt CE (4.42 ± 0.26%). Figure 8 shows the statistics of the DSSC parameters. APPJ processing gradually increased the FFs and PCEs of DSSCs, consistent with the results obtained from EIS and Tafel measurement. The improved FF and efficiency with APPJ processing time could result from the better conversion of metallic Pt from the precursor solution.
performance. XPS results show that almost no metallic Pt was converted with 5 s and 15 s APPJ processing, indicating the catalytic effect of oxidized Pt and Sn compound CEs in DSSCs, and this agrees with previous reported findings [30,32]. As the APPJ treatment time increases, the PCE of DSSCs with 30 s, 60 s, and 120 s APPJ-processed CEs reaches 4.01 ± 0.34%, 4.20 ± 0.41%, and 4.46 ± 0.29%, respectively. The performance of DSSC with a 120 s APPJ-processed Pt-SnOx CE was comparable to that with a conventional furnace-processed Pt CE (4.42 ± 0.26%). Figure 8 shows the statistics of the DSSC parameters. APPJ processing gradually increased the FFs and PCEs of DSSCs, consistent with the results obtained from EIS and Tafel measurement. The improved FF and efficiency with APPJ processing time could result from the better conversion of metallic Pt from the precursor solution.

Conclusions
We analyze the time evolution of Pt-SnOx nanoparticle catalysts that are converted from a mixture of chloroplatinic acid and tin(II) chloride using DC-pulse nitrogen APPJ. XPS analyses indicate the conversion of a large portion of the metallic Pt and tin oxide. EIS and Tafel