Ag-deposited Ti gas diffusion electrode in proton exchange membrane CO2 electrolyzer for CO production
Journal Pre-proof Ag-deposited Ti Gas Diffusion Electrode in Proton Exchange Membrane CO2 Electrolyzer for CO Production Seonhwa Oh, Young Sang Park, ...
Journal Pre-proof Ag-deposited Ti Gas Diffusion Electrode in Proton Exchange Membrane CO2 Electrolyzer for CO Production Seonhwa Oh, Young Sang Park, Hyanjoo Park, Hoyoung Kim, Jong Hyun Jang, Insoo Choi, Soo-Kil Kim
PII:
S1226-086X(19)30579-9
DOI:
https://doi.org/10.1016/j.jiec.2019.11.001
Reference:
JIEC 4843
To appear in:
Journal of Industrial and Engineering Chemistry
Received Date:
6 August 2019
Revised Date:
26 October 2019
Accepted Date:
1 November 2019
Please cite this article as: Oh S, Park YS, Park H, Kim H, Jang JH, Choi I, Kim S-Kil, Ag-deposited Ti Gas Diffusion Electrode in Proton Exchange Membrane CO2 Electrolyzer for CO Production, Journal of Industrial and Engineering Chemistry (2019), doi: https://doi.org/10.1016/j.jiec.2019.11.001
Ag-deposited Ti Gas Diffusion Electrode in Proton Exchange Membrane CO2 Electrolyzer for CO
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Production
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Seonhwa Oha, Young Sang Parka,b, Hyanjoo Parka, Hoyoung Kima, Jong Hyun Jangb,c,*, Insoo
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Choid,*, and Soo-Kil Kima,*
School of Integrative Engineering, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul
Center for Hydrogen·Fuel Cell Research, Korea Institute of Science and Technology (KIST), 5
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b
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06974, Republic of Korea
Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea c
Division of Energy & Environment Technology, KIST school, University of Science and
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Technology, Seoul 02792, Republic of Korea
Division of Energy Engineering, Kangwon National University, 346 Jungang-ro, Samcheok,
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25913, Republic of Korea
*Correspondence should be addressed to S.-K. Kim (email: [email protected], Tel.: +82-8205770, Fax: +82-2-813-8159), I. Choi (email: [email protected], Tel.: +82-33-570-6315, Fax: +82-33-570-6319), and J. H. Jang (email: [email protected], Tel.: +82-2-958-5287)
Graphical abstract
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Abstract
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A gas diffusion electrode composed of Ag catalysts/Ti substrate was prepared for CO2 electrolyzer. Pre-treatments on Ti gas diffusion layer affected the properties of electrodeposited Ag catalysts. HCl-H2SO4 pre-treatment resulted in the highest CO production efficiency in PEM-based MEA-type electrolyzer.
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Highlights
Electrochemical CO2-to-CO conversion is techno-economically effective for utilizing CO2. Although numerous studies are available on CO2 conversion catalysts, many of them are limited
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to a half-cell or conventional H-type apparatus in aqueous mediums, providing insufficient CO2 feeding. In this study, as a part of pioneering works on gas-feeding reactors, a gas diffusion electrode consisting of a Ti substrate with affixed Ag electrocatalysts was suggested; this enables the mass conversion of CO2 via direct feeding of CO2. Herein, Ag catalysts were electrodeposited on a Ti gas diffusion layer for a proton exchange membrane-based CO2 electrolyzer. Pre-treatment
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of the Ti crucially influenced the deposition profile, adhesiveness, morphology, and electrochemical surface area of the Ag deposit, which influence the CO2/CO conversion efficiency of the catalyst. Pre-treatment with HCl-H2SO4 conferred the highest roughness and hydrophilicity to the Ti substrate, leading to the highest surface area of the Ag catalyst and a consequent substantial increase in the CO2/CO conversion efficiency (45% at Vcell = −2.2 V), which is a 5.7fold increase when compared with the un-treated counterpart. The fabrication of Ag/Ti gas
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diffusion electrode via simple Ag electrodeposition and optimized Ti pre-treatments reported
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herein provides a guide for manufacturing proton exchange membrane-based CO2 electrolyzers.
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Keywords: Carbon dioxide, Electrochemical reduction, Silver, Titanium, Gas diffusion electrode,
The effects of global warming had been postulated by Mark Lynas in his predictive work titled “Six degrees: Our future on a hotter planet” [1]. In addition, to alleviate the global warming,
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several proactive endeavors were suggested by the 21st Conference of the Parties (COP21, in 2015) and by the special report from the Intergovernmental Panel on Climate Change (IPCC, in 2018). These considerations have led to the development of impactful and commercially-viable technologies that can remarkably mitigate and utilize the excess atmospheric CO2. The electrochemical reduction of CO2 and production of value-added chemical fuels, i.e., C1 or C2
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species, are one of the prospective routes to substantially reduce the excess atmospheric CO2 [2−7]. Previous techno-economic analysis confirmed that among the myriad carbonaceous chemicals generated via the electrolysis of CO2, CO appears to be the most viable one [8–10]. The thermodynamics and kinetics of the CO2 reduction reaction (CO2RR), which selectively forms CO, are greatly affected by the cathodic electrocatalyst [11–13]. Starting with the early work by Hori et al. [14], many studies have focused on developing metallic and non-metallic catalysts
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(or their combinations) and on elucidating the composition-structure-activity correlation
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[11,15−21]. The majority of these studies utilized a three-compartment H-cell to measure the CO2RR activity. However, it is not possible to achieve a high partial CO current density at a
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relatively low cell overpotential (ηcell < 1 V), which is a critical parameter for assessing the techno-
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economic viability of the CO2RR, with such systems without a gas diffusion electrode. Such a restriction is fundamentally attributed to the low solubility of CO2 in aqueous medium (about 35
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mM at 298 K, 1 atm), which limits the mass transport when dissolved CO2 is utilized as the reactant, in combination with the high activation energy barrier associated with formation of the reaction
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intermediate. Therefore, the flow-cell is suggested as a means of overcoming the mass transport limitation by continuously feeding reactant to the electrode [22–24]. In a typical flow-cell, the gas phase CO2 is directly delivered to the cathode or dissolved in mildly-basic solution. The membrane-embedded reactor is the most commonly utilized flow-cell architecture for the CO2RR,
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and is also known as a membrane reactor or zero-gap reactor [25]. These systems are based on the well-established low-temperature water electrolysis, which drives the power-to-gas (P2G) scheme, and can be effectively translated from H-cell experiments using liquid electrolytes. Three membrane types are generally used for the CO2RR: proton exchange [26–29], anion exchange [10,22,30,31], and bipolar [32–34] membranes. The proton exchange membrane (PEM), often
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termed the polymer electrolyte membrane, has been highlighted in PEM-based fuel cell or water electrolyzer technology, and can be easily adapted to the PEM-based CO2RR. In such reactors, the cathodic and anodic reactions are separated by the PEM. Membrane reactors can be built on the laboratory-scale, and their designs are amenable to larger stacks for scale-up. Despite the importance of the PEM-based membrane-electrode-assembly (MEA) type electrolyzer in the study of CO2RR [35–37], only a few studies have been reported when compared with those on H-cells.
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The combination of a highly porous and catalyst-supported gas diffusion electrode (GDE) with
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the polymer membrane completes the MEA. This is placed between the anode and cathode current collectors and separates the oxygen evolution reaction (OER) and CO2RR. The GDE allows a
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prolonged contact of CO2 with the catalyst during electrolysis, which ultimately serves to increase
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the reaction rate, thereby increasing the current density substantially when compared with that of the H-cell system. The GDE offers several advantageous features such as porosity, electrical
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conductivity, mechanical strength, wettability, and chemical stability. Conventionally, a dense array of carbon fibers, typically known as carbon paper, is used as the gas diffusion layer in the
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GDE because of the high porosity and large surface area of the paper [38–42]. In fact, we recently demonstrated the use of chemically-treated carbon paper containing a hierarchical dendritic electrodeposited Ag catalyst as the diffusion electrode in a membrane electrolyzer; here, the carbon paper electrode contributed to the conversion of CO2 to CO with high current density [35]. As
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aforementioned, the use of carbon paper in the GDE is quite effective for the CO2RR. However, GDEs comprising a metallic gas diffusion layer, such as Ti, should be developed for water or CO2 electrolysis, where a high cell voltage is normally required. Ti-based GDEs can offer chemical stability and resistance to corrosion, thus conferring durability to the system.
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In this study, Ag is used as a CO2-reducing electrocatalyst because Ag is known to be highly active for CO2 to CO conversion [14,43,44]. Au is another candidate material for producing CO, exhibiting relatively higher selectivity than that of Ag. However, based on the economic feasibility, Ag is preferred over Au for CO2RR. Ag catalysts with unique shapes, such as nano-coral or triangular nanoplates [45,46], and different particle sizes [11] have been utilized to enhance the catalytic activity of CO2/CO conversion. Our research group proposed many methods for
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synthesizing dendritic Ag catalysts, such as employing additives in the plating solution [47] or
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controlling the deposition potential and composition of the deposit [48], thereby increasing the portion of facets that enhance the CO production efficiency. A Ag-deposited Ti GDE was
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fabricated by electrodeposition, a technique whereby the metal catalyst is directly deposited on the
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surface of a gas diffusion substrate. Electrodeposition method is widely used for electrode fabrication in applications of PEM-based electrolyzers (including our work) [49–53]. Herein, both
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Ti mesh and Ti paper are used as substrates in consideration of the effect of the substrate morphology on the CO2RR activity and selectivity. We fabricated a Ag-deposited Ti GDE for a
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vapor-fed, PEM-based electrolyzer for the electrochemical conversion of CO2 to CO. To prepare the GDE, various pre-treatment steps are applied to the Ti substrate in pursuit of the optimal conditions for achieving a highly efficient and selective CO2RR catalyst. The substrate pre-treatment may influence the deposition profile, adhesiveness, morphology, and real surface
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area of the deposit. For several decades, the effects of pre-treatment of metal surfaces on the ensuing deposition and on the characteristics of the deposits have been examined in multiple ways [54–58]. Zhao et al. reported that pre-treatment of the Mg substrate by immersion in zinc solution clearly affected the surface morphology, composition, and microstructure of the electrodeposited Ni layer [54]. It is also known that pre-treatment may improve the anti-corrosion properties [55]
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and electrochemical activity [56] of the deposited layer. Accordingly, pre-treatment of the Ti substrate should affect the CO2RR activity. First, the catalytic activities of GDEs subjected to various pretreatments for the CO2RR are evaluated and compared with the H-cell scheme, and then
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the optimal GDE is verified in the PEM-based MEA-type electrolyzer.
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2. Experimental 2.1. Pre-treatment of gas diffusion layer Two different types of gas diffusion substrates were examined in the current study: Ti mesh (Bolin, purity = 99.6%) and Ti paper (CNL Energy, purity = 99.9%). For the Ti mesh, three different sizes (#18, #60, and #80, based on ASTM standard) were considered. Prior to the electrodeposition of Ag on the gas diffusion substrate, the substrate was pretreated with acidic or
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basic solution to remove the surface oxide and organic species adsorbed on the substrate. Five pre-
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treatment methods were employed in this study: (1) un-treated or no treatment; (2) organic removal, i.e., sonication for 2 min in 30% ethanol (C2H5OH, 94.5%, Daejung); (3) 6 M HCl, i.e., heating at
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70 C for 40 min in 6 M hydrochloric acid (HCl, 38%, Daejung); (4) NH4OH-H2O2-H2O, i.e.,
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heating at 60 C for 40 min in a mixture of aqueous ammonium hydroxide (NH4OH, 28%, Fujifilm Wako), hydrogen peroxide (H2O2, 35%, Junsei), and de-ionized water with a volume ratio of 1:2:5;
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and (5) HCl-H2SO4, i.e., heating at 80 C for 20 min in 20 wt% HCl(aq) and further heated at 150 C for 15 min in 20 wt% aqueous sulfuric acid (H2SO4, 95%, Junsei). Methods (2) and (3) were
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used to remove pre-adsorbed organics and native oxides, respectively [59]. Method (4) was used to form a micro/nano hierarchical structure, thereby increasing the surface roughness and obtaining hydrophilicity after the chemical etching process [60]. In method (5), H2SO4 was utilized as the etching agent and the temperature was raised. Because the etch rate is accelerated at elevated
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temperatures, the Ti oxide can be removed to a greater extent, with definite roughening of the surface [61]. Note that method (2) preceded methods (3), (4), and (5). The conditions for each pretreatment method are summarized in Table 1.
2.2 Fabrication of Ag-deposited GDE
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As aforementioned, the Ag-deposited Ti GDE was fabricated by electrodeposition using a potentiostat (CS350, Wuhan CorrTest Instrument Corp., Ltd.). Ag was electrodeposited in the conventional three-electrode system, where the Ti substrate (areageo = 10 cm2), Pt gauze, and a saturated calomel electrode (SCE) were used as the working, counter, and reference electrodes, respectively. The electrolyte consisted of 0.01 M silver nitrate (AgNO3, 99.9%, Alfa Aesar), 0.6 M ammonium sulfate ((NH4)2SO4, 99%, Alfa Aesar), and 0.04 M ethylenediamine (C2H8N2, 99%,
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Sigma-Aldrich). Deposition was performed at −0.49 VSCE for 15 min. The deposition conditions
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of potential, time, and electrolyte were determined in our previous study [47], where optimal
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deposition conditions for Ag were determined using linear sweep voltammetry.
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2.3 Physicochemical characterization
The influence of the pre-treatment methods on the wettability of the Ti substrates was
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investigated by measuring the contact angle (FM40Mk2 EasyDrop, KRUSS GmbH, Germany). Angle-resolved photoelectron spectroscopy (AR-XPS, Axis Supra, Kratos, U.K.) was used to
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evaluate the surface oxide composition of the pre-treated Ti substrates using representative samples. The morphology and crystallinity of the samples before and after the Ag deposition were characterized by field-emission scanning electrode microscopy (FESEM) and X-ray diffraction (XRD, New D8-Advance, BRUKER-AXS), respectively. The gaseous products (CO and H2)
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generated as a result of the electrolysis of CO2 were analyzed by using gas chromatography (GC, 7890A, Agilent) equipped with a dual-detector (a thermal conductivity detector (TCD) and methanizer-combined flame ionization detector (FID)).
2.4 Electrochemical measurement
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The manufactured Ag-deposited Ti GDEs were embedded in an H-type cell for the electrolysis of CO2. A PEM (Nafion 212, DupontTM) was used to separate the cathodic and anodic compartments. CO2-saturated 0.5 M potassium hydrogen carbonate (KHCO3, 99.5%, Daejung) and Ar-purged 0.5 M KHCO3 were used as the catholyte and anolyte, respectively. During electrolysis, CO2 was continuously supplied to the cathodic compartment at a rate of 10 mL min−1 by a mass flow controller (MFC; MKS Instruments Inc.). Pt gauze and SCE were utilized as the
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counter and reference electrodes, respectively. The reduction reaction was performed at −1.5 VSCE
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for 30 min. The gaseous product was delivered to the sampling loop of the GC system with the aid of a CO2 carrier. The catalytic activity was measured based on the reduction current produced
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during the constant-potential electrolysis and the concentration of the produced gases determined
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from gas analysis, which ultimately provided the partial current density and Faradaic efficiency. The detailed information is provided in the previous papers [12]. The electrochemical surface area
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of the Ag-deposited Ti GDE was determined based on the charge consumed for the formation of the oxidative monolayer of Ag2O or AgOH [62]. In detail, each pre-treated Ag-deposited Ti GDE
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was first reduced by applying −1.41 VSCE for 10 min in 0.1 M potassium hydroxide (KOH, 93%, Daejung), and then oxidized at 0.14 VSCE. The charge required for oxidation of the Ag monolayer is assumed to be 400 μC cm−2 [63,64]. Prior to the measurement, the KOH solution was de-aerated
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by purging with Ar for 30 min.
2.5 Preparation of MEA and operation of PEM-based electrolyzer The best-performing Ag-deposited Ti GDE (areageo = 9 cm2), screened in the H-cell experiment,
was transferred to the PEM-based electrolyzer. The cathodic reduction of CO2 was coupled with the anodic evolution of O2 on a Ti paper (1 mg cm−2) coated with iridium oxide (IrO2). The IrO2-
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coated Ti paper was prepared by spraying IrO2 stock solution (1 mg cm−3) onto the oxalic-acidpretreated Ti paper using an electrospray system (eS-robot, NanoNC). The IrO2 stock solution comprised a mixture of IrO2 (99.99%, Alfa Aesar), a small quantity of de-ionized water, Nafion perfluorinated resin binder (5 wt%, Sigma-Aldrich), and isopropyl alcohol (70%, Honeywell Research Chemicals). This mixture was sonicated for 30 min before spraying. The MEA was prepared by embedding the PEM (Nafion 212, DuPontTM) between the cathodic and anodic GDEs.
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For this purpose, 1.0 M KHCO3 (99.7%, Sigma-Aldrich) and 1.0 M KOH (90%, Sigma-Aldrich)
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were used as the catholyte and anolyte, respectively, and were circulated by using a pump system (SIMDOS 10, KNF). The flow rates of the catholyte and anolyte were 4 and 10 mL min−1,
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respectively. CO2 was supplied to the cathode at a rate of 16.2 mL min−1 and was pressurized (4
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bar) using a regulator (TESCOM). PEM-based CO2 electrolysis was performed at cell voltages (Vcell) ranging from −1.8 to −3.4 V by applying a constant voltage for 30 min at an interval of −400
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mV using a potentiostat (HCP-803, Bio-Logic Science Instruments). The gaseous products (CO
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and H2) were analyzed by using a gas chromatograph (7890B, Agilent), with Ar as the carrier gas.
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3. Results and Discussion 3.1. Fabrication of Ag-deposited Ti GDE and physicochemical characterization The surface properties of the Ti substrate (morphology, roughness, oxidation level, and hydrophilicity) vary according to the pre-treatment method. Figure 1 shows the FESEM images of the Ti mesh (#18) and representative samples subjected to pre-treatment methods (1)−(5).
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The un-treated Ti mesh is also presented for comparison (Fig. 1a). The substrate surfaces
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obtained from the organic removal (Fig. 1b), 6 M HCl (Fig. 1c), and NH4OH-H2O2-H2O (Fig. 1d) methods were similar to that of the un-treated material. However, the Ti substrate was notably
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rougher after being pre-treated with HCl-H2SO4 (Fig. 1e). Although this is simply a morphological
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analysis using FESEM, the pre-treatment using a strong acid combination of HCl and H2SO4 was expected to have the greatest physical and chemical effects on the substrate, which will be dealt
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with in a later section.
The Ag catalyst was deposited on the Ti substrate via electrodeposition to generate the Ag-
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deposited Ti GDE. Ag deposition on the Ti substrates that were pretreated under different conditions was performed at −0.49 VSCE for 15 min. Figure 2a (1st row) presents the FESEM images of the bare Ti mesh (#18) and the Ag deposited on the meshes that were pre-treated with
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methods (1)−(5).
For comparison, Ag-deposited GDEs were also fabricated using different Ti mesh sizes. The
electrodes manufactured using various Ti substrates and the corresponding pre-treatment are shown in Fig. 2b−c. A GDE was also prepared by using a metallic Ti paper, which is similar to the carbon paper; the surface image is presented in Fig. 2d. In the un-treated substrate, the Ag catalyst
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was deposited as cones and aggregates, regardless of the mesh size and substrate type (2nd column of Fig. 2a−d, indicated by (1)). Further, some areas were not deposited with Ag catalyst when the substrate was un-treated. These observations indicate that the un-treated substrate was unfavorable for an efficient metal nucleation. In the organic removal method, Ag catalyst dendrites were formed on the mesh and paper of all sizes (3rd column of Fig. 2a−d, indicated by (2)). This dendritic morphology was observed for 6 M HCl, NH4OH-H2O2-H2O, and HCl-H2SO4 pre-treatments
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(4th−6th columns of Fig. 2a−d, indicated by (3)−(5)). However, it was evident that the 6 M HCl and
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HCl-H2SO4 pre-treatments provided relatively high density and growth of dendritic Ag.
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Furthermore, the mesh size did not appear to have a significant impact on the electrode fabrication.
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3.2. CO2RR on Ag-deposited GDE
The CO2RR activities of the Ag-deposited Ti GDEs fabricated using different pre-treatments
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and various Ti mesh sizes were evaluated in the H-cell scheme (−1.5 VSCE, 30 min), as shown in Fig. 3. The total and CO Faradaic efficiencies of the un-treated Ag-deposited Ti GDE were
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markedly lower than those of the other electrodes. The organic removal pre-treatment caused a slight increase in the CO Faradaic efficiency (5−7%) when compared with that of the un-treated case. Regardless of the mesh size and type, the NH4OH-H2O2-H2O pre-treatment provided a very low CO Faradaic efficiency (<5%). However, both acidic pre-treatments led to a drastic increase
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in the efficiency. Specifically, in the case of pretreatment with 6 M HCl, the efficiencies of Ti mesh #18, #60, #80, and Ti paper were 1.5, 7.3, 5.6, and 2.8 times greater than that of un-treated case, respectively. Moreover, these efficiencies increased by 9.8, 10.0, 6.4, and 4.0 times, respectively, in the case of HCl-H2SO4 pre-treatment. The HCl-H2SO4 pre-treatment resulted in the highest CO2/CO conversion efficiency (>58%). In short, the CO2/CO conversion efficiency
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increased in the following order: NH4OH-H2O2-H2O, organic removal, 6 M HCl, and HCl-H2SO4 (Fig. 3a). Figures 3b and c present the total current density and CO partial current density, respectively for the Ag-deposited Ti GDEs fabricated using different pre-treatments and various Ti mesh sizes. The total current density of the electrode pre-treated with HCl-H2SO4 was lower than that
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achieved with the other methods. However, as shown in Fig. 3a, most of the current was consumed
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to convert CO2 to CO, and thus, the CO partial current density of the electrode was enhanced for all Ti meshes and Ti paper. This trend is opposite for the GDEs subjected to other pre-treatments,
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i.e., high total current density and low CO partial current density. This is attributed to the dominant
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hydrogen evolution under the corresponding circumstances. Thus, this experiment demonstrates that the type of Ti substrate, i.e., mesh or paper, does not have a significant effect on the CO2RR
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activity or on the CO2/CO selectivity. However, the pre-treatment has a clear influence; thus, it can be concluded that HCl-H2SO4 provided a high CO2/CO conversion efficiency and CO-
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generating ability.
To determine the influence of the pre-treatment on the properties of the Ti substrates, contact angle measurement was used to test the wettability of the surface (Fig. 4).
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In an aqueous electrolyte-based electrochemical system, the ability of the electrolyte to wet the
solid electrode is important because electrochemical reactions such as Ag deposition occur through a liquid-solid heterogeneous interface. For the contact angle analysis, a Ti plate was used instead of Ti mesh or Ti paper as the droplets were not formed well on the latter two because of their porous nature. The un-treated and organic-removed Ti plates were found to be highly hydrophobic
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based on the contact angles (Fig. 4a−b). However, 6 M HCl pre-treatment increased the hydrophilicity (Fig. 4c). The hydrophilicity was further enhanced and a completely wet surface was achieved after the NH4OH-H2O2-H2O and HCl-H2SO4 pre-treatments (Fig. 4d−e). The data obtained from this analysis combined with the FESEM analysis (Fig. S1a−e) indicate that the surface attained a polygonal topography after the NH4OH-H2O2-H2O and HCl-H2SO4 pretreatments, implying that these surfaces might exhibit high roughness (Fig. S1d−e). Experimental
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studies on metal surfaces have reported that the hydrophilicity increases as the roughness increases
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[65,66], which is in accordance with the Wenzel’s model [67]. Enhanced hydrophilicity of the surface should facilitate interfacial contact with the electrolyte or penetration of the precursor
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solution into the electrode structure, which would in turn affect the formation of the Ag catalyst.
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The variation in contact angle implies a change in the surface composition after pre-treatment. To confirm the same, XRD analysis was conducted by using Ti mesh #18 as a representative
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sample (Fig. 5).
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After HCl-H2SO4 pre-treatment, which resulted in the highest CO current shown in Fig. 3, the intensity of the Ti(100) peak (35.10°, blue dashed line) increased 3.3-fold when compared with that of the un-treated congener, whilst the intensity of the TiO2 peak (27.48°, orange dashed line) decreased 0.7-fold. Thus, it could be inferred that HCl-H2SO4 pre-treatment removed the surface
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Ti oxide, exposing the underlying Ti. A similar effect was observed for the other Ti substrate, i.e., Ti mesh #60 (Fig. S2). To support the above claim, AR-XPS analysis of the sample before and after HCl-H2SO4 pre-treatment was performed with emission angles of 0−80° relative to the surface normal, at 20° intervals, thereby providing clearer evidence of the changes in the surface composition (Fig. 6).
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In the AR-XPS analysis, Ti paper was used to minimize the signal alteration due to the sidewall of the meshed structure during the angle change. At all angles, signals of metallic Ti were hardly detectable for un-treated Ti paper. However, when the HCl-H2SO4 pre-treatment was performed, the content of metallic Ti increased by 17.1, 19.2, 20.4, 21.0, and 18.2% when compared with that of the un-treated sample, as the analyzed spot was moved from the bulk to the surface, i.e., as the
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emission angle changed from 0 to 80°, respectively. Further, the content of Ti(IV) decreased by
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41.5, 45.5, 45.5, 42.9, and 47.3%. From the AR-XPS analysis, it was observed that Ti(IV), which previously accounted for most of the titanium content, was replaced with metallic Ti or Ti in the
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reduced state, i.e., Ti(II)/Ti(III), after the pre-treatment. At all angles, it was confirmed that
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approximately 20% of the surface TiO2 was replaced with metallic Ti after HCl-H2SO4 pretreatment.
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The changes in the surface state indicated by the contact angle, XRD, and AR-XPS analyses may affect the subsequent Ag electrodeposition. This could change the properties (crystallinity
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and morphology) of the deposit, which in turn influences the CO2RR activity and real surface area of the catalyst. It was previously reported that the crystallinity of Ag plays a crucial role in determining the CO2RR activity and selectivity [64,68]. In fact, the XRD analysis in Fig. S3 did not show any substantial changes in the crystalline structure before vs. after HCl-H2SO4 pre-
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treatment, except for the appearance of the Ti peak due to the oxide removal and a slight increase in the intensity of the Ag peak due to the increased amount of Ag deposit (Fig. 2) for the samples after pre-treatment. The electrochemical surface area of the Ag catalyst was also measured by using an electrochemical method [63,64] to confirm the contribution of the pre-treatment to the active area.
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Figure 7 shows the charge consumed during oxidation to form a monolayer of metal oxide on the Ag-deposited Ti paper. The charge required to oxidize the surface and the electrochemical surface area derived from these data is summarized in Table 2. The electrochemical surface followed a trend quite similar to that of the CO2/CO selectivity and activity shown in Fig. 3a and c. Notably, the electrochemical
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surface area of the Ag-deposited Ti GDE with HCl-H2SO4 pre-treatment was 3.6 times greater
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than that of the un-treated congener. A larger surface area of the Ag catalysts would be advantageous for CO2/CO conversion.
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Using the HCl-H2SO4 pre-treatment, where the CO2RR activity and selectivity were maximal,
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the amount of Ag deposited on the Ti paper was optimized by increasing the deposition time from 15 to 100 min (Fig. 8).
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The reason for selecting Ti paper as the substrate for final optimization is discussed in the
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subsequent section. The total current density from electrolysis gradually increased with the prolonged deposition time and the amount of Ag deposited increased accordingly. However, the CO partial current density increased from 15 min, reached a maximum at 75 min, and decreased at 100 min (Fig. 8a). The CO Faradaic efficiency also increased with the deposition time and
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reached 67.3% at 75 min, which is higher than that at any other time (Fig. 8b). Therefore, the optimized time for Ag deposition on the Ti GDE was considered to be 75 min. The optimized Ag-deposited GDE, in terms of deposition and pre-treatment, was utilized in the
PEM-based MEA-type electrolyzer to validate and reflect the electrochemical performance observed with the H-cell scheme. As aforementioned, only a few studies are available on the PEM-
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based MEA-type CO2 electrolyzer that produces CO. As representative examples, Delacourt et al. [26] used PEM-based CO2 electrolyzer with commercial Ag catalysts to obtain approximately 18 and 32 mA/cm2 of CO partial currents at Vcell = 2.8 and 3.4 V, respectively. Wu et al. [27] reported CO and HCOO- partial currents of approximately 2 and 17 mA/cm2, respectively, at Vcell = 2.5 V by using commercial Sn powder catalysts and PEMFC-like cell configuration. Dufek et al. [28] also reported a substantially high CO partial current of approximately 140 mA/cm2 at Vcell = 2.9 V
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using porous Ag sheet in a pressurized (18.5 atm) PEM-based MEA-type CO2 electrolyzer.
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Figure 9 shows the total reduction current density and the ratio of CO versus the other gaseous
treatment in the applied Vcell range of −1.8 to −3.4 V.
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products when the Ag catalysts were used on Ti paper both with and without HCl-H2SO4 pre-
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From the results shown in Fig. 8, two samples with Ag deposition for 50 and 75 min were
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selected because they exhibited the highest CO current and Faradaic efficiencies. The Ag catalysts on Ti mesh were also tested; however, the Nafion membrane in the MEA structure was damaged
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because of the stiffness caused by the Ti mesh GDL when compressed during cell assembly. This occurred repeatedly for all mesh Ti GDEs, and the failure of the MEA structure resulted in shortcircuiting, as shown in Fig. S4. Therefore, the use of a thicker membrane is recommended when Ti mesh GDEs are used; this requires a compromise between the mechanical strength and
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membrane resistance.
Figure 9a shows the increase in the total current density with Vcell; however, HCl-H2SO4 pre-
treatment caused no substantial difference up to −3.0 V. At −3.4 V, the un-treated Ag catalysts exhibited a higher current than those pre-treated with HCl-H2SO4. When combined with the GC analysis, the ratio of CO to the other gaseous products of electrolysis could be represented as
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shown in Fig 9b. The CO ratio plot exhibited a volcano-shaped relation with Vcell, and the highest CO2/CO selectivity was observed at −2.2 V, regardless of the pre-treatment or deposition time. At −2.2 V, the CO2/CO conversion efficiency followed this order: Ag-deposited (deposition time = 50 min) Ti paper with HCl-H2SO4 pre-treatment (44.7%) ≫ Ag-deposited (75 min) Ti paper untreated (14.3%) > Ag-deposited (75 min) Ti paper with HCl-H2SO4 pre-treatment (10.6%) > Ag-
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deposited (50 min) Ti paper un-treated (7.8%). The effect of pre-treatment was evident for the 50 min-deposited electrodes, where the CO ratio increased 5.7-fold. The effect of the deposition time
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on the CO production is shown in Fig. 8, indicating that a deposition time of 75 min provided the optimal electrode performance. However, in the PEM-based MEA-type electrolyzer experiment,
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the electrode failed to deliver the same CO-producing ability. The discrepancy observed with the
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75-min deposited electrode is attributed to the weak adhesion of the thickly deposited Ag catalyst on the Ti substrate, as shown in Fig. S5. It was observed that most of the Ag catalyst was exfoliated
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and detached after operation of the electrolyzer, when compared with the as-prepared Agdeposited GDE. In the high Vcell region, the competing cathodic hydrogen evolution reaction was
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accelerated to a much greater degree than the CO2RR, and thus, the CO2/CO selectivity gradually decreased. Most importantly, however, with the optimal surface pre-treatment developed in this study, the Ag-deposited Ti-based GDE could be successfully fabricated and applied to the PEM-
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based MEA-type electrolyzer for the CO2RR.
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4. Conclusions Five pre-treatment methods were applied to a Ti substrate, which is a potential candidate as a gas diffusion electrode for the PEM-based CO2 electrolyzer. In-depth investigation of the effects of the pretreatments on electrochemical deposition of the Ag catalyst showed that the properties influence the electrochemical CO2/CO conversion. Both Ti mesh and Ti paper were employed, and Ti paper was found to be more suitable in terms of fabrication and processability. Contact
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angle measurement and FE-SEM analyses showed that the surface wettability and morphology
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varied substantially depending on the pre-treatment methods. Combined acid (HCl-H2SO4) pretreatment conferred high hydrophilicity to the Ti substrate. XRD analysis confirmed successful
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removal of the surface oxide after the HCl-H2SO4 pre-treatment, in accordance with the AR-XPS
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analysis. The very rough and hydrophilic Ti substrate generated by HCl-H2SO4 pre-treatment increased the electrochemical surface area of the Ag catalyst, which was quantitatively supported
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by oxidation current measurements. In the H-cell scheme, the Ag-deposited Ti GDE pre-treated with HCl-H2SO4 exhibited improved CO Faradaic efficiency (>58%) when compared with the un-
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treated electrode: 9.8 and 4.0 times for Ti mesh #18 and Ti paper, respectively. The ratio of CO to the total gaseous product in the PEM-based MEA-type electrolyzer scheme was defined for the optimally pre-treated Ag-deposited Ti GDE by combined electrochemical measurement and GC analysis. The data revealed that the highest CO2/CO conversion efficiency (~45%) was achieved
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at Vcell = −2.2 V for the Ag-deposited Ti GDE subjected to HCl-H2SO4 pre-treatment, representing a 5.7-fold increase when compared with that of the un-treated counterpart. The Ag deposition time was optimized and restricted to 50 min, except for when the Ag catalyst was exfoliated and detached from the substrate after CO2 electrolysis in the PEM-based MEA-type electrolyzer.
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Declarations of interest: none
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Acknowledgments This research was supported by the Chung-Ang University Research Scholarship Grants in 2018.
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This work was also supported by the National Research Foundation (NRF) of Korea Grant funded
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by the Korea CCS R&D Center (KCRC) (grant number 2014M1A8A1049349).
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Figure Captions Figure 1. FESEM image of Ti mesh (#18) substrate subjected to the following pretreatments: (a) un-treated, (b) organic removal, (c): (b) + 6 M HCl, (d): (b) + NH4OH:H2O2:H2O, and (e): (b) +
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HCl-H2SO4.
Figure 2. FESEM image of Ag catalysts fabricated by electrodeposition on Ti mesh pretreated by different methods: (a) #18, (b) #60, (c) #80, and (d) Ti paper at −0.49 VSCE for 15 min (from left to right, the pre-treatment methods are: (1) un-treated, (2) organic removal, (3): (2) + 6 M HCl,
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(4): (2) + NH4OH:H2O2:H2O, and (5): (2) + HCl-H2SO4.
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Figure 3. Effect of pre-treatment methods (1)−(5) on (a) Faradaic efficiency, (b) total current
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density, and (c) CO current density of Ag-deposited Ti mesh (A) #18, (B) #60, (C) #80, and (D)
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Ti paper. CO2RR was evaluated in H-cell scheme.
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Figure 4. Contact angle of Ti plate pre-treated as follows: (a) un-treated, (b) organic removal, (c):
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(b) + 6 M HCl, (d): (b) + NH4OH:H2O2:H2O, and (e): (b) + HCl-H2SO4.
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Figure 5. X-ray diffraction patterns of Ti mesh (#18) substrate before/after organic removal + HCl-
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H2SO4 pre-treatment.
Figure 6. AR-XPS data for (a) un-treated Ti paper and (b) Ti paper pre-treated with organic
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removal + HCl-H2SO4 (emission angle with respect to surface normal ranging from 0° to 80°).
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Figure 7. Oxidation current profile of Ag-deposited Ti GDEs with various pretreatments.
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Oxidation was performed at 0.14 VSCE until saturation of the current.
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Figure 8. (a) Total current density and CO current density and (b) Faradaic efficiency of Ag-
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deposited Ti GDEs with various deposition times.
Figure 9. Performance of PEM-based MEA-type electrolyzer with applied Vcell: (a) total current
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density and (b) CO ratio. The cell was equipped with Ag-deposited Ti GDE subjected to organic
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removal + HCl-H2SO4 pre-treatment and Ag was electrodeposited for 50 and 75 min.
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Table Captions Table 1. Detailed Ti substrate pre-treatment conditions for methods (1)−(5) Label
