Generation of a Highly Efficient Electrode for Ethanol Oxidation By

catalysts

Article Generation of a Highly Efﬁcient Electrode for Ethanol Oxidation by Simply Electrodepositing Palladium on the Oxygen Plasma-Treated Carbon Fiber Paper

Houg-Yuan Pei 1, Chen-Han Lin 2, Wei Lin 1 and Chiun-Jye Yuan 1,2,3,*

1 Institute of Molecular Medicine and Bioengineering, National Chiao Tung University, Hsinchu 300, Taiwan; [email protected] (H.-Y.P.); [email protected] (W.L.) 2 Department of Biological Science and Technology, National Chiao Tung University, Hsinchu 300, Taiwan; [email protected] 3 Center for Intelligent Drug Systems and Smart Bio-devices (IDS2B), National Chiao Tung University, Hsinchu 300, Taiwan * Correspondence: [email protected]; Tel.: +886-3-5731735

Abstract: In this study, a highly efficient carbon-supported Pd catalyst for the direct ethanol fuel cell was developed by electrodepositing nanostructured Pd on oxygen plasma-treated carbon fiber paper (Pd/pCFP). The oxygen plasma treatment has been shown to effectively remove the surface organic contaminants and add oxygen species onto the CFP to facilitate the deposition of nano-structured Pd on the surface of carbon fibers. Under the optimized and controllable electrodeposition method, nanostructured Pd of ~10 nm can be easily and evenly deposited onto the CFP. The prepared Pd/pCFP electrode exhibited an extraordinarily high electrocatalytic activity towards ethanol oxidation, with a current density of 222.8 mA mg−1 Pd. Interestingly, the electrode also exhibited a high tolerance Citation: Pei, H.-Y.; Lin, C.-H.; Lin, to poisoning species and long-term stability, with a high ratio of the forward anodic peak current W.; Yuan, C.-J. Generation of a Highly density to the backward anodic peak current density. These results suggest that the Pd/pCFP catalyst Efficient Electrode for Ethanol may be a promising anodic material for the development of highly efficient direct alcohol fuel cells. Oxidation by Simply Electrodepositing Palladium on the Keywords: carbon fiber paper; ethanol oxidation; Pd catalyst; fuel cell Oxygen Plasma-Treated Carbon Fiber Paper. Catalysts 2021, 11, 248. https: //doi.org/10.3390/catal11020248 1. Introduction Academic Editor: Vincenzo Baglio Energy supply by consumption of fossil fuels, such as petroleum, coal and natural

Received: 29 December 2020 gas, raises severe problems for the environment and public health, including depletion Accepted: 8 February 2021 of natural resources, air pollution and global warming [1,2]. Hence, finding clean and Published: 12 February 2021 reliable power sources for various mechanical, electrical, and medical devices and vehicles without increasing stress on the environment and public health is a top priority worldwide. Publisher’s Note: MDPI stays neutral Compared to conventional power generators, fuel cells convert chemical energy directly with regard to jurisdictional claims in to electrical energy by electrochemical reactions and operate with high efficiency, low published maps and institutional affil- emissions and clean processes [3–6]. Among various fuel cells, direct liquid fuel cells have iations. attracted much attention because they exhibit high energy density and high efficiency and can be operated under ambient conditions [7,8]. In addition, liquid fuels, such as ethanol, methanol and formic acid, exhibit many advantages, such as safe, easy handling and storage and convenient refill. Among commonly used liquid fuels, ethanol is particularly Copyright: © 2021 by the authors. attractive due to its high specific energy (8.0 kWh/kg), low toxicity, renewability and Licensee MDPI, Basel, Switzerland. low crossover problems [9–11]. Moreover, the large-scale production of ethanol is readily This article is an open access article available [12,13]. Hence, the direct ethanol fuel cell (DEFC) is recognized as one of the most distributed under the terms and efficient power systems for powering portable and mobile devices [14–16]. DEFC works conditions of the Creative Commons by electrochemical oxidation of ethanol at the anode, while at cathode oxygen is reduced. Attribution (CC BY) license (https:// Ideally, ethanol can be completely oxidized on the anode of DEFC to H2O and CO2 with creativecommons.org/licenses/by/ sequential transfer of 12 electrons, and it delivers high energy density [17,18]. However, 4.0/).

Catalysts 2021, 11, 248. https://doi.org/10.3390/catal11020248 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, 248 2 of 13

high overpotential, intermediate to high temperatures and a highly active catalyst are usually required for the complete oxidation of ethanol. Two basic types of DEFC were developed on the basis of the usage of a permeable membrane, i.e., proton-exchange membrane (PEM) and anion-exchange membrane (AEM), that separates the anode and cathode. The PEM-based DEFC works well under the acidic condition, with platinum (Pt) and various Pt-based bimetallic materials having long been used as highly efficient catalysts for the ethanol oxidation reaction (EOR) [19–21]. As a catalyst, Pt exhibits several appealing properties, such as low operating temperatures, high power densities, high stability and relative ease of scale-up. However, the requirement of a relatively large amount of scarce and expensive Pt, and low poisoning tolerance to the CO-like species generated during EOR [22,23], are major disadvantages of this type of DEFC. Alloying Pt with less expensive metals, such as ruthenium (Ru), tungsten (W), tin (Sn), gold (Au) and 3d transition metals, helps to reduce the CO intolerance and improve the catalytic activity of Pt [19,20,22,24,25]. The second metal in the alloy may enhance the electro-oxidation of ethanol by activating water molecules and providing preferential sites for–OH adsorption (–OHads) at lower potentials than Pt. The –OHads species are required to completely convert the poisoning intermediates to CO2. However, the dissolution of these metals under the acidic condition remains the major reason for the severe degradation of the catalytic performance of these alloyed catalysts [25,26]. In comparison, the AEM- based DEFC works well under the alkaline condition and exhibits some advantages over the PEM-based process by showing faster kinetics in both ethanol oxidation and oxygen reduction reactions. In addition, palladium (Pd) can be adopted as the catalyst for the oxidation of ethanol. Pd is a good alternative for the development of an active catalyst for DEFC because it is more abundant, cost-effective and has relatively high catalytic activity in alkaline media and tolerance to CO-like species poisoning [27–30]. Although Pd is a good electrocatalyst for EOR [31], its performance can be further enhanced by changing its size, morphology and the supporting substrate [31–34]. Carbon materials have long been used to support metal catalysts because of their inertness to chemical reactions, resistance to corrosion, large surface area and cost-effectiveness [35–37]. Various carbon-based nano- or micro-materials, such as carbon nanotube (CNT) [29,38], graphene or graphene oxide [39–43], carbon microspheres [44] and helical carbon nanofibers [45], have been used as Pd-based catalyst supports to increase its catalytic activity and efficiency. However, these carbon-based materials are typically produced by methods with harsh synthetic conditions, complicated synthetic procedures and low production yields, and their economical and mass production are limited [46–48]. In addition, the presence of organic contaminants and/or the surface hyper-hydrophobicity of these carbon supports may cause weak interaction with noble metal nanoparticles and eventually lead to their detachment and uneven distribution [49,50]. Carbon fiber paper (CFP) is a thin sheet of laminated short carbon fibers with high electrical conductivity [51]. CFP has been used for the development of various fuel cells or biofuel cells due to its high surface area, high porosity, strong mechanical property, good electrical and thermal conductivity and excellent chemical stability [52–57]. CFP has also been adopted for the development of a Pd-supported anode (Pd/CFP) in DEFCs by directly electrodepositing Pd onto the CFP without any treatment [43]. Unfortunately, its performance, in terms of its poisoning tolerance to intermediate species, was poor [43]. Although the true reason for the poor performance of Pd/CFP is unknown, the inefficient deposition, loose attachment and non-uniform distribution of nano-scaled Pd on CFP have been postu- lated. Recently, plasma was shown to improve the surface chemical and electrical properties of carbon-based electrodes such as the screen-printed carbon paste electrode, graphite nanofiber electrode, graphite felt electrode, glassy carbon electrode and CFP [49,50,58–63]. The great improvement in electrical properties of carbon-based electrodes, such as electrochemical reactivity and capacitance, by plasma treatment may be due to the removal of the organic contaminants and the increase in oxygen species and other functional groups on the surface of electrodes [49,50,58–63]. The attachment of electrochemical active mi- Catalysts 2021, 11, 248 3 of 13

croorganisms on the glassy carbon electrode and carbon/graphite felt electrode can also be enhanced after the pre-treatment of electrodes by oxygen plasma [60,61]. Interestingly, the removal of the organic contaminants may increase hydrophilicity of the electrode surface and allow the easy access of metal ions. Adding oxygen species to the surface of carbon-based electrodes can further enhance the surface adsorption of metal ions [35,36], leading to the enhancement of chemical deposition and loading of Pt-Ru on the graphite nanofibers [63], and the improvement of electrodeposition and even distribution of gold nanoparticles on CFP [64]. Based on these observations, the development of a highly efficient electrode for DEFC by direct electrodeposition of Pd on the oxygen plasma-treated CFP was proposed. In this study, we demonstrated that Pd could be effectively electrodeposited onto the surface of the oxygen plasma-treated CFP (pCFP). The deposition of Pd onto pCFP was analyzed by a scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS) and thermogravimetric analysis (TGA). The generated Pd-deposited pCFP (Pd/pCFP) exhibited great catalytic activity towards ethanol oxidation in alkaline media, with high specific current density and good tolerance to poisoning intermediates.

2. Results and Discussion 2.1. Effect of Plasma Treatment in the Electrodeposition of Pd on CFP Oxygen plasma treatment was previously shown to improve the electrodeposition of Au nanoparticles onto CFP [64]. In this study, oxygen plasma treatment was proposed to improve the electrodeposition of Pd onto CFP, as well as the catalytic activity of the electrodeposited Pd catalyst towards ethanol oxidation. As shown in Figure S1, without oxygen plasma treatment, the Pd nanoparticles (PdNP) could barely be electrodeposited onto the CFP by CV in 100 mM phosphate buffer, pH 7.0, containing 2 mM K2PdCl4 in a potential range from −0.6 to +1.0 V (vs. Ag/AgCl) with a scan rate of 50 mV s−1 for 20 cycles (panels A and B). In contrast, after oxygen plasma treatment, the PdNPs could be electrodeposited onto the CFP under the same conditions with a size distribution between 5 and 90 nm (panels C and D). The Pd signals on the Energy Dispersive X-ray Spectrometry of the PdNP/pCFP further demonstrated the electrodeposition of Pd onto the surface of CFP (Figure S2). These results indicate that oxygen plasma treatment can improve the deposition of the Pd onto CFP. However, the low deposition rate of Pd onto pCFP suggests that optimization for Pd electrodeposition is required.

2.2. Preparation of Pd/pCFP Electrode for Ethanol Oxidation The effect of pH on the electrodeposition of Pd on pCFP was ﬁrst investigated by the catalytic activity of electrodes in H2O2 oxidation. The electrodeposition of Pd onto pCFP was performed by CV in 2 mM K2PdCl4 at various pH values (pH 2.9, 5.0, 5.8, 7.0 and 8.0) in a potential range from −0.5 V to +0.9 V for 10 cycles. The electrochemical responses of the Pd/pCFP electrodes towards H2O2 oxidation showed that the electrode prepared at pH 5.8 exhibited the highest peak response (Figure S3). The electrochemical responses of Pd/pCFP electrodes to H2O2 were moderately decreased when they were prepared at the pHs of 2.9 and 5.0, while the electrochemical responses of the electrodes signiﬁcantly decreased when they were prepared at higher pH values, i.e., pH 7.0 and 8.0. The SEM images showed that the surface of the Pd/pCFP electrode prepared at pH 5.8 was covered by a layer of Pd nanoparticles, with an average size of about 10 nm (Figure1A,B). This result suggests that the electrodeposition of Pd onto pCFP at pH 5.8 is optimal and can be used for the future preparation of Pd/pCFP electrodes. The catalytic activity of the Pd/pCFP electrode prepared at pH 5.8 to 1 M ethanol under the alkaline condition (1 M KOH) was further analyzed by CV in a potential range from −0.8 V to +0.6 V. As shown in Figure1C, the prepared Pd/pCFP electrode exhibited a broad oxidation peak corresponding to ethanol oxidation in a forward scan starting from −0.6 V to the onset potential peak at −0.08 V (vs. Ag/AgCl). A small oxidative peak corresponding to the depletion of the adsorbed poisonous species appeared at −0.41 V in the reverse scan Catalysts 2021, 11, 248 4 of 13

with forward and backward peak current densities of 118.67 mA cm−2 and 29.20 mA cm−2, respectively. The ratio of the forward anodic peak current (If) to the backward anodic peak current (Ib) was calculated as If/Ib = 4.02. This redox proﬁle is similar to that reported previously [65]. Although the electrocatalytic activity (118.67 mA) of the prepared Pd/pCFP towards ethanol oxidation is lower than that of Pd/CFP reported previously (413 mA mg−1, − Catalysts 2021, 11, 248 Pd corresponding to 165.2 mA cm 2)[43], the prepared Pd/pCFP electrode exhibited a 4 of 13 higher tolerance to the poisonous intermediates during the ethanol oxidation [38,66] than that of Pd/CFP, with an If/Ib ratio of 2.14 [43].

Figure 1. The effect of plasma treatment on the preparation of Pd/CFP electrodes. The Pd/pCFP was Figure 1. The effect of plasma treatment on the preparation of Pd/CFP electrodes. The Pd/pCFP prepared by CV in 0.1 M phosphate buffer, pH 5.8, containing 2 mM K2PdCl4 in a potential range betweenwas prepared−0.5 V andby +0.9CV in V (vs.0.1 Ag/AgCl)M phosphat withe a buffer, scan rate pH of 505.8, mV containing s−1 for 10 cycles. 2 mM SEM K2PdCl images4 in a potential −1 ofrange Pd electrodeposited between −0.5 onto V and pCFP +0.9 at lowV (vs. (20,000 Ag/AgCl)×)(A) and wi highth a magniﬁcations scan rate of (200,00050 mV ×s)( Bfor). The 10 cycles. SEM scaleimages bars of in Pd panels electr (A)odeposited and (B) are 1 µontom and pCFP 100 nm, at low respectively. (20,000×) (C) ( TheA) and cyclic high voltammogram magnifications of (200,000×) 1(B M). ethanolThe scale oxidation bars onin thepanels Pd/pCFP (A) electrodeand (B) inare 1 M1 μ KOHm and in a potential100 nm, rangerespectively. of −0.8 V to(C +0.6) The V cyclic voltam- atmogram scan rate of of 1 50 M mV ethanol s−1. oxidation on the Pd/pCFP electrode in 1 M KOH in a potential range of −0.8 V to +0.6 V at scan rate of 50 mV s−1. To generate the electrode with both high catalytic activity to ethanol and high tolerance to poisonous intermediates from ethanol oxidation, the preparation of Pd/pCFP was furtherThe catalytic optimized activity by using of different the Pd/pCFP potential cyclingelectrode numbers prepared (10, 20, 30at andpH 405.8 cy- to 1 M ethanol −1 cles)under and the scan alkaline rates (100, condition 50, 40, 30,(1 20,M KOH) 10 mV swas). fu Asrther shown analyzed in Figure by2A, CV the in I f aof potential range Pd/pCFPfrom −0.8 prepared V to +0.6 at pHV. As 5.8 shown increased in in Figure response 1C to, the the decreaseprepared in Pd/pCFP scan rates, electrode with exhibited −1 thea broad highest oxidation response peak appearing corresponding at the scan rate to ethanol of 10 mV oxidation s with a in current a forward density scan of starting from ± −2 224.3−0.6 V 28.1to the mA onset cm potentialand an If/I bpeakratio at of 1.77.−0.08 Next, V (vs. the Ag/AgCl). effect of different A small potential oxidative cy- peak corresponding to the depletion of the adsorbed poisonous species appeared at −0.41 V in the reverse scan with forward and backward peak current densities of 118.67 mA cm−2 and 29.20 mA cm−2, respectively. The ratio of the forward anodic peak current (If) to the backward anodic peak current (Ib) was calculated as If/Ib = 4.02. This redox profile is similar to that reported previously [65]. Although the electrocatalytic activity (118.67 mA) of the prepared Pd/pCFP towards ethanol oxidation is lower than that of Pd/CFP reported previously (413 mA mg−1, Pd corresponding to 165.2 mA cm−2) [43], the prepared Pd/pCFP electrode exhibited a higher tolerance to the poisonous intermediates during the ethanol oxidation [38,66] than that of Pd/CFP, with an If/Ib ratio of 2.14 [43]. To generate the electrode with both high catalytic activity to ethanol and high tolerance to poisonous intermediates from ethanol oxidation, the preparation of Pd/pCFP was further optimized by using different potential cycling numbers (10, 20, 30 and 40 cycles) and scan rates (100, 50, 40, 30, 20, 10 mV s−1). As shown in Figure 2A, the If of Pd/pCFP prepared at pH 5.8 increased in response to the decrease in scan rates, with the highest response appearing at the scan rate of 10 mV s−1 with a current density of 224.3 ± 28.1 mA cm−2 and an If/Ib ratio of 1.77. Next, the effect of different potential cycling numbers on the

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clingpreparation numbers of onPd/pCFP the preparation was studied. of Pd/pCFP The electrodes was studied. were prepared The electrodes using 2 were mM prepared K2PdCl4, −1 −1 usingpH 5.8, 2 mMwith K a2 PdClscan 4rate, pH of 5.8, 10 withmV s a scan for 10, rate 20, of 30 10 mVand s40 cycles.for 10, As 20, shown 30 and 40in Figure cycles. As2B, shownthe Pd/pCFP in Figure prepared2B, the Pd/pCFPat a scan rate prepared of 10 mV at a s scan−1 for rate40 cycles of 10 mVreached s −1 forthe 40highest cycles response reached −2 − theto ethanol highest oxidation response towith ethanol a current oxidation density with of a477.9 current ± 29.6 density mA cm of 477.9 and ±If/29.6Ib ratio mA of cm 3.37.2 andTheseIf/ observationsIb ratio of 3.37. suggest These observationsthat anodes suggestfor future that experiments anodes for future could experiments be prepared could on bepCFP prepared by CV onin pCFP100 mM by phosphat CV in 100e, mM pH phosphate,5.8, containing pH 5.8,2 mM containing K2PdCl4 2in mM a potential K2PdCl range4 in a potentialof −0.5 V rangeto +0.9 of V− (vs.0.5 Ag/AgCl) V to +0.9 Vat (vs. a scan Ag/AgCl) rate of 10 at mV a scan s−1 ratefor 40 of cycles. 10 mV s −1 for 40 cycles.

Figure 2. TheThe cyclic cyclic voltammograms voltammograms of ethanol of ethanol oxidat oxidationion onto onto Pd/pCFP Pd/pCFP electrodes electrodes prepared prepared un- underder different different conditions. conditions. The Theeffect effect of the of cycling the cycling numbers numbers and scan and rates scan of rates the CV of on the the CV prepa- on the preparationration of Pd/pCFP of Pd/pCFP was studied. was studied. The performance The performance of the prepared of the prepared electrodes electrodes was analyzed was analyzed by the byelectrochemical the electrochemical oxidation oxidation of 1 M ofethanol 1 M ethanol in 1 M inKOH. 1 M ( KOH.A) The (A preparation) The preparation of Pd/pCFP of Pd/pCFP was car- was ried out by CV in 100 mM phosphate buffer, pH 5.8, containing 2 mM K2PdCl4 in a potential range carried out by CV in 100 mM phosphate buffer, pH 5.8, containing 2 mM K PdCl in a potential range of −0.5 V to +0.9 V at different scan rates (100, 50, 40, 30, 20, 10 mV s−1) for2 10 cycles.4 (inset) The of −0.5 V to +0.9 V at different scan rates (100, 50, 40, 30, 20, 10 mV s−1) for 10 cycles. (inset) The average maximum forward (closed bars) and backward (open bars) current densities of Pd/pCFPs averageto 1 M ethanol maximum were forward obtained (closed from three bars) indepe and backwardndent experiments (open bars) and current presented densities as mean of Pd/pCFPs ±S.D. to(B) 1 The M ethanol Pd/pCFP were electrodes obtained were from prepared three independent as described experiments in (A) except and that presented different aspotential mean ± cy-S.D. (clingB) The numbers Pd/pCFP (i.e., electrodes 10, 20, 30 were and prepared40 cycles) as were described adopted. in ((insA) exceptet) The that average different maximum potential forward cycling numbers(closed bars) (i.e., and 10, 20,backward 30 and 40(open cycles) bars) were current adopted. densities (inset) of ThePd/pCFPs average to maximum 1 M ethanol forward were ob- (closed bars)tained and from backward three independent (open bars) experi currentments densities and presented of Pd/pCFPs as mean to 1 M±S.D. ethanol were obtained from three independent experiments and presented as mean ± S.D.

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2.3. Characterization of Pd/pCFP Electrode 2.3. CharacterizationThe surface of characteristics Pd/pCFP Electrode of the prepared Pd/pCFP under the optimized condition wasThe monitored surface characteristics under the ofSEM. the prepared It was found Pd/pCFP that under Pd theformed optimized a uniform condition layer with numer- wasous monitored nano-granules under the on SEM. the It wassurface found of that pCFP Pd formed (Figure a uniform 3A,B). layer The with EDS numerous analysis of the surface nano-granules on the surface of pCFP (Figure3A,B). The EDS analysis of the surface of Pd/pCFPof Pd/pCFP exhibited exhibited a characteristic a characteristic energy peak energy of Pd and peak some of minor Pd and peaks some representing minor peaks represent- theing C the and C O and elements, O elements, demonstrating demonstrating the deposition the of Pd deposition on the CFP (Figureof Pd 3onB). the CFP (Figure 3B).

Figure 3. 3.Scanning Scanning electron electron microscopic microscopic (SEM) images (SEM) and images energy dispersiveand energy X-ray dispersive spectrometry X-ray spectrometry of Pd/pCFP electrodes. The Pd/pCFP electrodes were prepared by CV in 2 mM K2PdCl4/100 mM of Pd/pCFP electrodes. The Pd/pCFP electrodes were prepared by CV in 2 mM K2PdCl4/100 mM phosphate buffer, pH 5.8, in a potential range of −0.5 V to +0.9 V at a scan rate of 10 mV s−1 for 40 cy- −1 cles.phosphate The SEM buffer, images ofpH Pd/pCFP 5.8, in undera potential magniﬁcation range powers of −0.5 of V 5000 to× +0.9(A) andV at 30,000 a scan× ( Brate) were of 10 mV s for 40 recorded.cycles. The The SEM scale barimag in (esA) of and Pd/pCFP (B) is 1 µm under and 200 magnification nm, respectively. powers (C) The energyof 5000× dispersive (A) and 30,000× (B) X-raywere spectroscopyrecorded. The was carriedscale bar out inin the (A region) andon (B the) is Pd/pCFP 1 μm and (inset). 200 nm, respectively. (C) The energy dispersive X-ray spectroscopy was carried out in the region on the Pd/pCFP (inset). The loading content of Pd onto pCFP was determined by thermogravimetric analysis. In this study, the prepared Pd/pCFP electrode was thermally decomposed in air at a heating rate ofThe 10 ◦ Cloading min−1. content The Pd/pCFP of Pd exhibited onto pCFP almost was no determined weight loss until by aboutthermogravimetric 600 ◦C analysis. (FigureIn this4 ).study, A major the weight prepared loss occurred Pd/pCFP between electrod 615 ◦Ce and was 850 thermally◦C, which was decomposed due to the in air at a heat- decompositioning rate of 10 of °C carbon min materials.−1. The Pd/pCFP The remaining exhibited residue almost after thermal no weight decomposition loss until about 600 °C 2 represents(Figure 4). the A loading major of weight Pd onto pCFP,loss whichoccurred was estimatedbetween to 615 be 2.43 °C mg/cmand 850. °C, which was due to the decomposition of carbon materials. The remaining residue after thermal decomposition represents the loading of Pd onto pCFP, which was estimated to be 2.43 mg/cm2.

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Figure 4. Thermogravimetric analysis of Pd/pCFP electr electrodesodes in in air. air. The The Pd/p Pd/pCFPCFP was was prepared prepared as as described in Methodology and Figure3 3.. TheThe thermalthermal decompositiondecomposition of of Pd/pCFP Pd/pCFP waswas conductedconducted inin −1 air heated from 30 ◦°CC to 1000 °C◦C at a rate of 10 °C◦C min min−.1 .

2.4. Electro-Oxidation of Et Ethanolhanol on Pd/pCFP Electrodes The prepared prepared Pd/pCFP Pd/pCFP electrode electrode was was designed designed for for the the ethanol ethanol oxidation oxidation under under the alkalinethe alkaline condition. condition. Therefore, Therefore, CV of CV the of prepared the prepared electrode electrode in 1 M in 1KOH M KOH was wasfirst ﬁrstper- formedperformed to determine to determine the thebasal basal responses responses of the of Pd/pCFP the Pd/pCFP without without ethanol ethanol (Figure (Figure S4). The S4). electrochemicalThe electrochemical response response of the of Pd/pCFP the Pd/pCFP electrode electrode to 1 M to KOH 1 M KOH was performed was performed at a scan at a −1 ratescan of rate 50 mV of 50 s− mV1 in the s potentialin the potential range of range −1.0 V of to− +1.01.0 VV. to The +1.0 prepared V. The prepared Pd/pCFP Pd/pCFPexhibited similarexhibited electrochemical similar electrochemical behavior behavioras that of as the that reported of the reported carbon-based carbon-based Pd catalysts. Pd catalysts. Three responsiveThree responsive peaks peakswere observed were observed in the in cyc thelic cyclic voltammogram. voltammogram. Pd has Pd been has been known known to ab- to sorbabsorb hydrogen hydrogen at atroom room temperature temperature and and atmospheric atmospheric pressure pressure [67]. [67]. Hence, Hence, the anodicanodic peak A A in in the the potential potential range range between between −0.65−0.65 V and V and−0.45− V0.45 represents V represents the oxidation the oxidation of the absorbedof the absorbed hydrogen hydrogen on the surface on the of surface active ofPd active [65,68,69]. Pd [65 Peak,68, 69B emerged]. Peak B from emerged about from −0.1 − Vabout to +0.70.1 V and V to was +0.7 due V and to the was formation due to the of formation Pd(II) oxide of Pd(II)on the oxidesurface on of the the surface catalyst of [68- the 70].catalyst The [potential68–70]. The and potential current and increased current successively increased successively over time overuntil time reaching until reachinga plateau. a Theplateau. process The beyond process beyondthe potential the potential range could range be could attributed be attributed to the tooxygen the oxygen evolution evolution reac- reaction [71]. In the reverse scan, a cathodic peak C, situated at −0.6 V, was attributed to tion [71]. In the reverse scan, a cathodic peak C, situated at −0.6 V, was attributed to the the reduction of Pd(II) oxide on the surface of the Pd nanoparticles [65,72]. reduction of Pd(II) oxide on the surface of the Pd nanoparticles [65,72]. The electrochemical responses of ethanol oxidation on the prepared Pd/pCFP were The electrochemical responses of ethanol oxidation on the prepared Pd/pCFP were further investigated by CV in different potential ranges (i.e., −0.8 V to +0.6 V, −0.8 V to further investigated by CV in different potential ranges (i.e., −0.8 V to +0.6 V, −0.8 V to +0.8 +0.8 V, −0.8 V to +1.0 V, and −0.8 V to +1.2 V) to ﬁnd the optimal operational condition V, −0.8 V to +1.0 V, and −0.8 V to +1.2 V) to find the optimal operational condition (Figure (Figure5). Interestingly, the oxidative current of ethanol on Pd/pCFP reached a maximum 5). Interestingly, the oxidative current of ethanol on Pd/pCFP reached a maximum in the in the potential range of −0.8 V to +0.8 V (dotted curve), with the peak potential at potential range of −0.8 V to +0.8 V (dotted curve), with the peak potential at +0.29 V (vs. +0.29 V (vs. Ag/AgCl) and a current density of 222.8 mA mg−1 Pd or 541.3 mA cm−2, Ag/AgCl) and a current density of 222.8 mA mg−1 Pd or 541.3 mA cm−2, whereas an anodic whereas an anodic peak in the backward scan appeared at −0.4 V, with a current density peak in the backward scan appeared at −0.4 V, with a current density of 40.6 mA mg−1 Pd of 40.6 mA mg−1 Pd or 98.7 mA cm−2. The I /I ratio for the prepared Pd/pCFP was −2 f b orabout 98.7 5.48. mA Incm comparison,. The If/Ib ratio the ethanol for the oxidationprepared responsesPd/pCFP acrosswas about different 5.48.potential In comparison, ranges (Figurethe ethanol5) were oxidation lower with responses the current across densities different of 168.2potential mA mgranges−1 Pd,(Figure 187.7 5) mA were mg −lower1 Pd, −1 −1 −1 withand 146.0the current mA mg densities−1 Pd for of the 168.2 potential mA mg ranges Pd, 187.7 of −0.8 mA V mg to +0.6 Pd, V and (solid 146.0 line), mA− mg0.8 V Pd to +1.0for the V (dashpotential line), ranges and − of0.8 −0.8 V toV to +1.2 +0.6 V V (dash-dotted (solid line), line),−0.8 V respectively. to +1.0 V (dash Interestingly, line), and − the0.8 V to +1.2 V (dash-dotted line), respectively. Interestingly, the If/Ib ratios of ethanol oxida- If/Ib ratios of ethanol oxidation in the potential ranges of −0.8 V to +1.0 V (If/Ib = 7.24) tion in the potential ranges of −0.8 V to +1.0 V (If/Ib = 7.24) and −0.8 V to +1.2 V (If/Ib = 8.04) and −0.8 V to +1.2 V (If/Ib = 8.04) were much higher than that in the potential range of were−0.8 Vmuch to +0.8 higher V. This than may that be in duethe potential to the decomposition range of −0.8 ofV theto +0.8 poisoning V. This speciesmay be at due high to theanodic decomposition potentials. of Considering the poisoning the species possible at high occurrence anodic ofpotentials. unnecessary Considering electrochemical the pos- siblereactions occurrence at high of potentials, unnecessary the potential electrochemica range ofl reactions−0.8 V to at +0.8 high V potentials, was suggested the potential to be the optimal condition for the ethanol oxidation reaction.

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Catalysts 2021, 11, 248 8 of 13 rangerange of of − −0.80.8 V V to to +0.8 +0.8 V V was was suggested suggested to to be be the the optimal optimal condition condition for for the the ethanol ethanol oxida- oxida- tiontion reaction. reaction.

FigureFigure 5. 5.5. The The electrochemical electrochemical responses responses of of ethanol ethanol ox oxidationoxidationidation across across different different potential potential ranges. ranges. The The electrochemicalelectrochemicalelectrochemical responses responses of of 1 1 1M M M ethanol ethanol ethanol oxidat oxidat oxidationionion in in in 1 1 M 1M M KOH KOH KOH on on on the the the Pd/pCFP Pd/pCFP Pd/pCFP electrode electrode electrode were were were −1 performedperformedperformed by by CV CV CV at at at a a ascan scan scan rate rate rate of of of50 50 50mV mV mV s s−1 in sin− the1 thein potential potential the potential ranges ranges ranges of of − −0.8~0.60.8~0.6 of −0.8~0.6 V V (solid (solid V line), line), (solid line), −0.8~0.8 V (dotted line), −0.8~1.0 V (dash line), and −0.8~1.2 V (dash-dotted line). −−0.8~0.80.8~0.8 V V (dotted (dotted line), line), −0.8~1.0−0.8~1.0 V (dash V (dash line), line), and and −0.8~1.2−0.8~1.2 V (dash-dotted V (dash-dotted line). line).

TheThe presence presencepresence of ofof poisonous poisonous intermediates intermediates during duringduring ethanol ethanolethanol oxidation oxidation may maymay block block the the electrodeelectrodeelectrode surface, surface,surface, poison poisonpoison the thethe catalyst, catalyst,catalyst, and and reduce reduce the the oxidation oxidation of of ethanol. ethanol. The The long-term long-term electrochemicalelectrochemicalelectrochemical stability stability and and and poisoning poisoning poisoning resistance resistance resistance capability capability capability of of Pd/pCFP ofPd/pCFP Pd/pCFP catalyst catalyst catalyst was was wasfur-fur- therfurtherther evaluatedevaluated evaluated byby chronoamperometry bychronoamperometry chronoamperometry (CA)(CA) (CA) [7[73].3]. [73 InIn]. thisthis In this study,study, study, thethe theelectrochemicalelectrochemical electrochemical re-re- sponsesresponsessponses of of of1 1 M 1M Methanol ethanol ethanol in in inan an an alkaline alkaline alkaline conditio conditio conditionnn were were were monitored monitored monitored at at at different different different potentials potentials (i.e., (i.e., −−0.10,0.10,0.10, − −0.15,−0.15,0.15, −−0.20,0.20,−0.20, −−0.250.25−0.25 andand and −−0.300.30− 0.30V) V) forfor V) 30003000 for 3000 s.s. AsAs s. shownshown As shown inin FigureFigure in Figure 6,6, thethe6, Pd/pCFP thePd/pCFP Pd/pCFP elec-elec- trodeelectrodetrode exhibited exhibited exhibited a a slow slow a slowcurrent current current decay decay decay at at the the atpotential potential the potential of of − −0.150.15 of V V− (do 0.15(dotttted Ved (dottedline), line), with with line), a a current current with a densitycurrentdensity of densityof 30.130.1 mAmA of 30.1 cmcm−−2mA2 atat thethe cm end−end2 at ofof the 30003000 end s.s. InofIn comparison,3000comparison, s. In comparison, thethe initialinitial current thecurrent initial ofof ethanol currentethanol oxidationofoxidation ethanol decayeddecayed oxidation quicklyquickly decayed inin chronoamperometry quicklychronoamperometry in chronoamperometry analysisanalysis atat analysisthethe restrest of atof thethe restpotentialspotentials of the tested,potentialstested, i.e.,i.e., tested, −−0.100.10 V i.e.,V (solid(solid−0.10 line),line), V (solid −−0.200.20 line), VV (short(short−0.20 dashdash V (short line),line), dash −−0.250.25 line), VV (dash-dotted(dash-dotted−0.25 V (dash-dotted line)line) andand −line)−0.300.30 andVV (long (long−0.30 dash dash V (longline).line). These dashThese line).result result Thesess suggest suggest results that that suggestat at the the potential potential that at theof of − − potential0.150.15 V, V, the the of Pd/pCFP Pd/pCFP−0.15 V, electrodetheelectrode Pd/pCFP exhibited exhibited electrode better better exhibited catalytic catalytic better activity activity catalytic an andd resistance resistance activity andagainst against resistance poisoning poisoning against intermediates intermediates poisoning duringintermediatesduring ethanol ethanol during oxidation. oxidation. ethanol oxidation.

Figure 6. Electrochemical characterization of Pd/pCFP. Chronoamperometric curves of 1 M ethanol was carried out in 1 M KOH at various ﬁxed potentials (−0.10, −0.15, −0.20, −0.25 and −0.30 V) (vs. Ag/AgCl) for 3000 s.

Catalysts 2021, 11, 248 9 of 13

Figure 6. Electrochemical characterization of Pd/pCFP. Chronoamperometric curves of 1 M etha- Catalysts 2021, 11, 248 9 of 13 nol was carried out in 1 M KOH at various fixed potentials (−0.10, −0.15, −0.20, −0.25 and −0.30 V) (vs. Ag/AgCl) for 3000 s.

TheThe poisoning effect effect of of intermediates, intermediates, such such as as acetaldehyde, acetaldehyde, from from the the incomplete incomplete ox- oxidationidation of ofethanol ethanol was was demonstrated demonstrated by the by theelectrochemical electrochemical oxidation oxidation of 1 M of ethanol 1 M ethanol with- withoutout (solid (solid line) line) and andwith with 0.5 M 0.5 acetaldehyde M acetaldehyde (dash (dash line) line) (Figure (Figure 7). 7The). The anodic anodic peak peak po- potentialtential blue-shifted blue-shifted from from 0.49 0.49 V V to to 0.26 0.26 V, V, and and the the peak peak current density (j) reducedreduced aboutabout 33%33% fromfrom 205.5205.5 mAmA mgmg−−11 PdPd to to 138.3 138.3 mA mA mg mg−−1 1Pd.Pd.

FigureFigure 7.7. TheThe inﬂuenceinfluence of of acetaldehyde acetaldehyde on on ethanol ethanol oxidation. oxidation. The The CV CV of of 1 M 1 ethanolM ethanol on theon the Pd/pCFP electrodePd/pCFP waselectrode carried was out carried in 1 KOH out in in 1 theKOH potential in the potential range of range−0.8 Vof to−0.8 +0.8 V to V with+0.8 V (dash with line)(dash or withoutline) or without (solid line) (solid 0.5 line) M acetaldehyde. 0.5 M acetaldehyde. The scan The rate scan was rate 50 mV was s −501. mV s−1.

3.3. MaterialsMaterials andand MethodsMethods 3.1. Materials 3.1. Materials Carbon fiber paper (TGP-H-060) was purchased from Toray Inc. (Tokyo, Japan). Cop- Carbon fiber paper (TGP-H-060) was purchased from Toray Inc. (Tokyo, Japan). Cop- per foil, potassium tetrachloropalladate (II) (K2PdCl4), ethanol (99.8%) and acetaldehyde per foil, potassium tetrachloropalladate (II) (K2PdCl4), ethanol (99.8%) and acetaldehyde were obtained from Sigma-Aldrich (St. Louis, MO, USA). Potassium dihydrogen phos- were obtained from Sigma-Aldrich (St. Louis, MO, USA). Potassium dihydrogen phosphate, potassium hydrogen phosphate, sodium chloride, potassium chloride and potassium phate, potassium hydrogen phosphate, sodium chloride, potassium chloride and potas- hydroxide were obtained from Showa. Acetic acid and 2-propanol were obtained from J.T. sium hydroxide were obtained from Showa. Acetic acid and 2-propanol were obtained Baker (Randor, PA, USA). The rest of the reagents were analytical grade. from J.T. Baker (Randor, PA, USA). The rest of the reagents were analytical grade. 3.2. Apparatus 3.2. Apparatus The electrochemical measurements were carried out on a CHI6116E (CH Instruments, Austin,The TX, electrochemical USA) by using themeasurements standard three-electrode were carried system out on with a CHI6116E an Ag/AgCl (CH electrode, Instru- platinumments, Austin, wire andTX, Pd/pCFPUSA) by using as the the reference standard electrode, three-electrode counter system electrode with and an workingAg/AgCl electrode,electrode, respectively.platinum wire Oxygen and Pd/pCFP plasma as treatment the reference was performed electrode, incounter the plasma electrode cleaner and (Zepto,working Diener electrode, electronic, respectively. Ebhausen, Oxygen Germany). plasma Scanning treatment electron was performed microscopy in (SEM)the plasma and EDScleaner were (Zepto, carried Diener out using electronic, a high-resolution Ebhausen, Germany). thermal field Scanning emission electron scanning microscopy electron microscope,(SEM) and EDS JSM-7610F were carried (JEOL Ltd.,out using Tokyo, a Japan),high-resolution of the Instrumentation thermal field Centeremission at Nationalscanning Tsingelectron Hua microscope, University. JSM-7610F Thermal Gravimetric(JEOL Ltd., To Analysiskyo, Japan), (TGA) of was the Instrumentation performed using Center 2-HT (Mettler-Toledo,at National Tsing Greifensee, Hua University. Switzerland) Thermal of theGravimetric Instrumentation Analysis Center (TGA) at was National performed Tsing Huausing University 2-HT (Mettler-Toledo, and was conducted Greifensee, in airSwit heatedzerland) from of 30the◦ CInstrumentation to 1000 ◦C at aCenter rate ofat 10National◦C min Tsing−1. Hua University and was conducted in air heated from 30 °C to 1000 °C at a rate of 10 °C min−1. 3.3. Preparation of Working Electrode for Ethanol Oxidation 3.3. PreparationThe working of Working electrodes Electrode were preparedfor Ethanol from Oxidation pCFP (working area of 3 × 5 mm2) as described previously with some modification [50]. The plasma treatment of CFP prior to the Pd electrodeposition was carried out in the chamber of the plasma cleaner at an air pressure of 0.4 bar, power of 75 W, and oxygen flow rate of 40 N h−1 for 15 s. Electrodeposition of Pd was performed in 0.1 M phosphate buffer, pH 5.8, containing 2 mM K2PdCl4 at a

Catalysts 2021, 11, 248 10 of 13

scan rate of 10 mV s−1. The potential cycling between −0.5 V and +0.9 V was performed at room temperature for 40 cycles.

3.4. Electrochemical Analysis Unless deﬁned, the electrochemical measurements of the oxidation of 1 M ethanol were performed by cyclic voltammetry (CV) in 1 M KOH between the potential range of −0.8 V and +0.8 V with a scan rate of 50 mV s−1. The electrolyte was deaerated with N2 gas at ambient temperature for 10 min before measurements were obtained. The chronoamperometric measurement (CA) of ethanol oxidation on the Pd/pCFP electrode was performed in 1 M KOH containing 1 M ethanol at the overpotentials of −0.10, −0.15, −0.20, −0.25 and −0.30 V for 3000 s.

4. Conclusions In this study, a highly efﬁcient Pd-based electrode for ethanol oxidation was simply fabricated by electrodepositing Pd on the oxygen plasma-treated CFP. The direct deposition of Pd catalyst on the plasma-treated CFP exhibits several advantages, including easy preparation, inexpensiveness and high catalytic activity. The condition for the preparation of Pd/pCFP was also optimized by changing pH values, scanning rates and the cycling numbers for CV. The electrochemical response of Pd/pCFP, prepared under optimized conditions for ethanol oxidation, was 222.6 mAmg−1 Pd loaded, which was about 54% of the reported Pd/CFP [41]. However, the If/Ib ratio of 5.49, an indicator of the capability of a catalyst to tolerate the poisonous intermediate species during reactions, was much higher than that of Pd/CFP (If/Ib ratio = 2.14). In addition, the Pd/pCFP showed a long stability. In conclusion, a highly efﬁcient Pd-supported carbon-based electrode with high catalytic activity towards ethanol and high tolerance to poisonous species was developed and demonstrated in this study. These excellent characteristics made Pd/pCFP a more favorable anodic electrode for the development of DEFC.

Supplementary Materials: The following are available online at https://www.mdpi.com/2073-4 344/11/2/248/s1, Figure S1: Scanning electron microscopic images of palladium-electrodeposited CFP electrodes, Figure S2: Energy dispersive X-ray spectrometry of the Pd/pCFP. Figure S3: Effect of pH value on the electrodeposition of Pd on pCFP, Figure S4: Electrochemical behavior of Pd/pCFP electrode to 1 M KOH. Author Contributions: Conceptualization, C.-J.Y.; Methodology, H.-Y.P. and C.-H.L.; Investigation, H.-Y.P. and C.-H.L.; Resources, C.-J.Y.; Data curation: H.-Y.P. and C.-H.L.; Supervision, C.-J.Y.; Formal analysis, C.-J.Y. and W.L.; Writing—original draft, H.-Y.P.; Writing—review and editing, C.-J.Y.; Funding acquisition, C.-J.Y. All authors have read and agreed to the published version of the manuscript. Funding: This research was financially supported by the research grants MOST 107-2314-B-009-002 and MOST 109-2314-B-009-002 from the Ministry of Science and Technology, Taiwan (ROC). Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: The authors would like to thank the Ministry of Science and Technology (MOST), Taiwan (ROC) for financially supporting this research and the Instrumentation Center at National Tsing Hua University for helping to perform the SEM and chronoamperometric analysis. Conflicts of Interest: The authors declare no conflict of interest.

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