Elektrokatalis Berbasis-Nano dan Perilaku Perakitan Elektroda Membran dalam Sel Bahan Bakar Gliserol Langsung Pasif

Hasil dan Diskusi

Karakterisasi Fisik Elektrokatalis

Pola difraksi sinar-X untuk ketiga sampel elektrokatalis hasil sintesis yaitu Au/VGCNF, Pd/VGCNF, dan Pd₁ Au₁ /VGCNF ditunjukkan pada Gambar. 1, sedangkan data analisis dikompilasi pada Tabel 1. Pola difraksi sinar-X dari elektrokatalis menunjukkan struktur kubik (gugus ruang Fm-3m) tanpa fase kedua. Puncak difraktogram pertama pada 26,0° dapat ditetapkan untuk karbon berstruktur grafit (002) dari bidang difraksi VGCNF mentah dari grafit heksagonal (file kartu JCPDS 41-1487) (Rajarao dan Bhat [30]). Setiap pola difraktogram yang dipamerkan memiliki empat puncak difraksi yang dapat diindeks ke difraksi dari bidang (1 1 1), (2 0 0), (2 2 0), dan (3 1 1) fcc.

Difraktogram dari berbagai elektrokatalis Pd₁ Au₁ /VGCNF, Pd/VGCNF, dan Au/VGCNF

Gambar 1 menunjukkan puncak difraksi sampel elektrokatalis Au/VGCNF pada sekitar 38,3 °C, 44,5 °C, 64,7 °C dan 77,7 °C dan dapat diindeks ke bidang kristalografi (1 1 1), (2 0 0), ( 2 2 0), dan (3 1 1), masing-masing, yang dapat diklasifikasikan sebagai Fm-3m (JCPDS No. 65-8601, a = b = c = 0.4072 nm). Gambar 1 menunjukkan pola difraksi sampel elektrokatalis Pd/VGCNF di sekitar nilai 2θ 40,12 °C, 46,66 °C, 68,12 °C, dan 82,10 °C yang sesuai dengan bidang-bidang berikut (1 1 1), (2 0 0) , (2 2 0), dan (3 1 1) (JCPDS No. 05-0681, a = b = c = 0.3889 nm). Pada Gambar. 1, untuk sampel bimetal PdAu, Pd dan Au tampak berbagi puncak difraksi tunggal yang menunjukkan bahwa telah terjadi pembentukan paduan homogen.

Nanopartikel PdAu tipikal yang didukung pada VGCNF pada dasarnya dibuat dengan reduksi bersama dari prekursor logam (yaitu, \( {\mathrm{AuCl}}_4^{-} \) dan \( {\mathrm{PdCl}}_4^{ 2-} \)) dengan adanya natrium borohidrida sebagai zat pereduksi. Gambar 2a–c menunjukkan gambar TEM khas elektrokatalis Au/VGCNF, Pd/VGCNF, dan PdAu/VGCNF. Terlihat bahwa morfologi nanopartikel Au, Pd, dan PdAu menunjukkan perilaku aglomerasi derajat sedang dengan distribusi homogen pada VGCNF. Pola struktur kristal dianalisis dengan analisis difraksi elektron area terpilih (SAED) (dimasukkan pada Gambar 2a–c) dan menunjukkan bahwa Au/VGCNF, Pd/VGCNF, dan PdAu/VGCNF biasanya adalah struktur polikristalin fcc. Gambar TEM (HRTEM) resolusi tinggi yang diamati pada Gambar. 2d, e menunjukkan jarak spasi 0,24 nm dan 0,22 nm yang sesuai dengan jarak interplanar dari bidang (111) untuk Au dan Pd, masing-masing. Gambar 2f mengungkapkan d -nilai spasi dari pinggiran kisi yang ditentukan dari daerah yang ditandai adalah 0,230 nm, sesuai dengan bidang (111) dari paduan PdAu kubik berpusat muka (fcc).

a –c Gambar tipikal pola TEM dan SAED untuk Au/VGCNF, Pd/VGCNF, dan Pd₁ Au₁ /VGCNF. d –f Gambar HRTEM untuk Au/VGCNF, Pd/VGCNF, dan Pd₁ Au₁ /VGCNF elektrokatalis

Analisis luas permukaan dan porositas BET adalah alat yang efektif untuk evaluasi kemampuan Au/VGCNF, Pd/VGCNF, dan Pd₁ Au₁ /VCGNF untuk bertindak sebagai adsorben. BET mengungkapkan jumlah bahan teradsorpsi (adsorbat) dan derajat adsorpsi ke permukaan adsorben pada padatan (adsorben) [31]. Luas permukaan BET dari Au/VGCNF, Pd/VGCNF, dan Pd₁ Au₁ /VGCNF sampel dianalisis menggunakan isoterm adsorpsi/desorpsi nitrogen seperti yang ditunjukkan pada Gambar. 3a. Kurva isoterm mencerminkan bentuk isoterm dan akan memberikan penilaian kualitatif dari porositas bahan. Pada penelitian ini, bentuk isoterm yang diamati menunjukkan bahwa Au/VGCNF, Pd/VGCNF, dan Pd₁ Au₁ /VCGNF ditemukan sebagai tipe IV (berdasarkan klasifikasi IUPAC) dan menunjukkan lereng yang curam pada kisaran tekanan relatif 0,83–0,99 P/P₀ , yang merupakan ciri adanya material mesopori [32]. Bahan dengan struktur mesopori menunjukkan stabilitas morfologi yang baik dan katalis aktif karena peningkatan volume adsorpsi, sehingga meningkatkan aktivitas katalitik.

a N₂ isoterm adsorpsi/desorpsi Au/VGCNF, Pd/VGCNF, dan Pd₁ Au₁ /VGCNF. b Kurva distribusi ukuran pori Au/VGCNF, Pd/VGCNF, dan Pd₁ Au₁ /VGCNF

Luas permukaan, volume pori, dan nilai diameter pori elektrokatalis disajikan pada Tabel 1. Dari tabel tersebut, VGCNF memiliki luas permukaan 30,56 m ² /g, volume pori 0,415 cm ³ /g, dan diameter pori 7,0 nm. Kehadiran katalis logam (Au, Pd, dan PdAu) di atas dukungan katalis ditemukan untuk mengurangi luas permukaan, volume pori, dan diameter pori. Hal ini disebabkan logam katalis telah tercerai-berai dan memenuhi permukaan penyangga katalis. Bahkan setelah logam katalis tersebar pada VGCNF, loop histeresis tipe H1 (P/Po 0,83-0,99) menunjukkan struktur mesopori material tidak terpengaruh. Dapat disimpulkan bahwa logam katalis telah menyebar dengan baik pada permukaan VGCNF dan sesuai dengan gambar TEM yang menunjukkan bahwa logam katalis tersebar merata pada permukaan penyangga katalis.

Dilihat dari distribusi ukuran pori BJH Au/VGCNF, Pd/VGCNF, dan Pd₁ Au₁ /VCGNF disajikan pada Gambar. 3a, ini menunjukkan diameter pori yang sempit atau diameter pori sekitar 4 nm (jari-jari pori), sedangkan rentang ukuran pori yang diamati di semua katalis terletak antara 2 dan 5 nm, yang menjadikan material sebagai penyerap yang unik menengah [31]. Au/VGCNF, Pd/VGCNF, dan Pd₁ Au₁ /VGCNF elektrokatalis memiliki volume pori kecil dibandingkan dengan VGCNF, karena beberapa ruang berpori di dalam media pendukung diblokir dengan nanopartikel katalis logam atau reaksi kimia lainnya selama proses dekorasi nanopartikel dan akibatnya mengakibatkan penurunan volume pori. Namun, luas permukaan BET yang dilaporkan tidak mengacu pada seluruh luas permukaan elektrokatalitik. Oleh karena itu, analisis elektrokimia perlu dilakukan secara detail untuk mendapatkan luas permukaan aktif.

Analisis XPS dilakukan untuk menyelidiki C 1s , Hal 3d , dan Au 4f dalam sampel elektrokatalis Au/VGCNF, Pd/VGCNF, dan PdAu/VGCNF. Spektrum puncak resolusi tinggi dari C 1s puncak ditunjukkan pada Gambar. 4a; Empat komponen yaitu 284,6 eV (C-C), 285.4 eV (C-O-C), 286,6 eV (C-OH), dan 287,8 eV (C = O), dapat diamati yang menentukan struktur kimia VGCNF dalam sampel elektrokatalis. XPS digunakan untuk mengkarakterisasi kimia, keadaan elektronik, dan komposisi permukaan Au/VGCNF, Pd/VGCNF, dan PdAu/VGCNF. Tabel 2 menunjukkan persentase berat dan energi ikat untuk Pd/VGCNF, Au/VGCNF, dan Pd₁ Au₁ /VGCNF. Gambar 4b menggambarkan Au 4f regional regional spektrum Au/C dan PdAu/C. Di Au 4f spektrum Au/VGCNF, puncak ganda mengandung pita energi rendah (Au 4f _7/2 ) dan pita energi tinggi (Au 4f _5/2 ) pada 84.1 dan 87.7 eV, yang mengkonfirmasi pengurangan \( {\mathrm{AuCl}}_4^{2-} \) menjadi Au ⁰ . Pada Gambar 4c untuk Pd/VGCNF, puncaknya terletak di 335.4(Pd 3d _5/2 ) dan 340,7 eV (Pd 3d _3/2 ) dikaitkan dengan Pd 3d yang mengkonfirmasi pengurangan \( {\mathrm{PdCl}}_4^{2-} \) menjadi Pd ⁰ . Sedangkan untuk bimetal PdAu/VGCNF dapat diamati bahwa kedua energi ikat Au 4f dan Pd 3d sedikit menyimpang ke sudut yang lebih rendah dibandingkan dengan Au/VGCNF dan Pd/VGCNF. Pergeseran energi ikat memberikan pembenaran yang kuat dari transfer elektron dari Pd ke Au, yang mungkin dapat dikaitkan dengan interaksi elektronik yang terganggu antara orbit atom Pd dan Au dan selanjutnya memverifikasi pembentukan struktur nano paduan. Jumlah Pd dan Au dapat ditentukan melalui intensitas puncak dan menunjukkan bahwa rasio Pd terhadap Au adalah 79:21, jauh lebih tinggi dari nilai yang dihitung (50:50). Ini berarti atom Pd diperkaya di bagian luar permukaan katalis. Hal ini terjadi karena perbedaan laju reduksi Au(III) dan Pd(II). Dapat diasumsikan bahwa inti Au terbentuk lebih dulu, diikuti dengan penambahan atom Pd ke inti, dan mengarah pada pembentukan struktur nano paduan dengan permukaan yang diperkaya Pd. Pengamatan ini menegaskan keberadaan logam Pd dan Au di permukaan luar Pd₁ Au₁ /sampel nanokatalis VCGNF.

Spektrum XPS resolusi tinggi dari Pd/VGCNF, Au/VGCNF, dan Pd₁ Au₁ /VGCNF. a Hal 4f _5/2 dan Pd 4f _7/2 . b Au 4f _5/2 dan Au 4f _7/2 dan c C 1s

Analisis Elektrokimia untuk Oksidasi Gliserol dalam Larutan Basa

Pemindaian CV katalis dilakukan setelah pemuatan katalis pada elektroda karbon kaca selama kurang lebih 24 jam. Waktu pembebanan secara tidak langsung mempengaruhi rapat arus dan potensial puncak (Ep). Dengan bertambahnya waktu pemuatan, lebih banyak waktu yang dibutuhkan untuk jumlah total hidrogen yang berevolusi dari sampel. Kepadatan arus sangat meningkat dari potensi awal yang terkait dengan desorpsi hidrogen. The typical cyclic voltammetry curve in basic solution is generally used for the determination of hydrogen and oxygen adsorption on Pd/VGCNF, Au/VGCNF, and Pd₁ Au₁ /VGCNF in 0.5 M NaOH solution as depicted in Fig. 5. Region A in the potential range from − 0.8 to − 0.5 V can be assigned solely to adsorption and desorption of the H ⁺ ion on electrocatalyst surface area. However, the adsorption and desorption peaks cannot be clearly distinguished. This is possibly caused by the blocking of certain electrocatalyst active sites upon the incorporation of the second metal [33]. In region B, in the intermediate potential between − 0.4 V and − 0.2 V, the catalyst behaves as an ideally polarizable catalyst (double layer region) where the surface has free-adsorbed H ⁺ dan OH ⁻ . Meanwhile, the formation of oxides occurs in the forward scan at the higher potential more than − 0.2 V between 0.1 and 0.3 V. The oxide reduction peak at − 0.34 V is also observed during the reverse scan in region C [33]. Meanwhile, Au/VGCNF electrocatalyst shows a slightly different voltammetry standard curve, where region A is the large capacitive current associated with the double layer region and is a typical electrochemical behavior of the Au catalyst followed by area B where the absorption of hydroxyl OH ⁻ species and oxide formation is at higher potential above − 0.2 V vs Ag/AgCl, while area C is observed as a reduction region of Au oxides.

Cyclic voltammogram for Pd/VGCNF, Au/VGCNF, and Pd₁ Au₁ /VGCNF in 0.5 M glycerol

Figure 6 shows the anodic peak current density for Au/VGCNF, Pd/VGCNF, and Pd₁ Au₁ /VGCNF electrocatalysts for the oxidation of glycerol in alkaline medium. The performance in cyclic voltammetry curve of the prepared electrocatalysts towards the oxidation of the glycerol is seen in terms of four parameters:(1) onset oxidation potential, (2) forward anodic peak potential, (3) forward anodic peak current, and (4) CO tolerance as can be seen in Table 1 are investigated. The oxidation peak in forward scan can be attributed to the oxidation of freshly chemisorbed species from glycerol adsorption, and the oxidation peak in negative scan means the removal of carbonaceous species incompletely oxidized the forward scan. As a result, the ratio of the forward current density peak (I _f ) to the backward current density peak (I _b ) reflects the tolerance ability to carbonaceous species accumulation. A high ratio I _f /Aku _b indicates efficient oxidation of glycerol during the forward scan and little accumulation of carbonaceous residues. In terms of peak current density, Pd₁ Au₁ /VGCNF provided the best performance compared to Pd/VGCNF and Au/VGCNF. Besides that, Pd₁ Au₁ /VGCNF also shows the lowest onset potential (− 0.75 V vs.> − 0.60 for glycerol). The worst results were obtained with methanol, especially in terms of current density and oxidation overpotential. After a detailed perusal of the literature and estimating that the metal catalyst loading is very low (0.1 mg cm ⁻² ), one may readily realize that Pd₁ Au₁ /VGCNF can be classified among the most active electrocatalysts ever reported for the oxidation of glycerol in half-cells. The current density for glycerol oxidation with electrocatalysts Pd/C, Pt/C, and Au/C is significantly higher compared to Zhang et al. [26] due to higher catalyst loading and temperature dependence.

Cyclic voltammogram for Pd/VGCNF, Au/VGCNF, and PdAu/VGCNF in 0.5 M glycerol + 0.5 M NaOH

In Table 3, it exhibits that the bimetallic PdAu/VGCNF has high electrocatalytic activity towards the glycerol oxidation with respect to monometallic electrocatalyst. The presence of a second metal which is Au leads to a slight increase of the activity between 0.45 and 0.7 V versus Ag/AgCl, as higher current densities are achieved. Looking at CV curve (Fig. 6) recorded at Pd/VGCNF and Au/VGCNF electrocatalysts, both materials presented almost the same onset potential for the glycerol oxidation close to − 0.4 V vs Ag/AgCl. The onset potential for PdAu/VGCNF shifted towards a lower value due to the bifunctional mechanism which gives the catalytic improvement. In agreement with the bifunctional mechanism, Au is an oxyphilic material which has oxygen adsorbing character which promotes the chemisorbed species and assists the electrooxidation of glycerol leading to the decrease of the onset potential of glycerol electrooxidation and enhanced catalytic activity [34]. Besides that, bimetallic electrocatalysts PdAu/VGCNF also examined excellent CO tolerance, when the ratio I _f /Aku _b is superior than others, which reveals the higher catalytic efficiency for direct oxidation during the forward scan.

Tafel Analysis

Figure 7a–c shows a Tafel plot of all the electrocatalysts tested in the reaction of the glycerol oxidation. Each Tafel plot shows two linear areas with an increase in Tafel slope. The Tafel slope is one of the kinetic parameters associated with the determination of a reaction that occurs on the surface of the electrode. At low oxidation potential (E_v ), the Tafel slope is low which indicates the kinetic transfer charge is fast during the reaction of the glycerol oxidation. At higher oxidation potential (E_v ), the value of the second Tafel slope is greater which indicates that the electrocatalytic reaction mechanism changes. The change in Tafel slope can be attributed to the kinetic rate of the charge transfer in the reaction decrease due to the presence of intermediate species which can poison the surface of the electrocatalysts (Kang et al. [35]).

Tafel slope for Au/VGCNF, Pd/VGCNF, and Pd₁ Au₁ /VGCNF electrocatalysts in solution (0.5 M NaOH + 0.5 M Glycerol) at scanning rate 50 mVs ^− 1

Table 4 lists the values of Tafel slope obtained on low and high potential oxidation. The Tafel slope for Pd₁ Au₁ /VGCNF electrocatalyst is lower than Pd/VGCNF and Au/VGCNF indicating that the charge transfer kinetic on bimetallic electrocatalyst is faster than single metal electrocatalyst for the glycerol oxidation reaction. Once the potential oxidation (E_v ) is increased, the Tafel slope also increased from low to high. This indicates that the reaction mechanism of the glycerol oxidation changes over the first one. Similar to the phenomenon occurring in other alcohol oxidation such as methanol [36] and ethanol [37], the maximum mass transfer of glycerol occurs in high potential oxidation (E_v ) which makes the hydroxyl adsorption and formed the oxide layer at electrocatalysts surface.

Performance of Au/VGCNF, Pd/VGCNF, and PdAu/VGCNF in Direct Glycerol Fuel Cell

Electrochemical I-V Characterization

Reliability of any synthesized electrocatalyst can generally be defined as the probability of the electrocatalyst to carry out its desired activity, which in this study is the catalysis for direct glycerol fuel cell applications. Although the electrocatalyst was analyzed earlier using a half-cell and showed good performance, the application of the electrocatalyst in direct glycerol fuel cells does not show outstanding performance. Figure 8 illustrates the cell polarization and power density curves of DGFC with FAA-3 (FuMA-Tech) membrane which show the passive DGFC single cell test results for Au/VGCNF, Pd/VGCNF, and Pd₁ Au₁ /VGCNF. From the DGFC performance test, results clearly show that the OCV and power density of Pd₁ Au₁ /VGCNF are higher compared to Pd/VGCNF and Au/VGCNF electrocatalysts. Table 5 lists the performance of all electrocatalysts in DGFC application in this study. In terms of electrocatalysts prices, roughly the price for Pd₁ Au₁ /VGCNF is USD 0.26/mg cheaper than the price for black Pd commercial which is USD 0.41/mg.

Polarization and power density curves for direct glycerol fuel cell (DGFCs) using PdAu/VGCNF at 25  ^° C

Passive-performance alkaline fuel cell may not be able to challenge the performance of the active fuel cell. One of the main problems for the passive alkaline fuel cell is related to the liquid electrolyte such as NaOH or KOH. This is due to the very spontaneous reaction with the NaOH electrolyte solution which is very sensitive to the presence of CO₂ from the air [38]. The use of air instead of oxygen will expose the hydroxyl ions to CO₂ contained in the air and will lead to the formation of Na₂ CO₃ or K₂ CO_3. The presence of carbonates will indirectly reduce the amount of OH ions in the anode and thereby reduce the ionic conductivity [39]. Too much carbonate will block the pores of the gas diffusion layer and can severely degrade the performance of the single cell [31,32,33] .

Electrochemical Impedance Spectroscopy

The performance of the direct glycerol fuel cell assessed by polarization curves does not reflect and give enough information about the condition of the inner component. Thus, electrochemical impedance spectroscopy (EIS) has to be carried out together with polarization curve measurements to evaluate the overall kinetic performance such as distinguishing the influences (voltage drop caused by charger transfer, membrane or mass transfer resistance) of the different processes (kinetic, ohmic, or mass transport dominated) in the whole electronic system [40]. Figure 9a–c shows the Nyquist plots which are one typical curve of electrochemical impedance spectroscopy (EIS) for three different types of electrocatalysts. Generally, the diameter of the semicircle arc is to measure the charge–transfer resistance for glycerol oxidation reactions. Apparently, the DIA in Fig. 9 showed the order as follows:Au/VGCNF > Pd/VGCNF > PdAu/VGCNF. This suggests that PdAu/VGCNF has the greatest charge transport performance, which was consistent with the abovementioned outstandingly excellent electrochemical catalytic activity. Besides that the equivalent circuit which was fit to EIS spectra shown in Fig. 9d in which Rs represents the solution resistance and Rct represents the charge–transfer resistance for the process of glycerol electrocatalytic oxidation. Overall, the Rct value for PdAu/VGCNF is much smaller compared to other electrocatalyst which reveals that the electron transfer for glycerol oxidation on PdAu/VGCNF is much easier, which could enhance the electrocatalytic activity of the electrocatalyst for glycerol oxidation.

Nyquist plots of electrochemistry impedance spectra for glycerol oxidation reaction of a Au/VGCNF, b Pd/VGCNF, c PdAu/VGCNF, and d circuit.

Since there is no difference in the MEA and cell assembly between the two cells, the lower resistance of PdAu/VGCNF cell can be attributed to the catalyst layer. In the PdAu/VGCNF-coated layer, small catalyst particle size can allow sufficient humidifying water to be held in the electrode and the uniform particle distribution can prevent the agglomeration effect, which may lead to the increasing boundary resistance. Furthermore, the small particle and uniform distribution can decrease the migration path of ions, which can further reduce the distributed resistance. The high-frequency EIS results provide the information that in the real fuel cell operation, the smaller cathode catalyst particle size will not only facilitate the ORR kinetics, but also reduce the performance loss that is due to the ohmic and contact resistances.

MEA Morphology Analysis

Basically, the low performance of the fuel cell may be due to the inherent properties of the materials used in fuel cell construction, defects in the materials, and the assembly process of the fuel cell. Below, we further discuss the potential effects that give rise to the lower performance of direct glycerol fuel cell.

Microporous Layer (MPL) Cracking

To justify the low performance of direct glycerol fuel cell, the morphology of some MEA were evaluated by SEM after the performance evaluation in the fuel cell. Figure 10a shows the typical SEM surface image for the microporous layer (MPL) consisting of the carbon black layer and the catalyst layer. Cracks were observed on the catalyst surface. The formation of wrinkles and cracks that occur in the MPL is likely due to a chemical reaction that cannot be controlled during the operation of single cell. This reaction probably causes dimensional changes in the polymer electrolyte membrane such as volume expansion and contraction that occur due to the hydrothermal stresses during the fuel cell operation [41]. Catalyst drying process occurs irregularly, causing cracks in the catalyst layer [42]. The fuel cell still can be operated in the presence of catalyst cracking. However, this indirectly degrades the fuel cell performance.

a Delamination. b MPL cracking. c Catalyst agglomeration

Delamination

The SEM image presented in Fig. 10b clearly shows an obvious gap between the membrane and the electrodes after the operation of the direct glycerol fuel cell. This could be caused by the thermal expansion mismatch between the membrane and electrodes occurring during the operation of the fuel cell. In addition, this may also occur during the manufacturing process of the MEA such as owing to a too high temperature used during the catalyst layer drying process, which may form a vapor which will be trapped and thereby create an area where the adhesion between the two layers is poor [43]. Nevertheless, the electrolyte in the catalyst ink is expected to act as a binder during the lamination process. However, this may not occur in practice because the manual catalyst casting will lead to wrinkles on the electrode resulting in the existence of voids between the layers and subsequent degradation in the performance. Another reason for the gap in the interface may be the fact that mechanical hot pressing is difficult to ensure a fully intimate membrane–electrode interface. This is another factor which affects the fuel cell performance. This condition will increase the contact resistance and will require a longer time for the proton to travel to the catalyst sites and the existence of dead zone can occur which indirectly led to the loss of catalyst activity.

Orientation of Catalyst Layer

The catalyst layer was cast by manual casting. This method is difficult to control, and it has the potential to cause the agglomeration of undispersed catalyst powder [44, 45]. Figure 10c shows the areas corresponding to the agglomeration of nanoparticles on the MPL layer. This can create contact resistance over the entire MEA and therefore adversely impact the porosity or proton conductivity.

AEM Stability

The anion exchange membrane (AEM) shows some technological and scientific limitation. AEM suffers from a poor chemical stability in alkaline media arising from the hydroxide attack on the cationic group and resulting in the decreased anionic conductivity [46]. The AEM with OH ⁻ ion is less stable because AEM is easily converted to the less conductive \( {\mathrm{CO}}_3^{2-} \) and even less conductive \( {\mathrm{HCO}}_3^{-} \) forms when exposed to air even for short time. Figure 10d, e shows the comparison between before and after the treatment with alkali, indicating that AEM faced the problem of high swelling with the increased hydration [47]. This is a challenge in the handling of AEM. AEM is used to overcome the Nafion membrane degradation problem. However, AEM has a tendency to be hydrocarbon or aromatic based which gives rise to poor oxidation stabilities [41].