Katalis Berbasis Platinum pada Berbagai Pendukung Karbon dan Polimer Konduktor untuk Aplikasi Sel Bahan Bakar Metanol Langsung:Tinjauan
Abstrak
Logam nanopartikel berbasis platinum (Pt) telah menerima banyak perhatian dan merupakan katalis paling populer untuk sel bahan bakar metanol langsung (DMFC). Namun, biaya katalis Pt yang tinggi, oksidasi kinetik yang lambat, dan pembentukan molekul antara CO selama reaksi oksidasi metanol (MOR) merupakan tantangan utama yang terkait dengan katalis Pt logam tunggal. Studi terbaru berfokus pada penggunaan paduan Pt, seperti logam Fe, Ni, Co, Rh, Ru, Co, dan Sn, atau bahan pendukung karbon untuk meningkatkan kinerja katalitik Pt. Dalam beberapa tahun terakhir, katalis paduan Pt dan Pt yang didukung pada potensi besar bahan karbon seperti MWCNT, CNF, CNT, CNC, CMS, CNT, CB, dan graphene telah menerima minat yang luar biasa karena sifat signifikannya yang dapat berkontribusi pada MOR yang sangat baik. dan kinerja DMFC. Makalah tinjauan ini merangkum pengembangan paduan dan bahan pendukung di atas yang terkait untuk mengurangi penggunaan Pt, meningkatkan stabilitas, dan kinerja elektrokatalitik Pt yang lebih baik di DMFC. Terakhir, pembahasan masing-masing katalis dan pendukung dalam hal morfologi, aktivitas elektrokatalitik, karakteristik struktural, dan kinerja sel bahan bakarnya disajikan.
Pengantar
Teknologi sel bahan bakar telah mendapat perhatian luas di seluruh dunia. Fuel cell (FCs) merupakan teknologi pembangkit listrik alternatif yang menjanjikan yang mengubah energi kimia menjadi energi listrik melalui reaksi elektrokimia [1, 2]. Selain itu, untuk teknologi sel bahan bakar, fokus utama dalam teknologi sel bahan bakar adalah untuk menghasilkan produksi berbiaya rendah, sehingga mencapai kinerja yang kuat dari sistem sel bahan bakar dan menemukan bahan yang tahan lama. Namun demikian, masalah umum yang muncul dalam teknologi sel bahan bakar saat ini adalah bahwa sistem melibatkan biaya intrinsik yang tinggi dan daya tahan yang buruk [1]. Terlepas dari janjinya sebagai sel bahan bakar, sel bahan bakar metanol langsung (DMFC) memiliki tantangan dan keterbatasan, para peneliti terkemuka untuk mempelajari metode untuk meningkatkan efisiensi dan kinerja DMFC. Banyak masalah dengan DMFC telah diidentifikasi dan tetap belum terpecahkan, termasuk crossover bahan bakar metanol dari elektroda anoda ke elektroda katoda [3,4,5] kinerja yang buruk disebabkan oleh laju kinetika yang lambat, ketidakstabilan katalis, dan manajemen termal dan air [6,7,8].
Baru-baru ini, ada banyak penyelidikan pada sel bahan bakar, termasuk DMFC, sel bahan bakar membran pertukaran proton (PEMFC), sel bahan bakar oksida padat (SOFC), dan sebagainya, yang merupakan teknologi sel bahan bakar yang populer. Sebagai sumber energi baru, DMFC dapat digunakan untuk aplikasi mobile dan stasioner [9, 10]. Banyak kemajuan penelitian telah dicapai di bidang sel bahan bakar. Di antara sel bahan bakar, DMFC telah dipelajari secara ekstensif dalam beberapa tahun terakhir [11,12,13,14,15,16] karena banyak keuntungannya, seperti kepadatan daya yang tinggi, kemudahan penanganan bahan bakar, kemudahan pengisian, dan lingkungan yang rendah. dampak [17, 18]. Namun, beberapa tantangan teknis untuk komersialisasi DMFC tetap belum terselesaikan, termasuk persilangan metanol, laju reaksi kimia yang rendah, dan keracunan katalis. Namun, DMFC masih mendapat perhatian dari banyak peneliti dan telah menjadi sel bahan bakar paling populer karena operasi suhu rendahnya (sistem DMFC beroperasi pada 373 K). Karena keunggulan DMFC yaitu efisiensi energi yang tinggi dan sistem start-up yang cepat, teknologi DMFC sangat cocok untuk diterapkan sebagai sumber listrik perumahan, baterai pada perangkat mobile, dan sebagai bahan bakar kendaraan [19,20,21,22]. Selain itu, konsep DMFC dapat dipelajari lebih lanjut untuk menemukan sumber bahan bakar alternatif seperti dari gas alam dan biomassa, serta fermentasi produk pertanian untuk menghasilkan etanol, untuk meminimalkan ketergantungan pada sumber energi yang tidak aman [14].
Pada DMFC, sisi anoda disuplai dengan larutan metanol yang akan mengalami elektrooksidasi menjadi karbon dioksida (CO2 ) melalui reaksi di bawah ini:
Dalam sistem DMFC, ada dua jenis mode DMFC:mode aktif dan pasif [23,24,25]. Dalam sistem DMFC aktif, aliran keluar dari tumpukan DMFC disirkulasi ulang melalui kontrol loop tertutup dari umpan metanol cair. Sementara itu, metanol cair pada aliran anoda dikendalikan oleh sensor konsentrasi metanol yang berperan penting dalam memberikan injeksi tambahan metanol dan air yang cukup untuk memulihkan bahan bakar ini berdasarkan konsentrasi target. Ada beberapa jenis sensor konsentrasi metanol yang digunakan dalam sistem DMFC untuk mengontrol dan mempertahankan konsentrasi umpan metanol [17]. Biasanya, metanol cair dikirim ke sisi anoda oleh pompa peristaltik sedangkan udara di sekitarnya yang mengandung oksigen disuplai ke sisi katoda oleh blower atau kipas [16]. Dalam mode pasif sistem DMFC, metanol cair diumpankan terus menerus ke sistem. Konsep pasif ini sangat menarik untuk sistem DMFC [26,27,28]. Konsep pasif berarti sistem beroperasi sepenuhnya secara mandiri tanpa perangkat pendukung. Konsep DMFC pasif berarti sistem beroperasi sepenuhnya secara mandiri tanpa bantuan perangkat eksternal untuk memompa metanol dan meniupkan udara ke dalam cerobong. Dalam mode pasif sistem DMFC, lapisan katalis akan disuplai oleh metanol dan oksigen sebagai reaktan. Selama reaksi oksidasi metanol (MOR), CO2 dan air akan dikeluarkan dari sel dengan cara pasif, yaitu difusi, konveksi alami, aksi kapiler, dll. [20]. Mode pasif DMFC tampaknya lebih menguntungkan dibandingkan dengan mode aktif DMFC dalam hal desain yang lebih sederhana, lebih kompak, dan biaya rendah. Desain dan kontrol sistem yang kompleks dapat menjadi kelemahan DMFC mode aktif [21]. Dari aspek penggunaan praktis, mode aktif DMFC tampaknya lebih sesuai untuk sistem daya tinggi, sedangkan mode pasif DMFC lebih cocok digunakan pada kebutuhan daya rendah [22].
Gambar 1 menunjukkan pengaturan dan desain untuk DMFC sel tunggal. Tumpukan DMFC sel tunggal terdiri dari perakitan elektroda membran lima lapis (MEA) yang diapit oleh dua pelat yaitu anoda dan katoda. Di sisi anoda, metanol cair (mengandung metanol dan air deionisasi) dan metanol absolut dialirkan ke saluran dengan pompa peristaltik. Di sisi katoda, udara dipompa ke sel bahan bakar oleh rotameter. Pengontrol suhu di tumpukan DMFC digunakan untuk menjaga suhu kerja di dalam sel dengan perangkat pemanas tambahan. Perangkat beban elektronik digunakan untuk mengubah kerapatan arus ke tingkat yang berbeda dan mengukur nilai tegangan yang sesuai. Kinerja sel dipantau oleh stasiun kerja elektrokimia, sedangkan produksi CO2 sebagai produk akhir dari reaksi keseluruhan diukur dengan CO2 detektor konsentrasi [23]. Dalam sel bahan bakar metanol langsung, ada beberapa parameter operasi penting yang harus diperhatikan selama studi eksperimental, yaitu (i) suhu kerja, (ii) konsentrasi metanol, dan (iii) laju alir masukan larutan metanol umpan dan udara [23] . Gambar 2a, b masing-masing menunjukkan mode aktif dan pasif DMFC.
Pengaturan eksperimental umum untuk DMFC sel tunggal [23]
Diagram skema dari a mode aktif [24] dan b mode pasif [25] dari DMFC
Tinjauan ini akan fokus pada kemajuan terbaru dalam penelitian dan pengembangan dukungan katalis berbasis katalis Pt sebagai katalis mulia di DMFC. Kami menyertakan aktivitas katalis berbasis Pt yang dikombinasikan dengan paduan, logam, logam transisi, karbida logam, nitrida logam, dan berbagai spesies karbon, seperti graphene/graphene oxide (G/GO), carbon nanotube (CNT), carbon nanofiber ( CNF), carbon nanocoil (CNC), carbon black (CB), multiwall carbon nanotube (MWCNT), dan carbon mesopori (CMS), serta polimer konduktif, seperti polianilin (PANi) dan polipirol (Ppy) sebagai pendukung bahan. Banyak metode sintesis dapat diterapkan untuk membuat katalis berbasis Pt. Metode yang paling umum diterapkan untuk mendapatkan partikel Pt skala nano adalah impregnasi [29,30,31,32,33,34], teknik hidrotermal [35,36,37,38,39,40,41], mikroemulsi [42,43, 44,45], dan reduksi [46, 47]. Umumnya, metode preparasi dapat mempengaruhi morfologi dan ukuran partikel katalis; oleh karena itu, pemilihan metode sintesis katalis sangat penting.
Kinerja Berbagai Jenis Katalis Berbasis Pt
Selama dekade terakhir, banyak peneliti telah memfokuskan penelitian mereka pada pengembangan elektrokatalis untuk meningkatkan aktivitas elektrokatalitiknya dalam MOR metanol untuk sistem DMFC [37, 38]. Platinum (Pt) adalah katalis logam tunggal yang menunjukkan aktivitas katalitik yang sangat tinggi untuk MOR. Namun, Pt murni saja dalam sistem DMFC dapat dengan mudah diracuni oleh spesies antara, yaitu karbon monoksida (CO), dan biaya katalis Pt yang tinggi membatasi aplikasi komersialnya sebagai elektrokatalis, sehingga menurunkan laju kinetik oksidasi metanol. dalam sistem DMFC [48,49,50]. Ketiga titik tersebut merupakan kendala dan keterbatasan utama penggunaan Pt saja sebagai elektrokatalis untuk DMFC. Namun, untuk mengatasi hambatan tersebut, beberapa penelitian telah dilakukan untuk mensintesis elektrokatalis paduan berbasis Pt untuk mencapai kinerja elektrokatalitik yang lebih baik dengan penggunaan Pt yang lebih sedikit [11, 47, 51, 52]. Biasanya, ukuran rata-rata partikel Pt dan morfologinya dapat ditentukan melalui analisis scanning emission micrograph (SEM) atau transmission electron micrograph (TEM), yang merupakan metode paling umum di bidang katalisis yang dapat digunakan untuk mengkarakterisasi sifat fisik elektrokatalis. Tabel 1 menunjukkan ukuran partikel rata-rata partikel Pt dengan berbagai metode sintesis, sifat, dan kinerjanya.
Bimetal PtRu dianggap sebagai katalis paling aktif karena mekanisme bifungsional dan efek ligan [48, 53]. PtRu menjadi paduan katalis yang menarik, dan telah digunakan sampai saat ini dengan banyak penyangga karbon. Namun, efek toksikologi dari penambahan logam rutenium (Ru) masih belum pasti [49]. Oleh karena itu, penelitian tentang paduan yang lebih murah yang mencampur Pt dengan logam nonmulia lainnya telah dilakukan [49,50,51,52, 54,55,56,57], seperti yang dibahas di bagian “Kinerja paduan berbasis Pt”.
Kinerja Paduan Berbasis Pt
Arico dkk. [35] menemukan bahwa banyak penelitian telah dilakukan untuk meningkatkan aktivitas katalitik katalis Pt di MOR. Dalam banyak penelitian, rasio Pt-Ru optimal telah diidentifikasi sebagai 1:1, dan ukuran partikel pada skala nano adalah ukuran ideal untuk meningkatkan pemanfaatan katalis. Namun, Shi et al. [38] mengidentifikasi bahwa 3:2 adalah rasio optimal untuk Pt-Ru dalam eksperimen mereka untuk meningkatkan aktivitas katalitik MOR. Selain itu, aktivitas elektrokatalitik untuk aktivitas elektrooksidasi metanol juga dapat ditingkatkan jika partikel elektrokatalis PtRu berukuran nano pada kisaran 2–4 nm. Paulas dkk. [39] setuju dengan pernyataan ini. Seperti kita ketahui, Pt menunjukkan reaktivitas yang tinggi terhadap bahan bakar metanol, menjadikan logam Pt sebagai elektrokatalis yang ideal untuk elektroda anoda dalam sistem DMFC. Namun demikian, selama MOR katalis Pt, karbon monoksida (CO), yaitu spesies antara, akan terbentuk pada permukaan partikel Pt, yang dengan demikian meracuni permukaan katalis [58,59,60,61]. Oleh karena itu, diperlukan beberapa upaya untuk mengatasi masalah yang terkait dengan pembentukan spesies beracun pada permukaan partikel Pt, sehingga tidak menutupi area situs aktif Pt. Umumnya paduan biner, seperti PtRu [62,63,64,65,66], PtRh [67,68,69,70,71], PtAu [72,73,74], PtSn [62, 63, 75, 76,77], PtNi [64,65,66,67,78,69], PtCo [70, 71, 78,79,80], dan PtFe [81,82,83,84,85], sering digunakan sebagai kombinasi elektrokatalis untuk elektroda anoda pada sistem DMFC. Penambahan logam-logam tersebut, seperti rutenium (Ru), timah (Sn), dan rhodium (Rh), diyakini dapat menghasilkan aktivitas katalitik yang lebih tinggi.
Penggabungan Nikel (Ni) ke dalam katalis berbasis platinum memberikan kinerja yang unggul untuk MOR dan DMFC. Dalam penelitian terbaru, Guerrero-Ortega dan rekan kerja menjelaskan penambahan Ni dalam dukungan Pt-Vulcan mempromosikan peningkatan penting dalam arus faradik selama MOR dari satu urutan besarnya, meskipun penggunaan Pt lebih rendah dalam katalis bimetal [ 55] . Hasil eksperimen mereka juga menyarankan bahwa penambahan Ni mempromosikan beberapa modifikasi struktural dan elektronik yang meningkatkan kinerja reaksi yang lebih baik pada antarmuka elektroda. Dalam karya lain, penggabungan Au ke paduan Pt meningkatkan aktivitas elektrokatalitik karena perubahan struktur elektronik dan peningkatan area aktif elektrokimia (ECSA) [47]. Sedangkan penambahan Timah (Sn) ke paduan berbasis Pt menunjukkan peningkatan aktivitas elektrokatalitik, yang sangat dipengaruhi oleh penggabungan Sn dalam sistem paduannya dan bentuk teroksidasi, meningkatkan reaksi lebih mudah karena potensi oksidasi yang lebih rendah [ 56]. Juga, penambahan Cobalt (Co) ke paduan berbasis Pt meningkatkan sifat katalitik secara signifikan oleh katalis PtCo (1:9)/rGO yang ditemukan sepuluh kali lebih tinggi daripada Pt/rGO [51]. Peningkatan densitas arus dikaitkan dengan dispersi yang lebih tinggi dari nanopartikel PtCo pada sifat hidrofilik dari dukungan rGO yang mendorong aktivasi air dan menyebabkan oksidasiiklan CO di situs Pt. Selanjutnya, menurut mekanisme bifungsional Co, ia mempromosikan H2 Aktivasi O menghasilkan lebih banyak ion -OH dan O2 . lainnya -mengandung spesies untuk mengoksidasi CO- spesies intermediet di situs Pt [57]. Mekanisme bifungsional Co ini juga dapat digunakan untuk logam transisi katalitik lainnya menuju MOR. Mekanisme oksidasi katalitik untuk spesies CO menjadi CO2 dengan adanya katalis PtCo dapat diringkas sebagai berikut:
Selanjutnya, Löffler et al. [86] berhasil mensintesis PtRu sebagai katalis anoda untuk DMFC yang menghasilkan elektrokatalis paling aktif untuk elektrooksidasi metanol pada kira-kira 50 at.% Ru. Sementara itu, Dinh et al. melaporkan [87] bahwa PtRu dengan rasio PtRu 1:1 memiliki sifat logam yang lebih kuat dan aktivitas elektrokatalitik yang lebih tinggi untuk oksidasi metanol (MOR). Kinerja terkait dengan dua faktor utama ini:(i) luas permukaan katalis yang dimaksimalkan dan (ii) permukaan katalis dengan jumlah maksimum situs paduan logam dengan rasio atom mendekati 1:1. Kedua kelompok ini juga menunjukkan sangat. Berdasarkan mekanisme bifungsional, Aricò et al. [58] dan Goodenough et al. [62] menyarankan bahwa spesies antara CO yang terbentuk pada situs aktif permukaan Pt dapat dioksidasi menjadi karbon dioksida (CO2 ) oleh atom oksigen aktif yang terbentuk pada unsur sekunder, misalnya Ru, Sn, dan Mo, di daerah potensial yang lebih rendah. Tabel 1 merangkum kinerja berbagai jenis katalis paduan Pt yang dilakukan oleh peneliti untuk MOR. Menurut mekanisme bifungsional [88,89,90], MOR pada katalis paduan PtRu yang didukung dapat diringkas sebagai persamaan berikut. Pt adalah katalis yang lebih aktif untuk adsorpsi metanol daripada Ru. Oleh karena itu, reaksi keseluruhan pada elektrokatalis PtRu untuk reaksi oksidasi metanol mengikuti mekanisme bifungsional.
Mengacu pada mekanisme bifungsional ini, metanol pada awalnya terdisosiasi dan teradsorpsi pada Pt, selanjutnya terurai menjadi COiklan dan/atau spesies mirip formil -CHOiklan dengan reaksi dehidrogenasi (7). Pada saat yang sama, air terdisosiasi menjadi OHiklan dan teradsorpsi pada situs Ru (8). Kemudian spesies tersebut teradsorpsi pada situs Pt dan Ru dan bergabung bersama untuk membentuk CO2 molekul (9) dan (10). Reaksi antara Pt–COiklan dan Ru–OHads menyebabkan CO2 evolusi, menghasilkan situs Pt dan Ru yang diperbarui (reaksi 10). Sedangkan, pekerjaan lain yang dilakukan oleh Ewelina Urbanczyk et al. [48] melakukan reaksi oksidasi metanol untuk katalis PtNi dalam medium basa (1,0 M KOH). Secara teoritis, reaksi oksidasi metanol dalam medium basa adalah:
Reaksi dimulai pada elektroda Pt dari DMFC menjadi karbon dioksida. Selama proses ini, molekul perantara (CO), dapat terbentuk yang dapat menyebabkan racun dan penonaktifan sisi aktif Pt. Molekul CO ini adalah produk oksidasi tidak sempurna dari metanol. Oksidasi metanol yang tidak sempurna membentuk CO sebagai produk antara (Persamaan 11). Permukaan elektrokatalis juga dapat mengadsorbsi gugus hidroksil (Persamaan 6). Akhirnya, karena desorpsi produk utama, karbon dioksida terbentuk (13). Racun kedua yang mungkin dihasilkan selama oksidasi metanol adalah metana. Dalam hal ini, reaksi berikut dapat terjadi (8). Oksidasi total bentuk antara karbon menjadi karbon dioksida dalam reaksi elektrokimia adalah sebagai berikut:
Saat ini, para peneliti masih mempelajari teknik paduan untuk meningkatkan aktivitas katalitik elektrokatalis berbasis Pt dengan membuat paduan terner dan kuaterner dari Pt, seperti PtRuSn [91, 92], PtRuNi [93,94,95], PtRuMo [70, 96 , 97], PtRuOsIr kuaterner [79, 80], dan PtRuIrSn [97, 98], karena perilakunya yang sangat baik dalam MOR dan penghilangan spesies perantara (CO) yang terbentuk di situs permukaan Pt. Namun, penambahan logam ketiga dan keempat pada katalis terner dan kuaterner ini masih belum diketahui. Selain itu, ada beberapa keterbatasan dan tantangan dalam memproduksi paduan terner dan kuaterner. Optimalisasi morfologi katalis dan komposisi katalis menjadi sulit diperoleh karena banyaknya kemungkinan kombinasi logam dan komposisi. Namun, banyak penelitian membuktikan bahwa penambahan logam ketiga dan keempat sangat meningkatkan aktivitas katalitik, meningkatkan stabilitas katalis, dan toleransi CO yang baik terhadap elektrooksidasi metanol dan aplikasi DMFC.
Tsiouvaras dkk. [99] melakukan pengukuran elektrokimia katalis PtRuMo/C dan menemukan bahwa meskipun semua katalis terner lebih aktif terhadap oksidasi CO dan metanol daripada katalis biner, katalis diberi perlakuan dengan H2 menunjukkan peningkatan kinerja sekitar 15% sehubungan dengan katalis terner yang diolah dalam He atau tanpa perlakuan. Pada tahun 2012, Hu et al. [100] berhasil mensintesis elektrokatalis bimetalik (PtNi) yang unggul, yaitu PtNi nanospheres (HMPNNs) mesopori berongga. Katalis menunjukkan kinerja katalitik yang luar biasa dalam MOR dengan efisiensi pemanfaatan Pt yang ditingkatkan secara signifikan karena struktur unik HMPNN dan luas permukaan elektrokimia yang besar. Sedangkan sekitar tahun 2016, pekerjaan yang dilakukan oleh Yang et al. [101] juga menyelidiki reaktivitas elektrokatalis PtFe bimetalik di mana mereka mengamati bahwa interaksi yang kuat antara logam Pt dan besi (Fe) dapat menurunkan energi adsorpsi NP bimetal. Mereka juga menemukan bahwa nanopartikel PtFe bimetalik lebih suka teradsorpsi pada graphene kekosongan tunggal melalui atom Fe ketika atom Pt dan Fe keduanya berada di permukaan, karena interaksi antara atom Fe dan graphene kekosongan tunggal lebih kuat daripada interaksi antara atom Pt dan grafena lowongan tunggal. Gambar 3 mengilustrasikan posisi partikel Pt dan Fe yang terdispersi pada penyangga graphene seperti yang disarankan oleh Yang et al. [101].
Posisi katalis PtFe pada dukungan graphene diilustrasikan oleh Yang et al. [101]
Kinerja Katalis Berbasis Pt dan Karbida Logam Transisi
Karbida logam transisi (TMC), dengan stabilitas mekanik dan kimia yang tinggi terhadap korosi, ketahanan yang baik terhadap lingkungan asam, stabilitas jangka panjang, dan toleransi CO yang tinggi, dapat bertindak sebagai katalis anoda [88,89,90, 102,103,104]. Selain itu, TMC juga memberikan banyak keunggulan dibandingkan logam induknya dalam hal aktivitas, selektivitas, dan ketahanan terhadap racun, misalnya tungsten carbide (WC) menunjukkan sifat khusus, seperti konduktivitas listrik yang baik, ketahanan terhadap lingkungan asam, biaya rendah. , dan toleransi terhadap keracunan CO dalam proses elektrooksidasi metanol [88, 105, 106].
Wang dkk. [103] melaporkan sintesis luas permukaan yang tinggi (256 m
2
g
−1
) mikrosfer tungsten karbida melalui metode hidrotermal sederhana. K2 C ditemukan sebagai fase utama dalam sampel yang disintesis. Saat ini, para peneliti sedang mengeksplorasi potensi Pt yang didukung pada WC sebagai katalis ideal untuk DMFC [38, 88, 107, 108]. Kristen dkk. [106] menyimpulkan bahwa relatif terhadap unsur logam transisinya, TMC berperilaku seperti logam mulia seperti Pt, Pd, Rh, dan Ru untuk reaksi kimia dan elektrokimia tertentu, termasuk reaksi oksidasi hidrogen, karbon monoksida, dan alkohol dan reduksi oksigen. [109, 110]. Dalam studi lain, Liu et al. [107] mempresentasikan bahwa molibdenum karbida (Mo-Karbida) dapat bertindak sebagai promotor untuk tungsten karbida dan meningkatkan aktivitas elektrokatalitik di DMFC. Namun, tanpa penggabungan logam Pt, aktivitas elektrokatalitik WC murni terhadap MOR untuk sistem DMFC masih rendah. Oleh karena itu, sejumlah kecil logam platinum ditambahkan ke komponen WC sangat nyaman untuk mendapatkan keuntungan dari efek sinergis antara Pt dan WC [91, 111, 112]. Sementara itu, Hasan dkk. [109] mengungkapkan bahwa pengotor umum (spesies CO) yang terbentuk selama oksidasi metanol memiliki energi ikat yang kuat pada permukaan Pt; oleh karena itu, ia harus dioksidasi sehingga dapat dihilangkan dari situs aktif Pt. Penambahan komponen WC pada elektrokatalis Pt/WC menunjukkan toleransi CO yang tinggi terhadap MOR, yang menunjukkan adanya efek sinergis antara logam Pt dan WC sebagai komponen pendukung. Penelitian juga dilakukan oleh peneliti lain dengan menggunakan lebih sedikit logam Pt untuk mengurangi biaya Pt, sambil mempertahankan kinerja elektrokatalitik yang baik.
Selain itu, komponen WC lebih aktif dalam pembentukan gugus metoksi (CH3 O-) daripada Pt murni [113, 114]. Kinerja CV (Pt:Ru)4-WC/RGO menunjukkan kinerja katalitik yang luar biasa dengan kepadatan arus 330,11 mA mg
−1
Pt dibandingkan dengan lima katalis lainnya, menunjukkan bahwa elektrokatalis hasil sintesis memiliki aktivitas katalitik yang sangat baik terhadap MOR. Selain itu, kombinasi Ru dan WC pada katalis Pt meningkatkan jumlah permukaan OH dan memungkinkan CO yang teradsorpsi pada permukaan Pt teroksidasi pada potensial yang lebih rendah [39].
Kinerja Katalis Berbasis Pt dan Nitrida Logam Transisi
Transition metal nitride (TMN) merupakan kandidat ideal sebagai pendukung katalis Pt karena memiliki konduktivitas listrik (metalik) yang baik, kekerasan, stabilitas elektrokimia yang tinggi, dan ketahanan korosi pada kondisi operasi fuel cell [115.116.117.118]. Nitrida logam transisi, seperti CrsN, TiN, dan VN, yang didukung katalis Pt telah dilaporkan dan menunjukkan kinerja katalitik yang tinggi dan stabilitas yang lebih baik dibandingkan dengan dukungan karbon tradisional [112]. Semua logam transisi dapat membentuk nitrida, kecuali baris kedua dan ketiga dari logam golongan 8, 9, dan 10 (Ru, Os, Rh, Ir, Pd, dan Pt). Perilaku dan sifat struktural nitrida logam transisi dapat ditemukan dalam literatur [92,93,94]. Xiao dkk. [112] menyiapkan elektrokatalis Pt yang didukung kobalt nitrida titanium yang menunjukkan kinerja dan stabilitas yang sangat baik terhadap reaksi reduksi oksigen (ORR). Ti0,9 Co0.1 Katalis Pt yang didukung N menunjukkan ukuran partikel yang kecil dan dispersi logam yang baik. Elektrokatalis yang disiapkan ini juga mempertahankan luas permukaan elektrokimia (ECSA) Pt dan sangat meningkatkan pelestarian ECSA, dengan hanya penurunan 35% pada penurunan ECSA awal setelah 10.000 siklus ADT. Doping kobalt secara signifikan meningkatkan aktivitas dan daya tahan ORR. Sementara itu, elektrokatalis berkinerja tinggi dan tahan lama dalam sistem DMFC dapat diperoleh dengan menggunakan elektrokatalis Pt(Ru)/TiN yang luas permukaannya besar, yang juga menunjukkan aktivitas elektrokimia yang tinggi terhadap MOR dengan peningkatan aktivitas katalitik 52% dan stabilitas/daya tahan yang baik dibandingkan untuk komersial JM-Pt(Ru). Sementara itu, kinerja sel tunggal DMFC mencapai kepadatan daya maksimum yang lebih baik sebesar 56% dan menunjukkan stabilitas elektrokimia yang luar biasa untuk elektrokatalis CSG-Pt(Ru)/TiN [115].
Penelitian saat ini pada nanopartikel Pt didukung pada nanotube besi titanium nitrida dengan struktur berongga dan berpori dan luas permukaan yang tinggi disintesis oleh Li et al. [116]. Ini menunjukkan peningkatan aktivitas elektrokatalitik yang signifikan terhadap MOR dalam kondisi asam dan memiliki daya tahan yang lebih baik. Alasan untuk sifat-sifat ini adalah karena pekerjaan data eksperimental mereka yang memverifikasi bahwa penambahan Fe dapat menyesuaikan struktur elektronik atom Pt, yang berkontribusi pada aktivitas yang diperkuat dan stabilitas katalis Pt untuk MOR. Sementara itu, pada pekerjaan sebelumnya yang dilakukan oleh Xiao et al. [117], Pt/Ti0,8 Mo0.2 Katalis N menunjukkan struktur berpori dan luas permukaan tinggi, nanopartikel Pt berukuran kecil dan terdispersi dengan baik. Sistem katalis ini mempertahankan stabilitas elektrokimia intrinsik dari struktur nano TiN dan sangat meningkatkan aktivitas dan daya tahan MOR. Namun, informasi yang tersedia saat ini tentang stabilitas elektrokimia tungsten nitrida (WN) masih belum mencukupi [109].
Sementara itu, MoxN (x = 1 atau 2) pada substrat Ti menunjukkan stabilitas elektrokimia dalam elektrolit asam 4,4 M H2 JADI4 hingga potensial anodik + 0.67 V (vs. SHE) selama 50 siklus berulang [110]. Namun, elektrokatalis ini menunjukkan kerusakan permukaan, seperti retak dan remuk, di daerah potensial katodik tinggi (di bawah 0.1 V vs. SHE) dan anodik (di atas + 0.67 V vs. SHE) karena korosi katodik dan anodik. Di wilayah potensial anodik tinggi di atas + 0.67 V (vs. SHE), komposisi oksigen meningkat karena pembentukan oksida MoOx, yang dapat menyebabkan penonaktifan. Hasil ini menunjukkan bahwa MoxN bereaksi dengan spesies oksigen yang ada dalam elektrolit berair dan tidak stabil di atas +-0,67 V (vs. SHE). Mustafha dkk. [111] menemukan bahwa Pt yang dimuat ke TiN sebagai pendukung menunjukkan elektroaktivitas untuk oksidasi metanol, dengan rasio If/Ib yang tinggi mewakili resistensi CO yang tinggi dalam voltammogram yang dilakukan pada laju pemindaian 20 mV/s dalam 0,5 M CH 3 OH + 0,5 M H2 JADI4 sebagai elektrolitnya. Efek bifungsional antara Pt dan TiN disebut-sebut sebagai penyebab resistensi CO dari Pt/TiN. Selain itu, Ottakam Thotiyl et al. [91] mencapai hasil yang baik untuk katalis TiN bermuatan Pt, menunjukkan toleransi CO yang sangat baik untuk oksidasi elektrokimia metanol. Mereka menyimpulkan bahwa karakteristik khusus TiN yang membuatnya cocok sebagai pendukung Pt untuk MOR dalam media alkali adalah bahwa ia menunjukkan stabilitas yang luar biasa, ketahanan korosi yang ekstrim, konduktivitas elektronik yang baik, dan perilaku adhesi yang kuat. Katalis yang didukung TiN bermanfaat dalam hal stabilitas jangka panjang, kerapatan arus tukar, dan arus stabil pada potensi berlebih yang rendah. Pemuatan platinum sebesar 40% berat pada TiN digunakan dalam eksperimen.
Dalam beberapa tahun terakhir, Liu et al. [118] successfully synthesized platinum on titanium nickel nitride decorated 3D carbon nanotubes which reduced graphene oxide (TiNiN/CNT-rGO) support by solvothermal process followed by nitriding process. Pt with small particle size is well-dispersed on TiNiN/CNT-rGO support. The 3D shape of CNT-rGO support gives a fast route for charge transfer and mass transfer as well as TiNiN NPs with good synergistic effect and the strong electronic coupling between different domains in TiNiN/CNT-rGO support. Thus, it greatly improved the catalytic activity of this catalyst. In another research, the non-carbon TiN nanotubes-supported Pt catalyst done by Xiao et al. [119] also displayed enhanced catalytic activity and durability toward MOR compared with the commercial Pt/C (E-TEK) catalyst.
Performance of Pt-Based Catalysts with Transition Metal Oxide
Pan dkk. [92] reported the synthesis of platinum–antimony-doped tin oxide nanoparticles supported on carbon black (CB) as anode catalysts in DMFC, which exhibited better improvement in catalytic activity toward MOR compared to Pt-SnO2 /C or commercial Pt/C electrocatalyst. The enhancement in activity was attributed to the high electrical conductivity of Sb-doped SnO2 , which induced electronic effects with the Pt catalysts. Another work done by Abida et al. [93]described the preparation of Pt/TiO2 nanotube catalysts for methanol electrooxidation. TiO2 nanotubes-supported Pt catalyst (Pt/TiO2 nanotubes) exhibited excellent catalytic activity toward MOR and had good CO tolerance. They also reported that the use of hydrogenotitanate nanotubes as a substrate for the Pt catalyst considerably improved the COads oxidation on Pt, but the MOR still occurred at high potential. Then, several years later, Wu et al. [94] synthesized Pt-C/TiO2 with MOR activity 1.6 higher than commercial Pt-C and the stability of Pt-C/TiO2 was also enhanced by 6.7 times compared to Pt-C. The excellent performance of this catalyst was a contribution of mesopores and partially coated carbon support. Zhou dkk. [95] prepared hollow mesoporous tungsten trioxide microspheres (HMTTS) using the spray-drying method to yield Pt/HMTTS. The electrocatalyst exhibited excellent electrocatalytic activity and high stability toward MOR than Pt/C and Pt/WO3 electrocatalysts, which may be attributed to the well-ordered Pt particles (with an average size of 5 nm) on the HMTTS surface. Wu dkk. [120] used polystyrene spheres as templates to obtain pore-arrayed WO3 (p-WO3 ). The Pt nanoparticles with an approximate size of 3.3 nm dispersed on pore-arrayed WO3 (Pt/p-WO3 ) exhibited high catalytic activity toward MOR.
Li dkk. [121] used Sn-doped TiO2 -modified carbon-supported Pt (Pt/Ti0.9 Sn0.1 O2 –C) as an electrocatalyst for a DMFC system. The synthesized Pt/Ti0.9 Sn0.1 O2 –C electrocatalyst revealed high catalytic activity and CO tolerance toward MOR. The enhanced catalyst activity was due to the high content of OH groups on the Ti0.9 Sn0.1 O2 electrocatalyst sample and the strengthened metals and support interactions. In addition, Lv et al. [122] also reported in their work that the addition of TiO2 could not only facilitate CO removal and hinder CO formation on Pt surface during methanol oxidation, but it can also prevent the agglomeration and corrosion of Pt, which can be concluded from strong metal-supports interaction between TiO2 –C and Pt. Huang et al. [123] revealed that a TiO2 -coated carbon nanotube support for Pt electrocatalysts could be prepared via a one-step synthesis. Hao et al. [124] developed a new catalyst composed of Pt nanoparticles deposited on graphene with MoO3 . These catalysts exhibited high catalytic activity toward MOR and high resistance to CO species. However, the size of MoO3 must be tuned by controlling the metal oxide loading.
The selection of metal oxide such as MnO, RuO, CeO, SnO2 , MgO, and V2 O5 as additional component in electrocatalyst of Pt because of their low cost, good electrochemical properties, and have proton-electron intercalation properties [125]. From the catalytic activity aspect, it can be summarized that the addition of these metal oxides can enhance the electrocatalytic activity of DMFC and other fuel cells. The incorporation of these conducting metal oxides together with Pt catalyst could also facilitate the oxidation process of CO intermediate molecules. Hence, these types of metal have high potential to be used together with platinum as anode electrode.
Carbon support
To improve the utilization of the Pt catalysts, the carbon support is also another useful approach to be used together with Pt. Carbon materials are largely used as catalyst support because of its special properties such as relatively stable in both acid and basic electrolyte, good conductivity, and provide high surface area for dispersion of metal catalyst. It is believed that carbon materials have a strong effect that can influence the electrocatalysts properties such as metal particle size, morphology, metal dispersion, alloyed degree, and stability. Carbon supports can also affect the performance of supported catalysts in fuel cells, such as mass transport and catalyst layer electronic conductivity, electrochemical active area, and metal nanoparticle stability during the operation.
Currently, a great concern of the development in the nanotechnology field, especially carbon nanomaterials synthesis, is to create more stable and active supported catalysts. Support materials of nanoparticles are believed to be the most promising materials for catalytic activity in fuel cells, including the DMFC system. Pt has been traditionally used as nobel catalysts for many fuel cells application [126,127,128]. However, the high cost and low reserve are hindering commercialization of fuel cells and driving researchers to make the utmost of the catalyst. According to this problem, the major effort has been done toward nanoscaling of the catalyst nanoparticles to form more active sites per mass unit. The morphology, structure, and activity of the catalyst, and correspondingly the whole lifetime of a cell, thus strongly depend on the catalyst support [129]. Table 2 shows the preparation, physical properties, performance, and activity of Pt-based supported carbon done by groups of researchers. The details of Pt-based supported carbon will be performed in the following sections:“Graphene Support” to “Carbon Nanocoils”.
Graphene support
Graphene has many extraordinary properties; it exists as a two-dimensional carbon (2-D) form, which is called a crystalline allotrope, one-atom-thick planar flat sheet of sp2 tightly bonded carbon atoms with a thickness of 0.34 nm. Its carbon atoms are packed in a regular atomic-scale chicken wire (hexagonal) pattern [92, 119]. The theoretical specific surface area of graphene is 2630 m
2
g
−1
, which is much larger than that of carbon black (typically less than 900 m
2
g
−1
) and carbon nanotubes (100 to 1000 m
2
g
−1
) and similar to that of activated carbon [130]. Graphene has high potential as a metal support [131, 132, 133] [33] due to its high surface area [134] for better catalyst/metal dispersion [135], high electrical conductivity [136], and good thermal properties [137, 138]. Moreover, the functionality of graphene support can be modified by changing it surface structure, and hence contribute to its potential applications, such as in fuel cells, energy storage, electrochemistry, supercapacitors, and batteries [138,139,140,141,142]. Figure 4 illustrates the preparation steps to obtain the graphene nanosheets (GNS), while Fig. 5 shows their TEM images [143]. It can be clearly observed that the thickness of the GO with many typical wrinkles obviously decreases compared to graphite. This can be explained by the presence of the rich oxygen-containing functional groups over the surface of GO [132]. Besides, both resulting GN-900 and GN-900-C contained of a large size of nanosheets structure, but the GN-900-C comprised more transparent than the GN-900.
illustration of the preparation of graphite oxide to graphene nanosheets (GNS) by using oxalic acid [143]
TEM images of graphite (a ), GO (b ), GN-900 (c ), and GN-900-C [143]
The discovery of graphene sheets began around 2000 by mechanical extracting process from 3D graphite source [133]. Graphene can be obtained by several synthesis methods such as hydrothermal [144], chemical reduction [143], chemical vapor deposition, and electrochemical. Ma et al. [145] enhanced the electrocatalytic activity of Pt nanoparticles by supporting the Pt nanoparticles on functionalized graphene for DMFC. Functionalized graphene was prepared by methyl viologen (MV) and Pt/MV–rGO electrocatalyst was synthesized by a facile wet chemical method. They also reported that the higher catalytic activity of Pt/MV–RGO was attributed to the synergetic effect between MV and rGO.
Meanwhile, Zhang et al. [146] modified the graphene support with graphene nanosheets through Hummer’s method, followed by polymerization of aniline (as nitrogen source). The TEM images for Pt/NCL-RGO and Pt/RGO electrocatalysts show that the aggregation between separated graphene sheets was decreased by nitrogen-doped carbon layer (NCL), leading to a better dispersion of the Pt catalyst on the graphene nanosheets support and better electroactivity and stability toward methanol electrooxidation (MOR). Presence of NCL successfully prevented the aggregation of graphene nanosheets as the Pt nanoparticles supporting material.
In 2011, Qiu et al. [135] successfully synthesized nanometer-sized Pt catalyst via sodium borohydride reduction method with an average particle size of only 4.6 nm. These Pt nanoparticles showed an even dispersion of Pt catalyst on graphene oxide support and very high electrocatalytic activity toward MOR by controlling the percent deposition of Pt loaded on the graphene. In another study conducted by Ojani et al. [147], for synthesized Pt-Co/graphene electrocatalyst, it was shown that graphene nanosheets improved the electrocatalytic behavior and long-term stability of the electrode. In addition, the Pt-Co/G/GC electrocatalyst showed great stability toward MOR. The catalytic performance toward MOR can also be improved by using cobalt core–platinum shell nanoparticles supported on surface functionalized graphene [148]. This enhanced catalytic activity could be attributed to the poly (diallyldimethylammonium chloride) (PDDA) that plays a crucial role for dispersion and stabilization of Co@Pt catalyst on graphene support. PDDA-functionalized graphene provided the higher electrochemical active surface area [149, 150]. Huang et al. [138] also studied a PtCo-graphene electrocatalyst with outstanding catalytic performance and high CO tolerance toward the MOR, which far outperformed Pt-graphene and PtCo-MWCNT electrocatalysts with the same ratio of Pt and carbon content. Figure 4 shows the formation of a graphene-PtCo catalyst prepared from a graphite source. Sharma et al. [57] synthesized Pt/reduced graphene oxide (Pt/RGO) electrocatalyst using a microwave-assisted polyol process, which sped up the reduction of GO and formation of Pt nanocrystals. They compared Pt/RGO to a commercial carbon support (Pt/C), which exhibited high CO tolerance, high electrochemically active surface area, and high electrocatalytic activity for the MOR. In a previous study, Zhao et al. [139] reported that the unique 3D-structured Pt/C/graphene aerogel (Pt/C/GA) exhibited greater stability toward MOR with no decrease in electrocatalytic activity. Moreover, the Pt/C/graphene aerogel also exhibited significantly higher stability to scavenge crossover methanol at high potential in an acidic solution compared with the commercial Pt/C electrocatalyst. At the initial catalytic stage, the Pt/C electrocatalyst lost approximately 40% after 1000 CV cycles. In contrast, the Pt/C/graphene aerogel only lost 16% of the initial catalytic activity. After 200 cycles of CV, the current density of Pt/C/graphene aerogel was much higher with a remarkably higher stability than that of Pt/C electrocatalyst. Meanwhile, Yan et al. [151] demonstrated highly active mesoporous graphene-like nanobowls supported Pt catalyst with high surface area of 1091 m
2
g
−1
, high pore volume of 2.7 cm
3
g
−1
, and average pore diameter of 9.8 nm obtained by applying a template synthesis method. In addition, the Pt/graphene bowls also achieved high performance toward MOR with a current density value of 2075 mA mgPt
−1
, which was 2.87 times higher than that of commercial Pt/C (723 mA mgPt
−1
). The onset potential for the Pt/graphene bowls toward methanol electrooxidation was negatively shifted by approximately 160 mV compared with that to the latter and showed CO resistance. Figure 6 shows the proposed schematic for the formation of PtCo catalyst on reduced-GO (rGO) support [51]. It is described that the formation of graphene oxide nanosheets from oxidation of graphite powder leads to the increase in interlayer “d” spacing of stacked graphitic sheets from 0.34 to 0.78 nm due to the presence of various oxygen-containing functional groups. The oxygen-containing functional groups act as anchor sites for the well-dispersed Pt and PtCo nanoparticles on rGO sheets, and used for efficient electrooxidation of methanol.
Illustrates the schematic formation of graphene supported Pt-Co catalyst [51]
We can conclude that the reduce graphene oxide (rGO), graphene, modified graphene as supporting material exhibited high electrocatalytic activity toward methanol electrooxidation process. A lot of studies have been reported related to the particle size distribution and size, morphologies, and catalytic activities of Pt and Pt alloys using graphene as supporting material, which showed great improvement in fuel cell performance as mentioned and discussed above. Thus, graphene support can be further studied for better fuel cell performance.
Multiwall Carbon Nanotube and Single-Wall Carbon Nanotube Support
Several years ago, Jha et al. [140] prepared multiwall carbon nanotube (MWCNTs) via chemical vapor deposition using an AB3 alloy hydride catalyst. Platinum-supported MWCNT (Pt/MWCNT) and platinum-ruthenium-supported MWCNT (Pt-Ru/MWCNT) electrocatalysts were prepared by chemical reduction. The performance of these electrodes was studied at different temperatures, and the results demonstrated a very high power density of 39.3 mW cm
−2
at a current density of 130 mA cm
−2
, which could be attributed to the dispersion and accessibility of the MWCNT support and Pt-Ru in the electrocatalyst mixture for the methanol oxidation reaction. This was also done by other researchers that using different catalyst supported MWCNT for DMFC system [152,153,154,155]. Meanwhile, Wu and Xu [156] compared MWCNT-supported Pt and single-wall carbon nanotube (SWCNT)-supported Pt. Figure 7 shows that the TEM images of Pt catalyst was deposited on MWNT and SWNT electrodes through the electrodeposition technique. The Pt particles in Pt-SWNT (Fig. 7b) looked closer contact with the network of entangled and branched bundles of SWNT support, and the shape is closer to highly exposed sphere. The benefits of the SWCNT support are due to its greater electrochemical surface-active area and easier charge transfer at the electrode/electrolyte interface because of the graphitic crystallinity structure, rich amount of oxygen-containing surface functional groups, and highly mesoporous and unique 3D-structure of SWNT. The electrodeposition technique carried out by them contributed to higher utilization and more uniform dispersion of Pt particles on its support.
TEM images for the Pt on MWCNT(a ) and SWCNT (b ) [156]
Then, Wang et al. [157] reported the high performance of modified PtAu/MWCNT@TiO2 electrocatalyst prepared via deposition-UV-photoreduction for DMFC, which also exhibited high CO tolerance. Zhao dkk. [126] studied 3D flower-like platinum-ruthenium (PtRu) and platinum-ruthenium-nickel (PtRuNi) alloy nanoparticle clusters on MWCNTs prepared via a three-step process, and the best ratios obtained from their experiments for the PtRu and PtRuNi alloys were 8:2 and 8:1:1, respectively. Another group, i.e., Zhao et al. [158], found a higher current density toward MOR and better activity for MWCNT-supported PtWC compared with Pt/C electrocatalyst. These results were attributed to the factors of the synergistic effect between the Pt catalyst and the WC component, high CO tolerance from the bifunctional effect of the Pt catalyst and the WC component, and strong interaction between metals and WC in the electrocatalyst composite.
As a summary, both of MWCNT and SWNT support in terms of structural, surface, and electrochemical properties have their own characteristics as supporting material that remarkably enhanced their performance in catalysis of methanol oxidation process. However, as a comparison, SWCNT possess a high degree of graphitization, highly mesoporous 3D structure, and contain more oxygen-containing functional groups at its surface sites. In relation with these properties, the SWCNT exhibits a higher electrochemically accessible surface area and faster charge transfer rate at the electrode/electrolyte interface.
Carbon Nanotube Support
Wen dkk. [144] proposed that carbon nanotubes (CNTs) support could improve fuel cell performance; for example, Pt can be fixed to the inner wall and the outer wall of CNTs and may cause improvement in the electrocatalytic properties of platinum-CNTs. Yoshitake et al. [159] proposed that fuel cells using CNTs as the catalyst support produced larger current densities. The addition of binary or other components to the electrocatalysts for methanol electrooxidation overcomes the problems related to catalyst poisoning caused by CO during the reaction. Therefore, new electrocatalyst carbon supports, such as carbon nanotubes [160, 161], are being actively developed to significantly improve fuel cell performance. Kakati et al. [128] successfully synthesis the PtRu on CNT/SnO2 for anode catalyst DMFC via hydrothermal process. It has been found that the presence of SnO2 provide a high durability property for the catalyst and the presence of SnO2 in the district of Pt could supply oxygen-containing functional groups for the removal of CO intermediate molecules from the Pt surface sites during electrooxidation of methanol. Generally, the decomposition methanol occurs at Pt surface sites; meanwhile, the decomposition of water occurs at SnO2 surface sites to form oxygen-containing species which then react with CO intermediate molecules. However, as support material, the conductivity property of SnO2 still needs to be enhanced. Kakati et al. [128] also proposed the schematic diagram of the formation of PtRu on CNT/SnO2 composite as shows in Fig. 8, and FESEM images of CNT/SnO2 composite support (a and b) and PtRu/SnO2 /CNT composite electrocatalyst (c and d) in Fig. 9.
Illustrates the schematic diagram for the formation of PtRu/SnO2/CNT composite [128]
FESEM images of CNT/SnO2 composite support (a , b ) and PtRu/SnO2/CNT composite electrocatalyst (c , d )
Chien et al. [127] proposed that the high catalytic performance of Pt-Ru/CNT for MOR can be attributed to the presence of CNT as the carbon support material with several factors:(i) the as-synthesized Pt-Ru/CNT electrocatalyst owns the ideal nanosized particles and composition to increase catalytic activity, (ii) the presence of functional group on the CNT surface results in high hydrophilicity of CNT, which produces better electrochemical reaction on the electrode area, and (iii) the high electronic conductivity of the CNT support lowers the resistance in MOR. Jeng et al. [150] prepared Pt-Ru/CNT electrocatalyst via a modified polyol with a PtRu composition ratio of 1:1, exhibiting high catalytic activity toward MOR and better performance than that of commercial PtRu/C. Show et al. [162] reported that Pt catalyst with a size of less than 10 nm can be obtained by dispersing the Pt particles on a CNT surface using the in-liquid plasma method, and excellent performance was demonstrated by the electrical power achieving 108 mW cm
−2
[162]. The in-liquid plasma method was also used by Matsuda et al. [163] that can applied to obtain nanometer-sized Pt catalyst on support material that remarkably enhanced the fuel cell performance.
To be concluded, high electric conductivity, large surface area, excellent chemical and electrochemical stabilities, quasi one-dimensional structure, and good morphology as the supporting materials are the key factors of carbon nanotubes (CNTs) in enhancing the DMFC performance. In addition, carbon support materials such as CNTs which contribute a large effect on metal distribution and size have also been proven to be an essential to the electrocatalysts to achieve high catalytic activity during methanol oxidation process.
Carbon Nanofiber Support
Steigerwalt et al. [164] reported the successful synthesis of PtRu alloy that was widely dispersed on a graphene carbon nanofiber (CNF) support as an electrocatalyst in DMFC. The catalytic activity was enhanced by ~ 50% relative to that recorded for an unsupported PtRu colloid anode electrocatalyst. Meanwhile, Wang et al. [152] reported that Pt/CNF nanocomposites obtained by the reduction of hexachloroplatinic acid (H2 PtCl6 ) precursor with formic acid (HCOOH) in aqueous solution containing electrospun CNFs at room temperature showed a higher current density than other prepared Pt/CNFs and was approximately 3.5 times greater than that of the E-TEK Pt/C electrocatalyst. Another research carried out by Giorgi et al. [153] described a CNF and bimetallic PtAu electrode with a single layer and both diffusive and catalytic functions using a decreased noble metal amount (approximately five times less) with a consequent large cost reduction. In addition, the bifunctional electrocatalytic properties were also active for the MOR on the PtAu nanoparticle catalysts [154]. Calderón et al. [155] reported PtRu/CNF prepared via reduction using sodium borohydride (NaBH4 ), methanol, and formate ions. This electrocatalyst synthesized by SFM was heat-treated (denoted as SFM TT), which improved its electrocatalytic activity during MOR. Later, Maiyalagan [165] reported that the addition of silicotungstic acid acted as a stabilizer for the PtRu particles on CNT support. The PtRu-supported CNT was prepared by microwave heating of an ethylene glycol (EG) solution of STA, H2 PtCl6 .6H2 O (as Pt precursor), and RuCl3 .xH2 O (as Ru precursor) with CNF suspended in the solution. The Pt and Ru precursors were loaded on CNF by conventional impregnation method. The results revealed that the PtRu nanoparticles are uniformly dispersed on carbon nanofiber support, with an average particle size of 3.9 nm enhanced the catalytic activity toward methanol electrooxidation. As a conclusion, the carbon nanotubes supporting material with high electronic conductivity and high surface area gives an advantage of better dispersion for the Pt or Pt alloys deposition. The higher the surface area of supporting material can reduce the agglomeration of metal particles on it, thus can produce better catalyst morphology for better fuel cell performance.
Mesoporous Carbon Support
Mesoporous carbon (MPC) support is another ideal candidate as an electrocatalyst support material in DMFC and fuel cell. Generally, mesoporous carbons are divided into two classes based on their structures which are ordered mesoporous carbons (OMCs), with highly ordered pore structure and uniform pore size, nonordered mesoporous carbons with irregular pores. Other than that, OPC can be produced by using high quality of SBA-15 silica and sucrose as carbon source template. To prepare the high quality of SBA-15 SBA-15 sample, triblock copolymer, EO20-PO70EO20 (Pluronic P123, BASF), as the surfactant and tetraethyl orthosilicate (TEOS, 98%, Acros) as the silica source are used, as reported by literature [166,167,168]. The synthesis of MPC starts from synthesis of SBA-15, followed by calcination process.
A well-dispersed and ultralow Pt catalyst (PtFe) supported on ordered mesoporous carbon (OMC) was prepared via a simple route and showed superior catalytic activity. The PtFe alloy with a size range of 3–5 nm was homogeneously dispersed on the CMS with a very high specific surface area of more than 1000 m
2
g
−1
[169]. The incorporation of Fe was discussed in the previous section (“Performance of Various Types of Pt-Based Catalysts” section and “Performance of Pt-Based Alloys” section). The high specific surface area of mesoporous carbon support can be produced by carbonization process of a resorcinol-formaldehyde polymer with a cationic polyelectrolyte as a soft template [160]. The performance of Pt/MPC also related to the synthesis/preparation method as done by Kuppan and Selvam. Kuppan and Selvam [167] synthesized four type of Pt/mesoporous carbon by using different reducing agent which are NaBH4 , EG, hydrogen, and paraformaldehyde. From there, the Pt/mesoporous carbon synthesized using paraformaldehyde as reducing agent for showed highest current density. The highest in catalytic was attributed to the use of paraformaldehyde that gives the smallest Pt particle size (4.5 nm), and the highest ECSA (84 m
2
/g) belongs to Pt/mesoporous carbon.
Wang dkk. [161] synthesized a Pt@WC/OMC electrocatalyst composite, in which the composite was platinized using a pulsed microwave-assisted polyol technique. The OMC produced in this synthesis exhibited high surface area property. The Pt@WC/OMC electrocatalyst also showed high activity, desirable stability, and CO tolerance toward MOR. In another work done by Zhang et al. [170], the ordered CMS had a unique hierarchical nanostructure (with a 3-D structure) with ordered large mesopores and macropores that facilitated the dispersion of Pt nanoparticles and rapid mass transport during the reactions.
To maximize the use of Pt particles, the support materials should have uniform dispersion, high utilization efficiency, and desirable activity and stability. Moreover, the good supporting materials must be suitable for surface chemistry, high loading of Pt dispersion, and some functional roles. Additionally, based on the previous studies as discussed above, the ordered mesoporous carbons with large pore sizes are highly desirable for fast mass transfer and, thus, enhance the catalytic activity especially in the reaction involve large reactants molecules.
Carbon Black
Carbon black (CB) is one of the commercial carbon support that has been used till now. There are many types of CB such as Vulcan XC-72, Black Pearl 2000, Denka Black, Shawinigan Black, Ketjen EC-300J, etc. [171, 172]. CB is commonly used as a carbon support material for electrocatalysts because it possesses high porosity properties, which make it suitable as a potential support material for the catalyst layer in PEMFCs and DMFCs as reported in provided literatures [173,174,175,176,177,178,179,180]. The comparison of the several carbon black support was reported by Wang et al. [181] who investigated the effect on DMFC performance using several types of carbon black such as Vulcan XC-72R, Ketjen Black EC 300J, and Black Pearls 2000 carbon black as additives/support for the Pt cathode catalyst. From the experiments, the results showed that Ketjen Black EC 300J was the most useful carbon support for increasing the electrochemical surface area and DMFC performance of the cathode catalyst.
Nowadays, CB is commercial carbon support for many fuel cell systems. Generally, it is used for the comparison with new or modified catalyst [125]. The following Table 3 summarizes the commercial carbon black and its properties for fuel cell application. There are so many modifications among carbon support materials and development of new carbon support for enhance fuel cell performance; however, commercial carbon black still is used in many fuel cell applications especially for the comparison with new or modified catalyst.
Carbon Nanocoils
Celorrio et al. [182] proposed carbon nanocoils (CNCs) as a PtRu support in their experiment, indicating that the electrocatalyst performance was strongly dependent on the synthesis method. CNC-supported electrocatalysts showed better electrochemical behavior than E-TEK electrocatalysts, and better electrocatalytic behaviors toward CO and methanol oxidation were achieved using CNC as a support material [182]. Sevilla et al. obtained highly graphitic CNCs from the catalytic graphitization of carbon spherules via the hydrothermal treatment of different saccharides which are sucrose, glucose, and starch [183]. They demonstrated that the high electrocatalytic activity of the CNCs is due to the combination of good electrical conductivity of their graphitic structure and high porosity property, which allows much less diffusional resistance of reactants/products. Two years later, Sevilla et al. [184] reported highly dispersed Pt nanoparticles on graphitic CNCs with diameters in the range of 3.0–3.3 nm and a very fine particle size distribution. The electrocatalyst possessed large active Pt surface area (up to 85 m
2
g
−1
Pt), high catalytic activity toward MOR (up to 201 A g
−1
Pt), and high resistance against oxidation, which was noticeably greater than that of the Pt/Vulcan electrocatalyst. Celorrio et al. [185] obtained Pt/CNC electrocatalysts via the impregnation method, which showed that a combination of Pt and CNCs facilitated the CO oxidation process.
Conductive Polymer Supports
Choi et al. [186] synthesized PtRu alloy nanoparticles with two types of conducting polymers, i.e., poly(N -vinyl carbazole) and poly(9-(4-vinyl-phenyl)carbazole), as the anode electrodes. Electrochemical and DMFC tests showed that these nanocomposite electrocatalysts were beneficial in a DMFC system, but their catalytic performance was still lower than that of a carbon supported electrode. Thus, they suggested that higher electrical conductivity of the polymer and lower catalyst loss are required in nanocomposite electrodes to achieve better performance in a DMFC. Choi et al. [171] and Kim et al. [172] prepared polyaniline (PANi) as a support material for PtRu catalyst in a DMFC system. PANi is a group of conductive polymers with high electronic conductivity and a methanol oxidation current similar to that of carbon-supported PtRu catalyst. Then, Kim et al. [172] conducted catalytic tests to compare PtRu/PANi support with PtRu/carbon support, showing that the enhanced catalytic activity of PtRu/PANi was due to (i) the high electrical conductivity of the polyaniline support, (ii) the increase of electrochemical surface area of the prepared electrocatalyst, and (iii) the higher ion diffusion behavior. In another study, Amani et al. [74] synthesized PtSn supported by C-PANI as an electrocatalyst with different Pt:Sn atomic ratios using the impregnation method. The PtSn/C-PANI electrocatalyst with a ratio of 30:70 showed outstanding performance in the methanol electrooxidation, and the current density was approximately 40% higher than PtRu/C and 50% higher than Pt/C-PANi. The CO tolerance and stability were improved compared to that of PtRu/C, and the methanol crossover was reduced. Yaldagard et al. [173] studied the electrocatalytic performance of Pt/PANi/WC/C electrocatalyst for methanol electrooxidation (MOR) and oxygen electro-reduction (ORR), and it exhibited higher MOR activity, high CO resistance, and improved stability compared to Pt/C electrocatalyst in the presence of methanol.
Wu dkk. [174] presented polypyrrole nanowire networks (PPNNs) as the anodic microporous layers (MPLs) of passive DMFC. In passive DMFC system, the novel MPL achieved a 28.3% increase in the power density from 33.9 to 43.5 mW cm
−2
compared with the conventional layer with a similar PtRu (1:1). The high performance was due to the presence of PPNNs, which expressively improved the catalyst utilization and mass transfer of methanol on the anode. Besides, Selvaraj and Alagar [175] prepared Pt-Ru nanoparticle-decorated polypyrrole/multiwalled carbon nanotubes (Ppy/CNT) via the in situ polymerization of Ppy on CNTs containing ammonium peroxydisulphate (NH4 )S2 O8 as an oxidizing agent at the temperature range of 0–5 °C, followed by deposition of Pt particles on PPy-CNT composite films via chemical reduction to produce Pt/PPy-CNT. It was found that the PtRu particles deposited on PPy–CNT composite films exhibited higher catalytic activity and stability toward MOR compared to Pt/PPy-CNT. So far, the investigation on polymer as supporting materials is not much as carbon support materials. From aspect as supporting materials, the performance of polymer support was not good/excellent as carbon support. Further studies are needed in the future for better electrocatalytic activity and DMFC performance.
Problems and Limitations of Using Pt for DMFC Systems
There are two major challenges in the development of new DMFC catalysts:(i) performance, including the catalytic activity, reliability, and durability; and (ii) catalyst cost reduction. Two major problems arise in DMFC when using pure Pt alone as the anode catalysts:(1) slower kinetics oxidation of methanol, even on some state-of-the-art anode catalysts, and methanol crossover through the membrane, which not only lowers cathode performance but also reduces fuel efficiency. To develop successful fuel cell technology, including DMFC technology, new catalysts must be investigated to improve the performance and reduce the cost. Reduction of the catalyst cost remains a major challenge. Currently, platinum is one of the most effective electrocatalysts for DMFC due to its high catalytic activity for methanol oxidation, but because it is a precious metal, platinum usage is challenging and limited [176, 177]. Therefore, many scientists have attempted to find materials that can behave like Pt catalysts. One problem with the MOR in DMFCs is that CO is produced as an intermediate reaction product when using Pt catalyst and has strong binding energy on platinum particles, poisoning the active sites of the platinum surface area [58]. Therefore, CO must be removed by oxidizing it from the Pt surface using another material with high resistance to CO poisoning. For example, Hwu et al. proposed Pt-modified WC catalyst that has remarkable resistance to CO poisoning [178]. On the other hand, they also suggested that CO tolerance originates from the lower CO desorption temperature on pure and Pt-modified WC compared to pure Pt.
There are many solutions that can be applied to reduce the cost of Pt, overcome or minimize the formation of CO species during methanol oxidation, and increase the kinetics of methanol oxidation, such as alloying with other metals or transition metals, the incorporation of metals, metal nitrides, and metal oxides and the use of carbon supports as discussed in this paper. However, to overcome this problem, we need to understand the formation of CO on Pt sites particle, and understanding of the mechanism of the anode reaction in DMFCs. Unfortunately, it has limited amount of mechanistic insight to be studied, because this reactions involve complex mechanism path with many possible intermediate molecules and also competing reaction pathways [179]. For Pt catalytic mechanism, it has been suggested by a direct reaction path. Unfortunately, the use of Pt on other metals has limited mechanistic information available. Figure 10 represents the reaction path for methanol electrooxidation and their possible intermediates molecules formed during the process. Black arrows show direct path, while green arrows show the indirect mechanism for CO2 formation as a final product. In the direct mechanism, the reaction path does not involve a CO intermediate, and CO2 is formed directly from methanol. In contrast, indirect mechanism forming a CO intermediate molecule and subsequently it is oxidized to CO2 product. Notably, CO is the most stable molecule of all the intermediates on Pt during MOR. The stability of CO causes it to be a main reason for the extensive CO poisoning problem that is often found on Pt catalyst.
Schematic of the reaction paths and possible intermediates molecules considered in methanol electrooxidation [237]
First step in the mechanism of methanol decomposition reaction on Pt is the activation of methanol molecule. It can take place via hydrogen abstraction from either the carbon or the oxygen atoms. Further step, hydrogen abstraction creates formaldehyde (CH2 O) or hydroxymethylene (CHOH), followed by formyl (CHO) or COH. In the direct mechanism, instead of stripping off the final hydrogen from CHO or COH molecule to CO, a water molecule will release a proton/electron pair and resulting to OH species that can further bind with the carbonaceous species to form dihydroxycarbene (C(OH)2 ) or formic acid (HCOOH). This step is called hydroxyl addition process. The next step is followed by dehydrogenation to form either formate (HCOO) or carboxyl (COOH) molecule, with subsequent dehydrogenation to form CO2 as the final product of reaction. In addition, an alternative direct mechanism involve the stripping of a proton/electron pair from water and addition of the resulting hydroxyl to CH2 O, subsequently to H2 COOH, which then undergoes dehydrogenation to form HCOOH or dioxymethylene (Hs COO). The Hs COO molecule can then undergoes dehydrogenation to HCOO and finally to CO2 . Besides, in the indirect mechanism, CHO or COH species are directly dehydrogenated to CO. Water is dissociated separately on the surface to form OH, and the two surface species react together to form CO2 gas in a way similar to the water-gas-shift reaction [187]. This indirect mechanism occurs because less energy is required to form CO than CO2 . Strong adsorbed CO intermediate form on the Pt surface sites revealed a major problem at the anode site of DMFC. Formation of this intermediate species can cause deactivation Pt catalyst. Furthermore, the rate of kinetic methanol oxidation for DMFC is slower. Therefore, to increase the resistance of Pt catalyst to CO poisoning on the electrodes, Pt alloy or hybrids, such as PtRu, PtSn, PtMO, PtPb, PtFe, PtCo, PtNi, PtRuOs, PtRuMo, PtRuSn, PtRuNi, etc. (as mentioned and discussed in “Performance of various types of Pt-based catalysts” section), are usually employed as electrocatalyst materials on DMFC anodes. Addition/incorporation of these alloys to Pt can prevent the adoption of CO on Pt surface by decreasing the oxidation overpotential of the anode [84].
Conclusion and Prospects
Great progress has been made in recent years in the development and optimization of new catalysts using Pt-based catalysts and carbon and conductive polymer supports for DMFC anode catalyst. Some new carbon materials, such as nano- or mesostructured carbons, have been demonstrated as highly potential catalyst support materials, although their applications face challenges in terms of synthesis, metal loading, and electrode preparation. The combination of platinum as the best metal catalyst for DMFC and an excellent carbon support could produce breakthroughs in the investigation of a new DMFC anode catalyst in the future. Since platinum is an expensive metal, it is necessary to reduce the amount of Pt used in the electrocatalyst. Therefore, this paper presented more than 100 studies on the electrocatalytic activity and performance related to Pt-based electrocatalysts and various carbon and conductive polymer supports. The main problems related to the platinum electrocatalyst, such as carbon monoxide formation during the methanol oxidation reaction and the poor kinetics of methanol oxidation, could be overcome using additional materials and various supports, as reported in the research presented in this paper.
Many studies conducted in the recent years to reduce the loading amount of Pt catalyst and to increase the percentage utilization efficiency, and hence, enhance the electrocatalytic activity of Pt toward the oxygen reduction reaction (ORR) and methanol electrooxidation reaction (MOR), were discussed in this paper. Pt has been alloyed with many transition metals such as Fe, Co, Ni, Ir, Ru, Rh, and Pd, resulting in higher catalytic activity for the DMFC system. The incorporation of these materials also resulted in good dispersion on the carbon and polymer supports, which showed higher performance in the DMFC test compared to the use of Pt metal alone. Various carbon support sources, namely activated carbon (AC), carbon black (CB), multiwall carbon nanotubes (MWCNTs), carbon nanofibers (CNFs), carbon nanotubes (CNTs), graphene, and conductive polymer supports, have been used with Pt-based catalysts to improve their catalytic performance. Additionally, Pt-based alloy catalysts have been designed as hollow mesoporous PtNi, nanowire PtRu, and nanodendritic PtRh, which showed improved electrocatalytic activity and superior electrocatalytic performance. Meanwhile, 3-D Pt/C/graphene aerogel demonstrated enhanced stability toward methanol electrooxidation. The work performed by researchers showed that the electrocatalytic activities of nanoparticles Pt alloy catalysts depend on several factors such as the synthesis method, condition of experiments (such as temperature and pH), alloy composition/ratio, precursors, and thermal treatment. For the future study, it should be extended to the optimization of the geometry and structure of previous studies that revealed active Pt alloys can increase their electrocatalytic activity and stability and the application of support materials for fuel cell applications. For example, current research that have been done by Liu et al. 2017 [188] shows the excellent performance of platinum. From theoretical calculations, it revealed that the main effective sites on platinum single atom electrocatalysts are single-pyridinic-nitrogen-atom-anchored single-platinum-atom centers, which ascribed to the tolerant CO in MOR. They also suggested that carbon black supported used together with Pt single atom is effective in cost, efficient, and durable electrocatalyst for fuel cell application. According to the above study, herein, we can conclude that the modification on structure and morphology of precious metal such as platinum could also remarkably increase the performance of electrocatalyst, but in the same time can reduce the overall cost of fuel cell for commercialization.
To improve the morphologies of Pt and Pt alloys, carbon support material also need further study. Nanoporous metals become an interesting part of catalyst to be studied for fuel cell application. It is determined very suitable for fuel cell catalysts because they possess high surface area, three-dimensional (3D) network structures with adjustable ligament/pore sizes suitable for mass transport, and electron conduction. Around 2017, Li et al. successfully carried out modification on Pt-Pd-Au trimetallic surface as cathode for oxygen reduction reaction [189]. The surface evolution of 3-D Pt-Pd-Au trimetallic greatly enhanced the ORR activity and highly stable as ORR catalyst. The modification of PtNi alloy also done by Li et al. 2016 [190] showed ultrafine jagged platinum nanowire with highly large ECSA that exhibits enhanced mass activity of ~ 50 times higher than state-of-the-art commercial Pt/C catalyst, while Bu et al. 2016 [191] reported highly uniform PtPb/Pt core/shell nanoplate with biaxially strain extremely active, stable for anodic oxidation reactions, and great performance compared to commercial Pt/C in both methanol oxidation reaction (MOR) and ethanol oxidation reaction (EOR). Since the nanostructured platinum becomes an efficient catalyst for fuel cells as well as various industrial chemical reactions. Thus, these modifications on surface of Pt particles electrocatalysts could also to be applied in MOR for future DMFC.
On the other hand, to reduce the consumption of the Pt catalysts, the modification of the carbon support is also another useful way. This not only improves the transport capacity of protons but also reduces the usage of Nafion, which can cut the cost of the fuel cell. Moreover, with regards to the carbon support for the ORR catalysis, the hydrophobic carbon support material is required to allow water (product) to be quickly removed from the catalyst surface sites, and oxygen (reactant) to access the active sites. In contrast, the MOR catalysis requires a certain degree of hydrophilic carbon support. It can be achieved by the modification of the carbon support materials. By combination of modified carbon support materials and development of new carbon support with Pt metal catalyst, it is possible to get an ideal electrocatalysts for direct methanol fuel cell technology. Combination of Pt metal with varied carbon supports with different specific surface areas, structures, pore sizes, electronic properties, and morphologies could be great catalyst to be studied for future DMFC.
Carbon support also influence the overall performance for DMFC. Vulcan XC-72R, which is a commercial carbon support, has a large surface area, appropriate particle size, and good electrical conductivity for good support. However, in the process of depositing metal particle on these support with loading of 40% or more, the particle size of metal increased quickly, which is a disadvantage for DMFC, because a higher metal loading is used to give a better performance. In addition, multiwalled carbon nanotubes (MWCNTs) and carbon nanofibers (CNFs) with relatively smaller surface area, large diameter, and high aspect ratio could be very difficult to deposit a catalyst with high loading metal (40% and more). Therefore, modification of MWCNTs and CNFs support must be done to improve its surface area, surface functional groups, and reduce the wall thickness to achieve outstanding performance for direct methanol fuel cell even though high loading metal catalyst is consumed. As well, a great and important part to be further studied in DMFC system is about the anode and cathode catalyst preparation approaches.
