Jurnal Kejuruteraan SI 1(2) 2018: 9-17http://dx.doi.org/10.17576/jkukm-2018-si1(2)-02

A Short Overview Current Research of Catalyst for Methanol Oxidation Reaction in Direct Methanol Fuel Cell (DMFC) from Experimental and Theoretical Aspect

(Tinjauan Ringkas Mangkin Tindak Balas Pengoksidaan Metanol dalam Sel Fuel Metanol Langsung (DMFC) dari Aspek Eksperimen dan Teori)

Nabila A. Karim*

Fuel Cell Institute, Universiti Kebangsaan Malaysia, Malaysia

Norilhamiah YahyaMalaysian Institute of Chemical and Bioengineering Technology, Universiti Kuala Lumpur, Malaysia

ABSTRACT

Direct Methanol Fuel Cell (DMFC) has shown a great development to replace fossil fuel as new alternative of power sources. However, there are still many issues need to overcome to achieve the dream of world use no more fossil fuel for power sources. One of the main issues is slow catalytic activity of methanol oxidation reaction in anode DMFC. There are several studies of catalysts for methanol oxidation reaction other than Platinum-Ruthenium (PtRu). Finding the catalyst that is cheaper and high reactivity towards methanol oxidation reaction is recommended. The catalyst synthesized should be tested on durability for a long period of time. The selection of catalyst support also affect in methanol oxidation reaction activity in DMFC. Catalyst support must have characteristics in high surface area, conductivity and strong interaction between support and catalyst to prevent leaching problem during long-term operation in DMFC. Several metal alloys and support catalyst has been review in this paper together with discussion about durability of the catalyst. In addition, the DFT study to find the most stable condition of methanol adsorption to occur is highly encouraged. The determination of mechanism reaction using DFT is important in which different pathways will give different activation energy during methanol oxidation reaction. This short overview presented the current research of new catalysts for methanol oxidation reaction.

Keywords: Electrocatalyst; methanol oxidation reaction; Direct Methanol Fuel Cell

ABSTRAK

Sel Fuel Metanol Langsung (DMFC) telah menunjukkan perkembangan maju untuk menggantikan bahan bakar fosil sebagai alternatif sumber tenaga baru. Walau bagaimanapun, masih terdapat banyak isu yang perlu diatasi untuk mencapai impian di mana tiada lagi penggunaan bahan api fosil untuk sumber tenaga. Salah satu isu utama adalah aktiviti mangkin yang perlahan dalam tindak balas pengoksidaan metanol di bahagian anod DMFC. Terdapat beberapa kajian mangkin untuk tindak balas pengoksidaan metanol selain Platinum-Ruthenium (PtRu). Penemuan mangkin yang lebih murah dan mempunyai aktiviti yangi tinggi terhadap tindak balas pengoksidaan metanol adalah disyorkan. Mangkin yang disintesis hendaklah diuji pada ketahanan untuk tempoh masa yang panjang. Pemilihan penyokong mangkin turut mempengaruhi aktiviti tindak balas pengoksidaan metanol di DMFC. Penyokong mangkin perlulah mempunyai ciri-ciri seperti luas permukaan yang tinggi, kekonduksian dan interaksi yang kuat antara sokongan dan mangkin untuk mencegah masalah larut lesap semasa operasi jangka panjang di DMFC. Beberapa aloi logam dan penyokong mangkin telah dikaji semula dalam makalah ini bersama-sama dengan perbincangan mengenai ketahanan mangkin. Di samping itu, kajian DFT juga dijalankan untuk mengetahui penjerapan yang stabil semasa tindak balas pengoksidaan metanol amat digalakkan. Penentuan mekanisme tindak balas menggunakan DFT adalah penting di mana mekanisme yang berbeza akan memberikan tenaga pengaktifan yang berbeza semasa tindak balas pengoksidaan metanol. Gambaran ringkas ini membentangkan penyelidikan terkini mangkin baru untuk tindak balas pengoksidaan metanol.

Kata kunci: Elektromangkin; tindak balas pengoksidaan methanol; Sel Fuel Metanol Langsung

INTRODUCTION

Fuel cell is one of the alternative energy that have advantages such as higher efficiency than diesel or gas engines, less pollution emission and can operates at silently. Application of fuel cell is believe can reduce environmental impact due to

current energy conversion system that use direct combustion of fossil fuel. Fuel cells have several application for example use for portable and backup power, transportation and stationary power. Fuel cell produce electrical energy using chemical energy through electrochemical reaction occur both at anode and cathode without combustion. There are several

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type of fuel cells such as Polymer Electrolyte Membrane Fuel Cell (PEMFC) (Kamarudin et al. 2007), Direct Methanol Fuel Cell (DMFC), Solid Oxide Fuel Cell (SOFC) (Rahman et al. 2010), Molten Carbonate Fuel Cell (MCFC) and Microbial Fuel Cell (MFC). DMFC have advantages over other fuel cell for portable power sources due to fuel ease in handling, transportation and storage, convenience and practicality, high efficiency, and low pollutant.

The electrochemical reaction occur in DMFC is methanol oxidation reaction at anode DMFC and oxygen reduction reaction at cathode DMFC as shown in equation (1) and (2), respectively. The product of methanol oxidation reaction at anode side are protons, electrons and carbon dioxide. The protons will travel through membrane electrolyte to cathode side while the electron will be flowing through external circuit to cathode side. At cathode side, the oxygen reduction reaction need the protons and electrons to reduce the oxygen molecule to form water. The complete reaction both anode and cathode as shown in equation (3). There are several issues in DMFC need to be solved such as methanol crossover, heat management, low power density, slow reaction of electrochemical reaction and others (Kamarudin et al. 2007).

CH OH H O H e CO

O H e H O

CH OH O H O CO

3 2 2

2 2

3 2 2 2

6 6

3

26 6 3

3

22

+ → + +

+ + →

+ → +

+ −

+ −

(1)

CH OH H O H e CO

O H e H O

CH OH O H O CO

3 2 2

2 2

3 2 2 2

6 6

3

26 6 3

3

22

+ → + +

+ + →

+ → +

+ −

+ − (2)

CH OH H O H e CO

O H e H O

CH OH O H O CO

3 2 2

2 2

3 2 2 2

6 6

3

26 6 3

3

22

+ → + +

+ + →

+ → +

+ −

+ −

(3)

The slow kinetics reaction of methanol oxidation reaction in anode side lead to increasing activation anode potential, reduce cell voltage and efficiency DMFC system (Kamarudin et al. 2009). As shown in equation (1), the methanol oxidation reaction produce six protons, six electrons and dioxide. While in Figure 1 shows the mechanism reaction of methanol oxidation reaction in DMFC. During the methanol oxidation reaction, there are high possibilities that by-product formaldehyde, formic acid and carbon monoxide. From Figure 1, the pathway in black color shows that formation of carbon monoxide (CO) before formation of final product of carbon dioxide (CO2). While at the top blue color pathway lead formation of formaldehyde and at the bottom represent formation of formic acid (Neurock et al. 2009). Catalyst that has high reactivity towards methanol oxidation reaction will produce final product at anode side which is protons, electrons and carbon dioxide.

The size of metal particles and distribution on supporting materials will determine the catalytic activity of electrochemical reaction in DMFC. Thus, metal must be in nanoparticle to increase surface catalytic area and well distributed on catalyst support. Platinum based catalyst is commonly used for methanol oxidation reaction in DMFC due to platinum durability and catalytic efficiency. To prevent carbon monoxide (CO) poisoning that occur as by-product

of methanol oxidation reaction is by adding second or third alloy elements such as Ru, Sn, and Rh. Addition of another elements in Pt catalyst not only to reduce CO adsorption on Pt surface but to reduce the high cost of Pt metal. Thus, the major challenging is to design catalyst for methanol oxidation reaction that has high reactivity, low cost, steady, high durability and sustainable from any poisoning catalyst.

CATALYSTS FOR METHANOL OXIDATION REACTION

The catalyst that high surface areas, more edges and corner are able to increase reaction reactivity and ultra-sized noble metal PtRu is the best catalyst for methanol oxidation reaction because it has high selectivity and reactivity compared to other materials. Finding the replacement of Pt or Ru that can give maintaining the reactivity of methanol oxidation reaction or increase reactivity compared to PtRu is searching actively not only to reduce poisoning catalyst that occur in DMFC but also to find cheaper material for DMFC. For example, a new replacement of PtRu catalyst to reduce the cost has been done using combination of four metal to form PtRuFeNi catalyst. The loading of PtRu has reduced from 0.048 mg cm-2 in PtRu/C to only 0.027 mg cm-2 in PtRuFeNi/C. The cost of PtRu per mg is 0.054 USD and with this new alloy catalyst has reduced the cost more than 21% (Basri et al. 2014). It is expected that the new catalyst would reduce the cost of material in DMFC.

The octahedral structure of Platinum-Silver (PtAg) has shown increase in reactivity of methanol oxidation reaction in DMFC (Li et al. 2018). The structure of PtAg has typical XRD plane of fcc Pt on (111), (200) and (220) and the octahedral has 7-10 nm particle size. The molar ratio of Pt to Ag is changing to find the best loading for methanol oxidation reaction. The electrochemical surface active area (ECSA) value for the molar ratio of Pt:Ag 2:1, 1:1 and 1:2 is 57.48 m2 g-1, 47.89 m2 g-1 and 45.64 m2 g-1, respectively. While the ECSA for commercial Pt/C is only 38.26 m2 g-1. Not only that, the synthesized PtAg that has molar ratio 1 has mass density 372.3 mA mg-1

Pt much higher than commercial Pt/C, 178.6 mA mg-1

Pt. Other researchers that study PtAg but on different structure and support for methanol oxidation reaction such

FIGURE 1. The mechanism electrocatalytic oxidation of methanol in DMFC

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as PtAg dendrites nano-forests (Lin et al. 2015), PtAg/rGO (Yousaf et al. 2016) and PtAg/Silica (Wisniewska and Ziolek 2017).

Another substitute for Ru metal is platinum-gold (PtAu) prepared through co-reduction of H2PtCl6 and HAuCl4 supported on reduced graphene oxide (rGO). Introducing Au in Pt has increase the activity of methanol oxidation reaction in which the value of mass-specific activity of PtAu is 0.72 A mg-1

metal compared to Pt/rGo that has only 0.30 A mg-1metal.

However, the result in CO stripping voltammogram shows that PtAu has higher value of onset or peak potential compared to Pt/rGO. The catalyst for methanol oxidation reaction shows higher CO resistance if the onset or peak potential has lower value in CO stripping voltammogram. In other word, the PtAu prior to poison by CO adsorption on catalyst surface (Feng et al. 2017). Both catalyst of PtAu superlattice arrays (Feng et al. 2016) and PtAu nano-alloy/hybrid nanocarbon (Bhaghavathi et al. 2016) show increasing catalytic performance in methanol oxidation reaction.

Not only that, iron (Fe) substitute of Ru also giving promising result to be used as catalyst in methanol oxidation reaction. The PtFe supported in rGO is synthesized using chemical reduction has less than 100 nm of particle size (Eshghi et al. 2018). The XRD analysis showed that presence of Pt, Fe and Fe2O3 in PtFe/rGO sample. The typical diffraction peak of face-centered cubic Pt at plane (111), (200), (220) and (311) while the plane of (110), (200) and (211) represent diffraction peaks at 42°, 64.5° and 82°, respectively. The α-Fe2O3 have diffraction peaks at 36°, 50°, 57°, 60° and 64.5° for plane (110), (113), (116), (018) and (214), respectively. In methanol oxidation reaction, the PtFe/rGO has the highest current density at 4.67 mA cm-2, compared to Pt/C and Pt/rGO has only 2.48 and 1.74 mA cm-2, respectively.

PtRh nano-sponges is develop to overcome the issues of difficulty to controlling the particle morphology, composition and electrochemical property of catalyst for methanol oxidation reaction. The authors (Lu et al. 2018) believe that applying nano-sponge structure in catalyst for methanol oxidation reaction not only accelerating the mass transfer but also can increase the accessible reactant to surface of catalyst. From that, the Pt54Rh46 nano-sponge has increase the reactivity of methanol oxidation reaction by having mass current density of 404.9 mA mg-1

Pt compared to Pt/C that has only 102.8 mA mg-1

Pt.Single metal such as silver (Ag) has been explored to

find any possibilities to be low cost catalyst for methanol oxidation reaction. Synthesis of Ag anchored on reduced graphene oxide sheets (Ag/rGO) as support as new catalyst for methanol oxidation reaction in alkaline DMFC. XRD analysis shows that the diffraction peaks of Ag metal that has average particle size about 10.7 nm are 38.14°, 44.32°, 64.42° and 77.52° represent for (111), (200), (220) and (311) planes, respectively. The reaction current for methanol oxidation reaction is 32 mA cm-2 while for rGO only without catalyst is 3 mA cm-2. The ratio of forward anodic peak current to backward anodic peak current represent poor conversion of alcohol to produce carbon dioxide and Ag/rGO has ratio

4.4 compared to rGO only has ratio about 2.4 (Kumar et al. 2018).

The cobalt-metal organic framework (Co-MOF) supported on GO is being study their reactivity toward methanol oxidation reaction in alkaline medium. From diffraction peaks shows that the plane at (111), (200) and (220) for peaks at 28°, 34° and 53° indicate the presence of cobalt species. Increasing in loading has increase the peak current density of Co-MOF/GO become 29.1 mA cm-2. However, this value still cannot outperform Pt catalyst supported on rGO that has current density of 32.01 mA cm-2 (Mehek et al. 2017).

Replacement of Pt on PtRu using Palladium (Pd) metal has been investigated for methanol oxidation reaction in alkaline medium. The use of Pd based catalyst due to high possibilities that Pd based can give higher performance compared Pt based catalyst at the same times can minimize the material cost in DMFC (Lo Vecchio et al. 2018). The potential more than 0.6 V shows that Ru-core-Pd-shell has high reactivity in methanol oxidation reaction. The authors believe that this increasing in reactivity due the electronic effect and bifunctional mechanism. However, analysis of differential electrochemical mass spectrometry (DEMS) shows that there are detection of formaldehyde, formic acid or methyl formate referring to incomplete methanol oxidation reaction in which same mechanism as shown in Figure 1 (Kübler et al. 2018).

CATALYST SUPPORT MATERIAL

Catalyst synthesis at nanosize particle need medium to support and disperse the catalyst. Catalyst support is use in electrochemical reaction to help distribution of catalyst. The important of catalyst support not only to prevent the adsorption of CO but also to increase rate reaction of methanol oxidation and durability of catalyst and support. Carbon black have properties such as high specific surface area (Kaluža et al. 2015), low cost and high activity (Show and Ueno 2017). Modification of support surface can increase activity and catalytic activity of methanol oxidation reaction. Some research have been done to increase active surface area of supporting material, electrical conductivity, heterogeneous electron transfer rate and uniform distribution on nanoparticle by functionalized surface carbon support (Luo et al. 2013, Eris et al. 2018). Pt catalyst supported on functionalized carbon black (Pt/fCB) showed increasing catalytic current density in magnitude of 5 (50 mA cm-2) compared to commercial Pt/C. The interaction between Pt and support catalyst also increase due believing effect from functionalization process on surface carbon black (Eris et al. 2018).

Carbon nanotube (CNT) made of carbon is a tube-shaped which have many structures, differing in length, thickness and number of layers. CNT has been used as support in DMFC for example PtRu supported on CNT has reduce about 83% loading PtRu and enhancement of power density about 68% (Li et al. 2006). Recently, a hybrid of Titanium cobalt nitride (TiCoN) and CNT is used as support catalyst for methanol

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oxidation reaction to increase corrosion resistance. The Pt catalyst is supported on TiCoN-CNT surface show an increasing activity and corrosion resistance after operating at 5000 s (Zhan et al. 2016). Study using hybrid catalyst of Pt and nickel phosphide (Ni2P) support on CNT has produce current density per mass of Pt as high as 1400 mA mg-1 (Li et al. 2018). Other study that modified or addition other material to CNT is TiN-CNT (Liu et al. 2017), a special network connecting Pt and MWCNT (Huang et al. 2017) and dihydroxy-polybenzimidazole (2OH-PBI)-MWCNT (Yang and Luo 2017) as support for methanol oxidation reaction in DMFC.

Graphene and graphene based also have unique properties such as large surface area, high durability and thermal stability, good conductivity and high mechanical stability which are suitable as a catalyst support in acidic environment DMFC. Graphene that high surface area (~2630 m2 g-1 compared to carbon nanotube (CNT) and graphite is suitable for energy application like in DMFC (Shaari and Kamarudin 2017). Reduced graphene oxide (rGO) matrix by chemical reduction method has been used for support catalyst in methanol oxidation reaction. Nanoparticle platinum-cobalt (PtCo) alloy is use as catalyst has 2-5 nm particle size. The molar ratio of Pt:Co is being tested to find the optimum value for methanol oxidation reaction with value of catalyst to support is 20 wt.%. Having low ratio Pt to Co (1:9) is the optimum value in this research has produced peak power density as high as 118.4 mW cm-2 compared to commercial Pt/C has power only 42 mW cm-2 (Baronia et al. 2017). Pt supported on holey reduced graphene oxide framework (Yu et al. 2017), graphene-fullerene (Zhang et al. 2018), graphene oxide-PVP (Daşdelen et al. 2017), graphene-nanofiber (Mu et al. 2017) and PtCu-rGO (Yang et al. 2017) also show high efficient electrocatalyst for methanol oxidation reaction.

Metal oxide is one of the growth material to be used as support in methanol oxidation reaction for DMFC. There are two ways metal oxide facilitate to increase reaction rate in methanol oxidation reaction by increases the resistance of poisoning effect and also helping increase in dispersion nanoparticle as well as decrease migration and agglomeration of nanoparticle. Cerium Oxide (CeO2) with addition of tin (Sn) mixed with Vulcan XC-72 carbon black is use as catalyst support in methanol oxidation reaction. The combination of CeO2 and Sn is synthesized using solvothermal method and Pt catalyst which having size of 2 nm is deposited on support surface. The Ce0.8Sn0.2O2-δ catalyst support provide high specific surface area for deposition of catalyst, enhanced conductivity. The Pt/ Ce0.8Sn0.2O2-δ-C has mass activity about 502 mA mg-1 and higher in magnitude of 1.2 times and 1.4 times in comparison with Pt/CeO2-C and commercial Pt/C, respectively (Gu et al. 2014).

Titanium oxide (TiO2) is another example application of metal oxide as catalyst support in DMFC. TiO2 has unique properties such as thermal and electrochemical stable in composite and economical making it suitable as support catalyst (Abdullah and Kamarudin 2015, Abdullah and Kamarudin 2017). The electronic conductivity of TiO2 can be alter by mixing with carbon materials. A research to improve

catalyst activity and durability by introduced a hybrid support of TixSn1-xO2-C for methanol oxidation reaction. Pt nanoparticle of 2.3 nm deposited on TixSn1-xO2-C show well improvement in catalyst activity as well as low adsorption of CO on catalyst surface. The inhibition of CO adsorption on catalyst surface due to the high content of OH group support surface and strong interaction of metal and catalyst support. The power performance of single cell using Pt/Ti0.9Sn0.1O2-C is 97.0 mW cm-2 higher than commercial Pt/C which only has power of 73.0 mW cm-2. In comparison with Pt/TiO2-C which only has power performance of single cell 83.2 mW cm-2 even though the structure from TEM and HRTEM showed that both prepared catalyst almost having identical structure (Li et al. 2015). Optimization study of loading between TiO2 Nanotube (TiO2-NT) and PtRu catalyst has been done using method one-factor-at-a-time (OFAT) and Response Surface Methodology (RSM). The best loading ratio between PtRu loading and TiO2-NT is 20.92 wt.% and 40.11 wt.%, respectively. Other research using TiO2-based as catalyst support such as TiO2 nanofiber (Abdullah et al. 2017) and ultrafine TiO2–RF (resorcinol-formaldehyde) (Li et al. 2017).

A recent study that use benzene tetra-carboxylic acid doped polyaniline (BDP) tube as support catalyst for methanol oxidation reaction. The nanoparticle that has 2.5 nm particle size deposited on BDP surface by solution dipping method. To find the optimum value of diameter size and Pt catalyst, the loading density of Pt on BDP is controlled by changing the w/w ratio of BDP to H2PtCl6 from 0.8 to 3.5. A broad peak in XRD analysis at 15 – 30° shows that the BDP has amorphous structure and typical Pt crystalline peak at 40°, 46.3°, 67.8°, and 81.5° corresponding to crystal plane (111), (200), (220) and (311). Ratio of BDP to Pt that has the highest ECSA is 1 to 3.5 with the value of ECSA is 69.54 m2 g-1 compared to Pt/C that has only 14.62 m2 g-1 (Mondal and Malik 2016).

DURABILITY OF ELECTROCATALYST

Durability of catalyst can be tested using cyclic voltammetry (CV). The electrochemical surface active area (ECSA) of catalyst is tested after several cycle of CV and being compared between first cycle and 1000 cycles for example. The ECSA can be calculated using equation (4) below:

ECSAQ

mQH

ref

= (4)

The QH (mC cm-2) represents the columbic charges passed for hydrogen adsorption, m (mg cm-2) represents the Pt loading on the glassy carbon electrode, and Qref is the charge required for a monolayer adsorption of hydrogen on Pt catalyst surface.

Pt nanoparticle supported on graphene oxide (GO) and polyvinylpyrrolidone (PVP) synthesized by one-pot approach is tested on its electrocatalytic activity and durability. The electrochemical surface area (ECSA) represented from hydrogen adsorption and desorption regions in nanocatalyst

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shows that the synthesis catalyst has higher more than 1.53 and 2.87 time compared to Pt-GO and Pt-PVP. In durability test of CV, the catalytic activity is not changes after 1000 cycles (Daşdelen et al. 2017).

A research of Pt deposited on multiwalled carbon nanotube (MWCNT) and dihydroxy-polybenzimidazole (2OH-PBI) in which using protocol of the Fuel Cell Commercialization Conference of Japan (FCCJ) to measure the durability of catalyst. The ECSA measurement is carried out after 1000 cycles. The ECSA value after durability test is 41.5 m2 g-1

Pt compared to before is 47.2 m2 g-1Pt. The

percentage decreasing in ECSA value in Pt/CB has lost its reactivity fast compared to Pt/MWCNT-2OH-PBI (Yang and Luo 2017). Table 1 represent summary from studies that shows the value of ECSA before and after methanol oxidation reaction to determine the durability of synthesis catalyst.

The operating condition and long-term operation in DMFC make metal alloy tend to dissolve. Ru metal is dissolved to form Ru ions and thus crossover to cathode side through nafion membrane (Sawy and Birss 2017, Alia et al. 2015). The dissolution of metal occur due to the surface segregation

effect. The heat of segregation and surface mixing energy may influence the surface region of the alloy metal. This process may enhance or suppress the electrocatalytic reaction of MOR. The effect of Pt segregation in PtRu/C has been studied. The heat treatment at 200°C has changed the surface concentration of Pt in PtRu/C. This process of the surface modification make the Ru become a host energetically segregates the Pt which act as solute. The XPS analysis confirm that metallic Ru was no observed on the surface of PtRu/C (Jeon et al. 2010). From this studied inspired to prepare a core-shell structure in which the Ru metal is the core such as Ru-Pt (Xie et al. 2016) and Ru-Pd (Kübler et al. 2018). The core-shell have high durability and stability compared to PtRu/C nanoparticle. The surface segregation energy for transition-metal alloy have been calculated from theoretical aspect (Ruban et al. 1999). The result from these calculation showed that components that have low surface energy tends to segregate towards the surface of the alloy. The movement of the metal in alloy may affect the reactivity of the MOR by reducing the active site for the reaction to occur. This segregation process thus reduce the performance in long-term operation of DMFC.

TABLE 1. Comparison electrochemical surface active area (ECSA) before and after several cycles

Catalyst Number of ECSA First ECSA at nth cycle Ref Cycle (n) Cycle (m2 g-1)

Pt/CNT@TiCoN 10,000 55.9 76% from the initial value of ECSA (Zhan et al. 2016)

Pt/CNTs@TiN 5,000 50.5 60% from the initial value of ECSA (Liu et al. 2017)

Pt/CNTs 1000 26 93% from the initial value of ECSA (Li et al. 2018)

BDP@Pt4 100 69.54 If/Ib is reduced from 1.09 (initial cycle) to 0.86 (Mondal and Malik 2016) to 0.86Pt/Ti0.9Cu0.1N NFs 10,000 57.3a 87% from the initial value of ECSA (Yu et al. 2018)Pt/TiN NFs - 59.3a -PtAu/f-G-MWNTs - 96 - (Bhaghavathi et al. 2016)Pt54Rh46 - 32.9a - (Lu et al. 2018)PtAg - 57.48 - (Li et al. 2018)PtAu/rGO - 13.12a - (Feng et al. 2017)PtAu Supperlattice Array - 61.9 - (Feng et al. 2016)ArrayPtFe/rGO - 18 - (Eshghi et al. 2018)

If/Ib – ratio of current density forward scan to backward scana m2 g-1Pt

DENSITY FUNCTIONAL THEORY (DFT) OF METHANOL OXIDATION REACTION

Density Functional Theory (DFT) is a powerful computational quantum mechanical modelling method that used to investigate the mechanism reaction of methanol oxidation. DFT can be used in physics, chemistry, and materials science to investigate the electronic structure of many-body systems, in particular atoms, molecules, and the condensed phases. The develop catalyst model and adsorption orientation are some of the important factors to determine the mechanism of methanol oxidation reaction. Figure 2 shows the mechanism

reaction of methanol decomposition from CH3OH molecule to final product of CO adsorbed on Pt3Sn catalyst using potential energy surface diagram. The dissociation of methanol is believed can be in three pathways of C-H, O-H and C-O bond scission as shown in Figure 2. Only C-H scission has the lowest activation energy compared to O-H and C-O scission. The mechanism of methanol oxidation reaction on Pt3Sn occurs via CH3OH→CH3O→CH2O→CHO→CO (Lu et al. 2016). The determination of mechanism reaction of MOR using DFT is a powerful method for catalyst and product analysis from theoretical aspect.

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Synthesized of Platinum-Cobalt (PtCo) alloy supported on MWCNT has been tested on methanol oxidation reaction (Huang et al. 2012). In the XRD analysis, the typical diffraction peaks of Pt are at plane (111), (200) and (220). However, to compare the methanol oxidation in the DFT study, the model catalyst of PtCo (111) (Orazi et al. 2017) is developed. There are also several study on adsorption orientation of methanol that being adsorbed on surface of PtCo (111) for example on top of the platinum atom, top of the cobalt atom, several site on between of platinum and cobalt atom and also on between of three atoms. The value of adsorption energy of methanol adsorbed on top of Co atom is -0.92 eV and this adsorption is most stable compared to other orientations. There is another example of methanol adsorbed on the surface Pt3Sn as shown in Figure 3. In Figure 3a, several adsorption of methanol can be seen here as number 1 and 2 referring to top adsorption of methanol on Pt and Sn atom, respectively. While for B

is referring to bridge adsorption and F and H referring to different hollow adsorption of methanol on Pt3Sn catalyst. While Figure 3b shows the stable adsorption configuration of reaction intermediates after several testing on adsorption orientations (Lu et al. 2016). The negative value of adsorption energy is referring the adsorption is stable. The distance between Co atom (model catalyst of PtCo (111)) and oxygen atom in methanol molecule is 2.24 Å. Aluminum (Al-) or Silicon (Si-) decorated on graphene oxide is being tested on methanol adsorption using DFT method. The modified Go surface shows that the CH3OH and CH2O molecule effectively adsorbed on surface of Al-GO (Esrafili and Dinparast 2018). Other current DFT studies for methanol oxidation reaction such as model catalyst of mono- and bi- layer Pt-modified WCs (Sheng et al. 2015), PtRu (111) (Zhao et al. 2015) and Pt3Sn (111) (Lu et al. 2016).

FIGURE 2. Potential energy surface (PES) of methanol decomposition on Pt3Sn (111). The black, green and red line represent the C-H, O-H and C-O bond scission pathway, respectively. (Reprinted with permission

from Lu et al. 2016. Copyright (2018) American Chemical Society)

FIGURE 3. (a) The gigh symmetric adsorption site in Pt3Sn; 1: TPt; TSn; 3: B2Pt; 4: BPtSn; 5: F3Pt; 6: H3Pt; 7: F2PtSn; 8: H2PtSn. (b) The stable adsorption configuration of reaction intermediates along reaction pathway of methanol decomposition on Pt3Sn(The stable adsorption configuration of reaction intermediates along reaction pathway of methanol decomposition on Pt3Sn(111) and other metastable adsorption configurations of reaction intermediates. (Reprinted with permission from Lu et al. 2016. Copyright (2018) American Chemical Society)

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CONCLUSION

Application of DMFC has high potential and still has long ways to make it into reality. One of the main issues in DMFC is sluggish rate reaction of methanol oxidation reaction. The best and high selectivity of methanol oxidation reaction is PtRu catalyst. However, the high cost of noble metal is actually preventing the commercialization of DMFC. Finding the new catalyst that is cheaper but still can maintain the reactivity or better can improve the reactivity is highly expected. Every catalyst has own advantages and challenging. The durability study shown catayst deteriorate over the time. The catalyst structure that has nanosize tends to detach from support or agglomerate at a long period time. Synthesized the larger particle then has disadvantages on reduce the surface area of catalyst and increase the loading of catalyst leads to higher cost of noble metal. Thus the finding of optimum stucture that balance both cost, reactivity and durability should be investigate. Up to recent studies, the PtRu still has the best reactivity in MOR. The possible way to lowering catalyst cost by reducing the PtRu loading. The core-shell structure seen have several advanteges for MOR in term of stability and durability. Finding new combination of non-precious metal and PtRu for core and shell is believe not only can reduce the loading of PtRu but high possibilities improving MOR activity as well as maintaining stability and durability of the catalyst, resectively. While for support catalyst, the carbon based have more advantages over other materials in term of low cost and high conductivity. However, doping carbon such as nitrogen-doped carbon will increase the strength adsorption between catalyst and support and thus preventing the leaching problem in DMFC and improving stability as weel as durability of catalyst. Moreover, the study of methanol oxidation reaction could lead to improving reactivity and which catalyst is the best at molecular level.

ACKNOWLEDGEMENT

The authors would like to thank Universiti Kebangsaan Malaysia for their financial support under the grant GGPM-2018-054.

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*Nabila A. Karim

Fuel Cell Institute,Universiti Kebangsaan Malaysia, Malaysia *Corresponding author; email: nabila.akarim@ukm.edu.my

Received date: 9th April 2018Accepted date: 13th September 2018Online First date: 31st September 2018Published date: 1st October 2018