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Comparison of Carbon Supports in Anion Exchange Membrane Fuel Cells

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Comparison of Carbon Supports in Anion Exchange Membrane Fuel Cells materials Article Comparison of Carbon Supports in Anion Exchange Membrane Fuel Cells Van Men Truong 1,* , Ngoc Bich Duong 1 and Hsiharng Yang 2,3,* 1 School of Engineering and Technology, Tra Vinh University, Tra Vinh City 87000, Tra Vinh Province, Vietnam; [email protected] 2 Graduate Institute of Precision Engineering, National Chung Hsing University, 145 Xingda Road, South District, Taichung City 402, Taiwan 3 Innovation and Development Center of Sustainable Agriculture (IDCSA), National Chung Hsing University, Taichung City 402, Taiwan * Correspondence: [email protected] (V.M.T.); [email protected] (H.Y.); Tel.: +84-294-3855246 (V.M.T.); +886-04-22873629 (H.Y.) Received: 26 October 2020; Accepted: 24 November 2020; Published: 26 November 2020 Abstract: Anion exchange membrane fuel cells (AEMFCs) are attractive alternatives to proton exchange membrane fuel cells due to their ability to employ nonprecious metals as catalysts, reducing the cost of AEMFC devices. This paper presents an experimental exploration of the carbon support material effects on AEMFC performance. The silver (Ag) nanoparticles supported on three types of carbon materials including acetylene carbon (AC), carbon black (CB), and multiwalled carbon nanotube (MWCNT)—Ag/AC, Ag/CB, and Ag/MWCNT, respectively—were prepared using the wet impregnation method. The silver loading in the catalysts was designed as 60 wt.% during the synthesizing process, which was examined using thermogravimetric analysis. The elemental composition of the prepared Ag/AC, Ag/CB, and Ag/MWCNT catalysts was confirmed using X-ray diffraction analysis. The nanoparticle size of Ag attached on carbon particles or carbon nanotubes, as observed by scanning electron microscopy (SEM), was around 50 nm. For the performance tests of a single AEMFC, the obtained results indicate that the maximum power density using Ag/MWCNT as the cathode catalyst (356.5 mW cm 2) was higher than that using Ag/AC (329.3 mW cm 2) and Ag/CB · − · − (256.6 mW cm 2). The better cell performance obtained using a MWCNT support can be ascribed to · − the higher electrical conductivity and the larger electrochemical active surface area calculated from cyclic voltammetry measurements. Keywords: anion exchange membrane fuel cell; carbon support; cathode catalyst; multiwalled carbon nanotube 1. Introduction Fuel cell technology is considered a promising alternative to power generation for the near future [1]. For fuel cell devices, low cost, durability, and reliability are the main issues that need to be addressed to commercialize this technology [2]. Among the different types of fuel cells, anion exchange membrane fuel cells (AEMFCs) have been introduced to the fuel cell research community [3] due to their advantages compared to proton exchange membrane fuel cells (PEMFCs), which are well developed. The faster kinetics of the oxygen reduction reaction (ORR) in a basic environment than in an acidic environment [4,5], allowing the use of nonprecious metals as electrode catalysts and the reduction in the corrosion problem faced by fuel cell stack hardware, which are the main drawbacks of PEMFCs, demonstrate the potential of AEMFCs as an alternative to PEMFCs. Basically, the structural design of an AEMFC stack is usually similar to that of a PEMFC stack. A typical structure of an AEMFC, Materials 2020, 13, 5370; doi:10.3390/ma13235370 www.mdpi.com/journal/materials Materials 2020, 13, x FOR PEER REVIEW 2 of 12 Materials 2020, 13, 5370 2 of 12 structure of an AEMFC, consisting of anode and cathode electrodes and an anion exchange membrane (AEM) in between, is illustrated in Figure 1. During operation, H2 fuel and water are consistingsupplied to of the anode anode and side cathode, and electrodes O2 gas and and water an anion are delivered exchange membraneto the cathode (AEM) electrode. in between, The iselectrochemical illustrated in Figure reactions1. During occurring operation, at the Hcatalyst2 fuel surface and water in an are AEMFC supplied with to the a direct anode four side,-electron and O2 gaspathway and water can be are described delivered as to follows the cathode [6]: electrode. The electrochemical reactions occurring at the catalystAt surfaceanode: in an AEMFC with a direct four-electron pathway can be described as follows [6]: At anode: − − 0 2H2 + 4OH 4H2O + 4e ; E0 = −0.828 V; (1) 2H + 4OH− 4H O + 4e−;E = 0.828 V; (1) 2 ! 2 − At cathode: At cathode: − − 00 O2 +O22H+ 2OH2+O4e+−4e 4OH 4OH−;E; E == 0.4010.401 V.V. (2) (2) ! Figure 1. Schematic diagram of an anion exchange membrane fuel cell (AEMFC) GDL, gas diffusion Figure 1. Schematic diagram of an anion exchange membrane fuel cell (AEMFC) GDL, gas diffusion layer; CL, catalyst layer. layer; CL, catalyst layer. For most electrocatalysts employed in fuel cell devices, support materials play a critical role in determiningFor most catalytic electrocatalysts activity employed and durability, in fuel as cell well devices, as mass support transfer materials and water play management a critical role [ 3in]. Carbondetermining materials catalytic have activity long been and useddurability in heterogeneous, as well as mass electrocatalysts transfer and as water a support management due to their [3]. specificCarbon featuresmaterials such have as long being been stable used in in both heterogeneous acidic and basic elect environments,rocatalysts as a satisfying support due most to of their the desirablespecific features properties such required as being for stable a suitable in both support acid [ic7 ,8and], and basic being environment removables from, satisfying electrocatalysts most of the by burningdesirable o ffproperties, allowing required an effective for collectiona suitable ofsupport noble catalytic[7,8], and metals being [removable9]. In general, from carbon electrocatalysts materials, characterizedby burning off, by theirallowing high specifican effective surface collection area, high of electricalnoble catalytic conductivity, metals and [9] appropriate. In general, porosity, carbon canmaterials suitably, characterized disperse metal by nanoparticles,their high specific have asurface larger electrochemicalarea, high electrical active conductivity, surface area, andand haveappropriate better electron porosity transfer, can sui andtably water disperse management, metal nanoparticles, resulting in have enhanced a larger performance electrochemical of fuel active cell devices.surface area, Various and carbon have materialsbetter electron such as transfer carbon blackand water (CB), activatedmanagement, carbon, resulting graphene in (GR),enhanced and carbonperformance nanotubes of fuel (CNTs) cell device have beens. Various employed carbon in dimaterialsfferent electrocatalystssuch as carbon [ 10black–12]. (CB), Although activated the carboncarbon, support graphene plays (GR), an important and carbon role innanotubes the electrocatalysts (CNTs) employedhave been in fuelemployed cell devices, in different limited comparisonselectrocatalysts have [10– been12]. Although conducted the to carbon evaluate support and compare plays an fuel important cell performance role in the usingelectrocatalysts different carbonemployed supports in fuel in cell electrocatalysts. devices, limited For comparisons example, Bjorn have etbeen al. [condu13] experimentallycted to evaluate investigated and compare the influencefuel cell performance of carbon supports using different such as biochar carbon (BC),supports CB, in GR, electrocatalysts. and CNTs on the For electrocatalytic example, Bjorn properties et al. [13] ofexperimentally Pt–Ru catalysts investigated toward hydrogen the influence oxidation of carbon reaction supports used for such PEMFCs. as biochar Their (BC), results CB, showed GR, and that CNTs the electrocatalyticon the electrocatalyt activityic isproperties affected by of the Pt– crystallineRu catalysts phase, toward as well hydrogen as the point oxidation of zero reaction charge. Inused a later for study,PEMFCs. Anuar Their et results al. [14] showed conducted that experiments the electrocatalytic to observe activity the electrocatalyticis affected by the activity crystalline of iron phase/cobalt, as (FeCo)well as supported the point of on zero various charge. carbon In a materials later study including, Anuar CB, et al. CNTs, [14] conducted and reduced experiments graphene oxide to observe (rGO) forthe theelectrocatalytic ORR in an acid activity environment of iron/cobalt via cyclic (FeCo) voltammetry supported (CV) on measurements.various carbon FeComaterials/rGO including exhibited theCB, highestCNTs, catalyticand reduced activity graphene according oxide to the (rGO) CV results.for the ToORR the bestin an of acid our knowledge,environment the via eff ectcyclic of divoltammetryfferent carbon (CV supports) measurements. on the ORR FeCo/rGO in alkaline exhibited environments the highest was catalytic only explored activity according by Miguel to and the CV results. To the best of our knowledge, the effect of different carbon supports on the ORR in Materials 2020, 13, 5370 3 of 12 Neil [15]. In their work, Pt supported on CB, MWCNT, graphene oxide (GO), or rGO was prepared via a chemical reduction process and evaluated using the rotating ring-disc electrode technique. In the last decade, different non-noble metals have been intensively studied as cathode catalysts in AEMFCs; among them, silver (Ag) exhibits excellent catalyst activity for the ORR in alkaline environments and relatively high stability at different working temperatures [16,17]. In alkaline media,
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