Anode structure with double-catalyst layers for improving the direct ethanol fuel cell performance

The conventional anode design of direct ethanol fuel cells (DEFCs) usually encounter a problem on the performance stability and ethanol mass transport, i.e., ethanol crossover. Aiming to alleviate these issues, in this study, the anode with different configurations for DEFC was designed and fabricated with different catalyst layer (CL) and microporous layer (MPL) arrangements. The four types of membrane electrode assembly (MEA) is named with MEA-1 (with pretreated carbon paper (PCP) and PtCL), MEA-2 (with PCP, MPL and PtCL), MEA-3 (with PCP, MPL, PtCL and PdCL) and MEA-4 (with PCP, MPL, PtCL, MPL and PdCL). The performance, stability and ethanol crossover of MEAs were tested and measured for continuous long-term operation for 120 h, while the morphological characterization was analyzed. Based on the results, power density for each MEA decreased with time, while ethanol crossover increased gradually. The MEA-3 with additional PdCL shows a highest performance and stability about 20 W/m², and has a lowest ethanol crossover's magnitude. The highest ethanol crossover was obtained using MEA-1 at 3.7 mg/m²·s. Higher ethanol crossover had caused low stability of DEFC performance which result higher irreversible degradation. Moreover, based on characterization, elemental mapping and EDX illustrated phenomena of membrane swelling, delamination of electrode from membrane, and CL loss after stability test for 5 days for all MEAs. The significance of anode structure design was proven in this current study. The anode design of double-layered CL has potential to use at anode structure to reduce ethanol crossover rate, thereby improving DEFC performance and stability.     

Direct ethanol fuel cell anode simulation model

The electrochemical behavior of a direct ethanol feed proton exchange membrane fuel cell (DEFC) operating under steady-state isothermal conditions at 1 atm at both anode and cathode sides is considered. A mathematical model that describes in one phase and one dimension the ethanol mass transport throughout the anode compartment and proton exchange membrane is developed. The influence of the operation parameters such as current density, temperature, catalyst layer thickness and ethanol feed concentration on both anode overpotential and ethanol crossover rate has been examined. According to the simulation results, it was found that the anode overpotential is more sensitive to the protonic conductivity than to the diffusion coefficient of ethanol in the catalyst layer. It was concluded that in the case of low current density values and high concentrations of ethanol aqueous solutions, ethanol crossover is a serious problem for a DEFC performance. Finally, it was found a good agreement between simulation and experimental results.     

Effect of variation of hydrophobicity of anode diffusion media along the through-plane direction in direct methanol fuel cells

Reducing methanol crossover from the anode to cathode in direct methanol fuel cells (DMFCs) is critical for attaining high cell performance and fuel utilization, particularly when highly concentrated methanol fuel is fed into DMFCs. In this study, we present a novel design of anode diffusion media (DM) wherein spatial variation of hydrophobicity along the through-plane direction is realized by special polytetrafluoroethylene (PTFE) coating procedure. According to the capillary transport theory for porous media, the anode DM design can significantly affect both methanol and water transport processes in DMFCs. To examine its influence, three different membrane-electrode assemblies are fabricated and tested for various methanol feed concentrations. Polarization curves show that cell performance at high methanol feed concentration conditions is greatly improved with the anode DM design with increasing hydrophobicity toward the anode catalyst layer. In addition, we investigate the influence of the wettability of the anode microporous layer (MPL) on cell performance and show that for DMFC operation at high methanol feed concentration, the hydrophilic anode MPL fabricated with an ionomer binder is more beneficial than conventional hydrophobic MPLs fabricated with PTFE. This paper highlights that controlling wetting characteristics of the anode DM and MPL is of paramount importance for mitigating methanol crossover in DMFCs.     

Mitigating ethanol crossover in DEFC: A composite anode with a thin layer of Pt50–Sn50 nanoparticles directly deposited into Nafion membrane surface

A composite anode comprising an outer and an inner catalyst layer is proposed to 1) suppress the ethanol crossover in direct ethanol fuel cell (DEFC), and 2) improve the cell performance as well as the utilization efficiency of ethanol fuel. The inner catalyst layer contains a thin layer of Pt₅₀–Sn₅₀ nanoparticles directly deposited on the Nafion® membrane surface through impregnation-reduction (IR) method, and acts as the reactive ethanol filter. In this paper, several aspects of the research are reported. First, the mitigation of ethanol crossover and the performance of membrane electrode assembly (MEA) of the proposed structure are compared to those with normal structure. Next, a candidate mechanism of the mitigation of ethanol crossover and the improvement of MEA performance is investigated. Third, SEM, X-ray, EDS and EPMA analysis are used to characterize microstructures, phases, chemical composition and distributions of the obtained Pt₅₀–Sn₅₀ layer. Finally, the ethanol crossover rate in a DEFC is determined through measuring the CO₂ concentration at the cathode exhaust in real time. Experimental results demonstrate that the composite anode with an inner layer of Pt₅₀–Sn₅₀ nano-catalyst particles on Nafion membrane surface suppresses ethanol crossover up to 17% more than the anode without the inner layer, and yield a 6% better MEA performance than the normal-MEA. The inner Pt₅₀–Sn₅₀ catalyst layer serves both as an ethanol filter and an electrode. Its dual-role contributes to the suppression of ethanol crossover, and improvement of both cell performance and the utilization efficiency of ethanol fuel, both of which are dependent on the catalyst activity of the ethanol electro-oxidation over the thin catalyst layer directly deposited on Nafion membrane surface.     

Addition of rhenium (Re) to Pt-Ru/f-MWCNT anode electrocatalysts for enhancement of ethanol electrooxidation in half cell and single direct ethanol fuel cell

Pt-Ru and Pt–Re and Pt-Ru-Re nanoparticles supported on functionalized multi-walled carbon nanotubes (f-MWCNT) were synthesized via modified polyol reduction method and tested thoroughly in a half cell and single direct ethanol fuel cell for ethanol electrooxidation in acidic medium. The MWCNTs were functionalized in a mixture of HNO₃/H₂SO₄ solution for depositing a more active metal alloy nanoparticle on support material. The alloy formation of bi-metallic and tri-metallic electrocatalysts were examined by XRD analysis and more clearly explained by FE-SEM element mapping. The TEM analyses reveal that electrocatalysts nanoparticles are well dispersed on f-MWCNT, with spherical shapes and nano sizes range of 1.5–4 nm. The electrochemical analyses by cyclic voltammetry and chronoamperometry measurements reveal that tri-metallic electrocatalyst Pt-Ru-Re (1:1:0.5)/f-MWCNT exhibits the highest electrocatalytic activity and stability towards ethanol electrooxidation among all the synthesized electrocatalysts. The same electrocatalyst as anode in single DEFC results in excellent performance in comparison to all other synthesized electrocatalysts, with a maximum power density of 9.52 mW/cm² at a cell temperature of 30 °C. The bi-metallic Pt-Ru (1:1)/f-MWCNT and Pt–Re (1:1)/f-MWCNT produced power density of 7.48 mW/cm² and 4.74 mW/cm² at room temperature of 30 °C. The power density of DEFC enhanced 2.44 times, when cell operating temperature was increased from 30 °C to 80 °C using anode electrocatalyst Pt-Ru-Re (1:1:0.5)/f-MWCNT and keeping other parameters constant. The best result obtained in half cell and single DEFC using Pt-Ru-Re (1:1:0.5)/f-MWCNT electrocatalyst may be attributed to the synergistic effect of Pt, Ru and Re combined with bi-functional and ligand effects.     

Effects of anode microporous layers made of carbon powder and nanotubes on water transport in direct methanol fuel cells

The effects of the design parameters of the anode diffusion layer (DL), including the PTFE loading in the backing layer (BL), and the carbon and PTFE loading in the microporous layer (MPL), on water transport through the membrane and the performance of a liquid-feed direct methanol fuel cell (DMFC) are experimentally investigated. The results indicate that increasing the PTFE loading in the BL and introducing a MPL could decrease water crossover through the membrane without sacrificing cell performance when the feed methanol concentration is increased. It is also found that changing the PTFE loading in the MPL has little effect on water crossover, whereas increasing the carbon loading in the MPL could noticeably decrease the water-crossover flux. Nevertheless, the ability of the MPL to reduce water crossover is limited by the presence of a number of mud cracks. To reduce further the water-crossover flux, a crack-free MPL made of multi-walled carbon nanotubes (MWCNTs) and PTFE is proposed. Tests indicate that the DMFC with the nanotube MPL results in a much lower water-crossover flux than a conventional carbon-powder MPL. More importantly, the use of the nanotube MPL allows the DMFC to be operated with a higher methanol concentration, and thereby increases the fuel cell system energy density.     

High performance direct ethanol fuel cell with double-layered anode catalyst layer

Double-layered anode catalyst layers with two reverse configurations, which consist of 45 wt.% Pt₃Sn/C and PtRu black catalyst layers, were fabricated to improve the performance of a direct ethanol fuel cell (DEFC). The in-house 45 wt.% Pt₃Sn/C catalyst was characterized by XRD and TEM. The cross-sectional double-layered anode catalyst layer was observed by SEM. In DEFC performance test and anode linear sweep voltammetry measurement, the anode with double-layered catalyst layer exhibited better catalytic activity for ethanol electro-oxidation than those with single-layered 45 wt.% Pt₃Sn/C and PtRu black catalyst layers. In terms of anode product distribution, the DEFC with double-layered anode catalyst layer showed a higher yield of acetic acid than that with single-layered PtRu black catalyst layer and a higher yield of CO₂ than that with single-layered 45 wt.% Pt₃Sn/C catalyst layer, respectively. These results suggest that the double-layered anode catalyst layer possessed the advantages of both Pt₃Sn/C and PtRu black catalysts for ethanol electro-oxidation, and thus showed a higher ethanol electro-oxidation efficiency and DEFC performance in the practical polarization potential region.     

Promising anode candidates for direct ethanol fuel cell: Carbon supported PtSn-based trimetallic catalysts prepared by Bönnemann method

To find an efficient anode catalyst for ethanol electrooxidation, several trimetallic PtSnM/C (M = Ni, Co, Rh, Pd) and their corresponding bimetallic PtX/C (X = Sn, Ni, Co, Rh, Pd) catalysts were synthesized by Bönnemann's colloidal precursor method and evaluated by comparing their electrocatalytic activity using conventional electrochemical techniques. For better understanding of the catalyst deactivation during the ethanol electrooxidation, chronoamperometric test was also combined to X-ray photoelectron spectroscopy (XPS) analysis. A significant finding is that trimetallic compositions PtSnCo/C and PtSnNi/C have enhanced activity compared to that of PtSn/C, with lower onset potential for ethanol electrooxidation and notably improved peak current densities. Thus the presence of Ni and Co heteroatom seems to promote C–C bond cleavage and facilitate the removal from the catalyst surface of adsorbed intermediates. These trends are satisfactorily confirmed by testing in a direct ethanol fuel cell (DEFC), since trimetallic PtSnNi/C and PtSnCo/C anode catalysts have significantly higher overall performance and peak power density than Pt/C, PtSn/C or other trimetallic catalyst compositions PtSnRh/C or PtSnPd/C. Furthermore, the presence of Ni or Co helps to improve the weak stability of PtSn/C by providing a stronger Pt–carbon support interaction. XPS results revealed that the surface Pt/Sn atomic ratio of PtSnNi/C catalyst only slightly decreased even after 12 h at 500 mV. On the other hand, a higher concentration of oxide species appeared on the treated PtSn/C surface as a result of a high degradation of carbon support.     

A model for direct ethanol fuel cells considering variations in the concentration of the species

The fuel cell is an electrochemical device that converts chemical energy directly into electricity and is more efficient than traditional power generators. In this work, we developed a mathematical model for a direct ethanol fuel cell (DEFC), which considers the flow and concentration of species dependent on time and space for the calculation of losses overpotentials. In addition, the concentration of each species is modeled according to the current density of the DEFC. The finite element method is used to calculate the flow and concentration of the species in different layers of the cell (inlet and outlet channels, diffusion layer and catalyst layer). The model takes into account the losses overpotentials at the anode and at the cathode and the passage of ethanol through the membrane. The voltage and power density of the cell are calculated with different catalysts, temperatures and concentrations of ethanol. A result is shown for limiting current density for low ethanol concentrations. The results obtained compare favourably with the data found in the literature.     

Preparation and electrocatalytic characteristics of the Pt-based anode catalysts for ethanol oxidation in acid and alkaline media

The current study reports the preparation and investigation of several Pt-based anode catalysts loaded on reduced graphene oxide (rGO) as electrocatalysts in both acid and alkaline media for ethanol electrooxidation. The synthesized catalysts are evaluated by the method of XRD, Raman spectroscopy, XPS and TEM. Electrocatalytic properties of these catalysts for ethanol oxidation were investigated by cyclic voltammetry and chronoamperometry. It was found that the as-prepared nanocatalysts doped by metals and oxide metals showed the improvement of catalytic performance compared to Pt-only supported on graphene catalyst. The results indicated that the presence of Al favoured Pt nanoparticles dispersing on the surface of rGO sheets. Indeed, the PAG catalyst exhibits the highest mass activity for the ethanol oxidation of 1194 mA mg^?1_Pt in acid medium and 3691 mA mg^?1_Pt in alkaline medium. In addition, the PAG catalyst also shows good antipoisoning ability for ethanol electrooxidation in both media. This catalyst could be a potential catalyst for direct ethanol fuel cell (DEFC).     

Catalysts for direct ethanol fuel cells

By comparing the performance of fuel cells operating on some low molecular weight alcohols, it resulted that ethanol may replace methanol in a direct alcohol fuel cell. To improve the performance of a direct ethanol fuel cell (DEFC), it is of great importance to develop anode catalysts for ethanol electro-oxidation more active than platinum alone. This paper presents an overview of catalysts tested as anode and cathode materials for DEFCs, with particular attention on the relationship between the chemical and physical characteristics of the catalysts (catalyst composition, degree of alloying, and presence of oxides) and their activity for the ethanol oxidation reaction.     

Effect of design of multilayer electrodes in direct methanol fuel cells (DMFCs)

In a direct methanol fuel cell (DMFC), optimized multilayer electrode design is critical to mitigate methanol crossover and improve cell performance. In this paper, we present a one-dimensional (1-D) two-phase model based on the saturation jump theory in order to explore the methanol and water transport characteristics using various multilayer electrode configurations. To experimentally validate the 1-D model, two different membrane electrode assemblies (MEAs) with and without an anode microporous layer (MPL) are fabricated and tested under various cell current density and methanol feed concentration conditions. Then, 1-D DMFC simulations are performed and the results compared to the experimental data. In general, the numerical predictions are in good agreement with the experimental data; thus, the 1-D DMFC simulations successfully model the effects of the anode MPL that were observed experimentally. In addition to the comparison study, additional numerical simulations are carried out to precisely examine the role of the anode and cathode MPLs and the effect of the hydrophobicity of the anode catalyst layer on the water and liquid saturation distributions inside the DMFCs. This paper demonstrates the quantitative accuracy of the saturation jump model for simulating multilayer DMFC MEAs and also provides greater insight into the operational characteristics of DMFCs incorporating multilayer electrodes.     

A model-based parametric analysis of a direct ethanol polymer electrolyte membrane fuel cell performance

In the present work, a model-based parametric analysis of the performance of a direct ethanol polymer electrolyte membrane fuel cell (DE-PEMFC) is conducted with the purpose to investigate the effect of several parameters on the cell's operation. The analysis is based on a previously validated one-dimensional mathematical model that describes the operation of a DE-PEMFC in steady state. More precisely, the effect of several operational and structural parameters on (i) the ethanol crossover rate from the anode to the cathode side of the cell, (ii) the parasitic current generation (mixed potential formation) and (iii) the total cell performance is investigated. According to the model predictions it was found that the increase of the ethanol feed concentration leads to higher ethanol crossover rates, higher parasitic currents and higher mixed potential values resulting in the decrease of the cell's power density. However there is an optimum ethanol feed concentration (approximately 1.0 mol L⁻¹) for which the cell power density reaches its highest value. The platinum (Pt) loading of the anode and the cathode catalytic layers affects strongly the cell performance. Higher values of Pt loading of the catalytic layers increase the specific reaction surface area resulting in higher cell power densities. An increase of the anode catalyst loading compared to an equal one of the cathode catalyst loading has greater impact on the cell's power density. Another interesting finding is that increasing the diffusion layers’ porosity up to a certain extent, improves the cell power density despite the fact that the parasitic current increases. This is explained by the fact that the reactants’ concentrations over the catalysts are increased, leading to lower activation overpotential values, which are the main source of the total cell overpotentials. Moreover, the use of a thicker membrane leads to lower ethanol crossover rate, lower parasitic current and lower mixed potential values in comparison to the use of a thinner one. Finally, according to the model predictions when the cell operates at low current densities the use of a thick membrane is necessary to reduce the negative effect of the ethanol crossover. However, in the case where the cell operates at higher current densities (lower ethanol crossover rates) a thinner membrane reduces the ohmic overpotential leading to higher power density values.     

Parametric study on direct ethanol fuel cell (DEFC) performance and fuel crossover

A parametric study was carried out to investigate the effect of fuel concentration (0.5 M–3.0 M), operating temperature (ambient temperature to 85 °C), flow rate of ethanol (0.5–5.0 mL min⁻¹) and air (100–600 mL min⁻¹) on the direct ethanol fuel cell (DEFC) performance. The operations were conducted in three operational modes, namely, passive, semi passive, and active modes, and power generation were measured. Ethanol crossover was indicated by the carbon dioxide (CO₂) concentration present at the cathode outlet and measured by using a CO₂ analyzer. Results indicated that DEFC performance increased with the increase of ethanol concentration, and ethanol and oxidant flow rate increased with temperature until DEFC reaches the optimum conditions, i.e., concentration and flow rate. Meanwhile, the DEFC performance significantly and proportionally increased with operation temperature and reached values of up to 8.70 mW cm⁻² and 85 °C at stable conditions. Furthermore, fuel crossover, that is, ethanol flux, increased in proportion to the ethanol concentration, i.e., 3.71 × 10⁻⁴ g m⁻² s⁻¹ and 8.79 × 10⁻⁴ g m⁻² s⁻¹ for 0.5 M and 3.0 M ethanol concentration, respectively. At different modes of operation, the active DEFC system exhibited the highest performance, followed by the semi passive and passive DEFC system. These results indicated that optimizing ethanol, oxidant flow rate and temperature would enhance the mass transport in anodes and cathodes, and hence improve the electrochemical reactions and DEFC performance.     

Mathematical modeling of alkaline direct ethanol fuel cells

A one-dimensional model is developed for alkaline direct ethanol fuel cells (DEFC) by considering the complicated physicochemical processes, including mass transport, charge transport, and electrochemical reactions. The model is validated against experiments and shows good agreement with the literature data. The model is then used to investigate the effects of various operating and structural design parameters on the cell performance. Numerical results show that the cell performance increases with increasing the ethanol concentration from 1.0 M to 3.0 M and with increasing the OH⁻ concentration from 1.0 M to 5.0 M. The model is further applied to the study of the effect of the design of the anode diffusion layer (DL) on the performance; it is shown that the cell performance improves when the porosity of the DL is increased and the thickness of the DL is decreased.     

Evaluation of Pt–Ru–Ni and Pt–Sn–Ni catalysts as anodes in direct ethanol fuel cells

In this study, the electrooxidation of ethanol on carbon supported Pt–Ru–Ni and Pt–Sn–Ni catalysts is electrochemically studied through cyclic voltammetry at 50 °C in direct ethanol fuel cells. All electrocatalysts are prepared using the ethylene glycol-reduction process and are chemically characterized by energy-dispersive X-ray analysis (EDX). For fuel cell evaluation, electrodes are prepared by the transfer-decal method. Nickel addition to the anode improves DEFC performance. When Pt₇₅Ru₁₅Ni₁₀/C is used as an anode catalyst, the current density obtained in the fuel cell is greater than that of all other investigated catalysts. Tri-metallic catalytic mixtures have a higher performance relative to bi-metallic catalysts. These results are in agreement with CV results that display greater activity for PtRuNi at higher potentials.     

Effect of addition of rhenium to Pt-based anode catalysts in electro-oxidation of ethanol in direct ethanol PEM fuel cell

Breaking of C–C bond at low temperature to completely oxidize ethanol in direct ethanol fuel cell (DEFC) is the limiting factor for the development of DEFC as alternative source of power in portable electronic equipment. Binary and ternary Pt based catalysts with addition of Re, Pt–Re/C (20:20), Pt–Sn/C (20:20), Pt–Re–Sn/C (20:10:10) and Pt–Re–Sn/C (20:5:15) catalysts were prepared from their precursors by co-impregnation reduction method to study electro-oxidation of ethanol in DEFC. The electrocatalysts characterized by transmission electron microscope, scanning electron microscope, energy dispersive X-ray, and X-ray diffraction shows the formation of above mentioned bi- and tri-metallic catalyst with size ranges from 6 to 16 nm. Electrochemical analyses by cyclic voltammetry, linear sweep voltammetry and chronoamperometry show that Pt–Re–Sn/C (20:5:15) gives higher current density compared to that of Pt–Re/C (20:20) and Pt–Sn/C (20:20). The addition of Re to Pt–Sn/C is conducive to electro-oxidation of ethanol in DEFC. The power density obtained using Pt–Re–Sn/C(20% Pt, 5% Re, 15% Sn by wt) (30.5 mW/cm²) as anode catalyst in DEFC is higher than that for Pt–Re–Sn/C(20% Pt, 10% Re, 10% Sn by wt) (19.8 mW/cm²), Pt–Sn/C (20% Pt, 20% Sn by wt) (22.4 mW/cm²) and Pt–Re/C (20% Pt, 20% Re by wt) (9.8 mW/cm²) at 100 °C, 1 bar, with catalyst loading of 2 mg/cm² and 5 M ethanol as anode feed.     

Development of an advanced MEA to use high-concentration methanol fuel in a direct methanol fuel cell system

Despite serious methanol crossover issues in Direct Methanol Fuel Cells (DMFCs), the use of high-concentration methanol fuel is highly demanded to improve the energy density of passive fuel DMFC systems for portable applications. In this paper, the effects of a hydrophobic anode micro-porous layer (MPL) and cathode air humidification are experimentally studied as a function of the methanol-feed concentration. It is found in polarization tests that the anode MPL dramatically influences cell performance, positively under high-concentration methanol-feed but negatively under low-concentration methanol-feed, which indicates that methanol transport in the anode is considerably altered by the presence of the anode MPL. In addition, the experimental data show that cathode air humidification has a beneficial effect on cell performance due to the enhanced backflow of water from the cathode to the anode and the subsequent dilution of the methanol concentration in the anode catalyst layer. Using an advanced membrane electrode assembly (MEA) with the anode MPL and cathode air humidification, we report that the maximum power density of 78 mW/cm² is achieved at a methanol-feed concentration of 8 M and cell operating temperature of 60 °C. This paper illustrates that the anode MPL and cathode air humidification are key factors to successfully operate a DMFC with high-concentration methanol fuel.     

Predictive mathematical modeling and computer simulation of direct ethanol fuel cell

The challenges of finding a better substitute of energy as well as the shortcomings identified with direct ethanol fuel cell, includes high anode over potential and crossover necessitate the need to investigate the influence operating parameters on the performance of fuel through computer simulation. This study focus on the development of a predictive mathematical modeling for direct ethanol fuel cell for the purpose of investigating the influence pressure, temperature cathode, and reactants concentration on the performance, efficiency, and heat generated by the cell. Results obtained indicate that an increase in operating temperature led to a decrement in output voltage and cell efficiency, while the same condition of increasing the temperature positively favors the heat generated from the cell. Simulated results also show that cell performance is improved with an increase in concentration of the fuel (ethanol) and oxidant (oxygen). It can be inferred from this study that the cell performance of DEFC can be theoretically predicted with the developed model.