A Membraneless Direct Isopropanol Fuel Cell - ACS Publications

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A Membraneless Direct Isopropanol Fuel Cell (DIPAFC) Operated with a Catalyst-Selective Principle Xingwen Yu, Long Cheng, Yuanyue Liu, and Arumugam Manthiram J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12535 • Publication Date (Web): 22 Jan 2018 Downloaded from http://pubs.acs.org on January 29, 2018

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The Journal of Physical Chemistry

A Membraneless Direct Isopropanol Fuel Cell (DIPAFC) Operated with a Catalyst-Selective Principle Xingwen Yu, Long Cheng, Yuanyue Liu, and Arumugam Manthiram* Materials Science and Engineering Program & Texas Materials Institute, The University of Texas at Austin, Austin, TX 78712, USA Corresponding Author *Email: [email protected].

ABSTRACT: The concept of catalyst-selective membraneless DLFC has recently been proposed and has successfully been demonstrated with a number of alcohol or formate fuels. However, there is a critical anolyte-poisoning issue in the previous membraneless alkaline DLFC systems due to CO2 formation upon the oxidation of liquid fuels. This study demonstrates a new membraneless DLFC system with isopropanol as an anode fuel. The dehydrogenation of isopropanol generates mainly acetone rather than CO2 as a product, which eliminates the CO2poisoning concerns. Operation of the membraneless direct isopropanol fuel cells (DIPAFCs) is principally based on the high catalytic selectivity of the cathode catalyst which exhibits high

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catalytic activity for the oxygen reduction reaction (ORR) but no activity for the isopropanol oxidation reaction (IPAOR). The highly selective cathode catalysts demonstrated in this study are spinel MnCo2O4 or MnNiCoO4 oxide nanoparticles coupled to a multi-wall carbon nanotube (MWCNT) surface. Experimental results reveal that the MnNiCoO4/MWCNT catalyst exhibits a higher ORR activity than the MnCo2O4/MWCNT catalyst. Computational studies based on density functional theory (DFT) indicates that the addition of Ni to MnCo2O4 improves the electrical conductivity of the metal oxide nanoparticles, thereby enhancing the ORR activity of MnNiCoO4/MWCNT. The membraneless DIPAFC demonstrated in this study can deliver a high power density of ca. 20 mW cm-2 at room temperature and ca. 80 mW cm-2 at 80 oC.

INTRODUCTION Fuel cell technologies can play an important role in the efforts to switch to green energy, and they are considered as one of the key energy technologies in the future for both transportation and household heating systems.1-3 Low-temperature fuel cells operated with a proton exchange membrane (PEM) are currently in the forefront among the different types of fuel cells.4,5 The PEM-based fuel cells are commonly operated with two types of fuels, gaseous hydrogen and liquid-phase small molecular organics. In comparison to H2 gas, liquid fuels offer significant advantages with respect to storage, transportation, and safety.6-8 In addition, liquid fuels are able to provide much higher volumetric and gravimetric energy densities than the gaseous fuels.9-11 Furthermore, quite a lot of small molecule organic liquids are able to be harvested/obtained from renewable resources. Therefore, direct liquid fuel cells (DLFCs) have


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been regarded as one of the most promising energy conversion systems for future clean energy technologies.12,13 The traditional direct liquid fuel cells (DLFCs) based on PEM membranes have to be operated under a weakly acidic condition due to the use of proton (H+) exchange membranes. However, the acidic operating condition raises a key challenge with regard to the slow kinetics of oxygen reduction reaction (ORR).14,15 It requires a high-loading platinum-based cathode catalysts to facilitate the ORR under acidic environments to minimize the cell overpotential.16,17 On the other hand, due to the facile ORR kinetics in alkaline media, the alkaline DLFCs have recently attracted much attention.18,19 Under an alkaline condition, less expensive cathode catalysts can be used to drive the ORR.20,21 However, the operation of an alkaline DLFC requires the use of hydroxide (OH-) exchange membranes to separate the anode fuel and the air cathode. At the current development stage, the hydroxide (OH-) exchange membrane technologies are not yet practically reliable enough for the development of alkaline DLFCs in terms of cost, stability, and efficiency.22-25 The development of membraneless DLFCs can avoid the use of hydroxide exchange membranes. However, the membraneless fuel cells previously developed based on a micro-flow phenomenon cannot be built in large scales. Thus, their practical application is limited.26 According to the above issues associated with the DLFC technologies, development of membraneless DLFCs with alternative approaches have been pursued by employing strategically designed catalysts.27-29 We recently proposed a membraneless alkaline DLFC concept based on a catalyst-selective strategy.30-32 Small molecule alcohols, such as methanol and ethanol, are always the primary fuel candidates for the development of DLFCs because of their high volumetric and gravimetric energy densities.33,34 However, the oxidation


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products of methanol or ethanol are a mixture of carbon dioxide and some other intermediates.11,35 Under an alkaline fuel cell operating condition, the CO2 reacts with the hydroxide to precipice carbonates. Thus, it requires supplying hydroxide to the anode fuel to replenish the consumed hydroxide.36-38 To avoid the hydroxide-consumption problem by CO2, isopropanol has been studied as an alternative to methanol or ethanol in operating alkaline direct liquid fuel cells. The dehydrogenation of isopropanol yields majorly acetone as a product, which does not react with hydroxide.39,40 In view of the above discussions, it would be interesting to develop an alkaline membraneless DLFC system with isopropanol as the anode fuel to overcome the CO2-poisoning issue. Therefore, in this study, we present a membraneless alkaline direct isopropanol fuel cell (DIPAFC) operated under a catalyst-selective principle. Highly selective cathode catalysts employed in this study are MnCo2O4 and MnNiCoO4 oxide nanoparticles coupled onto multiwall carbon nanotube (MWCNT) surface. These two cathode catalysts exhibit a high catalytic selectivity that catalyzes only the ORR but does not catalyze the isopropanol oxidation reaction. The catalyst-selective feature of the cathode catalysts enables the membraneless operation of the DIPAFCs.

The

MnNiCoO4/MWCNT

catalyst

shows

a

higher

ORR

activity

than

MnCo2O4/MWCNT examined by the electrochemical experiments. To fundamentally understand the effect of Ni-doping on the ORR activity enhancement, computational studies were performed. Density functional theory (DFT) calculations imply that the ORR activity enhancement of the MnNiCoO4/MWCNT is due to the improvement in electric conductivity through the addition of Ni to MnCo2O4.


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EXPERIMENTAL SECTION Catalyst synthesis. The MWCNT used in this study was provided by US Research Nanomaterials, Inc. The MWCNT was first pretreated by refluxing 0.25 g of MWCNT powder in 25 mL of 10 M HNO3 solution at 80 °C for 5 h. After cooling down, the precipitated MWCNT powder was collected and washed with de-ionized (DI) water for five times. Then the MWCNT powder was dried at 45 oC under vacuum for 24 h. To synthesize the MnNiCoO4/MWCNT composite catalyst. 0.5 mL of 0.3 M Mn(CH3COO)2, 0.75 mL of 0.2 M Ni(CH3COO)2, and 0.5 mL of 0.3 M Co(CH3COO)2 solutions were added to a 25 mL CH3CH2OH bath together with 50 mg of the pretreated MWCNT powder. Then, 1.5 mL of ammonia hydroxide (NH4OH) was added to the above mixture at room temperature. The mixture was continuously stirred for 24 h at a constant temperature of 80 °C. Then, the mixture was transferred into an 80-mL autoclave to perform a hydrothermal reaction for 3 h in an oven at 150 °C. The resulting powder product was then collected and adequately washed with ethanol and water. Finally, the obtained MnNiCoO4/MWCNT composite catalyst product was dried at 45 oC under vacuum for 24 h. The MnCo2O4/MWCNT catalysts were synthesized the same way by adjusting the amount of Mn(CH3COO)2, Co(CH3COO)2 precursors. Electrochemical experiments. Rotating disk electrode (RDE) experiments were conducted on a Pine MSR Rotator System in combination with an Autolab potentiostat (PGSTAT128N). The working electrode was a catalyst-modified glassy carbon electrode (5 mm in diameter). The catalyst was loaded by spreading a catalyst ink (a homogeneous mixture of catalyst powder, Nafion binder, and water) onto the glassy carbon electrode. The experiments were performed with a three-electrode electrochemical cell using a saturated calomel electrode (SCE) as the reference electrode and a Pt mesh as the counter electrode. The electrochemical experiments


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were controlled by the Autolab potentiostat. The catalyst-modified working electrode was scanned from open-circuit potential (OCV) in a cathodic direction at a 5 mV s−1 scan rate. The rotating speed of the working electrode was maintained at 1600 rpm. The ORR activities of the catalysts were also examined with gas-diffusion electrodes, which were prepared by depositing 0.5 mg of catalyst (with a homogeneous catalyst ink comprising a mixture of catalyst powder, Nafion binder, and water) onto a 1 cm2 carbon fiber paper (purchased from Fuel Cell Store, Toray paper 060). The linear sweep voltammograms were recorded with an Autolab potentiostat (PGSTAT128N). The experiments were performed with a three-compartment electrochemical cell using a saturated calomel electrode (SCE) as the reference electrode and a Pt mesh as the counter electrode. The electrolyte was 1.0 M KOH solution with a constant flow of oxygen (to make sure the solution was saturated with oxygen). The activity of the isopropanol oxidation on different catalysts was tested with linear sweep voltammetry (LSV) measurements. The experiments were also performed on the Autolab potentiostat (PGSTAT128N) with a three-electrode electrochemical cell (with SCE as the reference electrode and Pt mesh as the counter electrode). The working electrodes were prepared by depositing 0.5 mg of catalyst onto a 1 cm2 Toray carbon fiber paper. The electrolyte was 1.0 M isopropanol in 1.0 M KOH solution. Fuel cell performance test. An in-house designed membraneless DLFC was employed for the cell performance evaluation. The detailed configuration of this fuel cell has been previously presented in our recent publications.30-32 The active flow area of this cell was 5.0 cm2. PtRu/C (40 wt.% Pt-Ru (Pt : Ru = 1 : 1) on Vulcan® XC-72, E-TEK) was employed as the anode catalyst in this study. The highly selective MnNiCoO4/MWCNT was used as the cathode catalyst. The anode and cathode were prepared the same way by homogenously depositing 10.0


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mg of catalyst onto a 5 cm2 Toray carbon fiber paper. The loading of catalyst (including all components, such as PtRu metal, carbon support, and Nafion binder in the anode, MnNiCoO4, MWCNT, and Nafion binder in the cathode) was 2.0 mg cm-2 in all cases (either in the anode or in the cathode). During the fuel cell operation, an alkaline solution comprising 2.0 M isopropanol and 1.0 M KOH (as supporting electrolyte) was fed into the cell chamber between the cathode and anode. At the cathode side, oxygen was delivered at a flow rate of 100 mL min-1. The cell was operated with a standard fuel cell test instrumentation produced by Scribner Associates Inc. (850E). Computational studies. Electronic structure of the catalysts was analyzed with density functional theory (DFT) calculations.

RESULTS AND DISCUSSION Insight into the ORR activity enhancement of the MnCoNiO4/MWCNT catalyst This section majorly deals with an insight into the ORR activity enhancement of the MnCoNiO4/MWCNT catalyst through the addition of Ni to the MnCo2O4/MWCNT catalyst. The characterizations of the MnCoNiO4/MWCNT catalyst can be accessed in our previous publication.31 The MnNiCoO4/MWCNT composite comprised ca. 5 nm MnNiCoO4 supported on MWCNT. The molar ratio of Ni : Co : Mn was ca. 1 : 1 : 1 in the catalyst. MnNiCoO4 was single-phase with the cubic spinel structure.31 Figure 1a compares the RDE characteristics of the MnCoNiO4/MWCNT and the MnCo2O4/MWCNT catalysts in 1.0 M KOH solution (saturated with O2). The voltammograms were obtained with a linear sweep mode. The rotating speed of the electrodes was maintained at 1600 rpm. The MnCoNiO4/MWCNT catalyst exhibited superior ORR activity than the


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MnCo2O4/MWCNT catalyst with respect to the half-wave potential and the disk current density, as indicated in Figure 1a. There was ca. 20 mV difference in the half-wave potentials between the MnCoNiO4/MWCNT and the MnCo2O4/MWCNT catalyst. Figure 1b compares the electrochemical performances of the MnCoNiO4/MWCNT and the MnCo2O4/MWCNT catalysts under the conditions resembling those in alkaline fuel cells. The electrodes were prepared by loading the MnCoNiO4/MWCNT or the MnCo2O4/MWCNT catalyst onto the carbon paper. As seen in Figure 1b, the MnNiCoO4/MWCNT catalyst yielded significantly higher ORR current density than MnCo2O4/MWCNT throughout the testing potential frame.

Figure 1. (a) Rotating-disk electrode (RDE) voltammetries of the MnNiCoO4/MWCNT and MnCo2O4/MWCNT catalysts in 1.0 M KOH electrolyte (saturated with oxygen). The scan rate was 5.0 mV s-1. The rotating speed of the electrodes was 1600 rpm. (b) Linear sweep voltammetries (LSV) of the MnNiCoO4/MWCNT and MnCo2O4/MWCNT catalysts. The electrodes were prepared by loading the catalysts onto carbon-fiber papers. The electrolyte was 1.0 M KOH solution saturated with O2.

In order to understand the effect of Ni-addition on the ORR activity of the catalyst, density functional theory (DFT) calculations were performed. The superior electrochemical performance of MnCoNiO4 over MnCo2O4 may be due to the enhanced charge transfer from the substrate electrode to the active sites on the surface through the catalyst. To examine this scenario, density


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functional theory calculations were performed by employing the Vienna Ab-initio Simulation Package (VASP)41-43 with projector augmented wave pseudopotentials44,45 and Perdew–Burke– Ernzerhof (PBE) exchange-correlation functional.46 To account for the on-site Coulomb repulsion among the localized 3d electrons, we added Hubbard U47 parameters for Mn (4.0 eV), Co (6.5 eV), and Ni (6.4 eV).48,49 The energy cutoff was set at 520 eV and the Brillouin zone was sampled with 8 × 8 × 8 Monkhorst-Pack50 k meshes. The systems were fully relaxed until the final force on each atom was less than 0.01 eV/Å. For the calculation of density of states, a much denser k mesh of 15 × 15 × 15 was employed. Figure 2 shows the calculated electronic density of states (DOS) of the MnCo2O4 and the MnCoNiO4 catalysts. We found that MnCo2O4 had a band gap of 1.3 eV (Figure 2a) while MnCoNiO4 exhibited a metallic feature (Figure 2b). Therefore, MnCoNiO4 has a higher electrical conductivity than MnCo2O4, which could give rise to the higher electrochemical performance. To understand the origin of this high conductivity, we projected the total DOS onto individual atoms and found a significant contribution from Ni 3d states to the MnCoNiO4 “conduction electrons” (i.e., the states near Fermi level; Figure 2b). The contribution of the Ni 3d electrons was further confirmed by the charge density distribution, as presented in Figure 2c. Therefore, the difference in electronic structure between the MnCoNiO4 and the MnCo2O4 can be attributed to the difference in d electrons between the Ni and Co. In the MnCo2O4, the Co3+ cations occupy the octahedral sites; due to the crystal field splitting, their six d electrons are paired up and fulfill the t2g level. When the Co3+ cation is substituted by Ni3+, the extra d electron from Ni3+ occupies the eg level. This unpaired electron results in unoccupied states near the Fermi level, thus improving the conductivity of MnCoNiO4.


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Figure 2. Calculated electronic density of states (DOS) of (a) MnCo2O4 and (b) MnCoNiO4. The total DOS is shown by black lines and the contributions from Mn, Co, O, and Ni are shown, respectively, by cyan, green, blue and red lines. The positive and negative values correspond, respectively, to the spin up and spin down states. The Fermi level is set as 0 eV with the dashed line. (c) Charge density distribution of the states within 0.05 eV below and above the Fermi level of MnCoNiO4.

Catalytic selectivity of the MnNiCoO4/MWCNT The activity of the catalysts for the isopropanol oxidation reaction was examined with electrochemical experiments. Figure 3a shows the cyclic voltammetry (CV) curves of the


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MnNiCoO4/MWCNT catalyst in a solution comprising 1.0 M isopropanol and 1.0 M KOH. For a comparison, the CV profile obtained with a PtRu/C catalyst was also plotted here. As seen in Figure 3a, the oxidation current of isopropanol on the PtRu/C catalyst almost increases in a linear manner within the potential frame of -0.7 V to -0.1 V (vs. SHE). This selected potential frame is the potential domain of anode (oxidation) reaction of the fuel for the operation of the alkaline fuel cells. On the other hand, the MnNiCoO4/MWCNT catalyst does not show any activity for the anode reaction of isopropanol within this potential range. The high ORR activity and the extreme inactivity for IPAOR hint that MnNiCoO4/MWCNT would be a suitable cathode catalyst for the development of a membraneless alkaline DIPAFC.

Figure 3. (a) Linear sweep voltammograms (5.0 mV s−1) of the MnNiCoO4/MWCNT and the PtRu/C catalysts in 1.0 M IPA + 1.0 M KOH electrolyte. (b) RDE voltammetries of the MnNiCoO4/MWCNT catalyst in 1.0 M KOH or 1.0 M KOH + 1.0 M isopropanol electrolyte (saturated with oxygen). The scan rate was 5.0 mV s-1. The rotating speed of the electrodes was 1600 rpm. (c) LSV profiles of the MnNiCoO4/MWCNT catalyst in 1.0 M KOH or 1.0 M KOH + 1.0 M isopropanol electrolyte. The electrodes were prepared by loading the catalysts onto carbon-fiber papers. The electrolyte was saturated with O2.

In order to investigate the effects of anode fuel isopropanol on the oxygen reduction reaction at the cathode, the same experiments as those in Figure 1 were performed by adding 1.0 M isopropanol to the 1.0 M KOH solution. The RDE characteristics and the LSV characteristics of the MnCoNiO4/MWCNT catalyst in a 1.0 M KOH solution and in a 1.0 M KOH/1.0 M


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isopropanol solution are, respectively, compared in Figure 3b and c. There were no significant impacts observed from isopropanol for the ORR on the MnCo2O4/MWCNT catalyst.

Membraneless Direct Isopropanol Fuel Cell (DIPAFC) Based on the catalyst-selective feature of the MnNiCoO4/MWCNT catalyst with respect to its activity for the ORR and its inactivity for the IPAOR, a membraneless alkaline DIPAFC was explored. Figure 4 shows the schematic of an in-house designed cell configuration. In this fuel cell, the highly selective MnNiCoO4/MWCNT was used as the cathode catalyst. PtRu/C was employed as the anode catalyst. An aqueous solution comprising 2.0 M isopropanol (fuel) and 1.0 M KOH (supporting electrolyte) was delivered through a 2 mm thick chamber between the two electrode layers. The supporting electrolyte KOH provides OH- ions to sustain both the cathode and the anode reactions. In conventional DLFCs, it usually requires an ionic membrane (either a H+-membrane or an OH--membrane) to prevent the liquid fuel from entering into the cathode. In the DIPAFC shown in Figure 4, since the cathode catalyst MnNiCoO4/MWCNT does not have any activity to catalyze the fuel oxidation reaction, the IPA fuel can freely enter into the cathode. Therefore, it does not require an ionic membrane in this fuel cell. Figure 5a and b show, respectively, the polarization behavior and the corresponding power density curves of the membraneless alkaline DIPAFCs collected at 25 oC, 50 oC, and 80 oC. The open-circuit voltage (OCV) of the cell was ca. 0.85 V, which was similar to those of direct methanol fuel cell (DMFC) and direct ethanol fuel cell (DEFC). Under the testing conditions in this study, the membraneless alkaline DIPAFCs can deliver specific powers of ca. 20 mW cm-2, ca. 55 mW cm-2, and ca. 80 mW cm-2, respectively, at 25 oC, 50 oC, and 80 oC. These performance data of the membraneless alkaline DIPACs are comparable to those of the


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previously demonstrated catalyst-selective DLFCs with methanol (DMFC), ethanol (DEFC), ethylene glycol (DEGFC), or glycerol (DGFC) fuels.30-32 Since the oxidation product of isopropanol is mainly acetone, the membraneless DIPAC system provides significantly important advantages without CO2-poisoning concerns over the DMFC, DEFC, DEGFC, and DGFC systems. On the other hand, the fuel cell performances presented in Figure 5 are also comparable (or even superior) to the previously demonstrated membrane-based DIPAFCs (with either a proton exchange membrane or an anion-exchange membrane) that are operated with a high-loading Pt cathode catalyst.33

Figure 4. Schematic configuration of the membraneless direct isopropanol fuel cell (DIPAFC).


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Figure 5. (a) Polarization curves and (b) corresponding power density plots of the membraneless direct isopropanol fuel cells (DIPAFCs) operated at different temperatures.

In the membraneless alkaline DIPAFC demonstrated above, facile cathode ORR reactions can be assessed on the non-noble metal MnNiCoO4/MWCNT catalyst. Exclusion of the ionic membrane from the fuel cell avoids the need for expensive and difficult-to-develop OH-exchange membranes. As a matter of fact, previous attempts to operate membraneless direct liquid fuel cell were always based on maintaining a laminar flow regime in the cell. Therefore, it is not feasible to scale-up such laminar-flow membraneless fuel cells. The catalyst-selective operating principle enables the membraneless DLFCs to be fabricated in flexible configurations without any scale-up concerns. This catalyst-selective strategy provides a versatile approach for the development of inexpensive, safe, and scalable membraneless alkaline DLFCs with a broad range of renewable liquid fuels.

CONCLUSIONS In summary, this study presents the catalytic selectivity of a MnCoNiO4/MWCNT catalyst with respect to its high activity for the oxygen reduction reaction and its inactivity for the isopropanol oxidation reaction. By taking this unique advantage, a membranelass alkaline direct


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isopropanol fuel cell (DIPAFC) has been demonstrated with MnCoNiO4/MWCNT as a cathode catalyst. The membraneless DIPAFC can deliver a high power density of ca. 20 mW cm-2 at room temperature and ca. 80 mW cm-2 at 80 oC. In addition to demonstrating the membraneless DIPAFC, insight into the ORR activity enhancement of the MnCoNiO4/MWCNT catalyst through the addition of Ni to the MnCo2O4/MWCNT catalyst has been computationally studied. The density functional theory (DFT) calculations indicate that the addition of Ni to MnCo2O4 improves the electric conductivity of the metal oxide nanoparticles, thereby enhancing the ORR activity of MnNiCoO4/MWCNT.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the Welch Foundation grant F-1254. This work used the computational resources at the National Renewable Energy Laboratory that was sponsored by the Department of Energy Office of Energy Efficiency and Renewable Energy, and the computational resources at the Texas Advanced Computing Center (TACC) at the University of Texas at Austin.


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A Membraneless Direct Isopropanol Fuel Cell - ACS Publications

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