An Acid–Base Battery with Oxygen Electrodes: A Laboratory

Demonstration Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

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An Acid−Base Battery with Oxygen Electrodes: A Laboratory Demonstration of Electrochemical Power Sources Guo-Ming Weng,*,†,‡ Chi-Ying Vanessa Li,† and Kwong-Yu Chan*,† †

Department of Chemistry, The University of Hong Kong, Pokfulam Road, Pokfulam, Hong Kong Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06511, United States

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S Supporting Information *

ABSTRACT: Utilizing an acid and a base to generate electricity is a valuable experience for students to (i) realize the importance of electrochemical power sources and (ii) develop fundamental knowledge related to energy and electrochemistry. We apply an acid−base battery with oxygen electrodes to demonstrate electrochemical power sources in the laboratory. This battery consists of an acidic oxygen reduction reaction at the cathode and an alkaline oxygen evolution reaction at the anode. This demonstration provides an attractive and instructive activity for laboratory courses in electrochemistry. KEYWORDS: First-Year Undergraduate/General, Physical Chemistry, Electrochemistry, Acids/Bases, Ion Exchange, Materials Science, Oxidation/Reduction, Hands-On Learning/Manipulatives

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reaction (Eocathode = 1.23 V) and producing H2O (reaction i). The overall reaction consumes two H2 molecules and one O2 molecule in the production of two H2O molecules (reaction iii). According to eq 1, this acidic fuel cell can deliver a cell voltage of 1.23 V. In an alkaline fuel cell, H2 is oxidized at the anode (Eoanode = −0.83 V), producing H2O and releasing electrons (reaction v). The electrons move from the anode to the cathode in the external circuit, reducing oxygen in the reaction (Eocathode = 0.40 V) and producing hydroxide ions (reaction iv). H2/air fuel cells have the same overall reaction (reactions iii and vi) with a Gibbs free energy change of ΔG° = −237.18 kJ mol−1,9 regardless of whether acidic or alkaline electrolyte is used.

lectrical energy generation with fossil fuels is a major source of air pollution in the world today as it produces harmful waste like carbon oxides, nitrogen oxides, and sulfur oxides. Clean energy conversion and storage technologies using renewable sources are therefore receiving increased attention.1−4 However, broad application of these technologies is still limited by intermittent energy sources and costly integrated energy storage devices.5 A recent report pointed out that the amount of solid waste generated in the United States, Asia, and Europe reached approximately 2.5 billion tons in 2015.6 This number forces us to realize the importance of environmental protection and develop electrochemical power sources using waste. An interesting example has been demonstrated by Goncalves,7 in which a discarded hard drive platter is used as an electrocatalyst for water splitting to produce clean hydrogen fuel. However, we know that there are many other types of waste, and multiple processes (e.g., combustion and anaerobic digestion) are used to convert waste into energy. Various innovative laboratory demonstrations8 utilizing waste to generate fuels or electricity can stimulate students to learn the fundamentals of energy science and address our waste management challenges. The hydrogen/air (H2/air) fuel cell (including both acidic and alkaline fuel cells) is an electrochemical device that converts oxygen (from the air) and hydrogen (from a supply) into electrical energy. The electrochemistry of fuel cells is illustrated in Scheme 1. In an acidic fuel cell, H2 is oxidized at the anode (Eoanode = 0.00 V), producing protons and releasing electrons (reaction ii). The electrons move from the anode to the air cathode in the external circuit, reducing oxygen in the © XXXX American Chemical Society and Division of Chemical Education, Inc.

o o o Ecell = Ecathode − Eanode

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Here, we demonstrate an acid−base battery (also termed acid−base fuel cell) with oxygen electrodes, aiming to utilize waste acid and base from industrial processes. As shown in Figure 1A and Scheme 1, this electrochemical cell has an air cathode (where an oxygen reduction reaction takes place, reaction vii or i, Eocathode = 1.23 V) in an acidic compartment coupled with a nickel foil anode (where an oxygen evolution reaction takes place, reaction viii, or the reverse of reaction iv, Eoanode = 0.40 V) in an alkaline compartment. A bipolar membrane (its structure is shown in Figure 1B, consisting of a cation-selective layer and an anion-selective layer laminated together) is used as an ionic conductor for K+ and HSO4− ions, Received: November 4, 2018 Revised: June 6, 2019

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DOI: 10.1021/acs.jchemed.8b00901 J. Chem. Educ. XXXX, XXX, XXX−XXX

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Scheme 1. Electrochemistry of H2/Air Fuel Cells and the Acid−Base Battery with Oxygen Electrodes

also as a barrier for H+ and OH− ions. The electrochemistry of the proposed acid−base battery is illustrated in Scheme 1. At the anode, hydroxide ions are oxidized in alkaline medium, producing O2 and H2O while releasing electrons (reaction viii). The electrons move from the anode to the air cathode in the external circuit, reducing oxygen (from the air) in an acidic medium producing H2O (reaction vii). The internal current flow is from the anode to the cathode carried by K+ and HSO4−/SO42− ions. The overall reaction consumes one proton and one hydroxide ion in the production of one H2O molecule (reaction ix or x). According to eq 1, this acid−base battery theoretically delivers a cell voltage of 0.83 V (corresponds to a Gibbs free energy change ΔG° = −79.89 kJ mol−1 based on the electroneutralization reaction, see section 1 of the Supporting Information). Compared with H2/air fuel cells, the acid−base battery is an electrochemical device that converts H+ and OH− ions into electrical energy in the presence of oxygen (from the air). This demonstration is used to (i) provide fundamental understanding of fuel cells and acid−base battery systems (other than conventional energy devices such as lithium-ion batteries), (ii) demonstrate an advanced electricity generator using neutralization energy, and (iii) elaborate basic electrochemistry and physical chemistry to students. To the best of our knowledge, this is the first demonstration using neutralization energy to appear in this Journal. This demonstration can be used as an attractive and instructive activity for laboratory courses in electrochemistry.

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contact with a 25 mL H2SO4 aqueous solution. The alkaline chamber has a nickel foil anode immersed in a 25 mL KOH aqueous solution. The two chambers are separated by a 0.1 cm thick bipolar membrane CMI7000/AMI7001 (Membrane International Inc., USA), and its active area is approximately 25 cm2. During discharge (as reported in ref 13), both HSO4− (acid chamber) and K+ (base chamber) ions move toward to the AEM/CEM junction and partially penetrate into the other side to complete the internal circuit (section 2, Supporting Information). The bipolar membrane is immersed in 5 wt % K2SO4 aqueous solution overnight before use. The dimension of each acid and alkaline electrolyte acrylic chamber is about 4.6 cm (L) × 6.8 cm (H) × 1 cm (W) (Figure 1C; a draft drawing is shown in section 3, Supporting Information). The interelectrode gap is around 2 cm. Gaskets (e.g., rubber and silicone) are used between the chambers to prevent the leakage of liquids (assembly instruction is shown in section 4, Supporting Information). The cell design can be easily fabricated by a 3D printer or via laser cutting with pHresistant materials (e.g., acrylonitrile butadiene styrene for 3D printing but with acrylics for laser cutting). Here, combined courses in electrochemistry and chemical engineering are highly recommended. In addition, an easy-to-assemble cell setup is shown in section 5 of the Supporting Information. Electrode Preparation

The cathode is prepared as follows: 50 mg of 10% Pt/C powder (E-Tek) and 30 wt % PTFE emulsion (freshly diluted from a 60 wt % PTFE emulsion in H2O, Sigma-Aldrich) are mixed to form a slurry, which is then pasted and pressed onto one side of a piece of 40% Teflon-treated carbon cloth (Fuel Cell Earth). The active area of the cathode is around 9 cm2. Before use, the anode (nickel foil, 99.9+%, Sigma-Aldrich) is pretreated with the following steps to improve its electrochemical activity:14 (i) the nickel foil is electrochemically

EXPERIMENTAL SECTION

Acid−Base Battery Design

The acid−base battery has two chambers (Figure 1A). The acid chamber has an air cathode (gas diffusion electrode), in which one side is open to air and the catalyst layer side in B

DOI: 10.1021/acs.jchemed.8b00901 J. Chem. Educ. XXXX, XXX, XXX−XXX

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Figure 1. (A) Schematic diagram of the acid−base battery with oxygen electrodes. Ep = ⌀cathode − ⌀Hg/Hg2SO4 is the potential difference between the gas diffusion electrode (GDE, cathode) and the reference electrode (Hg/Hg2SO4). En = ⌀anode − ⌀Hg/HgO is the potential difference between the nickel electrode (anode) and the reference electrode (Hg/HgO). Vm = ⌀Hg/Hg2SO4 − ⌀Hg/HgO is the potential difference between reference electrodes Hg/Hg2SO4 and Hg/HgO across the bipolar membrane. Vcell = ⌀cathode − ⌀anode is cell voltage. (B) Schematic diagram of the structure of the bipolar membrane. Possible ion transport during the discharging process is also shown. Reproduced with permission from ref 10. Copyright 2016 The Electrochemical Society. (C) Photographs of the acrylic compartments used for the cell assembly and (D) the acid−base battery in which an alternative three-electrolyte configuration (i.e., both cation- and anion-exchange membranes are used as separators)11,12 for a bipolar membrane is used.

reduced in 1 mol L−1 KOH aqueous solution at −1.2 V for 1 min; (ii) there is a 15 min idling period at open-circuit voltage; (iii) the nickel foil is electrochemically oxidized at 0.465 V for 3 min; (iv) the nickel foil is further electrochemically oxidized at 0.885 V for another 2 min. The above activation steps are conducted with a traditional three-electrode setup. Nickel foil is used as the working electrode. The counter electrode is a platinum plate, and the reference electrode is a Hg/HgO electrode with 1 mol L−1 KOH filling solution. The pretreated nickel foil is stored in 1 mol L−1 KOH for later use. The active area of the anode is around 9 cm2. Commercial fuel cell electrodes are also available online (e.g., Fuel Cell Store).

For the discharge polarization curve, the cell is discharged at a specified current density for 30 s and then allowed to relax for up to 2 min at its open-circuit voltage. This cycle can then be repeated by discharging at the next desired current density which is set by the potentiostat. Potential measurements are averaged over the 30 s of each current step to provide a point on the polarization curve.

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HAZARDS

Potassium hydroxide and concentrated sulfuric acid (98%) are extremely corrosive and will cause severe burns if they come into contact with eyes, skin, or mucous membranes. If strong acid/base is not preferred, weak acid/base (e.g., citric acid and potassium carbonate) could also be used for the proof-ofconcept. However, using weak acid/base with low ionic strength and concentration can significantly reduce the output power due to the increase of the internal resistance. Mercurybased reference electrodes (i.e., Hg/Hg2SO4 and Hg/HgO) contain toxic substances that cause harmful effects on the nervous, digestive, and immune systems, lungs, and kidneys. Therefore, care should be taken when handling these chemicals. Mercury-based electrodes are not necessary if monitoring the potential change of individual components is not needed.

Electrochemical Tests

Galvanostatic discharge tests are carried out with a Biologic VMP3 potentiostat in the air at 23 °C. A Hg/Hg2SO4 electrode filled with 1 mol L−1 H2SO4 and a Hg/HgO electrode filled with 1 mol L−1 KOH are employed as reference electrodes in the acidic and alkaline chambers, respectively (Figure 1A,D). Alternatively, hydrogen reference electrode can be substituted.13 If an electrochemical station is not available, resistors with different resistances and voltmeter (or multimeter) can be used. Stainless steel clips (Digi-key, U.S.) are preferred in this demonstration due to their relatively good corrosion resistance to acids and bases. C

DOI: 10.1021/acs.jchemed.8b00901 J. Chem. Educ. XXXX, XXX, XXX−XXX

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Figure 2. Discharge performance of the acid−base battery at 2 A m−2 (current per electrode active area): (A) Ep is cathode vs Hg/Hg2SO4; (B) En is anode vs Hg/HgO; (C) Vm is the voltage across the bipolar membrane; and (D) Vcell is the cell voltage.

Figure 3. Polarization curve of the acid−base battery at electrolyte concentration of H2SO4/KOH = 2 M/2 M (A) and corresponding V−I curve of each component (B).

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RESULTS AND DISCUSSION

6, in Supporting Information, and the discussion of polarization curves). Better electrode materials and cells with a supply of pure O2 are preferred to achieve higher open-circuit voltage. The stability of the electrochemical cell during idling is also reflected in the curves of Ep and En (Figure 2A,B). At 2 M/2 M, the Ep remains stable at a higher electrode potential (0.19 V vs Hg/Hg2SO4) while En is stable at a lower electrode potential (0.31 V vs Hg/HgO) as compared to the cases at 1 M/1 M. Vm curves at both electrolyte concentrations (Figure 2C) remain stable at 0.44−0.48, indicating that the bipolar membrane can effectively block the diffusion of H+ and OH− ions. This test shows that higher open-circuit voltage can be obtained with more concentrated electrolytes. As discussed in

Discharge Performance

The breakdown of voltage losses in the acid−base battery and performance of individual components at two different electrolyte concentrations can be determined by examining the four voltage profiles in Figure 2A−D. During idle, the cell voltages (Vcell, Figure 2D) with 1 M/1 M H2SO4/KOH (lower electrolyte concentration) and 2 M/2 M H2SO4/KOH (higher electrolyte concentration) are stable at about 0.27 and 0.36 V, respectively. The discrepancy between theoretical and practical open-circuit voltages is due to the low practical cathode potential and relatively high practical anode potential (section D

DOI: 10.1021/acs.jchemed.8b00901 J. Chem. Educ. XXXX, XXX, XXX−XXX

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section 1 in Supporting Information and described by eq 2, the cell voltage of such a battery is proportional to the pH differential between catholyte and anolyte. Ecell = −0.0592 V × (pH acid − pH base)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00901. Detailed description, discussion, and characterization of the acid−base battery (PDF, DOCX)

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During discharge (Figure 2D), Vcell at 2 M/2 M remains stable at 0.13 V which is about 0.08 V higher than that at lower electrolyte concentration (i.e., 1 M/1 M). Ep (Figure 2A) is stable at 0.1 V (1 M/1 M) ∼ 0.12 V (2 M/2 M) vs Hg/ Hg2SO4, and En (Figure 2B) is stable at 0.48 V (1 M/1 M) ∼ 0.46 V (2 M/2 M) vs Hg/HgO. The increment in operating voltage is mainly due to the contribution from voltage across the bipolar membrane when using a higher acid/base concentration (Figure 2C). Both Vm curves in Figure 2C remain stable during discharge, indicating a stable ionic transport (i.e., HSO4− and K+ ions) across the membrane. As discussed in section 7 of the Supporting Information, a higher concentration can also significantly lower the internal resistance. The voltage drop of Vcell (Figure 2D) is mainly induced by the activation loss of En (Figure 2B). Students are also encouraged to calculate the theoretical specific energy for different electrochemical cells (section 8, Supporting Information) and the potential values versus a standard hydrogen electrode (section 6, Supporting Information).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Guo-Ming Weng: 0000-0002-4710-8815 Chi-Ying Vanessa Li: 0000-0003-2559-9550 Kwong-Yu Chan: 0000-0002-4124-2562 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region (Project T23-601/17-R). The authors thank anonymous reviewers for their valuable comments and Christopher Karpovich for his generous help on this manuscript.

Polarization Curves

Polarization curves shown in Figure 3 are obtained using a constant current method where the duration of each current step is 30 s. As shown in Figure 3A, the peak power density of the present acid−base battery is 700 mW m−2 at the current density of 8 A m−2. It is seen that the discharge cell voltage drops sharply at the second current step, which is mainly attributed to the activation loss of the anode component (En) as shown in Figure 3B. This large activation loss is due to the highly irreversible four-electron oxygen evolution reaction at the nickel electrode, which is the main issue with this acid− base battery. For the activation loss at cathode (Ep in Figure 3B), the loss is smaller as Pt/C serves well as an electrocatalyst for the oxygen reduction reaction. For Vm in Figure 3B, the delta change in voltage between each current step is quite small. This means that the bipolar membrane performs well as an ionic conductor and barrier at these current densities. The electrochemical cell is mainly ohmically controlled, meaning that the V−I curve becomes linear.

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REFERENCES

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CONCLUSIONS An acid−base battery with oxygen electrodes is presented as an attractive laboratory demonstration to give students practical experience in electrochemistry, membrane science, and energy. Various clean energy technologies and environmental issues should be introduced to students before this demonstration, especially fuel cell and flow battery systems. A more thorough understanding can be acquired by performing additional experiments involving both acidic and alkaline H2/air fuel cells. The present demonstration is shown with static electrolytes. Extension to a flowing electrolyte setup will be closer to a practical application as described in ref 10 and help students relate to redox flow batteries using neutralization energy. While waste has been acknowledged as one of the promising energy sources in our future, this unique demonstration can intrigue students’ passion for science and stimulate them to explore pertinent energy solutions using waste as a source of electrochemical fuel. E

DOI: 10.1021/acs.jchemed.8b00901 J. Chem. Educ. XXXX, XXX, XXX−XXX

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(13) Weng, G. M.; Li, C.-Y. V.; Chan, K. Y. Hydrogen Battery Using Neutralization Energy. Nano Energy 2018, 53, 240−244. (14) Lyons, M. E. G.; Brandon, M. P. The Oxygen Evolution Reaction on Passive Oxide Covered Transition Metal Electrodes in Aqueous Alkaline Solution. Part 1-Nickel. Int. J. Electrochem. Sci. 2008, 3, 1386−1424.

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DOI: 10.1021/acs.jchemed.8b00901 J. Chem. Educ. XXXX, XXX, XXX−XXX