Chapter 2. Experimental techniques
2.3 Full-cell measurements
The working electrodes for full-cell measurements, such as metal-air batteries, metal-CO2 cells, and water electrolysis, can be prepared by depositing the catalysts onto the gas diffusion layers, such as, nickel foam, carbon paper, carbon felt, and titanium mesh. Catalyst inks can be prepared by
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dispersing catalysts in binder solutions made by dissolving organic binders such as Nafion, PVdF, PTFE into organic solutions such as ethanol, isopropyl alcohol, DMF, and NMP. In this work, the binder solution was prepared by mixing ethanol, isopropyl alcohol, and 5 wt.% Nafion solution (Sigma-Aldrich) with a volumetric ratio of 45:45:10. Then, the working electrode can be prepared by drop-coating, brushing, air-brushing, or electrospray processes depending on the experimental conditions. For the full tests, the appropriate current collector should be used, which does not participate in electrochemical reactions and does not self-react to electrolyte solutions. Thus, the use of Pt wire as a current collector should be refrained due to its high electrochemical activities.
Normally, the use of Ag wire is recommended instead of Pt wire.
2.3.2 Hybrid Li-air cells
A lithium foil with a thickness of 0.2 mm was obtained from Honjo metal, and disks with a diameter of 1 cm were cut for use as the anode. 1 M lithium hexafluorophosphate (LiPF6,Sigma- Aldrich Co.) in tetraethylene glycol dimethyl ether (TEGDME, Sigma-Aldrich Co.) was used as an organic liquid electrolyte, and 0.1 M lithium hydroxide (LiOH, Sigma-Aldrich Co.) in pure water was used as the aqueous liquid electrolyte. The anode and cathode were separated by a Li1+x+yTi2-xAlxP3- ySiyO12 (0.15 mm thickness, OHARA Inc., Japan) solid Li-ion conducting ceramic glass. This solid electrolyte glass was first placed on the top of the anode assembly and sealed by epoxy. Then the sealed anode assembly was placed in an argon filled glove box where the water and oxygen concentrations were kept to less than 1 ppm. The prepared lithium metal foil disk was loaded onto the stainless-steel current collector and the organic electrolyte were filled. After assembling the anode part with proper sealing, the assemblage was moved out of the glove box. The catalyst spray-coated gas diffusion layer was placed on top of the solid electrolyte and the aqueous electrolyte was filled between them. Nickel metal mesh was used as current collector onto the gas diffusion layer and electrochemical measurements were conducted at ambient air condition.
2.3.3 Seawater batteries
A cubic shaped sodium metal was obtained from Sigma-Aldrich, and disks with a diameter of 1.6 cm were cut for use as the anode. 1 M NaCF3SO3 (Sigma-Aldrich) in tetraethylene glycol dimethyl ether (TEGDME, Sigma-Aldrich) was used as an organic liquid electrolyte, and seawater taken from Ulsan was used as the aqueous liquid electrolyte. The anode and cathode were separated by Na super ionic conductor (NASICON) ceramic electrolytes (Na1+xZr2SixP3−xO12, x = 2) fabricated according to the procedure described elsewhere. Na coin cell assembly process was performed in a glovebox under a high-purity Ar atmosphere where O2 and H2O concentrations were kept less than 1 ppm. The catalyst drop-coated gas diffusion layer was placed in chamber of the test kit and the
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aqueous electrolyte of seawater was filled. A titanium wire was used as a current collector. Seawater battery testing kit was purchased from 4TOONE. All electrochemical measurements were conducted at ambient air condition.
2.3.4 Water electrolysis
The overall water electrolysis tests were proceeded in three-electrode configuration using Ag/AgCl reference electrode. The cathode and anode were prepared by electro-spraying the prepared catalyst ink onto a Ni mesh current collector with a catalyst loading density of 1 mg cm-2. The measurements were proceeded in deaerated 1 M KOH aqueous solution. The current density was normalized with the geometric area of the catalyst. All electrochemical tests were conducted at ambient air condition.
2.3.5 Hybrid Na-CO2 cells
The hybrid Na-CO2 system is composed of Na metal / organic electrolyte / solid electrolyte / aqueous electrolyte / cathode. For the organic electrolyte, 1 M Sodium trifluoromethanesulfonate (NaCF3SO3, Sigma-Aldrich Co.) in tetraethylene glycol dimethyl ether (TEGDME, Sigma-Aldrich Co.) was used. And NASICON-type (NASICON: Na super ionic conductor) Na3Zr2Si2PO12 was used as the solid electrolyte. For the aqueous electrolyte, 0.1 M NaOH and seawater were used. The cathode was prepared by drop-coating the catalyst ink onto a gas-diffusion layer (carbon felt, Fuel Cell Store Co.) with a loading density of 2 mg cm-2. The current density was normalized with the loading density of the catalysts. The anode part was assembled in Ar-filled glove box where the water and oxygen concentrations were kept less than 1 ppm. The sodium metal (Sigma-Aldrich Co.) was loaded on the stainless-steel current collector and the organic electrolyte was filled between sodium metal and NASICON. After assembling the anode part with proper sealing, the assemblage was moved out from the glove box. A titanium wire was used as a current collector of the cathode and the aqueous electrolytes were saturated by CO2 for electrochemical measurements in hybrid Na-CO2
system.
2.3.6 Aqueous Zn- or Al-CO2 cells
The zinc- and aluminum-CO2 cells were tested in H-type cell. Zn and Al metal (purity higher than 99.99 %) were purchased at Alfa Aesar Co.. The system is composed of Zn or Al metal / alkaline electrolyte / glass membrane / quasi-neutral electrolyte / cathode. For the alkaline electrolyte, 6 M KOH and 4M NaOH aqueous solution was used for Zn and Al system, respectively. For glass membrane, a porous grade of G4 membrane was used. For the quasi-neutral electrolyte, CO2-saturated 1 M KOH and seawater was used. The cathode was prepared by electro-spraying the catalyst ink onto
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a gas-diffusion layer (Toray carbon paper TGP-H-090, Fuel Cell Store Co.) with a loading density of 2 mg cm-2. And silver wire was used as a current collector. The current density was normalized with the geometric area of the catalysts. The aqueous electrolytes were kept saturated by CO2 for electrochemical measurements.
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