Effect of Carbonate Ion Species on Direct Ammonia Fuel Cell Employing Anion Exchange Membrane

Abstract

The effect of carbonate ion species on the direct ammonia fuel cell (DAFC) employing anion exchange membrane (AEM) was evaluated. The OH-form and HCO 3 -form AEMs were applied as electrolytes for DAFCs. The obtained current density of DAFC with HCO 3 -form AEM was 1/7 of that with OH-form AEM. This is because carbonate ion species in the AEM suppress the ammonia oxidation reaction on Pt/C anode. Under the presence of carbonate ion species in the electrolyte, the ammonium ion and carbamate ion were formed accompanied with a reduction in the ammonia concentration in the vicinity of anode. Recently, ammonia has been attracted much attention as a fuel for fuel cells due to its low production cost, ease in liquefaction at ambient temperature, and high energy density.1-3 Ammonia can be oxidized over platinum electrocatalysts in an alkaline aqueous solution at room temperature. [4][5][6][7][8] The anion exchange membrane (AEM) is an alkaline polymer containing quaternary ammonium or pyridinum function groups as ion exchange sites. In the previous report, we have demonstrated the direct ammonia fuel cell (DAFC) employing the OH-form AEM as an electrolyte and Pt/C as a cathode at 50• C. 9The OCV and performance were strongly dependent on the catalytic activity of anode for ammonia oxidation; e.g., the OCVs of cells with Pt/C and Pt-Ru/C anodes were 0.37 V and 0.54 V, respectively (anode gas; NH 3 humidified at 50 o C, cathode gas; 21% O 2 -N 2 humidified at 50• C). Furthermore, the permeation of ammonia through AEM also affected the OCV and performance. This finding suggested that the permeation of ammonia is inevitable even if the anode with high activity is developed. In the practical operating condition, moreover, the ambient air containing CO 2 will be used as an oxidant. However, CO 2 is the degradation factor for the cell performance, especially anode activity on H 2 oxidation.10 This is because the carbonate ion species (CO 2− 3 , HCO − 3 ) produced in AEM by CO 2 dissolution migrate toward the anode under a current loading condition, resulting in the condensation of these anions in the vicinity of anode.10-14 So, it is of importance to reveal the influence of carbonate ion species on the ammonia oxidation reaction over the anode.In this study, the effect of carbonate ion species on the performance of the direct ammonia fuel cell was evaluated by applying the OH-form and HCO 3 -form AEMs as electrolytes. Furthermore, the electrocatalytic activity of Pt/C anode for ammonia oxidation was studied under the coexistence of carbonate and bicarbonate ions in the electrolyte. ExperimentalFuel cell test.-For the electrocatalysts, Pt/C (Pt 46.4 wt. %, Tanaka Kikinzoku Kogyo) was used as received. The membrane electrode assembly (MEA) was constructed by screen-printing the mixture of Pt/C and anion exchange ionomer (AS-4, Tokuyama Co.) on AEM. The basic properties of AEM (A201, Tokuyama Co.) used in this study and the detailed procedure of screen-printing were described in the previous report. 9 The electrode area was 5.0 cm...

Anion-Exchange Membrane Fuel Cells with Improved CO2 Tolerance: Impact of Chemically Induced Bicarbonate Ion Consumption

“…OCV of ca. 0.45 V for 100% NH 3 (RH100%), see Table S1), 14,17,31 indicating that at this condition, the influence of the NH 3 oxidation reaction at the anode catalyst can be considered to be negligible. A closer look at Figure 1, however, shows that the current density was slightly improved when the small amount (less than 7%) of NH 3 was contained in the fuel.…”

“…It has been reported that the reduction in the activity of HOR in the presence of CO 2 -derived anion species causes serious degradation in the fuel cell performance 30 . 31 To understand the fundamental degradation mechanism, in situ observation of the Pt catalyst surface during the electrochemical reaction was performed by using attenuated total reflection infrared spectroscopy (ATR-IR). Furthermore, the effect of coexistence of NH 3 and HCO 3 − was studied extensively using both in situ ATR-IR and electrochemical methods.…”

Electrocatalytic Activity of Heteroatom-Doped Graphene for Oxidation of Hydroquinones

“…In recent years, developing energy storage and supply systems combined with renewable energies such as solar cells and windpowered electricity have attracted attention to utilize renewable energies effectively, 1 since environmental and energy problems, known as global warming and the depletion of fossil fuels, became critical issues. Direct-type fuel cell (DFC), where alcohols [2][3][4][5][6] and hydrogen-containing small molecules [7][8][9][10][11][12][13][14][15][16][17] are used as fuels, is one of the candidates of environmental friendly and efficient energy supply systems. DFC has similar constitution to the polymer electrolyte fuel cell (PEFC), 18,19 which consists of five components, electrolyte membrane, anode for electro-oxidation of fuels, cathode for oxygen reduction reaction (ORR), anode and cathode bipolar plates.…”

“…As a fuel for DFC, hydrogen carrier molecules hydrogenated by surplus renewable energies have been examined to use in energy storage and supply systems. In previous researches, many molecules such as methylcyclohexane, 17,20,21 ammonia, 7,8 sorbitol, 13 hydroquinone derivatives, [22][23][24][25][26][27][28] were tested as fuels. Among these molecules, methyl cyclohexane and ammonia have been intensively investigated, because they have high energy density and liquid state at room temperature.…”

“…Among these molecules, methyl cyclohexane and ammonia have been intensively investigated, because they have high energy density and liquid state at room temperature. 7,8,17,20,21 However, both molecules are toxic and flammable. On the other hand, sorbitol and hydroquinones are biological materials, and aqueous solutions of these organic molecules as fuels are non-toxic, safe, and easy to handle.…”