Hydroxide Ion Conducting Antimony(V)‐Doped Tin Pyrophosphate Electrolyte for Intermediate‐Temperature Alkaline Fuel Cells

Abstract

Alkaline fuel cells have received significant interest in recent years relative to acid fuel cells, because of advantages when operating under alkaline conditions, which include enhancement of the electrode reaction kinetics, especially at the cathode, as the cathode catalyst is not subjected to corrosion.[1] Consequently, non-noble metals or inexpensive metal oxides can be used as catalysts. [2] In addition, high energy density liquids and gases such as ethanol, hydrazine, and ammonia can be adopted as fuels.[3] Anion exchange polymers are widely viewed as promising candidates for electrolyte membranes; however, a major challenge in the development of such polymers is their stability at high pH, [4] because both the main chain and functional groups are easily degraded by hydroxide ion attacks. In addition, the poor chemical stability of current anion exchange polymers means that the operating temperature is limited to 80 8C or less; therefore, this type fuel cell is typically operated between 50 and 60 8C.[5]Operating a fuel cell at elevated temperatures provides the anode catalyst with high tolerance to CO, which is useful for both acid and alkaline fuel cells.[6] Additional benefits include small polarization loss, good drainage at the anode, and effective heat dissipation from the fuel cell system. A few anion-conducting electrolyte materials capable of operating at intermediate temperatures (100-200 8C) have been reported, such as KOH-doped polybenzimidazole (PBI) [6] and hydroxide ion-intercalated Mg-Al layered double hydroxide (LDH). [7] However, the reaction of KOH in the former electrolyte with CO 2 present in the air to form K 2 CO 3 is a possibility, and the hydroxide ion conductivity of the latter (0.03 S cm À1 at 200 8C) is not sufficiently high to achieve satisfactory cell performance. Further increases in chemical stability and conductivity would enhance the position of intermediate-temperature anion exchange membranes as the preferred electrolyte material for practical alkaline fuel cells.In this study, metal pyrophosphates (MP 2 O 7 ) were studied as anion-conducting electrolytes for intermediate temperature applications. The metal pyrophosphate structure can be described as a network of MO 6 octahedra sharing corners with P 2 O 7 units, characterized by the presence of intersecting zigzag tunnels delimited by pentagonal windows.[ .[9]The resultant proton conductivity reaches approximately 0.1 S cm À1 at 200 8C.[10] An opposite effect is expected by the partial substitution of Sn 4+ cations with high-valency cations, which would result in hydroxide ion exchange capability because of charge compensation for the high-valency cations. We demonstrate the hydroxide ion conduction of pentavalent cation-doped SnP 2 O 7 at intermediate temperatures using electrochemical measurements, including complex impedance, gas concentration cells, and H/D isotope replacement methods. Moreover, Sn 0.92 Sb 0.08 P 2 O 7 , which exhibits the highest hydroxide ion conductivity among the tested compounds, is characteri...

Ammonia as a Suitable Fuel for Fuel Cells

“…However, a good inorganic OH − ion conductor, which can be used at high temperature, is envisaged to be developed, which will be an ideal electrolyte for ammonia fuel cells. It has been reported Sb (V) and Mo(VI) doped SnP 2 O 7 exhibit hydroxide conduction at a temperature around 200°C (Hibino et al, 2012;Hibino and Kobayashi, 2013). It should be noted that it is controversial that the ionic conduction of doped SnP 2 O 7 could be related to the residual H 3 PO 4 under the circumstance the chemical compatibility between ammonia and these electrolytes would be a problem.…”

A Direct Urine Fuel Cell Operated at Intermediate Temperatures

“…0.05 S cm ¹1 in the temperature range above 0.01 S cm ¹1 between 100 and 300°C. 5 Sn 0.92 Sb 0.08 P 2 O 7 was prepared in a similar manner to that reported previously. 6 A total of 0.04 g of poly(tetrafluoroethylene) (PTFE) powder was added to 1.00 g of Sn 0.92 Sb 0.08 P 2 O 7 powder, followed by kneading with a mortar and pestle, and then cold-rolling to a thickness of 150¯m using a laboratory rolling mill.…”

“…As a consequence of the reduced P H2O in the fuel, the ohmic resistance of the electrolyte membrane increased because the ionic conductivity of Sn 0.92 Sb 0.08 P 2 O 7 is sensitive to the P H2O in the atmosphere. 5 The decomposition of urea to ammonia and isocyanic acid occurs at moderate temperatures 16 and the hydrolysis of isocyanic acid proceeds at temperatures as low as 150°C:…”

Hydroxide-ion incorporation and conduction mechanisms in tin pyrophosphate – a first-principles study

“…[1][2][3][4][5][6][7] For instance, trivalent In-doped SnP 2 O 7 (Sn 0.90 In 0.10 P 2 O 7 ) is reported to exhibit the highest proton conductivity in this system (0.195 S cm À1 at 250 C), 3 while hexavalent Mo-doped SnP 2 O 7 (Sn 0.85 Mo 0.15 P 2 O 7 ) has the highest hydroxide-ion conductivity (0.123 S cm À1 at 250 C). [1][2][3][4][5][6][7] For instance, trivalent In-doped SnP 2 O 7 (Sn 0.90 In 0.10 P 2 O 7 ) is reported to exhibit the highest proton conductivity in this system (0.195 S cm À1 at 250 C), 3 while hexavalent Mo-doped SnP 2 O 7 (Sn 0.85 Mo 0.15 P 2 O 7 ) has the highest hydroxide-ion conductivity (0.123 S cm À1 at 250 C).…”

“…The anode and cathode reactions in hydroxide-ion-conducting fuel cells are proposed as follows: 1,2 Anode: 2H 2 + 4OH À / 4H 2 O + 4e À (1) The anode and cathode reactions in hydroxide-ion-conducting fuel cells are proposed as follows: 1,2 Anode: 2H 2 + 4OH À / 4H 2 O + 4e À (1)…”