A Carbon Dioxide Tolerant Aqueous‐Electrolyte‐Free Anion‐Exchange Membrane Alkaline Fuel Cell

Abstract

Despite over a century of study and decades of intensive research, few fuel cell products have appeared on the market. [1] The major inhibitor to mass commercialisation is cost. [2] H 2 /air alkaline fuel cells (AFCs) containing KOH(aq) electrolyte promise the lowest cost devices, [3,4] with the ability to use non-Pt catalysts. The fundamental problem with AFCs is that the KOH(aq) electrolyte reacts with CO 2 (cathode air supply) to form carbonate species, which lowers cell performance and lifetime (formation of carbonate precipitates in electrodes and reduction of OH -concentration in electrolyte). [4,5] However, the carbonate content of a aqueous-electrolyte-free (metal-cation-free) alkaline anion-exchange membrane (AAEM), that was pre-exchanged to the CO 3 2-form, decreased when operated in H 2 /air and methanol/air fuel cells. This remarkable result is contrary to prior wisdom; AAEMs inherently prevent carbonate performance losses when incorporated into AFCs. This experiment was made possible only by the recent breakthrough development of an alkaline interface ionomer, [6] which allows fabrication of membrane electrode assemblies that do not require incorporation of metal hydroxides species to perform well. [7] The widely perceived advantages of alkaline membrane direct alcohol fuel cells (AMDAFC) are the potential use of relatively inexpensive and abundant non-Pt electrocatalysts (as with the H 2 /air AFCs), [8][9][10] reduced alcohol crossover, [11,12] and enhanced electro-oxidation of high energy density alcoholic fuels. [13] However, metal hydroxides have traditionally been used as an additive, either in the electrode architectures [7] or in the aqueous alcohol supplies [11] due to the previous lack of an alkaline analogue to the perfluorosulfonic acid dispersions, [14] required for high-performance membrane electrode assemblies (MEAs) for proton-exchange membrane fuel cells (PEMFCs). Concerns persist about the effect of carbonate formation with such AMDAFCs. [15] The hypothesis that was tested in this study is that the tendency to form CO 3 2-can be reduced on the elimination of M n+ from AAEM-based solid alkaline fuel cells (SAFCs); precipitates of metal carbonates [4] cannot then form (the counter -N + R 3 cations are covalently bound to the polymer electrolyte analogous to the -SO 3 -counter anions in PEMs). The data presented in Table 1 compare the ex situ properties of the AAEM in CO 3 2-and OH -forms; the properties do not vary substantially. Importantly, the through-membrane conductivities at 30°C in a static relative humidity (RH) = 100% atmosphere were similar. The ionic performance of AAEMs would not be seriously compromised even on substantial formation of carbonate.Peak power densities of 37.9 ± 1.4 mW cm -2 were obtained in H 2 /air fuel cell tests with the AAEM MEAs in CO 3 2-form ( Figure 1); this was higher than the 32.9 ± 1.6 mW cm -2 obtained with the OH -benchmarks. The respective in situ cell resistances of 1.5 ± 0.2 Ω cm 2 and 1.7 ± 0.2 Ω cm 2 showed that there was only a small in...

Highly Stable, Anion Conductive, Comb-Shaped Copolymers for Alkaline Fuel Cells

“…Indeed, it has even been shown that power densities can be as good or better when CO 2 is introduced into an AFC due to improved electrode kinetics in the presence of carbonate. 45,46 The correlation between HCO 3 − conductivity and temperature for comb-shaped CyDx and BTMAx membranes is shown in Figure 7b. The conductivity steadily increased with temperature due to the enhanced water motion, and exhibited values of 8.7− 17.2 mS cm −1 for CyDx membrane at 80°C which is comparable to the previous reported values for AEMs having high water uptake (WU > 50 wt %).…”

Development of imidazolium-type alkaline anion exchange membranes for fuel cell application

“…The membranes were then converted into the SO 4 2− form by immersion in aqueous Na 2 SO 4 (0.5 mol dm -3 ) solution for 8 h. The Cl − ions, released from the membranes, were then titrated with aqueous AgNO 3 (0.1 mol dm -3 ) aqueous solution using K 2 CrO 4 as a colorimetric indicator. The IECs (expressed as mmol g -1 ), were calculated from the amount of AgNO 3 (1:1 Ag + :Cl -reaction) consumed in the titration and the mass of the dry membrane. Hydroxide Conductivity The measurement of the ionic conductivity of an AAEM was conducted in a similar manner to the measurement of the proton conductivities of PEMs using the four-point probe technique.…”

“…[1][2][3][4][5][6] For example, the fuel oxidation and oxygen reduction reactions exhibit inherently faster kinetics in the high pH environments in APEFCs, thus allowing a wide range of non-precious-metal catalysts (e.g., nickel and manganese oxides) to be employed. Moreover, the direction of alkaline anion (e.g.…”

“…3,[14][15][16][17] However, in order to introduce such QA groups, extremely volatile reagents, such as trimethylamine (TMA), are employed, resulting in an environmentally unfriendly synthetic process. The low alkali thermochemical stabilities of QA-type AAEMs limit their long-term durability of APEFCs.…”

Alkali resistant and conductive guanidinium-based anion-exchange membranes for alkaline polymer electrolyte fuel cells

“…Over the last 10 years, alkaline anion-exchange membranes (AAEM), the core component of APEFCs, have been extensively studied and considerable advances have been made [1,8]. Among the variety of methods used for AAEM synthesis, an important and common method is to modify an existing base polymer, such as poly(phthalazinon ether sulfone ketone) (PPESK) [9], polysulfone (PS) [10] and poly(ether ketone) (PEK) [11].…”