ReviewRecent advances in direct formic acid fuel cells (DFAFC)
Introduction
Polymer electrolyte membrane (PEM)-based fuel cells are generally considered as viable candidates to replace batteries in portable power devices. Traditionally, H2-fed PEMFCs and direct methanol fuel cells (DMFCs) are the dominant choices [1], [2], [3], [4], [5], [6], [7], [8], [9]. However, despite many years of intensive research into these technologies, inherent limitations still remain.
The H2-PEMFC is limited by the high cost of miniaturized hydrogen containers, the potential dangers in the transport and use of hydrogen, and its low gas-phase energy density. For DMFCs, liquid methanol has an impressive energy density (approximately 4900 Wh L−1), but its electrocatalytic oxidation rate is very low relative to that of H2. The limited compatibility of methanol with Nafion® membranes allows only low concentrations (generally 1–2 M) to be fed to a DMFC [3], [10], [11]. Exceeding this limit leads to a high rate of fuel crossover, which simultaneously reduces fuel utilization and decreases cell performance. In addition, the inherent toxicity of methanol, particularly in the vapor phase, remains an issue for commercialization of DMFC technology [9]. It is these limitations of hydrogen and methanol that have in recent years increased interest in direct formic acid fuel cells (DFAFCs) [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38].
Formic acid is a liquid at room temperature and dilute formic acid is on the US Food and Drug Administration list of food additives that are generally recognized as safe [20]. Formic acid exhibits a smaller crossover flux through Nafion® than methanol [16], [19], allowing the use of highly concentrated fuel solutions and thinner membranes in DFAFCs. DFAFCs also have a higher electromotive force (EMF), as calculated from the Gibbs free energy, than either hydrogen or direct methanol fuel cells [9].
In only a few years of research, DFAFC technology has shown electrocatalytic oxidation activity far superior to DMFCs and in some cases performances approaching those of H2-PEM fuel cells [22]. The major disadvantage of formic acid as a fuel is that its volumetric energy density is only 2104 Wh L−1, considerably lower than that of neat methanol. However, this disadvantage can be compensated for by using a high concentration of formic acid. Thus, for many systems, especially smaller power systems, the advantages of DFAFC can outweigh those of its primary direct-liquid fuel cell contender, the DMFC.
In this review, recent advances in DFAFC research are reviewed, with a focus on progresses that has been made on catalysts for formic acid electro-oxidation. The fundamental DFAFC chemistry, formic acid crossover through Nafion® membranes, and DFAFC configuration development are also discussed.
Section snippets
Fundamentals of DFAFC chemistry and the mechanism of electro-oxidation of formic acid
As with all polymer electrolyte membrane-based fuel cells, the direct formic acid fuel cell also uses an air cathode. Oxygen reduction, through a 4-electron reaction at the cathode, is usually facilitated by a platinum based catalyst. At the anode, direct oxidation of formic acid releases two electrons per molecule. The cathode, anode and overall reactions of a direct formic acid fuel cell are described as:Anode: HCOOH → CO2 + 2H+ + 2e−, E0 ∼ −0.25 V (vs. SHE)Cathode: 1/2O2 + 2H+ + 2e− → H2O, E0 = 1.23 V (vs. SHE)
Unsupported Pt-based catalysts
In the early stages of DFAFC development, Pt-based catalysts were employed in the anode layer. In the first report of a DFAFC [12], Weber et al. found that formic acid was electrochemically more active than methanol on both Pt-black and Pt/Ru catalysts. In addition, the Pt/Ru catalyst was more active than Pt-black for formic acid oxidation. The results are summarized in Fig. 1 [12]. Despite the significance of these results, there were surprisingly no further reports on DFAFCs until 2002,
Crossover of formic acid through Nafion® membranes
For any type of PEM-based fuel cell, the fuel fed to the negative electrode (anode) can permeate the membrane to the positive electrode (cathode). This phenomenon reduces fuel utilization, results in a detrimental mixed potential, competes for and potentially poisons the cathode catalyst and thereby decreases the efficiency of the oxygen reduction reaction [78], [79]. For liquid fuels it can also cause flooding of the cathode catalyst layer [79].
One of the claimed advantages of formic acid for
DFAFCs designs
Direct formic acid fuel cells are generally considered as attractive alternatives to replace batteries in portable power devices. For this purpose, most DFAFCs are designed as small portable fuel cells. In 2004, Ha et al. [17] successfully demonstrated a 2 cm × 2.4 cm × 1.4 cm passive miniature air breathing DFAFC. This cell operated very well with a wide range of formic acid concentrations and generated a current density of up to 250 mA cm−2 and a maximum power density of 33 mW cm−2. After this first
Conclusions
This paper reviews recent advances in direct formic acid fuel cells, including the fundamental DFAFC chemistry, anodic catalysts for formic acid oxidation, formic acid crossover through Nafion® membranes, and DFAFC configuration development.
The DFAFC has a high theoretical open circuit voltage (OCV = 1.48 V) due to the low standard potential of formic acid. Formic acid oxidation generally follows the so-called ‘dual pathway’ mechanism involving both a dehydrogenation process leading to direct
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