Technical CommunicationEffect of deactivation and reactivation of palladium anode catalyst on performance of direct formic acid fuel cell (DFAFC)
Highlights
► We investigate effect of Pd deactivation and reactivation on DFAFC performance. ► Oxidation of Pd into Pd–OH mainly results in Pd deactivation and DFADC performance degradation. ► DFADC performance is recovered by activity revival of Pd at high potentials. ► Pd–OH is transformed into Pd–O followed by Pd for activity revival. ► ATR-FTIR and XPS measurements are used to verify transformations of Pd redox reactions.
Introduction
Recently, there has been significant demand for miniaturized fuel cell systems as battery replacements for consumer and military electronic devices. Direct formic acid fuel cells (DFAFCs), which use liquid formic acid to generate power, have great potential for use as fuel cell systems [1], [2], [3]. Indeed, formic acid has better edges than other fuel candidates such as methanol and hydrogen. For example, the electrochemical oxidation of formic acid is faster than that of methanol due to a simple molecular structure of the formic acid with high open circuit voltage of formic acid. Also, the fuel crossover rate of formic acid is lower than that of methanol, making it possible to use highly concentrated formic acid solutions and thinner membranes. The toxicity of formic acid is also very low [2], [3], [4], [5], [6], [7]. Compared to polymer electrolyte membrane fuel cells (PEMFCs), DFAFC can avoid potential danger of explosive hydrogen and save cost for additional expenditure such as hydrogen gas container. It has also higher gas-phase energy density than PEMFCs [8]. DFAFCs using formic acid fuel are therefore being considered more proper baseline fuel cell system than direct methanol fuel cells (DMFCs) and PEMFCs.
Although platinum (Pt) has been used as a main catalyst for DFAFCs [4], [6], [8], [9], to date the cell performances of DFAFCs incorporating Pt do not satisfy market needs due to Pt poisoning by carbon monoxide (CO). The subsequent retardation of electrochemical reactions caused by the Pt poisoning of formic acid results in low efficiency and power density. Reaction (1) shows the main electrochemical reaction of formic acid under Pt.
Here, the CO intermediate is adsorbed strongly on Pt, obstructing its active sites and inhibiting further reactions, thereby limiting the activity of Pt [4], [5], [6].
To address this problem, it is necessary to develop catalysts capable of promoting the electrochemical reactions of formic acid, thereby enhancing fuel cell efficiency and power density. To this end, palladium (Pd) has been regarded as alternative catalyst [3], [5], [10]. Importantly, DFAFCs using Pd exhibited superior cell performance to their Pt counterparts [2], [11]. The enhancement in performance of Pd-containing DFAFCs is due to the role Pd plays in expediting electrochemical reactions of formic acid. Specifically, formic acid is electrooxidated to produce carbon dioxide (CO2) through dehydrogenation under Pd (see reaction (2)).
Since there is little chance of Pd deactivation by CO absorption, Pd typically exhibit better activity than Pt in DFAFCs [4], [10], [11], [12], [13], [14]. Notwithstanding, it remains unclear what is really happening in between Pd and formic acid during DFAFC operation. Further, although cell performance of DFAFCs incorporating Pd is better than that of Pt, the former is prone to steady degradation in multiple-cycle polarization curve [11]. These results imply that the electrochemical reactions related to the Pd in DFAFCs may be controlled by other reactions, and not just the dehydrogenation of reaction (2).
However, in spite of a critical role of the Pd on cell performance of DFAFCs, there has been little known about how anodic Pd catalyst affects the redox reaction of formic acid during operation of DFAFCs, although some research groups have been suggested the reaction models of formic acid occurring under the existence of Pd catalyst. Amid the reports, Zhou et al. [15] and Pan et al. [16] proposed that initial decomposition of formic acid (HCOOH) into formate (COOH) ion that was adsorbed on the surface of Pd. The COOH ion was then decomposed into CO2. Miyake et al. [17] reported that HCOOH and water molecule (H2O) included in the formic acid solution were adsorbed as COOH ion and hydroxyl (OH) ion on the surface of Pd, respectively. The Pd-adsorbed COOH and OH then reacted to produce CO2 and H2O. Zhou et al. [18] also published that HCOOH and H2O were adsorbed as CO and OH on the surface of Pd, respectively, producing CO2 as a result of subsequent reaction between the CO and OH.
In the present study, we reveal a new Pd electrochemical reaction route that is different from the aforementioned reactions. The role of the proposed reactions was evaluated using surface analytical techniques such as attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) as well as electrochemical characterization like cyclic voltammetry (CV).
Section snippets
Experimental
For cell testing, membrane electrode assemblies (MEAs) with active area of 9.0 cm2 were prepared by catalyst-coated membrane (CCM) spraying. Pd black catalyst (Sigma–Aldrich, High Surface Area) and Pt black catalyst (Johnson Matthey, HiSPEC™ 1000) were used to prepare anode and cathode electrodes, respectively. Inks were made by dispersing the catalyst powders into a 5% recast Nafion solution (Solution Technology, Inc., 1100EW) in Millipore-purified water. The inks were then airsprayed onto
Results and discussion
Fig. 1 shows the cell polarization curves of the DFAFC. To investigate how the electrochemical reactions of anode Pd catalyst affected cell performance of DFAFC, the polarization curve were measured four times in a consecutive manner. As the multiple-cycle of polarization curve was performed, the cell performance of DFAFCs steadily degraded [19].
Although the dehydrogenation (reaction (2)) that occurs between Pd and formic acid is well known, its mechanism does not explain why cell performance
Conclusions
In this study, we investigate degradation and recovery in cell performance of DFAFCs when Pd and Pt are used as anode and cathode catalysts, respectively. As multiple-cycle DFAFC polarization curves are performed, the DFAFC cell performance is steadily degraded. That is attributed to the Pd deactivation by oxidation of Pd into Pd–OH that takes place in between 0.1 and 0.55 V during operation of the polarization curve. In CV experiments performed to further evaluate the effect of DFAFC cell
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