Can Silver Be a Reliable Current Collector for Electrochemical Tests?

Abstract

The true functionality of a current collector employed in electrochemical cells is to ensure a low-resistance steady electrons flow between the cell and instrumentation without involving in any local electrochemical reactions of the electrode. In this study, we investigated the effect of curing temperature of a common current collector, silver, on the polarization area specific resistance (ASR) of a cathode. The results explicitly showed that at least one order of magnitude lower ASR for a cathode with Ag cured at 800 • C than that cured at 650 • C of the same cathode configuration. Microscopic analysis of the 800 • C-cured cells revealed a deep penetration and abundant distribution of Ag into the cathode/electrolyte interfacial region. These finely dispersed and highly conductive Ag particles/agglomerates are ORR (oxygen reduction reaction)-active, thus engaging in the local electrochemical reaction and overshadowing the true properties of the cathode under investigation. Based on these results, we call for caution when using Ag as a current collector for electrochemical measurements, particularly at a temperature ≥650 • C. © 2012 The Electrochemical Society. [DOI: 10.1149/2.010301eel] All rights reserved.Manuscript submitted September 11, 2012; revised manuscript received October 23, 2012. Published November 6, 2012 Recent efforts in the development of solid oxide fuel cells have been focusing on exploring new electrode materials and optimizing microstructure.1-4 A general strategy to evaluate the performance of an electrode in a laboratory setting is to study its electrochemical behaviors in symmetrical cell or single cell configuration with either DC conductivity or AC impedance technique. In order to fulfill the electrical connection between the electrode and external instrumentation, a pair of metallic films necessary for collecting current is typically applied to the surfaces of the electrode. A good current collector should act solely as an interfacial electronic conductor to relay electrons from the cell to the measuring instrument; it should not participate in any local electrochemical reactions of the electrodes. 5,6 So far, the most popular current collectors of choice in electrochemical characterizations are found among noble metals such as Ag, Pt, and Au. In particular, Ag is a favored choice for cathode studies below 800• C because of its affordable cost and high electronic conductivity. 7-15On the other hand, silver is also known for its excellent oxygen reduction reaction (ORR) activity. Therefore, it has been intentionally utilized as a performance enhancer for cathodes La(Sr)Fe(Co)O 3 (LSCF), La(Sr)FeO 3 (LSF) and La(Sr)CoO 3 (LSCo) in SOFC research. 8,[16][17][18][19] However, due to its low melting temperature (962 • C) and high vapor pressure, 10 silver has a great propensity to be volatile and mobile at elevated temperatures. 20 If Ag migrates into the cathode microstructure as a result of its high mobility, and gets involved in the local electrochemical reactions, its known excellent ORR ...

Performance improvement and redox cycling of a micro-tubular solid oxide fuel cell with a porous zirconia support

“…It is to be noted that current leads containing cheaper materials such as Ni for the anode and Ag for the cathode may also be used. However, considering the high testing temperatures, at which Ag is not suitable due to its low melting temperature and high vapor pressure [41], and exposure of the anode current leads to the oxidizing environment in the test setup, Pt was selected for both the anode and cathode current leads. H 2 humidified with 3 vol.% H 2 O (20 cm 3 min À1 ) and ambient air (20,000 cm 3 min À1 ) were supplied as the fuel and oxidant gases, respectively.…”

La0.6Sr0.4Co0.2Fe0.8O3 Perovskite: A Stable Anode Catalyst for Direct Methane Solid Oxide Fuel Cells

“…The ohmic resistance in CH 4 is lower than that in H 2 . This could be due to the gradual penetration of Ag, which has been used for current collection, into the cathode microstructure leading to an increase in electronic conductivity [43]. The cross-sectional EDX mapping indicated the presence of Ag across the cathode (result not shown).…”

Molten Carbonates as an Effective Oxygen Reduction Catalyst for 550–650°C Solid Oxide Fuel Cells

“…The frequency was swept from 10 5 to 0.01 Hz with 20 mV of stimulus AC amplitude. In order to avoid the interference from silver paste, 33 the highest temperature of all the tests was limited to 650 • C. The long-term stability of both original and MC-added cathodes was simultaneously evaluated at 600 • C for a total of 1,500 hours with a side-by-side cell arrangement.…”

“…9 One possible source of discrepancy could arise from how the current collector is treated during the electrochemical testing. We have recently argued that silver metal commonly used as a current collector in electrochemical tests become highly mobile and ORRactive at t > 650 • C. 33 The volatile silver species then transport to the porous backbones of the cathode and promote the ORR kinetics as a catalyst. The involvement of silver in ORR thus exaggerates the true performance of the original cathode under evaluation.…”