Degradation Mechanisms of Carbon Supports under Hydrogen Passivation Startup and Shutdown Process for PEFCs

Abstract

The use of a hydrogen purge for startup and shutdown (H 2 -SU/SD) process of polymer electrolyte fuel cells has been proposed, which suppresses the generation of internal current during the SU/SD, process so-called "reverse current", and the severe carbon oxidation reaction (COR) in the cathode. However it was found that the COR was still caused during this H2-SU/SD process, even though it was less severe than that during the usual SU/SD process, i.e., the anode gas was successively cycled between air and H 2 . In order to clarify the mechanisms of the COR, we investigated (1) the effect of the presence of Pt catalyst, (2) the timing, and (3) the effect of Pt oxidation state. These results indicated that the COR was accelerated by the Pt catalyst in the cathode and was decelerated with increasing cathode potential during the H 2 -SU/SD process. We propose that the COR is caused by a shortage of protons associated with both the reduction of the Pt oxide and the oxygen reduction reaction at the reduced Pt. Polymer electrolyte fuel cells (PEFCs) convert chemical energy directly to electrical energy with low emissions and high energy efficiency and have shown promise to be an eco-friendly power source for fuel cell vehicles (FCVs) and residential co-generation systems. [1][2][3] Nevertheless, PEFCs still have several problems to be solved, such as limited lifetime and reliability and high cost, before large-scale commercialization can be realized. [4][5][6] The minimization of the PEFC cost can be achieved by improving the specific mass activity (MA) of the catalyst for the oxygen reduction reaction (ORR) at the cathode. The conventional cathode catalysts have consisted of Pt nanoparticles dispersed on high surface area carbon black supports (Pt/CB) to maximize the electrochemically active surface area (ECSA) for the ORR. 4 However, as is widely known, Pt/CB cathode catalysts are degraded under PEFC operating conditions such as load change cycles and startup/shutdown (SU/SD) cycles due to a combination of processes, which include ECSA loss due to the agglomeration or dissolution of Pt nanoparticles, [6][7][8][9][10][11][12] and the corrosion of the CB support material. 7,[11][12][13][14][15] During the SU/SD cycles, air and H 2 coexist transiently in the anode until the replacement of the former with the latter (or vice versa) is completed. Reiser et al. showed that this situation causes the cathode potential to climb to more than 1.5 V due to the so-called "reverse current mechanism", which significantly accelerates the degradation of the catalyst due to the oxidation of the CB and the agglomeration or dissolution of the Pt nanoparticles. 12 The latest papers on this topic have been reviewed. 13 Several approaches have been taken to both understand and mitigate the decrease of PEFC performance during operation.13 One approach to mitigate the decrease of the cell performance is to use transition metal oxide support materials, for example, titanium-based oxides [16][17][18][19][20] and tin-based oxides. [21][22][23][...

Engineering Catalyst Layers for Next‐Generation Polymer Electrolyte Fuel Cells: A Review of Design, Materials, and Methods

“…To emulate carbon corrosion, which happens especially during cell start‐up/shutdown where the cell is subject to high voltages caused by a hydrogen/air wavefront, [ 181,182 ] cells are cycled at higher voltages between 1 and 1.5 V for around 5000 cycles. [ 174 ] The result of extended cycling at extreme voltages includes the loss of oxygen‐containing functional groups on the surface of supports, [ 159 ] formation of gases like CO and CO 2 , [ 183 ] as well as increased cracking of the CL [ 184,185 ] and overall loss of carbon material.…”

Electrocatalysts for PEM Fuel Cells

“…During this transient phenomenon, the local cathode potential downstream (i.e., the cathode potential vs the reference electrode downstream) became higher, and the measured highest value exceeded 1.5 V, as shown in Figure 9a. As strategies in the balance of plant system during start-up and shut-down, controlling the cathode atmosphere to reduce the amount of oxygen in the cathode, [80,81] and controlling the cell voltage to prevent the cathode potential from rising [82] were proposed. As the fuel gas is introduced into the anode flow field, the local anode potential upstream decreases from ≈1 V to ≈0 V. This induces an increase in the cell voltage from ≈0 V to 1 V. Since the cell voltage is shared throughout the cell, the potential difference between the anode and cathode downstream is forced to increase to ≈1 V. Until the fuel gas appears, both the anode and cathode sides contain oxygen downstream, and the applied voltage drives this area to act as an oxygen pump, that is, oxygen reduction at the anode side and oxygen evolution at the cathode side.…”

“…Based on the insight of this mechanism, several strategies that involved the operational technique and the material design were proposed to mitigate cathode degradation. As strategies in the balance of plant system during start‐up and shut‐down, controlling the cathode atmosphere to reduce the amount of oxygen in the cathode, and controlling the cell voltage to prevent the cathode potential from rising were proposed. As strategies that involve the material design, the addition of an oxygen evolution reaction (OER) catalyst, and the use of a corrosion‐resistant catalyst support at the cathode side were proposed.…”

Hydrogen Oxidation Artifact During Platinum Oxide Reduction in Cyclic Voltammetry Analysis of Low-Loaded PEMFC Electrodes

“…Consequently, some carbon oxide species could form and be available for oxidation during the reduction of PtOx causing the artifact peak. Another alternative explanation could be linked to the study by Yamashita et al [41]. The authors investigated carbon corrosion mechanisms during shut-down and start-up procedures while varying the atmosphere in the WE and formulated three different carbon corrosion mechanisms correlated to PtOx reduction.…”