Pt Degradation Mechanism in Concentrated Sulfuric Acid Studied Using Rotating Ring−Disk Electrode and Electrochemical Quartz Crystal Microbalance

Abstract

In this paper, we describe our electrochemical investigation of platinum degradation in concentrated sulfuric acid. The Pt electrode weight is found to decrease by an alternating potential cycle between <−0.5 V vs Ag2SO4 and >+0.5 V vs Ag/Ag2SO4. No marked weight decrease occurs by excluding the former or latter region. Also, electrochemical reactions that participate in Pt dissolution and occur between the two potential regions were studied by a rotating ring−disk electrode (RRDE) method. The obtained voltammogram indicates (i) the electrodeposition of sulfur at <−0.5 V vs Ag/Ag2SO4, (ii) the dissolution of sulfur at (+0.6)−(+0.8) V vs Ag/Ag2SO4, (iii) the formation of a precursor to Pt dissolution at (+0.9)−(+1.2) V vs Ag/Ag2SO4, and (iv) Pt dissolution at +0.5 V vs Ag/Ag2SO4. Then, (v) the dissolved Pt is reduced at −0.2 V vs Ag/Ag2SO4. Further investigation was conducted to elucidate the relationships between each reaction by employing an electrochemical quartz crystal microbalance (EQCM). Consequently, the amount of sulfur dissolution at (+0.6)−(+0.8) V vs Ag/Ag2SO4 in step ii affects the amount of subsequent precursor formed that causes Pt dissolution in step iii. A large amount of precursor formed results in a large amount of Pt dissolving at around +0.5 V vs Ag/Ag2SO4 in step iv. These results strongly indicate that the deposition and dissolution of sulfur affect the amount of precursor formed that causes Pt dissolution. After a 30 h potential sweep, which ensured the corrosion of Pt, no dissolved Pt was found in the electrolytic solution, but Pt deposited on the electrode as metallic Pt in particle form. These phenomena imply that the dissolved Pt in step iv then deposits in the neighborhood of the Pt surface in step v. The dependence of Pt degradation on the electrode potential was clearly explained by the results of RRDE and EQCM measurements.

Potential Dependence of Pt and Co Dissolution from Platinum-Cobalt Alloy PEFC Catalysts Using Time-Resolved Measurements

“…The following list can be expanded to include hydroxy and aquo-hydroxy complexes including four-electron dissolution reactions proposed in the literature. 8,44,45 However, the rate constants for these additional species cannot be derived from the ICP-MS measurements of dissolved Pt alone. …”

“…[5][6][7][8][9][10] For example, earlier aqueous studies have established that a) Pt oxides play a role in Pt dissolution at potentials higher than 0.9 V, 5,7,11,12 b) Pt oxides are passivating but are also susceptible to dissolution at higher potentials, 9,11,13 and c) Pt dissolution can be accelerated by potential cycling to a degree that depends on the potential wave form, including the upper and lower potential limits, scan rate in triangle waves, and hold time in square waves. 8,12,[14][15][16] Utilizing rotating ring disk electrode (RRDE) and channel-flow double-electrode (CFDE) methods, 7,10,11 dissolution of platinum during both anodic (increasing potential) and cathodic (decreasing potential) sweeps has been distinguished, albeit semi-quantitatively. Employing RRDE, Mitsushima et al, 7 observed that at high upper potentials (1.8 V) and using slow cathodic sweep rates, the Pt dissolution rate was enhanced by an order of magnitude as opposed to symmetric waves.…”

Potentiostatic and Potential Cycling Dissolution of Polycrystalline Platinum and Platinum Nano-Particle Fuel Cell Catalysts

“…[1][2][3][4][5][6][7][8][9][10][11][12] Early studies of platinum dissolution in electrochemical systems were motivated by the desire to determine the origins of the evolution of the voltammetric signature of platinum electrodes with potential cycling, 3 termed "electrochemical activation", and of the faceting of polycrystalline platinum with rapid potential cycling and stepping. 13 More recently, studies have emerged related to the potentiostatic and potential cycling dissolution of polycrystalline platinum, 4,[14][15][16][17] single crystal platinum, 18 platinum black, 19 nano-particle films, [20][21][22] and of nano-particle platinum supported on high surface area carbon (Pt/C). 17,[23][24][25][26][27] These studies were motivated by the desire to determine the origins of the observed loss of electrochemically-active surface area (ECA) of phosphoric acid fuel cell and polymer electrolyte fuel cell (PEFC) electrocatalysts, as this is a dominant cause of irreversible loss of Pt/C-based cathode performance.…”

“…Some authors speculate that Pt dissolution primarily occurs during the positive-going potential scan/step and is assisted by the formation of oxide, either directly or as a result of surface roughening incurred during oxide reduction 20,[52][53][54] or to chemical dissolution of Pt oxides. 16,19,26,55 Others attribute Pt loss to Pt metal dissolution with oxide playing a passivating and protective role.…”

Platinum Oxide Growth on Pt/C Fuel Cell Catalysts Using Asymmetric Scan Electrochemical Quartz Crystal Nanogravimetry

“…EQCN studies of Pt-coated quartz crystals have probed oxide formation during cyclic voltammetric scans at rates between 5 and 70 mV s −1 [3,42,43], imposed perturbation profile at a single upper potential limit [3,28], and mass loss over time during potentiostatic holds [30,44]. A limited number of EQCN studies have been done on Pt/C nanoparticles limited to mass loss during potential holds [45,46].…”