Oxygen reduction reaction mass activity

Electrocatalysts One of the positive features of the supported electrocatalyst is that stable particle sizes in PAFCs and PEMFCs of the order of 2-3 nm can be achieved. These particles are in contact with the electrolyte, and since mass transport of the reactants occurs by spherical diffusion of low concentrations of the fuel-cell reactants (hydrogen and oxygen) through the electrolyte to the ultrafine electrocatalyst particles, the problems connected with diffusional limiting currents are minimized. There has to be good contact between the electrocatalyst particles and the carbon support to minimize ohmic losses and between the supported electrocatalysts and the electrolyte for the proton transport to the electrocatalyst particles and for the subsequent oxygen reduction reaction. This electrolyte network, in contact with the supported electrocatalyst in the active layer of the electrodes, has to be continuous up to the interface of the active layer with the electrolyte layer to minimize ohmic losses. 

Abstract In this chapter, we review recent works of dealloyed Pt core-shell catalysts, which are synthesized by selective removal of transition metals from a transition-metal-rich Pt alloys (e.g., PtMs). The resulted dealloyed Pt catalysts represent very active materials for the oxygen reduction reaction (ORR) catalysis in terms of noble-metal-mass-normalized activity as well as their intrinsic area-specific activity. The mechanistic origin of the catalytic activity enhancement and the stability of dealloyed Pt catalysts are also discussed. 

Table 6.1. Pt/VC mass activities at 0.9 V as a function of 0s,i (sulfur coverage) [41]. (Reproduced by permission of ECS—The Electrochemical Society, from Garsany Y, Baturina OA, Swider-Lyons KE. Impact of sulfur dioxide on the oxygen reduction reaction at IhA ulcan carbon electrocatalysts.)...

The basic function of a CL is to provide a place for electrochemical reactions. The main processes occurring in a CL include mass transport of the reactants, interfacial reactions of the reactants at the electrochemically active sites, proton transport in the electrolyte phase, and electron conduction in the electronic phase. The hydrogen oxidation reaction (HOR) takes place in the anode CL and the oxygen reduction reaction (ORR) occurs in the cathode CL. Both anodic and... 

Fig. 7 CeU G2 cathode catalyst-layer and GDL mass-transport overpotentials, oxygen reduction reaction (ORR) overpotential, and iR-corrected cell voltage as a function of current density at different durability testing times (1,054 total hours at constant 0.90 A cm Gore PRIMEA series 5620 MEA (50.0-cm active area, 0.40/0.60mg Pt cm, 35- am Gore SELECT membrane) SGL SIGRACET GDL 24BC (5wt%/23wt% PTFE substrate/MPL) at both sides. See the Appendix for durability testing conditions. RH Relative humidity, A Anode, C Cathode...

In alkaline solutions, electrochemistry of oxygen is not as greatly affected by purification of the solutions as in acidic media [45]. Since the earlier RRDE experiments, it has been shown that H2O2 is the main product of the reduction in the activation domain but is not detected anymore at lower potentials, when the electrode reaction is limited by mass transport [59, 64, 65]. The existence of two potential regions for ORR in alkaline media is also illustrated by the Tafel plots, as those reported in Fig. 6 [66]. The existence of two... 

Since hypoxic cells located remotely from the vascular supply of a tumor mass may have a greater capacity for reductive reactions than their normal, well-oxygenated counterparts, hypoxia could provide an opportunity for the design of selective cancer chemotherapeutic agents that could be reductively activated in these hypoxic cells.24 Several classes of agents are presently known which exhibit preferential cytotoxicity toward hypoxic cells through reductive activation. They... 

These examples are based on both electrodes operating in the activation polarization regime, in which the logarithm of the current is proportional to the overpotential. However, there are situations - particularly at low concentrations - in which the electrochemical reaction is limited by mass transport to the electrode surface. This is referred to as concentration polarization, and is illustrated in Figure 13.2d. In this case, above a critical overpotential the current becomes constant, which appears as a vertical line in the plot. A new mixed potential is established at the intersection of this vertical line and the cathode polarization for the oxygen reduction. This potential depends on the gas concentration, and thus can be used for the chemical sensor signal. 

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