Electrochemically active surface area ECSA

The catalyst layers (the cathode catalyst layer in particular) are the powerhouses of the cell. They are responsible for the electrocatalytic conversion of reactant fluxes into separate fluxes of electrons and protons (anode) and the recombination of these species with oxygen to form water (cathode). Catalyst layers include all species and all components that are relevant for fuel cell operation. They constitute the most competitive space in a PEFC. Fuel cell reactions are surface processes. A primary requirement is to provide a large, accessible surface area of the active catalyst, the so-called electrochemically active surface area (ECSA), with a minimal mass of the catalyst loaded into the structure. 

DMFC performance loss due to catalyst degradation has been attributed to several factors a decrement of the electrochemically active surface area (ECSA) of the platinum electrocatalyst supported on a high-surface-area carbon, a loss of cathode activity towards the ORR by surface oxide formation, and ruthenium crossover [83, 85, 116, 117]. 

The electrochemical active surface area (ECSA) reflects the total catalyst surface that has the potential to participate in the fuel cell reaction. It is typically measured by the hydrogen adsorption/desorption peak area or the CO oxidative stripping peak area. A larger ECSA normally gives better fuel cell performance. The ratio of ECSA to the mass of the catalyst is an indication of how effectively the precious metal catalyst is used. The ratio of ECSA to the total geometrical surface area of the catalyst estimated by the particle size is an indication of how effectively the catalyst surface is used. The latter ratio can be used to gauge how well (high ratio) or bad (low ratio) a catalyst layer is made. 

The activity difference between the two electrodes based on can arise from difference either in the catalyst (e.g., different catalysts, or same catalyst but with different crystalline facets, or different particle size and geometry) or in the loading. What represents the intrinsic activity of a catalyst is the j°. With known catalyst loading and the total electrochemical active surface area (ECSA), i2°/ji° can be obtained with respect to specific mass or surface activities of the two catalysts used in the two electrodes. This ratio represents the intrinsic activity difference of the two catalysts. 

Since both the anode and the cathode reactions involve the transport of electrons, protons, and reactants/products, only the catalyst surfaces at the catalyst-ionomer-reactant three-phase regions are electrochemically active. This area is called the electrochemical active surface area (ECSA). The remaining catalyst surfaces that do not meet this three-phase boundary condition will not be able to participate in the electrochemical reaction. An ideal electrode should be able to use all the catalyst surfaces to achieve 100% surface utilization. Measuring the ECSA and comparing it with the total surface area of the catalyst particles based on the particle size and flic catalyst loading reveals the actual utilization of the catalyst surface. 

In general, there are two methods to introduce the PA to the CL. The first one rehes on a highly PA-doped membrane. The PA is transferred between membrane and catalyst layer upon MEA preparation and/or cell assembly, respectively. The second method involves depositing (spraying, painting, etc.) PA directly onto the CL. The PA content in the membrane is one of the decisive factors for the selection of one of the two preparation methods. Overall, the CL needs to provide electron and proton transport pathways, gas accessibility, high catalytic activity towards HOR and ORR, a high electrochemically active surface area (ECSA) and has to withstand the harsh HT-PEMFC environment (acidity close to pH = 0, temperature up to 180 °C, and electrochemical potentials up to 1.5 V). 

The objective of catalyst layer design is twofold from a materials scientist s perspective, the objective is to maximize the electrochemically active surface area (ECSA) per unit volume of the catalytic medium Secsa, by (i) catalyst dispersion in nanoparticle form or as an atomistically thin film and (ii) optimization of access to the catalyst surface for electroactive species consumed in surface reactions. From a fuel cell developers point of view, the objective is to optimize pivotal performance metrics like voltage efficiency, energy density, and power density (or specific power) under given cost constraints and lifetime requirements. These performance objectives are achievable by integration of a highly active and sufficiently stable catalyst into a structurally well-designed layer. 

Clearly the majority of the electro-catalysts explored in Table 16.2 lend themselves to fuel cell applications. A key parameter to the success of the electrocatalytic sensing of ammonia involves the design of the catalyst. That is, a surface which gives rises to a large active surface area which also stabilises active intermediates and is of a composition to induce changes in the activation energy. The performance of the electro-catalyst is characterised by the mass activity (MA activity mass ) which is the current density (at a specific potential) normalised by the mass of the electro-catalyst which is related to the specific electrochemically active area (SSA area mass ) and the specific activity (SA activity area ) which is the current density normahsed by the electrochemically active surface area (ECSA) of the electro-catalyst. As such, the mass activity is the key parameter given by ... 

In fuel cells, the CV technique is widely used to estimate the electrochem-ically active surface area (ECSA) of a catalyst such as Pt. In the potential region between 0.0 and 0.4 V, acidic electrolyte will normally show two pairs of peaks on Pt due to the underpotential hydrogen adsorption and desorption processes, as shown in Figure 4.5 and Reactions 4.1 and 4.2, respectively. 

The research is therefore focused at the cathode. The state-of-the-art catalyst Pt/ C shows only a low specific activity in the order of 0.2 mA cm Pt at 900 mV (IR-free, at 1 bar 80°C) This is compensated by the large Electrochemically Active Surface Area values (ECSA) obtained with these catalysts, which can be in the order of 60-90 m g. The corresponding mass activities, which are the product of the ECSA and the specific activity, are between 0.12 and 0.18 A mg about a factor 3 lower than the target, resulting in a required Pt loading in the order of 0.4-0.6 mg cm Pt [46]. 

The electrochemically active surface area is generally larger in the case of the interface with Aquivion ionomer than in the case of Nafion [26]. The gain in terms of electrochemical active surface area for the cathode in the MEA equipped with Aquivion membrane was close to 30 % compared to Nafion . In fact, the ECSA changed fi om 66 m g for the cathode equipped with Nafion ionomer to 84 m g for that containing Aquivirm ionomer [26]. The increase of electrochemical active surface area has been interpreted in terms of... 

Kinetic Losses on Anode and Cathode The exchange current density, electrochem-ically active surface area (ECSA), and catalyst utilization are desired. 

Electrochemical Active Surface Area The characterization of the ECS A is a measure of the efficiency of the catalyst loading. The ECS A is an extremely useful tool to compare the efficiency of one electrode to another and also to examine the degradation of the electrode over time resulting from a variety of physical and chemical mechanisms. Eor the ECSA measurement on a platinum electrode, the experimental configuration shown in Eigure 9.7 is used and two assumptions are made [13] ... 

Fig. 10.13 (a) Loss of electrochemical surface area (ECSA) of Pt/C (E-TEK), platinum-black (PtB E-TEK), and PtNT catalysts with number of potential cycles in Ar-purged 0.5 M H2SO4 solution at 60°C (0-1.3 V versus RHE, sweep rate 50 mV/s). (b) ORR curves (shown as current-voltage relations) of Pt/C, platinum black (PtB), PtNTs, and PdPtNTs in 02-saturated 0.5 M H2SO4 solution at room temperature (1,600 rpm, sweep rate 5 mV/s). Inset Mass activity (top) and specific activity (bottom) for the four catalysts at 0.85 V (Reproduced from [130]. With permission)... 

The primary optimization parameter of porous electrodes is the ideal electrochem-ically active surface area per unit volume sa- rough approximation, the value ECSA is proportional to the amount of the electrocatalytically active material Pt, in the case of PEFC electrodes. It is inversely proportional to the feature size d, which could represent diameters of catalyst particles, of pores in a porous catalytic medium, or of rod-like structures (nanotubes or nanorods), onto which a thin film of catalyst is deposited. On the other hand, is also roughly proportional to the energy density... 

To interrogate the origin of the activities in ORR of the Pt p electrocatalysts, their mass-averaged electrochemical surface areas (ECSAs) were examined with the hydrogen adsorption peak in their... 

---