Electrocatalyst control

Zhang J, Vukmirovic MB, Xu Y, Mavrikakis M, Adzic RR. 2005a. Controlling the catalytic activity of platinum-monolayer electrocatalysts for oxygen reduction with different substrates. Angew Chem Int Ed 44 2132-2135. 

The simple porphyrin category includes macrocycles that are accessible synthetically in one or few steps and are often available commercially. In such metallopor-phyrins, one or both axial coordinahon sites of the metal are occupied by ligands whose identity is often unknown and cannot be controlled, which complicates mechanistic interpretation of the electrocatalytic results. Metal complexes of simple porphyrins and porphyrinoids (phthalocyanines, corroles, etc.) have been studied extensively as electrocatalysts for the ORR since the inihal report by Jasinsky on catalysis of O2 reduction in 25% KOH by Co phthalocyanine [Jasinsky, 1964]. Complexes of all hrst-row transition metals and many from the second and third rows have been examined for ORR catalysis. Of aU simple metalloporphyrins, Ir(OEP) (OEP = octaethylporphyrin Fig. 18.9) appears to be the best catalyst, but it has been little studied and its catalytic behavior appears to be quite distinct from that other metaUoporphyrins [CoUman et al., 1994]. Among the first-row transition metals, Fe and Co porphyrins appear to be most active, followed by Mn [Deronzier and Moutet, 2003] and Cr. Because of the importance of hemes in aerobic metabolism, the mechanism of ORR catalysis by Fe porphyrins is probably understood best among all metalloporphyrin catalysts. 

A number of metal porphyrins have been examined as electrocatalysts for H20 reduction to H2. Cobalt complexes of water soluble masri-tetrakis(7V-methylpyridinium-4-yl)porphyrin chloride, meso-tetrakis(4-pyridyl)porphyrin, and mam-tetrakis(A,A,A-trimethylamlinium-4-yl)porphyrin chloride have been shown to catalyze H2 production via controlled potential electrolysis at relatively low overpotential (—0.95 V vs. SCE at Hg pool in 0.1 M in fluoroacetic acid), with nearly 100% current efficiency.12 Since the electrode kinetics appeared to be dominated by porphyrin adsorption at the electrode surface, H2-evolution catalysts have been examined at Co-porphyrin films on electrode surfaces.13,14 These catalytic systems appeared to be limited by slow electron transfer or poor stability.13 However, CoTPP incorporated into a Nafion membrane coated on a Pt electrode shows high activity for H2 production, and the catalysis takes place at the theoretical potential of H+/H2.14... 

Run(Hedta)(NO+)]° and [Fen(Hedta)(NO )] have been shown to be effective electrocatalysts for the reduction of N02 in acidic aqueous media, to yield N20, N2, NH3OH+, or NH4 339,340 An element of selectivity is available by control of pH and applied potential. Steps involved in the typical six-electron reduction of nitrite to ammonia catalyzed by [Run(Hedta)(NO+)]° are summarized in Equations (67)-(69). The mechanisms by which nitrite is reduced appeared to be similar to those identified for Fe-porphyrin331 and Ru or Os-polypyridyl complexes.337... 

Very recently a new kind of electrocatalyst has been propounded using the dinuclear quinone-containing complex of ruthenium (25).492,493 Controlled-potential electrolysis of the complex at 1.70 V vs. Ag AgCl in H20 + CF3CH2OH evolves dioxygen with a current efficiency of 91% (21 turnovers). The turnover number of 02 evolution increases up to 33,500 when the electrolysis is carried out in water (pH 4.0) with an indium-tin oxide(ITO) electrode to which the complex is bound. It has been suggested that the four-electron oxidation of water is achieved by redox reactions of not only the two Run/Ruin couples, but also the two semiquinone/quinone couples of the molecule. 

As discussed earlier, it is generally observed that reductant oxidation occurs under kinetic control at least over the potential range of interest to electroless deposition. This indicates that the kinetics, or more specifically, the equivalent partial current densities for this reaction, should be the same for any catalytically active feature. On the other hand, it is well established that the O2 electroreduction reaction may proceed under conditions of diffusion control at a few hundred millivolts potential cathodic of the EIX value for this reaction even for relatively smooth electrocatalysts. This is particularly true for the classic Pd initiation catalyst used for electroless deposition, and is probably also likely for freshly-electrolessly-deposited catalysts such as Ni-P, Co-P and Cu. Thus, when O2 reduction becomes diffusion controlled at a large feature, i.e., one whose dimensions exceed the O2 diffusion layer thickness, the transport of O2 occurs under planar diffusion conditions (except for feature edges). 

A way to circumvent the first problem is to ensure that all of the active material is present at the electrode surface. That is, employ a chemically modified electrode where a precursor to the active electrocatalyst is incorporated. The field of chemically modified electrodes Q) is approaching a more mature state and there are now numerous methodologies for the incorporation of materials that exhibit electrocatalytic activity. Furthermore, some of these synthetic procedures allow for the precise control of the coverage so that electrodes modified with a few monolayers of redox active material can be reproducibly prepared. Q)... 

A] Qiu, J.-D., et al., Controllable deposition of platinum nanoparticles on graphene as an electrocatalyst for direct methanol fuel cells. The Journal of Physical Chemistry C, 2011. 115(31) p. 15639-15645. 

Figure 6.20. Experimental linear sweep voltammogram of carbon-supported high surface area nanoparticle electrocatalyst in oxygen-saturated perchloric acid electrolyte (room temperature). Solid curve pure Pt dashed curve Pt50Co50 alloy electrocatalyst. Inset a blow up of the kinetically controlled ORR regime. Inset b comparison of the specific (Pt surface area normalized) current density of the Pt and the Pt alloy catalyst for ORR at 0.9 V.

These methods are also selective because of control of the electrochemical spectrum through selection of applied potential range. In addition, selectivity can be improved by modification of the electrode surface with membranes, electrocatalysts, etc. 

The development of new and improved electrocatalysts begins with the discovery of materials displaying improved intrinsic electrochemical activity. Intrinsic activity is best observed and compared in a well-controlled catalyst environment where variables that may disguise the intrinsic activity trends are minimized or absent. Particle size, particle size distribution, surface area, catalyst utilization and the distribution of crystallographic phases are parameters that are typically difficult to control. Vapor deposition of unsupported thin film electrocatalysts eliminates many of these variables. This method provides a controlled synthetic route to smooth, single-phase or multi-phase, ordered or disordered metal alloy phases depending on deposition and processing conditions. 

Evaluating dendrimer templated nanoparticles in the absence of the dendrimer provides opportunities for insights into these new materials. In order to pursue these investigations, it is first necessary to immobilize DENs onto an appropriate substrate and to gently remove the dendrimer shell see Scheme 5. Opportunities for controlling nanoparticle size and composition make DENs potentially important precursors for heterogeneous catalysts and electrocatalysts, and DEN deposition and thermolysis are similarly critically important steps in pursuing these applications [45]. 

The activities of CNTs have been evaluated by Girishkumar et al. [7] using ex situ EIS. Their study was conducted in a three-compartment electrochemical cell using a GDE electrode (a carbon fibre paper coated with SWCNTs and Pt black as an anode or cathode). Electrophoretic deposition was used to deposit both the commercially available carbon black (CB) for comparison and the SWCNT onto the carbon Toray paper. Commercially available Pt black from Johnson Matthey was used as the catalyst. In both cases, the loading of the electrocatalyst (Pt), the carbon support, and the geometric area of the electrode were kept the same. EIS was conducted in a potentiostatic mode at either an open circuit potential or controlled potentials. 

An alternative to using commercially available carbon for electrocatalyst carbon substrates is to build a specific carbon structure having controlled properties. Thus, carbons have been prepared by the controlled pyrolysis of polyacrylonitrile (PAN) and contain surface nitrogen groups that act as peroxide decomposing agents.62... 

Under concentration control, the reversible hydrogen electrode exhibits Nemstian reversibility. This provides for a potential shift of 29.75 mV at room temperature, which translates to a shift of 46.8 mV at 200 °C for each decade of change in hydrogen concentration. Under fuel-cell operating conditions with highly dispersed electrocatalysts, it is possible to approach the kinetic rate determined by the dual-site dissociation of the hydrogen molecule, viz. ... 

Whereas the rate-determining step for hydrogen molecule oxidation now is recognized69,70 to be the dissociative chemisorption of the hydrogen molecule on dual sites at the platinum surface, the rate of this step is so high that in most electrochemical environments platinum electrocatalysts are almost always operating under diffusion control. 

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