Cynthia A. Rice

A NANOPARTICLE CATALYST WITH SUPERIOR ACTIVITY FOR ELECTROOXIDATION OF FORMIC ACID

This paper reports the synthesis and characterization of a series of Pt-based nanoparticle catalysts with high activity for formic acid electrooxidation. The catalysts were prepared using spontaneous deposition to decorate platinum nanoparticles with controlled amounts of palladium and palladium/ruthenium. Among all the catalysts investigated, the Pt/Pd catalyst shows the best performance; the steady-state formic acid oxidation current is ca. at 0.27 V vs. RHE. This current is two orders of magnitude higher than that obtained from pure platinum, and the catalyst is ideally suited to be an anode in the direct oxidation formic acid fuel cell. The enhancement in formic acid oxidation by the admetal addition does not correlate with the threshold for CO oxidative stripping (the CO tolerance). The Pt/Pd catalyst requires the highest potential to remove the CO, yet it is the most active. We suggest, therefore, that within the dual path mechanism of formic acid oxidation, the direct CO2 formation channel on Pt/Pd is much less affected by the CO chemisorption than on Pt, or on the Pt/Pd/Ru catalyst, also studied in this report.

UHV and electrochemical studies of CO and methanol adsorbed at platinum/ruthenium surfaces, and reference to fuel cell catalysis

This paper reviews surface chemistry of carbon monoxide and methanol in ultra high vacuum (UHV) and in the electrochemical environment on clean and Ru modified Pt single crystal surfaces, and on Pt and Pt/Ru nanoparticles. The results show that CO behaves very similarly in UHV and in the electrochemical environment. Cyclic voltammetry (CV), temperature programmed desorption (TPD) and radioactive labeling all show similar behavior in terms of numbers of peaks, peak splitting etc. Both UHV and CV measurements show that there is about a 200-meV change in the potential for CO removal in the presence of ruthenium. Earlier 13C EC-NMR data indicated a 30% reduction in the Ef-LDOS of CO bound to Ru islands deposited on platinum, and 15% of CO bound to Pt sites, and TPD and CV also show that the binding of CO is modified. The present data confirm that Pt atoms away from Ru are only weakly affected, and the overall CO binding energy modification is quite moderate. We conclude that the changes in the CO binding energy only play a small role in enhancing methanol electrooxidation rates. Instead, the main effect of the ruthenium is to activate water to form OH. Quantitative estimates of the reduction in CO desorption barrier indicate that the effect of bifunctional mechanism is about four times larger than that of ligand effect. In contrast to the results for CO, methanol behaves quite differently in UHV and in an electrochemical environment. Pt(111) is unreactive at room temperature in UHV, while Pt(110) is quite reactive. Initially, clean Pt(111) is less reactive than clean Pt(110) even in the electrochemical environment. However, Pt(110) is quickly poisoned in the electrochemical environment, so at steady state, Pt(111) is more reactive than Pt(110). Another issue is that the mechanism of methanol decomposition is quite different in UHV and in the electrochemical environment. There are three pathways in UHV, a simple decomposition via a methoxonium (CH3O(ad)) intermediate, an SN1 pathway via a methoxonium cation ([CH3OH2]+), and an SN2 pathway via a methoxonium intermediate. So far, none of these pathways have been observed in an electrochemical environment. Instead, the decomposition goes mainly through a hydroxymethyl (CH2OH(ad)) intermediate. These results show that there are both similarities and differences in the behavior of simple molecules in UHV and in the electrochemical environment.

Kinetic Study of Electro-oxidation of Formic Acid on Spontaneously-Deposited Pt/Pd Nanoparticles CO Tolerant Fuel Cell Chemistry

Pt/Pd catalysts have recently been found to have exceptional properties in formic acid fuel cells. In this paper, the kinetics of formic acid electro-oxidation on spontaneously-deposited Pt/Pd nanoparticles were measured to provide a basis for fuel cell design. The results confirmed previous findings that palladium covered platinum catalysts showed exceptional activity for formic acid electro-oxidation. The Pt/Pd catalysts were a factor of thirty more active than Pt under the conditions examined. The catalysts also continued to function as CO built up on the catalyst surface and showed reasonable activity. Thus, the catalysts were CO tolerant. In addition, the results showed that the Tafel slope of formic acid electro-oxidation on Pt/Pd was time dependent. Generally, the Tafel slope decreased as COad was deposited on the surface, suggesting a change in rate determining step with CO coverage. The formic acid concentration effect was also studied in detail to give guidance to the practical fuel cell design. Similar studies on Pt nanoparticles were performed for comparison

Effects of Nafion as a binding agent for unsupported nanoparticle catalysts

We studied the effect of Nafion on the formic acid oxidation reactivity of three unsupported nanoparticle catalysts: Pt black, Pt/Ru black, and Pt/Pd. The catalysts were prepared by physisorption deposition of the nanoparticles on Au both with and without Nafion as an adhesive. The results indicate that Nafion lowers both the apparent surface area and the formic acid oxidation current by blocking surface sites. Surface areas obtained with Nafion binding agent were suppressed by 13, 8, and 22%, while formic acid oxidation currents were attenuated by 40, 13 and 27% for Pt black, Pt/Ru black and Pt/Pd, respectively. When comparing the effects of Nafion on the surface areas, and comparing these results to the respective voltammograms, Nafion displays some selective site blocking effects as well.

Recent Advances in Electrocatalysis of Formic Acid Oxidation

Direct formic acid fuel cells offer an alternative power source for portable power devices. They are currently limited by unsustainable anode catalyst activity, due to accumulation of reaction intermediate surface poisons. Advanced electrocatalysts are sought to exclusively promote the direct dehydrogenation pathway. Combination and structure of bimetallic catalysts have been found to enhance the direct pathway by either an electronic or steric mechanism that promotes formic acid adsorption to the catalyst surface in the CH-down orientation. Catalyst supports have been shown to favorably impact activity through either enhanced dispersion, electronic, or atomic structure effects.

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

The urban automotive drive cycle includes multiple rapid idle-to-peak power transients posing a significant durability challenge for proton exchange membrane fuel cells (PEMFCs) [1–4]. The power transients force the cathode catalyst to undergo potential transitions where Pt surface oxides are quickly formed and removed, resulting in Pt catalytic surface area loss and severe performance decay [5–7]. Considerable progress has been made to reduce performance loss by surface nanoengineering [8–14] and novel catalyst systems’ development [13, 15–23]. Despite all of the achievements in theoretical modeling of fuel cell processes [24, 25], the relative contributions of the known decay mechanisms and their dependence on the transient potential profile and operational conditions remain elusive and therefore limiting the development of mitigation strategies. The dominant decay mechanisms are Pt dissolution with or without redeposition [26–32], migration/ coalescence [33], catalyst nanoparticle size distribution change [34, 35], and detachment from the support due to carbon corrosion [36]. The factors influencing the transient potential profiles include transition rates, the upper and lower potential limits (range between 0.6 and 1.0 V), and time at either idle (∼1.0 V) or peak (∼0.6 V) power [37]. The transition rates allow for the extent to oxide formation and reduction that control surface reorganization [25]. Operational conditions that influence decay rates include temperature [38] and relative humidity (acidity) [39]. The electrochemical quartz crystal nanobalance (EQCN) technique has been developed to utilize the piezoelectric effect of a quartz crystal to quantify nanogram (<0.4 ng cm) mass changes [40, 41]. The sensitivity to mass change scales directly with the resonate frequency (ƒ0) of the crystal which is typically between 5 and 10 MHz. The mass change (Δm) is inversely proportional to the frequency change (−Δƒ) through the Sauerbrey equations, Δm=−Cf Δƒ, where (Cf) is the mass sensitivity coefficient. The mass change is complicated by the dynamics of the interfacial electrical double layer impacted by the orientation of the electrolyte and water dipoles that vary with potential, in addition to Pt dissolution and oxide formation and reduction at cathode relevant potentials. EQCN studies of Pt-coated quartz crystals have probed oxide formation during cyclic voltammetric scans at rates between 5 and 70 mV s [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]. Within the present study, the impact of single perturbation sweep rates on commercial Pt/C, common to accelerated stress tests and instantaneous automotive idle-to-peak power C. A. Rice :D. Betancourt Center for Manufacturing Research and Department of Chemical Engineering, Tennessee Tech University, Cookeville, TN 38501, USA

A nanoparticle catalyst with superior activity for electrooxidation of formic acid [Electrochem. Commun. 4 (2002) 599-603]

This paper reports the synthesis and characterization of a series of Pt-based nanoparticle catalysts with high activity for formic acid electrooxidation. The catalysts were prepared using spontaneous deposition to decorate platinum nanoparticles with controlled amounts of palladium and palladium/ruthenium. Among all the catalysts investigated, the Pt/Pd catalyst shows the best performance; the steady-state formic acid oxidation current is ca. 1μAcm−2 at 0.27 V vs. RHE. This current is two orders of magnitude higher than that obtained from pure platinum, and the catalyst is ideally suited to be an anode in the direct oxidation formic acid fuel cell. The enhancement in formic acid oxidation by the admetal addition does not correlate with the threshold for CO oxidative stripping (the CO tolerance). The Pt/Pd catalyst requires the highest potential to remove the CO, yet it is most active. We suggest, therefore, that within the dual path mechanism of formic acid oxidation, the direct CO2 formation channel on Pt/Pd is much less affected by the CO chemisorption than on Pt, or on the Pt/Pd/Ru catalyst, also studied in this report.

Electrocatalysis of Formic Acid Oxidation

Direct liquid fuel cells for portable electronic devices are plagued by poor efficiency due to high overpotentials and accumulation of intermediates on the electrocatalyst surface. Direct formic acid fuel cells have a potential to maintain low overpotentials if the electrocatalyst is tailored to promote the direct electrooxidation pathway. Through the understanding of the structural and environmental impacts on preferential selection of the more active formic acid electrooxidation pathway, a higher performing and more stable electrocatalyst is sought. This chapter overviews the formic acid electrooxidation pathways, enhancement mechanisms, and fundamental electrochemical mechanistic studies.