Browsing by Author "Zheng, Jie"
Now showing 1 - 4 of 4
Results Per Page
Sort Options
Item Anode catalyst development for low-temperature fuel cells: fundamentals and synthesis(University of Delaware, 2016) Zheng, JieCommercialization and mass adoption of low temperature fuel cells have been hampered by the large cell voltage loss, which can be largely blamed on the sluggish electrode reaction kinetics even with the state-of-the-art Pt catalysts. Significant progress has been made in the development of cathode catalysts for the oxygen reduction reaction (ORR), whereas the search for efficient anode catalysts has not been as fruitful. Therefore, the rational design and development of efficient anode catalysts are of vital importance, which hinge on two key factors: 1) fundamental understanding of the reaction mechanism and 2) synthesis of catalysts with well-defined structures. Hydrogen oxidation reaction (HOR, 𝐻2 ↔ 2𝐻+ + 2𝑒) is roughly two orders of magnitude slower in base than in acid electrolytes on Pt-group metal (PGM) catalysts, which demands either a substantial anodic overpotential or a high PGM loading for hydroxide exchange membrane fuel cells (HEMFCs). Fundamental understanding HOR kinetics is a prerequisite in the design of highly active HOR catalysts. To achieve this goal, my research established protocols to reliably remove the contribution of diffusion in the HOR/HER activity measurement with the rotating disk electrode (RDE) method, based on which intrinsic kinetic information can be extracted. The effect of particle size on HOR/HER activities were explored on carbon supported Ir and Pd nanoparticles: the specific HOR/HER activities increase as particle size increase. The most active sites for HOR/HER on Ir/C were identified to be the sites with lowest hydrogen binding energy (HBE) (most likely the low-index facets), based on the observation that the activities normalized to the surface area of weakly binding sites are independent of particle size. Consistent with the results on Ir, the increased HOR/HER activity on larger Pd nanoparticles correlates with an increased ratio of the sites with lower HBE. These findings suggest that future catalyst design should focus on increasing the density of sites with low HBE, e.g., low-index facets. To establish the generality of the pH effect on the HOR/HER activity, a reliable and easily accessible method to experimentally determine the pH-dependent HBE was developed. In addition, HOR/HER activities on monometallic PGM (Pt, Pd, Ir and Rh) nanoparticles were mapped out over a broad pH range (1-13), which are then correlated with HBE. A universal correlation between the HOR activity and HBE is obtained on all PGMs evaluated, which offers strong evidence that HBE is the dominating descriptor for the performance of HOR catalysts. It follows that tuning of HBE could a key strategy in the future design of HOR catalysts. Aside from hydrogen, methanol is a promising liguid fuel for fuel cells. A key challenge in the development of active catalysts for methanol oxidation reaction (MOR, 𝐶𝐻3𝑂𝐻 + 𝐻2𝑂 → 𝐶𝑂2 + 6𝐻+ + 6𝑒), which is the anode reaction of direct methanol fuel cells (DMFCs), is the structural sensitive nature of the catalytic performance. Hence, synthesis of catalysts with tailored structures is critical. Extended surface nanostructures, e.g., PtRu nanotubes (PtRuNTs) and PtRu coated Cu nanowires (PtRu/CuNWs), were synthesized by galvanically displacing the CuNWs template, which showed higher specific MOR activity than that of the benchmark PtRu/C. We attribute the enhanced activity to the weakened Pt-CO bonding through the modification of d-band center of Pt.Item Correcting the Hydrogen Diffusion Limitation in Rotating Disk Electrode Measurements of Hydrogen Evolution Reaction Kinetics(The Electrochemical Society, 2015-10-10) Zheng, Jie; Yan, Yushan; Xu, Bingjun; Jie Zheng, Yushan Yan, and Bingjun Xu; Zheng, Jie; Yan, Yushan; Xu, BingjunRotating disk electrode (RDE) method is widely employed in studies on the hydrogen oxidation/evolution reaction (HOR/HER) owing to its well-defined mass transport behaviors. While it is accepted that the measured HOR current is controlled by both the electrode kinetics and the diffusion of H2, HER is typically assumed to be free of diffusion limitation. Here we demonstrate that HER could also be diffusion limited when the electrode kinetics is fast, as evidenced by the rotation speed dependent HER current on Pt in acid (pH = 1) and the overlap of the HER polarization curve with the concentration overpotential curve. The HER diffusion limitation originates from the insufficient mass transport of produced H2 from the electrode surface to the bulk electrolyte and the highly reversible nature of HOR/HER. Kinetic analyses based on HER polarization curves on Pt in acid without correcting for the diffusion limitation could lead to inaccurate Tafel slopes and mechanistic interpretations, and significantly underestimated HER activities. A general data analysis protocol based on the reversible Koutecky-Levich equation is developed to obtain accurate kinetic information of HOR/HER even when electrode kinetics is facile. This new method is compared with other existing methods on Pt disk electrodes at different pHs and thin-film electrodes with different Pt loadings.Item Nickel supported on nitrogen-doped carbon nanotubes as hydrogen oxidation reaction catalyst in alkaline electrolyte(Nature Publishing Group, 2016-01-14) Zhuang, Zhongbin; Giles, Stephen A.; Zheng, Jie; Jenness, Glen R.; Caratzoulas, Stavros; Vlachos, Dionisios G.; Yan, Yushan; Zhongbin Zhuang, Stephen A. Giles, Jie Zheng, Glen R. Jenness, Stavros Caratzoulas, Dionisios G. Vlachos & Yushan Yan; Zhang, Zhongbin; Giles, Stephen A.; Zheng, Jie; Jenness, Glen R.; Caratzoulas, Stavros; Vlachos, Dionisios G.; Yan, YushanThe development of a low-cost, high-performance platinum-group-metal-free hydroxide exchange membrane fuel cell is hindered by the lack of a hydrogen oxidation reaction catalyst at the anode. Here we report that a composite catalyst, nickel nanoparticles supported on nitrogen-doped carbon nanotubes, has hydrogen oxidation activity similar to platinum-group metals in alkaline electrolyte. Although nitrogen-doped carbon nanotubes are a very poor hydrogen oxidation catalyst, as a support, it increases the catalytic performance of nickel nanoparticles by a factor of 33 (mass activity) or 21 (exchange current density) relative to unsupported nickel nanoparticles. Density functional theory calculations indicate that the nitrogen-doped support stabilizes the nanoparticle against reconstruction, while nitrogen located at the edge of the nanoparticle tunes local adsorption sites by affecting the d-orbitals of nickel. Owing to its high activity and low cost, our catalyst shows significant potential for use in low-cost, high-performance fuel cells.Item Universal dependence of hydrogen oxidation and evolution reaction activity of platinum-group metals on pH and hydrogen binding energy(American Association for the Advancement of Science, 2016-01-14) Zheng, Jie; Sheng, Wenchao; Zhuang, Zhongbin; Xu, Bingjun; Yan, Yushan; Jie Zheng, Wenchao Sheng, Zhongbin Zhuang, Bingjun Xu, Yushan Yan; Zheng, Jie; Zhuang, Zhongbin; Xu, Bingjun; Yan, YushanUnderstanding how pH affects the activity of hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) is key to developing active, stable, and affordable HOR/HER catalysts for hydroxide exchange membrane fuel cells and electrolyzers. A common linear correlation between hydrogen binding energy (HBE) and pH is observed for four supported platinum-group metal catalysts (Pt/C, Ir/C, Pd/C, and Rh/C) over a broad pH range (0 to 13), suggesting that the pH dependence of HBE is metal-independent. A universal correlation between exchange current density and HBE is also observed on the four metals, indicating that they may share the same elementary steps and rate-determining steps and that the HBE is the dominant descriptor for HOR/HER activities. The onset potential of CO stripping on the four metals decreases with pH, indicating a stronger OH adsorption, which provides evidence against the promoting effect of adsorbed OH on HOR/HER.