Atomic scale designing of materials for low temperature fuel cells

Abstract

There is a growing interest in the search for alternative energy resources due to the limited availability of fossil fuels and to reduce the emission of greenhouses gases (CO2, CH4, N2O, ozone, chlorofluorocarbons and so on). Therefore, clean energy resources like fuel cells, lithium ion batteries, supercapacitors, and photovoltaics have emerged as alternative ways. A fuel cell is an energy-conversion device that converts the chemical energy from hydrogen or hydrogen-rich fuels into electrical power through electrochemical reactions. The fuel cells are categorized as low-temperature or high-temperature fuel cells. The category with operating temperature below 150 °C can be considered as low-temperature fuel cells. Oxygen reduction reaction (ORR) at the cathode is the most important reaction in low-temperature fuel cells, where Pt-black electrocatalyst with Pt(111) exposed surface is used as conventional cathode electrode.Recently, metal nanoclusters (NCs) surrounded by multiple numbers of well-defined facets have shown its potential towards ORR in comparison to their bulk metal surfaces due to the presence of high surface unsaturation. These highly unsaturated sites possess higher d-band energies, which influences the overall activity of the NCs. Earlier theoretical studies are done either on small-sized metal NCs or bulk metal surfaces (slab model) to understand the catalytic activity of the experimentally synthesized NCs. However, the size of the NC is very important to its catalytic reactivity due to the finite-size effects. Moreover, the low-coordinated sites can’t be modelled in slab model study. Therefore, cluster model study with well-defined facet can only provide the real scenario about the experimental situation.The contents of each chapter included in the thesis are discussed briefly as follows:1. Introduction In this chapter, a brief overview of the fuel cells and their working principle has been discussed, putting much emphasis on low-temperature fuel cells. The potential applicability of NC-based electrode in the low-temperature fuel cell has also been discussed here. The recent advances on hollow materials like metallic nanocage, nanoframes have been reviewed. Furthermore, the applicability of nanosheets based catalysts and their possible application in fuel cells has been discussed. In addition, the roles of the shape of NCs and composition of the core-metal in the core-shell NCs towards the catalytic activity have also been surveyed. Besides, the adsorption behavior and decomposition pathways of methanol at the anode of direct methanol fuel cell have also been discussed. The last part of this chapter discusses the basis of the density functional theory which is used for the electronic structure calculations. The procedure for modelling the NCs, core-shell NC and nanosheets have also been discussed here. This chapter also covers the computational techniques which are used to explain the results of the computation.2. Pt3Ti (Ti19@Pt60)‑based cuboctahedral core-shell nanocluster favors the direct oxygen reduction reaction pathway over the indirect pathway In this chapter, the potential applicability of a cuboctahedral core-shell (Ti19@Pt60) NC towards ORR activity has been investigated and compared with that of a pure Pt NC (Pt79). The energetic stability, thermal stability, and dissolution limit of Ti19@Pt60 NC has been investigated for its possible synthesis and practical usages. Thermodynamic and kinetic parameters are explored to find out the most favored ORR pathway and product selectivity on the Ti19@Pt60 NC. Rate-determining steps (*O2 activation and *OH formation) are highly improved over the Ti19@Pt60 NC with respect to the cuboctahedral Pt NC (Pt79), pure metal (Pt, Pd, and Ag), and alloy (Pt3M; M = Ni, Co, Ti) based catalysts. The detailed investigation reveals that the *O2-induced structural changes favor direct *O2dissociation on the Ti19@Pt60 NC surface. Further, it has been found that a dual mechanism (ligand effect and charge transfer) plays an important role to improve the ORR activity. The results obtained in this study provide fundamental insight into the role of a core-shell NC towards ORR activity.3. Single-layered platinum nanocage as highly selective and efficient catalyst for fuel cells In this chapter, the ORR pathways are systematically studied on the (111) facet of an octahedral single-layered platinum nanocage (Pt66), enclosed by well-defined (111) facets. Energetic (cohesive energy), thermal (molecular dynamics simulation) and dynamic (phonon frequency) calculations are carried out to evaluate the stability of the nanocage. Thermodynamic (reaction free energies) and kinetic (free energy barriers, and temperature dependent reaction rates) parameters are investigated to find out the most favourable pathway for the ORR. The catalytic activity of the nanocage is investigated in greater detail toward its product selectivity (H2O vs. H2O2). Previous theoretical and experimental reports on bulk Pt(111) show that direct O–O bond dissociation and OH formation are very much unlikely due to the high-energy barrier. However, it is found that the direct O–O bond dissociation and OH formation are thermodynamically and kinetically favourable when catalysed by an octahedral Pt-nanocage. The microkinetic analysis shows that the nanocage is a highly selective catalyst for the four-electron reduction (*H2O formation) over two-electron reduction (*H2O2 formation). The excellent catalytic activity of the nanocage is explained from the surface energy, compressive strain, Bader charge and density of states analysis.4. Free-standing platinum monolayer as efficient and selective catalyst for oxygen reduction reaction In this paper, a two-dimensional platinum monolayer (platene) sheet is reported for ORR activity using first-principle calculations. Unlike the previous reports of supported hexagonal planar monolayer, platene exhibits an orthorhombic buckled structure, where each Pt-atom is coordinated with six Pt-atoms. State-of-the-artcalculation shows that the platene is energetically, thermally, dynamically and mechanically stable and thus can be synthesized. An orbital mixing between in-plane σ-orbital and out-of-plane π-orbital helps in stabilizing the buckling pattern. It has been found that the dz2 orbitals of the out-of-plane Pt atoms tilt themselves (by 30°) towards the dyz orbital of the in-plane Pt atoms to gain the maximum overlap, which in turn stabilizes the buckled structure. The potential applicability of platene towards ORR activity has been investigated and it is found that the ORR rate determining step (OH formation) is significantly improved when catalyzed by the platene compared to any catalysts reported to date. The unique adsorption pattern of adsorbed oxygen-atom helps to lower the activation barrier of the rate determining step. The potential dependent study shows that the ORR is thermodynamically favourable at 0.38 V and thus lowers the overpotential for ORR. Besides, platene is very much selective towards H2O formation over H2O2 formation.5. Insights into the shape-dependent (cuboctahedral vs. octahedral) catalytic activity of platinum nanoclusters for fuel cell applications In this chapter, the shape-dependent catalytic activities of two platinum NCs with cuboctahedral (Pt79) and octahedral (Pt85) shapes have been investigated toward ORR. The energetic stability, thermal stability, and dissolution limit of the NCs are investigated for their synthesis and practical usage. The four-electron (H2O formation) vs. two-electron (H2O2 formation) ORR mechanisms are systematically studied on the (111) facet of the NCs to gain more insight into the shape-dependent ORR activity and product selectivity (H2O vs. H2O2). Thermodynamic (reaction free energies) and kinetic (free energy barriers and temperature-dependent reaction rates) parameters are investigated to find out the most favored ORR pathway and product selectivity. The NC-based Pt catalysts are very efficient and selective with respect to the previously reported bulk metal (Pt, Pd, and Ag) based catalysts. The results show that the rate-determining step is no longer a rate-determining step when the reaction is catalyzed by the cuboctahedral NC. The excellent catalytic activity of the cuboctahedral NC isattributed to the surface energy, compressive strain and d-band center position of the catalyst. The results are very much consistent with experimental findings, and thereby such NC-based electrodes may serve as good candidates for fuel cell applications.6. A cuboctahedral platinum (Pt79) nanocluster favours di-sigma adsorption and improves the reaction kinetics for methanol fuel cells In this chapter, the methanol dehydrogenation steps are studied very systematically on the (111) facet of a cuboctahedral platinum (Pt79) NC enclosed by well-defined facets. The various intermediates formed during the methanol decompositions are adsorbed at the edge and bridge site of the facet either vertically (through C- and O-centres) or in parallel. The di-sigma adsorption (in parallel) on the (111) facet of the NC is the most stable structure for most of the intermediates and such binding improves the interaction between the substrate and the NC and thus the catalytic activity. The reaction thermodynamics, activation barrier, and temperature dependent reaction rates are calculated for all the successive methanol dehydrogenation steps to understand the methanol decomposition mechanism, and these values are compared with previous studies to understand the catalytic activity of the NC. It has been found that the catalytic activity of the NC is excellent while comparing with any previous reports and the methanol dehydrogenation thermodynamics and kinetics are best when the intermediates are adsorbed in a di-sigma manner.7. Conclusions The conclusions of research work described here are as follows: i) The O2-induced structural changes on Ti19@Pt60 NC make the NC as a very selective and efficient catalyst for H2O over H2O2 formation and thus could be a promising catalyst for fuel cell applications. ii) The rate determining steps of ORR (O–O bond dissociation and OH formation) are thermodynamically and kinetically favorable when catalyzed by an octahedral Pt-nanocage. The nanocage is a highlyselective catalyst for the four-electron reduction (H2O formation) over two-electron reduction (H2O2 formation). iii) Free-standing orthorhombic platinum monolayer (platene) could be a very promising catalyst for the efficient and selective reduction of oxygen. iv) The cuboctahedral NC improves the ORR activity and selectivity compared to the octahedral NC of similar size. v) The methanol decomposition activity of the Pt79 NC is excellent while comparing with any previous reports and the methanol dehydrogenation thermodynamics and kinetics are best when the intermediates are adsorbed in a di-sigma manner.