Nanostructured Metal Sulfides for Electrochemical Energy Conversion

Abstract

Storing the fluctuating renewable energy into synthetic fuels or in batteries is meaningful due to the emerging energy crisis. In this thesis, four nanostructured catalysts based on two kinds of metal sulfides, namely Cu2S and SnS2, were produced and optimized to improve their performance towards three key electrochemical energy conversion processes, namely electrochemical oxygen evolution, photoelectrochemical water splitting and lithium-ion batteries. Chapter 1 presented a general introduction to explain the motivation of the thesis topic. In chapter 2, a metallic copper substrate was used as current collector and chemical template to produce Cu2S nanorod arrays for electrochemical oxygen evolution reaction (OER). Suitable characterization tools were applied to investigate the chemical, structural and morphological transformation in OER operation, during which the initial Cu2S nanorod arrays would perform as a “pre-catalyst” that in-situ changed to CuO nanowires. Notably, the Cu2S-derived CuO showed significant improved OER performance compared with that of CuO prepared by directly annealing a Cu(OH)2 precursor, in terms of both activity and stability. Thus obtained electrocatalyst can be ranked among the best Cu-based OER catalysts reported so far. To take advantage of the unlimited solar energy, an ultrathin SnS2 NPL with a suitable band gap around 2.2 eV was produced via a hot-injection solution-based process in chapter 3. The unsatisfied photoelectrochemical (PEC) performance of bare SnS2 motivated me to deposit Pt NPs on its surface as cocatalyst via in-situ reduction of a Pt salt. The resulting SnS2-Pt heterostructures with optimal Pt amount showed significant improvement (six fold) towards PEC water oxidation. Mott-Schottky analysis and PEC impedance spectroscopy (PEIS) were used to analyze in more detail the effect of Pt on the PEC performance. The optimal SnS2-Pt heterostructure presented acceptable performance towards PEC water splitting. However, it still suffered from a moderate stability due to the peel-off of the catalyst layer from the FTO surface. To solve this problem, in chapter 4 we detailed a simple, versatile and scalable amine/thiol- based molecular ink to grow nanostructured SnS2 layers directly on conductive substrates such as FTO, stainless steel and carbon cloth. Such layers on FTO were characterized by excellent photocurrent densities. The same strategy was used to produce SnS2-graphene composites, SnS2-xSex ternary coatings and even phase pure SnSe2 layers. Finally, the potential of this precursor ink to produce gram scale amounts of unsupported SnS2 was also investigated. Apart from the application as a photocatalyst, SnS2 can also be a promising anode material for Li-ion batteries (LIB). In chapter 5, nanostructured SnS2 with different morphologies produced in chapter 3 were tested as LIB anodes firstly to find that thin SnS2 NPLs provided the highest performance. Thereafter, a colloidal synthesis strategy to grow the same SnS2 NPLs within a matrix of porous g-C3N4 (CN) and graphite plates (GPs) was developed and the obtained materials were tested for LIB application. Such hierarchical SnS2/CN/GP composites using SnS2-NPL as active materials, porous CN to provide avenues for electrolyte diffusion and ease

the volumetric expansion of SnS2, and GP as “highways” for charge transport displayed excellent rate capabilities (536.5 mAh g-1 at 2.0 A g-1) and an outstanding stability (~99.7 % retention after 400 cycles), which were partially associated with a high pseudocapacitance contribution (88.8 % at 1.0 mV s-1). The excellent electrochemical properties of these nanocomposites were ascribed to the synergy created between the three components. Overall, four nanostructured catalysts based on Cu2S and SnS2 were prepared, and proper optimizations/treatments were defined to improve their catalytic performance. The results shown in this thesis demonstrate the promising application of non-toxic, low cost metal sulfides in electrochemical energy conversion technologies.

En esta tesis, se produjeron y optimizaron cuatro catalizadores nanoestructurados basados en Cu2S y SnS2 para mejorar su rendimiento hacia la conversión de energía electroquímica. El Capítulo 1 presentó una introducción general para explicar la motivación del tema de tesis. En el capítulo 2, las matrices de las nanovarillas de Cu2S se sintetizaron in situ sobre un sustrato de cobre metálico para la reacción electroquímica de evolución de oxígeno (OER). Se aplicaron herramientas de caracterización adecuadas para investigar la transformación en la operación OER, durante la cual las matrices iniciales de las nanovarillas Cu2S in situ cambió a nanohilos de CuO. En particular, el CuO derivado de Cu2S mostró un rendimiento de OER significativamente mejor cuando comparado al de CuO preparado mediante el recocido. En el capítulo 3, se detalló un proceso basado en una solución de inyección en caliente para producir nanoplacas ultrafinas SnS2 (NPL). Posteriormente, se cultivóPt en su superficie mediante la reducción in situ de una sal de Pt. Posteriormente se probó el rendimiento fotoelectroquímico (PEC) de los fotoanodes hacia la oxidación del agua. Los fotoanodes de SnS2-Pt optimizados proporcionaron densidades de fotocorriente significativamente más altas que el SnS2 desnudo (seis veces). Se analizó el efecto de Pt. En el capítulo 4, se informó una tinta molecular simple para cultivar capas de SnS2 nanoestructuradas directamente sobre sustratos conductores. Tales capas nanoestructuradas en FTO se caracterizaron por excelentes densidades de fotocorriente. Se utilize la misma estrategia para producir compuestos de grafeno-SnS2, recubrimientos ternarios SnS2-xSex, capas de SnSe2 de fase pura e incluso polvo de SnS2 a gran escala. En el capítulo 5, el SnS2 nanoestructurado con diferentes morfologías se probaron como ánodos LIB en primer lugar para encontrar que los NPL de SnS2 delgados proporcionaban el mayor rendimiento. Posteriormente, se desarrolló una estrategia de síntesis coloidal para cultivar los mismos NPL de SnS2 dentro de una matriz de g-C3N4 (CN) poroso y placas de grafito (GP) y se probaron para la aplicación LIB. Tales compuestos jerárquicos SnS2/CN/GP mostraron excelentes propiedades electroquímicas, lo que se atribuye a la sinergia creada entre los tres componentes como se investigó.