Colloidal Metal Phosphide Nanocrystals for Electrochemical Energy Technologies

Abstract

Metal phosphides are an important class of functional materials that exhibit a wide range of applications for energy storage and conversion. However, the synthesis of metal phosphide nanoparticles and its scalability is often limited by the toxicity, air sensitivity and high cost of reagents used. In this thesis, a simple, scalable and cost-effective procedure was developed to produce metal phosphides using inexpensive, low-toxicity and air-stable triphenyl phosphite as phosphorus source. The use of chlorides or metal carbonyl as metal precursors and hexadecylamine as ligand allowed the synthesis of a variety of phosphide nanocrystals (NCs) including phosphides of Ni, Co, Cu, Fe, Mo. The use of NiCl2 and CoCl2 mixture as metal precursors, ternary metal phosphide Ni2-xCoxP (0≤x≤2) NCs could be obtained. The synthesis of Ni2-xCoxP involved the nucleation of amorphous Ni-P and its posterior crystallization and simultaneous incorporation of Co. Tuning the experimental parameters allows producing NCs with different composition, morphology and particle size. Ni2-xCoxP-based electrocatalysts exhibited enhanced electrocatalytic activity toward the hydrogen evolution reaction compared to binary phosphides. In particular, NiCoP electrocatalysts displayed very low overpotential of 97 mV at J = 10 mA cm-2 and an excellent long-term stability. Density functional theory (DFT) calculations of the Gibbs free energy for hydrogen adsorption at the surface of Ni2-xCoxP NCs showed NiCoP to have the most appropriate composition to optimize this parameter within the whole Ni2-xCoxP series. The use of chromium hexacarbonyl as metal precursor and high boiling point oleylamine as ligand, less-studied CrP NCs could also be produced. This method allows producing CrP with nanometric particle size and with a very high throughput and material yield. CrP NCs were mixed with carbon to prepare electrocatalysts, which exhibited remarkable activity and stability toward oxygen reduction reaction in an alkaline electrolyte and an absolute tolerance to methanol. DFT calculations demonstrated CrP to provide a very strong chemisorption of O2 which facilitates its reduction and explains the excellent performance. Besides, another phosphorus source hexamethylphosphorous triamide (HMPT) and new synthetic strategies were applied for the synthesis of SnP and PdP2 NCs. SnP NCs were produced from the reaction of HMPT and a tin phosphonate prepared from tin oxalate and a long chain phosphonic acid. SnP NCs obtained from this reaction displayed a spherical geometry and a trigonal crystallographic phase with a superstructure attributed to ordered diphosphorus pairs. Such NCs were mixed with carbon black and used as anode materials in sodium-ion batteries, which displayed a high reversible capacity of 600 mA h g-1 at a current density of 100 mA g-1 and cycling stability for over 200 cycles. The excellent cycling performance was associated with both the small size of the crystal domains and the particular composition and phase of SnP which prevent mechanical disintegration and major phase separation during sodiation and desodiation cycles. The synthesis of PdP2 involves the reaction of palladium acetylacetonate and HMPT to nucleate defective Pd5P2 nanoparticles that subsequently, with further phosphorus incorporation, crystallize into PdP2. The produced PdP2 NCs showed high mass activity and long-term stability toward the ethanol oxidation reaction in alkaline media. The activity and stability of the PdP2-based catalyst were further improved by supporting PdP2 NCs onto reduced graphene oxide (rGO). PdP2/rGO catalysts showed high current densities up to 51.4 mA cm-2 and mass activities of 1.60 A mg-1Pd, that is 4.8 and 15 times higher than Pd NCs. Overall, in this thesis serval metal phosphide nanomaterials were produced by colloidal synthetic strategy, and showed highly performance for electrochemical energy conversion and storage applications.

Los fosfuros metálicos son materiales funcionales de gran interés e impacto en la sociedad debido a su gran rendimiento en almacenamiento y conversión de energía. Sin embargo, la síntesis de nanopartículas de fosfuros metálicos y su escalabilidad es limitada. En la presente tesis, se ha desarrollado un proceso de producción para nanopartículas de fosfuros metálicos simple, escalable y rentable. El éxito de los resultados radica en el uso de fosfito de trifenilo, considerado como una fuente de fósforo barata, estable al contacto con el aire y no tóxica, que además permite la síntesis de una gran variedad de nanocristales (NCs) (fosfuros de Ni, Co, Cu, Fe, Mo, así como fosfuros ternarios de Ni2-xCoxP). Optimizando los parámetros experimentales, es posible producir NCs de diferente composición, morfología y tamaño. Además, el uso de solventes de alto punto de ebullición, facilita la síntesis de materiales menos estudiados como el CrP. Los NCs de Ni2-xCoxP y CrP NCs pueden ser combinados con carbono dando lugar a electrocatalizadores útiles para reacciones tales como evolución de hidrógeno y reducción de oxígeno, respectivamente. El uso de otras fuentes de fósforo, como por ejemplo la triamida de hexametilfósforo, permite a su vez producir NCs de SnP y PdP2. En el caso particular de SnP, los NCs demostraton poseer una geometría esférica y una fase cristalográfica trigonal con una superestructura atribuida a los pares difósforos ordenados. Mezclados con carbón negro y utilizados como ánodos en baterías de ion sodio, estos NCs demostraron una alta capacidad reversible de 600 mA h g-1 a una densidad de corriente de 100 mA g-1 y estables durante más de 200 ciclos. Por otro lado, los NCs de PdP2 soportados sobre óxido de grafeno se utilizaron como electrocatalizadores para la oxidación de etanol, mostrando una alta densidad de corriente de hasta 51.4 mA cm-2 y una actividad de masa de 1.60 A mg-1Pd. En resumen, se han desarrollado métodos de síntesis basados en estrategias coloidales para poder producir una gran variedad de fosfuros metálicos a escala nanométrica, mostrando estos un gran rendimiento para su uso en conversión y almacenamiento de energía electroquímica.