Nanocrystals boost electrochemical oxidation of biomass-derived compounds

Abstract

[eng] The global energy situation coupled with the indiscriminate use of fossil fuels is currently a major problem for both human health and the prosperity of the planet. Biomass represents a highly promising renewable resource, and electrochemical processes can serve as an efficient technology for energy conversion and the sustainable generation of a wide variety of high-value products. However, the main current challenge is the development of competitive, efficient, and cost-effective catalysts. Nanostructured materials are highly valuable in this field due to their large surface area and the ability to modulate their properties through structural, compositional, and morphological changes, providing them versatility across a wide range of application fields. Historically, noble metals such as Pt or Pd have been the primary choice as catalysts due to their remarkable electrochemical properties despite their high cost, limited availability, and susceptibility to contamination with CO. In this context, in the different chapters of the thesis, I detail the work I have undertaken to produce and optimize different catalysts for application in electrochemical systems, either for electricity generation or for the valorization of compounds extracted from biomass. Among the four types of materials produced, two are based on Pd alloys modified to enhance both activity and stability. In the other two catalysts, I have replaced noble metals with more cost-effective and abundant transition metals. In Chapter 2, the development of a bimetallic electrocatalyst based on cobalt-iron oxyhydroxide derived from cobalt-iron phosphide nanorods is detailed. This material is used in high-performance anodes for the OER at high current densities, exceeding 1 A cm-2. Under alkaline conditions and anodic polarization, phosphorous depletion and a morphological transition occur, converting the initial CoFeP nanorods to CoFe oxyhydroxide nanoplates with a high ECSA. This procedure enables the preservation of the metal homogeneous distribution on the support, achieving a high density of active sites. In Chapter 3, the application of the compound PdH0.58@C2N as a catalyst in the FOR reaction is studied. The introduction of hydrogen modifies the electronic energy levels, increasing the specific activity to achieve a current density of up to 5.6 A·mgPd−1, which is 5.2 times higher than that of Pd/C. Additionally, the introduction of hydrogen reduces the onset potential and enhances stability, as it exhibits the slowest current decay compared to the reference 16 electrocatalysts. Analysing the results using DFT, it is observed that the d-band of the PdH0.58@C2N surface is downshifted, weakening the adsorbate binding and thus accelerating the rate-limiting step of the FOR. In Chapter 4, the synthesis of the ternary compound Pd2Sn0.8P and the effect of phosphorus incorporation into the Pd-Sn alloy on the electrocatalytic response for formate oxidation are detailed. As a result, Pd2Sn0.8P exhibits very high catalytic activity, with record mass current densities of up to 10.0 A·mgPd−1. Additionally, compared to Pd1.6Sn, not only is the activity improved, but the stability of the catalyst is also enhanced. To understand how phosphorus affects the reaction mechanism, the system was studied using DFT calculations, which confirm that the presence of phosphorus favours the desorption of CO2, thus reducing the energy barrier of the rate-limiting step. In Chapter 5, I detail the effect of introducing an oxophilic element into Ni nanoparticles used as catalysts in the oxidation of glucose. We observed that incorporating Sn not only enhances the reaction kinetics but also that NiSn0.6 achieves excellent current densities and a Faradaic efficiency of 93% towards formic acid. A DFT study shows that Sn facilitates the adsorption of glucose on the Ni surface and promotes the formation of catalytically active Ni³⁺ species. At low concentrations and potentials, formic acid overoxidation to carbonates reduces the total Faradaic efficiency, while at high concentrations, the non-Faradaic glucose degradation pathway is promoted, increasing selectivity towards fructose, acetic acid, and lactic acid production.

[cat] La situació energètica global, juntament amb l'ús indiscriminat de combustibles fòssils, constitueix actualment un greu problema tant per a la salut humana com per a la prosperitat del planeta. La biomassa representa un recurs renovable altament prometedor, i els processos electroquímics poden servir com una tecnologia eficient per a la conversió d'energia i la generació sostenible d'una àmplia varietat de productes d'alt valor. Els materials nanoestructurats tenen un gran valor com a catalitzadors per la seva gran àrea superficial i la capacitat de modular les seves propietats. En els diferents capítols de la tesi, he detallat el treball que he dut a terme en la fabricació i optimització de diferents catalitzadors i la seva aplicació en sistemes electroquímics, tant per a la generació d'electricitat com per a la valorització de compostos extrets de la biomassa. Al Capítol 2, es detalla el desenvolupament d'un electrocatalitzador bimetàl·lic basat en oxihidròxid de cobalt-ferro derivat de nanorods de fosfur de cobalt-ferro. Aquest material és utilitzat en ànodes d'alt rendiment per a l'OER a densitats de corrent elevades, superiors a 1 A cm-2. Al Capítol 3, s'estudia l'aplicació del compost PdH0.58@C2N com a catalitzador en la reacció FOR. La introducció de l'hidrogen modifica els nivells d'energia electrònics, augmentant l'activitat específica. A més, la introducció de l'hidrogen redueix el potencial d'inici i augmenta l'estabilitat, ja que exhibeix la menor taxa de disminució de corrent en comparació amb els electrocatalitzadors de referència. Al Capítol 4, es detalla la síntesi del compost ternari Pd2Sn0.8P i l'efecte de la incorporació del fòsfor en l'aliatge de Pd-Sn sobre la resposta electrocatalítica per a l'oxidació de format. Com a resultat, Pd2Sn0.8P presenta una activitat catalítica molt elevada, amb densitats de corrent massiva rècord de fins a 10.0 A·mgPd−1. A més, a l’incorporar el fòsfor, no només es millora l'activitat, sinó també l'estabilitat del catalitzador. Al Capítol 5, s'estudia l'efecte d'introduir un element oxofílic en nanopartícules de Ni com a catalitzadors en l'oxidació de la glucosa. Observem que la incorporació de Sn no només millora la cinètica de la reacció, sinó que també el NiSn0.6 aconsegueix excel·lents densitats de corrent i una eficiència faradaica del 93% cap a l'àcid fòrmic.