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As a consequence of the transition from fossil fuels to renewable biofuels and in particular biodiesel, glycerol has shifted from being a commodity chemical to a by-product. Currently, glycerol is often either discarded or further oxidised into value-added 3-carbon products. Electrochemical oxidation also offers the potential to produce clean hydrogen gas. The main challenge from both the industrial and scientific viewpoints is in understanding the reaction mechanisms in order to develop effective and selective catalysts.

This thesis focuses on the development, characterization, and optimization of Pt- (and Pd-) based catalysts for the glycerol electrooxidation reaction (GEOR), with a particular emphasis on the effectiveness, the reaction mechanism, and the selectivity of the reaction. In the first part, the enhancement of the catalytic surface area of Pt is achieved through the use of different porogens where Pt is electrodeposited to fabricate mesoporous catalysts with linear, hierarchical or cubic pore structures. The mass diffusion of the reactant and products in the pores was the critical step where hierarchical pores allowed for an increased electrochemically available surface area without hindering out the diffusion of oxidation products. To tackle some of the drawbacks of pure platinum and to increase the catalytic activity, a Cu-Pt alloy with varying relative compositions was studied. Incorporating Cu into Pt improves the catalytic performance by straining and introduction of vacancies into the valence band of Pt atoms. An enhanced current density, catalytic activity, and stability enhanced activity was found close to the composition Cu₃Pt, which was also theoretically predicted. The composition of these alloys also had an influence on the product selectivity where the composition and potential-dependent mechanism was matched with DFT calculations. The performance and stability of the Cu-Pt catalysts was studied under operando conditions using grazing incidence diffraction with synchrotron radiation, for which a dual-chamber flow cell was specifically designed to mimic normal laboratory conditions. Analysis of the angle- and time-dependent data provided insights into the real-time structural dynamics as function of probing depth as well as the degradation of the electrocatalysts particularly under the first few “activation” cycles but also upon prolonged cycling. The results show that the activation of catalysts with an excess copper resulted unequivocally on the surface leaching of copper species and consequent surface dealloying towards Pt rich surface compositions. The last part of the thesis focused on the characterization of Pd_0.9Ni_0.1 and Pd electrocatalyst electrodeposited on a Ni rotating disk electrode for the GEOR to understand the structural and morphological changes of PdNi catalysts after electrolysis.

Experimental and theoretical results show that Pt- (and Pd-) based alloys are promising catalysts for the industrial GEOR although there is still work to do.

Using seawater to generate green hydrogen through electrolysis is a promising strategy for energy conversion. However, direct seawater splitting to form green hydrogen suffers drawbacks from electrode corrosion due to chlorine and other impurities. Herein, we demonstrate direct electrochemical seawater splitting using a forward osmosis membrane coupled with an electrolysis cell. By using this cell, high activity (270 mV at 10 mA/cm²) and decent stability (up to 6 days) are achieved by utilizing RuO₂-(Ni,Co)₃O₄ catalyst in a neutral electrolyte. This system is further studied in various electrolytes under neutral to alkaline conditions. This proof of concept shows that seawater splitting could be coupled with semipermeable membranes, allowing for direct utilization of seawater without pretreatment or purification and evading the challenges posed by impurities.

BACKGROUND: Sodium chlorate (NaClO3) is extensively used in the paper industry, but its production uses strictly regulated highly toxic Na2Cr2O7 to reach high hydrogen evolution reaction (HER) Faradaic efficiencies. It is therefore important to find alternatives either to replace Na2Cr2O7 or reduce its concentration.

RESULTS: The Na2Cr2O7 concentration can be significantly reduced by using Na2MoO4 as an electrolyte co-additive. Na2MoO4 in the millimolar range shifts the platinum cathode potential to less negative values due to an activating effect of cathodically deposited Mo species. It also acts as a stabilizer of the electrodeposited chromium hydroxide but has a minor effect on the HER Faradaic efficiency. X-ray photoelectron spectroscopy (XPS) results show cathodic deposition of molybdenum of different oxidation states, depending on deposition conditions. Once Na2Cr2O7 was present, molybdenum was not detected by XPS, as it is likely that only trace levels were deposited. Using electrochemical measurements and mass spectrometry we quantitatively monitored H-2 and O-2 production rates. The results indicate that 3 mu mol L-1 Na2Cr2O7 (contrary to current industrial 10-30 mmol L-1) is sufficient to enhance the HER Faradaic efficiency on platinum by 15%, and by co-adding 10 mmol L-1 Na2MoO4 the cathode is activated while avoiding detrimental O-2 generation from chemical and electrochemical reactions. Higher concentrations of Na2MoO4 led to increased oxygen production.

CONCLUSION: Careful tuning of the molybdate concentration can enhance performance of the chlorate process using chromate in the micromolar range. These insights could be also exploited in the efficient hydrogen generation by photocatalytic water splitting and in the remediation of industrial wastewater.

Glycerol is a renewable chemical that has become widely available and inexpensive due to the increased production of biodiesel. Noble metal materials have shown to be effective catalysts for the production of hydrogen and value-added products through the electrooxidation of glycerol. In this work we develop three platinum systems with distinct pore mesostructures, e.g., hierarchical pores (HP), cubic pores (CP) and linear pores (LP); all with high electrochemically active surface area (ECSA). The ECSA-normalized GEOR catalytic activity of the systems follows HPC > LPC > CPC > commercial Pt/C. Regarding the oxidation products, we observe glyceric acid as the main three-carbon product (3C), with oxalic acids as the main two-carbon oxidation product. DFT-based theoretical calculations support the glyceraldehyde route going through tartronic acid towards oxalic acid and also help understanding why the dihydroxyacetone (DHA) route is active despite the absence of DHA amongst the observed oxidation products.

Through glycerol electrooxidation, we demonstrate the viability of using a PdNi catalyst electrodeposited on Ni foam to facilitate industrially relevant rates of hydrogen generation while concurrently providing valuable organic chemicals as glycerol oxidation products. This electrocatalyst, in a solution of 2 M NaOH and 1 M glycerol at 80 & DEG;C, enabled current densities above 2000 mA cm(-2) (in a voltammetric sweep) to be obtained in atmospheres of both air and N-2. Repeated potential cycling under an aerated atmosphere to these exceptional current densities indicated a high stability of the catalyst. Through steady state polarisation curves, 1000 mA cm(-2) was reached below an anodic potential of 0.8 V vs RHE. Chronoamperometry showed glycerate and lactate being the major oxidation products, with increased selectivity for lactate at the expense of glycerate in aerated systems. Aerated atmospheres were demonstrated to consistently increase the apparent Faradaic efficiency to >100%, as determined by the concentration of oxidation products in solution. The excellent performance of PdNi/Ni in aerated solutions suggests that O-2 removal from the electrolyte is not needed for an industrial glycerol electrooxidation process, and that combining electrochemical and chemical glycerol oxidation, in the presence of dissolved O-2,O- presents an important process advantage.

The development of alcohol-based electrolysis to enable the concurrent production of hydrogen with low electricity consumption still faces major challenges in terms of the maximum anodic current density achievable. Whilst noble metals enable a low electrode potential to facilitate alcohol oxidation, the deactivation of the catalyst at higher potentials makes it difficult for the obtained anodic current density to compete with water electrolysis. In this work the effect of significant parameters such as mass transport, glycerol and OH- concentration and electrolyte temperature on the glycerol electrooxidation reaction (GEOR) in alkaline conditions on a bimetallic catalyst PdNi/Ni-RDE (Pd0.9Ni0.1) has been studied to discern experimental conditions which maximise achievable anodic current density before deactivation occurs. The ratio of NaOH:glycerol in the electrolyte highly affects the rate of the GEOR. A maximum current density of 793 mA cm(-2) at-0.125 V vs. Hg/HgO through steady state polarisation curves was achieved at a moderate and intermediate rotation rate of 500 RPM in a 2 M NaOH and 1 M glycerol (ratio of 2) electrolyte at 80 & DEG;C. Shown here is a method of catalyst reactivation for enabling the longterm use of the PdNi/Ni-RDE for electrolysis at optimal conditions for extended periods of time (3 h at 300 mA cm(-2) and 10 h at 100 mA cm(-2)). Through scanning electron microscopy (SEM), X-ray photon electron spectroscopy (XPS) and X-ray diffraction (XRD) it is shown that the electrodeposition of Pd and Ni forms an alloy and that after 10 h of electrolysis the catalyst has chemical and structural stability. This study provides details on parameters significant to the maximising of the GEOR current density and the minimising of the debilitating effect that deactivation has on noble metal based electrocatalysts for the GEOR.& nbsp;(c) 2021 The Authors. Published by Elsevier Ltd.& nbsp;

Electrochemical valorization of biomass waste (e.g., glycerol) for production of value-added products (such as formic acid) in parallel with hydrogen production holds great potential for developing renewable and clean energy sources. Here, a synergistic effort between theoretical calculations at the atomic level and experiments to predict and validate a promising oxide catalyst for the glycerol oxidation reaction (GOR) are reported, providing a good example of designing novel, cost-effective, and highly efficient electrocatalysts for producing value-added products at the anode and high-purity hydrogen at the cathode. The predicted CoMoO4 catalyst is experimentally validated as a suitable catalyst for GOR and found to perform best among the investigated metal (Mn, Co, Ni) molybdate counterparts. The potential required to reach 10 mA cm(-2) is 1.105 V at 60 degrees C in an electrolyte of 1.0 m KOH with 0.1 m glycerol, which is 314 mV lower than for oxygen evolution. The GOR reaction pathway and mechanism based on this CoMoO4 catalyst are revealed by high-performance liquid chromatography and in situ Raman analysis. The coupled quantitative analysis indicates that the CoMoO4 catalyst is highly active toward C-C cleavage, thus presenting a high selectivity (92%) and Faradaic efficiency (90%) for formate production.