Novel catalysts and applications for polymer electrolyte membrane cells

Polymer electrolyte membrane (PEM) water electrolysis represents a promising technology for the sustainable, emission-free hydrogen production from renewable energy, as it is able to quickly respond to fluctuations in the renewable energy supply. Nevertheless, natural scarcity of iridium, which is used as catalyst for the anodic water oxidation reaction, hinders wide-scale implementation of these cells. In this thesis we investigated how the performance of iridium catalysts can be improved by the addition of Sn and SnO2. Furthermore, despite their high efficiency, PEM cells are currently only employed for hydrogen production. We investigated whether it is possible to run other electrochemical transformations in PEM cells. Particularly, we focused on nitrate and nitric oxide reduction. These are pollutant molecules in ground water and in air, respectively. We investigated potential catalysts for the efficient transformation of these species to ammonia, which is a very important molecule for the fertilizer industry. Our approach can open new pathways for ammonia synthesis, replacing the energy-intensive, state-of-the-art Haber-Bosch process.ChemE/Catalysis Engineerin

PEM Electrolysis-Assisted Catalysis Combined with Photocatalytic Oxidation towards Complete Abatement of Nitrogen-Containing Contaminants in Water

Electrolysis-assisted nitrate (NO3−) reduction is a promising approach for its conversion to harmless N2 from waste, ground, and drinking water due to the possible process simplicity by in-situ generation of H2/H/H+ by water electrolysis and to the flexibility given by tunable redox potential of electrodes. This work explores the use of a polymer electrolyte membrane (PEM) electrochemical cell for electrolysis-assisted nitrate reduction using SnO2-supported metals as the active cathode catalysts. Effects of operation modes and catalyst materials on nitrate conversion and product selectivity were studied. The major challenge of product selectivity, namely complete suppression of nitrite (NO2−) and ammonium (NH4+) ion formation, was tackled by combining with simultaneous photocatalytic oxidation to drive the overall reaction towards N2 formation.ChemE/Catalysis Engineerin

Electrochemical Conversion of NO to NH₃ in a PEM Cell

The continuous electrochemical NO reduction to ammonia in a PEM cell was investigated in this work. We used a ruthenium-based catalyst at the cathode and an iridium oxide catalyst at the anode. The highest ammonia faradaic efficiency was observed at 1.9 V cell voltage. Adjusting the NO flow allowed to achieve 97% NO conversion and 93% ammonia faradaic efficiency for a 5.2% NO/He feed. The ammonia yield was 0.51 mmol cm-2 h-1, among the highest reported to date with the advantage of continuous operation. Experiments with a low NO concentration feed of 983 ppm showed 98% conversion at 0 V vs pseudo-RHE. Achieving this performance under such mild conditions indicates the great potential of the PEM cells for NOx abatement applications and the production of valuable NH3ChemE/Catalysis Engineerin

Electrified Conversion of Contaminated Water to Value: Selective Conversion of Aqueous Nitrate to Ammonia in a Polymer Electrolyte Membrane Cell

The application of a polymer electrolyte membrane (PEM) electrolytic cell for continuous conversion of nitrate, one of the contaminants in water, to ammonia at the cathode was explored in the present work. Among carbon-supported metal (Cu, Ru, Rh and Pd) electrocatalysts, the Ru-based catalyst showed the best performance. By suppressing the competing hydrogen evolution reaction at the cathode, it was possible to reach 94 % faradaic efficiency for nitrate reduction towards ammonium. It was important to match the rate of the anodic reaction with the cathodic reaction to achieve high faradaic efficiency. By recirculating the effluent stream, 93 % nitrate conversion was achieved in 8 h of constant current electrolysis at 10 mA cm−2 current density. The presented approach offers a promising path towards precious NH3 production from nitrate-containing water that needs purification or can be obtained after capture of gaseous NOx pollutants into water, leading to waste-to-value conversion.ChemE/Catalysis Engineerin

Towards Higher NH₃ Faradaic Efficiency: Selective-Poisoning of HER Active Sites by Co-Feeding CO in NO Electroreduction**

Direct electroreduction of nitric oxide offers a promising avenue to produce valuable chemicals, such as ammonia, which is an essential chemical to produce fertilizers. Direct ammonia synthesis from NO in a polymer electrolyte membrane (PEM) electrolyzer is advantageous for its continuous operation and excellent mass transport characteristics. However, at a high current density, the faradaic efficiency of NO electroreduction reaction is limited by the competing hydrogen evolution reaction (HER). Herein, we report a CO-mediated selective poisoning strategy to enhance the faradaic efficiency (FE) towards ammonia by suppressing the HER. In the presence of only NO at the cathode, Pt/C and Pd/C catalysts showed a lower FE towards NH3 than to H2 due to the dominating HER. Cu/C catalyst showed a 78 % FE towards NH3 at 2.0 V due to the stronger binding affinity to NO* compared to H*. By co-feeding CO, the FE of Cu/C catalyst towards NH3 was improved by 12 %. More strikingly, for Pd/C, the FE towards NH3 was enhanced by 95 % with CO co-feeding, by effectively suppressing HER. This is attributed to the change of the favorable surface coverage resulting from the selective and competitive binding of CO* to H* binding sites, thereby improving NH3 selectivity.ChemE/Catalysis EngineeringLarge Scale Energy Storag

Influences of Nanostructures of Sn and Ir for the Oxygen Evolution Reaction in Polymer Electrolyte Membrane Water Electrolysis

The influence of nanostructures and interaction of Sn and Ir in oxygen evolution catalysts in a polymer electrolyte membrane electrolyzer were investigated. For this aim, two synthesis methods, namely, the one-step solution combustion method and the precipitation-deposition method with sodium borohydride reduction, were evaluated to prepare distinct nanostructures. Sn addition to Ir-based oxygen evolution reaction catalysts has been reported to yield materials with higher activity; however, in our case, this was observed only for Sn/Ir catalysts prepared by the precipitation-deposition method. The nanolayer of Sn/SnO2 deposited over metallic Ir particles was identified to enhance the interfacial contacts, resulting in synergistic interactions. By deconvolution of the polarization curves into constituting contributions, the performance improvement was attributed to the higher exchange current density of the Sn/Ir powder as a consequence of a higher number of surface reaction sites created by the Sn-Ir interactions.ChemE/Catalysis Engineerin