• Li, Qingfeng
• Prag, Carsten Brorson
• Petrushina, Irina
• Bjerrum, Niels Janniksen
• Christensen, Erik

Abstract

Water electrolysis for hydrogen production has been predicted to get a prominent role in the energy system of the future. Current low temperature technologies rely on expensive noble metal catalysts and high temperature systems requires special construction materials to withstand the high temperatures. Electrolysis in the intermediate temperature (IT) region (200-400 °C) is of interest as it would allow for the use of non-noble metal catalysts, due to the improved kinetics, and a wide range of construction materials as a result of the more benign temperature. At these temperatures water is supplied as steam.This work centred on the design and development of a novel steam electrolysis concept based on phosphate electrolytes capable of operating in the IT range. Central for the work was the selection and evaluation of the materials and components for the test setup and cells as well as the technological issues and challenges faced.<br/>A setup suitable for intermediate temperature electrolysis has been constructed in order to accommodate testing in the IT region. This included the evaluation of multiple generations of components such as end plates and flow plates.Chemical vapour deposition of tantalum was used to protect stainless steel components from the highly oxidative environment of the oxygen side of the electrolyser. While such protection should not be necessary on the hydrogen side, it was found that the best results were obtained using tantalum coated stainless steel flow plates not only on the oxygen side but at the hydrogen side as well.<br/>Additional key steps and components for electrolysis testing are detailed in this thesis. This includes gas diffusion layers (GDL), sealing, cell assembly techniques, test operation, electrolytes and electrocatalysts.Gas diffusion layers of carbon with a PTFE bound micro-porous layer was used for the cathode side and tantalum coated stainless steel felt was used for the anode side due to the need of corrosion protection. For the cathode side a platinum electrocatalyst was used as benchmark (Pt-black ≈ 8 mg/cm²) and iridium oxide was used for the anode (≈ 3 mg/cm2). Symmetrical cell testing for hydrogen pumping at 200 _C revealed the cathode gas diffusion layers to be unstable over time. After 60 hours, the electrode resistance was more than tripled. The most prominent reason for this was thought to be a softening of the PTFE in the cathode micro-porous layer.<br/>CsH₂PO₄ and Sn_0.9In_0.1P₂O₇ were used as proof-of-concept electrolytes, with emphasis on the latter electrolyte. Evaluation of electrolysis cells with these electrolytes was done with a range of tools constantly under development.These tools included regression analysis of I-V curves, reference electrode measurements and electrochemical impedance spectroscopy (EIS). While reference electrode measurements were found hard to optimise, EIS, and especially complex non-linear least-square (CNLS) fitting, was found very useful. CNLS allowed for the estimation of electrolyte resistance and polarisation resistances giving a detailed view of the novel system.<br/>Electrolysis with CsH₂PO₄ as electrolyte revealed a need for steam on both cathode and anode in order to prevent dehydration of the electrolyte. Additional stabilisation in the form of SiC fibres was found to increase longevity considerably. Highest achieved current density was 60 mA/cm² at 2.0 V and 250 °C.<br/>Measurements using Sn_0.9In_0.1P₂O₇ as electrolyte, Pt black as cathode electrocatalyst and IrO₂ as anode electrocatalyst gave current densities as high as 313 mA/cm² at 1.9 V and 200 °C. The stability of the electrolyte was found to be high at 200 °C and a water partial pressure of 0.05 atm. For stabilisation of the electrolyte at 250 °C a higher water partial pressure is needed. Variation of temperature from 200-250 °C showed both signs of activation of electrode processes and electrode degradation.<br/>Efforts were done to optimise the synthesis of Sn_0.9In_0.1P₂O₇ in order to establish a reproducible synthesis procedure. The synthesis used in this work required two heat treatment steps. Fourier transform infrared spectroscopy (FT-IR) shows an O-H band in the IR spectrum from 1500 cm^-1 to 3800 cm^-1 strongly dependent on the first heat treatment step of the synthesis. It was found that initial heating of the synthesis precursors to 270 _C gave a high quality sample in a reproducible fashion.<br/>Investigations of two additional novel phosphates was attempted. These were phosphoric acid treated Nb₅P₇O₃₀ and a mixture of Bi₂P₄O₁₃, BiPO₄ and 2 wt.% Polybenzimidazole (PBI). Both were found to be lacking in stability.As a cent...

Topics

• Deposition
• porous
• density
• Carbon
• stainless steel
• corrosion
• Oxygen
• Platinum
• Hydrogen
• electrochemical-induced impedance spectroscopy
• activation
• current density
• Fourier transform infrared spectroscopy
• tantalum
• Iridium