Cost-effective electrodes for alkaline water electrolysis
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Green hydrogen production using water electrolysis has potential to be the key component in storing and coupling renewable energy to the transportation and chemical sectors. Alkaline water electrolysis offers a relatively low capital cost and simple device for producing hydrogen from electrical energy. The alkaline environment has the advantage of enabling the use of a wider selection of materials and longer life times than the acidic and high temperature alternatives. However, a further reduction in capital and operating costs are highly desired and can be mitigated through scaling up of manufacturing plant, using cheap, durable and active materials, and optimize cell design and operation. All these fields are currently intensively investigated. For instance, stainless steel is considered as a cost-effective electrode alternative in alkaline water electrolysis. However, untreated stainless steel is not active, which calls for implementing efficient surface activation procedures. The purpose of this PhD work is to contribute to the production of cost effective, active and stable stainless steel and nickel-based electrodes for both oxygen and hydrogen evolution in alkaline water electrolysis. The performance of the modified electrodes are assessed in 1 M KOH electrolytes at room temperature using a 3-electrode electrochemical cell. Selected electrodes are then assessed in a commercial alkaline water electrolysis test station and compared to ordinary nickel electrodes. An activation procedure that includes holding the potential at 1.70 V for 18 hours in KOH electrolyte is found to increase the oxygen evolution reaction performance of all electrodes. The activated electrodes are characterized using XPS, GD-OES, Raman and SEM in addition to electrochemical characterization. The Ni content in the surface depends on the KOH concentration, and increases from 41% in 1 M KOH to 73% in 7.5 M KOH, or higher concentrations. The resulting surface compositions are explained in terms of the point defect model. The KOH concentration in the activation procedure is used to tailor the surface composition of Ni, Fe and Cr for optimized oxygen evolution reaction activity. Activated stainless steels using 7.5 M KOH gives a surface composition comprising of 73% Ni and 27 % Fe and leads to the highest OER activity of all samples, with a stable performance at 10 mA cm-2 over a 48 h test in 1 M KOH. We also show that Cr in the surface oxide is beneficial towards the OER. Activated stainless steel 316 and stainless steel 304 perform better than activated Incoloy 800, Inconel 718 and pure Ni electrodes. A Ni mesh along with untreated and activated stainless steel 316 meshes are investigated insitu in 30 wt% KOH at various temperatures and pressures in an alkaline water electrolyzer test station. The activated stainless steel 316 anode is tested in the test station for 255 hours at a constant current of 20 A (0.8 A cm-2 ) at 80°C and 9 bar. Negligible degradation in the performance of the cell is observed. Moreover, the morphology and change in composition of both anode and cathode after the 255 hours in-situ operation are investigated showing formation of deposits at both electrodes. Successful activation of stainless steel 316 is also obtained during in-situ operation in the alkaline water electrolysis cell applying 44 A (1.76 A cm-2 ) for 18 hours in 30 wt% KOH, 2 bar and 25°C. Iron and copper deposition from the KOH electrolyte is found after use and affects the HER activity. The deposition and impact on HER activity depend on the KOH purity, concentration and applied potential. The HER activity of Ni-based and stainless steel electrodes are improved when pre-treated 24 h at constant potential of -0.6 V vs. RHE in low grade 7.5 M KOH electrolyte. An alkaline water electrolysis cell employing an activated stainless steel 316 anode (activated for 18 hours at 1.70 V in 7.5 M KOH) combined with a pre-treated stainless steel 316 cathode (pre-treated 24 hours at -0.6 V in 7.5 M KOH) show an improvement in overall cell current density by ≈ 330 mA cm-2 at 2.0 V compared to a cell with pure nickel electrodes in 30wt% KOH at 80°C and 9 bar.