Elsevier

Applied Surface Science

Volume 473, 15 April 2019, Pages 746-749
Applied Surface Science

Full Length Article
High-temperature steam electrolysis combined with methane partial oxidation by solid oxide electrolyzer cells

https://doi.org/10.1016/j.apsusc.2018.12.128Get rights and content

Highlights

  • High temperature steam electrolysis was combined with methane partial oxidation.

  • The operating voltage of the electrolyzer cell was significantly reduced.

  • Hydrogen was produced both from water vapor (cathode) and methane (anode).

Abstract

Solid oxide electrolyzer cells consisting of scandia-stabilized zirconia (ScSZ) electrolyte, Ni-yttria-stabilized zirconia (YSZ), and lanthanum strontium ferrite (LSCF)-gadolinia-doped ceria (GDC) were fabricated, and their performance in high-temperature steam electrolysis combined with methane partial oxidation was investigated. The voltage of the electrolyzer cell operated with methane/steam input was significantly reduced compared to that of the cell with air/steam input. The hydrogen production rates at 1.3 V (850 °C) were 4.53 and 1.74 sccm for the electrolyzer cell with methane/steam and air/steam inputs, respectively. On-line gas chromatography analysis confirmed the production of hydrogen and carbon monoxide in the anode side, indicating that methane partial oxidation occurred at the anode side.

Introduction

Electrolytic production of hydrogen is known to be an environmentally benign technique for hydrogen production because it does not consume fossil fuels and emits no greenhouse gases such as carbon dioxide or nitric oxide [1], [2], [3]. Hydrogen can be produced from water (at low temperature) or steam (at high temperature) using a simple electrochemical device. In general, hydrogen production through electrolysis can be performed with significantly higher efficiency by operating in the steam phase than in the water phase [4], [5]. In addition, a water electrolyzer that is operated at room temperature requires precious metals whose high costs make their widespread use prohibitive. In this regard, high-temperature steam electrolysis (HTE) using a solid oxide electrolyzer cell (SOEC), consisting of an oxygen ion-conducting electrolyte and oxide catalytic electrodes, has attracted much attention [6].

The main drawback of electrolytic hydrogen production is the large amount of electricity required. Although the efficiency of the electrolytic process itself is higher than 90% above 800 °C [7], [8], the overall efficiency of the electrolyzer is low, considering the fact that the production of electricity has an average efficiency of less than 40%. On the other hand, the energy efficiency of the steam methane reforming process is known to be as high as 70%, depending on the reaction temperature and steam/carbon ratio [9]. As a result, electrolytic hydrogen production is more expensive than steam-reformed hydrogen.

In an SOEC, water vapor is supplied to the cathode side of the cell, oxygen ions are transported to the anode through the electrolyte, and hydrogen is produced at the cathode side. If the air in the anode is replaced by methane, the thermodynamic conditions are more favorable; specifically, the molar Gibbs energy (ΔG) and enthalpy change (ΔH) of the reaction drop from 187.033 and 248.46 kJ·mol−1 for the H2O/O2 system to −17.505 and 75.541 kJ·mol−1, respectively, for the H2O/CH4 system at 827 °C [10]. The methane supplied to the anode of an SOEC can react with oxygen pumped through the solid electrolyte and become oxidized to yield a mixture of carbon monoxide, carbon dioxide, water vapor, and hydrogen, depending on the operating conditions such as temperature and methane to oxygen ratio [11]. By using a catalyst that drives the partial oxidation of methane in the anode, it is expected that the hydrogen production efficiency of SOECs can be further improved.

In this study, an electrolyte supported solid oxide electrolyzer cell consisting of a Sc2O3-stabilized ZrO2 (ScSZ) electrolyte, nickel/yttria-stabilized zirconia (Ni/YSZ) cermet cathode, and lanthanum strontium cobalt ferrite (LSCF) anode was fabricated and stream/hydrogen and methane were fed to the cathode and anode, respectively. The electrolysis performance of the cell and its methane oxidation behavior were investigated.

Section snippets

Experimental procedure

An electrolyte-supported Ni-YSZ/ScSZ/GDC/LSCF cell was fabricated in this study. Commercially available ScSZ (89 mol% ZrO2-10 mol% Sc2O3-1 mol% Al2O3, Daiichi Kigenso Kagaku Kogyo Co., Ltd, Osaka), La0.6Sr0.4Co0.2Fe0.8O3-d (LSCF10, Fuel Cell Materials, LA), Ni-YSZ (Fuel Cell Materials, LA) and gadolinium-doped ceria (GDC) powders were used to prepare the cell.

First, the ScSZ powder was uniaxially pressed into a pellet and sintered at 1400 °C for 5 h in air. ScSZ disks with a diameter of 22 mm

Results and discussion

A cross-sectional SEM image of the LSCF-GDC (anode)/GDC (buffer)/ScSZ (electrolyte)/Ni-YSZ (cathode) electrolyzer cell is shown in Fig. 2. The LSCF-GDC anode, GDC buffer layer, and Ni-YSZ cathode were approximately 30, 10, and 20 μm thick, respectively. The LSCF-GDC and Ni-ScSZ layers were sufficiently porous, whereas the ScSZ electrolyte was relatively dense (98% of the theoretical density). The GDC buffer layer exhibited good interfacial contact with the LSCF-GDC layer and ScSZ electrolyte,

Conclusions

Electrolyte-supported solid oxide electrolytic cells were made with ScSZ electrolyte, Ni-YSZ, and LSCF-GDC, and their steam electrolysis performance combined with methane partial oxidation was investigated. The operating voltage of the electrolyzer cell was significantly reduced when methane instead of air was supplied to the anode side. The methane reacted with evolved oxygen which was produced by steam electrolysis, reducing the chemical potential of the electrolyzer cell. A hydrogen

Acknowledgements

This work was funded by an Inha University research grant.

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