A mixed electronic and protonic conducting hydrogen separation membrane with asymmetric structure

Abstract

In this work, highly doped ceria with lanthanum, La_0.5Ce_0.5O_2−δ (LDC), are developed as hydrogen separation membrane material. LDC presents a mixed electronic and protonic conductivity in reducing atmosphere and good stability in moist CO₂ environment. LDC separation membranes with asymmetrical structure are fabricated by a cost-saving co-pressing method, using NiO + LDC + corn starch mixture as substrate and LDC as top membrane layer. Hydrogen permeation properties are systemically studied, including the influence of operating temperature, hydrogen partial pressure in feed stream and water vapor in both sides of the membrane on hydrogen permeating fluxes. Hydrogen permeability increases as the increasing of temperature and hydrogen partial pressure in feed gas. Using 20% H₂/N₂ (with 3% of H₂O) as feed gas and dry high purity argon as sweep gas, an acceptable flux of 2.6 × 10⁻⁸ mol cm⁻² s⁻¹ is achieved at 900 °C. The existing of water in both sides of membrane has significant effect on hydrogen permeation and the corresponding reasons are analyzed and discussed.

Introduction

In modern society, H₂ as a sustainable and clean energy source is in great demand, which has driven the research of hydrogen production [1], [2], [3]. Hydrogen separation from the fossil fuel gas is one of the most important hydrogen-producing technologies and has attracted extensive interest in last few decades [4], [5], [6]. A dense ceramic membrane with mixed protonic and electronic conductors (MPEC) is cost-saving and high-selectivity, compared with other membrane separation technologies. Due to existing of electrons and protons in membranes, the H₂ separation is achieved in a non-galvanic mode without the need for electrodes or electrical circuitry. The driving force for the hydrogen transport via coupled diffusion of protons and electrons through MPEC membranes is hydrogen partial pressure gradient across membranes [7].

In current study, there are two types of ceramic hydrogen separation membranes. One is the metal–ceramic composite membrane, in which metal is used as electron conducting phase and ceramic oxide is only a single protonic conductor, e.g., Ni-BaZr_0.1Ce_0.7Y_0.2O_3−δ [8], [9], [10], Ni-La_0.5Ce_0.5O_2−δ [11], [12] separation membrane. The other is the single ceramic oxide, which meanwhile can transport protons and electrons, e.g. doped-ACeO₃(A = Ba, Sr) hydrogen separation. Doped SrCeO₃ perovskite membranes have been extensively studied because of their excellent mixed protonic and electronic conductivity at high temperature, in which electronic conductivity is achieved by doping the B site with a multivalent cation [13], [14], [15]. Unfortunately, BaCeO₃, SrCeO₃ showed badly chemical stability in acidic gas (e.g. CO₂ or H₂S) due to existing of alkali element [16], [17]. The seeking of new proton-conducting material has never been broken off because it is of great significance for application of SOFC and separation membrane. Although some high temperature proton conductor (e.g. LaNbO₄ [18] and BaCa_1.18Nb_1.82O₉ [19], Ln₆WO₁₂ [20]), have been investigated as SOFC electrolyte or hydrogen separation membrane, the low conductivity blocks their further development.

In this paper, a single-phase La_0.5Ce_0.5O_2−δ (LDC) with mixed electronic and protonic conduction is developed as hydrogen separation membrane. The fact that, highly doped ceria, La_0.5Ce_0.5O_2−δ, owns proton conductivity, has been reported. Wang et al. applied Ca-doping La_0.5Ce_0.5O_2−δ as electrolyte to ammonia synthesis electrochemically [21]. In our experimental group, Fang and Yan also successfully prepared Ni-La_0.5Ce_0.5O_2−δ hydrogen separation membrane and obtained good hydrogen permeation performance respectively [11], [12]; Tao et al. [22] assembled a solid oxide fuel cell with La_0.4875Ca_0.0125Ce_0.5O_2−δ electrolyte and found that the open circuit voltages (OCVs) are below theoretical values at each temperature, 0.832 V at 700 °C, 0.847 at 650 °C and 0.861 at 600 °C. The reaction, Ce⁴⁺ + e = Ce³⁺, occurs at elevated temperatures in a reducing atmosphere, such as at the anode, which leads to electronic conductivity and then significantly lowers the OCV of the SOFC. Therefore, LDC is a mixed electronic and protonic conductor and has potential application in hydrogen separation.

Usually an asymmetrical structure is generally attempted for many gas separation membranes to improve permeability of separation membrane, for example, oxygen separation [23], Pd hydrogen separation membrane [24], as well as SrCeO₃ hydrogen separation membrane [25]. Herein, considering that thin LDC membrane contributes to promoting hydrogen permeability, the LDC asymmetrical membranes composed of porous Ni + LDC substrate and dense LDC top membrane were prepared by co-pressing and co-firing process and the effect of temperature, water and H₂ partial pressure on hydrogen permeability was studied and discussed.