Abstract
The solid oxide electrolysis cell (SOEC) technology has a huge potential for future mass production of hydrogen, mainly due to its high electrical-to-chemical energy conversion efficiency. However, the durability and the performance of SOEC devices are inferior to that of other competitive electrolysis technologies inhibiting the commercialization of SOECs. Despite the fact that Ni-based cermets are currently the most widely used cathode materials for SOEC, change of the nickel oxidation state has been accused as a major issue limiting the performance of these devices. In this work we provide operando experimental evidence of the active surface oxidation state and composition of nickel/doped-ceria cermets under water electrolysis conditions using ambient pressure X-ray photoelectron and near edge X-ray absorption fine structure spectroscopies, combined with quantitative spectra simulation. Remarkably under specific operational conditions, nickel is maintained in a partially oxidized state which, counterintuitive to the expected behavior, can be beneficial to the cell performance. This finding may initiate new improvement strategies for SOEC electrodes based on thorough optimization of the operational conditions, in order to engineer in situ the most propitious electrode configuration.
Highlights
Renewable electricity sources, such as wind and solar, are usually intermittent in the sense that are difficult to predict and are not continuously available
We show that the surface of the electrode during steam electrolysis is determined by a complex interplay between the thermochemical oxidation due to water vapor and the electrochemical reduction of nickel
Preparation of solid oxide electrolysis cell (SOEC) cathodes involves high temperature sintering in air (>1000 °C) which results in an oxidized electrode
Summary
Renewable electricity sources, such as wind and solar, are usually intermittent in the sense that are difficult to predict and are not continuously available. The main reason is the fact that by increasing the operating temperature the demand of electrical energy is significantly reduced and in some cases the electrical-to-chemical energy conversion efficiency can exceed 100 % This can lead to a reduced cost of hydrogen production, especially if the required heat energy is provided by Joule heat that is inevitably produced by the internal electrical resistance of the cell, or by waste heat of high-temperature industries [4],[7]. In practical applications, mixing ceria with metals (typically nickel) provides the adequate electronic and ionic conductivities as well as the electrocatalytic activity at intermediate temperatures, which is an ideal combination for the fuel (steam) electrode in IT-SOECs [15],[16]. In contrast to the common conception we provide a new paradigm, which demonstrates that nickel oxide is not always detrimental for the electrode performance, but in some cases it can be beneficial for its stability
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