How the surface state of nickel/gadolinium-doped ceria cathodes influences the electrochemical performance in direct CO2 electrolysis
Graphical abstract
Introduction
CO2 is the most common environmental pollutant and is considered largely responsible for global warming through the “greenhouse effect”. In recent years, there is increasing interest to consider CO2 as resource rather than waste, and transform it catalytically to value-added chemicals and fuels [1], [2]. In order to convert CO2 to higher-value products, it should be first transformed to a more chemically active molecule, for example carbon monoxide. Electrolysis of CO2 to CO at elevated temperature using solid oxide electrolysis cells (SOEC) appears as a very promising solution for large-scale energy storage [3], [4], [5], [6], [7], [8], since it offers higher energy efficiency as compared to low temperature conversion methods. In addition, simultaneous electrolysis (co-electrolysis) of steam and CO2 is an alternative route to directly produce synthesis gas (i.e. CO and H2) from non-fossil fuel sources [9], [10].
Despite the great efforts devoted to the design and preparation of new electrocatalysts with high activity and stability, composites of ceramic and metal (cermets) that have been previously optimized for solid oxide fuel cells (SOFC), remain the state-of-the-art (SoA) cathode electrodes for SOEC as well [9], [11]. The two principal SoA cermet cathode electrodes are nickel composites with yttria stabilized zirconia (NiYSZ) or gadolinium doped ceria (NiGDC). Ni oxidation and carbon formation of Ni-based cermet cathodes during CO2 electrolysis are related to cells and stacks performance degradation [3], [9], [12]. To prevent Ni oxidation a reductive gas, such as H2 or CO, is typically mixed with CO2 [6], [10]. However, recent studies demonstrated a stable operation in direct CO2 electrolysis [13], [14], [15]. Please note that the term “direct” [16], or sometimes also called “pure” [13], or “without safe gas” [15], [17] CO2 electrolysis, refers to feed conditions where CO2 is not mixed with a reductant gas.
Carbon formation on Ni and ceria containing electrodes is attributed mainly to the Boudouard reaction (2CO → CO2 + C), while direct electrochemical carbon deposition reactions are also possible [18]. In this respect, C. Graves and coworkers demonstrated that cells with GDC cathodes can operate steadily in CO2/CO mixtures, while in similar conditions their Ni/YSZ equivalents are rapidly deactivated due to carbon deposition [5].
Although significant effort has been devoted to understand the carbon deposition mechanism during CO2 electrolysis over Ni-based cathodes [5], [12], [18], [19], [20], [21], [22], [23], [24], [25], the effect of the electrode surface state in the electrocatalytic performance is much less studied. Typically, literature considers that the active electrode area of NiYSZ is the convergence of Ni, YSZ and gaseous phase, the so-called three-phase boundaries (TPBs), while for NiGDC, the mixed ionic electronic conduction of GDC allows the reaction to take place at the two-phase boundary (2PBs) between GDC and the gas phase [3], [11]. Cells composed of porous doped-ceria fuel electrodes, i.e. without nickel, show considerable CO2 electrolysis activity and stability [5], while the primary role of 2PBs on GDC, as compared to TPB, was also proposed as well for H2O electrolysis [26] and H2 electroxidation [27] reactions. The aforementioned observations put into question the essential role of nickel in electrochemical reactions over NiGDC, which is somehow surprising considering that SoA NiGDC electrodes are composed of 65 wt% nickel.
We have previously studied the role of nickel surface state in the electrocatalytic performance of NiGDC electrodes during H2O electrolysis [26], [28], showing that partially oxidized nickel does not lead to deactivation, but on the contrary, can be even beneficial for the cell performance. The aim of the present work is to examine how nickel and ceria oxidation states affect the CO2 electrolysis performance and, in particular, to find evidences about the role of nickel in the reaction. To do so, we combine operando near ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) with online gas phase and electrical measurements over cells with porous NiGDC cathodes. It is important to underline that, contrary to the majority of CO2 electrolysis NAP-XPS studies which concentrate to the carbon deposition mechanism [5], [20], [23], [24], [29], [30], [31], [32], here we employed operational conditions bellow the carbon deposition threshold, in order to focus on the surface oxidation state and lift complications related to modifications due to irreversible carbon deposition.
Section snippets
Fabrication of cells for NAP-XPS experiments
The cell was composed of a 40 µm thick NiO/Gd0.1Ce0.9O2 (65/35 wt% ratio) cathode (fuel electrode) with mass loading of approx. 25 mg/cm2, deposited by screen printing method directly on a 300 µm thick 8YSZ electrolyte (Kerafol GmbH). A Pt layer on the reverse side, deposited by magnetron sputtering, was acting as the anode (O2 electrode). The samples were sintered in air at 1250 °C for 5 h. Due to limitations related to the NAP-XPS sample holder dimensions, fragments of button cells with
Interaction of CO2 with NiGDC without polarization
Initially, we follow the capacity of CO2 atmosphere to modify the surface oxidation state and composition of NiGDC under the specific conditions of the NAP-XPS experiment. To examine this, the NiGDC was pre-treated in 0.1 mbar H2, O2 or H2O and subsequently the gas atmosphere switched to 0.1 mbar CO2 at 700 °C. Since the effect of H2, O2 or H2O pre-treatment on the NiGDC surface state has been discussed in detail previously [35], here we focus on CO2. Fig. 1a shows Ni 2p and Ce 3d NAP-XPS
Conclusions
Summarizing, operando NAP-XPS was applied to examine NiGDC cermet cathodes during CO2 electrolysis at potentials below the threshold of carbon deposition. It was shown that the functional NiGDC surface consists of metallic nickel and partially reduced ceria. Activation of the CO2 electrolysis reaction is correlated to electro-reduction of NiO to Ni, which is feasible at relatively low CO2 electrolysis potentials. Partially reduced nickel particles are organized in a core-shell structure, with
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
DC would like to thank the China Scholarship Council (CSC) for supporting his studies at ICPEES. Y.T. Law help during BESSY synchrotron measurements is greatly appreciated. The authors would like to thank the electronic structure group and especially Dr A. Knop-Gericke, Dr M. Haevecker and Dr D. Teschner, of the Fritz-Haber Institute der MPG and Helmholtz Zentrum, Berlin Germany for providing access at ISISS beamline. The authors would also like to thank Mr. P. I. Giotakos, MSc. researcher at
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