A nanoarchitectured cermet composite with extremely low Ni content for stable high-performance solid oxide fuel cells

ABSTRACT

A strategy for improving the stability of nickel-based solid oxide fuel cell (SOFC) anodes via compositional and microstructural engineering is presented. Ni content was reduced to 2 vol%, and nanosized Ni particles were uniformly dispersed in a mixed ionic-electronic conducting matrix comprising gadolinium-doped ceria (GDC) using a thin-film technique. Remarkable stability with no performance deterioration even after 100 reduction-oxidation cycles could be observed for the optimized nanostructured anodes. Cell performance at 500°C was enhanced, exceeding 650 mW/cm². This study offers valuable insights for enhancing the durability, performance, and productivity of SOFCs.

Introduction

Solid oxide fuel cells (SOFCs) are highly efficient for the direct conversion to electricity from numerous chemical fuels, including hydrocarbons, natural gas, as well as renewable fuels [1], [2], [3], [4]. To respond to the intermittent availability of renewable energy resources, SOFCs should be stable during continuous operation, as well as instantaneously responsive in on-off modes [4,5]. However, the cessation of fuel supply leads to oxidation of the reduced anode during operation. Repeated on and off switching induces reduction–oxidation (RedOx) cycles at the anode, causing catastrophic failure of the SOFC cell [5,6]. Specifically, the SOFC anode consists of a porous Ni-solid oxide electrolyte cermet, fabricated from a NiO-solid oxide electrolyte composite via a reduction step prior to cell operation [7], [8], [9]. Recent X-ray ptychographic nanotomograpy observed less Ni/YSZ interface area in the reduced state than the NiO/YSZ area in the pristine state, revealing that the nickel network is detached from the YSZ [10]. In addition, Ni metal particles readily agglomerate during cell operation at high temperatures [8,9]. When operation ceases, they are readily re-oxidized [11], resulting in considerable volume expansion. The agglomeration of Ni and volumetric expansion of NiO during RedOx cycling results in the generation of substantial stress and leads to anode cracking, which can propagate through electrolytes [12], [13], [14], [15]. Therefore, failure suppression during RedOx cycling is crucial for the commercialization of SOFCs.

To address the challenges associated with RedOx cycling, two approaches can be applied. First, at the system operational level, RedOx cycling can be prevented by maintaining fuel supply or anti-oxidation gas supply until the internal temperature drops below a certain temperature during shutdown [16], [17], [18]. However, this tactic is not suitable for a depleted and/or interrupted gas supply in an emergency stop. In addition, it decreases energy efficiency. The second approach is to increase the stability of the anode against RedOx cycling by improving the material properties and/or its microstructure. In this regard, the construction of anodes devoid of a metal catalyst has been practiced to eliminate the stress induced by volume changes during RedOx cycling; [19], [20], [21] however, in this case, inadequate anode catalytic activity has hampered the development of an SOFC with reasonable performance. Thus, alternative structures and fabrication methods have been proposed for cermet anodes that can endure RedOx cycling [22,23]. For example, increasing the fracture strength of the anode prevents the destruction of the cell during RedOx cycling [24], [25], [26]. While increasing the number of voids in the anode matrix reduces the stress. Similarly, a thin oxide layer coating deposited over the metal catalyst by infiltration or ALD technique prevents agglomeration [23,27]. Hence, compositional and microstructural engineering can offer meaningful remedies; however, a sufficiently stable structure capable of enduring RedOx cycling has not been reported thus far.

Thin-film electrolyte-based SOFCs (TF-SOFCs) fabricated via vacuum deposition techniques are considered promising candidates for lowering the SOFC operating temperature [2,3,27,28]. Nanostructured electrodes and thin-film electrolytes have been employed, achieving excellent performances at low temperatures [29], [30], [31], [32], [33]. Such low operating temperatures can reduce mechanical and chemical failures in terms of long-term stability. However, most thin film cells exhibited very poor stability which only last for an hour [34]. To further sustain the structural stability of these cell components, a unit cell platform containing various structural scales was recently developed by employing a combination of the powder and vacuum deposition processes [35], [36], [37], [38], [39]. This structural improvement, termed as multiscale-architecturing, has enabled the attainment of high peak power densities (PPDs) of 2 W/cm² at 650°C [37] and structural stability sustained for over 50 thermal cycles [36]. Inspired by this achievement, we hypothesized that superior RedOx cycling stability and electrochemical performance could be realized via compositional and microstructural modulation of a cermet anode based on thin-film technology.

In this study, a novel anode comprising evenly distributed Ni nanoparticles in a highly conductive oxide matrix was designed for achieving high RedOx cycling stability. First, the anode Ni content was substantially reduced to below 10 vol%, which is considerably below the percolation limit (approximately 40 vol% in general) to prevent contact among Ni nanoparticles. Such an exceedingly low Ni content effectively prevented Ni coarsening by blocking direct contact between the particles and resulted in negligible volume changes, as compared a conventional anode containing 40 vol% Ni. Second, to compensate for the decrease in electronic conductivity and cell performance arising from this low Ni content, gadolinium-doped ceria (GDC), a mixed ionic-electronic conductor (MIEC) with high conductivity, was employed as the oxide matrix in the thin-film Ni-cermet anode by using pulsed laser deposition (PLD). As a result, the PLD-derived Ni-GDC nanoarchitecture with extremely low Ni content exhibited a performance comparable to that of a 40 vol% Ni-containing anode. In addition, multiscale-architectured unit cells with these novel anodes exhibited significantly improved cell performance. More importantly, the performance deterioration was negligible even after 100 RedOx cycles for a cell employing the optimal fabricated anode, unlike the cells utilizing conventional anodes. Our novel low-volume Ni-GDC anodes has significantly improved mechanical properties in the RedOx cycle and superior electrochemical performance. To our best knowledge, there is no result of satisfying stability and performance at this high level at the same time up to date. Surprisingly, the optimized 2 vol% Ni-GDC thin-film anode no longer required post-annealing, while conventional thin-film anodes with 40 vol% Ni necessitate an additional post-annealing step (1200 °C) to prevent crack generation caused by agglomeration of as-deposited small Ni particles [40], [41], [42]. This implies that the continuous vacuum deposition of all cell components is possible for anodes of this type. Therefore, this strategy offers insights for the development of stable SOFCs for practical applications and for the mass production of TF-SOFC via a continuous deposition process.