Boosting Oxygen Electrode Performance via a Redox-Treatment

Introduction The transition to a sustainable energy system complying with climate policy targets is a huge societal challenge. “Hard to electrify” sectors like aviation and global shipping will be especially difficult to transition. These sectors will need vast amounts of sustainable synthetic fuels. Key to their synthesis will be the deployment of large-scale electrolysis units to produce the hydrogen needed. Solid oxide electrolysis cells (SOEC) offer a distinct efficiency advantage compared to low-temperature analogs. Co-locating fuel production with the hydrogen production will further benefit efficiency via the use of waste heat from fuel synthesis in the endothermic steam splitting process. These years, R&D in SOEC is expanding, and investments in building production capacity are increasing. Today, the area-specific hydrogen production capacity and the lifetime of the cells are mostly dictated by the fuel electrode [1]. However, for future large-scale deployment, improvement of the oxygen electrode is also desirable. Perovskites like La 1-x Sr x Co y Fe 1-y O 3-δ (LSCF) and La 1-x Sr x CoO 3-δ (LSC) offer fast oxygen exchange and electrodes based on these materials provide good performance for cell types targeting operation at 750 o C and above. It will be advantageous though, to develop faster electrodes enabling a reduction in operation temperature, as this could reduce the rate of degradation processes and reduce the cost of auxiliary components. Moreover, it would be advantageous to reduce the amount of Co used, since this is in high demand for other technologies, it is carcinogenic, and raw materials are mined under criticizable conditions. To develop faster oxygen electrodes, we have recently started focusing on various pre-use treatments that could boost the performance of already utilized materials. The advantage of this approach is that one can stay with proven manufacturing routes and materials that have adequate electronic conductivity and suitable thermal expansion coefficient. Firing conditions can then be optimized to ensure strong adherence and good bulk transport properties as the electro-catalytic performance is boosted via the subsequent low-temperature treatments. At the same time, pre-use treatments which only modify the surface of the materials can be a route to develop a better understanding of the factors that dictate the oxygen exchange rate. Staying with one backbone material and doing various surface modifications, one can potentially ascribe changes in performance to the realized changes. A stronger focus on the surface composition and structure over the outer 1-10 unit cells of the electrode material [2,3] will likely lead to a better understanding of the electrode process, and can supplement the classical approach of relating performance to overall composition and bulk properties. Results from three different pre-use treatments shall be briefly discussed here; 1) infiltration with Pr-nitrates, 2) “washing” in ammonia water, and 3) redox treatment. The treatments were applied on several different electrode backbones including; LSF, LSF/CGO-composites, and LSCF/CGO-composites. In some cases, the treatments were combined. Experimental Most of the investigations were carried out on symmetrical cells, where the electrodes were applied to both sides of an electrolyte. The electrode performance was tracked in a one-atmosphere setup. For some of the more promising treatments, the performance has also been tracked at full cell level [4]. The symmetrical cells were prepared on 5·5 cm 2 pieces of 150 μm dense scandia- (KERAFOL, Zr doped with 10 mol% Sc 2 O 3 and 1 mol% CeO 2 ) or yttria-stabilized zirconia (8YSZ, KERAFOL) electrolytes. The electrolytes were coated with a 1 μm layer of Ce 0.9 G 0.1 O 2 (CGO) via PVD. Electrodes were fabricated via screen printing using 35/65 wt% (La 0.6 Sr 0.4 ) 0.98 FeO 3-δ /Ce 0.9 Gd 0.1 O 2 (LSF/CGO), 50/50 wt% La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O/Ce 0.9 Gd 0.1 O 2 (LSCF/CGO) or (La 0.6 Sr 0.4 ) 0.98 FeO 3-δ . The LSF/CGO was fired at 1000 o C for 4 hr., the LSF at 1050 o C for 5 hr., and the LSCF/CGO at 1080 o C for 2 hr. The electrode thickness was ~20 μm after firing. The fabricated cells were laser-cut into ~1 cm 2 pieces and tested in a rig enabling simultaneous test of 4 cells. Pt-paste and Pt-meshes were used for contacting [5]. The LSF and the LSF/CGO backbone electrodes discussed here were prepared at different times using slightly different procedures. The LSF powder was from the same batch in all cases. In one case, the electrode was fired at a slightly higher temperature of 1100 o C (data set 3, labelled “Tong”). In two cases the symmetrical cells were prepared on CGO electrolytes (data sets 3 and 4) also acquired from Kerafol. Infiltration precursors of 0.5 M were prepared by dissolving Pr(NO 3 ) 3 ·6H 2 O in MiliQ water. A “washing” solution with a pH of 11.5 was prepared by adjusting the pH of MiliQ water with ammonia-water. The small symmetrical cells were infiltrated by submerging them in the solutions for approximately 60 s. Excess solution was wiped away, and the cells were calcined in air for 30 min at 350 °C. The process was repeated at least twice. The cells were heated to 800 o C (for set 3 and 4 only to 750 o C) prior to performance characterisation at various temperatures from 750 o C to 550 o C. The redox treatments were implemented flowing a mixture of N 2 /H 2 /H 2 O over the cells (defining an Emf of less than -900 mV versus air) and subsequently returning to air. The realised oxygen partial pressure was measured using a zirconia sensor placed in the test compartment. The reduction was carried out at 650 o C for 10 hr. (“Tong”, data-sets 3 and 4) or at 700 o C for 10 hr. (“Humlebæk”, data-sets 1 and 6). After the reduction and a short purge in N 2 the samples were re-oxidized for 4 hours, prior to characterisation. Results and Discussion The results of various pre-use treatments are compared in Figure 1. In all cases, the values reported are the polarization resistances of one electrode measured at 700 o C. Evidently, the performance of the three slightly different LSF electrodes vary somewhat - the two better ones showing an Rp of 0.25 – 0.35 Ohm cm 2 (1050 o C firing) and the “worst” providing an Rp of 0.85 Ohm cm 2 (1100 o C firing, graphite pore-former). The numbers quoted in the figure are averages measured over at least three samples cut from the same bigger cell. The spread between the pieces is less than 20%. In terms of response of the electrodes to Pr-infiltration, washing and redoxing the results are consistent. These shall be discussed in turn below. Effects of surface decoration via nitrate-precursor infiltration. We have recently reported findings on modifying performance of LSF/CGO and similar composites via infiltrating with Pr-nitrate and several other infiltrates [6,7]. The approach works well in boosting performance both of LSF, LSF/CGO- and LSCF/CGO-composites, where a reduction of Rp by a factor 3-3.5 has been observed [7]. The treatment leaves a layer of nano-structured Pr-oxide on the electrode, which is well established to have high catalytic activity for oxygen reduction/ evolution [7]. Effects of washing The use of Pr is effective in boosting performance, however, it is not without drawbacks, as it is a relatively scarce and expensive element. Recently, Nicollet et al. [8] demonstrated on samples of porous Pr 0.1 Ce 0.9 O 2-δ (PCO) that cheap infiltrates like Ca-nitrates and Li-nitrates could result in dramatic changes in oxygen surface exchange rates. PCO, albeit a good oxide ion conductor and fair electro-catalyst, is not a practical material due to very large and very un-linear TEC. Inspired by the work, we have tried to use a range of different cheap infiltrates to identify possible beneficial effects on technological electrodes like LSF/CGO- or LSCF/CGO-composites. Results are presented in a separate contribution at this conference [9]. During this work, we also investigated the effects of a treatment where the electrodes are simply immersed in water, an acid, or a base to see if this part of the infiltration process itself could have an effect. Indeed, there is such an effect, and it seems to be even stronger than the effect of the decoration left [9]. The results of the washing in a strong base are summarized in Fig. 1. For both LSF and LSF/CGO an improvement by a factor of ~2-3 can be achieved, whereas the LSCF/CGO showed no marked effect. Hence, as a “pre-use” treatment the washing treatment is very effective for the LSF-based electrodes. Effects of a redox treatment. We have previously shown that classical oxygen electrodes like LSCF/CGO may “survive” a redox treatment prior to use, and that the treatment can in fact improve performance slightly [5]. Here, more investigations along that line are reported. Evident from the results in Fig. 1, all the electrodes respond well to a redox-treatment and performance is improved by a factor of ~3 (see sets 1, 3, 4, 6). It should be noted here that the performance of the LSCF/CGO (set 6) and the LSF (set 1) is comparable to state-of-the art electrodes based on these compositions. Hence, the reported improvement is noteworthy and could be of practical importance. Interestingly, the redox treatment also works to improve the performance of an LSF electrode, where the performance was already boosted via Pr-nitrate infiltration. After redox, the Rp of the Pr-infiltrated LSF electrode is ~35 mOhm cm 2 at 700 o C. Long term durability The long-term durability of the LSF electrode (set 3) and the Pr-infiltrated LSF electrode (set 4) was also investigated. The results are summarized in Figure 2, showing Rp as a function of time (upper part) and the Rp measured at 650 o C after an initial mild treatment in N 2 and the redox treatment. Evidently, a mild redox in N 2 hampers performance, whereas a significant improvement is observed after the N 2 /H 2 /H 2 O- redox. After redox, the reduced Rp increased slowly (at a rate of ~0.8 Ω·cm 2 /1000 hr.) for the subsequent 100 hr. over which it was tracked. This is less than observed for the untreated electrode. Post test, structural and chemical characterization of the electrodes revealed no structural damage. The LSF electrodes appeared structurally unaltered as investigated by microscopy, whereas on the LSCF, some exsolved Co 3 O 4 nanoparticles were observed. XPS investigations are ongoing and will be discussed in the presentation. Common to the treatments is that they leave the surface in a different state, than the one that forms during the final steps of electrode manufacture. The treatments result in a surface configuration (nanostructure and chemical composition) that is not the normal state at the test temperature, which benefits performance. The demonstrated benefit of “pre-use” redoxing of the oxygen electrode points to, that cells can actually be manufactured in a reduced state, which can facilitate modification the fuel electrode via infiltration and post manufacture quality control. Conclusion Several different pre-use treatments to improve LSF, LSCF/CGO, and LSF/CGO composite oxygen electrodes were investigated, including surface modification with Pr-O, a washing in a basic solution, and a redox treatment. All three treatments that leave the surface in a state different from the normal state after oxygen electrode firing, improve electrode performance very significantly and reduces polarization resistances by more than a factor of 3. The redox treatment was found to be the most effective resulting in very low polarization resistances at 700 o C of 0.07 Ohm cm 2 , 0.035 Ohm cm 2 , and 0.026 Ohm cm 2 for LSF, Pr-O-decorated LSF and LSCF/CGO, respectively. Moreover, we observed no adverse effect on electrode durability, making this a feasible route to boost electrode performance of state-of-the-art oxygen electrode materials at the technological level. Acknowledgments The Independent Research Fund of Denmark is gratefully acknowledged for financially support through the project “FASTER” (grant no. 2035-00308B). References S. Ovtar et al, Nanoscale, 11, 4394– 4406 (2019). Đ. Tripković et al., Chem. Mater. , 34 , 1722-1736 (2022). M. Kubicek et al., J. Electrochem. Soc. 158 B727 (2011). X. Tong et al., Electrochem. Soc. , 2020, 167, 024519. Khoshkalam et al., J Electrochem. Soc. , 167 , no. 2, p. 024505, (2020). M. Khoshkalam et al., J Power Sources , vol. 457, p. 228035, (2020). P. V. Hendriksen et al., ECS Trans. , 91 , 1, pp. 1413–1424, (2019). Nicollet et al., Nat Catal. , vol. 3, no. 11, pp. 913–920, (2020). V. Humlebæk Jensen, S. Aw-Ali, M. Khoshkalam and P. V. Hendriksen, ibid. Figure 1