Ag decorated (Ba,Sr)(Co,Fe)O3 cathodes for solid oxide fuel cells prepared by electroless silver deposition

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 2 4 1 3 e2 4 2 0

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Ag decorated (Ba,Sr)(Co,Fe)O3 cathodes for solid oxide fuel cells prepared by electroless silver deposition Ren Su a,b, Zhe Lu¨ a,**, San Ping Jiang c, Yanbin Shen b, Wenhui Su a, Kongfa Chen c,* a

Center for the Condensed-Matter Science and Technology, Department of Physics, Harbin Institute of Technology, Harbin 150001, China Interdisciplinary Nanoscience Center (iNANO), Aarhus University, DK-8000 Aarhus, Denmark c Fuels and Energy Technology Institute and Department of Chemical Engineering, Curtin University, Perth, WA 6102, Australia b

article info

abstract

Article history:

We reported nano-structured Ag modified Ba0.5Sr0.5Co0.6Fe0.4O3
d (Ag@BSCF) cathode for

Received 1 October 2012

solid oxide fuel cells (SOFCs) that is prepared by vacuum assisted electroless deposition

Received in revised form

technique. We show that the concentration of Ag can be easily adjusted by tuning the

13 November 2012

deposition time without altering the perovskite structure of the pristine BSCF. The effect of

Accepted 23 November 2012

Ag loading on the electrochemical performance of the material has been systematically

Available online 23 December 2012

studied by varying the Ag loading and the working condition (oxygen partial pressure). An optimized electrode performance is observed with an Ag loading of w2 wt%. We demon-

Keywords:

strate that the presence of Ag significantly reduces the electrode ohmic resistance and

Solid oxide fuel cells

enhances the catalytic O2 reduction performance of the BSCF cathode.

BSCF cathode

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

Electroless deposition

reserved.

Silver nanoparticles

1.

Introduction

Solid oxide fuel cells (SOFCs) are among the most efficient energy coversion technologies that electrochemically convert the chemical energy of fuels (e.g., hydrogen, methane) to electricity with very low emission of air pollutants [1e3]. Lowering the operation tempeature from high temperature (w1000  C) to an intermediate temperature range (600e800  C) will promote significant advantages, such as more choices of electrode and electrolyte materials, reduction of cost, increase of lifespan, and use of metallic interconnect materials [4,5]. However, the cell performance substantially decreases at reduced temperatures due to the lower ionic conductivity of the electrolyte and reduced activity of the anodes and in particular the cathodes. O2 reduction reaction on the cathodes

is the most sluggish process through the whole fuel cell reaction when the temperature is lower than 700  C [6,7]. The perovskite structured (La,Sr)MnO3 (LSM) is the most commonly used SOFC cathodes [8e10]. However, due to its high activation energy for O2 reduction reaction and negligible ionic conductivity, LSM is not suitable for intermediate temperature SOFCs. On the other hand, mixed ionic/electronic conducting (MIEC) materials such as (La,Sr)(Co,Fe)O3 (LSCF) [11,12], La(Ni,Fe)O3 [13,14], GdBaCo2O5 [15e17] and (Ba,Sr)(Co,Fe)O3
d (BSCF) [18e20] yield enhanced performances at reduced temperatures. Among the MIEC cathode materials, BSCF exhibits excellent electrocatalytic activity at low temperatures, e.g., the area specific resistance is less than 0.15 U cm2 at 650  C [18]. The excellent electrocatalytic activity is attributed to its high oxygen vacancy concentration

* Corresponding author. Tel.: þ61 8 9266 4159; fax: þ61 8 9266 1138. ** Corresponding author. E-mail addresses: [email protected] (Z. Lu¨), [email protected] (K. Chen). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.11.126

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and bulk oxygen diffusion coefficient, as BSCF is highly oxygen deficient and the oxygen stoichiometry reaches as low as w2.34 at 600  C [21]. However, the electrical conductivity of dense BSCF oxide is relatively low compared to other materials (i.e., w30 S cm
1 for BSCF and w300 S cm
1 for LSCF at 600  C [22,23]). The electrical conductivity of porous BSCF electrode could be even lower, which limits the interfacial electron transfer from the electrode to the adsorbed oxygen molecule [24]. From practical standpoint of the SOFC stack development, the low electrical conductivity of the BSCF oxides can also cause the significant increase in the contact resistance between the electrode coating and current collector [25]. The incorporation of materials such as LaCoO3 and (Sm,Sr) CoO3 with a higher electrical conductivity has been shown to signficantly improve the electrical conductivity of the BSCF cathodes [26,27]. Due to the nature of the composite structure, a high concentration (e.g., 30 vol%) of the electronic conducting phase is required to reach the percolation threshold for enhancing the electrical conductivity. Wet impregnation/ infiltration is an effective way to incorporate nanoparticles into rigid perovskite skeletons. The fabrication of nanostructured cathodes such as La2NiO4þd infiltrated BSCF [28], (La,Sr)CoO3 infiltrated LSM [29], LSM infiltrated (La,Sr)(Co,Fe) O3 [30], and BSCF infiltrated LSM [31] significantly improved the electrode performance and/or stability of the cathodes. Due to the unique nano-structure, a well connected network can be formed even at a loading below the percolation threshold [32]. However, the main disadvantage of the infiltration method is the non-uniform distribution of the nanoparticles with little control over the infiltration process [33]. Moreover, the infiltration process is also time-consuming, as a great number of infiltration and heatetreatment cycles have to be repeated to achieve a desirable loading. Therefore a simple method with better control of the deposition for improving the performance of BSCF-based electrodes is desirable. Electroless deposition is a well established method to deposite metal coatings. It has been applied to deposite nanostructured metal with various identities (i.e., Ni and Cu) on porous ceramic supports for use as the anodes of SOFC [34e38]. Deposition of Ag has been widely applied in modifying the cathode materials due to its superior catalytic activity in oxygen reduction reaction (ORR) as well as its high electrical conductivity [39e47]. Recently, Shao et al. has deposited Ag nanoparticles into the porous BSCF cathodes via a modified electroless deposition process. In their work, AgNO3 was first infiltrated into the BSCF skeleton and the Agþ was then reduced by the addition of reductants such as N2H4 and HCHO [48e50]. They observed that the reductant has a strong effect on the composition of BSCF and choosing the right chemical is essential in optimizing the performance of the Ag decorated BSCF. We have shown that vacuum assisted electroless deposition can be used to deposit nano-structured Ag network in BSCF cathodes [51]. The cell with the Ag modified BSCF cathode exhibits a peak power density of 0.95 W cm
2 at 600  C [51], which is w30% higher than the cell using the pristine BSCF cathode [52]. However, there is still a lack of understanding on how the Ag loading and microstructure affect the electrocatalytic activity of BSCF cathodes.

Moreover, whether the vacuum assisted electroless deposition process has an effect on the microstructure and composition of the BSCF skeleton is unclear. Here we prepared a series of Ag modified BSCF cathodes by the vacuum assisted electroless deposition technique. The physical properties of the cathodes were analyzed to evaluate the influence of the deposition process. We also present a systematical study on how the Ag loading influences the electrochemical performance of the BSCF cathode.

2.

Experimental

2.1.

Sample preparation

Ba0.5Sr0.5Co0.6Fe0.4O3 (BSCF) and Ce0.8Sm0.2O1.9 (SDC) powders were prepared by combined EDTA-citrate complexing solegel method and Pechini method [22,53], respectively. 0.5-mmthick electrolyte pellets were prepared by die pressing of the as prepared SDC powder and sintered at 1400  C for 4 h. BSCF cathodes were symmetrically fabricated on both sides of the electrolytes by slurry coating and sintered at 1000  C for 4 h. The effective cathode area was 0.28 cm2. The thickness and porosity of the BSCF cathode was w12 mm and 33%, respectively. Ag decorated BSCF cathodes (Ag@BSCF) were prepared by electroless deposition. The solution for electroless Ag deposition was prepared from 0.15 M formaldehyde (Analytical Reagent, A.R.), 0.1 M AgNO3 (A.R.)-NH3∙H2O (A.R.) solution, ethanol (A.R.) and deionized water. Prior to the electroless deposition, 0.2 ml of 5 wt% silver nitrate solution was evenly injected to the cathodes and the electrodes were heat-treated at 450  C for w2 min to activate the BSCF skeleton. The activation process promotes the deposition rate and therefore reduces the deposition time. Electroless Ag deposition was then carried out in a vacuum environment (<20 kPa) for 5e180 s. The electroless deposition reaction is considered as a combined result of two independent electrode reactions which are the reduction of metal ions and the oxidation of the reductant [54]. The electroless Ag deposition takes place according to the following equation:
AgðNH3 Þþ 2 þ e ¼ Ag þ 2NH3

(1)

The samples were immersed in deionized water for 24 h after the deposition process, followed by heat-treating at 500  C for 20 min to remove the remaining impurities. The details of the electroless Ag plating can be found elsewhere [51]. For comparison, a traditional infiltration process was also conducted to incorporate Ag into the BSCF structure (Ag/BSCF) using AgNO3 solution and heat-treated at 500  C for 20 min.

2.2.

Characterization & electrochemical performance

The microstructure of the electrodes was characterized using a scanning electron microscope (SEM, Hitachi S-4700SE/N). The chemical composition of the near surface region and the oxidation states of the containing elements were analyzed using X-ray photoelectron-emission spectroscopy (XPS, PHI-

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 2 4 1 3 e2 4 2 0

5700 PE). A monochromatic Al Ka source was operated at 10 mA emission current at an accelerating voltage of 15 kV. The binding energy (BE) scale was calibrated using the C 1s binding energy of 285 eV of adventitious carbon. Depth profiles of the porous cathode were obtained using argon (Arþ) sputtering for 5 min. The bulk composition of the cathodes was examined using inductance coupled plasmaatomic emission spectroscopy (ICP-AES, PE-5300DV, PE). Xray diffraction (XRD) is employed to characterize the crystal structure of the materials. The diffraction patterns were collected using a diffractometer (D/MAX-2550, Rigaku) with Cu Ka radiation. The diffraction patterns were measured over the 2q range of 20e80 . The unit cell parameters were calculated by the least square fitting of the XRD data in FullProf. The structure was refined in the Pnma space group [55]. Electrochemical impedance responses of the symmetrical electrodes were measured under open circuit conditions, using a 10 mV amplitude ac signal over a frequency range from 0.01 Hz to 91 kHz with a Solartron SI 1260 impedance analyzer combined with an SI 1287 electrochemical interface. Silver paste was utilized to serve as the current collector for all measurements. To study the effect of oxygen partial pressure on the impedance responses, the cells were placed into a sealed chamber. Nitrogen was used to dilute the oxygen to achieve oxygen partial pressures from 0.001 to 0.21 atm, which were monitored by a yttria-stabilized zirconia oxygen sensor.

3.

Results and discussion

3.1.

Physical properties

Fig. 1 depicts the effect of deposition time on the loading of Ag in the Ag@BSCF determined by ICP-AES analysis. Prior to the electroless Ag deposition, a small amount of Ag (0.4 wt%) was introduced by infiltration to activate the electrode surface. This process avoids the introduction of impurities during the initial sensitization process [56,57]. A linear correlation between the Ag loading and the deposition time was observed, which suggests that the Ag concentration can be feasibly tuned by adjusting the deposition time. It is also noticed that

Fig. 1 e The effect of electroless deposition time on the loading of Ag in the Ag@BSCF determined by the ICP-AES analysis. The initial 0.4 wt% of Ag was introduced by infiltration to activate the electrode surface.

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10 wt% of Ag can be obtained at a deposition time of 180 s, which demonstrates the efficiency of the method in incorporation of Ag to the porous BSCF structure. The top-view (Fig. 2aed) and cross-sectional (Fig. 2e) SEM images of the electrodes modified with different loadings of Ag are representatively depicted in Fig. 2. The pre-deposited BSCF electrode with a Ag loading of 0.4 wt% is characterized by a well connected BSCF skeleton with very few Ag nanoparticles formed on the surface (Fig. 2a). A significant increase in the density of Ag nanoparticles is observed following the increase of the deposition time (Fig. 2b and c). We also observed that the BSCF particles are covered uniformly by Ag nanoparticles with a diameter of w20 nm with a Ag loading of 2.1 wt% (Fig. 2b and d). The porous BSCF electrode is eventually covered by a layer of Ag film as the loading increased to 10 wt% (Fig. 2c). The distribution of Ag nanoparticles within the porous Ag@BSCF with a Ag loading of 2.1 wt% is exemplarily demonstrated in Fig. 2e. It is clear that the Ag nanoparticles show a homogeneous distribution from surface to bulk of the electrode region. The bare surfaces of the BSCF particles result from fracturing of the samples. It is also worth noting that the Ag nanoparticles are deposited homogeneously at the SDC electrolyte/electrode interface region, which is essential in benefiting the O2 reduction reaction. Compared to the inhomogeneous distribution of nanoparticles introduced by conventional impregnation approaches, e.g., in the case of (Gd,Ce)O2 (GDC)-infiltrated LSM cathodes [58], all electrodes retain the highly porous microstructure regardless of the Ag loading. As discussed in our previous work, the oxidation states of Ag within Ag@BSCF prepared by electroless deposition and Ag/BSCF by infiltration are different [51]. The Ag in Ag@BSCF exhibits different chemical states in the near surface region (Ag2O) and in the bulk (Ag), whereas the Ag in Ag/BSCF presents only the oxidation state (Ag2O). We have further examined the chemical states of the BSCF skeleton by XPS to evaluate the effect of different preparation methods. Fig. 3 shows the high resolution scan of the overlapped Co 2p and Ba 3d spectra of pristine BSCF, 3 wt% infiltrated Ag/BSCF, and 2.1 wt% Ag@BSCF. For both pristine BSCF and infiltrated Ag/ BSCF, the peaks are located at 780.2 eV, which indicates the infiltration process has a negligible effect on the BSCF substrate. The peak position of Ba 3d and Co 2p allows us to conclude that Ba presents in its highest oxidation states (Ba2þ), while Co is mainly in its trivalent state (Co3þ) [59,60]]. In the case of Ag@BSCF, the oxidation states of Ba and Co in the near surface region remain consistent with pristine BSCF and Ag/BSCF. However, a 1 eV shift to a higher BE of the peaks is observed in the bulk, which implies the electroless deposition process has an effect on the BSCF skeleton. Since Ba is more likely to remain at the divalent state, the appearance of the new peak at 781.2 eV can be associated with the partial variation of Co from 3þ to 4þ. The relative molar ratios of the cations located in A and B sites of the perovskite structure as a function of the electroless deposition time are shown in Fig. 4a. After a deposition time of 5 s, the ratio of Ba/Sr, Fe/Co and (Ba þ Sr)/(Fe þ Co) are 0.84, 0.63 and 0.88, respectively, which are slightly lower than the calculated number from the stiochiometry of Ba0.5Sr0.5Co0.6Fe0.4O3
d. Significant decreases of both the Ba/Sr and Fe/Co ratios are

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Fig. 2 e Top-view SEM images of (a) BSCF with 0.4 wt% of Ag by infiltration; Ag@BSCF with (b) 2.1 wt% and (c) 10 wt% of Ag; (d and e) Cross-sectional SEM images of the Ag@BSCF electrodes with 2.1 wt% of Ag.

observed with a deposition period of 10 s, and then the composition remains stable with further deposition. The decrease of the Ba/Sr and Fe/Co ratios and the increase of the (Ba þ Sr)/(Fe þ Co) ratio indicate the BSCF skeleton is selectively etched during the Ag deposition, more specifically, Ba in the A-site and Fe in the B-site tend to be etched preferentially. The mechanism for the preferential etching is not clear at the present stage. Similar results have been reported by Zhou et al. [48], where the formation of BaxSr1-xCO3 is observed when using HCHO as reductant and results in the A-site cation deficient of BSCF. However, the crystal structure change is negligible according to the XRD pattern comparison of the pristine BSCF and the 2.1 wt% Ag@BSCF (Fig. 4b). The unit cell of the Ag@BSCF remains similar to the pristine BSCF based on the refinement (a ¼ 5.63  A, b ¼ 7.97  A, c ¼ 5.63  A for Ag@BSCF and a ¼ 5.63  A, b ¼ 7.95  A, c ¼ 5.63  A for BSCF). The close unit cell parameters also indicate that Ag most likely stays on the surface of BSCF in its metal form rather than in the lattice of BSCF.

3.2.

Fig. 3 e XPS spectra of Co 2p and Ba 3d of (a) a pristine BSCF, (b) a 3.0 wt% Ag/BSCF and (c) a 2.1 wt% Ag@BSCF.

Electrochemical behavior

The effect of the Ag loading on the electrochemical activity of the Ag@BSCF cathodes is shown in Fig. 5a by means of the electrochemical impedance spectroscopy. In general, the performance of the cathodes can be evaluated by comparing the polarization resistance of the electrodes. Surprisingly, the impedance of the 1 wt% Ag@BSCF increases significantly compared to that of the pristine BSCF, which indicates the 1 wt% Ag@BSCF exhibits a reduced reactivity. The dropping of the performance points to the leaching out Ba and Fe of the BSCF material (Fig. 4a), which implies the chemical composition of BSCF has a huge impact on the electrochemical performance. The performances of Ag@BSCF cathodes are gradually improved following the increase in Ag loading till 2.1 wt%, and then slightly drop at a high loading (10 wt%). To extract the effect of Ag loading, the impedance responses of all electrodes are fitted using the following equivalent circuit:

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Fig. 4 e (a) Molar ratios of (Ba D Sr)/(Fe D Co), Ba/Sr and Fe/ Co as a function of deposition time, and (b) XRD pattern of BSCF and a 2.1 wt% Ag@BSCF electrode on SDC electrolytes.

LRU ðRH Q H ÞðRL Q L Þ

(2)

where L corresponds to an inductance that is associated with the Ag current/voltage probes. RU is the ohmic resistance between the cathode and reference electrode. QH, QL and RH, RL denote the constant phase element and electrode polarization resistance at high and low frequencies, respectively. The aggregation of RH and RL summarises the electrode polarization resistance (RE). The fitting results indicate that the oxygen reduction reaction is dominated by at least two electrode processes, which will be further discussed in this study. Fig. 5b addresses the effect of Ag loading on RH, RL, RE and RU derived from the fitting results. RH depicts a decrease trend as the loading of Ag increases. Interestingly, RL shows a different trend upon Ag decoration. It initially increases at 1 wt% of Ag loading, and then follows a gradual decease with further increase of Ag loading. The optimum result is obtained on the 2.1 wt% Ag@BSCF. RE follows a similar trend as shown in RL, which indicates the oxygen reduction process can be effectively enhanced with the 2.1 wt% of Ag loading. Furthermore, a clear decreasing trend of the RU is observed up

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Fig. 5 e (a) Impedance responses for the O2 reduction reaction on the Ag electroless deposited BSCF cathodes with different silver loadings, measured under open circuit at 650  C. Symbols are the experimental data and lines are the fitted data. The high frequency intercepts have been subtracted for better comparison. (b) The effect of Ag loading on RH, RL, RE and RU. RH, RL denote the electrode polarization resistance at high and low frequencies, respectively, RE is the sum of RH and RL, and RU is the electrode ohmic resistance.

to 2.1 wt% of Ag loading. A remarkable 40% reduction of RU is observed for the 2.1 wt% Ag@BSCF compared to the pristine BSCF. Further increase of the Ag loading to 10 wt% shows a negligible effect on RU. As a loading of 2.1 wt% of Ag is not likely to form a continuous Ag network throughout the electrode, the decrease of ohmic resistance most likely results from the enhanced current collecting ability of BSCF by the presence of Ag nanoparticles on the electrode surface. In order to get a qualitative insight of each individual electrode process, the impedance responses of the pristine BSCF and the 2.1 wt% Ag@BSCF cathodes are systematically investigated under various oxygen partial pressures ( pO2), as representatively shown in Fig. 6. The high frequency intercepts, namely RU, have been subtracted for better comparison. There is a very slight increase of the RU for both the cathodes when decreasing the oxygen partial pressure. Both cathodes show a similar behavior under ambient pO2 that are demonstrated in the insets, however, their impedance responses at low pO2 (0.01 atm) exhibit completely different features. In the case of pristine BSCF

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Fig. 6 e Impedance responses of (a) a pristine BSCF cathode and (b) a 2.1 wt% Ag@BSCF cathode at 650  C under low (0.01 atm) and ambient (0.21 atm) oxygen pressures. The red dashed lines are simulated results using the corresponding equivalent circuits. The high frequency intercepts have been subtracted for better comparison. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

(Fig. 6a), two-arc impedance responses are observed, whereas the 2.1 wt% Ag@BSCF (Fig. 6b) shows three-arc impedance responses. Meanwhile, it is also observed that the relaxation frequency of the low-frequency arc of both electrodes has significantly dropped to 0.13e0.42 Hz, which indicates a new electrode process emerging at low pO2. The impedance responses at low pO2 (<0.1 atm) can be fitted by the following equivalent circuits: BSCF : LRU ðRH Q H ÞðRD Q D Þ

(3)

Ag@BSCF : LRU ðRH Q H ÞðRL Q L ÞðRD Q D Þ

(4)

where the RD and QD denote the polarization resistance and constant phase element respectively that correspond to the arc addressed at the lowest frequency. We have further analyzed the dependence of RH, RL and RD on pO2 to explain how the Ag nanoparticles improve the electrocatalytic oxygen reduction reaction of BSCF, as demonstrated in Fig. 7. The individual electrode process can be estimated from the slopes (k) of the fitting results [58]. For both the electrodes, RD is highly dependent on pO2 (k <
0.80), which indicates the additional-low frequency arc at low pO2 is most likely related to the diffusion of molecular oxygen species [58,61]. In the case of Ag@BSCF, the weak dependence of RH on pO2 (k ¼
0.07) indicates that the high frequency arc is most

Fig. 7 e The dependence of fitted RH, RL and RD on pO2 for pristine BSCF and Ag@BSCF cathodes.

likely related to the ion migration process [58]. The slope of RL is
0.51, which implies the low frequency arc is related to the oxygen dissociation and/or surface diffusion processes [49,58,61]. On the other hand, the slope of RH (k ¼
0.38) of the pristine BSCF suggests the single arc at high frequency is most likely correlated to a mixed process consisting of both ion migration and dissociation and/or surface diffusion processes. Thus the dominant promotion of the process at low frequency under ambient air (Fig. 5) indicates that the presence of the electroless deposited Ag nanoparticles primarily improves the oxygen dissociation and/or surface diffusion processes. Since silver is well known for its superior oxygen solubility and mobility [46], the presence of Ag nanoparticles not only creates a large number of effective surface sites for the adsorption and dissociation of oxygen species, but also supplies a shorter path for the diffusion of oxygen to the Ag and BSCF interface. In addition, according to the work from Wang et al., the reaction barrier of O2 dissociation process on silver surfaces can be signficantly reduced when the support has oxygen vacancies [62]. This suggest the highly oxygen deficient BSCF skeleton also has a promotion effect on the catalytic activity of silver. Overall, we can conclude that the presence of Ag nanoparticles improves the electrocatalytic performance of the cathode mainly by enhancing the efficiency of oxygen dissociation and surface diffusion processes.

4.

Conclusions

We have fabricated a series of Ag nanoparticles decorated porous BSCF cathodes with various loadings via vacuum assisted electroless deposition technique. The incorporation

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 2 4 1 3 e2 4 2 0

of Ag into the BSCF skeleton has a minor effect on the chemical composition of the skeleton but without changing the perovskite structure of the pristine BSCF. We demonstrate the presence of Ag nanoparticles significantly improves the electrocatalytic oxygen reduction performance by promoting the dissociation and surface diffusion of oxygen species on the BSCF electrode. More importantly, we show that the presence of Ag nanoparticles also substantially enhances the current collecting ability of BSCF and thereby reduces the electrode ohmic resistance of the BSCF cathodes, enabling the applications of BSCF-based materials as cathodes in practical SOFC stacks and systems.

Acknowledgments This work is supported by the Ministry of Science and Technology of China under contract no. 2007AA05Z139 and the Curtin Research Fellowship Program, Curtin University. We acknowledge financial support from the Center of Energy Materials (CEM), the Danish Strategic Research Council, and the Carlsberg Foundation.

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