# Detailed Study of Sulfur Poisoning and Recovery of Ni-YSZ-Based Anodes Operating up to 1.8 W cm-2 in a Biogas Fuel

Authors: Jianjun Ma; Yao Jiang; Paul A. Connor; Stephen R. Gamble; Mark Cassidy; Cairong Jiang; John T. S. Irvine
Journal: International Journal of Energy Research
 (2023)

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## Abstract

Ni-YSZ (nickel-yttrium-stabilized zirconia) is a common anode for solid oxide fuel cells (SOFCs) because of its excellent catalytic performance and electronic conductivity. It shows that the nickel anode-supported cell exhibits good cell performance in a biogas fuel of 36CH4-36CO2-20H2O-4H2-4CO. Unfortunately, natural biogas fuels often contain sulfur, so using nickel anodes is not always straightforward. This paper investigates the sulfur poisoning and the recovery of BaCe0.7Zr0.1Y0.1Yb0.1O3-δ- (BCZYYb-) (Ce, Y, and Yb codoped barium zirconate) impregnated nickel anode-supported cells operating up to 1.8 W cm-2 in the biogas. The in situ gas analysis reveals that the suppression of the reforming reactions might cause sulfur poisoning in a 4 ppm (v) H2S (hydrogen sulfide) in open circuit conditions, whereas the current degradation in working conditions could be attributed to the deactivation of reforming reactions and catalyst activity. The incidence of water-gas shift reactions is associated with the degradation rate of these two reactions. After removing the H2S, the recovery is accelerated by a steam hydrogen fuel, indicating that steam facilitates the efficient release of sulfur from nickel sites.


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## Body

## 1. Introduction

A solid oxide fuel cell (SOFC) attracts more attention being an energy conversion device due to its fuel flexibility and environmental friendliness [1]. Many types of gas, liquid, and solid biomass can be used as fuel in SOFCs [2]. Some successful cases have been reported on the direct use of butane [3] and methane [4]. As a renewable energy, biomass might be a solution to the global warming effect. The abundant biomass resources, like water primrose [5], rice straw [6], and buffalo grass [7], have been converted into biogas, which might provide enough electricity for the rural area, supposing stacks [8] can be in industrialization [9]. However, the fuel impurities have to be solved before industrial application. Nickel tends to react with the impurities, such as sulfur, chlorine, or silicon, triggering a performance loss [10], subsequent degradation [11], or even deactivation [12] of SOFCs [13]. Although no apparent degradation was found in a single SOFC using biogas fuel, add a trace amount of HCl (4, 8, or 12 ppm (v)) under a galvanostatic test at 850°C [14]. However, the whole stack could break due to the corrosion of the sealant by the chlorine compounds [15]. It is, therefore, required to remove these impurities. A chemical purification method [16] has been applied to remove H2S [17] and HCl [18], with a significant increase in the methane concentration in biogas from 64.76% to 92.35% [19]. As an impurity, sulfur can cause the deactivation, or even the failure of the nickel electrode, over long-term operation [20]. It is challenging to operate SOFCs in any natural biogas containing sulfur impurities [21]. Sulfur poisoning is controlled by many parameters, e.g., working temperature, impurity concentration, and working time [22]. Some studies have addressed the degradation of SOFCs in sulfur in various conditions, such as in pure hydrogen, hydrocarbon, or a gas mixture [23]. In pure H2, adsorption/oxidation in the presence of a low concentration of sulfur is the main reason for performance loss [24]. High concentration sulfur leads to irreversible performance degradation.It is more complicated in a mixture of gases due to the interactions between gases and more chemical and electrochemical reactions than in pure hydrogen [25]. Shiratori et al. [26] added 1 ppm (v) H2S in a 3 : 2 CH4/CO2 mixture. They observed a rapid reduction in the reforming reactions within 2 hours of supplying H2S to a mixture of biogas with a ratio of 1.5 to 1 (CH4 : CO2 (25 ml/min)) and N2 (25 ml/min, respectively). Kuramoto et al. found a slight degradation over 1000 hours of durability test under 1 ppm (v) H2S, but some particles started to sinter in the diffusion layer [10]. Consequently, the carbon monoxide (CO) yield and the reaction rate drop by about 40% [26]. It indicates that the reforming reactions and electrochemical oxidation with H2 or CO as the fuel are affected by sulfur chemisorption on the active sites in the triple-phase boundary (TPB). However, the performance loss is not fatal and can be recovered entirely after stopping H2S.Hauch et al. [27] demonstrated the influence of 2 ppm (v) H2S in a gas mixture of CH4:H2:H2O=3:1:6 on cell performance and found that sulfur poisoning under low potential was reversible, even if the cell was exposed to 2 ppm (v) H2S for 500 hours. Li et al. [28] found that the performance was not recoverable in a mixture of 35H2-46CO-16N2-3H2O with 12.5 ppm (v) H2S. The performance regeneration could be improved by adding 10% H2O or CO2 to the above gas mixture. Rasmussen and Hagen [29] investigated the reforming activity of anodes with various amounts of H2S. They found that a ratio of (CO+CO2) to CH4 dropped by 90% with a low H2S concentration, indicating that steam reforming of CH4 was strongly affected even if there was only a small amount of H2S. There are usually two stages in sulfur poisoning, a quick voltage drop and a slow degradation [27]. The impedance resistance, cell microstructure, and X-ray diffraction (XRD) patterns were identified to investigate possible mechanisms of sulfur poisoning. These characterization methods are mostly posttests and sometimes cannot represent the actual reactions in poisoning [30]. In situ characterization is strongly recommended to differentiate the chemical and electrochemical reactions regarding sulfur poisoning. Up to date, in situ Fourier transform infrared (FTIR) spectroscopy has been used to evaluate the possible responses in H2S, but a few reports are on the in situ gas analysis in terms of long-term durability. Rasmussen and Hagen and Chlipala et al. confirmed in the experiments that the cell could operate stably for 24 hours under a maximum concentration of 7 ppm (v) H2S [29, 31]. Besides sulfur poisoning, carbon deposition is another crucial factor that cannot be ignored when carbon-containing gas is used as fuel for SOFCs. Density functional theory calculation shows that the carbon growth mechanisms at the Ni-YSZ interface depend on the interface structure [32]. In considering the possibility of carbon deposition, the sulfur poisoning mechanisms are more complicated.Some anode materials, such as doped titanate [33], molybdate [34], La0.75Sr0.25Cr0.5Mn0.5O3 [35], vanadate [36], and copper-based oxide [37], show high tolerance to sulfur. However, the cell performance using nonnickel anodes is not high enough to be applied in the industry compared to those with nickel electrodes. The low catalytic activities of the other anode materials need to be improved, and suitable thin electrolyte membranes must be solved before the materials can be used widely [33]. Some extremely high power output has been achieved in Ni-based fuel cells even at low temperatures [38]. Therefore, nickel remains the most used material for carbon-containing fuels like biogas. In an attempt to use a nickel-based electrode in a sulfur-containing fuel, many ways of modifying the Ni-YSZ (nickel-yttrium-stabilized zirconia) system have been developed [23]. Using ScSZ (Sc-doped zirconia) electrolytes to replace YSZ increased the ability of sulfur tolerance, no fatal voltage drop was observed by adding 100 ppm (v) H2S to the hydrogen fuel [39]. Coating or infiltration is another effective way of protecting nickel from direct exposure to H2S [40]. A layer of Nb2O5 was coated on the Ni-YSZ surface, resulting in good electrical conductivity, enhanced sulfur resistance, and excellent catalytic activity [41]. Ceria-based materials, such as CeO2 [42], Sm0.2Ce0.8O2 [43], and Sm0.2Ce0.8Y0.1Yb0.1O1.9 [44], were also introduced to Ni-YSZ electrodes exposed to sulfur. Ni-YSZ with infiltrated Sm0.2Ce0.8O2 operated more stably in H2 containing 60 ppm (v) H2S fuel than without Sm0.2Ce0.8O2. The cell did not fail at a high H2S concentration, up to 500 ppm (v) [43]. Recently, barium zirconate-based anode [45] and BaO [46] were reported to have an excellent ability to resist carbon coking and sulfur poisoning.Previous research showed that Ni-YSZ exhibited a good cell performance operating in biogas composed of 36CH4-36CO2-20H2O-4H2-4CO, especially infiltrated with BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BCZYYb) [47]. We aim to investigate the sulfur poisoning and the recovery of a bare nickel-based cell and the cells impregnated with BCZYYb operating up to 1.8 W cm-2 in a biogas fuel of 36CH4-36CO2-20H2O-4H2-4CO both in operational conditions and in equilibrium states by in situ gas analysis.


## 2. Experimental

### 2.1. Single-Cell Preparation

The cell used in the experiment was homemade in this study. Details of the preparation process and BCZYYb-based cells have been described in our previous papers [2, 47]. The cell was impregnated with BCZYYb 3 times, 6 times, and 9 times to obtain different amounts of proton conductors. Correspondingly, the proton content was 0.6 wt%, 1.0 wt%, and 1.6 wt% based on the total weight of each cell. The cell was defined as Ni-YSZ-3BCZYYb, Ni-YSZ-6BCZYYb, and Ni-YSZ-9BCZYYb. The impregnated BCZYYb and the (La0.8Sr0.2)0.95MnO3-YSZ (LSM-YSZ) cathode were thermal-treated at 1100°C for two hours. Finally, the current collector for the cathode and anode was Pt and Au paste, respectively.


### 2.2. Electrochemical Test

The cell (sealed with Aremco 552 sealant) was heated to 800°C with a thermocouple to detect the temperature, remaining at 100°C and 250°C for one hour to obtain good sealing. Argon gas was purged to protect the cell from being oxidized. Hydrogen gas started to supply when the target temperature was reached. The cathode was supplied with flowing air. The open circuit voltage (OCV) was monitored to reach an equilibrium state before testing. Initially, cell performance at 800°C was characterized by linear sweeps and electrochemical impedance spectra on Solartron 1280B (UK). The electrochemical impedance spectra (EIS) were collected in open circuit conditions.After the open circuit voltage (OCV) reached a constant value, the anode gas bubbled through the water to get humidified hydrogen. The I-V-P (current-voltage-power density) curves and EIS of the cell were tested in humidified H2 (3%H2O). And then, 20% steam was applied by a steam generator. The cell performance was tested under 20%H2O-H2. Afterward, a biogas mixture of 36% CH4, 4% H2, 4% CO, 36% CO2, and 20% H2O balanced with 15 ml min-1 argon reference gas to obtain a 300 ml min-1 in total was fed to the anode. There was 5% argon in the feed gas. The volume of outlet gases might increase after reforming and electrochemical reactions. Therefore, the percentage of argon would further decrease. The cell performance was tested again. A 4 ppm (v) H2S was then added (using 100 ppm (v) in hydrogen, 4% hydrogen in the total stream) for the next 24 hours. The OCV was first monitored for 2 hours, and the gases from the outlet of the anode chamber were also detected. After that, the current of cells was recorded, while a 0.7 V voltage load was added to the cell for the next 22 hours. The sulfur was removed, and the EIS and the I-V-P curves were measured again. The recovery of the current density in the biogas for 20 hours and the 20% steam hydrogen for 24 hours was carried out at 0.7 V.


### 2.3. Gas Characterization

A gas chromatograph (GC, HP 6890, Japan) was used for the in situ measurement of the tailor gas in this study. The gas detection was carried out continuously during the test process operating under open circuit conditions or a load. A reference gas, argon, was added to aid the measurement of the amount of hydrogen gas, as the detection by the GC may not have been accurate enough from the experimental test in our previous paper. More details about the gas analysis were shown in our previous paper [47]. Gases like methane, carbon monoxide, carbon dioxide, and hydrogen were detected by the GC, while the water content (before the trap) was tested using a dewpoint sensor (Vaisala, HMT330). The microstructure of the anodes was analyzed using a field emission scanning electron microscope (JEOL JSM-6700, Japan).


## 2.1. Single-Cell Preparation

The cell used in the experiment was homemade in this study. Details of the preparation process and BCZYYb-based cells have been described in our previous papers [2, 47]. The cell was impregnated with BCZYYb 3 times, 6 times, and 9 times to obtain different amounts of proton conductors. Correspondingly, the proton content was 0.6 wt%, 1.0 wt%, and 1.6 wt% based on the total weight of each cell. The cell was defined as Ni-YSZ-3BCZYYb, Ni-YSZ-6BCZYYb, and Ni-YSZ-9BCZYYb. The impregnated BCZYYb and the (La0.8Sr0.2)0.95MnO3-YSZ (LSM-YSZ) cathode were thermal-treated at 1100°C for two hours. Finally, the current collector for the cathode and anode was Pt and Au paste, respectively.


## 2.2. Electrochemical Test

The cell (sealed with Aremco 552 sealant) was heated to 800°C with a thermocouple to detect the temperature, remaining at 100°C and 250°C for one hour to obtain good sealing. Argon gas was purged to protect the cell from being oxidized. Hydrogen gas started to supply when the target temperature was reached. The cathode was supplied with flowing air. The open circuit voltage (OCV) was monitored to reach an equilibrium state before testing. Initially, cell performance at 800°C was characterized by linear sweeps and electrochemical impedance spectra on Solartron 1280B (UK). The electrochemical impedance spectra (EIS) were collected in open circuit conditions.After the open circuit voltage (OCV) reached a constant value, the anode gas bubbled through the water to get humidified hydrogen. The I-V-P (current-voltage-power density) curves and EIS of the cell were tested in humidified H2 (3%H2O). And then, 20% steam was applied by a steam generator. The cell performance was tested under 20%H2O-H2. Afterward, a biogas mixture of 36% CH4, 4% H2, 4% CO, 36% CO2, and 20% H2O balanced with 15 ml min-1 argon reference gas to obtain a 300 ml min-1 in total was fed to the anode. There was 5% argon in the feed gas. The volume of outlet gases might increase after reforming and electrochemical reactions. Therefore, the percentage of argon would further decrease. The cell performance was tested again. A 4 ppm (v) H2S was then added (using 100 ppm (v) in hydrogen, 4% hydrogen in the total stream) for the next 24 hours. The OCV was first monitored for 2 hours, and the gases from the outlet of the anode chamber were also detected. After that, the current of cells was recorded, while a 0.7 V voltage load was added to the cell for the next 22 hours. The sulfur was removed, and the EIS and the I-V-P curves were measured again. The recovery of the current density in the biogas for 20 hours and the 20% steam hydrogen for 24 hours was carried out at 0.7 V.


## 2.3. Gas Characterization

A gas chromatograph (GC, HP 6890, Japan) was used for the in situ measurement of the tailor gas in this study. The gas detection was carried out continuously during the test process operating under open circuit conditions or a load. A reference gas, argon, was added to aid the measurement of the amount of hydrogen gas, as the detection by the GC may not have been accurate enough from the experimental test in our previous paper. More details about the gas analysis were shown in our previous paper [47]. Gases like methane, carbon monoxide, carbon dioxide, and hydrogen were detected by the GC, while the water content (before the trap) was tested using a dewpoint sensor (Vaisala, HMT330). The microstructure of the anodes was analyzed using a field emission scanning electron microscope (JEOL JSM-6700, Japan).


## 3. Results and Discussion

### 3.1. Sulfur Poisoning of the Bare Nickel Electrode

The poisoning of Ni-YSZ exposed to a 4 ppm (v) sulfur-containing biogas is shown in Figure1. Figure 1(a) presents the open circuit voltage in biogas after the sulfur supply was recorded. There are four regions in the graph. In region I, the cell is stable in the biogas atmosphere without sulfur. Sulfur is added to the biogas in region II. Unlike previous reports that initial exposure to several ppm (v) H2S resulted in a rapid drop in the voltage [27], the OCV in this study slightly increased for 2 hours after adding H2S in region II, which could be because of an increase in hydrogen concentration in the feed gas. The uncertainty of measurement data cannot be ignored. The hydrogen content is calibrated, presuming a constant volume of argon in the inlet and outlet.Figure 1
Cell performance of the Ni-YSZ on exposure to 4 ppm (v) H2S. (a) OCV evolution. (b) Anode exhaust gas in open circuit conditions after adding H2S to the biogas. (c) Impedance spectra patterns in the biogas (black), in the sulfur-containing biogas for 11 hours (red), and in the sulfur-containing biogas for 20 hours (blue).
(a)(b)(c)Another possibility could be that sulfur is oxidized, and the reaction is shown in(1)H2S+O2−=12S2+H2O+2e−(2)H2S+3O2−=SO2+H2O+6e−When H2S is oxidized into S2 or SO2, the OCV is given in equations (3) and (4), respectively [36].
(3)E=E0−RT2FlnPS21/2PH2OPH2SPO21/2,(4)E=E0−RT6FlnPSO2PH2OPH2SPO23/2.Aguilar et al. also found a quick response after H2S is introduced into fuel [36]. The current density increased from 400 mA cm-2 to 500 mA cm-2 at 0.44 V in the first two to three hours and then decreased. Our results are well agreed with their report. The adsorption process of some species (S or S2) at the nickel catalyst surface enhanced cell performance in 20 ppm (v) H2S. A low concentration of H2S in this study is used so that the OCV increases slightly from 1.043 V to 1.047 V.A quick voltage drop in the next 7 hours in region III is noticed after sulfur had been supplied for 2 hours. The addition of sulfur brings about a rapid voltage drop, which could be caused by sulfur adsorption. The corresponding result is the prevention of methane conversion (Figure1(b)). We also see an increase in polarization resistance (Figure 1(c)). Nickel sulfide is not thermodynamically favored under the investigated conditions [48]. Slow degradation of cell OCV was then observed over the next 15 hours in region IV. No stable cell voltage was achieved during the whole exposure period in the H2S.Figure1(b) shows the anode exhaust during the test. It was found that the amount of CH4 in the output stream increases continuously in sulfur, which indicates that the reforming reaction was prevented by exposure to H2S. This result is similar to the experimental observation of a 90% drop in the ratio of CO/CH4, CO2/CH4, and (CO+CO2)/CH4 at a 4 ppm (v) H2S by Rasmussen and Hagen [29]. The gas change was claimed to be caused by the deactivation of steam reforming in sulfur.Steam reforming is possible.(5)CH4+H2O⟶CO+3H2Dry reforming could take place as well.(6)CH4+CO2⟶2CO+2H2It is found that CO2 and H2O increase over time, which means equations (5) and (6) might be altered. In other words, steam and dry reforming are possibly being prevented from exposure to sulfur in open circuit conditions.Figure1(c) shows the EIS data for a cell recorded under biogas and after a specific time of exposure to H2S biogas in open circuit conditions. It is noticed that the ohmic resistance in this study remains constant, and it is even smaller after adding H2S, which agreed with the report by Li and Wang [49]. They consider that the increased series resistance is attributed to the adsorption of sulfur species (like S or S2) on the nickel surface. The electrochemical reaction slows down due to less active sites [49]. Our results are not the same as some reports. The polarization and ohmic resistance increased in a CO/H2/H2O fuel, even with a low concentration of H2S [50]. The difference between our results and the report might be caused by the different gas compositions. In the present study, a more complicated system was involved, such as reforming reactions, including steam reforming (as shown in equation (1)) and dry reforming (as shown in equation (2)). These two reactions are involved in the whole process of the cell and are not related to the working conditions. Other reactions like electrochemical reactions and WGS (water-gas shift reaction) should be considered when the cell is working. The polarization resistance is 0.2Ωcm2 before sulfur is added, increasing to 0.3Ωcm2 exposure to H2S for 11 hours. The value increases to 0.5Ωcm2 after further exposure to the sulfur for 17 hours. Before the exposure to the H2S, the prevalent resistance was at 100 Hz. After exposure for 11 hours, the dominant resistances were at 1000 Hz and 100 Hz. A significant response to a higher frequency with the addition of H2S indicates that charge transfer contributes to performance loss, suggesting that the poisoning mechanism could also change [27]. It has been demonstrated that hydrogen oxidation in a hydrogen atmosphere is blocked by sulfur coverage. In a mixture of gases, the poisoning is more complicated.


### 3.2. Sulfur Poisoning of BCZYYb-Infiltrated Cells

The sulfur poisoning of the cell infiltrated with BCZYYb was investigated in this study as it was reported that BCZYYb has a high tolerance of sulfur (up to 100 ppm (v)) [51]. Previous research demonstrated that an optimal infiltration amount of BCZYYb was 0.6 wt% (Ni-YSZ-3BCZYYb) [47].Figure2 shows the performance of the Ni-YSZ-3BCZYYb cell. The power output at 800°C reaches 1.8 W cm-2 (Figure 2(a)). As the cell Ni-YSZ-3BCZYYb displays the best performance, the current density at 0.7 V would have been over the current limitation of the testing machine, and the stability was tested at 1 A cm-2. Along with the voltage evolution, the anode exhaust gases are also shown in Figure 2(b). The sulfur poisoning of BCZYYb-impregnated cells is similar to that of the noninfiltrated anodes, and no significant improvement of sulfur tolerance is found in the BCZYYb-infiltrated Ni-YSZ. Blinn and Liu [52] reported that no difference was found between a BaZr0.9Yb0.1O3-δ-modified electrode and a nonmodified one. In region I, the cell voltage gradually increased and reached a stable state at 1 A cm-2 current load over a two-hour testing time. After introducing 4 ppm (v) sulfur, the cell voltage steadily increases for about 3 hours in region II. Then, its voltage rapidly drops in region III, followed by a slow degradation in region IV. Even though not much difference was found in the cell voltage changes in the infiltrated Ni-YSZ, the gases have a different change trend than those in the noninfiltrated Ni-YSZ. The gases either increase or decrease over time. The increases in CH4 concentration during the test time might indicate that the sulfur has influenced the reforming reactions. An increase in CO2 and H2O is also noticed, meaning that dry and steam reforming reactions are affected. The hydrogen and carbon monoxide concentration decreases over the test period.Figure 2
(a) Electrochemical performance of the cell Ni-YSZ-3BCZYYb in the biogas operating at 800°C and after poisoning in sulfur for 20 hours. (b) Anode exhaust gases of the cell Ni-YSZ-3BCZYYb in the biogas and the sulfur-containing biogas, considered over a period of time at 1 A cm-2. The changes of the voltage and the ratio of H2 : CO are also shown.
(a)(b)The gas content change might be the result of both the poisoning of the reforming and electrochemical reactions. One of the parameters might be the dominant factor in this process; for example, if the electrochemical reaction dominates, the current density value will be the most important. If the reforming reaction dominates, the content of infiltrated BCZYYb will be critical as the reforming reaction is affected by the presence of BCZYYb proton conductors. Despite the deactivation of reforming reactions, there is still enough hydrogen to maintain a constant current of 1 A. Therefore, in this case, hydrogen decreased over time.Figures3(a) and 3(b) show the sulfur poisoning of Ni-YSZ-6BCZYYb and Ni-YSZ-9BCZYYb, respectively. There are four regions in the change of current density when the cell is loaded with 0.7 V (Figures 3(a) and 3(b)). The current density increases in region I steadily, while operating in the biogas atmosphere. The current density in region II increases for the first 2-3 hours of the initial exposure to H2S. A rapid current drop in region III and a slow current degradation in region IV are observed.Figure 3
(a) Anode exhaust gases of the cell Ni-YSZ-6BCZYYb in the biogas and in the sulfur-containing biogas, considered over a period of time at 0.7 V. (b) Anode exhaust gases of the cell Ni-YSZ-9BCZYYb in the biogas and in the sulfur-containing biogas, considered over a period of time at 0.7 V. The changes in the current density and the ratio of H2 : CO are also shown.
(a)(b)Four regions of change are also noticed in the gases. In region I, the CH4 content is constant as the equilibrium state of reforming is reached. Under electrochemical conditions, the increase of the water content by hydrogen oxidation promotes the WGS reactions. Therefore, the amount of CO2 increases. When the current density increases, the H2 and carbon monoxide decrease, and the carbon dioxide and H2O increase. The faster drop in the CO as compared to the other gases might suggest that the CO is involved in the electrochemical reactions. The deactivation of reforming reactions starts after introducing H2S in region II and therefore brings about a rapid increase in the CO2 and H2O. The additional increase in the current density results in a further increase in the content of CO2 and H2O. The current density reaches around 2 A cm-2 at the end of region II. More H2 and CO are consumed to maintain the current at this high level, and at the same time, more H2O and CO2 are produced due to the increased current density. In region III, the current density drops rapidly, indicating that the electrochemical reaction is affected by sulfur.Therefore, the content of hydrogen and CO is increased, which might provide evidence that CO is involved in the electrochemical reactions. Alternatively, more CO is produced because of the increased WGS reaction rate. The deviation of the electrochemical reaction causes the increased content of H2O and CO2. Once the current density reaches a high value of 1 A cm-2, the condition is similar to Ni-YSZ; the current density slowly degrades in region IV. In this region, the slow reforming reactions result in the content reduction of the H2 and CO.


### 3.3. Mechanism of Sulfur Poisoning

Table1 summarises the cells used in these experiments. The schematic diagram of the current density and the gases at 0.7 V is presented in Figure 4. The cell was tested for around 2 hours to reach the equilibrium state in the biogas (region I). The poisoning stage of the cell is divided into three regions, regions II, III, and IV. In the first stage of sulfur poisoning (region II), the cell performance increases after introducing H2S. H2S might be used as a fuel, taking part in the reactions shown in equations (1) and (2). Therefore, enhanced performance is found in region II. This process lasts about two hours, whether under a voltage/current load or in open circuit conditions.Table 1
Testing regimes for the various cells and current behaviour.
SampleElectrodeTesting conditionRegion IRegion IIRegion IIIRegion IVBiogasBiogas+H2SBiogas+2SBiogas+H2SCell ANi-YSZOpen circuit conditionsOCV stableOCV increasesRapid OCV dropSlow OCV dropCell BNi-YSZ0.7 VCurrent increasesCurrent increasesRapid current dropSlow current dropCell CNi-YSZ-BCZYYb1 ACurrent increasesCurrent increasesRapid current dropSlow current dropCell DNi-YSZ-6BCZYYb0.7 VCurrent increasesCurrent increasesRapid current dropSlow current dropCell ENi-YSZ-9BCZYYb0.7 VCurrent increasesCurrent increasesRapid current dropSlow current dropCell ANi-YSZOpen circuit conditionsOCV stableOCV increasesRapid OCV dropSlow OCV dropFigure 4
Schematic diagram of the change of current density or voltage on exposure to H2S.In the next 10 hours, the cell shows a significant drop in the OCV in open circuit conditions. In the case of a current/voltage load, the voltage/current drops dramatically in region III. The surface of nickel could be covered by the S with the continuous flow of sulfur. The electrochemical reaction active sites might be covered in this case. Therefore, the reforming reactions slow down due to less participation of nickel catalyst in the reactions. Appari et al. also [53] demonstrated that the drop in CH4 conversion in a mixture of 12.5CH4-8.4CO2-25.2H2O-53.9N2 with a 20 ppm (v) H2S could be related to the deactivation of nickel catalysts. The possible reaction is shown in
(7)Ni+H2Sg⇆Ni−S+H2gThe degradation rate decreases in region IV and sometimes lasts long. It gradually reaches the “saturation” coverage of S/S2 on nickel catalysts at a given H2S concentration.The chemisorption process is reversible. It takes longer for the cell to recover from the deactivation. The regeneration experiment was carried out in biogas, followed by 20% steamed hydrogen. It can be seen that 50% activity of the catalysts recovers after 10 hours in biogas by removing H2S. It is possible to fully recover after another 10 hours in 20% steamed hydrogen. These results are well agreed with Ashraf et al.’s report [54]. The catalyst activity loss might be due to the coverage of sulfur species (e.g., S or S2) on the nickel surface. A high adsorption ability of hydrogen removes the sulfur species and then the catalyst recovers its catalytic activity.The recovery of the cell in the biogas in region V and steam hydrogen in region VI is recorded after the removal of the hydrogen sulfur. The recovery rate in the steam hydrogen in region VI is much faster than in the biogas in region V. This could be because of the increased hydrogen concentration (80% vs. 4%), leading to faster sulfur desorption. It was shown to be the case in SOFC with internal methane steam reforming by Riegraf et al. [55]. It takes time for the cell to allow the electrochemical reactions to take place in the biogas atmosphere. As far as reforming reactions are concerned, it takes longer to reach equilibrium. However, no reforming reactions occur in the steam hydrogen. Therefore, the dissociative adsorption of different species of sulfur (like S or S2) on the surface of the electrode is reasonably fast. The current density almost reverts to its initial value after running in the steam hydrogen for 10 hours. It is also found that the current density continues to increase throughout the testing time (24 hours).In our previous study, we demonstrated that methane content remains almost at the same level under various current loads, suggesting that methane reforming reactions are not significantly affected by the current load [47]. Figure 5(a) presents the conversion rate of methane during its exposure to H2S. The best overall conversion rate is obtained with 3BCZYYb-infiltrated Ni-YSZ; however, all the other samples also show a decreasing rate. The inset graph in Figure 5(a) shows the conversion rate of methane in the first few hours. The electrochemical (under current/voltage load) and reforming reactions (in open circuit conditions) are affected. In region I, this rate remains constant in biogas free of sulfur. The methane conversion rate decreases when sulfur is added to the biogas. In region II, the conversion rate begins to change as soon as sulfur is added. At the beginning of the addition of sulfur, 3% less methane is converted through dry reforming and steam reforming, thereby producing a notable increase of CO2 and H2O in region II (Figure 4). The variation of the gas components in the anode exhaust leads to a change in the OCV. The OCV decreases to 1.043 V after sulfur is added and then increases to 1.047 V, as shown in Figure 1. Under current/voltage load, the activities of the catalyst deactivate in the presence of sulfur. It suggests that the electrochemical reactions are halted. The drop in CH4 conversion represents the loss of catalyst activity. The methane conversion rate of the Ni-YSZ-6BCZYYb cell drops from 80% to 22.72% under open circuit conditions in 20 hours. A slightly higher degradation was found on the Ni-YSZ-9BCZYYb cell at 0.7 V from 80% to 9.76%. It suggests that sulfur poisons the reforming reactions. In region III, the open circuit voltage decreases quickly because of the sudden drop in CO and H2 in the available fuel. In region IV, the open circuit voltage further decreases, but the rate slows down because of the slow poisoning of the reforming reactions.Figure 5
(a) CH4 conversion rate of the cells changes over time in the presence of H2S. (b) Relation of gas conversion and the current load.
(a)(b)The electrochemical performance follows the same trend as the open circuit voltage in H2S. In region I, the current gradually increases in a biogas atmosphere containing no sulfur. The decrease in CO in this region (Figures 1(b), 2(a), 3(a), and 3(b)) would suggest that CO is involved in the electrochemical reactions. The addition of sulfur causes a slight decrease in region II and then increases the current in the following 2-3 hours. This change might result from the gas components by the poisoning of reforming reactions. In region III, the current of the cells decreases dramatically over time, presumably due to the poisoning of the electrochemical reactions.Faraday’s law of electrolysis is shown as [56]
(8)n=ItFυ,where n is the amount of substance with mole as the unit; t is the total time under a given current with s as the unit; I is the current with A as the unit; F is Faraday’s constant, 96500 Coulombs/mol; and υ is the valency of the ions.The reversible process of solid oxide electrolysis cells is SOFCs. Therefore, the above equation can be applied to calculate the generated current from fuels in SOFCs. The possible electrochemical reactions in SOFCs are the oxidation of hydrogen or carbon monoxide, as shown in(9)H2+O2−=H2O+2e−(10)CO+O2−=CO2+2e−Equation (8) can be written as
(11)I=ntFυ,where n/t is the gas flow rate with mole/min as the unit.Note:(1)
n is the mole of fuels consumed in the whole electrochemical reactions. It includes hydrogen oxidation and electrochemical oxidation of carbon monoxide in this study(2)
υ is the electron involved in the reactions. Here, it is 2 for the reaction of hydrogen oxidation or carbon monoxide oxidationThe volume of fuels consumed in the electrochemical reactions can be obtained by the ideal gas equation [57].
(12)PV=nRT,where P is the pressure of the tailor gas from the outlet of the anode (KPa), V is the volume of the mixed gas (ml), n is the mole involved in the electrochemical reactions (mole), R is the gas constant (8.314 J/(mol∗K)), and T is the temperature of the mixed gas (K).Combing equations (11) and (12), the current generated by the SOFCs can be expressed with the volume flow rate.
(13)I=ntFυ=QFυ,where Q is the flow rate of the gases involved in the electrochemical reactions (ml/min).As long as the gas flow rate is given, the current generation can be obtained using the above equation.According to equation (13), 7.60 ml/min of fuel is required to produce 1 A current. However, only 4.53 ml/min H2 is consumed from the gas content (Figure 5(b)). This consumption is obtained from the slope of curve 2 under 1 A current load (Figure 5(b)). Therefore, some other fuels must be taking part in the electrochemical reactions, as shown in equation (10).According to Figure5(b), the total consumption of both CO and H2 of 8.28 ml/min is consistent with the current generation according to equation (13). It suggests that the theoretical production of the current agrees with the electrochemical oxidation of hydrogen and carbon monoxide. In this case, hydrogen could be produced through WGS and then oxidized into H2O or by the direct oxidation of CO into CO2. This theoretical value of 8.28 ml/min fitting from curve 1 is slightly higher than the theoretical value of 7.60 ml/min calculated from equation (13), which is 8.8% away from ideal value. The differential could be from the experimental error, which could be improved by repeating the same experimental test on a few samples.There is no evidence of which path it goes through in the present study. It is reported in the literature that rather than being oxidized directly, CO might be producing hydrogen through WGS, and then, the as-produced hydrogen is oxidation by oxygen ions [50, 58, 59]. The performance loss results from the poisoning of these electrochemical reactions, which is significant at a high current level. It can be seen that CO and H2 decrease in region III when the current displays a fast degradation (Figure 5(b)). The concentration of H2O and CO2 shows the degree of the poisoning of the electrochemical and reforming reactions.Under open circuit conditions, the level of H2O and CO2 increases due to the poisoning of the reforming reactions. Under a constant current, the level of H2O and CO2 shows the poisoning of the dry reforming and steam reactions, and at the same time, it remains a constant value to sustain a certain level of current. Therefore, the slope of H2O and CO2 under a constant current is consistent with that of the value under open circuit conditions. Under the applied voltage, the actual current values change over time, so the content of H2O and CO2 changes over time as well.When considering the possible poisoning of all kinds of reactions, WGS is an essential reaction involved in mixed gas and has to be considered. It has been shown in the literature that WGS is at the same sites where methane reforming reactions take place. The reaction sites for the above reactions are different from electrochemical hydrogen oxidation sites [59]. We eliminated the influence of the reforming reactions and the electrochemical oxidation by subtracting any methane results in this study since we knew the effect of sulfur on the reforming and electrochemical reactions, as discussed above. Figure 6 shows the four gases of carbon monoxide, water, carbon dioxide, and hydrogen in the anode tailor gas. The corresponding calculated gas components are shown as well in Figure 6. In open circuit conditions, WGS deviates from the equilibrium state and shifts to the reversible WGS (RWGS) direction. WGS is deemed the most favorable reaction, even when added sulfur to hydrogen (Figure 6(a)). It is closest to the equilibrium conditions at the end of the sulfur poisoning test. Under a current load, electrochemical reactions are the priority. WGS is slower than the electrochemical reactions and shifts to the left as the gas components alter in the electrochemical reactions. The initial deviation with the addition of sulfur is insignificant, and it gradually becomes more distinct, indicating that the WGS poisoning in sulfur-containing biogas is getting more severe. In this study, a small anode chamber was used (volume=π×1.52×30). Therefore, there was not enough time for all the reactions to reach equilibrium in most conditions. The deviation of WGS might suggest that the electrochemical and reforming reactions do not get their equilibrium state exposed to sulfur. H2O has to be maintained at a high level by adjusting WGS to sustain a high current of 1 A. It is shown that the electrochemical and chemical reactions are related to the deviation of WGS.Figure 6
Experimental values of formation rate of four gases of CO, CO2, H2O, and H2 in the anode exhaust gas compared with values expected from equilibrium for this gas input composition. (a) The cell of Ni-YSZ cell in open circuit conditions. (b) The cell 3BCZYYb-NiO-YSZ under 1 A current load.
(a)(b)The enhancement of the cell performance with the infiltrated BCZYYb is significant, as shown in the previous paper [47]. After infiltration, there is less CO during the electrochemical process in the presence of H2S (Figure 6(b)), which might indicate that a proton conductor can act as an accelerator in WGS to some extent. More hydrogen was found in the proton conductor-infiltrated cell than in the noninfiltrated Ni-YSZ cell, which could be because of the enhancement of WGS induced by the presence of the BCZYYb. However, the initial target of enhancing the sulfur tolerance by impregnating the proton conductor of BCZYYb in the Ni-YSZ anode was not achieved. It might also have been due to an insufficient amount of proton conductors being added to the electrode. A maximum of 9BCZYYb was added in this study, presumed to be a small amount not to change the properties of the electrode significantly [45, 51].


### 3.4. Sulfur Recovery

Figure7 records the recovery of the Ni-YSZ cell in open circuit conditions after the H2S was stopped. The open circuit voltage in the biogas had quickly increased from 0.965 V to 1.014 V in 20 hours, indicating that 97% of the open circuit voltage had been recovered within that period after stopping the sulfur supply. The cell performance recovered even faster in the steam hydrogen; the open circuit voltage increased to 1.019 V in 1.6 hours and remained constant during the rest test time. With the removal of the H2S gas, the OCV started to increase, as shown in Figure 7(a). The recovery rate was slower than the degradation rate in the 4 ppm (v) H2S. The recovery of cell performance in sulfur-free fuel is a complicated process. If the sulfur poisoning is caused by sulfur adsorption, it needs to desorb after the sulfur removal. The catalyst needs to recover from the reforming and electrochemical reactions. The recovery in the steam hydrogen is much faster than in the biogas. In steamed hydrogen, it is much simpler, as there is only the hydrogen electrochemical reaction to be considered. Other researchers have already examined the function of steam in the recovery of sulfur poisoning [60]. Kim et al. found that in a steam atmosphere, there was a desirable condition for the cell performance to retrieve its original value in a highly concentrated sulfur fuel (5000 ppm (v) sulfur in 50 mol% n-decane) in a Cu-ceria anode [61]. Riegraf et al. found that a high concentration of hydrogen was the accelerator for the recovery from sulfur poisoning [55]. We also found a similar result that cell performance continuously improved in the steam hydrogen throughout the testing period (region VI, Figures 7(b) and 7(c)). Our results confirm that the nickel poisoning induced by sulfur adsorption is removable. It is, therefore, that the cell performance is recoverable. However, the cell voltage did not recover to its initial value, which could also be due to the carbon deposition. It is deemed that both sulfur poisoning and carbon position are the key factors that induce cell performance degradation in a biogas fuel with sulfur impurity. Ni-YSZ interfaces are the preferential position for carbon deposition [32]. In this study, the infiltrated BCZYYb occupies the Ni-YSZ interfaces, making carbon less likely to deposit. The barium-containing proton conductors have excellent water storage ability. These materials also exhibit high ionic and electronic conductivity. The water-gas shift reaction is promoted by the presence of BCZYYb. Therefore, infiltration of BCZYYb suppresses sulfur poisoning and carbon deposition [62]. Our results are in agreement with the report that less carbon deposition is detected at 800°C compared to that at 700°C. In other words, sulfur passivation is a potential strategy to suppress carbon deposition if sulfur poisoning is recoverable.Figure 7
Recovery of cells in different conditions. (a) OCV recovery of the Ni-YSZ cell in the biogas and the steam hydrogen in an open circuit condition after the removal of H2S. (b) Recovery of the cells in the biogas and steam hydrogen (24 hours in the biogas and the subsequent time was in the steam hydrogen (20%H2O-H2)). (c) Recovery of 3BCZYYb/Ni-YSZ in the biogas at 1 A cm-2 for 24 hours and in the steam hydrogen at 1 A cm-2.
(a)(b)(c)It has been shown that the recovery can take 24 hours of test time, regardless of the BCZYYb infiltration. In 24 hours, 70% of the cell performance can be recovered. The recovery properties might be related to the potential (or load). The results also indicate that the increase in the infiltration of BCZYYb will shorten the recovery time; for example, the recovery time of the cell Ni-YSZ-6BCZYYb, considered an optimal amount of infiltration, is shorter than that of a noninfiltrated cell. Adsorbed sulfur on an anode surface is quickly released as soon as the H2S is removed; this happens most rapidly in the initial stages of the recovery process. It is a long distance for the sulfur to transport from the TPB to the anode surface and then be released as the anode is thick (1 mm), especially after a lengthy period of sulfur poisoning. Figure 8 shows the resistance recovery of the cells in the biogas and 20% H2O-hydrogen. It is found that the cell performance recovers very slowly, and the cell resistance only reduces from 0.65Ωcm2 to 0.6Ωcm2 24 hours after the removal of the sulfur (as shown in Figure 8(a)).Figure 8
Impedance spectra of the cells. (a) In the biogas before the sulfur experiment and after recovery in biogas for 24 hours. (b) In 20%H2O-H2 before the sulfur experiment and after recovery in 20%H2O-H2 for 24 hours.
(a)(b)However, the resistance recovered in the steam hydrogen for 24 hours is close to the original value, as is shown in Figure8(b). It is believed that H2O is a positive factor for sulfur desorption from the Ni particles when the cell is exposed to sulfur [63].Figure9 shows the postimages of the cells in biogas (Figure 9(a)) and biogas with a 4 ppm (v) sulfur (Figure 9(b)). BCZYYb is covered on the surface of grains uniformly. The grains of nickel, BCZYYb, and YSZ are well distributed in the anode. We can see that the anode grains are well integrated with the triple-phase boundary (TPB). It seems that the TPB is not well integrated after the poisoning in the sulfur for 24 hours, even though it has been recovered in steamed hydrogen (Figure 9(b)). There is a more fraction of pore area in the triple-phase boundary. The performance loss is associated with the change in the microstructure, which is proven in Hagen et al.’s report [64]. Further efforts to improve the microstructure of the anode and the triple-phase boundary between the anode and the electrolyte should be made for the cell to be more stable in sulfur.Figure 9
Scanning electron microscopy (SEM) images of the cells. (a) Ni-YSZ-9BCZYYb in biogas. (b) Ni-YSZ-9BCZYYb in biogas with a 4 ppm (v) sulfur.
(a)(b)


## 3.1. Sulfur Poisoning of the Bare Nickel Electrode

The poisoning of Ni-YSZ exposed to a 4 ppm (v) sulfur-containing biogas is shown in Figure1. Figure 1(a) presents the open circuit voltage in biogas after the sulfur supply was recorded. There are four regions in the graph. In region I, the cell is stable in the biogas atmosphere without sulfur. Sulfur is added to the biogas in region II. Unlike previous reports that initial exposure to several ppm (v) H2S resulted in a rapid drop in the voltage [27], the OCV in this study slightly increased for 2 hours after adding H2S in region II, which could be because of an increase in hydrogen concentration in the feed gas. The uncertainty of measurement data cannot be ignored. The hydrogen content is calibrated, presuming a constant volume of argon in the inlet and outlet.Figure 1
Cell performance of the Ni-YSZ on exposure to 4 ppm (v) H2S. (a) OCV evolution. (b) Anode exhaust gas in open circuit conditions after adding H2S to the biogas. (c) Impedance spectra patterns in the biogas (black), in the sulfur-containing biogas for 11 hours (red), and in the sulfur-containing biogas for 20 hours (blue).
(a)(b)(c)Another possibility could be that sulfur is oxidized, and the reaction is shown in(1)H2S+O2−=12S2+H2O+2e−(2)H2S+3O2−=SO2+H2O+6e−When H2S is oxidized into S2 or SO2, the OCV is given in equations (3) and (4), respectively [36].
(3)E=E0−RT2FlnPS21/2PH2OPH2SPO21/2,(4)E=E0−RT6FlnPSO2PH2OPH2SPO23/2.Aguilar et al. also found a quick response after H2S is introduced into fuel [36]. The current density increased from 400 mA cm-2 to 500 mA cm-2 at 0.44 V in the first two to three hours and then decreased. Our results are well agreed with their report. The adsorption process of some species (S or S2) at the nickel catalyst surface enhanced cell performance in 20 ppm (v) H2S. A low concentration of H2S in this study is used so that the OCV increases slightly from 1.043 V to 1.047 V.A quick voltage drop in the next 7 hours in region III is noticed after sulfur had been supplied for 2 hours. The addition of sulfur brings about a rapid voltage drop, which could be caused by sulfur adsorption. The corresponding result is the prevention of methane conversion (Figure1(b)). We also see an increase in polarization resistance (Figure 1(c)). Nickel sulfide is not thermodynamically favored under the investigated conditions [48]. Slow degradation of cell OCV was then observed over the next 15 hours in region IV. No stable cell voltage was achieved during the whole exposure period in the H2S.Figure1(b) shows the anode exhaust during the test. It was found that the amount of CH4 in the output stream increases continuously in sulfur, which indicates that the reforming reaction was prevented by exposure to H2S. This result is similar to the experimental observation of a 90% drop in the ratio of CO/CH4, CO2/CH4, and (CO+CO2)/CH4 at a 4 ppm (v) H2S by Rasmussen and Hagen [29]. The gas change was claimed to be caused by the deactivation of steam reforming in sulfur.Steam reforming is possible.(5)CH4+H2O⟶CO+3H2Dry reforming could take place as well.(6)CH4+CO2⟶2CO+2H2It is found that CO2 and H2O increase over time, which means equations (5) and (6) might be altered. In other words, steam and dry reforming are possibly being prevented from exposure to sulfur in open circuit conditions.Figure1(c) shows the EIS data for a cell recorded under biogas and after a specific time of exposure to H2S biogas in open circuit conditions. It is noticed that the ohmic resistance in this study remains constant, and it is even smaller after adding H2S, which agreed with the report by Li and Wang [49]. They consider that the increased series resistance is attributed to the adsorption of sulfur species (like S or S2) on the nickel surface. The electrochemical reaction slows down due to less active sites [49]. Our results are not the same as some reports. The polarization and ohmic resistance increased in a CO/H2/H2O fuel, even with a low concentration of H2S [50]. The difference between our results and the report might be caused by the different gas compositions. In the present study, a more complicated system was involved, such as reforming reactions, including steam reforming (as shown in equation (1)) and dry reforming (as shown in equation (2)). These two reactions are involved in the whole process of the cell and are not related to the working conditions. Other reactions like electrochemical reactions and WGS (water-gas shift reaction) should be considered when the cell is working. The polarization resistance is 0.2Ωcm2 before sulfur is added, increasing to 0.3Ωcm2 exposure to H2S for 11 hours. The value increases to 0.5Ωcm2 after further exposure to the sulfur for 17 hours. Before the exposure to the H2S, the prevalent resistance was at 100 Hz. After exposure for 11 hours, the dominant resistances were at 1000 Hz and 100 Hz. A significant response to a higher frequency with the addition of H2S indicates that charge transfer contributes to performance loss, suggesting that the poisoning mechanism could also change [27]. It has been demonstrated that hydrogen oxidation in a hydrogen atmosphere is blocked by sulfur coverage. In a mixture of gases, the poisoning is more complicated.


## 3.2. Sulfur Poisoning of BCZYYb-Infiltrated Cells

The sulfur poisoning of the cell infiltrated with BCZYYb was investigated in this study as it was reported that BCZYYb has a high tolerance of sulfur (up to 100 ppm (v)) [51]. Previous research demonstrated that an optimal infiltration amount of BCZYYb was 0.6 wt% (Ni-YSZ-3BCZYYb) [47].Figure2 shows the performance of the Ni-YSZ-3BCZYYb cell. The power output at 800°C reaches 1.8 W cm-2 (Figure 2(a)). As the cell Ni-YSZ-3BCZYYb displays the best performance, the current density at 0.7 V would have been over the current limitation of the testing machine, and the stability was tested at 1 A cm-2. Along with the voltage evolution, the anode exhaust gases are also shown in Figure 2(b). The sulfur poisoning of BCZYYb-impregnated cells is similar to that of the noninfiltrated anodes, and no significant improvement of sulfur tolerance is found in the BCZYYb-infiltrated Ni-YSZ. Blinn and Liu [52] reported that no difference was found between a BaZr0.9Yb0.1O3-δ-modified electrode and a nonmodified one. In region I, the cell voltage gradually increased and reached a stable state at 1 A cm-2 current load over a two-hour testing time. After introducing 4 ppm (v) sulfur, the cell voltage steadily increases for about 3 hours in region II. Then, its voltage rapidly drops in region III, followed by a slow degradation in region IV. Even though not much difference was found in the cell voltage changes in the infiltrated Ni-YSZ, the gases have a different change trend than those in the noninfiltrated Ni-YSZ. The gases either increase or decrease over time. The increases in CH4 concentration during the test time might indicate that the sulfur has influenced the reforming reactions. An increase in CO2 and H2O is also noticed, meaning that dry and steam reforming reactions are affected. The hydrogen and carbon monoxide concentration decreases over the test period.Figure 2
(a) Electrochemical performance of the cell Ni-YSZ-3BCZYYb in the biogas operating at 800°C and after poisoning in sulfur for 20 hours. (b) Anode exhaust gases of the cell Ni-YSZ-3BCZYYb in the biogas and the sulfur-containing biogas, considered over a period of time at 1 A cm-2. The changes of the voltage and the ratio of H2 : CO are also shown.
(a)(b)The gas content change might be the result of both the poisoning of the reforming and electrochemical reactions. One of the parameters might be the dominant factor in this process; for example, if the electrochemical reaction dominates, the current density value will be the most important. If the reforming reaction dominates, the content of infiltrated BCZYYb will be critical as the reforming reaction is affected by the presence of BCZYYb proton conductors. Despite the deactivation of reforming reactions, there is still enough hydrogen to maintain a constant current of 1 A. Therefore, in this case, hydrogen decreased over time.Figures3(a) and 3(b) show the sulfur poisoning of Ni-YSZ-6BCZYYb and Ni-YSZ-9BCZYYb, respectively. There are four regions in the change of current density when the cell is loaded with 0.7 V (Figures 3(a) and 3(b)). The current density increases in region I steadily, while operating in the biogas atmosphere. The current density in region II increases for the first 2-3 hours of the initial exposure to H2S. A rapid current drop in region III and a slow current degradation in region IV are observed.Figure 3
(a) Anode exhaust gases of the cell Ni-YSZ-6BCZYYb in the biogas and in the sulfur-containing biogas, considered over a period of time at 0.7 V. (b) Anode exhaust gases of the cell Ni-YSZ-9BCZYYb in the biogas and in the sulfur-containing biogas, considered over a period of time at 0.7 V. The changes in the current density and the ratio of H2 : CO are also shown.
(a)(b)Four regions of change are also noticed in the gases. In region I, the CH4 content is constant as the equilibrium state of reforming is reached. Under electrochemical conditions, the increase of the water content by hydrogen oxidation promotes the WGS reactions. Therefore, the amount of CO2 increases. When the current density increases, the H2 and carbon monoxide decrease, and the carbon dioxide and H2O increase. The faster drop in the CO as compared to the other gases might suggest that the CO is involved in the electrochemical reactions. The deactivation of reforming reactions starts after introducing H2S in region II and therefore brings about a rapid increase in the CO2 and H2O. The additional increase in the current density results in a further increase in the content of CO2 and H2O. The current density reaches around 2 A cm-2 at the end of region II. More H2 and CO are consumed to maintain the current at this high level, and at the same time, more H2O and CO2 are produced due to the increased current density. In region III, the current density drops rapidly, indicating that the electrochemical reaction is affected by sulfur.Therefore, the content of hydrogen and CO is increased, which might provide evidence that CO is involved in the electrochemical reactions. Alternatively, more CO is produced because of the increased WGS reaction rate. The deviation of the electrochemical reaction causes the increased content of H2O and CO2. Once the current density reaches a high value of 1 A cm-2, the condition is similar to Ni-YSZ; the current density slowly degrades in region IV. In this region, the slow reforming reactions result in the content reduction of the H2 and CO.


## 3.3. Mechanism of Sulfur Poisoning

Table1 summarises the cells used in these experiments. The schematic diagram of the current density and the gases at 0.7 V is presented in Figure 4. The cell was tested for around 2 hours to reach the equilibrium state in the biogas (region I). The poisoning stage of the cell is divided into three regions, regions II, III, and IV. In the first stage of sulfur poisoning (region II), the cell performance increases after introducing H2S. H2S might be used as a fuel, taking part in the reactions shown in equations (1) and (2). Therefore, enhanced performance is found in region II. This process lasts about two hours, whether under a voltage/current load or in open circuit conditions.Table 1
Testing regimes for the various cells and current behaviour.
SampleElectrodeTesting conditionRegion IRegion IIRegion IIIRegion IVBiogasBiogas+H2SBiogas+2SBiogas+H2SCell ANi-YSZOpen circuit conditionsOCV stableOCV increasesRapid OCV dropSlow OCV dropCell BNi-YSZ0.7 VCurrent increasesCurrent increasesRapid current dropSlow current dropCell CNi-YSZ-BCZYYb1 ACurrent increasesCurrent increasesRapid current dropSlow current dropCell DNi-YSZ-6BCZYYb0.7 VCurrent increasesCurrent increasesRapid current dropSlow current dropCell ENi-YSZ-9BCZYYb0.7 VCurrent increasesCurrent increasesRapid current dropSlow current dropCell ANi-YSZOpen circuit conditionsOCV stableOCV increasesRapid OCV dropSlow OCV dropFigure 4
Schematic diagram of the change of current density or voltage on exposure to H2S.In the next 10 hours, the cell shows a significant drop in the OCV in open circuit conditions. In the case of a current/voltage load, the voltage/current drops dramatically in region III. The surface of nickel could be covered by the S with the continuous flow of sulfur. The electrochemical reaction active sites might be covered in this case. Therefore, the reforming reactions slow down due to less participation of nickel catalyst in the reactions. Appari et al. also [53] demonstrated that the drop in CH4 conversion in a mixture of 12.5CH4-8.4CO2-25.2H2O-53.9N2 with a 20 ppm (v) H2S could be related to the deactivation of nickel catalysts. The possible reaction is shown in
(7)Ni+H2Sg⇆Ni−S+H2gThe degradation rate decreases in region IV and sometimes lasts long. It gradually reaches the “saturation” coverage of S/S2 on nickel catalysts at a given H2S concentration.The chemisorption process is reversible. It takes longer for the cell to recover from the deactivation. The regeneration experiment was carried out in biogas, followed by 20% steamed hydrogen. It can be seen that 50% activity of the catalysts recovers after 10 hours in biogas by removing H2S. It is possible to fully recover after another 10 hours in 20% steamed hydrogen. These results are well agreed with Ashraf et al.’s report [54]. The catalyst activity loss might be due to the coverage of sulfur species (e.g., S or S2) on the nickel surface. A high adsorption ability of hydrogen removes the sulfur species and then the catalyst recovers its catalytic activity.The recovery of the cell in the biogas in region V and steam hydrogen in region VI is recorded after the removal of the hydrogen sulfur. The recovery rate in the steam hydrogen in region VI is much faster than in the biogas in region V. This could be because of the increased hydrogen concentration (80% vs. 4%), leading to faster sulfur desorption. It was shown to be the case in SOFC with internal methane steam reforming by Riegraf et al. [55]. It takes time for the cell to allow the electrochemical reactions to take place in the biogas atmosphere. As far as reforming reactions are concerned, it takes longer to reach equilibrium. However, no reforming reactions occur in the steam hydrogen. Therefore, the dissociative adsorption of different species of sulfur (like S or S2) on the surface of the electrode is reasonably fast. The current density almost reverts to its initial value after running in the steam hydrogen for 10 hours. It is also found that the current density continues to increase throughout the testing time (24 hours).In our previous study, we demonstrated that methane content remains almost at the same level under various current loads, suggesting that methane reforming reactions are not significantly affected by the current load [47]. Figure 5(a) presents the conversion rate of methane during its exposure to H2S. The best overall conversion rate is obtained with 3BCZYYb-infiltrated Ni-YSZ; however, all the other samples also show a decreasing rate. The inset graph in Figure 5(a) shows the conversion rate of methane in the first few hours. The electrochemical (under current/voltage load) and reforming reactions (in open circuit conditions) are affected. In region I, this rate remains constant in biogas free of sulfur. The methane conversion rate decreases when sulfur is added to the biogas. In region II, the conversion rate begins to change as soon as sulfur is added. At the beginning of the addition of sulfur, 3% less methane is converted through dry reforming and steam reforming, thereby producing a notable increase of CO2 and H2O in region II (Figure 4). The variation of the gas components in the anode exhaust leads to a change in the OCV. The OCV decreases to 1.043 V after sulfur is added and then increases to 1.047 V, as shown in Figure 1. Under current/voltage load, the activities of the catalyst deactivate in the presence of sulfur. It suggests that the electrochemical reactions are halted. The drop in CH4 conversion represents the loss of catalyst activity. The methane conversion rate of the Ni-YSZ-6BCZYYb cell drops from 80% to 22.72% under open circuit conditions in 20 hours. A slightly higher degradation was found on the Ni-YSZ-9BCZYYb cell at 0.7 V from 80% to 9.76%. It suggests that sulfur poisons the reforming reactions. In region III, the open circuit voltage decreases quickly because of the sudden drop in CO and H2 in the available fuel. In region IV, the open circuit voltage further decreases, but the rate slows down because of the slow poisoning of the reforming reactions.Figure 5
(a) CH4 conversion rate of the cells changes over time in the presence of H2S. (b) Relation of gas conversion and the current load.
(a)(b)The electrochemical performance follows the same trend as the open circuit voltage in H2S. In region I, the current gradually increases in a biogas atmosphere containing no sulfur. The decrease in CO in this region (Figures 1(b), 2(a), 3(a), and 3(b)) would suggest that CO is involved in the electrochemical reactions. The addition of sulfur causes a slight decrease in region II and then increases the current in the following 2-3 hours. This change might result from the gas components by the poisoning of reforming reactions. In region III, the current of the cells decreases dramatically over time, presumably due to the poisoning of the electrochemical reactions.Faraday’s law of electrolysis is shown as [56]
(8)n=ItFυ,where n is the amount of substance with mole as the unit; t is the total time under a given current with s as the unit; I is the current with A as the unit; F is Faraday’s constant, 96500 Coulombs/mol; and υ is the valency of the ions.The reversible process of solid oxide electrolysis cells is SOFCs. Therefore, the above equation can be applied to calculate the generated current from fuels in SOFCs. The possible electrochemical reactions in SOFCs are the oxidation of hydrogen or carbon monoxide, as shown in(9)H2+O2−=H2O+2e−(10)CO+O2−=CO2+2e−Equation (8) can be written as
(11)I=ntFυ,where n/t is the gas flow rate with mole/min as the unit.Note:(1)
n is the mole of fuels consumed in the whole electrochemical reactions. It includes hydrogen oxidation and electrochemical oxidation of carbon monoxide in this study(2)
υ is the electron involved in the reactions. Here, it is 2 for the reaction of hydrogen oxidation or carbon monoxide oxidationThe volume of fuels consumed in the electrochemical reactions can be obtained by the ideal gas equation [57].
(12)PV=nRT,where P is the pressure of the tailor gas from the outlet of the anode (KPa), V is the volume of the mixed gas (ml), n is the mole involved in the electrochemical reactions (mole), R is the gas constant (8.314 J/(mol∗K)), and T is the temperature of the mixed gas (K).Combing equations (11) and (12), the current generated by the SOFCs can be expressed with the volume flow rate.
(13)I=ntFυ=QFυ,where Q is the flow rate of the gases involved in the electrochemical reactions (ml/min).As long as the gas flow rate is given, the current generation can be obtained using the above equation.According to equation (13), 7.60 ml/min of fuel is required to produce 1 A current. However, only 4.53 ml/min H2 is consumed from the gas content (Figure 5(b)). This consumption is obtained from the slope of curve 2 under 1 A current load (Figure 5(b)). Therefore, some other fuels must be taking part in the electrochemical reactions, as shown in equation (10).According to Figure5(b), the total consumption of both CO and H2 of 8.28 ml/min is consistent with the current generation according to equation (13). It suggests that the theoretical production of the current agrees with the electrochemical oxidation of hydrogen and carbon monoxide. In this case, hydrogen could be produced through WGS and then oxidized into H2O or by the direct oxidation of CO into CO2. This theoretical value of 8.28 ml/min fitting from curve 1 is slightly higher than the theoretical value of 7.60 ml/min calculated from equation (13), which is 8.8% away from ideal value. The differential could be from the experimental error, which could be improved by repeating the same experimental test on a few samples.There is no evidence of which path it goes through in the present study. It is reported in the literature that rather than being oxidized directly, CO might be producing hydrogen through WGS, and then, the as-produced hydrogen is oxidation by oxygen ions [50, 58, 59]. The performance loss results from the poisoning of these electrochemical reactions, which is significant at a high current level. It can be seen that CO and H2 decrease in region III when the current displays a fast degradation (Figure 5(b)). The concentration of H2O and CO2 shows the degree of the poisoning of the electrochemical and reforming reactions.Under open circuit conditions, the level of H2O and CO2 increases due to the poisoning of the reforming reactions. Under a constant current, the level of H2O and CO2 shows the poisoning of the dry reforming and steam reactions, and at the same time, it remains a constant value to sustain a certain level of current. Therefore, the slope of H2O and CO2 under a constant current is consistent with that of the value under open circuit conditions. Under the applied voltage, the actual current values change over time, so the content of H2O and CO2 changes over time as well.When considering the possible poisoning of all kinds of reactions, WGS is an essential reaction involved in mixed gas and has to be considered. It has been shown in the literature that WGS is at the same sites where methane reforming reactions take place. The reaction sites for the above reactions are different from electrochemical hydrogen oxidation sites [59]. We eliminated the influence of the reforming reactions and the electrochemical oxidation by subtracting any methane results in this study since we knew the effect of sulfur on the reforming and electrochemical reactions, as discussed above. Figure 6 shows the four gases of carbon monoxide, water, carbon dioxide, and hydrogen in the anode tailor gas. The corresponding calculated gas components are shown as well in Figure 6. In open circuit conditions, WGS deviates from the equilibrium state and shifts to the reversible WGS (RWGS) direction. WGS is deemed the most favorable reaction, even when added sulfur to hydrogen (Figure 6(a)). It is closest to the equilibrium conditions at the end of the sulfur poisoning test. Under a current load, electrochemical reactions are the priority. WGS is slower than the electrochemical reactions and shifts to the left as the gas components alter in the electrochemical reactions. The initial deviation with the addition of sulfur is insignificant, and it gradually becomes more distinct, indicating that the WGS poisoning in sulfur-containing biogas is getting more severe. In this study, a small anode chamber was used (volume=π×1.52×30). Therefore, there was not enough time for all the reactions to reach equilibrium in most conditions. The deviation of WGS might suggest that the electrochemical and reforming reactions do not get their equilibrium state exposed to sulfur. H2O has to be maintained at a high level by adjusting WGS to sustain a high current of 1 A. It is shown that the electrochemical and chemical reactions are related to the deviation of WGS.Figure 6
Experimental values of formation rate of four gases of CO, CO2, H2O, and H2 in the anode exhaust gas compared with values expected from equilibrium for this gas input composition. (a) The cell of Ni-YSZ cell in open circuit conditions. (b) The cell 3BCZYYb-NiO-YSZ under 1 A current load.
(a)(b)The enhancement of the cell performance with the infiltrated BCZYYb is significant, as shown in the previous paper [47]. After infiltration, there is less CO during the electrochemical process in the presence of H2S (Figure 6(b)), which might indicate that a proton conductor can act as an accelerator in WGS to some extent. More hydrogen was found in the proton conductor-infiltrated cell than in the noninfiltrated Ni-YSZ cell, which could be because of the enhancement of WGS induced by the presence of the BCZYYb. However, the initial target of enhancing the sulfur tolerance by impregnating the proton conductor of BCZYYb in the Ni-YSZ anode was not achieved. It might also have been due to an insufficient amount of proton conductors being added to the electrode. A maximum of 9BCZYYb was added in this study, presumed to be a small amount not to change the properties of the electrode significantly [45, 51].


## 3.4. Sulfur Recovery

Figure7 records the recovery of the Ni-YSZ cell in open circuit conditions after the H2S was stopped. The open circuit voltage in the biogas had quickly increased from 0.965 V to 1.014 V in 20 hours, indicating that 97% of the open circuit voltage had been recovered within that period after stopping the sulfur supply. The cell performance recovered even faster in the steam hydrogen; the open circuit voltage increased to 1.019 V in 1.6 hours and remained constant during the rest test time. With the removal of the H2S gas, the OCV started to increase, as shown in Figure 7(a). The recovery rate was slower than the degradation rate in the 4 ppm (v) H2S. The recovery of cell performance in sulfur-free fuel is a complicated process. If the sulfur poisoning is caused by sulfur adsorption, it needs to desorb after the sulfur removal. The catalyst needs to recover from the reforming and electrochemical reactions. The recovery in the steam hydrogen is much faster than in the biogas. In steamed hydrogen, it is much simpler, as there is only the hydrogen electrochemical reaction to be considered. Other researchers have already examined the function of steam in the recovery of sulfur poisoning [60]. Kim et al. found that in a steam atmosphere, there was a desirable condition for the cell performance to retrieve its original value in a highly concentrated sulfur fuel (5000 ppm (v) sulfur in 50 mol% n-decane) in a Cu-ceria anode [61]. Riegraf et al. found that a high concentration of hydrogen was the accelerator for the recovery from sulfur poisoning [55]. We also found a similar result that cell performance continuously improved in the steam hydrogen throughout the testing period (region VI, Figures 7(b) and 7(c)). Our results confirm that the nickel poisoning induced by sulfur adsorption is removable. It is, therefore, that the cell performance is recoverable. However, the cell voltage did not recover to its initial value, which could also be due to the carbon deposition. It is deemed that both sulfur poisoning and carbon position are the key factors that induce cell performance degradation in a biogas fuel with sulfur impurity. Ni-YSZ interfaces are the preferential position for carbon deposition [32]. In this study, the infiltrated BCZYYb occupies the Ni-YSZ interfaces, making carbon less likely to deposit. The barium-containing proton conductors have excellent water storage ability. These materials also exhibit high ionic and electronic conductivity. The water-gas shift reaction is promoted by the presence of BCZYYb. Therefore, infiltration of BCZYYb suppresses sulfur poisoning and carbon deposition [62]. Our results are in agreement with the report that less carbon deposition is detected at 800°C compared to that at 700°C. In other words, sulfur passivation is a potential strategy to suppress carbon deposition if sulfur poisoning is recoverable.Figure 7
Recovery of cells in different conditions. (a) OCV recovery of the Ni-YSZ cell in the biogas and the steam hydrogen in an open circuit condition after the removal of H2S. (b) Recovery of the cells in the biogas and steam hydrogen (24 hours in the biogas and the subsequent time was in the steam hydrogen (20%H2O-H2)). (c) Recovery of 3BCZYYb/Ni-YSZ in the biogas at 1 A cm-2 for 24 hours and in the steam hydrogen at 1 A cm-2.
(a)(b)(c)It has been shown that the recovery can take 24 hours of test time, regardless of the BCZYYb infiltration. In 24 hours, 70% of the cell performance can be recovered. The recovery properties might be related to the potential (or load). The results also indicate that the increase in the infiltration of BCZYYb will shorten the recovery time; for example, the recovery time of the cell Ni-YSZ-6BCZYYb, considered an optimal amount of infiltration, is shorter than that of a noninfiltrated cell. Adsorbed sulfur on an anode surface is quickly released as soon as the H2S is removed; this happens most rapidly in the initial stages of the recovery process. It is a long distance for the sulfur to transport from the TPB to the anode surface and then be released as the anode is thick (1 mm), especially after a lengthy period of sulfur poisoning. Figure 8 shows the resistance recovery of the cells in the biogas and 20% H2O-hydrogen. It is found that the cell performance recovers very slowly, and the cell resistance only reduces from 0.65Ωcm2 to 0.6Ωcm2 24 hours after the removal of the sulfur (as shown in Figure 8(a)).Figure 8
Impedance spectra of the cells. (a) In the biogas before the sulfur experiment and after recovery in biogas for 24 hours. (b) In 20%H2O-H2 before the sulfur experiment and after recovery in 20%H2O-H2 for 24 hours.
(a)(b)However, the resistance recovered in the steam hydrogen for 24 hours is close to the original value, as is shown in Figure8(b). It is believed that H2O is a positive factor for sulfur desorption from the Ni particles when the cell is exposed to sulfur [63].Figure9 shows the postimages of the cells in biogas (Figure 9(a)) and biogas with a 4 ppm (v) sulfur (Figure 9(b)). BCZYYb is covered on the surface of grains uniformly. The grains of nickel, BCZYYb, and YSZ are well distributed in the anode. We can see that the anode grains are well integrated with the triple-phase boundary (TPB). It seems that the TPB is not well integrated after the poisoning in the sulfur for 24 hours, even though it has been recovered in steamed hydrogen (Figure 9(b)). There is a more fraction of pore area in the triple-phase boundary. The performance loss is associated with the change in the microstructure, which is proven in Hagen et al.’s report [64]. Further efforts to improve the microstructure of the anode and the triple-phase boundary between the anode and the electrolyte should be made for the cell to be more stable in sulfur.Figure 9
Scanning electron microscopy (SEM) images of the cells. (a) Ni-YSZ-9BCZYYb in biogas. (b) Ni-YSZ-9BCZYYb in biogas with a 4 ppm (v) sulfur.
(a)(b)


## 4. Conclusions

The durability of Ni-YSZ anode-supported cells impregnated with BaCe0.7Zr0.1Y0.1Yb0.1O3-δ (BCZYYb) operating up to 1.8 W cm-2 in sulfur-containing biogas with a composition of 36CH4-36CO2-20H2O-4H2-4CO was investigated. A 4 ppm (v) H2S was applied to study the sulfur poisoning in open circuit and electrochemical reaction conditions for around 24 hours. The gas analysis showed that the conversion rate of CH4 decreased on exposure to sulfur, indicating that sulfur poisoning in the biogas was caused by the reduction of reforming reactions in the equilibrium state. The sulfur poisoning could be seen in three regions: an increase in cell performance in the initial 2-3 hours (region II), a rapid drop of current density in the following 5-7 hours (region III), and slow degradation of current density in 8-10 hours (region IV). H2O and CO2 increased due to the quick poisoning of the reforming reactions in region II. A reversible change of all these gases was found in region III. More hydrogen and CO were observed due to the poisoning of electrochemical reactions in H2S. In region IV, hydrogen and carbon monoxide decreased again, while the amount of H2O and CO2 remained unchanged. It suggests that a possible equilibrium state was reached due to the slow degradation of current density. The impedance spectrum results indicated that the power output loss on the sulfur exposure was owing to increased polarization resistance in the charge transfer process at a high frequency. The recovery experiments in this study showed that the poisoning was reversible despite a 70% performance loss within 22 hours in sulfur. BaCe0.7Zr0.1Y0.1Yb0.1O3-δ- (BCZYYb-) infiltrated Ni-YSZ showed promising electrochemical performance and was highly resistant to sulfur poisoning. However, a small amount of BaCe0.7Zr0.1Y0.1Yb0.1O3-δ (BCZYYb) was applied, and the sulfur poisoning mechanism needs to be investigated in future research.



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*Source: 2339117-2023-02-16.xml*