Investigation of Perovskite Oxide SrCo0.8Cu0.1Nb0.1O3–δ as a Cathode Material for Room Temperature Direct Ammonia Fuel Cells

Abstract Single‐phase perovskite oxide SrCo0.8Cu0.1Nb0.1O3–δ was synthesized using a Pechini method. X‐ray diffraction (XRD) analysis indicated a cubic structure with a=3.8806(7) Å. The oxide material was combined with active carbon, forming a composite electrode to be used as the cathode in a room temperature ammonia fuel cell based on an anion membrane electrolyte and NiCu/C anode. An open circuit voltage (OCV) of 0.19 V was observed with dilute 0.02 m (340 ppm) ammonia solution as the fuel. The power density and OCV were improved upon the addition of 1 m NaOH to the fuel, suggesting that the addition of NaOH, which could be achieved through the introduction of alkaline waste to the fuel stream, could improve performance when wastewater is used as the fuel. It was found that the SrCo0.8Cu0.1Nb0.1O3−δ cathode was converted from irregular shape into shuttle‐shape during the fuel cell measurements. As the key catalysts for electrode materials for this fuel cell are all inexpensive, after further development, this could be a promising technology for removal of ammonia from wastewater.


Introduction
Fuel cells are ap romising technology for converting the chemical energy in fuels directly into electrical energy.A mmonia is an important energy conversion and hydrogen storagem aterial with ah ydrogen weight percento f1 7.8 wt %. [1,2] Compared with other fuels, such as hydrogen, methanol, and ethanol, ammonia has been regarded as ap romising potentialf uel because it is carbon-free with high volumetric and gravimetric energy densities. [2,3] In addition, there is ah igh concentration of ammonia in wastewater,s uch as sewage, landfill leachate, and spent lee from distilleries. Sewage wastewater may also contain urea;h owever,u rea naturallyh ydrolyses into ammonia, [4] and there are free ammonia or ammoniumi ons existing in wastewater. [5] Conventionally,a mmonia removal is done using ac ombined aerobic and anaerobic treatment process in wastewater treatment plants (WWTPs). This process requires energy for aeration, which can constitute up to 60 %o ft he total energy consumption of aW WTP. [6] On the other hand, ammonia is at ypical fuel that contains al arge quantity of chemicale nergy.I nstead of consuming energy,u seful electricity can be generatedf rom ammonia in wastewater if al ow-cost ammonia fuel cell is developeda nd applied; ammonia fuel cells are promising technologiest or emove ammonia in wastewater at low or negative energy consumption.
Recently,a mmonia fuel cells have attracted researchers' interests. [7][8][9] The main reports on ammonia fuel cells are based on solid oxide electrolytes. The high operating temperature of ammonia solid oxide fuel cells (SOFCs) or ammoniaa lkaline fuel cells (AFCs) makes them very difficult to be directly used for ammonia-containing wastewater treatment owing to the high thermalc apacityo fw ater. [7,[10][11][12] There are also reports on low-temperature ammonia fuel cellsb ased on alkaline membrane or acidic Nafion membrane electrolytes. [5,[13][14][15] In reported room temperature ammonia fuel cells, preciousm etals, such as Pt, werer egarded as one of the most activee lectrocatalysts for the ammonia oxidation reaction( AOR) and oxygen reduction reaction (ORR) owing to their high activity and low overpotential. [7,13,14,[16][17][18][19][20] Ad irect ammonia microfluidic fuel cell was also reported using KOH solutiona st he electrolyte. [21] The key to improving the workinge fficiency of ammonia fuel cells is to improvet he performance of the catalysts in the anode,c athode, and electrolyte, and to reduce the overall cost. One of the key challenges towards this goal is to develop low-cost electro-catalysts. In our research, we focusedo nd evelopinganew low-coste lectrocatalyst to improve performance of room temperature ammonia fuel cellsu nder alkaline conditions. Single-phase perovskite oxide SrCo 0.8 Cu 0.1 Nb 0.1 O 3-d was synthesized using aP echini method. X-ray diffraction (XRD) analysis indicated ac ubic structure with a = 3.8806 (7) .T he oxide material was combined with active carbon, forming ac omposite electrode to be used as the cathode in ar oom temperature ammonia fuel cell based on an anion membrane electrolyte and NiCu/C anode. An open circuit voltage (OCV) of 0.19 Vw as observed with dilute 0.02 m (340 ppm) ammonia solution as the fuel. The power density and OCVw ere improved upon the addition of 1 m NaOH to the fuel, suggesting that the addition of NaOH, which could be achieved through the introduction of alkaline waste to the fuel stream, could improvep erformance when wastewater is used as the fuel. It was found that the SrCo 0.8 Cu 0.1 Nb 0.1 O 3Àd cathode wasc onverted from irregular shape into shuttle-shape during the fuel cell measurements. As the key catalysts for electrode materials for this fuel cell are all inexpensive, after furtherd evelopment, this could be ap romising technology for removal of ammonia from wastewater.
In an ammonia fuel cell, AOR occurs at the anode: [5] and ORR occurs at the cathode: [5,22] At present, many non-precious metal catalysts with higher activity for ORR have been discovered to replace Pt group metals.F or example, metal/nitrogen/carbon composites prepared by the pyrolysis of inexpensive materials can effectively controlt he porosity and final structure of the catalysts. In addition, ORR is very sensitive to surface electronic properties and coordination of electron surface atoms or catalysts. Therefore, changing the metal organic framework ando ptimizing the catalyst atomic structure [23] has also becomeapopular research area. For instance,h eterogeneousa tom-doped carbon materials have been widely studied because of their low-costa nd abundant raw materials, high catalytic activity,h ighc hemical stability, and environmental friendliness. [24] Moreover,p orous morphology and larger electrochemical surfacea rea also improve catalytic activity. [25] The best combination of oxide and nanocarbon can achievee xcellent stability. In recent years, perovskite oxidesh ave become ah ighly efficient ORR catalystt o replacep reciousm etal catalysts because of their high catalytic activity,variety,a nd low cost. [26] Perovskite oxides have been widely used as both cathode and anode in SOFCs. It is reported that mixed ionic and electronic conducting La x Sr 1Àx Co 1Ày Fe y O 3Àd (LSCF) perovskites are currently the adopted ORR cathode in industry. [27] Other mixed conducting perovskite or perovskite-related oxides, such as La x Sr 1Àx CoO 3Àd [28] and SrCo 1Àx Nb x O 3Àd [29] ,h ave better ORR activity than LSCF but have higher thermal expansion coefficients. [27] From our previous work, we investigated Cu-doped SrFe 0.9 Nb 0.1 O 3Àd and found that SrFe 0.8 Cu 0.1 Nb 0.1 O 3Àd exhibits the highest conductivity in air when appliedi nS OFCs. [30] Therefore, the Cu-doped SrCo 1Àx Nb x O 3Àd ,which was not reported before,i se nvisaged to also exhibit good ORR activity in fuel cells and could be ap otentialc athode material for room temperature fuel cells.
To the best of our knowledge,r eports on using perovskite oxide as the cathode material in at wo-electrode fuel cell is scarce. In this study,w er eport the performance of a SrCo 0.8 Cu 0.1 Nb 0.1 O 3Àd cathode in atwo-electrode room temperature directammonia fuel cell.
Ag ood AOR anode is also desired to rigorously investigate with the developed cathode material. There is as ignificant amount of researcho nt he hydrogen oxidation reaction (HOR), which has similar reactions to AOR in alkaline medium. [31] Identifying non-Pt electrocatalysts for HOR andA OR is ac hallenge. [31,32] In our previouss tudies,w ei dentified that NiCu bimetal andh ierarchical nickel-copper hydroxide nanowires are excellent catalysts for electrochemical oxidation of ammonia. [33,34] These materials are expected to be good anode materials for ammonia fuel cells. Indeed,i tw as reportedt hat NiCu nanoparticles supported on carbon is ag ood anode for a direct ammonia microfluidic fuel cell. [21] Therefore, in this work, the NiCu/C catalysti sa lso used as an anode for ac onventional ammonia fuel cell. Additionally,i tw as found that Ni-based electrocatalysts exhibit high activity for hydrogen oxidation reaction (HOR); [35] specifically,N i 95 Cu 5 -alloy nanoparticless upported in carbon blacksw ere successfully applied in an anion exchange membrane fuel cell. [36] The key task of this work is to identify low-cost oxide cathode materials for room temperature direct ammonia fuel cells. The fuel cell performance of a room temperature ammonia fuel cell containing a SrCo 0.8 Cu 0.1 Nb 0.1 O 3Àd /C cathode and NiCu/Ca node is presented.

O 3Àd and NiCu
The perovskite oxide SrCo 0.8 Cu 0.1 Nb 0.1 O 3Àd was synthesized through aP echini method, [37] similar to the synthesis of SrFe 0.8 Cu 0.1 Nb 0.1 -O 3Àd . [30]  , with am olar ratio of 1.2:1 of citric acid to the total molar of metal ions, was added into the mixed solution. Ethylene glycol (Fisher Scientific), with am olar ratio of 1:1 to the citric acid, was added into the mixed solution and magnetically stirred at 120 8Cf or over 10 ho nahot plate. While stirring, a gel formed. Then the formed gel was dried at ac onstant temperature of 410 8Ct ob ei gnited for combustion. After the organic components in the mixture burned off during the drying process, the powder was ground and calcined in am uffle furnace at 500 8Cf or 2h with the heating/cooling rate of 5 8Cmin À1 .A fter regrinding, the powder was then fired at 1000 8Cf or 4h with ah eating/cooling rate of 3 8Cmin À1 .T he as-prepared SrCo 0.8 Cu 0.1 Nb 0.1 O 3Àd was used for materials characterization and fuel cell measurements. The NiCu nanoparticles supported on carbon black were synthesized by ahydrothermal method, [38] similar to the method for preparation of Ni 50 Cu 50 /C catalyst reported in aprevious paper. [21] Firstly, carbon black (50 mg, Cabot Vulcan XC-72R) was ultrasonicated in 5mLd eionized water for 30 min to form an ink. Then, NiSO 4 ·6 H 2 O (0.114 g) and CuSO 4 ·5 H 2 O( 0.099 g) were added to the prepared ink and ultrasonicated for another 20 min. The mixture was placed in an ice-water container.F resh NaBH 4 (0.1 g) solution, dissolved in 5mLd eionized water,w as added dropwise into the container under stirring for 2h.A fter reaction, the mixture was transferred to aT eflon-lined stainless autoclave that was then sealed and put in an oven at 150 8Cf or 4h.A fter cooling to room temperature, the powder was centrifuged at 10 000 rpm. for 10 min and washed several times with deionized water.T he final NiCu/C catalysts were collected after drying at 70 8Cf or 12 h. SUPRA 55-VP scanning microscope. Energy dispersive X-ray spectroscopy (EDS) was used to analyze the electrode and determine the element composition of the samples, including both point and area analyses. [39] Fuel cell fabrication Plain carbon fiber cloth (0.35 mm thickness, E-TEK) was used as the substrate for the catalysts. The carbon cloth electrode (1 2cm 2 ) was sonicated in dilute hydrochloric acid, water,a nd isopropanol, respectively,a sp retreatments. 1g prepared SrCo 0.8 Cu 0.1 Nb 0.1 O 3-d powder was mixed with 0.2 gc arbon black (Cabot Vulcan XC-72) and 0.2 gA mberlite IRA-402(OH) resin (Alfa Aesar) through ball-mill machine (Ortoalresa OABM 255) at 200 rpm for 24 h. Then polytetrafluoroethylene (PTFE) suspension with 0.2 gP TFE was added into the milled mixture using 5mLw ater and 5mLi sopropanol as the solvent. [40,41] The mixture was stirred at room temperature for 48 ht op repare the ink. Afterwards, the as-prepared ink was brushed onto the pre-treated carbon cloth and the electrode was put in af ume cupboard to naturally dry overnight. The loading of catalyst SrCo 0. 8  The fumapem FAAa nion exchange membrane (Fumatech, OHform) was used as the electrolyte. The anode material was NiCu nanoparticles supported on carbon black, which was synthesized by hydrothermal method, as described above. Plain carbon cloth was also used as the anode substrate. [42] The loading of NiCu/C catalyst was 2.5 mg cm À2 with the NiCu bimetal loading of 1.2 mg cm À2 .T he effective area of cell was 1 1cm 2 .

Electrochemical measurements
After all the components of fuel cell, including anode, cathode, and electrolyte membrane, were prepared, the fuel cell was assembled. Ammonia solution was slowly pumped into the anode chamber as the fuel, and compressed air was introduced into the cathode chamber from the opposite direction. The flow rate of ammonia solution was controlled by as mall pump that rotated at 20 rpm with af low rate of the ammonia solution of approximately 1mLmin À1 .T he flow rate of air was controlled at 10 mL min À1 .T he performance of the fuel cell was tested by aS olartron 1287A electrochemical interface controlled by electrochemical software Corr-Ware/CorrView. The fuel was prepared using concentrated ammonia solution (35 %, 0.88 gmL À1 ,F isher Chemical) and deionized water.N aOH (98 %, Alfa Aesar) was added to research the performance of fuel cell under alkaline conditions. At the beginning, the performance of the fuel cell was tested at different concentrations of ammonia solution at room temperature without the addition of NaOH. Then 1 m NaOH was added to the ammonia solution to adjust the pH value of the fuel. As the ammonia content in the wastewater is usually between 200 ppm ( % 0.01 m)a nd 2000 ppm ( % 0.1 m), it is important to test the performance of the cell when low concentration of ammonia solution is used as the fuel.

Results and Discussion
Characterizationo ft he catalyst XRD was used to determine the phase of the synthesized SrCo 0.8 Cu 0.1 Nb 0.1 O 3Àd .T he room temperature XRD pattern of the as-prepared SrCo 0.8 Cu 0.1 Nb 0.1 O 3Àd powder is showni n Figure 1, confirming that as ingle-phase perovskite oxide SrCo 0.8 Cu 0.1 Nb 0.1 O 3Àd was obtained. It can be indexed as ac ubic structure with a = 3.8806 (7) .T herefore, an ew perovskite oxide SrCo 0.8 Cu 0.1 Nb 0.1 O 3Àd was successfully synthesized.

Ammonia fuel cell performancewithout the addition of NaOH
In reported work on room temperature ammonia fuel cells, the addition of alkaline solutions, such as aqueous NaOH and KOH, increases the pH value, facilitating the oxidation of ammonia because of the increase in the OH À ion concentration. [13] The addition of NaOH/KOH will help the removal of ammonia, but it is desired to neutralize the alkaline wastewater in the following step before disposing to drainage, which means additional cost. Therefore, it would be ideal for the fuel cell to remove ammonia from wastewaterw ithout requiring alkaline additives.I nt his study,w ei nitially measure the ammonia fuel cell performance using ammonia solution without addition of alkaline. Figure 2s hows the current-voltage (I-V)c urves of the fuel cell at room temperature with different concentrations of ammonia aqueous solution. It was found that the open circuit voltage (OCV) increased when the ammonia concentration increased.Amaximum current and power density of 0.9 mA cm À2 and 0.053 mW cm À2 were achievedw hen 35 wt % concentrated ammonia was used as the fuel.
It should be noted that, when researchers investigate the activities of perovskite oxidesonORR in alkalinec onditions, addition of as trongb ase, such as KOH or NaOH is very common to create an alkaline environment. [26] However,f rom Figure 2, reasonable performance is achieved despite the absence of NaOH, especially at the lowest ammonia concentration of 0.02 m where an open circuit voltage (OCV) of 0.19 Vi sa chieved. These resultss uggest that, for normalw astewater containing 1000 ppm (i.e.,0 .06 m) ammonia,t his fuel cell can generate almost0 .19 VO CV and 0.08 mA cm À2 currentd ensity Ammoniafuel cell performancewiththe addition of 1 m NaOH In our previousstudies, it was found that the NiCu bimetal and nickel copper hydroxide catalysts work better forA OR under alkaline conditions. [33,34] On the other hand, most perovskite oxides work better under alkaline conditions as ORR catalysts when measured by three electrode methods. [26] Therefore, the addition of alkaline additives, such as NaOH, willi ncrease the pH value and facilitateb oth the anode and cathode reactions, thus the ammonia fuel cell is expected to performbetter. Figure 3s hows the fuel cell performance when 1 m NaOH was added to the ammonia fuel. Comparedw ith Figure 2, it is obvious that, when the ammonia concentration is lower than 1 m,a dding NaOH to increase the pH of fuel can improvet he OCV to 0.35 Va nd can increase the currentd ensity over 5times. For high concentration ammonia, the increaseo fp H does not contributet he OCV,b ut can also improve current density and powerd ensity almost7times. Am aximum current and powerd ensity of 5mAcm À2 and0 .25mWcm À2 wasa ch-ieved when 35 wt %c oncentrated ammonia with 1 m NaOH was used as the fuel. This is because the concentration of OH À ion is high when the ammonia concentrationi sh igh, resulting in av ery high pH value of the fuel, whichc an increaset he reaction rates of both AOR and ORR.
To quicklyr emove ammonia from wastewater,i ncreasing the pH value of the fuel is ag ood choice. This can be achieved by addinga lkaline waste, such as coal fly ash. The powerd ensity of the room temperature fuel cell is lower than those reported for hydrogen and hydrazine when Pt-or Pd-based catalysts were used in the electrodes, [43] but it is more suitable for large scale applicationb ecause of the low-cost catalystsu sed in this study.

XRD and SEM/EDS analysis after ammoniafuel cell measurements
To study the stability of the cathode materialu nder the fuel cell condition, XRD analyses were carried out on the cathode before after the fuel cell measurements.   www.chemsuschem.org at room temperature. The peaks of perovskite oxidesc an be observede asily in these two plots. Some obvious peaks, for example at 2 q of % 268 and 298,o ft he electrode after the test are affected by the carbon cloth as shown in Figure 4b.A fter the long-time test, the majority of SrCo 0.8 Cu 0.1 Nb 0.1 O 3Àd on the carbon cloth electrode was stuck to the membrane, which affected the observation and research on the electrode. The smallp eak at 2 q of % 378 belongs to the PTFE binder. [44] It was found that the intensity of the perovskite oxide SrCo 0.8 Cu 0.1 Nb 0.1 O 3Àd decreased after the fuel cell measurements.T his could be relatedt ot he quantity of the powder because most catalystw as stuck on the alkaline when separating the membrane electrode assembly (MEA). Another possibility is that, the SrCo 0.8 Cu 0.1 Nb 0.1 O 3Àd became less crystallized. There could be somei nteraction between ammonia and/or NaOH with the SrCo 0.8 Cu 0.1 Nb 0.1 O 3Àd crystals, weakening the bonds in SrCo 0.8 Cu 0.1 Nb 0.1 O 3Àd ,r esulting in reduced crystallinity.T his is also confirmed by the SEM observation of the cathode, as discussed below.
The SEM pictures of the cathode before and after the fuel cell measurements are shown in Figure 5. The    (Figure 4b). At 10 000 times magnification, it can be seen that the particle size of SrCo 0.8 Cu 0.1 Nb 0.1 O 3Àd was also reduced from 1t o0 .1 mma fter the fuel cell measurements (Figure 6and 7). The reason for this change in shape needs further investigation. Figure 6s hows the EDS analysis of the electrode before fuel cell testing. According to the energy spectrum, all of the chemical elementso fS rCo 0.8 Cu 0.1 Nb 0.1 O 3Àd can be observed clearly. In addition, elements of C, F, and Oa re visible, which come from chemicals in the ink, such as PTFE. Figure 7s hows the EDS analysiso ft he cathode after fuel cell test. The EDSr esults have proved that the nanoscale crystal contains all of the chemical elements of SrCo 0.8 Cu 0.1 Nb 0.1 O 3d .I na ddition, Na wasv ery clearly observed as the tested fuels consist of NaOH, which may indicate fuel crossover from the anode to the cathode.F uel cross-over is very commoni nf uel cells based on polymeric electrolytes, particularly for liquid fuels. The fuel cross-over can reduce the potentialb etween anode and cathode, leading to decreased performance of fuel cell. This can be alleviated by adding additives to the electrolyte membrane to improvefuel cell performance.

Conclusions
The perovskite oxide SrCo 0.8 Cu 0.1 Nb 0.1 O 3Àd was successfully synthesized using aP echini method. X-ray diffraction (XRD) analysis indicated that it exhibits ac ubic structure with a = 3.8806 (7) .T his study indicates that SrCo 0.8 Cu 0.1 Nb 0.1 O 3Àd is a good oxygen reduction reaction( ORR) catalystt hat can be used as the cathode for ar oom temperature ammonia fuel cell. Reasonable open circuit voltage (OCV) andp ower density were observedf or ad irect ammonia fuel cell using a SrCo 0.8 Cu 0.1 Nb 0.1 O 3Àd /C composite cathode and NiCu/Ca node when ammonias olutionsw ithoutt he addition of NaOH were used as the fuel. An OCV of 0.19 Vw as observed when the ammonia concentration is as low as 0.02 m (340 ppm). The power density can be improved 7times when adding1 m NaOH to the fuel, and the OCV can be improved through the addition of NaOH when the ammonia concentration is lower than 1 m. Therefore, to obtain good OCV for fuel cells with low concentration ammonia in wastewater,a dding NaOH is necessary, which can be achieved through the addition of alkaline waste, such as coal fly ash. The ammonia fuel cell also functions without the addition of NaOH, but will certainly take longer time to remove ammonia owing to the lower power density.I nterestingly, it was found that the SrCo 0.8 Cu 0.1 Nb 0.1 O 3Àd in the cathode is converted from irregular shape into shuttle-shape during the fuel cell measurements, which needs further investigation.Ast he key catalysts in this fuel cell are all non-precious, after further development, this couldb eap romising technology for removalo fammonia from wastewater.