Nanosheets of Cuco2o4 As a High-Performance Electrocatalyst in Urea Oxidation

applied sciences

Article Nanosheets of CuCo2O4 As a High-Performance Electrocatalyst in Urea Oxidation

Camila Zequine 1, Fangzhou Wang 2, Xianglin Li 2 , Deepa Guragain 3, S.R. Mishra 3, K. Siam 1, P. K. Kahol 4 and Ram K. Gupta 1,5,*

1 Department of Chemistry, Pittsburg State University, Pittsburg, KS 66762, USA; [email protected] (C.Z.); [email protected] (K.S.) 2 Department of Mechanical Engineering, University of Kansas, Lawrence, KS 66046, USA; [email protected] (F.W.); [email protected] (X.L.) 3 Department of Physics and Materials Science, The University 3Department of Physics and Materials Science, The University of Memphis, Memphis, TN 38152, USA; [email protected] (D.G.); [email protected] (S.R.M.) 4 Department of Physics, Pittsburg State University, Pittsburg, KS 66762, USA; [email protected] 5 Kansas Polymer Research Center, Pittsburg State University, Pittsburg, KS 66762, USA * Correspondence: [email protected]; Tel.: +1-620-2354763

Received: 24 January 2019; Accepted: 20 February 2019; Published: 24 February 2019

Abstract: The urea oxidation reaction (UOR) is a possible solution to solve the world’s energy crisis. Fuel cells have been used in the UOR to generate hydrogen with a lower potential compared to water splitting, decreasing the costs of energy production. Urea is abundantly present in agricultural waste and in industrial and human wastewater. Besides generating hydrogen, this reaction provides a pathway to eliminate urea, which is a hazard in the environment and to people’s health. In this study, nanosheets of CuCo2O4 grown on nickel foam were synthesized as an electrocatalyst for urea oxidation to generate hydrogen as a green fuel. The synthesized electrocatalyst was characterized using X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy. The electroactivity of CuCo2O4 towards the oxidation of urea in alkaline solution was evaluated using electrochemical measurements. Nanosheets of CuCo2O4 grown on nickel foam required the potential 2 of 1.36 V in 1 M KOH with 0.33 M urea to deliver a current density of 10 mA/cm . The CuCo2O4 electrode was electrochemically stable for over 15 h of continuous measurements. The high catalytic activities for the hydrogen evolution reaction make the CuCo2O4 electrode a bifunctional catalyst and a promising electroactive material for hydrogen production. The two-electrode electrolyzer demanded a potential of 1.45 V, which was 260 mV less than that for the urea-free counterpart. Our study suggests that the CuCo2O4 electrode can be a promising material as an efﬁcient UOR catalyst for fuel cells to generate hydrogen at a low cost.

Keywords: urea oxidation reaction; fuel cells; cyclic voltammetry; hydrogen production

1. Introduction In recent years, the utilization of urea as an alternative fuel in fuel cells has grown signiﬁcantly, since it is highly available, stable, and nonﬂammable [1]. Fuel cells are devices that convert chemical energy into electrical energy, playing a vital role in overcoming the world energy crisis [2,3]. Urea is an abundant waste generated in agricultural land, and it is present in industrial and human wastewater as well. Urea is produced from natural gas or can be synthesized from ammonia. When in contact with atmosphere and groundwater, urea breaks down into ammonia and nitrate, causing health and environmental problems [4–6]. A solution to eliminate these problems caused by urea, and to

Appl. Sci. 2019, 9, 793; doi:10.3390/app9040793 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 793 2 of 9 also generate clean energy, is the urea oxidation reaction (UOR) [7]. This reaction is described as follows [4,5]: CO(NH2)2 + H2O → N2 + 3H2 + CO2 (1) Thermodynamically, water splitting occurs at 1.23 V to generate hydrogen, while urea oxidation could provide hydrogen at a lower potential of 0.37 V [4,8]. This means that urea oxidation provides cheaper hydrogen production than water electrolysis. In previous reports, it is common to see the intense utilization of nickel-based catalysts to oxidize urea efficiently at low overpotential [3,9,10]. Forslund et al. reported a study on the use of LaNiO3 as a catalyst for the electrooxidation of urea in alkaline conditions [4]. The nonstoichiometric spinel structure Ni1.5Mn1.5O4 synthesized by Periyasamy et al., using a hydrothermal synthesis process, showed excellent electrochemical properties with the highest activity toward electro-oxidation of urea in alkaline solutions [1]. The development of porous Ni3N nanosheet arrays on carbon cloth studied by Liu et al. also showed high performance and was a durable electrocatalyst for urea oxidation [11]. A possibility to use the same principles, but with a different component, led us to develop and produce copper-based catalysts that can oxidize urea efficiently to improve the properties of fuel cells. Electrode materials made of transition metal oxides are often used as electrocatalysts because of their high electrochemical performance, natural abundance, and low cost. In addition, copper and cobalt salts are considered a promising electrode material for energy generation and storage devices due to its nontoxicity and high electrical conductivity. Zheng et al. have shown that cobalt-based compounds are excellent for hydrogen adsorption [12]. Mesostructured CuCo2S4/CuCo2O4 nanoflowers were synthesized by Xu et al. and showed an excellent electrochemical performance for energy applications [13]. An asymmetric supercapacitor based on CuCo2S4/CuCo2O4 as the cathode and graphene aerogel as the anode delivered a high energy density of 33.2Wh/kg and power density of 13.3 kW/kg. Tong et al. used NiMoO4 nanosheets to study the performance in the urea oxidation reaction. They found a larger number of exposed active sites, faster electron transport, and lower adsorption energy of urea molecules. Comparing the cyclic voltammetry (CV) curve recorded in KOH solution, the anodic current density of NiMoO4 increased largely after adding urea, demonstrating its high catalytic response activity for electro-oxidation of urea [5]. As reported by Wu et al., a nickel hydroxide electrode with nanocup architecture offered better electrocatalytic performance during electrolysis of urea [14]. Nickel oxide−nickel hybrid nanoarrays on nickel foam (NiO−Ni/NF) was studied by Yue et al. for water splitting and urea oxidation [9]. The electrode showed excellent activity for urea oxidation with the need of a low potential in 1.0 M KOH with 0.33 M urea. Ji et al. reported that open-ended α-Ni(OH)2 nanotubes provided effective surface area and active sites when exposed to an electrolyte for urea electrolysis, ensuring a much higher current density during urea electrolysis [15]. Hence, in this work, CuCo2O4 was synthesized by the hydrothermal method, and the electrocatalytic activity was studied for the oxidation of urea. Materials with three-dimensional (3D) architectures are attractive because their high surface area provides many active sites, and, thus, better electron transfer. The material studied in this report is a 3D flower-like structure composed of CuCo2O4 nanosheets, which showed excellent electrochemical properties with a high activity.

2. Experimental Details

2.1. Preparation of CuCo2O4 on Ni Foam

The CuCo2O4 was synthesized on Ni foam using a one-pot hydrothermal method followed by an annealing process. A piece of Ni foam was cleaned with 3 M HCl solution in an ultrasound bath to remove the NiO layer on the surface, and was then washed with deionized water several times. In a typical synthesis, 528 mg of Co(NO3)2.6H2O, 93.8 mg of Cu(NO3)2.3H2O, and 721 mg of urea were dissolved in 34 mL of ethanol. The solution was transferred to a 45 mL Teﬂon autoclave reactor along with the pre-cleaned Ni foam and maintained at 120 ◦C for 8 h. After cooling to room temperature, the Ni foam was washed with water and dried under vacuum at 60 ◦C overnight. Finally, in order to obtain CuCo2O4 on the Ni foam, the sample was annealed at 350 oC with a temperature ramp rate of 5 oC/min for 2 h in an air atmosphere to obtain CuCo2O4 (Figure. 1).

2.2. Structural Characterization

The structural identity, phase purity, and physical morphology of the fabricated electrode were analyzed using X-ray diffraction (XRD), FEI Versa 3D Dual Beam scanning electron microscopy (SEM), FEI Tecnai F20 XT Field Emission transmission electron microscope (TEM), and Phi 5000 VersaProbe II X-ray photoelectron spectroscopy (XPS). CuKα1 (λ=1.5406 Å) radiation was used to record the X-ray diffraction patterns in 2θ-θ mode.

2.3. Electrochemical Characterization

To evaluate the catalytic activity of CuCo2O4 and investigate the function of OH- ions in the electro-oxidation of urea, cyclic voltammetry (CV) was performed in 1 M KOH solution containing Appl.0.33 Sci.M 2019urea., 9 ,These 793 measurements were performed using the standard three-electrode method.3 of A 9 Versastat 4-500 electrochemical workstation (Princeton Applied Research, USA) was used for all electrochemical measurements. A platinum wire and a saturated calomel electrode were used as a obtain CuCo O on the Ni foam, the sample was annealed at 350 ◦C with a temperature ramp rate of counter and 2a reference4 electrode, respectively. The synthesized sample on nickel foam was used as 5 ◦C/min for 2 h in an air atmosphere to obtain CuCo O (Figure 1). the working electrode. 2 4 2.2. Structural Characterization

2.3. Electrochemical Characterization Figure 1. Illustration of the formation of CuCo2O4 on Ni foam.

Figure 1. Illustration of the formation of CuCo2O4 on Ni foam. To evaluate the catalytic activity of CuCo2O4 and investigate the function of OH2.2.- i Structuralons in the Characterization electro-oxidation of urea, cyclic voltammetry (CV) was performed in 1 M KOH solution containing 3. ResultsThe structural and Discussion identity, phase purity, and physical morphology of the fabricated electrode were 0.33 M urea. These measurements were performed using the standard three-electrodeanalyzed method. using A X-ray diffraction (XRD), FEI Versa 3D Dual Beam scanning electron microscopy (SEM), Versastat 4-500 electrochemical workstation (Princeton Applied Research, USA) was3.1.FEI used TecnaiStructural for F20 all Characterization XT Field Emission transmission electron microscope (TEM), and Phi 5000 VersaProbe II electrochemical measurements. A platinum wire and a saturated calomel electrode were used as a X-ray photoelectron spectroscopy (XPS). CuKα1 (λ = 1.5406 Å) radiation was used to record the X-ray X-ray diffraction analysis confirmed a high phase purity of the synthesized CuCo2O4, as seen in counter and a reference electrode, respectively. The synthesized sample on nickel foamdiffraction was used patterns as in 2θ-θ mode. the working electrode. Figure 2a. All of the diffraction peaks matched well with the JCPDS card no. 78-2177, which corresponded2.3. Electrochemical to the Characterization pure cubic spinel phase of CuCo2O4 with a lattice parameter value of a = 8.128 Ǻ [16,17]. Scanning electron microscopy was employed to investigate the sample morphologically. - To evaluate the catalytic activity of CuCo2O4 and investigate the function of OH ions in the High-magnification SEM images confirmed that CuCo2O4 was uniformly grown on Ni foam, with a flower-shapedelectro-oxidation structure of urea, composed cyclic voltammetry of nanosheets (CV) wasin different performed orientations in 1 M KOH and solution a smooth containing surface, revealing0.33 M urea. a three-dimensional These measurements porous were structure performed (Figure using 2b). This the standarduniform flower-shaped three-electrode structure method. Amay Versastat have been 4-500 the electrochemicalresult of the Faradic workstation reaction. (Princeton The average Applied diameter Research, of the flower USA) structure was used was for aboutall electrochemical 5 µm, and it measurements. was interconnected A platinum with the wire nanosheets and a saturated that had calomel a thickness electrode of were160 nm. used TEM as a counter and a reference electrode, respectively. The synthesized sample on nickel foam was used as images of CuCo 2O4 powder scraped from the Ni foam at various magnifications are shown in Figure 3.the TEM working images electrode. reveal nanorod-like structures with good crystallinity. X-ray photoelectron Figure 1. Illustration of the formation of CuCo2O4 on Ni foam. spectroscopy3. Results and of Discussion the CuCo2O4 is shown in Figure 3. The XPS survey spectra confirmed the presence of 3. Results and Discussion copper, cobalt, oxygen, and carbon elements (Figure 4a). The high-resolution XPS spectrum for Cu 2p3.1. showed Structural the Characterization existence of 2p1/2 and 2p3/2 peaks of copper (Figure 4b) [8,18]. The high-resolution XPS spectrum for Co 2p is shown in Figure 4c. The two main peaks of Co 2p3/2 and Co 2p1/2 were observed 3.1. Structural Characterization X-ray diffraction analysis confirmed a high phase purity of the synthesized CuCo O , as seen around 779.6 and 794.7 eV, respectively, suggesting the existence of both Co2+ and 2Co43+ [8,18]. X-ray diffraction analysis confirmed a high phase purity of the synthesized CuCoin2O Figure4, as seen2 a.in All of the diffraction peaks matched well with the JCPDS card no. 78-2177, Considering the Co 2p peak shape, this was consistent with spectra of Co3O4 standards given by Gu Figure 2a. All of the diffraction peaks matched well with the JCPDS card no. 78-2177,which corresponded which to the pure cubic spinel phase of CuCo2O4 with a lattice parameter value et al. [19] and Yang et al. [20]. These studies showed a 2p3/2 peak binding energy of 779.9 eV and a corresponded to the pure cubic spinel phase of CuCo2O4 with a lattice parameter valueof ofa a= = 8.1288.128 Ǻ [16,17]. Scanning electron microscopy was employed to investigate the sample broad plateau-like satellite structure to higher binding energies between the 2p3/2 and 2p1/2 peaks. The [16,17]. Scanning electron microscopy was employed to investigate the sample morphologically.morphologically. High-magnification SEM images confirmed that CuCo2O4 was uniformly grown on High-magnification SEM images confirmed that CuCo2O4 was uniformly grown on NiNi foam, foam, with with a a flower-shaped structure composed3 of nanosheets in different orientations and a smooth flower-shaped structure composed of nanosheets in different orientations and a smooth surface, surface, revealing a three-dimensional porous structure (Figure 2b). This uniform flower-shaped revealing a three-dimensional porous structure (Figure 2b). This uniform flower-shapedstructure structure may have been the result of the Faradic reaction. The average diameter of the flower structure may have been the result of the Faradic reaction. The average diameter of the flowerwas structure about was 5 µm, and it was interconnected with the nanosheets that had a thickness of 160 nm. TEM about 5 µm, and it was interconnected with the nanosheets that had a thickness of images160 nm. of TEM CuCo 2O4 powder scraped from the Ni foam at various magnifications are shown in Figure 3. images of CuCo2O4 powder scraped from the Ni foam at various magnifications are shownTEM imagesin Figure reveal nanorod-like structures with good crystallinity. X-ray photoelectron spectroscopy 3. TEM images reveal nanorod-like structures with good crystallinity. X-ray of photoelectron the CuCo2O4 is shown in Figure 3. The XPS survey spectra confirmed the presence of copper, cobalt, spectroscopy of the CuCo2O4 is shown in Figure 3. The XPS survey spectra confirmed theoxygen, presence and of carbon elements (Figure 4a). The high-resolution XPS spectrum for Cu 2p showed the copper, cobalt, oxygen, and carbon elements (Figure 4a). The high-resolution XPS spectrumexistence for of Cu 2p 1/2 and 2p3/2 peaks of copper (Figure 4b) [8,18]. The high-resolution XPS spectrum for 2p showed the existence of 2p1/2 and 2p3/2 peaks of copper (Figure 4b) [8,18]. The high-resolutionCo 2p is shown XPS in Figure 4c. The two main peaks of Co 2p3/2 and Co 2p1/2 were observed around 2+ 3+ spectrum for Co 2p is shown in Figure 4c. The two main peaks of Co 2p3/2 and Co 2p1/2 were779.6 observed and 794.7 eV, respectively, suggesting the existence of both Co and Co [8,18]. Considering 2+ 3+ around 779.6 and 794.7 eV, respectively, suggesting the existence of both Co anthed Co Co 2p [8 peak,18]. shape, this was consistent with spectra of Co3O4 standards given by Gu et al. [19] and Considering the Co 2p peak shape, this was consistent with spectra of Co3O4 standardsYang given et al.by [ 20Gu]. These studies showed a 2p3/2 peak binding energy of 779.9 eV and a broad plateau-like et al. [19] and Yang et al. [20]. These studies showed a 2p3/2 peak binding energy of 779.9satellite eV structureand a to higher binding energies between the 2p3/2 and 2p1/2 peaks. The high-resolution broad plateau-like satellite structure to higher binding energies between the 2p3/2 and 2pspectrum1/2 peaks. forThe O 1s is shown in Figure 4d, which shows two peaks centered at 529.4 and 531.3 eV, high-resolution spectrum for O 1s is shown in Figure 4d, which shows two peaks centered at 529.4 and 531.3 eV, respectively. The peak around 529.4 eV corresponded to metal–oxygen bonding, while

Appl. Sci. 2019, 9, 793 4 of 9

high-resolution spectrum for O 1s is shown in Figure 4d, which shows two peaks centered at 529.4 high-resolutionhigh-resolution spectrum spectrum for for O O 1s 1s is isshown shown in in Figure Figure 4d, 4d, which which shows shows two two peaks peaks centered centered at at 529.4 529.4 andrespectively. 531.3 eV, respectively. The peak around The peak 529.4 around eV corresponded 529.4 eV corresponded to metal–oxygen to metal–oxygen bonding, whilebonding, the secondwhile andand 531.3 531.3 eV, eV, respectively. respectively. The The peak peak around around 529.4 529.4 eV eV corresponded corresponded to to metal–oxygen metal–oxygen bonding, bonding, while while thepeak second corresponded peak corresponded to a high number to a high of number defect sites of defect with low sites oxygen with low coordination oxygen coordination in the material in the with thethe second second peak peak corresponded corresponded to to a ahigh high number number of of defect defect sites sites with with low low oxygen oxygen coordination coordination in in the the materiala small with particle a small size [particle21]. size [21]. materialmaterial with with a asmall small particle particle size size [21]. [21]. [a] [a] [a] (311)

(311) (311) (440) (220) (440) (511) (440) (220) (511) (220) (511) (220) (400) Intensity (a.u.) (111) (220) (400) Intensity (a.u.) (220) (400) (111) (422) (533) Intensity (a.u.) (111) (422) (533) (422) (533)

10 20 30 40 50 60 70 80 10 20 30 40 50 60 70 80 10 20 30θ 40 50 60 70 80 2θ (degree) 2 2θ(degree) (degree) Figure 2. (a) XRD pattern and (b) SEM image of CuCo2O4. 2 4 FigureFigure 2. 2.(a ()a XRD) XRD pattern pattern and and (b ()b SEM) SEM image image of of CuCo CuCoO2O. 4. Figure 2. (a) XRD pattern and (b) SEM image of CuCo2O4.

Figure 3. TEM images of CuCo2O4 at various magnifications. Figure 3. TEM images of CuCo2O4 at various magnifications. Figure 3. TEM images of CuCo 2OO44 atat various various magnifications. magniﬁcations.

2+ Co 2p [a] [b] Cu2+ Cu 2p Co 2p [a] [b][b] Cu 2+ CuCu 2p 2p Co 2p [a] Cu

2+ Cu2+ Cu 2+ + Cu Cu+ 2+ Cu + O 1s Cu2+ Cu Cu 2+ O O1s 1s Cu Intensity (a.u.) Intensity Intensity (a.u.) Intensity Intensity (a.u.) Intensity (a.u.) Intensity Intensity (a.u.)

Intensity (a.u.) C 1s C C1s 1s

955 950 945 940 935 930 1000 800 600 400 200 0 955 950 945 940 935 930 10001000 800 800 600 600 400 400 200 200 0 0 955Binding 950 945 Energy 940 (eV) 935 930 Binding Energy (eV) BindingBinding Energy Energy (eV) (eV) Binding Energy (eV) Binding Energy (eV) Figure 4. Cont.

44 4

Appl. Sci. 2019, 9, 793 5 of 9

[c] [d] O1 [c] O1 Co3+ [d] 3+ Co Co2+ 3+ O2 Co 2+ Co Co2+ 3+ O2 Co Co2+ Intensity (a.u.) Intensity (a.u.) Intensity Intensity (a.u.) Intensity (a.u.) Intensity

800 795 790 785 780 775 540 537 534 531 528 525 Binding energy (eV) Binding Energy (eV) 800 795 790 785 780 775 540 537 534 531 528 525 Binding energy (eV) Binding Energy (eV)

FigureFigure 4. 4.XPS XPS spectra spectra of of the the CuCo CuCo2O2O4:(4: a(a)) full full survey survey spectra, spectra, ( b(b)) Cu Cu 2p 2p spectrum, spectrum, (c ()c Co) Co 2p 2p spectrum, spectrum, Figureandand ( d4. () dXPS O) O 1s 1sspectra spectrum. spectrum. of the CuCo2O4: (a) full survey spectra, (b) Cu 2p spectrum, (c) Co 2p spectrum, and (d) O 1s spectrum. 3.2.3.2. Electrocatalytic Electrocatalytic Activities Activities 3.2. Electrocatalytic Activities TheThe electrocatalytic electrocatalytic activities activities of of the the CuCo CuCo2O2O4 4electrode electrode were were examined examined using using a a three-electrode three-electrode setup in 1 M KOH, without and with 0.33 M urea. As seen in Figure 5a, the anodic current density setupThe electrocatalyticin 1 M KOH, without activities and of with the 0.33CuCo M2 Ourea.4 electrode As seen were in Figure examined 5a, the using anodic a three-electrode current density of setupofCuCo CuCo in 12 OM2O4 KOH,increased4 increased without significantly signiﬁcantly and with after0.33 after M the theurea. addition addition As seen of of in urea, Figure conﬁrmingconfirming 5a, the anodic aa highhigh current catalytic catalytic density activity activity of response of CuCo O for the electro-oxidation of urea. The CuCo O electrode demanded a potential CuCoresponse2O4 increased of CuCo 2significantly2O4 4 for the electro-oxidation after the addition of urea. of urea, The CuCoconfirming2 2O4 4 electrode a high demanded catalytic activitya potential 2 of 1.36 V to deliver a current density of 10 mA/cm ,2 which was much lower than that without urea. responseof 1.36 of V CuCo to deliver2O4 for a thecurrent electro-oxidation density of 10 ofmA/cm urea. The, which CuCo was2O 4much electrode lower demanded than that awithout potential urea. 2 ofThe 1.36The potential Vpotential to deliver required required a current to to deliver density deliver 10 of10 mA/cm 10 mA/cm mA/cm2using 2using, which the the was CuCo CuCo much2O2O4 4electrodelower electrode than was wasthat among amongwithout the the urea. more more favorable reported values. For example, the Ni(OH) nanosheet and nanocube required a potential of Thefavorable potential reported required values. to deliver For example, 10 mA/cm the2 Ni(OH)using 2the2 nanosheet CuCo2O4 and electrode nanocube was required among athe potential more of 2 1.52 V and 1.55 V, respectively, to produce a current density of 10 mA/cm [142,22]. Ni P, Rhodium-Ni, favorable1.52 V reportedand 1.55 values.V, respectively, For example, to produce the Ni(OH) a current2 nanosheet density and of nanocube10 mA/cm required [14,22].2 aNi potential2P, Rhodium- of and NiMo nanosheets demanded a potential 1.37 V, 1.47 V, and 1.37 V, respectively, to generate a 1.52Ni, V and 1.55NiMo V, nanosheets respectively, demanded to produce a potential a current 1.37 density V, 1.47 of 10V, andmA/cm 1.372 [14,22].V, respectively, Ni2P, Rhodium- to generate 2 Ni,current aand current NiMo density density nanosheets of 10 of mA/cm 10 demanded mA/cm[23–2 25[23–25]. a]. potential The TafelThe 1.37Tafel slope V, slope for 1.47 the V,for CuCo and the 2 1.37CuCoO4 electrode V,2 Orespectively,4 electrode was calculated wasto generate calculated using polarization data, and was observed to be 82 and 46 mV/dec in 1 M KOH without and with 0.33 M a currentusing polarizationdensity of 10 data, mA/cm and2 was[23–25]. observed The Tafel to be slope 82 and for 46 the mV/dec CuCo2 Oin4 1electrode M KOH waswithout calculated and with usingurea,0.33M polarization respectively urea, respectively data, (Figure and 5was(b).Figure Theobserved 5b lower). The to Tafel belower 82 slope and Tafel in 46 the slopemV/dec urea insolution inthe 1 ureaM KOH indicated solution without fasterindicated and catalytic with faster 0.33Mkineticcatalytic urea, energy kineticrespectively for energy the CuCo ( Figurefor2 Othe4 electrode. CuCo5b). The2O4 lowerelectrode. Tafel slope in the urea solution indicated faster catalytic kinetic energy for the CuCo2O4 electrode.

) 250

2 1.7 [a] CuCo2O4 [b] ) 250

2 1.7 200 [a] 82 mV/dec CuCo CuCo2O4 2O4 + urea 1.6[b]

200mA/cm ( CuCo O + urea 82 mV/dec 150 2 4 1.6 1.5 mA/cm ( 150 100 1.5 1.4 100 46 mV/dec 50 1.4 1.3 46 mV/dec 50 Potential (V, RHE) CuCo O 0 1.3 2 4 Current density

Potential (V, RHE) 1.2 0 CuCo CuCo2O4 2O4 + urea

Current density -50 1.2 CuCo O + urea 1.2 1.3 1.4 1.5 1.6 1.7 0.12 14 10 100 -50 2 1.2 1.3Potential 1.4 1.5(V, RHE) 1.6 1.7 0.1Current 1 density 10 (mA/cm 100 ) 2 Current density (mA/cm ) FigureFigure 5. 5. (a(Potentiala)) CyclicCyclic voltammetryvoltammetry (V, RHE) curvescurves inin 11 MM KOH KOH with with and and without without 0.33 0.33 M M urea, urea, and and ( b ()b) Figurecorrespondingcorresponding 5. (a) Cyclic Tafel Tafel voltammetry slopes. slopes. curves in 1 M KOH with and without 0.33 M urea, and (b) corresponding Tafel slopes. Furthermore,Furthermore, the the urea urea oxidation oxidation kinetics kinetics were were investigated investigated by by electrochemical electrochemical impedance impedance spectroscopyspectroscopyFurthermore, (EIS). (EIS). the The Theurea Nyquist Nyquist oxidation plots plots kinetics forfor thethe were CuCoCuCo investigated22OO44 electrodeelectrode by at at variouselectrochemical various potentials potentials impedance are are given given in in Figure S1. As seen in the Nyquist plots, CuCo O had a low impedance and, thus, markedly fast spectroscopyFigure S1. (EIS).As seen The in Nyquist the Nyquist plots forplots, the CuCo CuCo22O2O44 4had electrode a low atimpedance various potentials and, thus, are markedly given in fast kinetics toward urea electro-oxidation. Furthermore, the behavior of the Nyquist plots depended Figurekinetics S1. Astoward seen ureain the electro-oxidation. Nyquist plots, CuCoFurthermor2O4 hade, athe low behavior impedance of the and, Nyquist thus, plots markedly depended fast on kinetics toward urea electro-oxidation. Furthermore, 5the behavior of the Nyquist plots depended on 5

Appl. Sci. 2019, 9, 793 6 of 9 onthe the potential. potential. With With the the increase increase in in the the potential, potential, the the diameter diameter of the semicirclesemicircle waswas decreased,decreased, suggestingsuggesting a a lowering lowering in in charge charge transfer transfer resistance. resistance. An An important important method method to to evaluate evaluate the the long-term long-term stabilitystability of of the the obtained obtained electrodes electrodes in in urea urea oxidation oxidation is theis the chronoamperometric chronoamperometric response response test. test. Figure Figure S2 showsS2 shows the the superior superior catalytic catalytic stability stability with with low low degradation degradation in in current current density density after after 15 15 h. h. From From this this measurement, it was observed that CuCo O exhibited a stable and durable catalytic performance. measurement, it was observed that CuCo2 2O44 exhibited a stable and durable catalytic performance. The CV curves of the CuCo O electrode in 1 M KOH electrolyte with 0.33 M urea was measured at The CV curves of the CuCo2 2O44 electrode in 1 M KOH electrolyte with 0.33 M urea was measured at variousvarious scan scan rates. rates. Figure Figure S3 S3 shows shows the the variation variation of of measured measured potential potential versus versus scan scan rates rates determined determined using CV data at 50 mA/cm2. As seen in Figure S4, the CuCo O electrode showed almost no change using CV data at 50 mA/cm2. As seen in Figure S3, the CuCo2 2O44 electrode showed almost no change inin potential potential duringduring CVCV measurements at at various various scan scan rates, rates, indicating indicating superior superior rate rate capability capability for for the theUOR UOR process. process. Tests Tests performed performed by Shi by et Shi al. etdemonstrated al. demonstrated an increase an increase in the inanodic the anodiccurrent current density densityfor the forNi/Mo/graphene the Ni/Mo/graphene nanocatalysts nanocatalysts after addition after addition of urea in of the urea system in the [26]. system This [26 increase]. This increasewas also wasfound also by found Barakat by Barakatet al. in etthe al. urea in the oxidation urea oxidation test for testNi forand Ni Mn and nanoparticle-decorated Mn nanoparticle-decorated carbon carbonnanofibers nanofibers [27]. Sodium [27]. Sodium nickel nickel fluoride fluoride nanocubes nanocubes were were studied studied by by Kakati Kakati et et al. inin differentdifferent concentrationsconcentrations of of urea urea for for its its oxidation oxidation [ 28[28].]. It It was was found found that that the the peak peak potentials potentials of of urea urea oxidation oxidation increasedincreased linearly linearly with with increasing increasing urea urea concentration. concentration. Zeng Zeng et et al. al. studied studied the the effect effect in in NiCo NiCo layered layered doubledouble hydroxide hydroxide for for promoted promoted electrocatalytic electrocatalytic urea urea oxidation, oxidation, which which proved proved to to be be efficient, efficient, showing showing aa low low onset onset potential potential and and a a high high faraday faraday efficiency efficiency [29 [29].]. The electrocatalytic performance of the CuCo O electrode for hydrogen evolution reaction (HER) The electrocatalytic performance of the CuCo2 4 2O4 electrode for hydrogen evolution reaction was also studied. Figure 6a shows the polarization curves for a CuCo O electrode in 1 M KOH, (HER) was also studied. Figure 6a shows the polarization curves for a CuCo2 42O4 electrode in 1 M KOH, withoutwithout and and with with 0.33 0.33 M M urea. urea. AsAs observed,observed, thethe polarizationpolarization curvescurves werewere overlappingoverlapping eacheach other,other, indicating the little impact that urea had on the electrochemical activity of the CuCo O electrode indicating the little impact that urea had on the electrochemical activity of the CuCo2O2 4 4electrode in in the HER region. The CuCo O electrode required a potential of 168 mV to generate 10 mA/cm2. the HER region. The CuCo2O24 electrode4 required a potential of 168 mV to generate 10 mA/cm2. The The Tafel slope for the CuCo O electrode in 1 M KOH without and with urea was observed to be 113 Tafel slope for the CuCo2O42 electrode4 in 1 M KOH without and with urea was observed to be 113 and and112 112mV/dec, mV/dec, respectively, respectively, suggesting suggesting no nochange change in th ine the kinetics kinetics for for the the HER HER process process after after addition addition of of urea (Figure 6b). Liu et al. have reported a Tafel slope of 172 and 180 mV/dec for Ni N and Ni(OH) urea (Figure 6b). Liu et al. have reported a Tafel slope of 172 and 180 mV/dec for Ni3 3N and Ni(OH)2 2 nanosheetsnanosheets on on carbon carbon cloth cloth in in 1 1 M M KOH KOH + + 0.330.33 M M urea urea solution solution [ 11[11].].The Theefficiency efficiency( η(η)) for for the the urea urea electrolysiselectrolysis can can be be estimated estimated by by the the following following expression expression [ 14[14]:]: i� −−�i η = t OER× 100% (2)  = i × 100% (2) t� whereWhereit is the it is overall the overall current current density density during during electrolysis electrolysis of urea of (1urea M KOH(1 M KOH + 0.33 + M0.33 urea) M urea) and theand iOERthe iisOER the is currentthe current density density of OER of OER process process in 1 Min KOH1 M KOH at 250 at s 250 (Figure s (Figure S4). An S4). efficiency An efficiency of 95% of was 95% observedwas observed for the for CuCo the CuCo2O4 electrode.2O4 electrode.

) 2 0 [a] 0.0 [b] CuCo2O4 -20 CuCo O + urea

mA/cm 2 4 ( -40 -0.1 -60 112 mV/dec -80 -0.2 -100 CuCo2O4 Potential (V, Potential (V, RHE) 113 mV/dec Current density -0.3 -120 CuCo2O4 + urea -0.3 -0.2 -0.1 0.0 0.1 0.2 1 10 100 Potential (V, RHE) 2 Current density (mA/cm )

FigureFigure 6. 6.( a(a)) Linear Linear sweep sweep voltammetry voltammetry (LSV) (LSV) curves curves of of the the CuCo CuCo2O2O4 4electrode electrode toward toward HER HER in in 1.0 1.0 M M KOHKOH with with 0.33 0.33 M M urea, urea, and and ( b(b)) corresponding corresponding Tafel Tafel plots. plots.

The electrocatalytic activities of the CuCo2O4 electrode were further examined in a two-electrode electrolyzer configuration. The CuCo2O4 electrode was used as the anode for the UOR and the 6

Appl. Sci. 2019, 9, 793 7 of 9

The electrocatalytic activities of the CuCo2O4 electrode were further examined in a two-electrode cathodeelectrolyzer for conﬁguration.the HER process The CuCoin alkaline2O4 electrode media. wasFigure used 7a as shows the anode polarization for the UOR curves and of the the cathode two- electrodefor the HER electrolyzer process inin alkaline1 M KOH, media. without Figure and7 witha shows 0.33 polarizationM urea. Thecurves electrolyzer of the required two-electrode a cell voltageelectrolyzer of 1.45 in 1V Mto KOH,deliver without a current and density with 0.33of 10 M mA/cm urea. The2 in urea electrolyzer solution, required which was a cell 260 voltage mV less of 2 1.45than Vthe to cell deliver voltage a current required density without of 10 urea mA/cm in thein alkaline urea solution, solution. which The observed was 260 mVcell lessvoltage than was the bettercell voltage than many required other without reported urea systems, in the alkaline such as solution.Ni(OH)2-Ni(OH) The observed2 and Pt/C-RuO cell voltage2 [11]. was In better addition, than itmany had other a performance reported systems, in urea such solution, as Ni(OH) and2-Ni(OH) the CuCo2 and2O Pt/C-RuO4 electrode2 [11 showed]. In addition, outstanding it had electrochemicala performance indurability urea solution, (Figure and 7b the). CuCoThe chronopotentiometry2O4 electrode showed test, outstanding performed electrochemical for porous pomegranate-likedurability (Figure 7Ni/Cb). The in the chronopotentiometry urea oxidation reaction test, performed made by Wang for porous et al., pomegranate-likehad a stable performance Ni/C in overthe urea 12 h oxidation [30]. reaction made by Wang et al., had a stable performance over 12 h [30].

2.25 ) 2 [a] [b] 150

2.00 100

CuCo2O4 50 CuCo O + urea 1.75

2 4 (V) Potential

Current density (mA/cm Current 0 1.50 1.00 1.25 1.50 1.75 2.00 0 5 10 15 20 Time (h) Voltage (V)

FigureFigure 7. 7.(a ()a Polarization) Polarization curves curves for for the the CuCo CuCo2O24Oelectrode4 electrode in in 1 M1 M KOH KOH without without and and with with 0.33 0.33 M urea,M and (b) chronopotentiometricurea, and (b) chronopotentiometric curve for the CuCo2O curve4 electrode. for the CuCo2O4 electrode.

4. Conclusions 4. Conclusions In conclusion, it is possible to prove the two main advantages that the urea oxidation reaction In conclusion, it is possible to prove the two main advantages that the urea oxidation reaction can bring to the world: (1) it is a solution to eliminate the urea from groundwater systems and the can bring to the world: (1) it is a solution to eliminate the urea from groundwater systems and the environment that are harmful to people, and (2) the production of H2 uses less energy and has a environment that are harmful to people, and (2) the production of H2 uses less energy and has a lower lower cost, which helps to solve the energy crisis. CuCo2O4 grown on nickel foam was successfully cost, which helps to solve the energy crisis. CuCo2O4 grown on nickel foam was successfully synthesized, making it possible to utilize them as a highly efficient UOR electrocatalyst under alkaline synthesized, making it possible to utilize them as a highly efficient UOR electrocatalyst under alkaline conditions. The high surface area favors charge transfer capacity and improves the reaction kinetics. conditions. The high surface area favors charge transfer capacity and improves the reaction kinetics. The CV curves clearly show a significant significant decrease in potential after the addition of urea, which leads to cheaper hydrogen production. The The two-electrode two-electrode el electrolyzerectrolyzer need need a a potential potential of of 1.45 1.45 V, V, which is 260 mV less than that for for the the urea-free urea-free counterpart. From From the the overall overall investigation, investigation, the results suggest that CuCo2O4 can be used in fuel cells for low-cost hydrogen generation. that CuCo2O4 can be used in fuel cells for low-cost hydrogen generation. Supplementary Materials: The following are available online at http://www.mdpi.com/2076-3417/9/4/793/s1, AuthorFigure S1: Contributions: Nyquist plots of R.K.G. CuCo2 Oconceived4, Figure S2: the Stability project, performance designed ofthe the experiments, electrode using interpreted chronoamperometry, the data, andFigure finalized S3: Potential the manuscript. versus scan The rates first for CuCodraft2 ofO4 thelectrode,e manuscript Figure was S4: written Chronoamperograms by C.Z. C.Z. ofperformed CuCo2O4 allelectrode the electrochemical in 1 M KOH without measurements. and with 0.33 F.W. M urea. and X.L. provided SEM images, TEM images, and XPS Authordata. All Contributions: authors reviewedR.K.G. and conceived commented the project, on designedthe manuscript. the experiments, interpreted the data, and finalized the manuscript. The first draft of the manuscript was written by C.Z. C.Z. performed all the electrochemical measurements. F.W. and X.L. provided SEM images, TEM images, and XPS data. All authors reviewed and Acknowledgments:commented on the manuscript. Dr. Ram K. Gupta expresses his sincere acknowledgment to the Polymer Chemistry Program and the Kansas Polymer Research Center, Pittsburg State University for Acknowledgments: Ram K. Gupta expresses his sincere acknowledgment to the Polymer Chemistry Program providingand the Kansas financial Polymer and Research research Center, support Pittsburg to complete State University this project. for providing XL wants financial to andthank research the funding support supportto complete from this NSF project. (1833048). Xianglin SM Li wants thanks to thankFIT-DRON the fundingES and support Biologistic from NSF for (1833048).the financial S. R. support. Mishra thanks FIT-DRONES and Biologistic for the financial support. ConflictsConflicts of of Interest: InterestThe: The authors authors declare declare no conflict no conflict of interest. of interest.

References

Appl. Sci. 2019, 9, 793 8 of 9

References

1. Periyasamy, S.; Subramanian, P.; Levi, E.; Aurbach, D.; Gedanken, A.; Schechter, A. Exceptionally Active and Stable Spinel Nickel Manganese Oxide Electrocatalysts for Urea Oxidation Reaction. ACS Appl. Mater. Interfaces 2016, 8, 12176. [CrossRef][PubMed] 2. Lan, R.; Tao, S.; Irvine, J.T.S. A Direct Urea Fuel Cell—Power from Fertiliser and Waste. Energy Environ. Sci. 2010, 3, 438. [CrossRef] 3. Lan, R.; Tao, S. Preparation of Nano-Sized Nickel as Anode Catalyst for Direct Urea and Urine Fuel Cells. J. Power Sources 2011, 196, 5021. [CrossRef] 4. Forslund, R.P.; Mefford, J.T.; Hardin, W.G.; Alexander, C.T.; Johnston, K.P.; Stevenson, K.J. Nanostructured LaNiO3 Perovskite Electrocatalyst for Enhanced Urea Oxidation. ACS Catal. 2016, 6, 5044. [CrossRef] 5. Tong, Y.; Chen, P.; Zhang, M.; Zhou, T.; Zhang, L.; Chu, W.; Wu, C.; Xie, Y. Oxygen Vacancies Conﬁned in Nickel Molybdenum Oxide Porous Nanosheets for Promoted Electrocatalytic Urea Oxidation. ACS Catal. 2018, 8, 1. [CrossRef] 6. Yan, W.; Wang, D.; Botte, G.G. Nickel and Cobalt Bimetallic Hydroxide Catalysts for Urea Electro-Oxidation. Electrochim. Acta 2012, 61, 25. [CrossRef] 7. Zhu, X.; Dou, X.; Dai, J.; An, X.; Guo, Y.; Zhang, L.; Tao, S.; Zhao, J.; Chu, W.; Zeng, X.C.; et al. Metallic Nickel Hydroxide Nanosheets Give Superior Electrocatalytic Oxidation of Urea for Fuel Cells. Angew. Chem. Int. Ed. 2016, 55, 12465. [CrossRef][PubMed] 8. Yue, Z.; Zhu, W.; Li, Y.; Wei, Z.; Hu, N.; Suo, Y.; Wang, J. Surface Engineering of a Nickel Oxide-Nickel Hybrid Nanoarray as a Versatile Catalyst for Both Superior Water and Urea Oxidation. Inorg. Chem. 2018, 57, 4693. [CrossRef][PubMed] 9. Boggs, B.K.; King, R.L.; Botte, G.G. Urea Electrolysis: Direct Hydrogen Production from Urine. Chem. Commun. 2009, 4859–4861. [CrossRef][PubMed] 10. Xu, W.; Wu, Z.; Tao, S. Urea-Based Fuel Cells and Electrocatalysts for Urea Oxidation. Energy Technol. 2016, 4, 1329. [CrossRef]

11. Liu, Q.; Xie, L.; Qu, F.; Liu, Z.; Du, G.; Asiri, A.M.; Sun, X. A Porous Ni3N Nanosheet Array as a High-Performance Non-Noble-Metal Catalyst for Urea-Assisted Electrochemical Hydrogen Production. Inorg. Chem. Front. 2017, 4, 1120. [CrossRef] 12. Zheng, Y.; Jiao, Y.; Jaroniec, M.; Qiao, S.Z. Advancing the Electrochemistry of the Hydrogen-Evolution Reaction through Combining Experiment. Angew. Chem. Int. Ed. 2015, 54, 52. [CrossRef][PubMed]

13. Xu, X.; Liu, Y.; Dong, P.; Ajayan, P.M.; Shen, J.; Ye, M. Mesostructured CuCo2S4/CuCo2O4 Nanoﬂowers as Advanced Electrodes for Asymmetric Supercapacitors. J. Power Sources 2018, 400, 96. [CrossRef] 14. Wu, M.S.; Ji, R.Y.; Zheng, Y.R. Nickel Hydroxide Electrode with a Monolayer of Nanocup Arrays as an Effective Electrocatalyst for Enhanced Electrolysis of Urea. Electrochim. Acta 2014, 144, 194. [CrossRef] 15. Ji, R.Y.; Chan, D.S.; Jow, J.J.; Wu, M.S. Formation of Open-Ended Nickel Hydroxide Nanotubes on Three-Dimensional Nickel Framework for Enhanced Urea Electrolysis. Electrochem. Commun. 2013, 29, 21. [CrossRef] 16. Jadhav, H.S.; Pawar, S.M.; Jadhav, A.H.; Thorat, G.M.; Seo, J.G. Hierarchical Mesoporous 3D Flower-like

CuCo2O4/NF for High-Performance Electrochemical Energy Storage. Sci. Rep. 2016, 6, 31120. [CrossRef] [PubMed] 17. Settanni, G.; Zhou, J.; Suo, T.; Schöttler, S.; Landfester, K.; Schmid, F.; Mailänder, V. Protein Corona Composition of PEGylated Nanoparticles Correlates Strongly with Amino Acid Composition of Protein Surface. arXiv, 2016, arXiv:1612.08814.

18. Zhu, J.; Gao, Q. Mesoporous MCo2O4 (M = Cu, Mn and Ni) Spinels: Structural Replication, Characterization and Catalytic Application in CO Oxidation. Microporous Mesoporous Mater. 2009, 124, 144. [CrossRef] 19. Gu, D.; Jia, C.-J.; Weidenthaler, C.; Bongard, H.-J.; Spliethoff, B.; Schmidt, W.; Schüth, F. Highly Ordered Mesoporous Cobalt-Containing Oxides: Structure, Catalytic Properties, and Active Sites in Oxidation of Carbon Monoxide. J. Am. Chem. Soc. 2015, 137, 11407. [CrossRef][PubMed] 20. Yang, J.; Liu, H.; Martens, W.N.; Frost, R.L. Synthesis and Characterization of Cobalt Hydroxide, Cobalt Oxyhydroxide, and Cobalt Oxide Nanodiscs. J. Phys. Chem. C 2010, 114, 111. [CrossRef]

21. Wang, J.; Fan, S.; Luan, Y.; Tang, J.; Jin, Z.; Yang, M.; Lu, Y. Ultrathin Mesoporous NiCo2O4 Nanosheets as an Efﬁcient and Reusable Catalyst for Benzylic Oxidation. RSC Adv. 2015, 5, 2405. [CrossRef] Appl. Sci. 2019, 9, 793 9 of 9

22. Wang, D.; Yan, W.; Botte, G.G. Exfoliated Nickel Hydroxide Nanosheets for Urea Electrolysis. Electrochem. Commun. 2011, 13, 1135. [CrossRef] 23. Liu, D.; Liu, T.; Zhang, L.; Qu, F.; Du, G.; Asiri, A.M.; Sun, X. High-Performance Urea Electrolysis towards Less Energy-Intensive Electrochemical Hydrogen Production Using a Bifunctional Catalyst Electrode. J. Mater. Chem. A 2017, 5, 3208. [CrossRef] 24. Miller, A.T.; Hassler, B.L.; Botte, G.G. Rhodium Electrodeposition on Nickel Electrodes Used for Urea Electrolysis. J. Appl. Electrochem. 2012, 42, 925. [CrossRef]

25. Liang, Y.; Liu, Q.; Asiri, A.M.; Sun, X. Enhanced Electrooxidation of Urea Using NiMoO4·xH2O Nanosheet Arrays on Ni Foam as Anode. Electrochim. Acta 2015, 153, 456. [CrossRef] 26. Shi, W.; Ding, R.; Li, X.; Xu, Q.; Liu, E. Enhanced Performance and Electrocatalytic Kinetics of Ni-Mo/Graphene Nanocatalysts towards Alkaline Urea Oxidation Reaction. Electrochim. Acta 2017, 242, 247. [CrossRef] 27. Al-deyab, S.S. Ni & Mn Nanoparticles-Decorated Carbon Nanofibers as Effective Electrocatalyst for Urea Oxidation Applied Catalysis A: General. Appl. Catal. A Gen. 2015, 510, 180. 28. Kakati, N.; Maiti, J.; Lee, K.S.; Viswanathan, B.; Yoon, Y.S. Hollow Sodium Nickel Fluoride Nanocubes Deposited MWCNT as An Efficient Electrocatalyst for Urea Oxidation. Electrochim. Acta 2017, 240, 175. [CrossRef] 29. Zeng, M.; Wu, J.; Li, Z.; Wu, H.; Wang, J.; Wang, H.; He, L.; Yang, X. Interlayer Effect in NiCo Layered Double Hydroxide for Promoted Electrocatalytic Urea Oxidation. ACS Sustain. Chem. Eng. 2019, in press. [CrossRef] 30. Wang, L.; Ren, L.; Wang, X.; Feng, X.; Zhou, J.; Wang, B. Multivariate MOF-Templated Pomegranate-Like Ni/C as Efficient Bifunctional Electrocatalyst for Hydrogen Evolution and Urea Oxidation. ACS Appl. Mater. Interfaces 2018, 10, 4750. [CrossRef][PubMed]

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