Homogeneous Co3 O4 film electrode with enhanced oxygen evolution electrocatalysis via surface reduction

Xiang Li,Bo Yang,Yaqin Wu,Saisai Lin,Lin Zhang

1 Key laboratory of Biomass Chemical Engineering,Ministry of Education,College of Chemical and Biological Engineering,Zhejiang University,Hangzhou 310027,China

2 Research and Development Center of Water Treatment Technology Co.,Ltd.,Hangzhou 310012,China

Keywords:Co3 O4 thin film NaBH4 reduction Electrolysis Oxygen evolution reaction Stability

ABSTRACT Homogeneous NaBH4 -reduced Co3 O4 thin film electrodes with enhanced oxygen evolution electrocatalysis were obtained via a controlled-synthesis route.Firstly CoOx colloids were synthesized via ethylene glycol solvothermal method and cast on conductive glass substrates.The oxygen evolution reaction(OER)electrocatalysis of these asprepared Co3 O4 thin films were then significantly enhanced via a simple surface reduction by NaBH4 solution.The OER catalytic performance of the NaBH4 -reduced thin films was strongly dependent on the NaBH4 concentration.The use of NaBH4 -reduced thin film electrodes for OER in alkaline solution supported higher current density and consequently negative shifts of the onset potential compared to that of the pristine.The optimal B12.5, 20 -Co3 O4 thin films exhibited excellent OER catalytic performances:At the current density of 10 mA·cm?2,a low overpotential of 365 mV and a small Tafel slope of 59.0 mV·dec?1 were observed.In addition,these B12.5, 20 -Co3 O4 thin film electrodes possessed good stability that can well recover its OER performance in a 24-h chronoamperometric stability test.

1.Introduction

The oxygen evolution reaction(OER)in electrolysis has been considered as a crucial half-reaction for hydrogen generation,carbon dioxide and nitrogen electro-reduction,and metal-air batteries,because the kinetically sluggish OER significantly affects the overall reaction efficiency [1–4].Thus,the development of highly efficient and durable electrocatalysts for the OER will inevitably promote the commercialization these applications.To date,precious metal-based materials,such as IrO2[5],RuO2[6],Rh2O3[7],have been recognized as the most optimal catalysts for OER owing to their lowest overpotential at practical current densities.However,their widely practical applications have severely hindered by the scarce resource,exorbitant cost,and poor durability[5–9].In recent years,transition-metal-based electrocatalysts for the OER have attracted extensive attention because of their low cost,abundant reserves,environmental benignity,and resistance to corrosion in alkaline solutions [10–12].Among them,Co3O4,one of the few bifunctional catalysts for hydrogen and oxygen evolution reactions(HER and OER,respectively),has emerged as the promising alternative for precious metal-based catalysts[13–17].More importantly,Co3O4is capable to maintain stability over the course of water oxidation in virtue of the redox transformations between Co(III)and Co(IV)indicated by EPR(Electron Paramagnetic Resonance)studies[18].Thereby,Co3O4has a wide application in various catalytic fields,such as lowtemperature CO oxidation [16]，hydrogen photoproduction from water/ethanol[19],oxygen/hydrogen production from water splitting[13,14,20,21],and also oxygen reduction reaction[13,21–23].

Another attractive potential of Co3O4as water oxidation catalyst is its various synthesis methods.Controllable synthesis of micro-/nanomaterials with different morphologies and sizes is highly desirable to improve the catalytic performance of transition metal oxides.Many methods have been developed to prepare Co3O4catalysts such as electrodeposition [24],hydrothermal synthesis [25,26],solvent method[16],and atmospheric pressure CVD[19],etc..Among these,Co3O4prepared by calcination of CoOxprecursor has become more popular since in this way the original morphology of these precursor can be well maintained.However,the current common way is to anneal CoOxprecursor into Co3O4powder firstly,and then load annealed powder onto glassy carbon substrate to form Co3O4film electrodes.The as-prepared Co3O4film electrodes are always poor in uniformity and controllability,thereby difficult to scale up production for large-area electrodes[14,21,22,27].

In this study,we proposed a controlled-synthesis route to obtain homogeneous Co3O4thin film electrodes with efficient OER performance.Different from the commonly Co3O4catalysts preparation via loading annealed Co3O4powder on conductive substrates,we firstly synthesized CoOxcolloids via ethylene glycol solvothermal method and cast it on FTO conductive glass substrates,then annealed these CoOxcolloids into a homogeneous Co3O4thin films.By casting CoOxcolloids rather than loading Co3O4powder,the homogeneity of as-synthesized film electrodes can be well controlled and enhanced.It may open a new way to achieve the mass production for large-area electrodes.Furthermore,it has been reported that the reduction treatment of Co3O4catalysts will substantially enhance its OER activity and stability[15,28],since that the formation of CoO is kinetically facile to reach the active oxyhydroxide phase and provides an excellently mechanical protection to Co3O4.The previous works are also mainly focused to reduce the annealed Co3O4powder,what are more prone to deteriorate their original crystalline phase and catalytic stabilities.Considering that electrochemical reactions mainly occurs at the electrode surface,thus we implemented a simple reduction only on the very surface of these asprepared Co3O4thin films by NaBH4solution,effectively preventing the bulk deterioration.The electrochemical performances of these NaBH4-reduced Co3O4thin films as OER electrocatalysts were scientifically tested.It turned out that the NaBH4-reduced Co3O4thin film exhibited an excellent OER performance with a very small overpotential of 365 mV at the current density of 10 mA·cm?2and a low Tafel slope of 59.0 mV·dec?1.And the electrochemical stability of these reduced Co3O4thin film electrodes was also superior to that of the pristine.No irreversible performance degradation was found in a 24-h chronoamperometric stability test.

2.Experimental

2.1.Chemicals

All chemicals are analytical grade and require no further purification.Cobalt(II)acetate tetrahydrate(Co(CH3COO)2·4H2O),sodium borohydride (NaBH4),sodium carbonate,potassium hydroxide and glycol(>99%)were purchased from Aladdin.The water used in this experiment is deionized water.

2.2.Synthesis of CoOx colloids and Co3 O4 film electrodes

The CoOxcolloids were prepared via ethylene glycol solvothermal method.The cobalt acetate tetrahydrate was dispersed into 60 ml ethylene glycol and heated to boiling (180 °C).Then 200 ml 0.2 mol·L?1Na2CO3solution was added and kept boiling for 20 min.After filtration and wash,the dispersion was prepared into colloids loading with 6.8–7.2 wt%CoOx.Then cast CoOxcolloids onto FTO conductive glass substrates at room temperature and put into muffle furnace(SGM·M10/10,Shanghai Shsigma Co.,Ltd.)to heat at a rate of 5°C·min?1.After baking at 450°C for 4 h,the pristine Co3O4thin film was obtained,and then surface-reduced with NaBH4solution of variable concentration for 0–30 min.The NaBH4-reduced Co3O4thin film was denoted as Bx,y-Co3O4,where the subscript x indicated the NaBH4concentration,and y indicated the reduction time.

2.3.Materials characterization

The surface morphology of the obtained samples was characterized by scanning electron microscope(SEM,Ultra 55,CorlzeisD,Germany).The XRD pattern was tested by using a Bruker D8 Advance X-ray diffractometer.

2.4.Electrochemical measurements

The three-electrode test system with as-prepared samples as the working electrode,a Pt plate(99.95%,20 mm×20 mm,Wuhan Corrtest Instruments Co.,Ltd.)as the counter electrode and a saturated Hg/HgO(232-type,INESA Scientific Instrument Co.,Ltd.)as the reference electrode was analyzed on CHI660E electrochemical workstation(Shanghai Chinstruments Co.,Ltd.)at 25°C.The electrolyte was Ar-saturated 0.5 mol·L?1KOH (pH 13.6).The linear sweep voltammetry (LSV) was measured at sweep rate of 5 or 10 mV·s?1.Tafel slopes were calculated from LSV curves at the overpotential from 0.275 V to 0.6 V.Electrochemical impedance spectroscopy(EIS)data were obtained at a bias voltage of 0.1 V vs SCE with the frequency range of 0.01–10 MHz.The potential in this study was converted to RHE according to the following formula:ENHE=ESCE+0.0591 pH+0.241.

3.Results and Discussions

3.1.Crystallinity and morphology of Co3 O4 thin films

The XRD patterns of the pristine and the NaBH4-reduced Co3O4thin films(B12.5,20-Co3O4)were shown in Fig.1.Except for the diffraction peaks of FTO conductive glass substrate(marked with*),the diffraction peaks at 18.60°,32.42°,36.94°,37.70°,44.74°,54.50°,59.46°,and 65.46°in two catalysts all corresponded to the characteristic peaks of facedcentered cubic phase Co3O4(JCPDS 09-0418)[26,29].No change was found in the crystal phase of B12.5,20-Co3O4after 20 min reduction treatment in 12.5 mM NaBH4aqueous solution.It was suggested that under the low NaBH4concentration and short reduction time,such kind of the chemical reduction did not alter the bulk matrix of Co3O4thin films,as no boron doping or other impurity was found in the crystal phase.In addition,two new peaks emerged at 61.52°and 78.26°(marked with ■)only in B12.5,20-Co3O4thin films were matched to the CoO crystalline(JCPDS 75-0418)[30,31].That was referred to nanometers-thick CoO layers formed on the B12.5,20-Co3O4thin film after reduction treatment,which had been reported to be beneficial for the OER catalytic performance and stability in alkaline conditions[15].

Fig.2 showed the scanning electron microscope(SEM)images of the pristine and the NaBH4-reduced(B12.5,20-Co3O4)Co3O4thin films.A uniform,sponge-like morphology was observed in two films,both with some cracks caused by solvent evaporation.Quite different from the commonly reported nanowire morphology in the ethylene glycol solvothermal method where CoOxalways grows along the glycol chain at high temperature[16],in this work the thermal molecular motion of CoOxprecursors was too intense to readily grow along the glycol chain in such a short reaction time(20 min)since the CoOxsolution was kept boiling at 180 °C,and finally turned out to be a resultant sponge-like morphology.Further enlarged magnification to 50,000 times in Fig.2b and d,it could be found that the pristine and the B12.5,20-Co3O4films both mainly existed in the form of particle,and evenly dispersed on the surface of FTO substrates.Because of their good dispersibility,it was not easy to agglomerate together,which ensured the channels required for electron transportation and increased its electrochemical activity.While compared to the pristine Co3O4thin film,the sponge-like structure of B12.5,20-Co3O4films was slightly reinforced after NaBH4reduction,which resulted from the reductive etching of loose materials and reduced-products on the film surface.A proper surface etching of the film electrode actually was beneficial for exposing more active area for OER electrocatalysis[32–34].Overall,the high alignment with the above XRD results indicated that such kind of NaBH4-reduction treatment did not alter the surface or bulk structure too much.

Fig.1.XRD patterns of the pristine Co3 O4 and B12.5, 20 -Co3 O4 thin films.

Fig.2.SEM images of the pristine Co3 O4 thin films(a,b)and B12.5, 20 -Co3 O4 thin films(c,d).

3.2.Electrocatalytic OER performance of Co3 O4 film electrodes

3.2.1.Optimization of NaBH4 reduction conditions

The OER performance of Co3O4thin film electrodes was studied in 0.5 M KOH aqueous solution(pH 13.6)using a typical three-electrode device.The pristine Co3O4thin film electrodes were used for comparison.Firstly the effect of NaBH4concentration on OER performance was investigated,since it was of high importance in this electrochemical system.The chosen concentrations of NaBH4solution were 2.5,5.0,12.5,and 15.0 mmol·L?1,respectively.The LSV curves as shown in Fig.3a,the OER onset potential of NaBH4-reduced film electrodes gradually reduced with NaBH4concentration,and obtained the minimum value of 1.519 V at the concentration of 12.5 mmol·L?1.On the other hand,the pristine Co3O4film electrodes had the largest onset potential of 1.556 V and the current increased slowly,which showed an interior OER activity.Meanwhile,at the current density of 10 mA·cm?2,the potential was 1.615 V for the pristine and 1.569 V for the B12.5,20-Co3O4,respectively.By calculation,the overpotential was 415 mV for the pristine and 365 mV for the B12.5,20-Co3O4at this current density,which existed a negative shift of 50 mV.The reason why the overpotential of NaBH4-reduced Co3O4film electrodes less than pristine ones was speculated as shown in the SEM images of Fig.2,that is,the reinforced sponge-like film surface exposed more active area for OER electrocatalysis after reduction treatment [29–31].The Tafel slopes were then measured to investigate the electrode kinetics as shown in Fig.3b.The same trend was also observed of Tafel slopes that were reduced with NaBH4concentration,suggesting that the reduction treatment of NaBH4solution efficiently enhanced the kinetic performance.The corresponding Tafel slope values were 85.5,61.9 and 59.0 mV·dec?1for NaBH4concentration of 0,5 and 12.5 mmol·L?1,respectively.It meant that the B-Co3O4electrodes treated with 12.5 mmol·L?1NaBH4solution had faster electron transport speeds and was the most efficient OER electrocatalyst among them.Therefore,on basis of the above LSR and Tafel slope results,the concentration of NaBH4solution in current experimental conditions was optimized at 12.5 mmol·L?1.It was mainly because the proper NaBH4concentrations in the reduction reaction would increase the micro-structure and the of Co3O4film electrodes,since more loose materials etched off and simultaneously CoO layers formed[35–37].While the concentration of NaBH4solution increased above 12.5 mmol·L?1,the excessive surface etching on Co3O4electrodes in turn would destroy the original crystalline phase and unstable the film structure,further even reduce the generated CoO layer into Co metal [35,37,38];hence eventually degraded the OER performance.

In addition,under the optimum concentration of 12.5 mmol·L?1,the NaBH4reduction time was also investigated.It presented the same trend that first increased and then decreased as the NaBH4reduction time prolonging.On basis of the LSR results in Fig.4a,the reduction time in current experimental conditions was optimized at 20 min.

Fig.3.Effect of the NaBH4 concentration on the OER catalytic performance.(a)LSV curves;(b)Tafel slopes.(NaBH4 reduction time:20 min;sweep rate:5 mV·s -1 ;electrode area:0.20 cm2).

3.2.2.The influence of testing conditions on electrocatalytic OER performance

Compared to the role of NaBH4concentration,the factor of NaBH4reduction time seemed less impact on the OER catalytic performance.It should be noted that the sweep rate used in the above optimization cases of NaBH4concentration and reduction time was 5 mV·s?1and 10 mV·s?1,and the electrode area were 0.2 and 1.0 cm2,respectively.The influence of these two differentia on electrocatalytic OER performance of B12.5,20-Co3O4film electrodes was further figured out.Firstly,the scanning rate speeds among 5–50 mV·s?1almost did not have any effect on OER performance(Fig.S1),so that we can eliminate this differentia of sweep rate in the above studies.While for the electrode area shown in Fig.5a,an obviously positive shift was observed in the OER onset potential as the electrode area increased.It revealed that the geometric area of the electrode had a negative influence on OER performance in this test.Therefore,the results in Figs.3a and 4a that NaBH4concentration mattered more than NaBH4reduction time on OER performance was mainly attributed to the larger electrode area used in the reduction time optimization.In addition,the different concentrations of KOH electrolyte were also studied in Fig.5b with the electrode area of 0.42 cm2,and we found that the OER performance of the B12.5,20-Co3O4film electrode were always superior to that of the pristine.

3.2.3.Analysis of EIS results

Electrochemical impedance spectroscopy(EIS)can effectively reveal the transfer and recombination of charge at the electrode/electrolyte surface under OER conditions.Herein,the influence of the electrode area and reduction time on EIS were taken into account.The Nyquist plots in Fig.6 were obtained under open circuit potential(OCP),present semicircles with a diameter dependent on the electrode [39–41].The semicircle were not so obvious,but still could apparently do comparison on their diameters.In Fig.6a,the diameters increased with the electrode area,which meant that the charge transfer slowed down[42].It was coincident with the LSR results in Fig.5a that the current response decreased with the electrode area.Similarly,the diameters in Fig.6b decreased with the NaBH4reduction time that were also coincident with the LSR results in Fig.4a.The decrease of the resistance after NaBH4reduction represented the acceleration of charge transfer,which should be the main reason for the enhancement of OER performance[43].

3.3.Long-term OER performance of Co3 O4 film electrodes

Fig.4.Effect of the reduction time on the OER catalytic performance.(a)LSV curves;(b)Tafel slopes.(NaBH4 concentration:12.5 mmol·L?1;sweep rate:10 mV·s?1;electrode area:1.0 cm2).

Fig.5.Effect of the electrode area(a)and electrolyte concentration(b)on the OER catalytic performance of B12.5,20 -Co3 O4 thin film electrodes.(sweep rate:5 mV·s?1;a:Ar saturated 0.5 mol·L?1 KOH;b:electrode area 0.42 cm2).

The long-term electrocatalytic OER performance is another significant criterion to evaluate an electrocatalyst[44–46].Thus the electrochemical stability of as-prepared Co3O4catalysts was evaluated by chronoamperometry response in Fig.7.As shown in Fig.7a,after testing 20 min under three potential of 1.69 V,1.77 V,and 1.89 V,respectively,no performance degradation was observed in either the pristine or the B-Co3O4electrode,and the OER performance of the latter were all superior to that of the former.The long-term OER performance for the pristine one was carried out with the electrode area of 1.0 cm2shown in Fig.7b,where a significant drop in the current density caused by the charge–discharge phenomena and the adsorption contamination of the electrode surface was observed in the first 5 h.Continued after 5.8 h,the current density stabilized at 1.19 mA·cm?2with the electric quantity of 57.38 C;until after 10 h,the current density still at 1.27 mA·cm?2with the electric quantity of 75.96 C.To verify whether the pristine deactivated or not after 10 h testing,the KOH electrolyte was refreshed and a recovery of OER catalytic performance was observed.Continued running for 5.8 h,not much difference was found from the first cycle:j=1.83 mA·cm?2,Q=52.96 C.Thus the coincident results of these two cycles revealed that the degradation in current density was reversible over time,and mainly resulting from the generation of hydrogen peroxide and dissolved oxygen in the water splitting [47].Overall after 15 h testing,no irreversible attenuation of the OER electrocatalytic properties was observed in the pristine Co3O4film electrodes.As for the B12.5,20-Co3O4film electrode shown in Fig.7c,different from the sharp drop found in the pristine Co3O4electrodes,the current density was reduced gradually.Similarly after running for 5.8 h,the current density was 3.57 mA·cm?2with the electrode quantity of 52.96,three times higher than that of the pristine one(1.19 mA·cm?2).Thereby,a continuous declination in current density was triggered as much more oxygen was generated and adsorbed to the electrode surface.While after 12 h,the current density with the electric quantity of 154.05 C was 2.45 mA·cm?2,still two times higher than that of the pristine(running for 10 h,1.27 mA·cm?2).Then the KOH electrolyte was refreshed and simultaneously,the external bias voltage was increased to 1.89 V.A recovery of OER catalytic performance similar to the pristine was observed even at rather high potential.Continued running for 5.8 h,j=9.76 mA·cm?2,Q=225.78 C;after 12 h,j=9.50 mA·cm?2,Q=441.45 C.Overall after 24 h running,also no irreversible attenuation of the OER electrocatalytic properties was observed in B12.5,20-Co3O4film electrode,even under the increasing potential.Comparing the electrocatalytic performance in Fig.7,the efficiency and the stability of the OER electrocatalysis were simultaneously enhanced via the simple surface reduction of NaBH4solution,which supported the findings that CoO and Co3O4would play complementary roles to achieve an active and robust electrocatalyst[15].And these chronoamperometry response tested under the steady state would be more intuitive and supportive than the above OER performances obtained under the transient state.

Fig.6.Nyquist plots of B12.5, 20 -Co3 O4 thin film electrodes with different geometric area(a)and reduction time(b).(EIS performed at OCP,?0.1V vs SCE).

4.Conclusions

Herein,a controlled-synthesis route was proposed to obtain homogeneous NaBH4-reduced Co3O4film electrodes for enhanced OER electrocatalytic performance in alkaline solution.Firstly CoOxcolloids was synthesized via ethylene glycol solvothermal method and cast on FTO conductive glass substrates.Significant enhancement of OER electrocatalysis was obtained after the surface of the as-prepared Co3O4film electrode reduced by NaBH4solution.Optimization of the NaBH4concentration and reduction time was both possible under the prevailed experimental conditions.Analysis of the LSR curves and Tafel slopes revealed that the surface reduction by NaBH4solution had obviously reinforced OER electrocatalytic performance by means of exposing more surface area and the active sites.EIS data also confirmed the above conclusions.Meanwhile,the NaBH4-reduced Co3O4film electrodes exhibited a good electrocatalytic stability in a 24-h chronoamperometric stability test.

Fig.7.Electrochemical durability of Co3 O4 thin film electrodes.Chronoamperometric stability measurements for(a)the pristine Co3 O4 and B12.5, 20 -Co3 O4 thin flim electrode at different constant potentials;(b)the pristine Co3 O4 thin film electrode at potential of 1.74 V;(c)the B12.5, 20 -Co3 O 4 thin film electrode at potential of 1.74 V and 1.89 V.(Ar-saturated 0.5 mol·L-1 KOH solution;a:electrode area 0.42 cm?2;b&c:electrode area 1.0 cm2).

Acknowledgements

The work was supported by National Key Research and Development Program of China (2016YFB0600503,2016YFC0400503,2016YFC0400506).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.08.008.