Mns-Nanoparticles-Decorated Three-Dimensional Graphene

catalysts

Article MnS-Nanoparticles-Decorated Three-Dimensional Graphene Hybrid as Highly Eﬃcient Bifunctional Electrocatalyst for Hydrogen Evolution Reaction and Oxygen Reduction Reaction

Khalil ur Rehman, Shaista Airam, Long Song, Jian Gao, Qiang Guo, Yukun Xiao and Zhipan Zhang *

Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, Key Laboratory of Cluster Science, Ministry of Education of China, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, China; [email protected] (K.u.R.); [email protected] (S.A.); [email protected] (L.S.); [email protected] (J.G.); [email protected] (Q.G.); [email protected] (Y.X.) * Correspondence: [email protected]; Tel.: +86-10-6891-8608

Received: 6 September 2020; Accepted: 23 September 2020; Published: 3 October 2020

Abstract: The search for renewable energy resources has attracted considerable research interests in electrochemical reactions of hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) that are essential for fuel cells. Earth-abundant, eco-friendly and cost-effective transition metal compounds are emerging candidates as electrocatalysts in these reactions. Herein, we report the growth of manganese sulfide nanoparticles on three-dimensional graphene, through an easy, progressive successive ionic layer adsorption and reaction (SILAR) method, where manganese sulfide nanoparticles (MnS-NPs), diameter of 4–5 nm are homogeneously decorated on the 3D graphene matrix. The formed hybrid shows improved HER activity in 0.1 M KOH when compared to bulk MnS. Moreover, MnS-NPs@3DG is also active in catalyzing ORR, qualifying it as a new type of bifunctional electrocatalyst in alkaline media.

Keywords: manganese sulﬁde nanoparticles; 3D graphene; bifunctional electrocatalyst; hydrogen evolution reaction; oxygen reduction reaction

1. Introduction The ongoing depletion of fossil fuels has demanded an urgent development in more sustainable renewable energy sources. In particular, molecular hydrogen is short-listed as an eco-friendly energy conveyer to potentially meet the over-expanding energy needs [1,2]. The hydrogen evolution reaction (HER) is pivotal to electrochemical splitting of water into H2 and O2 as an inexpensive and viable route to subsequent hydrogen fuels [3–6]. Normally, catalysts are required to reduce the overpotential (η) to drive HER at a relatively low potential with an appreciable rate. To date, Pt remains as the most efficient HER electrocatalyst, but the price and scarcity of Pt seriously limit its practical applications [7–10]. Therefore, finding alternative cost-effective electrocatalysts from earth-abundant elements is of the utmost importance [11]. Recently, transition metals and their compounds including borides, carbides, nitrides, phosphides, oxides, sulfides and selenides emerged as efficient electrocatalysts for HER [12–18]. In particular, Mn-based compounds have attracted considerable interests owing to their low cost and relatively high abundance. Generally, electrocatalytic activities can be manipulated on two aspects, i.e., chemical compositional tuning by doping and morphology control for a judiciously designed nanostructurized electrode [19–23]. For example, Li et al. solvothermally synthesized hexagram

Catalysts 2020, 10, 1141; doi:10.3390/catal10101141 www.mdpi.com/journal/catalysts Catalysts 2020, 10, 1141 2 of 13 nanosheets of bimetallic cobalt manganese sulfide (CMS) supported by a three-dimensional (3D) Ni 2 foam and this free-standing electrode possessed an overpotential (η) of 210 mV at 100 mA cm− in the HER [24]. Tang et al. reported hydrothermally synthesized nanosheets of Co-Mn carbonate hydroxide 2 on a 3D nickel foam and the hybrid catalyst featured an overpotential of 180 mV at 10 mA cm− [20]. Meanwhile, Miao et al. set a scalable pathway to synthesize Ni-Mn-Co spinel ternary sulfide on the reduced graphene oxide (RGO), and the nanocomposite had an insignificant η of 151 mV at 2 10 mA cm− [25]. Moreover, catalytic activity of MnS has been enhanced unusually by supporting interconnected porous sheets into a 3D framework. Graphene has been demonstrated to be a suitable support for catalysts due to its high surface area and excellent conductivity [26–28]. To profit the electrocatalytic activity of MnS nanoparticles (Mn-NPs) in a 3D scenario, we herein report a hybrid catalyst of MnS nanoparticles supported on the porous 3D graphene foam (MnS-NPs@3DG) prepared via a convenient SILAR method. SILAR has been proven to be a facile, low-cost and eco-friendly method to grow nanomaterials on 3D conductive supports [29,30], and it is clearly different from previous MnS synthetic routes such as spray pyrolysis, microwave irradiation, chemical bath deposition, solvothermal and hydrothermal that required time-consuming procedures and harsh conditions [31–34]. Meanwhile, the 3D graphene network provides a large surface area with enriched catalytic sites to anchor MnS-NPs for efficient HER, while cordial contact of MnS-NPs with 3D conductive support ensures a facile electron transport at MnS-NPs/3DG interface, eliminating barriers for electron transport across highly resistive MnS. On the other hand, catalytic active sites are electronically connected through the underlying 3D graphene network, simultaneously permitting facile diffusion of gaseous molecules and electrolyte species for high HER performance. As a result, the MnS-NPs@3DG showed a catalytic performance of 2 274 mV (η) at 10 mA cm− . To the best of our knowledge, this material is reported for the first time for HER. Additionally, the MnS-NPs@3DG nanohybrid also actively participated in catalyzing the oxygen reduction reaction (ORR), making it a new kind of bifunctional electrocatalyst in alkaline media.

2. Results and Discussion The morphology of the resulted MnS-NPs@3DG was viewed by scanning electron microscopy (SEM). The SEM image in Figure1a reveals the interconnecting cross-linked feature of graphene sheets into a three-dimensional monolith, which was rich in pores of different sizes ranging from sub-micrometer to several micrometers. For nanoparticle loading by the SILAR process, the time of dipping/drying and the number of SILAR cycles were strictly controlled to ensure even distribution of nanoparticles on the graphene foam. As presented in Figure1b, the surface of the 3D graphene framework became rougher after the deposition of MnS-NPs, and element mapping in Figure1c endorses the subsistence of carbon, manganese, sulfur and oxygen in the as-prepared nanocomposite. MnS-NPs were further characterized by transmission electron microscopy (TEM, Figure1d). Owing to the transparent nature of thin graphene sheets in the composite, MnS-NPs can be well resolved on surface of graphene. The HRTEM images in Figure1e revealed that the size of nanoparticles was around 4-5 nm with 0.299 nm d-spacing paralleled to (101) plane of MnS [35]. In order to optimize the catalytic activity of the hybrid catalyst, SILAR cycles of 1, 3, 5 and 10 were applied to yield samples denoted as SILSR-1, SILAR-3, SILAR-5 and SILAR-10. It is found that the number of SILAR cycles is a crucial parameter to control aggregation of MnS-NPs on the 3DG network, and formation of MnS-NPs aggregates on the surface of the graphene framework may block pores of the graphene foam (Figure S1). Therefore, the number of SILAR cycles was optimized as 5, where sufficient MnS-NPs could be anchored on the 3DG network without interfering electron transport or electrolyte diffusion through the nanocomposite toward efficiently catalyzing the HER [36]. The structure of the MnS-NPs@3DG was probed by the x-ray diffraction technique. Figure2a compares XRD configurations of the as-prepared composite with the 3DG. A typical diffraction peak at 2θ of ~26◦ was observed in the 3DG, corresponding to the diffraction between the (002) plane of the graphene. In addition, the MnS-NPs@3DG also shows other characteristic peaks at 2θ of 27.6◦, 29.4◦, 45.7◦, 49.5◦ and 54.1◦, confirming the formation of γ-MnS (JPCD: 40-1289). XPS was further Catalysts 2020, 10, 1141 3 of 13 carried out to evaluate the bonding characteristics of MnS-NPs@3DG. The atomic ratio of Mn to S is 1.02: 1 (Figure2b), well matching the stoichiometric ratio of MnS. Additionally, the high-resolution S spectrum in Figure2c shows distinctive S2p 3/2 peak at the binding energy of 163.6 eV, which originated from the sulfur in MnS [37] and high-resolution Mn spectrum exhibits two main peaks of Mn2p3/2 and 2+ Mn2p1/2 at 641.8 eV and 653.7 eV, respectively, with a satellite feature of Mn situated at 647 eV in Figure2d [ 38,39]. Furthermore, N2 adsorption and desorption experiments were conducted (Figure2e) and the Brunauer–Emmett–Teller (BET) specific surface area of MnS-NP@3DG was determined as 2 1 2 1 134.3 m g− , which was significantly larger than that of the bulk MnS (32.5 m g− ) due to the benefit of porousCatalysts 3D support 2020, 10, x toFOR improve PEER REVIEW the specific surface area. Meanwhile, MnS-NPs@3DG3 hasof 14 a pore size of 4 nm (Figure2f), which may cause an easy flow of electrolytic species during the HER process.

Figure 1.FigureThe SEM 1. The images SEM images of 3DG of 3DG (a )(a and) and MnS-NPs@3DG MnS-NPs@3DG (b); (correspondingb); corresponding elemental elemental mapping of mapping of MnS-NPs@3DGMnS-NPs@3DG (c); TEM (c); TEM (d) and(d) and HRTEM HRTEM ( (ee)) image of of typical typical MnS-NPs MnS-NPs on graphene on graphene sheet. sheet. The structure of the MnS-NPs@3DG was probed by the x-ray diffraction technique. Figure 2a The electrocatalyticcompares XRD configurations HER activity of the of as-prepared catalysts wascomposite conducted with the in 3DG. alkaline A typical solution diffraction (0.1 peak M KOH) with a typical three-electrodeat 2θ of ~26° was observed system. in SEM the 3DG, images corresponding in Figure 3toa thed diffraction reveal that between the morphology the (002) plane of ofthe catalyst − changed fromthe graphene. slightly In rough addition, to largethe MnS-NPs@3DG aggregates withalso shows the increased other characteristic number peaks of SILAR at 2θ of cycles, 27.6°, and such a morphological29.4°, 45.7°, change 49.5° significantly and 54.1°, confirming influenced the formation the catalytic of γ-MnS performance (JPCD: 40-1289). in HER. XPS As was shown further in Figure3e, carried out to evaluate the bonding characteristics of MnS-NPs@3DG. The atomic ratio of Mn to S is as the loading1.02: 1 amount(Figure 2b), of well electrocatalytically matching the stoichiometric active MnS-NPs ratio of MnS. increased, Additionally, the the value high-resolution of the electrocatalytic S current densityspectrum (J catin) Figure dramatically 2c shows improved distinctive untilS2p3/2 SILARpeak at cyclesthe binding reached energy 5. Interestingly,of 163.6 eV, which a poor catalytic current wasoriginated observed from inthe SILAR-10, sulfur in MnS and [37] this and couldhigh-resolution be attributed Mn spectrum to the exhibits excessive two main stackings peaks of the highly 2+ resistive MnS-NPsMn2p3/2 and Mn2p that impede1/2 at 641.8 electroneV and 653.7 flow eV, acrossrespectively, the electrodewith a satellite to thefeature catalyst of Mn /electrolyte situated at interface. 647 eV in Figure 2d [38,39]. Furthermore, N2 adsorption and desorption experiments were conducted While the(Figure number 2e) ofand SILAR the Brunauer–Emmett–Teller cycles was optimized (BET as) 5,specific the e ffsurfaceect of area mass of loadingMnS-NP@3DG was furtherwas studied 2 in Figuredetermined S2, and an as optimal134.3 m2 g catalyst−1, which was loading significantly of ~0.28 larger mg than cm that− ofwas the bulk obtained. MnS (32.5 The m2 observedg−1) due inferior performanceto the with benefit a largerof porous catalytic 3D support loading to improve may the be specific attributed surface to area. impeded Meanwhile, electron MnS-NPs@3DG transport through a

Catalysts 2020, 10, 1141 4 of 13 Catalysts 2020, 10, x FOR PEER REVIEW 4 of 14 thicker catalysthas a pore layer size and of 4/ ornm agglomeration(Figure 2f), which may of catalytic cause an easy mesopores flow of electrolytic with ine speciesﬃcient during electrolytic the ﬂow through theHER catalyst process. on the electrode surface.

Figure 2. (aFigure) XRD 2. pattern(a) XRD ofpattern the MnS-NPs@3DG, of the MnS-NPs@3DG, 3DG 3DG and andγ-MnS γ-MnS (JCPDS (JCPDS no. no. 40-1289);40-1289); ( (bb) )XPS XPS spectral spectral data of MnS-NPs@3DG and high resolution XPS spectral data of S2p (c)and Mn2p (d); (e) N2 data of MnS-NPs@3DG and high resolution XPS spectral data of S2p (c)and Mn2p (d); (e)N2 adsorption adsorption and desorption isotherms and (f) corresponding pore size distribution of as-prepared bulk and desorptionMnS and isotherms MnS-NPs@3DG, and ( frespectively.) corresponding pore size distribution of as-prepared bulk MnS and MnS-NPs@3DG, respectively. The electrocatalytic HER activity of catalysts was conducted in alkaline solution (0.1 M KOH) Figurewith4a a plots typical comparative three-electrode system. polarization SEM images curves in Figure for 3a bulk−d reveal MnS, that 3DGthe morphology and MnS-NPs@3DG, of the catalyst changed from slightly rough to large aggregates with the increased2 number of SILAR cycles, respectively. The current density for bulk MnS was 8.8 mA cm− at an over-potential of 800 mV, and its poor HER catalytic activity may be attributed to lack of catalytic surface area and conductivity. In comparison, cathodic catalytic current for MnS-NPs@3DG initiated at 230 mV (Figure S3), and after 2 − this potential, Jcat sharply increased to about 142 mA cm− at an overpotential of 490 mV. Such an improvement in catalytic activity is a result of electrical coupling of catalytic active sites with 3D interconnected graphene sheets, thus enlarging the catalyst/electrolyte interface to promote HER and avoiding electron transport directly through intrinsically less-conductive MnS to the electrode [40]. The kinetics of the catalyzed HER were analyzed by measuring Tafel plots. As shown in Figure4b, 1 the calculated Tafel slope of MnS-NPs@3DG was 100 mV dec− approximately, while higher Tafel slopes 1 1 were found for 3DG (107 mV dec− ) and bulk MnS (150 mV dec− ), implying that the effective use of a Catalysts 2020, 10, 1141 5 of 13 large interfacial surface area and rapid electron transportation were responsible for the better HER observed in the MnS-NPs@3DG. Meanwhile, as Tafel slope is associated with the rate-limiting step in 1 HER, the measured Tafel slope value (100 mVdec− ) for MnS-NPs@3DG suggested a Volmer-Heyrosky mechanism [41]. In general, an over-potential of 274 mV was required to achieve a benchmark Jcat 2 of 10 mA cm− . In contrast, much higher overpotentials of 684 and 807 mV were required for 3DG 2 and bulk MnS to generate a current density of 10 mA cm− (Figure4c). Catalytic durability is another indispensable factor for effective practical usage of non-precious electrocatalysts for long-duration hydrogenCatalysts production. 2020, 10, x FOR All PEER samples REVIEW were measured by continuous cyclic voltammetric (CV) sweeps5 of 14 with a scan rate of 50 mV s 1 for 10,000 cycles at a selected potential range of 0.1 and 0.6 V. Figure4d shows and such a morphological− change significantly influenced the catalytic performance− in HER. As that theshown polarization in Figure 3e, curves as the ofloading the MnS-NP@3DG amount of electr inocatalytically Cycle 1 and active Cycle MnS- 10,000NPs increased, were almost the value identical, confirmingof the itselectrocatalytic excellent stability current indensity catalyzing (Jcat) dramatically HER. Furthermore, improved double until SILAR layer capacitancecycles reached (C dl5.) was estimatedInterestingly, for bulk a MnS, poor 3DGcatalytic and current MnS-NPs@3DG was observed (Figure in SILAR-10,4e) [ 42], and this a potential could be range attributed was selectedto the as 2 0.15 toexcessive0.25 V stackings due to the of presence the highly of resistive non-faradaic MnS-NPs current that [impede43]. In comparisonelectron flow toacross both the 3DG electrode (3.7 mF to cm − ) − 2 2 and bulkthe catalyst/electrolyte MnS (0.05 mF cm interface.− ), a larger WhileC dthel (10 number mF cm of− SILAR) was cycles found was for optimi the MnS-NPs@3DG,zed as 5, the effect proving of that itmass exposed loading more was active further sites studied for electrocatalyticin Figure S2, and activityan optimal in HER.catalyst Additionally, loading of ~0.28 the mg electrochemical cm−2 was impedanceobtained. spectroscopy The observed (EIS) inferior results performance in Figure4f revealwith a thatlarger MnS-NPs@3DG catalytic loading possessed may be attributed smaller faradaic to impeded electron transport through a thicker catalyst layer and/or agglomeration of catalytic impedance than bulk MnS, implying that the existence of the graphene support enhanced the interfacial mesopores with inefficient electrolytic flow through the catalyst on the electrode surface. surface area for rapid charge transfer in HER.

Figure 3. SEM images of MnS@3DG with different SILAR cycles: (a) SILAR-1, (b) SILAR-3, (c) SILAR-5 and (dFigure) SILAR-10; 3. SEM images (e) the of polarization MnS@3DG with curves different of MnS-NPs@3DG SILAR cycles: (a with) SILAR-1, different (b) SILAR-3, SILAR cycles. (c) SILAR- 5 and (d) SILAR-10; (e) the polarization curves of MnS-NPs@3DG with different SILAR cycles. The prepared hybrid was also active for catalyzing ORR in 0.1 M KOH solution. As depicted in Figure5a,Figure featureless 4a plots CV comparative response of polarization MnS-NP@3DG curves was for noticed bulk MnS, in nitrogen 3DG and saturated MnS-NPs@3DG, electrolyte, −2 whilerespectively. a strong cathodic The current reductive density peak for bulk appeared MnS was in oxygen8.8 mA cm saturated at an over-potential electrolyte, indicatingof 800 mV, and that the its poor HER catalytic activity may be attributed to lack of catalytic surface area and conductivity. In as-prepared hybrid has potential for electrocatalytic reduction in oxygen. ORR for MnS-NPs@3DG comparison, cathodic catalytic current for MnS-NPs@3DG initiated at −230 mV (Figure S3), and after started at ~0.9 V, with well resolved reductive loop bulging at ~0.75 V. In contrast, bulk MnS showed this potential, Jcat sharply increased to about 142 mA cm−2 at an overpotential of 490 mV. Such an inferiorimprovement ORR activity in ascatalytic compared activity to Mn-NPs@3DG,is a result of electrical which coupling again suggests of catalytic the improvedactive sites ORR with catalytic3D activityinterconnected of the as-prepared graphene hybridsheets, thus originated enlarging from the catalyst/electrolyte good compatibility interface between to promote MnS-NPs HER and and the underlyingavoiding 3DG electron network. transport For furtherdirectly confirmation,through intrinsically ORR kineticsless-conductive of MnS-NP@3DG MnS to the electrode was also [40]. noted in oxygenThe saturated kinetics of alkaline the catalyzed solution HER (0.1 were M KOH)analyzed by by using measuring the rotating Tafel plots. disk electrodeAs shown (RDE)in Figure system 4b, as presentedthe calculated in Figure Tafel5b.At slope various of MnS-NPs@3DG potentials, Koutecky was 100 mVLevich dec−1 approximately, (K-L) plots possessed while higher good Tafel linearity −1 − −1 whichslopes imply were a similar found electronfor 3DG (107 transfer mV dec number) and forbulk ORR MnS(Figure (150 mV5 c).dec For), implying ORR phenomena, that the effective the slope of Koutecky–Levichuse of a large interfacial plots surface was used area toand obtain rapid electron the number transportation of electrons were (responsiblen) required for to the reduce better one HER observed in the MnS-NPs@3DG. Meanwhile, as Tafel slope is associated with the rate-limiting O2 molecule. A value of n = 2 was calculated for MnS-NPs@3DG within the potential range of step in HER, the measured Tafel slope value (100 mVdec−1) for MnS-NPs@3DG suggested a Volmer- 0.2–0.5 V, signifying a specific two-electron transfer ORR process. Moreover, the MnS-NPs@3DG Heyrosky mechanism [41]. In general, an over-potential of 274 mV was required to achieve a nanohybrid catalyst exhibited the most positive ORR onset potential of 0.9 V and a current density of benchmark Jcat of 10 mA cm−2. In contrast, much higher overpotentials of 684 and 807 mV were 2 2.5 mArequired cm− atfor 1600 3DG rpmand (Figurebulk MnS5d). to Thegenerate methanol a current crossover density eofff 10ect mA was cm tested−2 (Figure in oxygen 4c). Catalytic saturated durability is another indispensable factor for effective practical usage of non-precious electrocatalysts for long-duration hydrogen production. All samples were measured by continuous cyclic voltammetric (CV) sweeps with a scan rate of 50 mV s−1 for 10,000 cycles at a selected potential range of 0.1 and −0.6 V. Figure 4d shows that the polarization curves of the MnS-NP@3DG in Cycle 1 and Cycle 10,000 were almost identical, confirming its excellent stability in catalyzing HER. Furthermore, double layer capacitance (Cdl) was estimated for bulk MnS, 3DG and MnS-NPs@3DG (Figure 4e) [42],

Catalysts 2020, 10, 1141 6 of 13 alkaline solution (0.1 M KOH) to investigate the methanol tolerance of the resulting MnS-NPs@3DG catalyst.Catalysts No apparent2020, 10, x FOR changes PEER REVIEW were observed for as-prepared MnS-NPs@3DG after adding methanol6 of 14 (10 vol%) showing its good methanol tolerance potential (Figure5e). In contrast, the Pt /C catalyst showedand an a potential obvious range oxidation was selected peak under as 0.15 theto − same0.25 V conditiondue to the presence (inset of of Figure non-faradaic5e), further current proving [43]. that theIn MnS-NPs@3DGcomparison to both nanohybrid 3DG (3.7 mF exhibited cm−2) and superiorbulk MnS methanol (0.05 mF cm tolerance−2), a larger compared Cdl (10 mF to cm a−2 standard) was platinum-basedfound for the catalyst. MnS-NPs@3DG, The CV measurementsproving that it exposed were further more active conducted sites for to electrocatalytic investigate the activity durability in HER. Additionally, the electrochemical impedance spectroscopy (EIS) results in Figure 4f reveal that of the electrocatalyst in alkaline solution at a 50 mV s 1 scan rate. As shown in Figure5f, polarization MnS-NPs@3DG possessed smaller faradaic impedance− than bulk MnS, implying that the existence of curves after 10,000 cycles negatively shifted 5 and 40 mV for Mns-NPs@3DG and Pt/C, respectively, the graphene support enhanced the interfacial surface area for rapid charge transfer in HER. highlighting the durability of the MnS-NPs@3DG catalyst in the ORR.

FigureFigure 4. (a) Hydrogen 4. (a) Hydrogen evolution evolution reaction reaction (HER) (HER) polarization polarization curves curves of bulk of MnS, bulk 3DG MnS, and 3DG MnS-NPs@3DG and MnS- 1 at scanNPs@3DG rate of 5 mVat scan s− inrate 0.1 of M 5 KOH;mV s−1 ( bin) 0.1 Tafel M plotsKOH; of (b the) Tafel bulk plots MnS, of 3DGthe bulk and MnS, MnS-NPs@3DG; 3DG and MnS- (c) the 2 −2 overpotentialNPs@3DG; at 10 (c) mA the overpotential cm− for bulk at MnS,10 mA 3DG cm andfor bulk MnS-NPs@3DG; MnS, 3DG and ( dMnS-NPs@3DG;) durability of ( MnS-NPs@3DGd) durability catalystof descriptMnS-NPs@3DG negligible catalyst HER descript activity negligible loss even HER after activity 10,000 loss cyclic even voltammetricafter 10,000 cyclic (CV) voltammetric cycles within 0.1 to 0.6 V at 50 mVs 1;(e) capacitive current measurements for bulk MnS, 3DG and MnS-NPs@3DG − − ∆J = Ja Jc); (f) Nyquist plots of bulk MnS, 3DG and MnS-NPs@3DG. 0 − Catalysts 2020, 10, 1141 7 of 13 Catalysts 2020, 10, x FOR PEER REVIEW 8 of 14

1 FigureFigure 5. 5.(a)( aCV) CV measurements measurements of of MnS-NPs@3DG MnS-NPs@3DG and bulkbulk MnSMnS inin alkaline alkaline solution solution at at 10 10 mV mV s− s,−1, wherewhere the the solid solid line andand dash dash line line represents represents the CVthe curves CV curves in oxygen in saturatedoxygen saturated and nitrogen and saturated nitrogen alkaline solution, respectively; (b) the RDE measurements for MnS-NPs@3DG in alkaline solution at saturated alkaline solution, respectively; (b) the RDE measurements for MnS-NPs@3DG in alkaline diﬀerent rotating speeds; (c) K–L plots attain at 0.5 to 0.2 V from the RDE measurements; (d) polarization solution at different rotating speeds; (c) K–L plots attain at 0.5 to 0.2 V from the RDE measurements; curves for MnS-NPs@3DG, 3DG, bulk MnS and Pt catalyst in oxygen saturated alkaline solution at (d) polarization1 curves for MnS-NPs@3DG, 3DG, bulk MnS and Pt catalyst in oxygen saturated 10 mV s− on a RDE (1600 rpm); (e) the crossover toleration test of MnS-NPs@3DG and Pt catalyst (inset) alkaline solution at 10 mV s−1 on a RDE (1600 rpm); (e) the crossover toleration test of MnS-NPs@3DG in the oxygen saturated alkaline solution at 0.2 V; (f) oxygen reduction reaction (ORR) polarization − andcurves Pt catalyst of the (inset) MnS-NPs@3DG in the oxygen and Ptsaturated/C after 10,000 alkaline cycles. solution at −0.2 V; (f) oxygen reduction reaction (ORR) polarization curves of the MnS-NPs@3DG and Pt/C after 10,000 cycles.

Performance-based comparative study for various Mn-based catalysts have been narrated in Figure 6a,b. Owing to bifunctional electrocatalysts (HER, ORR), the tailored nanocomposite was

Catalysts 2020, 10, 1141 8 of 13

CatalystsPerformance-based 2020, 10, x FOR PEER REVIEW comparative study for various Mn-based catalysts have been narrated9 of 14 in Figure6a,b. Owing to bifunctional electrocatalysts (HER, ORR), the tailored nanocomposite was proven provento excel to over excel other over Mn-based other Mn-based catalysts catalysts (Figure 6(Fig; Tablesure 6; S1 Tables and S2 S1 Supplementary and S2 Supplementary Materials). Materials). In a nut Inshell, a nut well-tailored shell, well-tailored nanocomposite nanocomposite (MnS-NPs@3DG) (MnS-NPs@3DG) showed impressive showed (HER, impressive ORR) functionalities(HER, ORR) functionalitiesthat though not that comparable though not to comparable Pt-based expensive to Pt-bas catalystsed expensive yet, may catalysts helpful yet, to may curtail helpful the expense to curtail of thesuch expense an expensive of such metal-based an expensive catalyst metal-based as alternatives catalyst as for alternatives the same purposes. for the same purposes.

Figure 6. (a) Comparison of recently reported highly active Mn-based HER electrocatalysts and (b) ORR Figureelectrocatalysts 6. (a) Comparison in the literature, of recently respectively. reported highly active Mn-based HER electrocatalysts and (b) ORR electrocatalysts in the literature, respectively. 3. Materials and Method 3. Materials and Method 3.1. Reagents 3.1. ReagentsManganese chloride tetra hydrate (MnCl 4H O) was received from Fuchen Chemical Reagents 2· 2 Factory (Tianjin, China), sodium sulfide (Na2S 9H2O) from Guangfu Fine Chemical industry (Tianjin, Manganese chloride tetra hydrate (MnCl2··4H2O) was received from Fuchen Chemical Reagents China), and 5 wt% Nafion solution (Aladdin, Shanghai, China). All reagents were utilized with no Factory (Tianjin, China), sodium sulfide (Na2S·9H2O) from Guangfu Fine Chemical industry (Tianjin China),further refining.and 5 wt% Nafion solution (Aladdin, Shanghai, China). All reagents were utilized with no further3.2. Synthesis refining. of GO

3.2. SynthesisThe modiﬁed of GO Hummer’s method was adopted to synthesize graphene oxide (GO) by oxidation leading exfoliation of natural graphite ﬂakes [44]. The modified Hummer’s method was adopted to synthesize graphene oxide (GO) by oxidation leading3.3. Synthesis exfoliation of 3DG of natural graphite flakes [44].

3.3. SynthesisOur previously of 3DG reported method was adopted for synthesis of 3DG. In nut shell, 3 mL 1 1 6 mg mL− GO aqueous solution with 0.25 mL 100 mg mL− sodium dodecyl sulfate (SDS) aqueous −1 solutionOur previously homogenized reported in a method 20 mL was propylene adopted isotopic for synthesis vial (internal of 3DG. In diameter nut shell, 22) 3 mL at 6 3000 mg mL rpm −1 GOstirring aqueous rate forsolution 60s. Then, with in0.25 this mL pre-homogenized 100 mg mL sodium solution, dodecyl 0.4 mL sulfate Nonidet (SDS) P-40 aqueous (NP-40 solution in 10% homogenizedoctylphenoxypolyethanol in a 20 mL propylene in aqueous isotopic media, vial Aladdin, (internal Shanghai, diameter China) 22) at 3000 was rpm added. stirring Then, rate direct for 60s. Then, in this pre-homogenized solution, 0.4 mL Nonidet P-40 (NP-40 in 10% quenching in liquid N2 for almost 30 min was required to solidify this solution then subjected to octylphenoxypolyethanol in aqueous media, Aladdin, Shanghai, China) was added. Then, direct freeze-drying for several hours to obtain GO foam (GOF) with subsequent calcination on 1000 ◦C for quenching in liquid N2 for almost 30 min was required to solidify this solution then subjected to 2h in inert (Ar) environment with 5 ◦C/min heating rate. freeze-drying for several hours to obtain GO foam (GOF) with subsequent calcination on 1000 °C for 2h3.4. in Synthesis inert (Ar) of environment MnS-NPs@3DG with 5 °C/min heating rate. In the present work, MnS-NPs were formed in-situ on the 3D graphene network via the SILAR 3.4. Synthesis of MnS-NPs@3DG method. Brieﬂy, for the deposition of MnS-NPs, 0.1 M MnCl2 4H2O solution (pH = 5.5) was used In the present work, MnS-NPs were formed in-situ on the 3D· graphene network via the SILAR method. Briefly, for the deposition of MnS-NPs, 0.1 M MnCl2·4H2O solution (pH=5.5) was used as a cationic precursor, and 0.05 M Na2S·9H2O solution (pH = 12) was used as an anionic precursor. After

Catalysts 2020, 10, 1141 9 of 13

Catalysts 2020, 10, x FOR PEER REVIEW 10 of 14 as a cationic precursor, and 0.05 M Na S 9H O solution (pH = 12) was used as an anionic precursor. 2 · 2 cleaningAfter cleaning by using by usingethanol-water ethanol-water (1:1 (1:1by volume), by volume), graphene graphene foam foam was was desiccated inin an an inert inert environmentenvironment at at 60 60 °C◦C for severalseveral hours. hours. After After graphene graphene foam foam became became completely completely dried, MnS-NPsdried, MnS-NPs were loaded as depicted in Scheme1 by the SILAR cycle. Here, the SILAR cycle was completed successfully were loaded as depicted in Scheme 1 by the SILAR2+ cycle. Here, the SILAR cycle was completed for the required outcomes in four steps: (1) Mn uptake: the 3DG2+ substrate was immersed into the successfully for the required outcomes2+ in four steps: (1) Mn uptake: the 3DG substrate was MnCl2 4H2O solution for 30 s for Mn ions adsorption. (2) 1st washing: washing of substrate for immersed· into the MnCl2·4H2O solution for 30 s for Mn2+ ions adsorption. (2) 1st washing: washing 50 s was required to remove detached Mn2+. (3) S2 uptake: 30 s complete immersion of substrate − 2+ 2− of substrate for 50 s was required to remove detached2 Mn . (3) S uptake:2+ 30 s complete immersion in Na2S 9H2O solution for ionic attachment of S − with ante-adsorbed Mn ions to form MnS-NPs. of substrate· in Na2S·9H2O solution for ionic attachment of S2− with ante-adsorbed Mn2+ ions to form (4) 2nd washing: washing of substrate for 50 s was required to drench as well as to remove unreacted MnS-NPs.2 (4) 2nd washing: washing2+ of substrate for 50 s was required to drench as well as to remove S − ions on the surface of Mn embellished 3DG. Lastly, before characterization of the sample, unreacted S2− ions on the surface of Mn2+ embellished 3DG. Lastly, before characterization of the the as-formed samples were desiccated at 60 ◦C for 3 h. For comparison, the SILAR process was sample,repeated the 1,as-formed 3, 5 and 10 samples times to were yield desiccated control samples at 60 °C of SILAR-1,for 3 h. For SILAR-3, comparison, SILAR-5 the and SILAR SILAR-10, process wasrespectively, repeated 1, to 3, optimize 5 and 10 the times size to of yield MnS-NPs control and samples the catalytic of SILAR-1, activity ofSILAR-3, the electrocatalyst. SILAR-5 and SILAR- 10, respectively, to optimize the size of MnS-NPs and the catalytic activity of the electrocatalyst.

SchemeScheme 1. 1. SyntheticSynthetic procedure procedure for for MnS-NPs@3DG bybythe the SILAR SILAR method. method. 3.5. Material Characterization 3.5. Material Characterization Scanning electron microscope (SEM, JSM-7500F, Hitachi, Tokyo, Japan) and transmission electron microscope Scanning (TEM, electron 7650B, microscope Hitachi, Tokyo, (SEM, Japan) JSM-7500 were appliedF, Hitachi, in order Tokyo, to confirm Japan) morphology and transmission of the electronas-formed microscope product. For(TEM, X-ray 7650B, analysis, Hitachi, Rigaku Tokyo, Smart Lab,Japan) X-ray were Diff ractometer,applied in Tokyo,order Japanto confirm was morphologyused to record of the data. as-formed While for product. X-ray photoelectron For X-ray analysis, spectroscopy Rigaku (XPS), Smart an ESCALab220i-XLLab, X-ray Diffractometer, electron Tokyo,spectrometer Japan (VGwas Scientific,used to Waltham,record data. MA, USA)While using for 300X-ray W Alphotoelectron Ka radiations wasspectroscopy used to record (XPS), data. an ESCALab220i-XL electron spectrometer (VG Scientific, Waltham, USA) using 300 W Al Ka radiations was3.6. used Electrochemical to record data. Characterization For electrochemical characterization of the as-formed nanohybrid, standard three-electrode set-up 3.6.(glassy Electrochemica carbon, graphitel Characterization rod and Ag/AgCl) was used with potentiostat (CHI 760D working station). ToFor prepare electrochemical catalytic ink ofcharacterization MnS-NP@3DG of as the a working as-formed electrode, nanohybrid, a 20 mg standard sample with three-electrode 3.6 mL solvent set- mixture (ethanol-isopropanol 3:1 by volume) and 0.4 mL nafion solution (5 wt%) was subjected to up (glassy carbon, graphite rod and Ag/AgCl) was used with potentiostat (CHI 760D working ultrasonic treatment till homogeneous ink formation. The catalyst ink (4 µL) was deposited on 3 mm station). To prepare catalytic ink of MnS-NP@3DG as a working electrode, a 20 mg sample with 3.6 mL solvent mixture (ethanol-isopropanol 3:1 by volume) and 0.4 mL nafion solution (5 wt%) was subjected to ultrasonic treatment till homogeneous ink formation. The catalyst ink (4 µl) was deposited on 3mm diametric glassy carbon electrode (GCE) (loading quantity ≈ 0.283 mg cm−2). Alkaline electrolyte (0.1 M KOH) was used for linear sweep voltammetry (LSV). Moreover, all data were in a reversed hydrogen electrode, and IR drop corrected polarization curves were recorded.

Catalysts 2020, 10, 1141 10 of 13 diametric glassy carbon electrode (GCE) (loading quantity 0.283 mg cm 2). Alkaline electrolyte ≈ − (0.1 M KOH) was used for linear sweep voltammetry (LSV). Moreover, all data were in a reversed hydrogen electrode, and IR drop corrected polarization curves were recorded. Catalytic resistance was measured within the frequency range 0.1–100 kHz by electrochemical impedance spectroscopy (EIS). By using the following Equation (1), the potential was converted to the RHE scale, where ERHE is the potential vs. RHE, while EAg/AgCl is the potential referring to Ag/AgCl. The overpotential (η) was attained by the intersection of the tangents of LSV current and the polarization curve baseline.

ERHE = E + 0.197 + 0.059 pH (1) Ag/AgCl × In order to test our as-formed catalyst for ORR, 5 µL catalytic ink was deposited onto another GCE (loading quantity 0.1 mg cm 2). The CV measurements were recorded in alkaline solution at 10 mV s 1, ≈ − − and RDE measurements were pursued at diﬀerent rotating speeds (0–2500 rpm). For comparative view, the standard Pt based catalyst was also studied. The transferred electron number (n) was calculated from the Koutecky–Levich (K–L) equation [45,46]. Mathematically, relation between current and electrode rotating speed is as follows:

1 1 1 = + (2) 0.5 j jk Bω

Where jk is the kinetic current, ω rotation rate speed of the electrode and B is the Levich slope, which is given by: 2/3 1/6 B = 0.2nF(Do2) V− Co2 (3)

Here, n describes the electron number for the reduction in one O2 molecule, F is the 1 Faraday constant (F = 96,485 C mol− ), Do2 is the diﬀusion coeﬃcient of O2 in 0.1 M KOH (Do = 1.9 10 5 cm2 s 1), ν is the kinematics viscosity (v = 0.01 cm2 s 1) and Co is O concentration 2 × − − − 2 2 in solution (Co = 1.2 10 6 mol cm 3). When the rotating speed of the electrode is expressed in rpm, 2 × − − the constant 0.2 has been used.

4. Conclusions In summary, MnS nanoparticles were synthesized and loaded on 3D graphene foam in one step by a cost-eﬀective and eco-friendly SILAR method for catalyzing HER and ORR. Taking advantage of the three-dimensional structure of graphene as a superior current acquirer and catalyst carrier, the MnS-NPs@3D graphene featured much lower overpotential than analogues of bulk MnS. Additionally, its ORR activity was well proved, enabling it as a new kind of bifunctional catalyst. The current work may pave a new way for other metal chalcogenide-based low-cost bifunctional catalyst alternatives to noble metal compounds for electrochemical applications.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4344/10/10/1141/s1, Figure S1: (a,b) SEM images of SILAR-1 and SILAR-10; Figure S2: the polarization curves of MnS-NPs@3DG with different loading weights; Figure S3: the Tafel plot of the MnS-NPs@3DG; Figure S4: (a–c) CV curves of bulk MnS, 3DG and MnS-NPs@3DG from 0.15 to 0.25 V vs. RHE at different scan rates; Table S1: comparison of recently reported highly active Mn-based HER electrocatalyst− in the literature; Table S2: comparison of recently reported highly active Mn-based ORR electrocatalyst in the literature. Author Contributions: K.u.R. conceived the conceptualization; S.A., L.S., J.G., Q.G., Y.X. participated in the software; and Z.Z. supervised the project. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Chinese Scholarship Council, grant number: [2016GXZF80; 2017GXZ019672]. Acknowledgments: Khalil ur Rehman and Shaista Airam thank Chinese Scholarship Council (No. 2016GXZF80, 2017GXZ019672) for financial support. Conflicts of Interest: The authors declare no conflict of interest. Catalysts 2020, 10, 1141 11 of 13

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