nanomaterials Article MOF-Derived Ultrathin Cobalt Molybdenum Phosphide Nanosheets for Efficient Electrochemical Overall Water Splitting Xiang Wang 1,2 , Linlin Yang 1,2 , Congcong Xing 1 , Xu Han 3 , Ruifeng Du 1,2 , Ren He 1,2 , Pablo Guardia 1 , Jordi Arbiol 3,4 and Andreu Cabot 1,4, * 1 Catalonia Institute for Energy Research (IREC), Sant Adrià de Besòs, 08930 Barcelona, Spain; wxiang@irec.cat (X.W.); lyang@irec.cat (L.Y.); congcongxing@irec.cat (C.X.); ruifengdu@irec.cat (R.D.); renhe@irec.cat (R.H.); pguardia@irec.cat (P.G.) 2 Departament d’Enginyeria Electrònica i Biomèdica, Universitat de Barcelona, 08028 Barcelona, Catalonia, Spain 3 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Catalonia, Spain; xu.han@icn2.cat (X.H.); arbiol@icrea.cat (J.A.) 4 ICREA, Pg. Lluis Companys, 08010 Barcelona, Catalonia, Spain * Correspondence: acabot@irec.cat Abstract: The development of high-performance and cost-effective earth-abundant transition metal- based electrocatalysts is of major interest for several key energy technologies, including water splitting. Herein, we report the synthesis of ultrathin CoMoP nanosheets through a simple ion etching and phosphorization method. The obtained catalyst exhibits outstanding electrocatalytic activity and sta- bility towards oxygen and hydrogen evolution reactions (OER and HER), with overpotentials down to 273 and 89 mV at 10 mA cm−2 , respectively. The produced CoMoP nanosheets are also characterized  by very small Tafel slopes, 54.9 and 69.7 mV dec−1 for OER and HER, respectively. When used as  both cathode and anode electrocatalyst in the overall water splitting reaction, CoMoP-based cells Citation: Wang, X.; Yang, L.; Xing, C.; require just 1.56 V to reach 10 mA cm−2 in alkaline media. This outstanding performance is at- Han, X.; Du, R.; He, R.; Guardia, P.; tributed to the proper composition, weak crystallinity and two-dimensional nanosheet structure of Arbiol, J.; Cabot, A. MOF-Derived Ultrathin Cobalt Molybdenum the electrocatalyst. Phosphide Nanosheets for Efficient Electrochemical Overall Water Keywords: MOF; phosphide; nanosheet; water splitting Splitting. Nanomaterials 2022, 12, 1098. https://doi.org/10.3390/ nano12071098 1. Introduction Received: 9 March 2022 Accepted: 25 March 2022 Hydrogen, with a high gravimetric energy density (142 MJ kg−1 ) and zero-carbon emis- Published: 27 March 2022 sions, is both a key component in the chemical industry and a very appealing energy carrier for clean and sustainable energy storage and supply [1,2]. Since molecular hydrogen is not Publisher’s Note: MDPI stays neutral freely available in nature, it needs to be extracted from hydrogen-containing compounds. with regard to jurisdictional claims in Currently, fossil fuels are the main source of H2 , which involves the release of large amounts published maps and institutional affil- of carbon. The electrochemical water splitting is the main green alternative to produce H2 , iations. but it is seriously hampered by the high cost and insufficient durability of current electro- catalysts, based on scarce novel metals such as Pt, Ir and Ru [3–6], and the slow kinetics of the hydrogen and oxygen evolution reactions (HER, OER), which makes water electrolysis Copyright: © 2022 by the authors. not competitive with steam reforming of natural gas or coal gasification processes [7–9]. Licensee MDPI, Basel, Switzerland. In order to overcome the current challenges and enable the massive production of This article is an open access article H2 by water splitting, substantial efforts were devoted to the development of high ac- distributed under the terms and tivity, stable and cost-effective electrocatalysts for overall water splitting (OWS). Among conditions of the Creative Commons the wide range of materials proposed for alkaline water electrolysis, including metal Attribution (CC BY) license (https:// oxides/hydroxides [10–12], chalcogenides [13–16], nitrides [17,18] and carbides [19,20], creativecommons.org/licenses/by/ metal phosphides demonstrated particularly attractive catalytic performances [21,22]. 4.0/). Nanomaterials 2022, 12, 1098. https://doi.org/10.3390/nano12071098 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2022, 12, 1098 2 of 12 The outstanding performance of phosphides was related to their high electrical conduc- tivity, favorable electronic structure and high stability against corrosion [23,24]. Amongst phosphides, cobalt phosphide (CoP) exhibits an exceptional HER activity associated with a proper electronic structure and specifically to the effective capture of protons by the negatively charged phosphorous atoms [25,26]. Besides, CoP is also characterized by a very high OER activity associated with the high ability of the positively charged cobalt cations to adsorb oxygen intermediates while the negatively-charged P facilitates the desorption of O2 molecules [27,28]. However, the OWS in CoP still requires too large overpotentials for practical applications, mainly ascribed to the high dissociation energy of water and the sluggish OER kinetics involving a multi-electron transfer process [29,30]. An effective strategy to optimize a material’s performance is the introduction of an additional element that provides additional degrees of freedom to modulate its elec- tronic structure and surface properties. Within transition metal phosphide electrocatalysts, additional metals enable a fine-tuning of the d-band position, optimizing the adsorption free energy of the reactants/intermediates/products and thus improving the catalytic activ- ity and even stability. In this direction, Xiao et al. reported the HER catalytic activity of CoP to be boosted by vanadium doping [31]. The introduced V strongly interacts with the hosted Co atoms, enhancing the VCoP electron density and thus accelerating the HER. Other ele- ments, such as Zn [32], Mn [33], Ni [34], Ce [35], Cr [36] and W [21], were also demonstrated to promote either the HER or OER through enhancing electron interactions [28]. Beyond composition, the structure, morphology and organization of the catalyst particles are key parameters defining the density, accessibility and activity of the catalytic sites, which ultimately determine the catalytic activity. In this regard, metal–organic frameworks (MOFs), with a crystalline and porous structure formed by metal ion/cluster bridged by organic ligands, were demonstrated to be excellent sacrificial templates to produce porous carbon-based nanomaterials with tuned composition and morphology [37–40]. Inspired by the above considerations, we rationally designed a novel and highly effective bifunctional electrocatalyst for OWS. This new catalyst is based on CoP, structured as 2D ultrathin nanosheets and derived from the ZIF-67 MOF. It includes Mo6+ as a high valence 4d transition metal ion, which ionic radius of 0.62 Å matches well with that of Co3+ (0.63 Å), thus allowing the substitution of Co3+ by Mo6+ within the CoP lattice [41,42]. We demonstrated here that the proposed porous nanosheet-based structure and the incorporation of Mo within the CoP lattice enable rapid water dissociation and effective and stable HER and OER performances. 2. Materials and Methods 2.1. Chemicals Ammonium molybdate tetrahydrate ((NH4 )6 Mo7 O24 ·4H2 O, 90%), cobalt nitrate hex- ahydrate (Co(NO3 )2 ·6H2 O, 99.9%), potassium hydroxide (KOH, 85%), iridium(IV) oxide (IrO2 , 99.9% metal basis) and Nafion (5 wt% within a blend of low aliphatic alcohols and water) were obtained from Sigma-Aldrich (St. Louis, MO, USA). 2-Methylimidazole (C4 H6 N2 , 99%) was purchased from Acros Organics (Antwerp, Belgium). Analytical grade methanol, ethanol and isopropanol were obtained from different sources. Milli-Q water was produced using an Elga Purelab flex. All chemicals were used as received. 2.2. Preparation of ZIF-67 ZIF-67 was produced following a previously reported procedure with some modifica- tions [16,43]. Briefly, 0.87 g Co(NO3 )2 ·6H2 O was dissolved in 30 mL of methanol to obtain a clear solution. Subsequently, the above solution was poured into 30 mL of methanol containing 1.97 g of 2-methylimidazole under vigorous stirring. After mixing completely, the solution was incubated for 24 h at room temperature. Purple precipitates were col- lected by centrifugation; they were washed with methanol three times and then dried at 60 ◦ C overnight. Nanomaterials 2022, 12, 1098 3 of 12 2.3. Preparation of Mo–Co MOFs One hundred and twenty milligrams of as-prepared ZIF-67 powder was ultrasonically re-dispersed in 20 mL of ethanol. This solution was poured into 100 mL of an aqueous solution containing 50 mg, 100 mg and 200 mg of ammonium molybdate under continuous magnetic stirring. The mixture was then stirred vigorously for 24 h at room temperature. Lavender precipitates were collected by centrifugation, washed with water at least three times and freeze-dried overnight. 2.4. Preparation of CoP and CoMoP The obtained ZIF-67 and Mo–Co MOFs powders were placed in a porcelain boat within a horizontal tube furnace. In another boat, a 20× mass amount of NaH2 PO2 ·H2 O was placed at the upstream side of the tube furnace. The material was then annealed at 350 ◦ C under N2 flow. After calcination for 2 h, the final black products were denoted as CoP and CoMoP, respectively. 2.5. Structural Characterization Powder X-ray diffraction (XRD) was performed on a Bruker AXS D8 Advance X-ray diffractometer (Bruker, Billerica, MA, USA) with Cu-Kα radiation (λ = 1.5406 Å). Scanning electron microscopy (SEM) analysis was conducted with a Zeiss Auriga microscope (Carl Zeiss, Jena, Germany) equipped with an energy dispersive spectroscope analyses (EDS) detector operating at 20 kV. Transmission electron microscopy (TEM), High-resolution TEM (HRTEM), Annular dark-field scanning transmission electron microscope (HAADF-STEM) and electron energy loss spectroscopy (EELS) analysis were obtained using a field emission gun FEI Tecnai F20 microscope (FEI, Hillsboro, OR, USA) with a Gatan Quantum filter (Pleasanton, CA, USA) at 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a SPECS using an Al anode XR50 source at 150 W and a 150 MCD-9 detector from Phoibos (SPECS, Berlin, Germany). 2.6. Electrochemical Measurements Electrochemical characterization was performed in a standard three-electrode sys- tem using an electrochemical workstation (CHI 760E, CH Instruments, Shanghai, China) in 1 M KOH solution (PH = 14). A graphite rod counter electrode and a Hg/HgO ref- erence electrode were employed. Electrochemical impedance spectroscopy (EIS) was measured within a frequency from 0.01 Hz to 10 kHz at 10 mV amplitude. The initial voltage was fixed at the overpotential required to obtain a current density of 10 mA cm−2 . The electrochemically active surface area (ECSAs) was determined using the electrochemi- cal double-layer capacitance (Cdl ) obtained with cyclic voltammetry data at different scan rates (v = 20–100 mV·s−1 ). Stability was determined by CV using 3000 cycles at 100 mV·s−1 and by chronopotentiometry at 10 mA·cm−2 . Overall water splitting tests were carried out in a two-electrode system with the voltage range of 0–2.0 V at a scan rate of 5 mV·s−1 in 1.0 M KOH electrolyte. 3. Results and Discussion 3.1. Characterization of Electrocatalysts CoMoP nanosheets were produced by a three-step process involving the synthesis of a Co–MOF, its etching and partial cation exchange and a final phosphorization step, as schematically illustrated in Figure 1a. First, a cobalt-based zeolitic imidazolate frame- work (ZIF-67), consisting of polyhedral-shaped micrometer-size particles, was produced as a self-sacrificial template (Figure 1b). The ZIF-67 was reacted with ammonium molyb- date with the double role of etching the structure and partially replacing Co3+ cations by Mo6+ , yielding a porous wrinkled nanosheet-based material that we refer to as Co–Mo MOF (Figure 1c). Finally, the Co–Mo MOF was annealed within a tube furnace contain- ing NaH2 PO2 at 350 ◦ C for 2 h to produce a porous phosphide with a similar wrinkled nanosheet-based morphology that we denoted as CoMoP (Figures 1d and S1). EDS analysis NaH2PO2 at 350 °C for 2 h to produce a porous phosphide with a similar wrinkled nanosheet-based morphology that we denoted as CoMoP (Figures 1d and S1). EDS anal- ysis of CoMoP showed a Co–Mo atomic ratio of Co/Mo = 4, and a phosphorus–metal atomic ratio of P/M =2.6 (M = Co + Mo) (Figure S2). In order to study the effect of ammonium molybdate, we replaced this chemical with an alternative Mo precursor. Using sodium molybdate as Mo source, the morphology of Nanomaterials 2022, 12, 1098 the final product was much more compact, consisting of partially porous cubes (Figure 4 of 12 S3). Besides, EDS analysis revealed the molybdenum content of this material to be much lower (Co/Mo = 18.8) than that of CoMoP. We will refer to this material as Mo–CoP. As a reference, a Mo-free CoP was obtained by directly annealing the ZIF-67 in the presence of the of phosphorous CoMoP showed source, a with no etching Co–Mo step (Figure atomic S4). ratio of The obtained Co/Mo = 4, material also dis- and a phosphorus–metal atomic played a more compact geometry than that ratio of P/M =2.6 (M = Co + Mo) (Figure S2). of the CoMoP nanosheets. Figure Figure1. (a) 1. Schematic illustration (a) Schematic of the CoMoP illustration synthesis of the CoMoP process. (b–d) SEM synthesis image of process. (b) ZIF-67 (b–d) SEM image of (b) ZIF-67 (c) Mo–Co MOFs and (d) CoMoP. (c) Mo–Co MOFs and (d) CoMoP. Figure 2a,b and S5 displays TEM images of CoMoP, further revealing their ultrathin In order nanosheet to study structure. the effect HAADF-STEM of ammonium analysis molybdate, and EELS chemical we maps composition replaced (Fig- this chemical with an alternative Mo precursor. Using sodium molybdate as Mo source, ures 2e, S6 and S7) displayed a homogenous distribution of C, Co, Mo and P within eachthe morphology of CoMoP nanosheet. HRTEM images (Figure 2c) and SAED patterns (Figure 2d) showed the final product was much more compact, consisting of partially porous cubes (Figure S3). CoMoP to present a weak crystallinity, with strong middle/long-range disordered [44– Besides, EDS analysis revealed the molybdenum content of this material to be much 46]. In this regard, while the XRD patterns of ZIF-67 and Na2MoO4-ZIF-67 displayed a lower good (Co/Mo(Figure crystallinity = 18.8) than 3a), that ofMOF the Co–Mo CoMoP. alreadyWe will refer presented to this a mostly material as Mo–CoP. As a amorphous reference, structure. a phosphorization, After Mo-free CoP was obtainedaby CoP maintained directly relatively annealinglattice, well-organized the ZIF-67 and in the presence CoMoP of the displayed phosphorous a weak source, crystallographic order, with no consistently etching step with HRTEM (Figure S4).results The (Fig- obtained material also ure 3b). displayed a more compact geometry than that of the CoMoP nanosheets. Figures 2a,b and S5 displays TEM images of CoMoP, further revealing their ultra- thin nanosheet structure. HAADF-STEM analysis and EELS chemical composition maps (Figures 2e, S6 and S7) displayed a homogenous distribution of C, Co, Mo and P within each CoMoP nanosheet. HRTEM images (Figure 2c) and SAED patterns (Figure 2d) showed CoMoP to present a weak crystallinity, with strong middle/long-range disordered [44–46]. In this regard, while the XRD patterns of ZIF-67 and Na2 MoO4 -ZIF-67 displayed a good crystallinity (Figure 3a), the Co–Mo MOF already presented a mostly amorphous structure. After phosphorization, CoP maintained a relatively well-organized lattice, and CoMoP displayed a weak crystallographic order, consistently with HRTEM results (Figure 3b). As expected, the XPS survey spectrum displayed the presence of C, N, O, P, Co and Mo elements on the surface of CoMoP (Figure 3c). The high-resolution P 2p XPS spec- trum displayed two doublets, which we associated with P within the metal phosphide lattice (P 2p3/2 = 129.7 eV), and a phosphate chemical environment (P 2p3/2 = 133.8 eV) (Figure 3d) [41,47]. The Co 2p XPS spectrum displayed six peaks (Figure 3e). The main Co 2p contribution was assigned to Co within the phosphide lattice (Co 2p3/2 = 779.3 eV). A second doublet was associated with Co within an oxide, hydroxide or phosphate chem- ical environment (Co 2p3/2 = 781.5 eV). The last two bands were assigned to satellite peaks [28–30,48,49]. Finally, the Mo 3d XPS spectrum displayed two doublets assigned to Mo within the metal phosphide lattice (Mo 3d5/2 = 228.2 eV) and an oxidized chemical environment (Mo 3d5/2 = 233.0 eV) (Figure 3f) [50–52]. Nanomaterials 2022, 12, 1098 5 of 12 Nanomaterials 2022, 12, x FOR PEER REVIEW 5 of 14 Figure 2. (a,b) TEM images, (c) HRTEM image, (d) SAED pattern and (e) HAADF STEM image and Figure 2. (a,b) TEM images, (c) HRTEM image, (d) SAED pattern and (e) HAADF STEM image EELS chemical composition maps obtained from the red squared area of the STEM micrograph. and EELS chemical composition Individual maps Co L2,3 edges at 779 obtained eV (red), fromatthe Mo M4,5 edges red 230 eV squared (green), areaatof P L2,3 edges 132the STEM micrograph. eV (blue), Nanomaterials 2022, 12, x FOR PEER REVIEW N K edge at 401 eV (pink) and C K edge at 284 eV (orange). 6 of 14 Individual Co L2,3 edges at 779 eV (red), Mo M4,5 edges at 230 eV (green), P L2,3 edges at 132 eV (blue), N K edge As at 401 eV (pink) expected, the XPSand C Kspectrum survey edge atdisplayed 284 eV the (orange). presence of C, N, O, P, Co and Mo elements on the surface of CoMoP (Figure 3c). The high-resolution P 2p XPS spectrum displayed two doublets, which we associated with P within the metal phosphide lattice (P 2p3/2 = 129.7 eV), and a phosphate chemical environment (P 2p3/2 = 133.8 eV) (Figure 3d) [41,47]. The Co 2p XPS spectrum displayed six peaks (Figure 3e). The main Co 2p contri- bution was assigned to Co within the phosphide lattice (Co 2p 3/2 = 779.3 eV). A second doublet was associated with Co within an oxide, hydroxide or phosphate chemical envi- ronment (Co 2p3/2 = 781.5 eV). The last two bands were assigned to satellite peaks [28– 30,48,49]. Finally, the Mo 3d XPS spectrum displayed two doublets assigned to Mo within the metal phosphide lattice (Mo 3d5/2 = 228.2 eV) and an oxidized chemical environment (Mo 3d5/2 = 233.0 eV) (Figure 3f) [50–52]. Figure Figure (a,b)pattern 3. XRD 3. (a,b) XRD of pattern the MOFsof(a) the MOFs and (a) and the the phosphorized phosphorized materials, materials, CoMoP, Mo–CoP and CoMoP, Mo–CoP and CoP. (c–f)(c–f) CoP. XPS survey and high-resolution XPS survey P 2p (d), Co 2p and high-resolution P(e) 2pand (d),Mo Co3d2p (f) XPS spectra (e) and Mo of 3d CoMoP. (f) XPS spectra of CoMoP. 3.2. Oxygen Evolution Reaction The OER activity of CoMoP was evaluated at room temperature using a three-elec- trode system in a 1.0 M KOH alkaline solution. As a reference, Mo–CoP, CoP and a com- mercial RuO2 catalyst were also evaluated in the same cell and reaction conditions. The LSV polarization curves displayed the CoMoP to be characterized by an outstanding OER activity, with an overpotential of only 273 mV at a current density of 10 mA cm−2 (Figures 4a and S8). This overpotential is well below that of Mo–CoP, CoP and the RuO2 electro- Nanomaterials 12, 1098 2022,2022, Nanomaterials 12, x FOR PEER REVIEW 7 of 14 6 of 12 The charge transport/transfer ability of the electrocatalysts was evaluated by electro- chemical impedance 3.2. Oxygen spectroscopy Evolution (EIS). An equivalent circuit model including a charge Reaction transfer resistance (Rct) and a solution resistance (Rs) during the OER process was used to The OER fit the Nyquist plotsactivity displayedof in CoMoP Figure 4d was evaluated [55,56]. at room CoMoP exhibited the temperature smallest Rct using a three-electrode (17.99 Ω), well system inbelow a 1.0that M ofKOHMo–CoP (Rct = 27.90 alkaline Ω), CoP (RAs solution. ct = 36.70 Ω) and RuO2 (45.47). a reference, Mo–CoP, CoP and a commercial These results reveal the CoMoP nanosheets to enable a faster charge transfer at the elec- RuO2 catalyst trode/electrolyte werethus interfaces, alsoaccelerating evaluated in the the OER same cellkinetics. electrocatalytic and reaction conditions. The LSV polar- ization curvesstability The long-term displayed of CoMoP thewasCoMoP to be characterized further analyzed by an outstanding OER activity, by CV and chronopotenti- ometry with measurements an overpotential [57,58]. of Figure only 4e 273 shows mVhowatthe LSV curvedensity a current of CoMoPof after mA cm−2 (Figures 4a and S8). 10 3000 CV cycles closely resembles that of the first cycle. The chronoamperometry test displayed This overpotential CoMoP is well to have an outstanding belowcatalytic long-term that of Mo–CoP, activity with justCoPa 3%and thedensity current RuO2 electrocatalyst tested here andafter decay outperforms thatatof 100 h of operation 273previously mV (Figure 4f). reported SEM images CoP-based OER catalysts, of the post-catalysts after as shown in Table S1. OER testing at high current showed the ultrathin CoMoP nanosheets to partially sinter As displayed in Figure 4b, CoMoP was not only characterized by the lowest overpotential at into a highly porous structure with thicker walls (Figure S10). At the same time, the EDX 10 mA −2 but also provided the lowest Tafel slope, 54.9 mV dec−1 . This value was signifi- cmthat result showed a loss of P occurred during the OER. These results are consistent with cantly below ofthat the reorganization of Mo–CoP the metal phosphide(60.4into amV metaldec −1 ), CoP (71.5 (oxy)-hydroxide the−OER duringdec 1 ) and RuO (86.4 mV dec−1 ), 2 reaction [59,60]. which indicates CoMoP to have associated a faster OER reaction kinetics [53,54]. Figure Figure 4. (a)4. OER (a)polarization OER polarization curves in 1.0 Mcurves KOH. (b)in 1.0 M KOH. Corresponding (b) Tafel Corresponding (c) Double-layer Tafel plots. (c) Double-layer plots. capacitances (Cdl). (d) Nyquist plots of the EIS data. (e) OER polarization curves before and after capacitances 3000 cycles. (f) OER (C dl ). (d) Nyquist chronoamperometric dataplots of the for CoMoP at anEIS data. (e) overpotential OER of 273 mV.polarization curves before and after 3000 cycles. (f) OER chronoamperometric data for CoMoP at an overpotential of 273 mV. The electrochemical active areas (ECSA) of CoMoP, Mo–CoP NCs, CoP and RuO2 were estimated according to the double-layer capacitance (Cdl ) determined via CV in the non-faradaic region at different scan rates, 20, 40, 60, 80 and 100 mV·s−1 (Figures S9 and 4c). CoMoP displayed larger Cdl (12.6 mF cm−2) than Mo–CoP (8.7 mF cm−2), CoP (4.9 mF cm−2) and RuO2 (2.3 mF cm−2 ). This result indicates that CoMoP offers a higher density of accessible electrochemical active sites, which we relate to the proper composition and nanosheet structure of CoMoP. The charge transport/transfer ability of the electrocatalysts was evaluated by electro- chemical impedance spectroscopy (EIS). An equivalent circuit model including a charge transfer resistance (Rct ) and a solution resistance (Rs ) during the OER process was used to fit the Nyquist plots displayed in Figure 4d [55,56]. CoMoP exhibited the smallest Rct (17.99 Ω), well below that of Mo–CoP (Rct = 27.90 Ω), CoP (Rct = 36.70 Ω) and RuO2 (45.47). These results reveal the CoMoP nanosheets to enable a faster charge transfer at the electrode/electrolyte interfaces, thus accelerating the OER electrocatalytic kinetics. The long-term stability of CoMoP was further analyzed by CV and chronopotentiome- try measurements [57,58]. Figure 4e shows how the LSV curve of CoMoP after 3000 CV Nanomaterials 2022, 12, 1098 7 of 12 cycles closely resembles that of the first cycle. The chronoamperometry test displayed CoMoP to have an outstanding long-term catalytic activity with just a 3% current density decay after 100 h of operation at 273 mV (Figure 4f). SEM images of the post-catalysts after OER testing at high current showed the ultrathin CoMoP nanosheets to partially sinter into a highly porous structure with thicker walls (Figure S10). At the same time, the EDX result showed that a loss of P occurred during the OER. These results are consistent with the reorganization of the metal phosphide into a metal (oxy)-hydroxide during the OER reaction [59,60]. 3.3. Hydrogen Evolution Reaction The HER performance of CoMoP was evaluated in 1.0 M KOH using a three-electrode system, and it was compared with that of Mo–CoP, CoP8 ofand Nanomaterials 2022, 12, x FOR PEER REVIEW 14 a commercial Pt/C cata- lyst. As shown in Figures 5a and S8b, the CoMoP electrocatalyst displayed a relatively low HER overpotential of 89 mV at the current density of 10 mA cm−2 , slightly above 3.3. Hydrogen Evolution Reaction that of Pt/C The HER (42 mV) performance and well of CoMoP below inthat was evaluated ofKOH 1.0 M Mo–CoP (154 mV), CoP (165 mV) and most using a three-elec- previously trode system, and reported phosphide-based it was compared with that of Mo–CoP, HER CoPelectrocatalysts and a commercial Pt/C (Table S2). The Tafel slope of CoMoP (69.7 mV dec−1S8b, catalyst. As shown in Figures 5a and ) was also much lower−2 than those of Mo–CoP (83.7 mV dec−1 ), the CoMoP electrocatalyst displayed a relatively low HER overpotential of 89 mV at the current density of 10 mA cm , slightly above that CoP(42(113.4 mV dec −1 and close to that of Pt/C (56.1 mV dec−1 ) (Figure 5b), which indicated of Pt/C mV) and well below )that of Mo–CoP (154 mV), CoP (165 mV) and most previ- rapid ously HERphosphide-based reported reaction kinetics following the HER electrocatalysts Volmer–Heyrovsky (Table S2). The Tafel slope ofmechanism [61,62]. CoMoP CoMoP (69.7 mV dec−1) was also much lower than those of Mo–CoP (83.7 mV dec−1), CoP also displayed the smallest semicircular diameter in the Nyquist plot of the EIS data among (113.4 mV dec−1) and close to that of Pt/C (56.1 mV dec −1) (Figure 5b), which indicated theHER rapid phosphide catalysts reaction kinetics followingtested (Figure 5c), showing the Volmer–Heyrovsky mechanism the lowest [61,62]. CoMoP charge transfer resistance dur- alsoing displayed the smallest catalytic semicircular processes. diameter In terms in the Nyquist of stability under plotHER of theconditions, EIS data Figure 5d displays how among the phosphide catalysts tested (Figure 5c), showing the lowest charge transfer re- CoMoP suffered a minor change in the LSV curves after 3000 sistance during catalytic processes. In terms of stability under HER conditions, Figure 5d cycles. Additionally, the CA measurement displays showed how CoMoP suffered thechange a minor current density in the LSV curvestoafter decrease just 3000 cycles. 6% after 100 h of continuous Addi- tionally, the CA measurement operation under HER showed the currentat conditions density to decrease just 6% an overpotential 89after mV 100(Figure h 5e). The morphology of continuous operation under HER conditions at an overpotential 89 mV (Figure 5e). The and composition of the catalyst after long-term HER are morphology and composition of the catalyst after long-term HER are displayed in Figure displayed in Figure S11. In this S11.case, In thisminor changes case, minor changes in instructure structure and aand a moderate moderate P loss were P loss were observed, which observed, which points to points to notable notable catalyst stability catalyst stability under HER. HER. under Figure 5. (a) HER Figure polarization 5. (a) curves in 1.0 M KOH. HER polarization (b) Corresponding curves in 1.0 M KOH. Tafel(b) plots. (c) Nyquist plots Tafel plots. (c) Nyquist plots Corresponding of the EIS data. (d) HER polarization curves before and after 3000 cycles. (e) Chronoamperometry dataof forthe CoMoP EISatdata. a 89 mV(d) HER polarization curves before and after 3000 cycles. (e) Chronoamperometry overpotential. data for CoMoP at a 89 mV overpotential. Nanomaterials 2022, 12, 1098 8 of 12 3.4. Overall Water Splitting Due to the excellent OER and HER performances demonstrated Nanomaterials 2022, 12, x FOR PEER REVIEW 9 of 14 by CoMoP, a two- electrode configuration electrolyzer with CoMoP both as the positive and negative elec- trodes was 3.4. Overall constructed Water Splitting and tested for OWS in 1.0 M KOH solution (Figure 6a). As shown fromDue the to the excellent OER and HERdisplayed polarization curves performances in Figure 6b, demonstrated the assembled by CoMoP, a two- device just required aelectrode cell voltage of 1.56 configuration V to reach electrolyzer a current with CoMoP both as density the positiveof 10negative and mA cm −2 , which is significantly elec- trodes was constructed and tested for OWS in 1.0 M KOH solution (Figure 6a). As shown below that of a cell containing Pt/C and RuO electrodes (1.61 V). More importantly, from the polarization curves displayed in Figure 6b, the assembled2 device just required a after −2 the CoMoP-based cell still maintained cell voltagehofof 40 continuous 1.56 operation V to reach a current at10100 density of mAmA cm−2, cm which is, significantly below an outstanding performance, with just a 14.8% loss at high that of a cell containing Pt/C and RuO 2 electrodes (1.61 V). More importantly, after 40current h of density (Figure 6c). continuous operation at 100 mA cm−2, the CoMoP-based cell still maintained an outstand- Thus, the as-prepared CoMoP can be considered as a highly ing performance, with just a 14.8% loss at high current density (Figure 6c). Thus, the as- competitive electrocatalytic cat- alyst preparedforCoMoP OWScan compared with be considered as athe previously highly competitive reported OWS electrocatalytic catalysts catalyst for (Figure 6d, Table S3). OWS compared Besides, with the previously its outstanding reported stability OWS catalysts (Figure demonstrates 6d, Table S3). its potential forBe-large-scale hydrogen pro- sides, its outstanding stability demonstrates its potential for large-scale hydrogen produc- duction from water tion from water splitting. splitting. Figure 6. (a) Schematic diagram of the OWS in a two-electrode system. (b) Polarization curves of Figure (a)the OWS cells6.with Schematic diagram electrode pairs: of theand CoMoPCoMoP OWS in a2 two-electrode Pt/CRuO system. (b) Polarization curves of in 1.0 M KOH. (c) Chronoam- OWS cells with the electrode pairs: CoMoP||CoMoP and Pt/C||RuO2 in perometric curve of CoMoP in the two-electrode system at 1.70 V polarization. (d) Comparison of 1.0 M KOH. (c) Chronoam- the CoMoP overpotential at 10 mA cm−2 with previously reported catalysts in 1.0 M KOH. perometric curve of CoMoP in the two-electrode system at 1.70 V polarization. (d) Comparison of the CoMoP overpotential at 10 mA cm−2 with previously reported catalysts in 1.0 M KOH. 4. Conclusions In conclusion, ultrathin CoMoP nanosheets were engineered using the Co–MOF ZIF- 4.67 Conclusions as a self-sacrificial template and ammonium molybdate as a shape-defining agent and Mo source. CoMoP nanosheets exhibited outstanding performance towards HER and OER inInalkaline conclusion, media, which ultrathin we associateCoMoP nanosheets with the proper were engineered transport properties and elec- using the Co–MOF ZIF-67 as alevels tronic energy self-sacrificial template provided by their and composition andammonium molybdate their porous nanosheet as a shape-defining agent structure. In particular, CoMoP presented low overpotentials of 89 and 273 mV at a current density and Mo source. CoMoP nanosheets exhibited outstanding performance towards HER of 10 mA cm−2 for HER and OER, respectively. Furthermore, CoMoP electrocatalysts also and OER in alkaline showed excellent long-termmedia, stabilities which we in alkaline associate electrolytes, with with thecurrent a minor proper den-transport properties and electronic sity decrease energy after 100 hlevels provided continuous byWhen operation. theirused composition for OWS, a celland their voltage porous nanosheet structure. of only 1.56 V was needed to reach a current density of 10 mA cm−2. This work provides a suitable In particular, CoMoP presented low overpotentials of 89 and 273 mV at a current density strategy to synthesize high-performance Co–Mo–P electrocatalysts with abundant ex- of 10 active mA sites cmand−2 for HER and OER, respectively. Furthermore, CoMoP electrocatalysts posed effective avenues for charge and electrolyte transport, and it can be also showed excellent long-term stabilities in alkaline electrolytes, with a minor current density decrease after 100 h continuous operation. When used for OWS, a cell voltage of only 1.56 V was needed to reach a current density of 10 mA cm−2 . This work provides a suitable strategy to synthesize high-performance Co–Mo–P electrocatalysts with abundant exposed active sites and effective avenues for charge and electrolyte transport, and it can be employed to further tune the structure and composition of other 2D nanostructures with optimized performance towards OWS and other electrocatalytic reactions. Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/nano12071098/s1, Figure S1: (a–f) SEM images of CoMoP; Figure S2: (a) SEM images of Co–Mo MOFs. (b,c) SEM images and (d) EDX spectrum of CoMoP. Figure S3: (a) SEM images of Na2MoO4-ZIF-67. (b,c) SEM images and (d) EDX spectrum Mo–CoP.; Figure S4: Nanomaterials 2022, 12, 1098 9 of 12 (a) SEM images of ZIF-67. (b,c) SEM images and d) EDX spectrum CoP. Figure S5: (a–d) TEM image of CoMoP. Figure S6: (a–d) HAADF-STEM micrographs of CoMoP. Figure S7: EELS chemical composition maps obtained from the red squared area of the STEM micrograph. Individual Co L2,3-edges at 779 eV (red), Mo M4,5-edges at 230 eV (green), P L2,3-edges at 132 eV (blue), N K-edge at 401 eV (pink) and C K-edge at 284 eV (orange). Figure S8: (a) OER and (b) HER polarization curves of CoMoP with different Mo content in 1.0 M KOH. Figure S9: Cyclic voltammograms for (a) CoMoP; (b) Mo–CoP; (c) CoP and (d) RuO2 in the non-faradaic region of 1.12–1.22 V vs. RHE at various scan rates. Figure S10: (a–c) SEM image and (d) EDX spectrum of CoMoP after long-term OER stability testing. Figure S11: (a–c) SEM image and (d) EDX spectrum of CoMoP after long-term HER stability testing. Table S1: Comparison of OER performance of CoMoP with some previously reported CoP-based catalysts in 1.0 M KOH solution. Table S2: Comparison of HER performance of CoMoP with some previously reported CoP-based catalysts in 1.0 M KOH solution. Table S3: Comparison of OWS performance of CoMoP with some previously reported CoP-based catalysts in 1.0 M KOH solution. References [63–81] are cited in the supplementary materials. Author Contributions: The manuscript was prepared through the contribution of all authors. A.C. guided the project and supervised the work. X.W., L.Y., C.X., R.D., R.H. and P.G. conceived and prepared the manuscript. X.W. and L.Y. produced the samples. X.W. and C.X. performed the electrochemical measurements and analyzed the results. X.H., J.A. and P.G. performed TEM, HRTEM and discussed these results. X.W. and R.H. performed XPS measurements and discussed these results. The manuscript was corrected and improved by all authors. All authors have read and agreed to the published version of the manuscript. Funding: This research was partially funded by the project Combenergy, PID2019-105490RB-C32, from the Spanish Ministerio de Ciencia e Innovación. X.W., L.Y., C.X., X.H. and R.D. thank the China Scholarship Council (CSC) for the scholarship support. IREC is funded by the CERCA Pro- gramme/Generalitat de Catalunya. Part of the present work has been performed in the frameworks of Universitat de Barcelona Nanoscience Ph.D. program. ICN2 acknowledges funding from Gen- eralitat de Catalunya 2017 SGR 327. The authors thank the support from the project NANOGEN (PID2020-116093RB-C43), funded by MCIN/AEI/10.13039/501100011033/. ICN2 is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706) and is funded by the CERCA Programme/Generalitat de Catalunya. Part of the present work has been performed in the framework of Universitat Autònoma de Barcelona Materials Science Ph.D. program. 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