DOI:
10.1039/D4QI00629A
(Research Article)
Inorg. Chem. Front., 2024,
11, 3446-3457
High ratios of Ni3+ and Co3+ facilitated by Mn-addition for enhanced oxygen evolution reaction and ethanol oxidation reaction†
Received
10th March 2024
, Accepted 27th April 2024
First published on 29th April 2024
Abstract
The design of dual-functional catalysts for the oxygen evolution reaction and ethanol oxidation reaction is essential for improving hydrogen production efficiency, and one of the strategies to improve catalytic performance is the incorporation of high-valence metals. However, the generation of high-valence metals has always been a challenge in the field of catalysis, and elucidating the involved mechanisms is also difficult. Herein, we synthesized a metal–organic framework (MOF) material on nickel foam (NF) using a straightforward one-step hydrothermal technique and subsequently transformed it into a metal oxide hydroxide through activation, denoted as Co1Ni0.5Mn1BDC@NF-A. The catalyst with Mn enhanced exhibits a curved sheet-like morphology, arranged in an ordered layered structure, with abundant active surfaces. Mn induces additional production of Ni3+ and Co3+ to accelerate the OER/EOR process. The catalyst exhibits outstanding electrochemical performance in both the OER, with an overpotential of 298 mV at 100 mA cm−2, and the EOR, with a potential of 1.30 V at 100 mA cm−2, demonstrating superior performance compared to other Ni-based and Co-based catalysts. Furthermore, to reach 100 mA cm−2, the combined EOR–HER process requires only 1.39 V, while the combined OER–HER process requires just 1.54 V. The remarkable performance is attributed to the inclusion of Mn, which can promote the transformation of morphology, mainly transforming the catalyst from disordered stacking to an ordered staggered arrangement, exposing more active sites. More importantly, XPS testing shows that Mn reduces the binding energy of the Ni/Co(II) to Ni/Co(III) transition by adjusting the hybridization effect between Ni 3d, Co 3d and O 2p orbitals, promoting the generation of Ni3+ and Co3+ for enhanced reaction kinetics. In summary, this work presents a simple yet effective strategy for the generation of high-valence transition metals, which can be used to accelerate the OER and EOR processes, thus offering promising prospects for advancing hydrogen production technology.
1. Introduction
The growing energy demand coupled with the decline of fossil fuels has spurred efforts to explore and advance renewable and eco-friendly energy sources.1,2 Over the last few decades, electrocatalytic water splitting has gained widespread recognition as a viable method for sustainable hydrogen production and addressing the global energy crisis.3–6 In water splitting, the OER occurs at the anode, involving the transfer of four electrons–protons and the formation of multiple intermediates with slow kinetics. It is considered an essential step in improving hydrogen production efficiency.7–10 IrO2/RuO2 is renowned for its superior activity in the OER, but its high cost, limited availability of resources, and poor stability impede its widespread industrial implementation.11,12 Hence, investigating affordable and abundant non-precious-metal catalysts with high catalytic performance and exceptional stability is essential. In addition to improving hydrogen production efficiency at the catalyst level, it has been found that adding some small molecule compounds (such as glycerol, methanol and acetone) to alkaline electrolysis tanks can significantly reduce the thermodynamic potential energy of anodic oxidation reactions, thereby reducing energy consumption in the hydrogen production industry.13–15 Among these compounds, ethanol stands out due to its plentiful availability, minimal toxicity, excellent safety profile and ease of handling, making it a preferred target.16,17 Similar to the OER catalyst, the catalysts suitable for the ethanol oxidation reaction are platinum and Pt-based materials, which are costly and relatively low in durability, limiting their commercial application.18,19 In this context, there is a collective effort to develop non-precious metal electrocatalysts as alternatives to Pt-based catalysts.
As a matter of fact, there have been many reports on non-precious metal catalysts for the OER or EOR individually,20–25 but there are few reports on bifunctional catalysts capable of both the oxygen evolution reaction and ethanol oxidation reaction. Therefore, designing dual-functional catalysts is crucial for electrocatalytic hydrogen production. Among the most advanced non-precious metal catalysts currently available, transition metals such as Fe, Co and Ni have garnered significant attention for their exceptional performance in both the OER and EOR in alkaline media.26,27 Previous studies have reported a synergistic effect between cobalt and nickel elements, which can effectively reduce the voltage required for both the OER and EOR. For instance, Shi et al. enhanced the OER performance with Co/N carbon nanotube catalysts by incorporating trace amounts of nickel,28 and Li et al. designed NiCoLDH to advance research on the EOR.29 Therefore, performance enhancement can be achieved through the utilization of NiCo bimetallic materials.
Currently, the activity of both the OER and EOR depends heavily on the presence of transition metal sites in high valence states.30 According to reports, certain high valence metals demonstrate superior OER activity compared to their counterparts in lower valence states. For instance, the Fe4+ based quadruple perovskite oxide CaCu3Fe4O12 exhibits higher activity than CaFeO3 containing Fe3+;31 Hg2Ru2O7 containing Ru5+ demonstrates better OER activity than commercial RuO2.32 The high valence compounds of metals play a pivotal role in both OER and EOR processes. However, the Pourbaix plot indicates that achieving complete oxidation of metals requires a higher potential. Additionally, considering Ni as an example, following the Hund rule, Ni2+ with semi-filled orbitals is relatively less stable compared to high valence Ni3+ and Ni4+. Simultaneously, highly oxidized metals also render oxygen unstable, predisposing it to form oxygen vacancies, potentially resulting in a reduction of the metal ions’ valence state.33 Therefore, while minimizing the energy required for formation and maximizing stability, dealing with highly oxidized metals poses an ideal yet challenging task.
So far, many studies have focused on constructing high-priced materials, for which heteroatom doping strategies are widely used due to their simplicity and significant effects.34,35 The crucial role of Mn in promoting the high-value oxidation of metals has attracted increasing attention. For example, Gan et al. promoted the generation of trivalent nickel in Mn-NiFe2O4 through the addition of Mn, resulting in outstanding OER performance with an overpotential of 200 mV at 50 mA cm−2.36 Similarly, Fan et al. employed a notable CoMOF precursor (CoBDC) to synthesize trace amounts of Mn-doped cobalt spinel oxide via a straightforward ion exchange method at room temperature. The doping of Mn can enhance OER performance by strengthening the stability of Co–O bonds.37 From this perspective, Mn has a significant promoting effect on the generation and stability of high-valence metals.
Herein, we have successfully synthesized an efficient OER/EOR electrocatalyst, named Co1Ni0.5Mn1BDC@NF-A, through hydrothermal and electrochemical activation methods. The addition of manganese induces the production of Ni3+ and Co3+ by adjusting the hybridization effect between the metal and oxygen. Co1Ni0.5Mn1BDC@NF-A exhibits outstanding performance in both the OER (with an overpotential of 298 mV at 100 mA cm−2) and EOR (with a potential of 1.30 V at 100 mA cm−2). This study aims to provide valuable references for the preparation of EOR and OER catalysts.
2. Experimental
2.1. Synthesis of Co1Ni0.5Mn1BDC@NF-A
Nickel foam (2 × 4 cm−2, 1.5 mm) was sonicated with 1 mol of hydrochloric acid and then with ethanol for 30 minutes. Subsequently, the cleaned NF was dried overnight in a vacuum oven at 60 °C. The preprocessed NF was then vertically immersed into a 30 mL mixed solution (24 mL DMF + 6 mL DI-water), containing 1 mmol of Co(NO3)2·6H2O, 0.5 mmol of Ni(NO3)2·6H2O, 1 mmol of MnCl2·4H2O and 1.25 mmol of terephthalic acid (BDC). The hydrothermal reaction was conducted in a 100 mL Teflon autoclave at 120 °C for 12 hours to obtain self-supported Co1Ni0.5Mn1BDC@NF. Next, the sample was washed extensively with water and ethanol and then dried under vacuum at 60 °C. After the electrochemical activation of the dried sample through CV, the final sample was successfully prepared.
2.2. Synthesis of Co1Ni0.5BDC@NF-A
It should be noted that the metal to ligand ratio should always be 1
:
1, so changing the amount of metal added will also change the amount of PTA.
Co1Ni0.5BDC@NF and Co1Ni0.5BDC@NF-A: except for not adding MnCl2·4H2O in the solution and adjusting the amount of PTA to 0.75 mmol, everything else is consistent with the synthesis of Co1Ni0.5Mn1BDC@NF and Co1Ni0.5Mn1BDC@NF-A.
2.3. Synthesis of Co1Mn1BDC@NF-A
Co1Mn1BDC@NF and Co1Mn1BDC@NF-A: except for not adding Ni(NO3)2·6H2O and adjusting the amount of PTA to 1 mmol, everything else is consistent with the synthesis of Co1Ni0.5Mn1BDC@NF and Co1Ni0.5Mn1BDC@NF-A.
2.4. Synthesis of Ni1Mn1BDC@NF-A
Ni1Mn1BDC@NF and Ni1Mn1BDC@NF-A: except for not adding Co(NO3)2·6H2O and changing the amount of PTA to 1 mmol, everything else is consistent with the synthesis of Co1Ni0.5Mn1BDC@NF and Co1Ni0.5Mn1BDC@NF-A.
3. Results and discussion
3.1. Structural characterization
The fabrication of the Co1Ni0.5Mn1BDC@NF-A material is shown in Fig. S1.† Firstly, the precursor catalyst was synthesized via the hydrothermal method and named Co1Ni0.5Mn1BDC@NF. Subsequently, cyclic voltammetry (CV) was employed to conduct 50 cycles of electrochemical activation on Co1Ni0.5Mn1BDC@NF, resulting in the final catalyst, named Co1Ni0.5Mn1BDC@NF-A (Fig. S2†).
The mechanism underlying the enhancement of the OER and EOR is illustrated in Fig. 1. The addition of Mn promotes the transformation of the structure from disorderly stacking to a layered ordered structure and the growth of tightly arranged flower-shaped microspheres, which increase the active area and enhance the reaction activity. Additionally, Mn reduces the binding energy of Co2+/Ni2+ oxidation to Co3+/Ni3+ by facilitating the hybridization of Co/Ni 3d and O 2p orbitals, thereby promoting the generation of Co3+ and Ni3+. Due to the addition of Mn, more Co3+ and Ni3+ species are generated in Co1Ni0.5Mn1BDC@NF-A, thereby accelerating the processes of oxygen evolution and ethanol oxidation reactions.
![image file: d4qi00629a-f1.tif](/image/article/2024/QI/d4qi00629a/d4qi00629a-f1.gif) |
| Fig. 1 Diagram illustrating the mechanism of Co1Ni0.5Mn1BDC@NF-A in the OER and EOR. | |
Various spectroscopic characterizations were performed to gain insight into the crystalline arrangement and chemical composition of the electrocatalysts. Initially, the primary chemical composition of Co1Ni0.5Mn1BDC@NF was confirmed through X-ray diffraction (XRD). Before activation, as shown in Fig. 2a, three characteristic peaks at 44.46°, 51.81° and 73.60° were well-assigned to metallic Ni in NF.38 The strong diffraction peaks at 17.797°, 30.676°, 32.713°, 37.054° and 45.489° were attributed to C8H6Co2O6 (PDF#00-034-1897) and the peaks at 15.699°, 31.808°, 35.346°, 40.469° and 41.786° were attributed to C8H6NiO5·H2O (PDF#00-035-1677). Additionally, the peaks at 14.053°, 18.991° and 28.228° indicate the successful incorporation of Mn. The above results indicate the successful synthesis of MOF structures. However, due to the influence of oxidation voltage, the composition of the MOF catalyst (Co1Ni0.5Mn1BDC@NF) changes. In Fig. S3,† after activation, the transformation of peaks belonging to MOFs into peaks indicative of an amorphous thin-layered structure was observed. Research has indicated that the surface reconstruction of MOF-based electrocatalysts leads to the formation of a metal hydroxide oxide (MOOH).39 Furthermore, Mn exhibits lower activity compared to Ni and Co; thus, NiOOH and CoOOH are considered the actual active substances for the OER and EOR in alkaline electrolytes.
![image file: d4qi00629a-f2.tif](/image/article/2024/QI/d4qi00629a/d4qi00629a-f2.gif) |
| Fig. 2 (a) XRD spectrum of Co1Ni0.5Mn1BDC@NF. (b) FT-IR spectra of Co1Ni0.5Mn1BDC@NF and Co1Ni0.5Mn1BDC@NF-A. | |
The FT-IR spectrum (Fig. 2b) reveals prominent carboxyl vibration peaks for Co1Ni0.5Mn1BDC@NF. Two strong carboxyl bands emerged due to the C
O vibration peak near 1650 cm−1 and the C–O bond stretching vibration peak near 1150 cm−1, respectively.40 Additionally, Co1Ni0.5Mn1BDC@NF exhibited O–H stretching vibrations at 3600 cm−1 for carboxylic acid derivatives.38 More notably, a broadband existed in the region between 1400 and 1600 cm−1 because of the benzene moiety in PTA.39 Peaks were observed in the 1100–1000 cm−1 region because of the hydrocarbon bending vibration of the in-plane ring.39 Furthermore, peaks in the range of 850–650 cm−1 were observed due to the out-of-plane C–H bending vibration. The bands at 450 cm−1 and 520 cm−1 typically corresponded to metal–O or metal–OH-related vibrations. These results signify the successful fabrication of MOF materials. After activation, the peak shape and position remained roughly the same as those before activation. However, there was some attenuation in the intensity of the characteristic peaks after activation, manifested by the reduction in the intensity of the O–H stretching vibration peak near 3600 cm−1 and the significant decrease in the characterization peaks of the benzene ring structure. It is crucial to note the emergence of a prominent C
C peak around 2200 cm−1, suggesting partial cleavage of the benzene ring structure following activation. Various phenomena can indicate the structural transformation of terephthalic acid MOFs after activation, namely the generation of MOOH41 (as observed through XRD). To investigate the role of Mn, FT-IR analysis was conducted on different samples (Fig. S4†). The FT-IR spectra of Co1Ni0.5Mn1BDC@NF-A, Co1Ni0.5BDC@NF-A, Co1Mn1BDC@NF-A and Ni1Mn1BDC@NF-A exhibit similar patterns, characterized by the decrease in carboxylic acid and benzene ring groups, along with the emergence of C
bonds. Due to the almost identical peak patterns in the FT-IR spectra of different catalysts, it can be inferred that the effect of Mn addition on catalysts cannot be analyzed at the functional group level. Instead, it is inferred that Mn tends to alter the electrochemical performance by modifying the electronic structure of the metal.
The synthesized electrocatalyst's spatial morphology and elemental distribution were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As depicted in Fig. 3b and c, Co1Ni0.5Mn1BDC@NF-A exhibits a curved sheet-like morphology and is arranged in an orderly layered structure, growing on the surface of NF. In addition, flower-shaped spheres composed of nanosheets grow in an orderly and tightly packed manner on the sheet-like material. Due to the small volume of the spheres, while increasing the active area, they will not block the gaps between the sheets. In Fig. S5a,† it is evident that the various curved surfaces in the layered structure are not obstructed by the nanospheres. Furthermore, as shown in Fig. S5b,† the catalyst grows uniformly and ideally on the surface of NF. The advantages of CoNiMnMOF (BDC) as a precursor are as follows:42 (1) the spatial confinement provided by BDC ligands enables the full dispersion and stability of active cobalt and nickel sites and (2) the presence of benzene rings in the ligand offers π electrons, ensuring high conductivity. As shown in Fig. 3a, the catalyst before activation (Co1Ni0.5Mn1BDC@NF) still exhibits a layered structure arranged in a curved sheet shape, which is conducive to providing a wider active area. However, comparing Fig. S6a and b,† it can be observed that before activation, flower-shaped small spheres grow less on the curved surface and are not arranged too tightly, resulting in a decrease in the active surface area. In general, the electrocatalytic performance of the synthesized catalysts primarily depends on the number and intrinsic activity of electroactive sites.43,44 The increase in active area after activation indicates performance enhancement,45 and the morphological transformation of the catalyst suggests the generation of MOOH. The TEM results show that the catalyst has a thin nanosheet structure with excellent electrocatalytic activity (Fig. 3d). High-resolution TEM images in Fig. 3e and f show the distinct lattice spacings of 0.218 nm and 0.209 nm marked in white, consistent with the (200) planes of CoOOH and the (105) planes of NiOOH, respectively, which matched the XRD results. This indicates that the true active substance of the catalyst after activation is Co/NiOOH. To investigate the role of Mn, we conducted SEM testing on Co1Ni0.5BDC@NF-A. According to Fig. S7,† it is disorderly stacked on the surface of NF in a sheet-like manner, which covers up some of the active areas compared to the ordered growth of curved sheet-like structures, and no flower-shaped small nanospheres are growing on the sheet-like surface. It can be inferred that the addition of Mn can promote the transformation of morphology, mainly transforming the catalyst from disordered stacking to an ordered staggered arrangement, which exposes more active sites.
![image file: d4qi00629a-f3.tif](/image/article/2024/QI/d4qi00629a/d4qi00629a-f3.gif) |
| Fig. 3 (a) SEM images of Co1Ni0.5Mn1BDC@NF. (b and c) SEM images of Co1Ni0.5Mn1BDC@NF-A. (d) TEM images of Co1Ni0.5Mn1BDC@NF-A. (e and f) HRTEM images of Co1Ni0.5Mn1BDC@NF-A. (g) SEM mapping images of Co1Ni0.5Mn1BDC@NF-A. | |
The SEM mapping test confirmed the uniform distribution of Co, Ni and Mn metal in the catalyst (Fig. 3g). The EDS results of Co1Ni0.5Mn1BDC@NF and Co1Ni0.5Mn1BDC@NF-A are shown in Fig. S8 and S9,† respectively, with a more significant change in the proportion of C, O and Ni atoms. The increase in Ni content may be due to the influence of Ni in NF, while the change in C and O should be due to the formation of MOOH after activation. TEM mapping and ICP-OES were also employed to determine the elemental content in the samples. Fig. S10† illustrates the uniform distribution of Co, Ni, Mn, C and O. The element ratio is presented in Fig. S11,† where the content of Ni and Mn shows a significant reduction compared to the SEM results. The TEM test results were more accurate than those obtained by SEM (which directly tests on NF) and XPS (which is limited to indicating testing), due to the process of sonicating the catalyst from NF and dissolving it in a dispersant for testing. Additionally, the proportion of metal element content tested by ICP-OES (Table S1†) closely matches that from TEM-EDS. Therefore, the TEM-EDS results should be used as the elemental content reference for the catalyst in this study.
The chemical composition of the surface and the electronic interaction of the samples were revealed using XPS. Fig. S12† presents the XPS survey spectra of Co1Ni0.5Mn1BDC@NF-A and Co1Ni0.5BDC@NF-A, indicating the coexistence of Ni, Co and Mn. Fig. S13† displays the Ni 2p and Co 2p spectra before and after the activation of Co1Ni0.5Mn1BDC@NF, revealing significant differences in peak shapes and binding energy positions. Compared to the MOF structure, the activated catalyst (Co1Ni0.5Mn1BDC@NF-A) exhibits a lower binding energy of Co/Ni 2p2/3, suggesting the formation of Co/NiOOH. Fig. S14† illustrates the percentage of near-surface elements in Co1Ni0.5Mn1BDC@NF and Co1Ni0.5BDC@NF obtained through XPS analysis (Table S2†). The addition of manganese partially replaces nickel and cobalt, with a greater inclination towards replacing Co-elements, as evidenced by the intensity of elemental peaks in the entire XPS spectrum. In Fig. S12,† the attenuation of the characteristic peaks of Co is more pronounced compared to that of Ni. More significantly, the metallic atomic ratio on the material surface is determined by examining the proportion of the fitted peaks’ areas, indicating an increase in the proportion of Ni3+ and Co3+ after the addition of the Mn element (Fig. 4a), which is the reason for the enhanced OER and EOR activity. These results indicate that the addition of Mn can modulate the delocalized electron density around Co and Ni atoms, thereby facilitating the formation of trivalent states of Ni and Co.46
![image file: d4qi00629a-f4.tif](/image/article/2024/QI/d4qi00629a/d4qi00629a-f4.gif) |
| Fig. 4 (a) The elemental contents of Ni3+ and Co3+ in Co1Ni0.5Mn1BDC@NF-A and Co1Ni0.5BDC@NF-A were determined by XPS peak areas. High-resolution spectra of (b) Co 2p, (c) Ni 2p, and (d) O 1s of Co1Ni0.5Mn1BDC@NF-A and Co1Ni0.5BDC@NF-A, respectively. (e) High-resolution Mn 2p spectrum of Co1Ni0.5Mn1BDC@NF-A. (f) High-resolution C 1s spectrum of Co1Ni0.5Mn1BDC@NF-A. | |
High-resolution Co 2p spectra (Fig. 4b) indicate the presence of both trivalent (Co3+ 2p3/2 780.57 eV, Co3+ 2p1/2 795.86 eV) and bivalent (Co2+ 2p3/2 782.50 eV, Co2+ 2p1/2 797.40 eV) forms in Co1Ni0.5Mn1BDC@NF-A.37 And Ni 2p spectra (Fig. 4c) demonstrate the presence of trivalent (Ni3+ 2p3/2 856.41 eV, Ni3+ 2p1/2 874.25 eV) and bivalent (Ni2+ 2p3/2 855.64 eV, Ni2+ 2p1/2 873.20 eV) forms in Co1Ni0.5Mn1BDC@NF-A.47 Additionally, in Fig. 4d, the O 1s spectrum of Co1Ni0.5Mn1BDC@NF-A is deconvoluted into three characteristic peaks at 530.28, 531.29 and 531.83 eV, corresponding to ligand skeleton oxygen (C–O), metallic lattice oxygen (M–O) and adsorbed oxygen.48 By comparing the XPS spectra of Co1Ni0.5Mn1BDC@NF-A and Co1Ni0.5BDC@NF-A, it is observed that the peak of Co2+ 2p3/2 undergoes a positive shift of 0.16 eV after the addition of Mn (Fig. 4b). Similarly, the peak of Ni2+ 2p3/2 experiences a positive shift of 0.11 eV (Fig. 4c). Combining XPS fitting peak area analysis, which shows that the catalyst added by Mn exhibits larger Co3+/Co2+ and Ni3+/Ni2+ atomic ratios than Co1Ni0.5BDC@NF-A, indicates that the incorporation of Mn can regulate the delocalized electron density around Co and Ni atoms. In addition, in Fig. 4d, the peak of M–O experiences a negative displacement of 0.13 eV. This shift can be attributed to the addition of Mn, which exerts a significant electronic regulatory effect on the M–O bond. Previous works have clearly demonstrated, using density functional theory, that catalysts without Mn exhibit a clear band gap in the distribution of the total density of states (TDOS). However, Mn doping causes two split orbitals in the band gap, thereby providing an intermediate energy level for electron transition. The partial density of states (PDOS) indicates that Ni/Co 3d and O 2p orbitals constitute this intermediate level.49 In this work, the addition of Mn promotes the hybridization of Ni 3d, Co 3d and O 2p orbitals in Co1Ni0.5Mn1BDC@NF-A, lowering the energy barrier for the Co/Ni(II) to Co/Ni(III) transition and thereby facilitating the oxidation of Co2+/Ni2+ into Co3+/Ni3+, thus enhancing the electrocatalytic kinetics of the OER and EOR.36,50
Fig. 4e displays two peaks at 639.44 eV and 652.20 eV, which can be assigned to Mn 2p3/2 and Mn 2p1/2 of Mn2+ species, and two peaks at 642.38 eV and 654.19 eV, corresponding to Mn 2p3/2 and Mn 2p1/2 of Mn3+ species, respectively.48 In Co1Ni0.5Mn1BDC@NF (Fig. S15†), Mn exists in both Mn2+ and Mn3+ states. After activation, Mn2+ and Mn3+ remain present in Co1Ni0.5Mn1BDC@NF-A, but the Mn2+ peaks in Mn 2p spectra shift towards higher energy levels. This shift is attributed to the oxidation of Mn2+ under prolonged oxidation potential and its interaction with Co/Ni.50 These findings suggest the presence of both Mn2+ and Mn3+ states in the final sample. Moreover, the peak area fitted by XPS and the shift in Mn2+ binding energy position indicate a higher content of Mn3+ compared to Mn2+. The changes in electron density around Mn due to activation and the shift in Co/Ni binding energy induced by Mn addition suggest a strong electronic interaction among Co, Mn and Ni. And in Fig. 4f, the C 1s spectrum of Co1Ni0.5Mn1BDC@NF-A is deconvoluted into three characteristic peaks at 284.78, 286.04 and 288.47 eV, corresponding to C–C, C–O, and C
C bonds.51 Overall, although Mn exhibits poorer activity in oxygen evolution and ethanol oxidation than Co and Ni, it can reduce the energy barrier for Co/Ni(II) to Co/Ni(III) transformation to reduce the reaction voltage of the OER and EOR.49 Hence, the inclusion of Mn significantly enhances the overall performance of the catalyst.
3.2 Electrocatalytic properties for the OER
The OER electrocatalytic performances of all samples were tested in 1 M KOH using a typical three-electrode system at room temperature. Firstly, the prepared catalyst undergoes electrochemical activation by CV, transitioning from a MOF structure to the active substance MOOH for the OER/EOR. A detailed analysis of the activation process reveals the following: during cycling, the CV curve gradually enlarges, and the current density increases with the number of cycles, peaking before the 50th cycle and then stabilizing, indicating the transformation of the catalyst into a more active material. It is well-known that Co2+/Ni2+ can be oxidized into high-valence species.52 Therefore, the increase in the oxidation peak area in Fig. S2† can be attributed to the redox reaction between Co2+/Ni2+ and Co3+/Ni3+. Based on this analysis, it can be inferred that CoNiBDC, composed of carboxylic acid ligands, undergoes facile hydrolysis on the surface in alkaline electrolyte, rapidly producing Co/Ni(OH)2 (CoNiBDC + 2OH− → Co/Ni(OH)2 + BDC2−),53 which subsequently further oxidizes to Co/NiOOH (Co2+/Ni2+ ↔ Co3+/Ni3+ + e−).54 The use of BDC as a ligand facilitates the coordination of Ni and Co in the catalyst with six oxygen species, enabling rapid dissociation into hydroxides in alkaline solutions and accelerating the reaction.52 As depicted in Fig. 5a, the CV curve confirmed the excellent OER activities of Co1Ni0.5Mn1BDC@NF-A at different current densities. To explore the role of the Mn element, electrochemical experiments were conducted on bimetallic systems containing Co1Mn1BDC@NF-A, Ni1Mn1BDC@NF-A and Co1Ni0.5BDC@NF-A. For Co1Mn1BDC@NF-A and Ni1Mn1BDC@NF-A, manganese enhances the OER electrochemical performance by promoting the generation of high-valence nickel or cobalt (Fig. S16 and S17†). However, unlike bimetallic systems, there exists a synergistic effect between nickel and cobalt in the three-metal systems (Fig. S18†).55 Then, the addition of Mn replaces the positions of Co and Ni, disrupting the synergistic effect, while the OER activity of Mn is inferior to that of Co and Ni. Therefore, considering the above two aspects, the addition of Mn should theoretically reduce OER performance. However, as shown in Fig. 5a, the voltage of Co1Ni0.5BDC@NF-A is significantly lower than that of Co1Ni0.5Mn1BDC@NF-A at different current densities, which underscores the role of Mn in promoting the generation of high-valent nickel and cobalt to enhance the electrochemical performance of the OER. By analyzing the CV plot, it was observed that Co1Ni0.5Mn1BDC@NF-A exhibits an overpotential of 298 mV at 100 mA cm−2 and 348 mV at 500 mA cm−2, which surpass those of Co1Mn1BDC@NF-A, Ni1Mn1BDC@NF-A, Co1Ni0.5BDC@NF-A (Fig. 5e) and other reported nickel-based and cobalt-based catalysts (Table S3†).
![image file: d4qi00629a-f5.tif](/image/article/2024/QI/d4qi00629a/d4qi00629a-f5.gif) |
| Fig. 5 Electrocatalytic OER in 1 M KOH. (a) The cyclic voltammetry curves of different catalysts. (b) Tafel plots obtained from (a). (c) EIS Nyquist plots. (d) Long-term stability curve for achieving 100 mA cm−2 with Co1Ni0.5Mn1BDC@NF-A. (e) Overpotentials needed at current densities of 100, 300 and 500 mA cm−2 for different samples. (f) Comparison of oxidation peaks on partial CV curves. | |
Furthermore, Co1Ni0.5Mn1BDC@NF-A demonstrates the quickest catalytic reaction kinetics, as evidenced by its lowest Tafel slope of 27 mV dec−1 (Fig. 5b). As illustrated in Fig. 5c, the beneficial electronic structure modification due to Mn addition leads to Co1Ni0.5Mn1BDC@NF-A having the lowest interfacial charge transfer resistance (Rct = 0.021 Ω) (Table S4†), a crucial factor in enhancing the OER. Mn can also reduce the solution impedance (Rs), which can also be observed in bimetallic systems (Fig. S16c, f and S17c, f†). The stability of electrocatalysts is also a crucial aspect to consider, especially in practical industrial applications. Chronopotentiometry is employed to measure the stability of the optimal sample. The catalyst maintains stability for 50 hours at 100 mA cm−2 (Fig. 5d). To further demonstrate its stability, the composition, morphology and electronic structure of the samples after stability testing were examined using XRD, SEM and XPS, respectively. The XRD analysis still reveals a peak corresponding to an amorphous thin-layered structure, indicating that the material composition of the catalyst has not changed and that the catalyst still consists of MOOH (Fig. S19†). As shown in Fig. S20b,† after stability testing, it retains its curved and orderly arrangement into a layered structure. The basic morphology of the catalyst remains mostly unaltered, confirming its stability. However, when working for a long time within the oxidation voltage range, the structure undergoes slight changes due to surface reconstruction and the generation of hydroxides, with most of the nanoflower-shaped balls on the flake disappearing (Fig. S20d†). In fact, the disappearance of small balls also has traces to follow. Before conducting stability testing on Co1Ni0.5Mn1BDC@NF-A, some small balls (Fig. S20c†) have already adhered to the surface. XPS analysis shows that after long-term stability testing, there is no significant shift in the electronic energy level, which further proves the profound stability (Fig. S21†). The excellent stability of the catalysts can be attributed to several factors: (1) utilizing an MOF as a precursor provides spatial confinement for BDC ligands, ensuring the complete dispersion and stability of active cobalt and nickel sites. (2) Co1Ni0.5Mn1BDC@NF-A synthesized through MOF reconstruction has a stable nanosheet like structure. (3) Before and after activation, the changes in electron density around Mn, Co and Ni indicate a strong electronic interaction between Co, Mn and Ni. (4) The hybridization effect between Co/Ni 3d and O 2p, enhanced by Mn addition, strengthens the electronic interaction between the metal and oxygen, thereby promoting catalyst stability. The above analysis proves that the sample can improve activity while still ensuring excellent stability.
Furthermore, as illustrated in Fig. 5f, the size of the oxidation peak corresponds to the oxidation process of Co and Ni (from Co2+/Ni2+ to Co3+/Ni3+), where a larger oxidation peak indicates a higher content of Co3+ and Ni3+. The notable disparity in oxidation peak size between Co1Ni0.5Mn1BDC@NF-A and Co1Ni0.5BDC@NF-A underscores the role of Mn in promoting the formation of high-valence Ni and Co states. This observation aligns with the electronic level analysis of XPS mentioned above, suggesting that the addition of Mn can lower this energy barrier by promoting the hybridization of Co/Ni 3d with O 2p orbitals, resulting in the generation of numerous Co/Ni(III) ions at low bias. The role of Mn addition in promoting the formation of high-valence metals has been demonstrated through both structural and electrochemical properties.
3.3 Electrocatalytic properties for the EOR
The EOR electrocatalytic performance of the catalysts was assessed using a typical three-electrode system, which had the same electrode configuration as that used to measure the OER activity, except that the electrolytic solution was replaced with 1 M ethanol in 1 M KOH. The LSV polarization curves presented in Fig. 6a at 2 mV s−1 were utilized to compare the catalytic performances of the various catalysts in the EOR. As anticipated, Co1Ni0.5Mn1BDC@NF-A exhibits a lower voltage of approximately 1.30 V vs. RHE (@100 mA cm−2) compared to Ni1Mn1BDC@NF-A (1.31 V), Co1Mn1BDC@NF-A (1.32 V) and Co1Ni0.5BDC@NF-A (1.33 V). This potential is also notably low compared with those of previously reported new nickel-based and cobalt-based EOR catalysts (Table S5†). In the electrochemical testing of the EOR, the comparison between Co1Ni0.5Mn1BDC@NF-A and Co1Ni0.5BDC@NF-A at different current densities is more significant. The reason for this is that the improvement in EOR performance is mainly attributed to the generation of high-valence Ni and Co. Because the mechanism of the EOR on the Ni surface is as follows56 (the reaction on the Co surface is the same as that on the Ni surface): (1) Ni(OH)2 + OH− → NiOOH + H2O + e−; (2) NiOOH + C2H5OH → CH3CHO + Ni(OH)2, the EOR reaction requires the generation of Ni3+/Co3+, and a higher content of Ni3+/Co3+ corresponds to a faster reaction rate. The focus is on the process and results of the generation of Ni3+ and Co3+, while the partial substitution of Co and Ni with Mn has a relatively minor impact. Therefore, whether it is a bimetallic system (Fig. S22 and S23†) or a triple metal system, Mn has a significant effect on enhancing ethanol oxidation reaction performance. The Tafel slope of Co1Ni0.5Mn1BDC@NF-A is 31 mV dec−1, lower than those of Ni1Mn1BDC@NF-A (35 mV dec−1), Co1Mn1BDC@NF-A (46 mV dec−1) and Co1Ni0.5BDC@NF-A (57 mV dec−1) (Fig. 6b), indicating that it has the fastest catalytic reaction kinetics in the EOR. In Fig. 6c, the Rct in Co1Ni0.5Mn1BDC@NF-A is 0.015Ω (Table S6†) due to the positive optimization of the electronic structure caused by Mn addition. Consistent with the OER, it can reduce the solution impedance (Rs) /Ω, which can also be reflected in the bimetallic system (Fig. S22c and S23c†). Stability was evaluated through chronopotentiometry, which shows that it was maintained for 50 hours at 100 mA cm−2 (Fig. 6d). Similarly, we conducted XRD, SEM, and XPS tests on the samples after EOR stability testing (Fig. S24–S26†). The results are similar to those of the OER, that is, the main active components are still CoOOH and NiOOH, and the layered structure and the electronic structure have not changed significantly. The above results once again prove the excellent stability of Co1Ni0.5Mn1BDC-A.
![image file: d4qi00629a-f6.tif](/image/article/2024/QI/d4qi00629a/d4qi00629a-f6.gif) |
| Fig. 6 Electrocatalytic EOR in 1 M KOH with 1 M ethanol. (a) The polarization curves of different catalysts at 2 mV s−1. (b) Tafel plots obtained from (a). (c) EIS Nyquist plots. (d) Long-term stability curve to drive 100 mA cm−2 of Co1Ni0.5Mn1BDC@NF-A. Comparison of the EOR and OER in (e) the polarization curves (f) and the potentials required at current densities of 100, 300 and 500 mA cm−2. | |
Adjusting the proportion of the three metals has always been a challenge in determining the optimal performance of the catalyst. We conducted multiple comparative experiments to adjust the optimal proportions of the three metals (Fig. S27a and c†). Due to the best OER performance of the bimetallic catalyst Co1Mn1BDC@NF-A, it was decided to use this as a basis to search for the best-performing bifunctional catalyst by changing the Ni element content (Fig. S27b and d†). Furthermore, as shown in Fig. 6e and f, there is a notable voltage disparity between the OER and EOR. The ethanol oxidation reaction requires only 1.30 V to achieve 100 mA cm−2 and 1.37 V to reach 500 mA cm−2, significantly lower than the oxygen evolution reaction, which requires 1.53 V to achieve 100 mA cm−2 and 1.58 V to reach 500 mA cm−2. To further reduce the energy requirement for hydrogen production, one solution is to substitute the OER with the EOR.
3.4 Overall water electrolysis efficiency
Inspired by the outstanding electrocatalytic performance and long-term stability exhibited by Co1Ni0.5Mn1BDC@NF-A in both the OER and EOR, it was employed as the anode in a two-electrode system to assess water-splitting voltage. The cathode of the two-electrode system is S, Fe-CoP, synthesized in the laboratory (Fig. S28†). The overall water decomposition diagram is illustrated in Fig. 7a. In Fig. 7b, the LSV curves of Co1Ni0.5Mn1BDC@NF-A during water splitting in 1 M KOH with and without 1 M ethanol for OER and EOR have been demonstrated. To attain a current density of 100 mA cm−2, the ethanol–water mixture requires just 1.39 V, markedly less than the voltage needed for the system lacking ethanol, which is 1.54 V. Similarly, comparable results are observed at higher currents. At 500 mA cm−2, the OER–HER system requires 1.85 V, considerably higher than the EOR–HER system (1.59 V) (Fig. 7c). This demonstrates that utilizing the EOR as the anode for hydrogen production via water electrolysis is one method to enhance hydrogen production efficiency.
![image file: d4qi00629a-f7.tif](/image/article/2024/QI/d4qi00629a/d4qi00629a-f7.gif) |
| Fig. 7 (a) Schematic diagram of a two-electrode system. (b) Comparison of the polarization curves for Co1Ni0.5Mn1BDC@NF-A during water splitting in 1 M KOH with and without 1 M ethanol and (c) the potentials at different current densities. | |
In summary, the robust catalytic activity of Co1Ni0.5Mn1BDC@NF-A can be attributed to the following reasons: (1) the high conductivity of Ni foam facilitates mass and electron transfer. (2) Benzene-1,4-dicarboxylic acid (PTA) with a para-conjugated benzene ring exhibits excellent electron delocalization and conductivity. (3) The uniform incorporation of Mn enables the in situ generation of MOFs with a well-organized layered nanosheet morphology,57 facilitating the accessibility of catalytic sites for the OER and EOR. (4) The addition of Mn promotes the hybridization effect between Ni, Co and O, facilitating the generation of Co/Ni(III) species, thereby accelerating the reaction kinetics of both the OER and EOR.
4. Conclusion
In summary, Co1Ni0.5Mn1BDC@NF, synthesized through a simple one-step hydrothermal method, was transformed into the actual active substances CoOOH and NiOOH after CV activation, exhibiting excellent electrochemical performance in both the OER (with an overpotential of 298 mV at 100 mA cm−2) and EOR (with a potential of 1.30 V at mA cm−2). Moreover, it showed stable operation in the EOR and OER during a 50-hour stability test. The addition of Mn not only adjusted the hybridization effect between Ni, Co and O, promoting the conversion of Ni2+ to Ni3+ and Co2+ to Co3+, but also assisted in structural transformation, thereby enhancing the efficiency of the OER and EOR. Additionally, the combination of the EOR and HER only needs 1.39 V and that of OER and HER only requires 1.54 V to reach 100 mA cm−2, which is much lower than those of the other two electrode hydrogen production systems. The successful synthesis of Co1Ni0.5Mn1BDC@NF-A, a non-precious metal catalyst, provides insights into achieving efficient and stable hydrogen production.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by grants from the Key R&D Program of Shandong Province, China (2021ZLGX06), and the National Natural Science Foundation of China (Grant No. 52274308, 52174283 and U22B20144).
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