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Junhua Song, Chengzhou Zhu, Bo Z. Xu, Shaofang Fu, Mark H. Engelhard, Ranfeng Ye, Dan Du, Scott P. Beckman, and Yuehe Lin*
and constructed to work in concert with the calling of “green evolution.” Facing the fact that the nonrenewable energy has been increasingly unsustainable, the energy harvested in the “green” ways greatly diversifies the source of current power supply systems and makes us less vulnerable to the possible environmental crisis. Among different renewable energy sources, wind, solar, and hydropower are considered the most accessible and relatively well-established ones. However, the unpredictable and intermittent nature of these green energy sources has introduced other concerns, which might cost even more energy waste than the traditional fossil fuel cycles.[1] For example, the electricity generated by hydropower is almost constant during day and night, but the power need varies during peak and off-peak times.[2] These concerns become renown in certain regions when the renewable energy makes up of the great portion of their power supply.[3] Incorporating a buffer section between the intermittent energy source and the power grid is necessary to compensate the peak time by supplying stored energy, and collecting the excess power during off-peak period.[4] The advancement in energy storage and conversion technologies is therefore of great importance to make the “green evolution” truly evolutional. Sustainable hydrogen fuel production driven by water electrolysis is one of the most practical solutions for energy storage and conversion.[5] To re-distribute the intermittent power, excess electrical energy is converted and stored in the chemical form (H2), which later can be used for power generation when needed. A typical water-splitting system involves two half-reactions: the oxygen evolution reaction (OER) at anode and hydrogen evolution reaction (HER) at cathode.[6] These two reactions combined determine the overall water-splitting efficiency. Currently, there are two major water-splitting media, acid and basis. Acidic electrolyte (low pH) is relatively well
Cobalt-based bimetallic phosphide encapsulated in carbonized zeolitic imadazolate frameworks has been successfully synthesized and showed excellent activities toward both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Density functional theory calculation and electrochemical measurements reveal that the electrical conductivity and electrochemical activity are closely associated with the Co2P/CoP mixed phase behaviors upon Cu metal doping. This relationship is found to be the decisive factor for enhanced electrocatalytic performance. Moreover, the precise control of Cu content in Co-host lattice effectively alters the Gibbs free energy for H* adsorption, which is favorable for facilitating reaction kinetics. Impressively, an optimized performance has been achieved with mild Cu doping in Cu0.3Co2.7P/nitrogen-doped carbon (NC) which exhibits an ultralow overpotential of 0.19 V at 10 mA cm–2 and satisfying stability for OER. Cu0.3Co2.7P/NC also shows excellent HER activity, affording a current density of 10 mA cm–2 at a low overpotential of 0.22 V. In addition, a homemade electrolyzer with Cu0.3Co2.7P/NC paired electrodes shows 60% larger current density than Pt/ RuO2 couple at 1.74 V, along with negligible catalytic deactivation after 50 h operation. The manipulation of electronic structure by controlled incorporation of second metal sheds light on understanding and synthesizing bimetallic transition metal phosphides for electrolysis-based energy conversion.
1. Introduction During the past few decades, tremendous amount of renewable energy-based infrastructures and facilities has been designed J. Song, Dr. C. Zhu, B. Z. Xu, S. Fu, R. Ye, Prof. D. Du, Prof. S. P. Beckman, Prof. Y. Lin School of Mechanical and Materials Engineering Washington State University Pullman, WA 99164, USA E-mail:
[email protected] M. H. Engelhard, Y. Lin Pacific Northwest National Laboratory Richland, WA 99352, USA
DOI: 10.1002/aenm.201601555
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Bimetallic Cobalt-Based Phosphide Zeolitic Imidazolate Framework: CoPx Phase-Dependent Electrical Conductivity and Hydrogen Atom Adsorption Energy for Efficient Overall Water Splitting
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received due to the compatibility of mass producible membrane, while basic media (high pH) has the advantage of low cost and less precious metal demanding. Conventionally, commercially available Pt and RuO2 are considered the state-of-theart HER and OER catalysts, which can efficiently increase the reaction rate and lower the overpotential.[7] The low natural abundance and prohibitive market price, however, are big hurdles for adopting Pt and RuO2 on the industrial scale electrolysis system.[8] Exploring alternative nonprecious metal and stable water-splitting catalysts with high efficiency becomes urgent and important. The promising alternatives for HER include transition metal-based sulfides, carbides, and phosphides, such as MoS2,[9] WS2,[10] NiS,[11] MoC,[12] CoP,[13] and NiP.[14] As for OER catalysts, transition metal and their oxide/ hydroxide species, including Co3O4/C,[15] CoO,[16] NiCo LDH (layered double hydroxide),[17] and MnO2,[18] etc., are usually preferred due to their high activities. Particularly, the formation of bimetallic HER and OER catalysts by introducing secondary metal ion is believed to be more active and stable due to tailored electronic and surface properties of the host structure. For example, Boettcher and co-workers have shown a dramatic increase in OER activity upon addition of Fe cations in first-row transition metals.[19] Zou and co-workers revealed that by incorporating Zn ion, the Co-based MOF (metal organic framework) catalysts demonstrate excellent HER performance comparable to commercial Pt/C.[20] However, transition metal phosphide based bimetallic catalysts are rarely studied and tested, despite their possible superiority in catalyzing OER and HER for overall water splitting. Herein, we report a bimetallic cobalt-based phosphide zeolitic imidazolate framework (BCP-ZIF) with optimized composition as a bifunctional catalyst for overall water splitting. Importantly, the low H* adsorption energy and high conductivity of our synthesized CuxCo3–xP/nitrogen-doped carbon (NC) catalyst, especially Cu0.3Co2.7P/NC, exhibit excellent catalytic performance in alkaline media (1.0 m KOH, pH = 13.5), featuring an ultralow overpotential of 0.19 V for OER and –0.22 V for HER at a current density of j = 10 mA cm−2. To the best of our knowledge, the OER overpotential (at j = 10 mA cm−2) of the Cu0.3Co2.7P/NC catalyst is the lowest reported to date measured by the rotating disk electrode (RDE) system. Resulting from dopant-induced CoPx phase behavior of the BCP-ZIFs, we demonstrate that upon mild Cu doping of CuxCo3–xP/NC (x = 0.3), an optimal electrocatalytic performance can be obtained with lowest charge-transfer resistance and highest reaction kinetics. More
impressively, our homemade electrolyzer with Cu0.3Co2.7P/NC as both cathode and anode catalysts has outperformed the Pt/C and RuO2 paired counterparts by showing 60% higher current density at a voltage of 1.74 V. In addition, it demonstrates a long-term durability with little deactivation after 50 h continuous operation in alkaline electrolyte.
2. Results and Discussions 2.1. Structure of BCP-ZIFs The synthesis route of BCP-ZIFs is illustrated in Scheme 1; two metal precursors and MeIM (2-methylimidazole) linkers are used to assemble ZIF self-templates, which subsequently undergo carbonization and phosphidation to obtain the final catalysts.[21,22] Figure 1a shows a typical scanning electron microscopy (SEM) image of Cu0.3Co2.7/NC-ZIFs with uniform polyhedrons (ca. 500 nm) and smooth surface. The morphology of as-obtained ZIFs is consistent with other reported literatures.[13b,23] After carbonization and phosphidation at elevated temperature, the size and shape polyhedral structures are maintained with relatively rougher surface, where bimetallic Cu/Co nanoparticles are encapsulated inside the nitrogendoped carbon matrix (Figure 1b,c). The distribution of elements in BCP-ZIFs is further investigated by elemental mapping. Figure 2d–h confirms the successful phosphidation and presence of bimetallic Cu/Co in Cu0.3Co2.7P/NC. The highly overlapped P and Co in SEM mapping also indicate the formation of CoPx species. The crystallinity and phase information of the bimetallic cobalt-based phosphide are confirmed by X-ray diffraction (XRD) in Figure 2 and Figure S2 of the Supporting Information. All investigated samples display highly crystallized structure with distinct characteristic peaks. The metallic cobalt and cobalt phosphides can be indexed as Co (PDF#15-0806), Co2Sitype Co2P (PDF#32-0306), and MnP-type CoP(PDF#65-2593).[24] It is worth mentioning that the incorporation of Cu2+ neither introduces any observable Cu peaks nor associated peak shifts in the XRD patterns, suggesting the homogeneous substitution of Cu ion into CoPx matrix without forming CuCo alloys.[25] The homogeneity of substituting Cu2+ ion might attribute to its similar ionic radius and electronegativity as Co2+ ion.[26] Alternating CoP, Co2P, and Co phase fraction, however, is clearly observed with increasing Cu2+ doping level. By measuring the
Scheme 1. Schematic drawing of the synthesis route of bimetallic cobalt-based phosphide Cu3Co2.7P/NC.
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Full paper Figure 1. a) SEM images of typical morphology of Cu0.3Co2.7/NC-ZIFs with well-defined polyhedral structure. b) After carbonization at 900 °C for 3 h for Cu0.3Co2.7P/NC. c) TEM image of Cu0.3Co2.7P/NC with bimetallic cobalt encapsulated in the nitrogen/carbon matrix. d–h) Elemental mapping of Cu0.3Co2.7P/NC confirming the presence of Co, Cu, P, and C.
ratio of peak intensities, it is clear that the metallic Co phase fraction is larger with higher Cu2+ doping level, accompanied by a decreasing Co2P phase present. For our CuxCo3–xP/ NC catalysts, the composition of Co, CoP, and Co2P mixed phases is believed to have a key effect in promoting electrochemical performance due to their different electroactivity and conductivity.[24] Building on the XRD survey of different phase behaviors, we further investigate the electronic states associated with Cu doping using X-ray photoelectron spectroscopy (XPS). Similar to other carbonized ZIF structures using MeIM as an organic linker, the nitrogen dopants in three major forms are clearly observed in N1s high-resolution spectra (Figure 3a).[27] In Figure 3b, the presence of deconvoluted peaks at 129.4 and
Figure 2. XRD patterns showing the progress of metallic cobalt and CoPx phases upon Cu doping.
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130.3 eV suggests the successful phosphidation of cobalt by forming CoP and Co2P, while a single peak at 133.9 eV represents some partially oxidized phosphate species.[13a,28] Besides confirming the presence of nitrogen and phosphide, the catalytically active Co element was further deconvoluted to gain more insight into the relationship between its complex oxidation states and transition metal (Cu) doping effect. It is noted that with increasing Cu content, from Cu0.3 to Cu1, stronger Co02p1/2 and Co02p3/2 peaks of metallic Co are present (Figure 3c). This is identical to what was observed in XRD patterns, indicating that the presence of more concentrated Cu ions in the Co host structure might favor the formation of the cobalt metal. Oxidized Co derivatives (Co2+ and Co3+) show more complicated but reasonable behaviors compared to the XRD results. Upon mild Cu doping (Cu0.3), the Co2p spectra show lower Co3+/Co2+ ratio for both Co2p1/2 and Co2p3/2 regions, corresponding to a more dominant Co2P than CoP phase. This is consistent with weaker CoP characteristic peaks in XRD of Cu0.3Co2.7P/NC. Interestingly, when introducing more Cu dopants, an increase of Co3+ is first observed for Cu0.6Co2.4P/NC, followed by a decreasing Co3+ content in Cu1Co1P/NC. Combining the XRD and XPS results, it is highly possible that the presence of Cu in CuxCo3–xP/NC can inhibit phosphidation of metallic cobalt, which leads to the phase transition of CoPx species, featured with higher Co2+ content upon mild Cu doping and the formation of Co metal with increasing Cu concentration. Another phenomenon associated with Cu doping is the peak shift for the oxidized Co species. All cobalt oxidation states, Co2+2p1/2, Co2+2p3/2, Co3+2p3/2, and Co3+2p1/2, exhibit a left shift of the bonding energy with increasing Cu content. The trend of oxidized Co binding energy indicates a complex CoP bonding behavior with weaken CoP bonds in the presence of Cu. A comparison of molar ratio of Cu/Co in CuxCo3–xP/NC between starting materials and surface composition obtained by XPS can be seen in Table S1 (Supporting Information). The mixed
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Figure 3. a) High-resolution XPS spectra of N2p, including pyridinic, pyrrolic, and quaternary N. b) XPS survey of P2p with the presence of phosphide and phosphate peaks. c) Deconvoluted spectra for CuxCo3–xP/NC with different Cu doping concentration.
Co oxidation states and associated phase behaviors are essential in determining their catalytic activity, which are shown in the electrochemical performance below.
2.2. Electrochemical Performance of BCP-ZIFs in Alkaline Media The incorporation of second transition metal into cobalt-based phosphide and thus induced mixed CoPx phase are of great importance to understand the relationship between elemental composition and their corresponding activities. The electocatalytic OER of CuxCo3–xP/NC was first evaluated by linear sweep voltammetry (LSV) using RDE in N2-saturated alkaline electrolyte (1 m KOH). As revealed in Figure 4a, Cu0.3Co2.7P/NC shows obviously superior performance compared to other Cu composition with the unprecedentedly low overpotential of η = 0.19 V at 10 mA cm−2 with a catalyst mass loading of 0.4 mg cm−2. Among different transition metal-doped BCP-ZIFs, Cu dopant presents the best OER catalytic activity, which is not only better than that of Zn0.3Co2.7P/NC (η = 0.33 V), Ni0.3Co2.7P/ NC (η = 0.28 V) at j = 10 mA cm−2, but outperforms commercial RuO2 (η = 0.27 V), as shown in Figure 4b. The optimal OER performance of Cu0.3Co2.7P/NC becomes clearer at high potential with rapid current increase and high current density j = 325 mA cm−2 at η = 0.77 V versus RHE (reversible hydrogen electrode). Besides such high catalytic activity, Cu0.3Co2.7P/ NC also displays excellent stability in electrochemical OER (Figure 4c). In fact, there is merely 0.05 V positive shift for η at 10 mA cm−2 and negligible change of current density for 1601555 (4 of 9)
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bias larger than V = 1.68 V after 1000 cycles. In Figure 4d, the kinetic analysis of OER is carried out by measuring the slope of linear part of Tafel plots. For CoP/NC, Cu0.3Co2.7P/NC, Cu0.6Co2.4P/NC, Cu1Co2P/NC, and RuO2, the Tafel slopes are 57, 44, 50, 65, and 63 mV dec−1, respectively. The exchange current density is derived from Tafel plots, using the equation η/b = log (j/j0), where b is the Tafel slope, j is the current density, and j0 is the exchange current density. The obtained j0 of Cu0.3Co2.7P/NC is 2.63 × 10−9 A cm−2, which is higher than the state-of-the-art commercial RuO2, j0 = 2.51 × 10−9 A cm−2. The low Tafel slope and high exchange current of Cu0.3Co2.7P/NC indicate that the Cu/Co molar ratio in CuxCo3–xP/NC plays an important role in determining OER catalytic activity. With mild Cu doping in Cu0.3Co2.7P/NC (x = 0.3), the electrochemical activity reaches its maximum, while continuously increasing the doping content results in decreasing catalytic activity. The composition-dependent electrochemical active surface area of CuxCo3–xP/NC is also taken into account for its contribution to OER activity. The trend similar to the Tafel slopes is observed by measuring the electrochemical double layer capacitance (CDL), where the CDL versus composition forms a volcanolike plot with Cu0.3Co2.7P/NC (30.9 F g−1) sitting at the tip of volcano, CoP (20.5 F g−1), Cu0.6Co2.4P/NC (19.2 F g−1), and Cu1Co2P/NC (9.1 F g−1) residing at the wings (Figure 4e). The measured electrochemical impedance spectroscopy (EIS) also confirms the effect of Cu dopant on the charge-transfer resistance increasing from 5.6 Ω for Cu0.3Co2.7P/NC to 5.8 Ω for Cu0.6Co2.4P/NC, 6.1 Ω for CoP/NC, and 6.6 Ω for Cu1Co2P/NC (Figure 4f). The low charge-transfer resistance demonstrates a
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Full paper Figure 4. a) LSV plots of CuxCo3–xP/NC and commercial RuO2 measured in 1 m KOH electrolyte using RDE. b) Comparison of OER performance of Zn0.3Co2.7P/NC, Ni0.3Co2.7P/NC, Cu0.3Co2.7P/NC, and RuO2. c) LSV plots showing strong stability of Cu0.3Co2.7P/NC after 1000 potential cycles. d) Tafel plots of CuxCo3–xP/NC and commercial RuO2 show superior reaction kinetic with Cu0.3Co2.7P/NC catalyst. e) Volcano-like plot of electrical double-layer capacitance CDL of CuxCo3–xP/NC with different Cu concentration. f) Nyquist plots derived from EIS measurements, showing lower charge-transfer resistance of Cu0.3Co2.7P/NC.
better conductivity with low Cu-doping content.[29] Moreover, the Cu0.3Co2.7P/NC displays a straighter line along the imaginary axis than the other composition, indicating a low diffusion resistance.[30] As expected, both EIS and kinetic analysis are high in accordance with CuxCo3–xP/NC OER performance displayed in LSV plots. In addition, the intrinsic catalytic activities with different Cu content are studied on the basis of turn over frequency (TOF), in which Cu0.3Co2.7P/NC shows the highest value of 5.82 × 10−3 s−1, larger than that of Cu0.6Co2.4P/ NC (2.72 × 10−3 s−1), Cu1Co2P/NC (1.96 × 10−3 s−1), CoP/NC (2.87 × 10−3 s−1), and RuO2 (0.61 × 10−3 s−1) (Table S2, Supporting Information). It is also worth noting that the electrochemical performance of Cu0.3Co2.7P/NC, including ultralow overpotential, high exchange current density, and low Tafel slope is among the best nonprecious metal OER catalysts (Table S3, Supporting Information).
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We next assess the HER performance of BCP-ZIFs in order to screen the optimal bifunctional catalyst for our final watersplitting tests. The HER experiments are first carried out by LSV in the same N2-saturated 1 m KOH solution resembled to OER testing. As expected in Figure 5a, Cu0.3Co2.7P/NC exhibits a low overpotential at 10 mA cm−2 of η = 0.22 V, which is smaller than that of Cu0.6Co2.4P/NC and Cu1Co2P/NC with higher Cu content. The TOF values calculated for different CuxCo3–xP/ NCs and Pt are shown in Table S2 (Supporting Information), where Cu0.3Co2.7P/NC exhibits a value of 24.29 × 10−3 s−1, much higher than other Cu composition. To compare the influence of different transition metal dopants, Zn0.3Co2.7P/NC and Ni0.3Co2.7P/NC catalysts are also investigated and shown in Figure 5b. The overpotential of Cu0.3Co2.7P/NC is found to be 0.06 V and 0.04 lower than its Zn-doped and Ni-doped counterparts, respectively. As previously reported, the transition metal
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Figure 5. a) LSV plots of CuxCo3–xP/NC and commercial Pt/C measured with RDE at 2000 rpm. b) Effect of different transition metal doping (Zn, Ni, and Cu) in determining HER performance of BCP/NC-ZIFs. c) Stability test of Cu0.3Co2.7P/NC before and after 1000 continuous potential cycling. d) Tafel plots of CuxCo3–xP/NC and commercial Pt/C.
can be more positively charged in the presence of P atoms, in which the negatively charged P can attract the hydrogen proton at low coverage and desorb H2 at high coverage in electrochemical HER.[31] Thus, one can expect that an increasing atomic percentage of P phase in CoPx (CoP vs Co2P) renders better HER performance. The electronic conductivity, however, is usually compromised for metal phosphides with low transition metal to P ratio, featured by weaker metallic character.[18,32] Thus a perfect balance between activity and conductivity is desired for maximizing electrochemical performance. In our bimetallic cobalt-based phosphides, such a balance is achieved by Cu0.3Co2.7P/NC with the smallest amount of Cu ions doped in the cobalt lattice. Moreover, as revealed by both XRD and XPS surveys, Cu0.3Co2.7P/NC shows the lowest amount of Co0, while Cu1Co2P/NC consists of a majority of metallic Co and Co2+ species. Therefore, we tentatively propose that the CoP and Co2P species are the decisive components for the optimized activity in CuxCo3–xP/NC catalyst. Other than high HER catalytic activity, Cu0.3Co2.7P/NC also exhibits strong stability, as shown by a small negative shift of 0.023 V at j = 10 mA cm−2, and indistinguishable difference of current density beyond η = 0.39 V after 1000 continuous cycling (Figure 5c). Although there is a lack of consensus on the HER reaction process in alkaline media, regardless of any possible routes the HER proceeds, hydrogen atom adsorbed on the catalyst surface (Hads) is always involved during the reaction. In order to examine the reaction pathway in our alkaline media, coverage of adsorbed hydrogen occurred in either Volmer–Tafel or Volmer–Heyrovsky pathway is adopted as an analogous model as in acidic 1601555 (6 of 9)
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media. The theoretical Tafel slopes of each of the following reactions are 120, 30, and 40 mV dec−1, respectively[33] H+(aq ) + e − → Hads ( Volmer reaction )
Hads + Hads → H2 ( g ) ( Tafel reaction )
H+(aq ) + Hads + e − → H2 ( g ) (Heyrovsky reaction )
In Figure 5d, the calculated Tafel slope of Cu0.3Co2.7P/NC is 122 mV dec−1, which is slightly larger than that of Pt/C (105 mV dec−1) and smaller than its doped and undoped counterparts (123 mV dec−1 for CoP/NC, 131 mV dec−1 for Cu0.6Co2.4P/NC, and 129 mV dec−1 for Cu1Co2P/NC). In spite of the hydrogen evolution mechanism in alkaline media that remains elusive, the reaction is more likely governed by Volmer–Heyrovsky, close to pure Ni with a similar Tafel slope of 120 mV dec−1.[34]
2.3. Optimized Electronic Structure and H* Adsorption Energy Density functional theory (DFT) analysis was also carried out to explore the origin of superior catalytic process upon foreign metal doping. The Nyquist plot discussed above shows a decreased charge-transfer resistance upon mild Cu doping, which is closely associated with improved electrical
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Full paper Figure 6. Red circles represent Co atoms; blue circles represent P atoms; purple circle represents Cu atoms; and yellow clouds represent charge accumulation. a) Bonding charge distribution for pristine CoP. Charge accumulation can be seen in between Co and P atoms. b) Bonding charge distribution for Cu-doped CoP and electron depletion around the Cu atom. c) Dramatic lower H* adsorption energy of both CoP and Co2P host structures upon Cu doping. d) Bonding charge distribution for pristine Co2P. e) Bonding charge distribution for Cu-doped Co2P.
conductivity of the catalysts. It is obvious that all four systems, CoP, Co2P, Cu-CoP, and Cu–Co2P, demonstrate the metallic feature since the Fermi level lies within the conduction band (Figures S9–S12, Supporting Information). Around the Fermi level, most of the densities of states in pristine and doped CoP come from the hybridization of metal d-states and phosphorous p-states, while in pristine and doped Co2P mainly from metal d-states. Figure 6a,b,d,e shows the change of bonding charge distribution in the crystal structure of Cu-doped CoP and Co2P from pristine CoP and Co2P. In pristine CoP and Co2P, electron accumulation (yellow cloud) can be seen in the space between a Co atom (red) and a P atom (red), which indicates the formation of covalent bonds between these atoms. By replacing one Co atom by a Cu atom (purple), it is easy to observe a certain amount of charge accumulation around Co atoms while less accumulation around Cu atoms in both Cu-doped CoP and Co2P systems. Moreover, no charge accumulation is found between Cu and P atoms. This may explain the enhancement in conductivity in Cu-doped systems since the introduction of Cu decreases the number of covalent bonds (CoP) in the systems by creating more free electrons. Notably, since Co2P possess more electrons per unit cell than CoP, more free electrons will be released with Cu doping in Co2P-riched phase, which are beneficial for a better electrical conductivity. Hydrogen adsorption energy is another critical factor in determining the ultimate catalytic activity. One of the agreements in searching for a good electrocatalyst is to possess a moderate H adsorption free energy, which greatly affects the efficiency of adsorption and desorption steps when splitting water. Commercial Pt has the most desirable H adsorption free energy around 0.09 eV,[20] approaching the thermoneutrality. Our DFT analysis reveals that the Cu doping significantly changes the value of hydrogen adsorption free energy in both CoP and Co2P systems. For instance, in Figure 6c, the hydrogen adsorption free energy is 0.48 eV for pristine CoP (001) surface, and 0.21 eV for Cu-doped CoP (001) surface, while the doping of Cu also changes the hydrogen adsorption free energy from 0.54 eV in pristine Co2P (001) surfaces to 0.28 eV in Cu-doped Co2P (001) surfaces. Contrary to its pristine counterpart, Cudoped Co phosphide shows excellent catalytic activity via a
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synergistic effect of superior electrical conductivity of Co2P phase and low hydrogen adsorption of CoP phase.
2.4. Symmetric Electrolyzer for Overall Water Splitting Based on the above analysis, we determine that Cu0.3Co2.7P/ NC is the most efficient BCP-ZIFs as a bifunctional electrocatalyst for both HER and OER, which possesses great potential in overall water splitting in alkaline solution. By pairing Cu0.3Co2.7P/NC as both cathode and anode catalysts, our homemade electrolyzer, operated in 1 m KOH electrolyte, shows superior activity, featured with ≈60% higher current density at 1.74 V (70 mA cm−2) compared to Pt/C/RuO2 pair (28 mA cm−2) at room temperature (Figure 7a). As revealed in Figure 7b, catalytic stability is also carried out at η = 10 mA cm−2 for Cu0.3Co2.7P/NC in the same electrolyte. After 50 h of operation, the electrolyzer exhibits 102% current retention, indicating the excellent robustness of our catalyst. Notably, there are very few reports using bimetallic transition metal phosphides as both cathode and anode catalysts, which paves a new way for designing efficient nonprecious metal catalysts for high-performance water splitting.
3. Conclusion A general synthesis method is adopted to uniformly incorporate a second transition metal into cobalt-based phosphide metal organic framework. Mixed CoPx phase behavior has been revealed to be associated with copper doping level. The subsequent electrochemical performance is also found to be optimized when introducing mild amount of Cu into the Co host structure. Moreover, DFT analysis shows that less covalently bonded elements and electron accumulations are found around the doped Cu atom, which are beneficial for enhanced electrical conductivity. Hydrogen atom adsorption energy is also determined to be closer to thermoneutrality with Cu doping, giving a catalytically more favorable environment for hydrogen adsorption and desorption. The excellent OER and HER performance
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Figure 7. a) Comparison of LSV curves of overall water splitting in 1 m KOH (pH = 13.5) for Cu0.3Co2.7P/NC paired electrodes and Pt/C/RuO2 paired electrodes (catalysts loading: 2 mg cm−2; inset: digital image at cut-off potential showing H2 and O2 bubbles escaping from Ni foam). b) Stability test of Cu0.3Co2.7P/NC electrodes operated at designed η (j = 10 mA cm−2) for 50 h.
render our Cu–CoP bimetallic phosphide one of the most efficient bifunctional catalysts for electrolysis.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements J.S. and C.Z. contributed equally to this work. This work was supported by a start-up grant from Washington State University. The authors thank Franceschi Microscopy & Image Center at Washington State University for TEM measurements. The XPS analysis was performed using Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for DOE by Battelle under Contract DE-AC05-76RL01830. Received: July 16, 2016 Revised: August 26, 2016 Published online:
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