Rational engineering of metal – organic coordination networks into facet-controlled phosphides for overall water splitting

Although transition metal phosphide electrocatalysts display unique electronic structure that serves as functional centers for hydrogen evolution reaction, the synthesis of this class of materials for oxygen evolution remains a challenge due to the complex multielectron transfer pathways and sluggish reaction kinetics. This study details an in-situ modification and transformation of cyanide-bridged nickel-iron (CN-NiFe) organometallic hybrid into the preferential Fe 2 P phase with prevailing exposed


| INTRODUCTION
Water splitting offers a promising and sustainable solution to alleviate future global energy demands for affordable and reliable energy generation and storage, such as through hydrogen production, rechargeable metal-air batteries, and fuel cells. [1][2][3][4] Thermodynamically, under standard conditions, water splitting is an endergonic reaction requiring an activation energy of 286 kJ mol À1 , which is achievable via electrolysis. Unfortunately, to attain appreciable current densities after reaction activation, high overpotentials are generally needed due to the sluggish half-cell reaction kinetics, namely, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). [5][6][7][8] Although noble metals exhibit unrivaled catalytic performances as benchmark water-splitting electrocatalysts (IrO 2 for OER and Pt for HER), their low abundance and high cost greatly hinder large-scale applications. 9,10 Besides, it can also be challenging and costly to pair two different monofunctional catalysts as the anodic and cathodic half-cells to form an integrated water-splitting cell. Hence, though challenging, it is highly important to rationally design non-precious and highly active bifunctional electrocatalysts.
Considerable progress has been achieved in the last decade to explore promising alternative nonprecious HER and OER electrocatalysts including transition metal oxides/hydroxides, 11,12 sulfides, 11,[13][14][15] phosphides, 14,16,17 selenides, 13,18 nitrides, 19,20 carbides, 21,22 and borides. 22 Among them, transition metal phosphides (TMPs) have been widely explored as HER catalysts owing to the resemblance of their electronic structure to hydrogenases, which are highly active H 2 generating enzymes, and their promising catalytic performances toward the hydrodesulfurization reaction. 23,24 Using detailed molecular calculations, the superb catalytic performance of TMPs toward HER has been ascribed to the ensemble effect that allows a moderate hydrogen binding environment on active sites due to the direct involvement of phosphorous in the reaction. In addition, the decreased poisoning effect of the metallic species creates favorable proton-acceptor (phosphorus) and hydride-acceptor (metal) sites. 23,24 However, compared to HER, OER has been the bottleneck because of the multielectron transfer pathways and complex reaction steps involving successive formation of three different adsorption intermediates (OOH*, O*, and OH*).
Nevertheless, prominent improvement in OER activity can be realized via atomic engineering since the crystallographic orientations, coordination number of atoms and number of dangling bonds greatly affect the catalytic reactivity of nanocrystals. 25,26 Essentially, highly active sites generally transpire in a local environment and orientation in the form of high surface energy facets. Nanocrystals meticulously elaborated by such active facets with favorable electronic structure can ensure thermodynamically and kinetically favorable surface adsorption and reaction, which warrants an utmost exploitation of the catalytic activity. However, typical nanocrystals are generally enclosed by diverse crystal planes, exposing different facets with dissimilar activities, and thereby reducing the overall electrocatalytic activity. Moreover, although creating highly reactive facets is a particularly enticing route for designing advanced electrocatalysts, they are difficult to be preserved as a result of these same high surface energies. Thus, the development of reliable and ingenious surface structural approaches to create a selective favorable surface atomic arrangement is an exciting breakthrough and fundamentally important to exploring new active and stable facets for high performing electrocatalysis. Specifically, rational synthesis of transition metal phosphides to explore and reveal the aforementioned surface atomic features that are beneficial for OER have not been well-explored thus far, despite the great progress achieved in investigating their HER activities.
Here, we devise a controllable reductive alkaline etching strategy to modify the framework chemistry and reactivity in the metal-organic coordination network of Prussian blue analogues (PBAs) by exploiting the lability of metal centers and metal-ligand bonds. The as-etched classical crystals of PBAs are conceived to comprise the original synergistic organometallic hybrid, that is, inorganic superstructures with atomically coordinated organic cyanide (CN) bridges in congruence with altered coordinate cleavage dynamics and compound reactivity. Based on the refined framework chemistry, bimetallic transition metal content and weak intramolecular bonds, we formulated a non-destructive phosphorization and preferential manifestation of a transition metal phosphide phase with predominantly active facets. Significantly, unlike the pristine PBAs that susceptibly reconstitute a variant of planes after phosphorization because of the comparable reactivity of its transition metal sites, the refined as-etched PBAs readily transform into a favorable phosphide phase with prevailing reactive centers. Explicitly, for the first time, we disclose a facile and efficient strategy to prepare the Fe 2 P phase with primarily exposed 120 È É facets that we prove to possess high OER activity, using proof-of-concept electrochemical tests and theoretical studies. Notably, using this methodology, it is possible to judiciously engineer similar iron-containing PBAs into iron phosphides with the same exposed active facets. Thus, the in-situ tailoring of the atomic bonding and crystal structure to synthetically modify phosphides and consequently unravel prominent OER active sites; can be recognized as a pioneering strategy for the design of new advanced OER electrocatalysts.

| RESULTS AND DISCUSSION
A template-engaged strategy is employed to prepare nickel-iron phosphide nanocrystals with primarily exposed high-index 120 È É facets of iron phosphide. The preparation of nickel-iron phosphide includes three successive steps: (i) preparation of self-sacrificial templates, (ii) etching and (iii) phosphorization ( Figure S1). Essentially, classical crystals of nickel-iron (Ni Fe) PBAs are selected as the self-sacrificial templates due to their location-dependent etching stability, robustness, lowdegree of defects, high-degree of lability, favorable dynamic framework and bimetallic transition metal content. 27,28 In this study, the self-sacrificial templates manifest sufficiently strong chemical stability to allow a kinetically controlled chemical etching process while being prone to changes in the microstructural features. In addition, it remains structurally intact during the phosphorization step at high temperatures. The PBAs are first synthesized via a facile coprecipitation process in an aqueous solution containing K 3 Fe(CN) 6 and Ni(NO 3 ) 2 Á6H 2 O. 29 The morphology of the as-synthesized PBAs are investigated by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) as shown in Figure 1A and D, respectively. The PBAs exhibit uniform cubic structure and a narrow size distribution with an average particle size of 180 nm ( Figures S2-S3). The flat external faces and edges of the nanocubes, as revealed by TEM, suggests the formation of NiFe-PBAs nanocubes through the classical ionic/ molecular crystallization pathway. This signifies the absence of mesocrystalline features dominated by clustered subunits that are formed by oriented crystallographic fusion of primary nanocrystal building units. Although mesocrystals and classical crystals of PBAs show identical single-crystal features, mesocrystals possess much weaker stability due to a void-rich, mostly defective structure with discontinuous and broken grain boundaries, which makes a chemical etching process difficult to control. Here, the compact and less-defective NiFe-PBAs crystals permit the gradual development of corner-etched PBAs ( Figure 1B, E) and finally cross-PBAs ( Figure 1C, F) through the chemical etching process ( Figures S1 and S2). The etching on PBAs is observed to be appreciably uniform and occurs preferentially on the corners and edges that possess high-surface energy and more reactivity ( Figure 1G). Figure 1H illustrates the structural evolution of the cross-shaped PBAs from PBAs nanocubes at 3 and 6 h, respectively. The average particle sizes of the corner-etched PBAs (166 nm) and cross-PBAs (163 nm) do not change significantly ( Figures S3-S7), indicating low etching activity on the 100 f g planes of the PBAs. The selective etching of the corners and edges of the PBAs is attained by a kinetically controllable alkaline etching approach ( Figure S8). By adding potassium hydroxide (KOH) and trisodium citrate (Na 3 Cit) to a suspension of PBAs, corner-etched PBAs and cross-PBAs are obtained after 3 and 6 h, respectively, at room temperature. The structural evolution of PBAs is kinetically driven by cooperative engraving of the etching components. Basically, treatment of PBAs with Na 3 Cit in the absence of KOH does not alter the size and morphology of the PBAs, while etching takes place rather slowly when the PBAs are treated using KOH without Na 3 Cit ( Figure S8). In particular, KOH alone slightly carves the edges and corners of the cubes revealing the regions less stable in alkaline solution, as also demonstrated in previous studies. 30,31 At the same regions, the etching rate of KOH can be effectively triggered by adding Na 3 Cit, a well-known complexing agent which can directly coordinate to various transition metal sites. [32][33][34] In this process, we postulate that while the alkaline etching proceeds through to cleave the labile Ni II [Fe II/III (CN) 6 ] complex in the framework, citrate molecules quickly interact with the nickel in the disassembled framework. This weakens the coordination strength of the cyanide ligand in the as-formed hemilabile PBAs due to the formation of a stronger metal-carboxylate π-bonding, and thus allows a facile subsequent alkaline etching process. [35][36][37] Moreover, the relative amounts of the KOH and Na 3 Cit is a crucial factor determining the kinetics of the etching. If either etchant component participates at low concentration, etching takes place rather slowly, producing cubes with poorly defined cavities ( Figure S9). In contrast, when the concentration of either etchant is high, broken particles are obtained ( Figure S9). This phenomenon shows that controllable etching depends on a balanced citrate coordination-alkaline carving process. Similarly, PBAs can be etched using an equivalent concentration of NaOH as an alternative to KOH, affirming the generalization of the citrate coordination-alkaline carving process ( Figure S10).
The effect of etching treatment on the microstructural bonding properties of the PBAs is examined by Fourier transform infrared spectroscopy (FTIR) and shown in Figure S11A. The FTIR spectrum of PBAs exhibits two prominent peaks at 2165 and 2099 cm À1 originating from the stretching vibrations of Fe III CN Ni II and Fe II CN Ni II , respectively. 38 As compared with the pristine PBAs, the corner etched-PBAs shows only one sharp stretching band at a wavenumber of 2092 cm À1 , indicating the sole presence of Fe II CN Ni II in the etched structures. Notably, there occurs a slight shift in the original Fe II CN Ni II peak position, which is related to the additional potassium ions introduced to the interstitial sites ( Figure S11B) to maintain the electroneutrality of the structure during the reductive etching process. 39 Here, the FTIR study clearly reveals that etching treatment reduces the oxidation state of Fe in PBAs from Fe III to Fe II . Moreover, X-ray photoelectron spectroscopy (XPS) investigation for the Fe 2p region of PBAs and corner-etched PBAs also confirms the reduction of the iron oxidation state after the etching process ( Figure S12). The crystal structure and phase purity of the PBAs and the etched products are examined by X-ray diffraction (XRD) as shown in Figure 1I. Both PBAs and etched PBAs (corner-etched PBAs and cross-PBAs) exhibit sharp and strong peaks with similar XRD patterns with slightly different lattice parameters (10.29 and 10.37 Å, respectively), showing that the pure facecentered cubic phase of nickel-iron PBAs (JCPDS card no. 86-0501) is mostly retained during the etching treatment. The change in lattice parameter can be attributed to the increase in the radius of CN-coordinated iron during their reduction from iron (III) to the iron (II), and to an increase in the number of potassium ions. 40,41 Moreover, the absence of a hydroxide phase in the XRD patterns of the etched products eliminates the possibility of an ion exchange reaction between the hydroxide ions with the PBAs. 42 The PBAs-P, corner-etched PBAs-P and cross-PBAs-P are converted from the as-obtained nickel iron PBA coordination networks via the decomposition of a phosphorous source, NaH 2 PO 2 , at 300 C in argon atmosphere, and its reaction with the PBAs, corner-etched PBAs and cross-PBAs, respectively. SEM images in Figures 2A and  S13 show the non-destructiveness of the phosphorization treatment under the selected reaction conditions by virtue of the kinetically labile metal-ligand linkingbreakage of the coordination compound. Further SEM ( Figure 2A) and TEM ( Figure 2B) investigations of the corner-etched PBAs-P also reveal that the morphology of the corner-etched PBA precursors is well-retained during the phosphorization treatment. The presence of Fe, Ni, and P elements and their homogeneous distribution in the products are confirmed by the scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) elemental mapping as shown in Figure 2C-F. In addition, energy dispersive X-ray spectroscopy (EDX) and XPS further corroborate the presence of Ni, Fe, and P elements as well as carbon and nitrogen in the corner-etched PBAs-P ( Figure S14).
To investigate the extent of topological transformation of the PBA coordination compound into a phosphide phase and examine the nature of the carbon in the corner-etched PBAs-P, Raman spectroscopy, XRD and high-resolution XPS are carried out. The Raman spectrum of the corner-etched PBAs in Figure 2G displays two distinct peaks at 2100 and 2143 cm À1 , which are assigned to the stretching vibration of Fe II -coordinated carbon nitrogen. 43 On the other hand, in the spectrum of corner-etched PBAs-P, no obvious cyanide bands are detected, supporting the complete decomposition of the carbon-nitrogen coordination. The bands displayed at 1350 and 1583 cm À1 indicate the typical D-and G-bands, respectively, 44,45 which, in conjunction with XPS and EDX, suggests the emergence of graphitic properties of N-doped-carbon with the phosphorization treatment. The crystal structures of the phosphorization products are analyzed using XRD ( Figure 2H). The XRD profiles of the products clearly reveal the disappearance of the NiFe-PBA phase ( Figure 1I) and formation of new crystal phases, further confirming the synergistic efficacy of the phosphorization treatment and favorable dynamics of the Ni-Fe coordination network. The XRD pattern of PBAs-P can be indexed to be a mixture of Ni 5 P 4 (JCPDS no. 18-0883) and Fe 2 P (JCPDS no. 88-1803) phases, with Ni 5 P 4 as the major component. 46,47 Although the XRD spectra of corner-etched PBAs-P and cross-PBAs-P also display the diffraction features of the mixed Ni 5 P 4 and Fe 2 P phases, the Fe 2 P phase emerges as the major component. Importantly, different from the XRD pattern of PBAs-P, two additional characteristic peaks occur at 17.6 and 25.4 , corresponding to the (100) and (001) crystal planes of hexagonal Fe 2 P. This activation of the iron phosphide phase in the etched-structures can be elucidated by the modification of the framework chemistry. The coordination stability of the metallic components and cyanide ligand modified in the course of the initial PBA etching process eventually affects the bond-breaking dynamics and reactivity of the components during the final phosphorization treatment. Particularly, metalligand bond strength in cyanide coordinated frameworks is directly affected by the oxidation states of the metals. 36,37,48 As Fe III CN exhibits larger metal-carbon and carbon-nitrogen force constants (strengths) than Fe II CN, since it experiences a stronger metal-carbon σ-bond and CN bond. 36,37,48 This relative weakness of the coordinate bond in the etched samples that contain Fe II CN, allows the readily cleavage of Fe C and C N linker bonds and a kinetically facile reaction of iron with phosphorus. Therefore, the Fe 2 P crystal phase actively develops in corner-etched PBAs-P and cross-PBAs-P, whereas the Ni 5 P 4 crystal structure occurs as the major component in PBAs-P. Similarly, by using the devised method, we also modified the crystalline properties of cobalt-iron PBAs to disclose the Fe 2 P crystal planes ( Figures S15-S16). The oxidation state of the iron in the cobalt-iron PBAs was completely reduced to 2+ from 3+ by the same etching treatment, and the as-etched material was subsequently converted to a mixture of Fe 2 P and CoP crystal phases, with Fe 2 P as the major component. This indicates the adaptability of the approach, which is highly significant considering the large diversity of bimetallic iron-based PBAs.
X-ray photoelectron spectroscopy measurements are further carried out to probe the surface composition and the oxidation state of the phosphide products as shown in Figures 2I-K and S17. The high-resolution Fe 2p spectrum in Figure 2I shows two peaks, at 707.5 and 711.0 eV located in the Fe 2p 3/2 region, whereas the Fe 2p 1/2 region consists of two peaks at 720.0 and 724.5 eV, along with a satellite peak. The peaks at 707.5 and 720.0 eV are assigned to binding energies of Fe 2 P, and the other peaks can be attributed to oxide binding energies due to superficial surface oxidation of phosphides. 49,50 In the Ni 2p spectrum of corner-etched PBAs-P ( Figure 2J), the two peaks at 852.6 and 870.2 eV correspond to the Ni 2p 3/2 and Ni 2p 1/2 energy levels of Ni 5 P 4 , respectively. Meanwhile, the peaks located at 857.3 and 875.4 eV, which are accompanied by a satellite peaks, are consistent with oxidized Ni species. 47,51 The high-resolution P 2p spectrum in Figure 2K is deconvoluted into two peaks, located at 129.2 and 129.9 eV, corresponding to the P 2p 3/2 and P 2p 1/2 binding energies of reduced phosphorous in metal phosphides. Moreover, the peak occurring at 133.5 eV reflects the P O bonding (P 5+ ) on the surface, which explains the conspicuous oxygen peak observed in the XPS survey scan. 47,50,52 Noticeably, the binding energies of nickel and iron are positively shifted compared to those of metallic nickel (852.5 eV) and iron (706.7 eV), while the binding energy of the measured phosphorus peak is lower than that of elemental phosphorus (130.1 eV). This indicates the partial positive charge of nickel and iron, and partial negative charge of phosphorus in the phosphorization products, confirming the formation of a bimetallic phosphide phase.
The electrocatalytic OER activities of the PBAs compounds and their phosphorization products are investigated in 1 M KOH aqueous electrolyte. Figure 3A shows the polarization curves of the PBAs, corner-etched PBAs, cross-PBAs, PBAs-P, corner-etched PBAs-P and cross-PBAs-P obtained by linear sweep voltammetry (LSV) at a scan rate of 5 mV s À1 in the potential window of 1.2-1.85 V (vs. RHE). Compared to PBAs and etched PBAs (corner-etched PBAs and cross-PBAs), which are presented by dashed-lines, their phosphide derivatives (solid lines) exhibit greatly improved OER activities, demonstrating that the phosphorization treatment effectively modulates the electrocatalytic inertness of PBA compounds and elaborates the final crystal structure with highly exposed active sites. Among the phosphide derivatives, the corner-etched PBAs-P shows the best performance, with a small onset oxidation potential of 1.46 V and an overpotential of 260 mV at the anodic current density of 10 mA cm À2 . Notably, it only requires overpotentials of 310 and 350 mV to reach the current densities of 50 and 100 mA cm À2 , respectively. Comparatively, to attain 10 mA cm À2 , cross-PBAs-P, PBAs-P, corner-etched PBAs, cross-PBAs and PBAs require relatively higher overpotentials of 300, 314, 384, 390, and 397 mV, respectively (Table S1). The electrochemical OER kinetics of the catalysts is assessed by the Tafel slopes obtained from the Tafel plots ( Figure 3B). The Tafel slope of corneretched PBAs-P is measured to be 45 mV dec À1 , which is superior to that of cross-PBAs-P (51 mV dec À1 ) and PBAs-P (57 mV dec À1 ), and significantly lower than that of PBAs and etched PBAs, revealing the favorable OER kinetics of corner-etched PBAs-P. Figure 3C explicitly presents the Tafel slope-overpotential (at 10 mA cm À2 ) relation of PBAs and their phosphide counterparts. The phosphides are located at a low Tafel slope-low overpotential region, which confirms the prominent activation of the PBAs with phosphorization treatment. Particularly, better electrocatalytic OER performances of the corneretched PBAs-P and cross-PBAs-P than that of PBAs-P reveals the further emergence of the catalytically active sites and enhancement of the intrinsic OER activity via the etching treatment. Specifically, the corner-etched PBAs-P, which showed the lowest overpotentials, had exhibited a mass activity of 27.3 A g À1 and a high turnover frequency (TOF) of 3.18 s À1 at 350 mV overpotential ( Figure S18). The electrochemical active surface area (ECSA) further serves as another relevant measure for an assessment of the catalytic activity. The ECSA values are obtained from the cyclic voltammograms of the PBAs-P, corner-etched PBAs-P and cross-PBAs-P collected at various scan rates (10-100 mV s À1 ) (Figures S19 and 3D). Corner-etched PBAs-P and cross-PBAs-P possess ECSA values of 19.1 and 7.5 mF cm À2 , which are 3.5 and 1.4 times higher than that of the PBAs-P, respectively. This clearly supports the enriched catalytically active sites in the etched-phosphides.
Nyquist plots obtained from the electrochemical impedance spectroscopy (EIS) tests are shown in Figure 3E. The phosphide derivatives exhibit smaller semicircles compared to the pristine counterparts, thus they possess smaller charge transfer resistances. This can be ascribed to the realization of more favorable charge transport kinetics with the in-situ generation of N-dopedgraphitic carbon during the high temperature phosphorization treatment as shown in Figure 2G. Moreover, the long-term electrochemical stability of PBAs-P, corner-etched PBAs-P and cross-PBAs-P electrocatalysts is examined by chronopotentiometric measurements by applying consecutive constant current densities of 10, 20, and 10 mA cm À2 , and investigating the changes between the initial and resulting potentials during 45 h of continuous OER operation. As presented in Figure 3F, in general, PBAs-P, corner-etched PBAs-P and cross-PBAs-P show negligible voltage degradation after 45 h. Notably, after the second 15 h of operation at the higher current density of 20 mA cm À2 , the electrocatalysts could deliver the same potential at 10 mA cm À2 as the initial potential delivered after 1 h, implying their outstanding electrochemical stability.
To gain insight into the enhanced electrochemical activity of the corner-etched PBAs-P, the crystalline nanostructures are investigated by high-resolution TEM (HR-TEM) measurements. Typical TEM images of the corner-etched PBAs-P are given in Figures 4A and S20, showing the homogeneity of the nanoparticles with sharp edges, corners and well-defined faces. HRTEM images taken from the regions A, B, C, and D are presented in  Figure 2H. This suggests that the primarily exposed facet of the corneretched PBAs-P is 120 È É ( Figure S22), which is further confirmed by the fast Fourier transform image ( Figure 4B inset). In comparison, for pristine PBAs that did not undergo chemical etching, HRTEM images of their phosphorization product (PBAs-P) show an assortment of planes in random orientations ( Figure S23). Moreover, it was noted that even after long-term OER testing, the neighboring (100) and (001) planes were still clearly visible in the HRTEM images of the corner-etched PBAs-P, suggesting good stability of the exposed facet, and further substantiates the electrochemical stability of corneretched PBAs-P ( Figure S24). Figure 4F presents the atomic arrangement of the etched PBA structures before and after the phosphorization process to illustrate etched PBAs with the two enclosed adjacent (100) and (001) facets of Fe 2 P.
To understand the compositional dependent OER activity of nickel-iron phosphides, where corner-etched PBAs-P (major component: Fe 2 P) is much superior to PBAs-P (major component: Ni 5 P 4 ), density functional theory (DFT) calculations were performed. Previous experimental and theoretical reports have confirmed that the adsorption of oxygenated intermediates (especially *OH) is the rate-determining step in OER for metal phosphides. [53][54][55] The optimization of *OH adsorption on metal phosphides is thus critical for the rational design of high-performance OER catalysts. 55 Therefore, we calculated the *OH adsorption energies on the surfaces of Fe 2 P and Ni 5 P 4 ( Figure 5A, B). The Fe 2 P 120 È É facet is chosen since it is primarily exposed in the corner-etched PBAs-P (Figures 4 and 5A). It is found that all of the surface Fe sites are unable to stabilize the *OH adsorbate. Upon structural relaxations, the *OH adsorbate will migrate to the surface phosphorus sites, implying that the latter are the active adsorption sites. Figure 5C shows the calculated *OH adsorption energy on distinct phosphorous sites (marked in Figure 5A). Site P 1 is the favorable adsorption site with an adsorption energy of À0.030 eV, whereas at the P 2 site it is 0.316 eV. This value is comparable to À0.058 eV that was previously calculated for strained FeP 2 . 55 Next, the adsorption energy of *OH on the same facet of Ni 5 P 4 was calculated for comparison ( Figure 5B). As shown in Figure 5D, the most stable *OH adsorption takes place on the site P 4 with an adsorption energy of À0.765 eV, such a negative adsorption energy however suggests that Ni 5 P 4 adsorbs *OH too strongly for OER. In contrast, the moderate *OH adsorption energy of Fe 2 P is more favorable and translates to a superior electrocatalytic performance. The relatively weaker *OH adsorption on Fe 2 P compared with Ni 5 P 4 can be understood by their different adsorption configurations ( Figure 5E, F). In an octahedron-like configuration for *OH adsorption on Fe 2 P ( Figure 5E), the orbitals of the surrounding five atoms ( Figure 5E, small blue atoms) are toward the central P atom ( Figure 5E, purple atom). Such a stable configuration in Fe 2 P results in its moderate *OH adsorption. In contrast, for the tetrahedron-like configuration of Ni 5 P 4 , the orbital directions are not toward the central atom ( Figure 5F), leading to a strong *OH adsorption on Ni 5 P 4 .
Phosphide-based transition metal electrocatalysts have been well-investigated for HER as the negatively charged phosphorous can offer extra active sites to trap the positively charged protons and modulate the transition metal-phosphide bonds for more favorable hydrogen adsorption on the transition metal site. 23,56 However, phosphide-based transition metals are less-often reported as superior electrocatalysts for OER in alkaline media due to their sluggish reaction kinetics. Considering the superior OER performance of the corner-etched PBAs in alkaline media and promising HER activity of metal phosphides, we now assess the HER electrocatalytic activity of corner-etched PBAs for developing a bifunctional OER and HER catalyst for overall water splitting. The HER activity of the corner-etched PBAs-P is evaluated by LSV in 1 M KOH aqueous electrolyte at a scan rate of 5 mV s À1 ( Figure 5G). It delivers a cathodic current density of 10 mA cm À2 at an overpotential of 145 mV, which was lower compared to that of cross-PBAs-P and PBAs-P which required 182 and 221 mV, respectively. Additionally, corner-etched PBAs also exhibits the lowest Tafel slope of 73 mV dec À1 , while Tafel slopes of 107 and 119 mV dec À1 were recorded for cross-PBAs-P and PBAs-P, respectively ( Figure 5H). In addition, the overpotential and Tafel slope of the corneretched PBAs-P are better or comparable to that of other similar phosphide-based electrocatalysts assessed in alkaline media, such as iron phosphide nanoparticles, 57 Ni 5 P 4 films, 58 porous iron-doped cobalt phosphide polyhedrons, 59 ultrasmall diiron phosphide nanodots anchored on graphene sheets, 60 and iron-tuned super nickel phosphide microstructures. 61 Thus, besides being a highly active OER electrocatalyst, the corner-etched PBAs-P is also revealed to exhibit a promising electrocatalytic performance toward HER. Encouraged by the promising bifunctional OER and HER performances of the corner-etched PBAs-P in alkaline media, we assembled an electrolytic cell by integrating two corner-etched PBAs-P electrodes as both the anode and cathode. Impressively, current densities of 10 and 20 mA cm À2 are attained at considerably low cell voltages of 1.64 and 1.73 V, respectively ( Figure 5I). On the other hand, for a similar current density of 10 mA cm À2 , cross-PBAs-P and PBAs-P required cell voltages of 1.72 and 1.75 V, respectively. Subsequently, chromoamperometric test was also conducted on the electrolytic cell setup with corneretched PBAs-P electrodes at constant potentials of 1.64 V over 45 h ( Figure 5J). The cell showed long term sustained performances with no depreciation in current densities. This indicates the impressive long-term durability of the electrocatalyst for overall water splitting, and unveils corner-etched PBAs-P to be not only highly active, but also a very stable bifunctional electrocatalyst in alkaline media.

| CONCLUSION
In summary, iron phosphide with highly active, exposed 120 È É facets is preferentially prepared, making use of the labile coordination network of PBAs. The obtained corner-etched PBAs-P exhibits outstanding activity toward OER with a small Tafel slope of 45 mV dec À1 and an overpotential of 260 mV at a current density of 10 mA cm À2 , as well as remarkable electrochemical stability after long operation. Such impressive OER performance originating from the active sites can be attributed to its optimal reaction intermediates adsorption energy. Furthermore, the catalyst also demonstrates activity toward overall water splitting, requiring a small cell voltage of 1.64 V to drive a current density of 10 mA cm À2 . This work paves a new avenue towards the rational design and exploitation of metal organic frameworks for selective synthesis of highly active phosphide OER electrocatalysts.