Highly Active and Abundant MAB Phases Ni n þ 1 ZnB n (n ¼ 1, 2) toward Hydrogen Evolution

Fossil fuels attenuation and their hazardous impact on the environment have made alternative renewable energy sources highly attractive. Among all the renewable clean energy sources, hydrogen has been one of the most promising candidates due to its high energy density and numerous production routes. Electrochemical water splitting through hydrogen evolution reaction (HER) is the most attractive method to produce hydrogen. However, high cost and scarcity of the most efficient electrocatalysts (platinum group metals) have made the process more challenging. Thus, the study of highly active and abundant electrocatalysts would provide a path for a more sustainable and clean energy supply. 2D materials such as graphene, borophene, phosphorene, h-BN, transition metal dichalcogenides (TMDs), etc. have recently received a great deal of attention due to their unique and enhanced properties if compared with their parental bulk forms. MoS2, which is one of the most studied 2D materials for the HER is known to have only active edge sites, thus its basal plane is inactive making it a poor catalyst in bulk form. In fact, most TMDs have the same behavior. Therefore, discovering other 2D/layered materials which show HER active basal plane is of high current interest. Recently, a new family of 2D carbides and nitrides named MXenes have seen high interest for a wide variety of applications including HER. These MXenes are directly exfoliated from their parent-layered MAX phases (Mnþ1AXn where M: early transition metal, A: main group element, X: C/N) by selectively etching out the “A” atoms. Similar to most TMDs, MAX electrocatalysts are not competitive toward HER. In fact, they exhibit overpotentials (η10 at 10mA cm ) ranging from 0.74 to 0.88 V to drive a current density of 10mA cm . A MAX-related family of layered ternary borides, called MAB phases is currently receiving increased interest. Compared with MAX phases, MAB phases hold a large variety of structures. Typical MAB phases contain aluminum as the “A” element and exhibit orthorhombic symmetry. However, they also include phases such as hexagonal Ti2InB2, [17] tetragonal Y5Si2B8, [18] tetragonal Ru2ZnB2, [19] hexagonal Cr5Si3B, [20] hexagonal i-MAB phases, and monoclinic Ninþ1ZnBn. [22] Even though these MAB phases are chemically and structurally different from the MAX phases, they have similar properties such as high electrical conductivity, stiffness, and resistance to thermal shock. In contrast to MAX phases whose chemical exfoliations to produce MXenes have been extensively studied, the exfoliation of MAB phases is still in its infancy, and only partially etched samples have been reported. Interestingly, nonetched and partially etched MoAlB samples are active electrocatalysts for HER with η10 values of 0.40 and 0.31 V, respectively. The activity was ascribed to the exposed active basal plane sites due to partial Al-etching. Similarly, bulk Fe2AlB2 was found to be an excellent OER electrocatalyst. Consequently, the studied A. A. Rezaie, Prof. B. P. T. Fokwa Materials Science and Engineering Program University of California, Riverside 92521 Riverside, CA, USA E-mail: bfokwa@ucr.edu E. Lee, Prof. B. P. T. Fokwa Department of Chemical and Environmental Engineering University of California Riverside, CA 92521, USA J. A. Yapo, Prof. B. P. T. Fokwa Department of Chemistry University of California, Riverside 92521 Riverside, CA, USA


Introduction
Fossil fuels attenuation and their hazardous impact on the environment have made alternative renewable energy sources highly attractive. Among all the renewable clean energy sources, hydrogen has been one of the most promising candidates due to its high energy density and numerous production routes. [1] Electrochemical water splitting through hydrogen evolution reaction (HER) is the most attractive method to produce hydrogen. [2] However, high cost and scarcity of the most efficient electrocatalysts (platinum group metals) have made the process more challenging. [3] Thus, the study of highly active and abundant electrocatalysts would provide a path for a more sustainable and clean energy supply.
2D materials such as graphene, [4] borophene, [5] phosphorene, [6] h-BN, [7] transition metal dichalcogenides (TMDs), [8] etc. have recently received a great deal of attention due to their unique and enhanced properties if compared with their parental bulk forms. MoS 2 , which is one of the most studied 2D materials for the HER is known to have only active edge sites, thus its basal plane is inactive making it a poor catalyst in bulk form. [9] In fact, most TMDs have the same behavior. Therefore, discovering other 2D/layered materials which show HER active basal plane is of high current interest. [10] Recently, a new family of 2D carbides and nitrides named MXenes have seen high interest for a wide variety of applications including HER. [11] These MXenes are directly exfoliated from their parent-layered MAX phases (M nþ1 AX n where M: early transition metal, A: main group element, X: C/N) by selectively etching out the "A" atoms. [12,13] Similar to most TMDs, MAX electrocatalysts are not competitive toward HER. [14,15] In fact, they exhibit overpotentials (η 10 at À10 mA cm À2 ) ranging from 0.74 to À0.88 V to drive a current density of À10 mA cm À2 . [14] A MAX-related family of layered ternary borides, called MAB phases is currently receiving increased interest. [16] Compared with MAX phases, MAB phases hold a large variety of structures. Typical MAB phases contain aluminum as the "A" element and exhibit orthorhombic symmetry. However, they also include phases such as hexagonal Ti 2 InB 2 , [17] tetragonal Y 5 Si 2 B 8 , [18] tetragonal Ru 2 ZnB 2 , [19] hexagonal Cr 5 Si 3 B, [20] hexagonal i-MAB phases, [21] and monoclinic Ni nþ1 ZnB n . [22] Even though these MAB phases are chemically and structurally different from the MAX phases, they have similar properties such as high electrical conductivity, [23] stiffness, [23] and resistance to thermal shock. [20] In contrast to MAX phases whose chemical exfoliations to produce MXenes have been extensively studied, the exfoliation of MAB phases is still in its infancy, and only partially etched samples have been reported. [24][25][26] Interestingly, nonetched and partially etched MoAlB samples are active electrocatalysts for HER with η 10 values of À0.40 and À0.31 V, respectively. [25] The activity was ascribed to the exposed active basal plane sites due to partial Al-etching. Similarly, bulk Fe 2 AlB 2 was found to be an excellent OER electrocatalyst. [27] Consequently, the studied DOI: 10.1002/aesr.202100052 Whilst MXenes (2D carbides and nitrides) have become highly popular in several research fields including the hydrogen evolution reaction (HER), unfortunately they are not competitive HER electrocatalysts in their bulk form (MAX phases). The related MAB (2D-like bulk borides) phases and the derived 2D MBenes, however, are less studied but show better HER properties. Herein, two highly HER-active and abundant MAB phases, Ni nþ1 ZnB n (n ¼ 1, 2), are studied experimentally and computationally. The pressed pellet electrodes from bulk polycrystalline powders of these phases drive a current density of 10 mA cm À2 at impressive overpotentials of η 10 ¼ À0.171 V (n ¼ 1) and η 10 ¼ À0.145 V (n ¼ 2) to efficiently produce hydrogen. Density functional theory (DFT) calculations prove that the most active site is the hollow site on the nickel basal plane, showing a free energy value comparable to that of the hollow site of Pt (111). This study paves the way for further development of bulk and nanoscale MAB phases for clean energy applications.
bulk MAB phases have shown far better electrocatalytic properties than their analogous MAX phases, hinting at an even greater potential for MBenes (fully exfoliated MABs) as future electrocatalyst candidates. While the synthesis of MBenes is still elusive they have been predicted as promising alternative candidates to substitute Pt as HER catalysts. [28] Further theoretical studies have suggested MBenes as potential catalysts for nitrogen reduction reaction (NRR) and HER upon embedding single atoms such as V and Zn. For example the calculated Gibbs free energy (ΔG H ) values for Zn-W 2 B 2 O 2 and V-W 2 B 2 O 2 are close to the upper point of the volcano plot (ΔG H ¼ À0.146 and 0.013 eV, respectively). [29] However, even the bulk MAB phases are still experimentally unexplored and warrant more attention given the aforementioned results and the fact that abundant bulk borides, especially those with layer-like AlB 2 -type structures have been demonstrated to have excellent HER activities. [30] Herein, we investigate Ni nþ1 ZnB n MAB borides as HER catalysts for the first time. We demonstrate that pellet electrodes of Ni nþ1 ZnB n (n ¼ 1, 2) exhibit abundant basal plane active sites that drive a 10 mA cm À2 current density at the lowest overpotential of all MAB, MAX phases, and their derivatives MXenes so far reported. In addition, we have carried out free energy density functional theory (DFT) calculations which have indeed identified many active sites on the Ni basal planes of these MAB phases.

Results and Discussion
The Ni-Zn-B system is very attractive and many ternary phases have been discovered. [31,32] Among these phases, Ni 2 ZnB and Ni 3 ZnB 2 have an atomically laminated structure similar to MAX/MAB phases (Figure 1, inset). On the one hand, these monoclinic structures differ from the typical MAX hexagonal symmetry and the common MAB orthorhombic symmetry. On the other hand, they hold a general chemical composition (Ni nþ1 ZnB n ) similar to MAX phases (M nþ1 AX n ). [22,32] Samples of Ni nþ1 ZnB n (n ¼ 1, 2) were synthesized as described previously (also explained in the Supporting Information). [22] As shown in Figure 1a,c, the Rietveld-refined powder X-ray diffraction (PXRD) patterns of both bulk phases show single-phase products. However, severe intensity mismatches were observed mainly for the (00l) reflections, but they were corrected using preferred orientation refinement along the c-axis, indicating a possible layer-like morphology of the samples. Indeed, scanning electron microscopy (SEM) images of the bulk products confirmed the expected stacking of thin sheets (Figure 1, inset). Furthermore, the presence of all elements and their homogenous distribution were confirmed through energy dispersive X-ray spectroscopy (EDS) and mapping, respectively ( Figure S1, Supporting Information).
X-ray photoelectron spectroscopy (XPS) was used to investigate the surface species on the synthesized Ni 3 ZnB 2 and Ni 2 ZnB powders. Figure 2 shows that the Ni 2p, Zn 2p, and B 1s spectra of the two phases are expectedly similar, and the peak fitting values are given in the Supporting Information (Table S2). The Ni 2p spectrum is deconvoluted into Ni 2p 1/2 and Ni 2p 3/2 , which are composed of two different peaks corresponding to Ni 2þ and Ni 0 species. [33] In both phases, the metallic Ni 0 peaks show by far the highest intensities if compared with the Ni 2þ peaks, indicating that the nickel oxide layer (from atmospheric exposure) present on the surface of these MAB phases is so thin that the bulk boride underneath dominates the XPS spectrum. A similar observation can be made for the Zn species, but the opposite situation is found for the B species. The Zn 2p spectrum is deconvoluted into Zn 2p 1/2 and Zn 2p 3/2 which are each composed of Zn 2þ peaks from the thin zinc oxide layer and the Zn 0 peaks of the boride underneath. [34] The B 1s spectra of both Ni 2 ZnB and Ni 3 ZnB 2 are each composed of two peaks corresponding to B 3þ and B 0 , the oxide peak being more intense than the boride one. These XPS results indicate that all the aforementioned species are present on the surfaces of these materials, thus suggesting them as potentially active species for HER.
To investigate the electrocatalytic HER activity of Ni nþ1 ZnB n , the crushed particles (Figure 3a,c) were drop casted on a carbon cloth electrode using Nafion as a binder and examined in a 1 M KOH solution (check the Supporting Information for more details). The linear sweep voltammetry (LSV) curves show good performances for Ni 3 ZnB 2 and Ni 2 ZnB, as they require overpotentials (η 10 ) of À0.25 and À0.31 V, respectively, to reach a current density of 10 mA cm À2 (Figure 4a). These values far  exceed those of other experimentally evaluated MAB/MAX phases for HER, to the best of our knowledge. MAX phases generally do not show good catalytical activity. Among all the MAX phases, Mo 2 Ga 2 C was reported as one of the most promising HER electrocatalysts with an overpotential η 10 ¼ À0.74 V, [11] and the two most studied MAX phases, Ti 2 AlC and Ti 3 AlC 2 , have even higher overpotential values (Figure 4d). [14] Interestingly, the few studied MAB phases are better electrocatalysts if compared with the MAX phases. MoAlB for example has the best HER activity so far with an overpotential of À0.40 V at 10 mA cm À2 for nonetched crystals. [25] The lower overpotential of Ni 3 ZnB 2 if compared to Ni 2 ZnB may be due to the different metal to boron ratios. Previously, our group discovered a boron dependency of molybdenum and vanadium borides electrocatalysts for   These arguments indeed support the 18% lower overpotential observed for Ni 3 ZnB 2 . As shown in Figure 1 and 3 and as reported previously, [22] the investigated bulk polycrystalline Ni 2 ZnB and Ni 3 ZnB 2 phases already hold a preferred orientation along the c direction (dominating basal planes), meaning that the basal planes may be preferably exposed during HER, hinting at a dominant role for the basal planes in the overall activity of these borides.
To study the effect of the basal plane on the HER activity, new electrodes were prepared by pressing the powdered MAB phases to pellets (using a hydraulic press at room temperature, see Supporting Information for more details). This method exposes more basal planes of the MAB particles parallel to the force direction, forming a pellet with even more pronounced preferred orientation along the [001] direction, as confirmed by PXRD ( Figure 1) and through SEM images of the electrodes (Figure 3b,d). In fact, the PXRD patterns of the pressed pellets showed a single-crystal-like behavior with mainly intensified (00l) peaks. Other peaks were observed in the PXRD but with much smaller intensities if compared with the (00l) peaks. For instance, the ð11 À 3Þ peak for Ni 3 ZnB 2 should have been the highest intensity peak based on the theoretical pattern. However, after pressing, only a small trace of this peak still prevailed while the (00l) peaks increased. To measure the HER activity of these pellet electrodes, the edges were covered with epoxy. Astonishingly, the overpotentials of these pellet electrodes were lowered by 42-44% if compared with the drop casted electrodes, reaching overpotentials of η 10 ¼ À0.14 V and À0.17 V for Ni 3 ZnB 2 and Ni 2 ZnB, respectively. These values are the lowest reported experimentally for any form of layered ternary metalrich borides, to the best of our knowledge. This finding was further confirmed by measurements of the electrochemically active surface area through the double layer capacitance values (C dl ¼ 22.3 mF cm À2 for n ¼ 1 and 35.4 mF cm À2 for n ¼ 2) which were noticeably higher if compared with those of the drop casted electrodes ( Figure S2 and S3, Supporting Information). This drastic increase in the pellet's active area sites points to the dominating role of the basal plane in the HER activity of these materials. The pellet also provides tightly packed MAB particles, thus facilitating the interlayer electron transfer in the electrode, [35] thereby increasing the activity. This was confirmed Figure 4. a) Polarization curves of various Ni nþ1 ZnB n MAB electrodes in 1 M KOH at a scan rate of 5 mV s À1 with ohmic potential correction. b) Tafel plots derived from the polarization curves. c) Electrochemical impedance spectra (Nyquist plots): The points represent the experimental data, and the solid curves indicate the fitting lines. d) Overpotential of reported bulk MAB and MAX phases compared with different Ni nþ1 ZnB n electrodes (* obtained from the study by Rosli et al. [14] ).
www.advancedsciencenews.com www.advenergysustres.com through charge transfer resistance (R ct ) measurements. In both cases, the pressed pellets had much lower R ct values if compared with the drop casted electrodes (Figure 4c and Table S1, Supporting Information). Also, the Tafel plots (Figure 4b) of the pellets show smaller Tafel slope values if compared with the drop casted electrodes. These values fall near 120 mV dec À1 for the drop casted electrodes indicating that the Volmer reaction strongly controls the HER process of these electrodes. [36] However, for the pellets, the Tafel slope values fall in between that of the Volmer reaction (Tafel slope %120 mV dec À1 ) and the Heyrovsky reaction (Tafel slope %40 mV dec À1 ), suggesting a more complex mechanism. Finally, the pellet electrodes were investigated for their long-term stability. The recorded continuous cyclic voltammetry (CV) curves for both pellet electrodes showed good stabilities after 3000 cycles with little change in the overpotential ( Figure S4, Supporting Information). Also, at high current density (À100 mA cm À2 ) the pellet electrodes showed better stabilities and activities if compared with the dropcasted ones ( Figure S5, Supporting Information).
To understand the origin of high basal plane HER activity in these MAB borides, DFT calculations were carried out using the Gibbs free energy (ΔG H ) as a descriptor. In fact, ΔG H for atomic hydrogen adsorbing on a catalyst's surface has been shown to correlate with the experimentally measured HER activity for a variety of systems. [37] Theoretically, the optimal HER activity is achieved when the ΔG H value is close to zero as this ensures that the reaction rates for both H adsorbing and H 2 desorbing onto/off the surface is maximized. In this study, DFT calculations (at 25% H coverage) were applied to survey the active sites on two surfaces for both Ni 2 ZnB and Ni 3 ZnB 2 . Experimentally, the (00l) planes were found to dominate the PXRD patterns (Figure 1), indicating that they are the most exposed surfaces. Therefore, we have studied the (001) Ni-and Zn-terminated surfaces for their HER activities. It should be noted that the (00l) layers containing B atoms were not studied as for both Ni 2 ZnB and Ni 3 ZnB 2 structures the B atoms bond covalently with each other and with Ni atoms, in contrast to the weaker metallic bonds between Ni and Zn. The results for the Zn-terminated layer show that the considered sites are not as active as those on the Ni-terminated layer, thus they are not further discussed. The computed results for the Ni exposed surface for both materials are shown in Figure 5a. The following three sites were found to be the most active for each surface (Figure 5b,c); the hollow site (Hol) between three Ni atoms, the top site on a Ni atom (Top), and the site bridging two Ni atoms (Brdg). The calculated ΔG H values for Ni 2 ZnB were: À0.12 eV (Hol), 0.19 eV (Brdg), and 0.44 eV (Top); whereas À0.14 eV (Hol), 0.21 eV (Brdg), and 0.31 eV (Top) were obtained for Ni 3 ZnB 2 . Consequently, the Hol site in both materials is the most HER active with ΔG H values similar to that calculated for Hol Pt (111). [38] While the Top sites were the least active, they were still low enough to be considered as active sites. The ΔG H values of Hol and Brdg sites are almost the same (0.02 eV difference) for the two phases, whereas those of the Top sites are significantly different (0.13 eV lower for Ni 3 ZnB 2 ). This may explain the significantly better HER activity found experimentally for Ni 3 ZnB 2 . These DFT results validate the experimental findings of high HER activity of both Ni 2 ZnB and Ni 3 ZnB 2 MAB phases, the latter being more active.

Conclusion
The HER properties of different electrodes (drop casted and pressed pellet) of Ni nþ1 ZnB n (n ¼ 1, 2) MAB phases were investigated for the first time. The electrodes, analyzed through PXRD and SEM, show high preferred orientation along the c-axis (exposed basal planes), the pressed pellet showing the highest. The pressed electrode had the best HER performance showing low overpotentials to drive a 10 mA cm À2 current density of -0.17 and -0.14 V, for Ni 2 ZnB and Ni 3 ZnB 2 , respectively. DFT free energy calculations showed that the nickel basal plane is more active than the zinc plane and that the hollow site is the most active site for both materials with free energy values close to that of the hollow site of Pt (111), thus supporting the experimental results. This study introduces Ni nþ1 ZnB n as active basal plane electrocatalysts for hydrogen evolution and paves the way for applications of these and other MAB phases in clean energy reactions.