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Aggregation-Resistant 3D MXene Based Architecture as Efficient Bifunctional Electrocatalyst for Overall Water Splitting Luyang Xiu, Zhiyu Wang, Mengzhou Yu, xianhong wu, and Jieshan Qiu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b02849 • Publication Date (Web): 26 Jul 2018 Downloaded from http://pubs.acs.org on July 26, 2018

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Aggregation-Resistant 3D MXene Based Architecture as Efficient Bifunctional Electrocatalyst for Overall Water Splitting Luyang Xiu, Zhiyu Wang,* Mengzhou Yu, Xianhong Wu and Jieshan Qiu* State Key Lab of Fine Chemicals, School of Chemical Engineering, Liaoning Key Lab for Energy Materials and Chemical Engineering, Dalian University of Technology, Dalian 116024, Liaoning, China E-mail: [email protected], [email protected]

Abstract The MXene combining high conductivity, hydrophilic surface and wide chemical variety has been recognized as a rapidly rising star on the horizon of two-dimensional (2D) material science. However, strong tendency to intersheet aggregate via van der Waals force represents a major problem limiting the functionalities, processability and performance of MXene-based material/devices. We report a capillary-forced assembling strategy for processing MXene to hierarchical 3D architecture with geometry-based high resistance to aggregation. Aggregate-resistant properties of 3D MXene not only double the surface area without loss of the intrinsic properties of MXene, but render the characteristics such as kinetics-favorable framework, high robustness and excellent processability in both solution and solid state. Synergistically coupling the 3D MXene with electrochemically active phases such as metal oxide/phosphides, noble metals or sulfur yields the hybrid systems with greatly boosted active surface area, charge transfer kinetics and mass diffusion rate. Specifically, the CoP-3D MXene hybrids exhibit high electrocatalytic activity towards oxygen and hydrogen evolution in alkaline electrolyte. As a bifunctional electrocatalyst, they exhibit superior cell voltage and durability to combined RuO2/Pt catalysts for overall water-splitting in basic solution, highlighting the great promise of aggregation-resistant 3D MXene in the development of high-performance electrocatalyst. 1 ACS Paragon Plus Environment

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Keywords:

MXene,

3D

architecture,

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aggregation-resistant

structure,

bifunctional

electrocatalyst, water splitting

The MXene has emerged as an attractive class of two-dimensional (2D) material with unusual structure and properties.1 They have a general formula of Mn+1XnTx (n = 1-3), where M is an early transition metal (such as Ti, V, Nb, Mo, etc.), X is the C and/or N element and T stands for the chemical groups such as -OH, -O, -Cl and -F.2, 3 So far, above 20 kinds of MXene have been obtained by selectively etching the layered MAX phase, where A is a group IIIA or IVA element (e.g., Al, Ga, etc.), and dozens more are predicted to exist.2 Their advantages lie in wide chemical and structural variety with a well combination of attractive properties such as high conductivity associated with high electron density of states near Fermi level, excellent hydrophily, good mechanical stability and rich surface chemistry enabled by chemical groups grafted.1,

2

Recently, the Ti3C2 and Nb2C MXene also exhibit good

biocompatibility and near-infrared adsorption for in vivo photothermal therapy.4 These merits make the MXene very promising in vast fields such as energy storage, catalysis, transparent electronic

devices,

separation

membrane,

sensors,

electromagnetic shielding and biomedicine, etc.1,

2, 5-8

reinforcement

for

composites,

On this basis, the commercial

availability of many MAX phases (e.g., Ti3AlC2, Ti2AlC, Ti3SiC2 and Nb2AlC, etc.) further guarantees the scalable production and practical use of MXene-based materials. Taking advantage of kinetic-favorable layered nanostructure, high conductivity, good surface reactivity and superior density to soft carbon, the MXene has attracted particular interests for electrochemical energy storage and conversion applications.2,

9

For example,

Ti3C2 MXene has exhibited outstanding volumetric capacitances up to 1500 F cm-3 in supercapacitors due to the accelerated pseudocapacitive charge storage and high density.2, 10, 11 Excellent high-rate capability (e.g., 36 C) could be also achieved for lithium storage due to low ionic diffusion barrier for Li+ (0.07 eV) and the pseudocapacitive behavior.12, 13 Various 2 ACS Paragon Plus Environment

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MXene-based hybrid systems have been also fabricated by coupling different MXene with electrochemically active phase such as carbon nanotubes, graphene, conductive polymers, graphitic C3N4, sulfur, MoS2, layered metal hydroxide, metal oxides/sulfides, metal-organic framework, Sn-PVP complex for high-performance supercapacitor, rechargeable batteries and electrocatalysis.2,

5, 6, 14-22

Like most 2D materials, however, the MXene also suffers from

strong tendency of intersheet aggregation via van der Waals (vdW) attraction and hydrogen bonding.23 This drawback causes the performance degradation associated with the loss of surface area and difficulty in processability of MXene-based materials. Freeze-drying and/or introducing interlayer spacers such as polymers, nanoparticles, large ions, nanotubes or even gaseous species between MXene nanosheets is effective to mitigate this problem.24-35 Nevertheless, the restacking of freeze-dried MXene may still occur in the process (e.g., drying and pressing) of electrode manufacturing. While the presence of the spacers may change or affect adversely the intrinsic properties and surface chemistry of MXene. Recently, a template-engaged strategy was developed for processing the MXene to 3D macroporous film assembled from hollow nanospheres.23 The hollow shells of MXene effectively inhibit the intershet aggregation, resulting in high ioinc accessibility and enhanced pseudocapacitive capacity for sodium storage. Nevertheless, the development of the strategy for easy and scalable production of MXene-based materials with favorable aggregation-resistant properties and good processability is still very challenging towards the practical use. Electrochemical water splitting for hydrogen/oxygen production has received extensive attention because of the high efficiency for sustainable yet clean energy harvesting especially when driven by solar energy.36-40 Usually, active electrocatalyst with high conductivity is indispensable to accelerate the charge transfer kinetics and reduce the overpotential for its half reactions, i.e., the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).41, 42

Precious metal based catalysts show desirable activity for both reactions but suffer from

unaffordable cost and limited performance for overall process of water splitting. It boosts 3 ACS Paragon Plus Environment

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great efforts in seeking for efficient yet cost-effective alternatives for practical use. With high conductivity, large surface area and wide chemical variety, the MXene represent the appealing active phase or matrix materials for developing high-performance electrocatalysts. Active

electrocatalysts

have

been

demonstrated

by

coupling

the

MXene

with

electrocatalytically active graphitic C3N4, layered metal hydroxide and Co-based MOF for catalyzing the OER in alkaline electrolyte.16, 20, 43 The nanohybrids of MXene and few-layered MoS2 were also fabricated as highly active catalysts towards HER in acidic electrolyte.25 Nevertheless, the aggregation of MXene nanosheets still limits the accessibility and exchange of ions at the electrode-electrolyte-gas triple-phase interface and hinders the full exploitation of the potential of MXene-based electrocatalysts. To fulfill the practical demands, the MXene-based electrocatalyst that can simultaneously catalyze the OER and HER in the same electrolyte is also highly desired to reduce the cost and complexity of overall water splitting system. In this work, we report a facile strategy for processing the MXene to 3D architecture with hierarchical structure and aggregation-resistant properties by isotropically capillary forced assembly of MXene nanosheets. The 3D MXene architectures not only inherit the attractive properties of MXene nanosheets, but also receive extra benefits such as large surface area, high robustness, 3D conductive network and geometry-based resistance to aggregation in both solution with different polarity and solid state (Figure 1a). Assembling the MXene nanosheets to particulate with submicrometer size may also help to deal with the drawback of nanomaterials induced by size effect such as poor processability, nano-pollution and possible biotoxicity with less sacrifice of their intrinsic properties. Substantial merits allow 3D MXene to be (e.g., Co3O4, SnO2, perovskite-type MnTiO3, Pt, Ag, and sulfur) for engineering the hybrid systems with a broad spectrum of functionalities. Specifically, an efficient electrocatalyst is fabricated by homogenously dispersing ultrafine CoP nanocrystallites on 3D Ti3C MXene architecture (denoted as CoP@3D Ti3C2-MXene). It renders highly exposed 4 ACS Paragon Plus Environment

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active surface, kinetics-favorable structure and high conductivity for accelerating oxygen/hydrogen evolution at the electrode-electrolyte-gas triple-phase interface. As a nonprecious bifunctional electrocatalyst, the catalyst exhibits high activity towards OER/HER, low cell voltage and good durability for driving overall water splitting in KOH solution.

Results and discussion The 3D MXene architecture is fabricated by ultrasonic-assisted aerosol spray drying of MXene colloids (Figure 1b). High-quality Ti3C2 MXene nanosheets with an average lateral size of hundreds of nanometers are firstly made by selectively etching the Al layers in Ti3AlC2 MAX phase with LiF/HCl followed by ultrasonic exfoliation in Ar-protected degassed water to minimize the oxidation (Figure S1). Subsequently, the MXene colloids with certain concentration (e.g., 5 mg mL-1) is ultrasonically nebulized to form aerosol droplets, which are carried by Ar gas through a tube furnace preheated at a desired temperature (e.g., 600oC). Because of the average size of the aerosol droplets is around several micrometers, the solvents could be rapidly evaporated upon heating. The resultant inward capillary force leads to the isotropic compression and fast assembly of MXene nanosheets to 3D architectures with fluffy shape after fully evaporation of the solvent (Figure 2a). Scanning electron microscopy (SEM) reveals the rough surface of 3D Ti3C2 MXene architectures with numerous ridges. It reduces the contact area and therefore results in minimal adhesion between individual particles (Figure 2b). Within 3D architecture, the aggregation of MXene nanosheets is also effectively inhibited because of too short time scale to restack the randomly distributed sheets and conquered vdW attraction between individual sheets by gases released from rapid evaporation of the droplets (Figure 2c and Figure 2d). The structure of 3D MXene architecture could be precisely tailored by tuning the properties of precursor colloids, which affects the size, stability and surface characteristics of ultrasonic generated aerosol droplets. For example, using appropriate amount of surfactant (e.g., polyvinylpyrrolidone, PVP) may help to reduce 5 ACS Paragon Plus Environment

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the surface tension and maintain the original shape of the aerosol droplets during drying. It leads to the sphere-like 3D MXene architectures with loose interior (Figure 2e-g). The average size of 3D MXene architectures could be also varied from 0.5 to 3.5 µm by tuning the initial concentration of MXene colloids from 0.5 to 5.0 mg mL-1 (Figure 2h). Generally, the 3D MXene architectures are much larger than the MXene nanosheets in particle size. They apparently are the assembly of many MXene sheets instead of a single crumpled one. Unlike flexible graphene,44,

45

the folding of a single MXene sheet with smaller size but higher

rigidity to complex structure with high curvature is much difficult. The structure and phase characteristics of 3D Ti3C2 MXene architecture is examined by Xray diffraction (XRD), as shown in Figure 3a. It indicates the formation of MXene phase by the strong (002) peak along with weak peaks from the (004), (006), (008), (0010), (0012) and (110) planes of Ti3C2 MXene.9 Compared to pristine sample, the peak from the (002) plane of Ti3C2 MXene in 3D architecture is shifted to a lower angle with greatly reduced intensity because of the bending and significantly suppressed stacking of MXene sheets with random orientation.23 No signals from possible impurities such as TiO2 were detected. The X-ray photoelectron spectroscopy (XPS) shows that 3D Ti3C2 MXene share similar feature with pristine Ti3C2 MXene in peak position and intensity of Ti 2p XPS spectrum, as characterized by two pairs of strong 2p3/2 and 2p1/2 doublets at 455.5 and 461.4 eV, respectively (Figure 3b). High-resolution C 1s XPS spectrum of 3D Ti3C2 MXene can be deconvoluted into five peaks at 281.4, 282.2, 284.6, 286.7 and 288.7 eV, corresponding to C-Ti, C-Ti-O, C-C, C-OH and O-C=O bonds (Figure S2).30, 43 High intensity of peak for sp2-hybridized carbon (284.6 eV) suggests the possible presence of carbide-derived carbon on MXene surface. They are generated by etching Ti3C2 structure during the preparation of MXene. In Raman spectrum of 3D Ti3C2 MXene (Figure S3), the modes at 212 and 732 cm-1 are A1g symmetry out-of-plane vibrations of Ti and C atoms in Ti3C2 MXene, respectively.46 While the modes at 284, 382 and 625 cm-1 corresponds to the Eg vibrations of in-plane Ti, C and surface functional group 6 ACS Paragon Plus Environment

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atoms.46 The signal of TiO2 is invisible, while weak bands at 1200-1690 cm-1 indicate the presence of small amount of carbide-derived carbon in the sample. The FT-IR spectra of 3D Ti3C2 MXene reveals the presence of the adsorption peaks at 3430 cm-1 for asymmetric stretch of -OH, 2971 cm-1 for asymmetric stretch of C-H, 2920 cm-1 for asymmetric vibration of -OH, 1623 cm-1 for bending vibration of -OH, 1144 cm-1 for C-O bonds and 1045-1080 cm-1 for C-F bonds (Figure 3c). Compared to pristine Ti3C2 MXene, the intensity of the signals from -OH groups are reduced as a result of the removal of water and a part of hydroxyl groups from the surface of 3D Ti3C2-MXene architecture upon spray drying. Nevertheless, rich amount (up to 23.64 wt.%) of the chemical groups could remain to guarantee high surface reactivity thank to the very short time scale for drying (typically in seconds) at relatively high temperature (Figure S4). Four-probe tests indicate the high electrical conductivity (ca. 120 S cm-1) of pellet made by compressing 3D Ti3C2 MXene architecture under 20 MPa (Figure 3d). This value is lower than that of pristine Ti3C2 MXene (175 S cm-1) under identical pressure but still sufficient for electrochemical applications. In fact, the intrinsic conductivity of 3D Ti3C2 MXene architecture is substantially underestimated owing the limited interparticulate contact. Since there is no intimate contact with each other, the vdW attraction between individual 3D MXene particles is so weak that they could be stable against the aggregation in both solution and solid state. Without further chemical modification, they can be easily dispersed in common solvents with different density and polarity such as water, ethanol, ethyl acetate, N-methylpyrrolidone (NMP), cyclohexane and pump oil by hand shaking and keep stable for over 4 h (Figure 3e). In contrast, the pristine Ti3C2 MXene is rapidly precipitated in some of these solvents, especially for cyclohexane and ethyl acetate, even after sonication. In addition to the improved processability in solution, the 3D Ti3C2 MXene architectures also exhibit good resistance against the aggregation in the solid state. Even being compressed under 150 MPa, their structure is still largely unaffected and the resultant pellet can be readily redispersed in water by hand shaking (Figure 3g). In contrast, the mechanical reorganization 7 ACS Paragon Plus Environment

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and extensively irreversible stacking of MXene sheets occurs after compressing the pristine Ti3C2 MXene under the same pressure. As a result, the formed pellets are hardly to be redispersed by ultrasonication, like the case happened in vacuum filtrated film comprised of densely packed MXene nanosheets (Figure 3h and Figure 3i). The aggregation-resistant 3D MXene architectures not only causes improved processability, but also greatly suppresses the loss of surface area. The N2 adsorption/desorption isotherm of 3D MXene architectures validates a typical mesoporous characteristic with a type IV isotherm and a type H2 hysteresis loop (Figure 3f). Much larger hysteresis loop of 3D MXene architectures than that of pristine Ti3C2 MXene and vacuum filtrated film indicates the generation of extra mesopores via effective suppression of intersheet aggregation, which is consistent with the pore size distribution analysis (Figure S5). As a result, the 3D MXene architectures deliver a high Brunauer-Emmett-Teller (BET) specific surface area of 165.3 m2 g-1 and pore volume of 0.346 cm3 g-1, outperforming the pristine Ti3C2 MXene (68.5 m2 g-1, 0.146 cm3 g-1) and vacuum filtrated film of Ti3C2 MXene (18.8 m2 g-1, 0.03 cm3 g-1). With high robustness, aggregate-resistant properties, large surface area and high conductivity, the 3D MXene architecture represents an appealing host for developing a huge family of hybrid systems with different functionalities. Typically, hierarchical hybrid systems are constructed by dispersing the nanoparticles of a variety of materials such as transitional and main-group metal oxides (e.g., SnO2, Co3O4), perovskite-type metal oxide (e.g., MnTiO3) and noble metals (e.g., Pt, Ag) on 3D MXene architectures via aerosol spray drying of the MXene colloids containing the corresponding metal salts. To this end, an appropriate amount of PVP is necessary to maintain the stability of MXene colloids in the presence of metal salts (Figure S6). Ultrafine nanoparticles with an average size of several nanometers are uniformly dispersed on the surface of 3D Ti3C2 MXene for Co3O4, SnO2, MnTiO3 and Pt (Figure 4a-h). For Ag, larger nanoparticles of tens of nanometers in size are anchored due to rapid electrostatic adsorption and reduction of highly-reactive Ag+ with MXene with negatively 8 ACS Paragon Plus Environment

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charge surface (Figure 4i and Figure 4j). A layer of sulfur can be also loaded on 3D MXene architecture without further chemical functionalization by popular melting-diffusion method at low temperature (Figure 4k and Figure 4l). The presence and homogenous distribution of Co, Sn, Mn, Ag, Pt or S element accompanied with Ti and C element from Ti3C2 MXene is identified by elemental mapping analysis of the corresponding structure. XRD analysis further confirm the formation of tetragonal SnO2 (JCPDS No. 41-1445), cubic Co3O4 (JCPDS No. 421467), rhombohedral MnTiO3 (JCPDS No. 29-902), cubic Pt (JCPDS No. 4-802), cubic Ag (JCPDS No. 4-783) and orthorhombic sulfur (JCPDS No. 8-247) on 3D MXene architecture. These materials are promising in a broad range of the applications. For example, the hybrid system comprised of 3D MXene and metal oxides is appealing electrode materials in rechargeable batteries, supercapacitor or solar cells.2 The Pt/3D Ti3C2-MXene system may acts as a catalyst in fuel cell systems to catalyze methanol and formaldehyde oxidation or oxygen reduction, while the Ag/3D Ti3C2-MXene hybrid holds the promise in gas sensor and electrocatalytic CO2 reduction.47, 48 The sulfur infiltrated 3D Ti3C2-Mxene is potentially useful for popular Li-S batteries due to the strong affinity of sulfur to MXene sheets.21 By appropriate choice of organic precursor and spray drying conditions, this method is also expected to be extended to MXene-polymer system. The multilevel characteristic of 3D MXene-based hybrid systems allows one to readily modulate their composition and functionalities, which further generates a broad family of hybrid materials. For instance, the Co3O4 nanoparticles could be in-situ converted to CoP counterpart via the phosphorization process at high temperature without damaging the 3D MXene architecture (Figure 5a-d). The CoP nanoparticles well inherit the ultrafine size and homogenous dispersion of parent Co3O4 particles without apparent fusion or aggregation (Figure 5e). The single crystalline nature of CoP nanoparticles is validated by clear lattice fringes from the (211) plane of CoP crystal (Figure 5f), being consistent with the XRD results (Figure S7). The co-existence and homogenous distribution of C, Co, Ti and P elements in 9 ACS Paragon Plus Environment

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CoP@3D Ti3C2-MXene is confirmed by elemental mapping and XPS analysis (Figure 5g and Figure S8a). The high-resolution Ti 2p XPS spectrum of CoP@3D Ti3C2-MXene can be deconvoluted into two pairs of strong 2p3/2/2p1/2 doublets for Ti-C (454.8/460.5 eV) and Ti2+ (455.8/461.6 eV) along with two other pairs of weak 2p3/2/2p1/2 peaks from Ti3+ (457.2/462.9 eV) and Ti-O (459.0/464.8 eV)(Figure S8b).21 The Co 2p XPS spectrum is resolved into two pairs of 2p3/2/2p1/2 doublets for Co(0) (778.9/793.9 eV) and Co(II) (782.0/798.4 eV)(Figure S8c).49, 50 The P 2p XPS spectrum shows the P 2p3/2 and 2p1/2 doublets for P in CoP at 129.5 and 130.4 eV, respectively (Figure S8d).49, 50 The content of Co, Ti and P in CoP@3D Ti3C2MXene is around 25.5, 49.2 and 13.5 wt.%, measured by inductively coupled plasma optical emission spectroscopy (ICP-OES), respectively (Table S1). It corresponds to an atomic ratio of around 1 : 1 between Co and P element and a weight ratio of 39 wt.% for CoP. The formation of 3D porous structure imparts the CoP@3D Ti3C2-MXene with much higher surface area (74 m2 g-1) than that of the nanohybrids made by dispersing CoP nanoparticles on pristine Ti3C2 MXene (43 m2 g-1) or MXene-free CoP (22 m2 g-1)(Figure S9). The promise of 3D MXene-based hybrid system in electrochemical applications is demonstrated by using CoP@3D Ti3C2-MXene as a bifunctional electrocatalyst for overall water splitting in alkaline solution. The OER performance of CoP@3D Ti3C2-MXene catalysts with various CoP content is first evaluated in 1.0 M KOH. The iR-corrected linear sweep voltammetry (LSV) curves are taken at a slow scan rate of 10 mV s-1 to minimize the capacitive current (Figure 6a and Figure S10). Among them, the catalyst with 39 wt.% CoP shows the lowest onset overpotential (ηonset = 220 mV), smallest Tafel slope (51 mV dec-1) and overpotential required to achieve a current density of 10 mA cm-2 (η

j=10 =

298 mV) with

respect to the catalysts with 54.3 wt.% CoP (ηonset = 270 mV, 56 mV dec-1, η j=10 = 340 mV) or 17.7 wt.% CoP (ηonset = 290 mV, 60 mV dec-1, η j=10 = 360 mV) and MXene-free CoP catalyst (ηonset = 370 mV, 81 mV dec-1, η

j=10 =

360 mV). Negligible OER activity is identified for 3D

Ti3C2 MXene architectures. The difference in electrocatalytic performance of these catalysts 10 ACS Paragon Plus Environment

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is related to their structure: the catalyst with 17.7 wt.% CoP has insufficient electrocatalytic active phase, while the one with 54.3 wt.% CoP and MXene-free CoP catalyst suffer from the presence of excess CoP with limited conductivity and surface area (Figure S11). Overall, the CoP@3D Ti3C2-MXene catalysts with various CoP content still exhibit superior performance to MXene-free CoP catalyst, highlighting the importance of MXene for electrocatalytic improvement. The OER performance of CoP@3D Ti3C2-MXene catalyst with 39 wt.% CoP is further compared with the benchmark catalysts including Ti3C2 MXene nanosheets with 42.5 wt.% CoP (CoP@Ti3C2-MXene), 3D rGO architectures with 46.2 wt.% CoP (CoP@3D rGO) and commercial RuO2 under identical conditions (Figure 6b and Figure S12). The controlled catalysts exhibit inferior activity and reaction kinetics towards OER, as characterized by much higher ηonset of 260, 310 and 270 mV, η j=10 of 320, 370 and 328 mV, and larger Tafel slope of 59, 69 and 66 mV dec-1 than that of CoP@3D Ti3C2-MXene catalyst, respectively (Figure 6c and Figure 6d). Consequently, high current density of 200 mA cm-2 can be achieved by CoP@3D Ti3C2-MXene catalyst at low overpotential of 395 mV, which is 3-9 folds higher than that of CoP@Ti3C2-MXene (67.03 mA cm-2), CoP@3D rGO (22.4 mA cm-2) and RuO2 (37.7 mA cm-2) catalysts at the same overpotential. Moreover, the CoP@3D Ti3C2-MXene catalyst also shows superior electrocatalytic activity to reported CoP-based OER catalysts (Table S2). When biased galvanostatically at 10 mA cm-2, the CoP@3D Ti3C2-MXene catalyst exhibits a nearly constant operating potential at around 1.54-1.55 V for about 10 h, manifesting good durability for OER (Figure 6e). In contrast, the operating potential of RuO2, CoP@Ti3C2-MXene, CoP@3D rGO and MXene-free CoP catalysts rise rapidly in several hours with much higher potential. After 7200s reaction, both the structure of CoP@3D Ti3C2MXene architecture and MXene nanosheets included is retained, showing high robustness against OER (Figure S13). Meanwhile, a thin shell is observed around the CoP nanoparticles perhaps due to the formation of oxy-hydroxides via partial oxidation/dephosphorization of 11 ACS Paragon Plus Environment

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CoP in KOH. The plots with a x-axis of the η axis of the η

j=10

j=10, a

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measure of the catalytic activity and a y-

at a time of 1h, a measure of catalyst stability, is drawn to reflect the

comprehensive performance of above catalysts (Figure 6f).41 The CoP@3D Ti3C2-MXene catalyst is located toward the bottom left corner of the plots that represent low overpotential but high stability over time, suggesting the best comprehensive performance among all benchmark catalysts. The electrocatalytic activity of CoP@3D Ti3C2-MXene catalyst is primarily attributed from the CoP because of the negligible activity of Ti3C2 MXene for OER. To uncover the origin of OER activity, comprehensive ex-situ characterization is performed on CoP@3D Ti3C2MXene catalysts after chronoamperometric (CA) tests of different time at 1.54 V. To facilitate the tests, the catalysts are loaded on carbon paper (CP) for the examination by energy dispersive spectrometer (EDS) and XPS (Figure S14). The EDS analysis reveal the everincreasing intensity of O signal over time and increased O/Co ratio from 8.8 to 20.2 in 2 h with OER proceeding. Meanwhile, the P/Co ratio is decreased from 1.0 to 0.1, indicative of the ongoing electrochemical oxidation/dephosphorization of CoP during OER. Highresolution XPS analysis record the gradual shift of Co 2p3/2 peaks to high binding energy and the reduced intensity of P 2p3/2 peaks with OER proceeding. Clear transition of O 1s peak from a surface dominated by adsorbed oxygen species (532.4 eV) to the state dominated by lattice oxygen species (530.6 eV) is simultaneously identified.51 All these results validate the transition of CoP to high-valence cobalt compounds such as cobalt oxy-hydroxides with OER proceeding, which may contribute to the OER activity of CoP@3D Ti3C2-MXene catalyst.52, 53

The CoP@3D Ti3C2-MXene catalyst also exhibits good performance towards HER in alkaline solution, which generally suffers from much higher energy barrier and sluggish kinetics with respect to the process in acidic electrolyte.54 Figure 7a shows the iR-corrected polarization curves of CoP@3D Ti3C2-MXene catalyst and the benchmark catalysts including 12 ACS Paragon Plus Environment

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commercial Pt/C (20 wt.% Pt), CoP@Ti3C2-MXene and MXene-free CoP for HER in 1 M KOH with a scan rate of 10 mV s-1. The CoP@3D Ti3C2-MXene catalyst delivers a current density of 10 mA cm-2 at an overpotential of 168 mV, which is higher than that of Pt/C catalyst (77 mV) but still outperforms the CoP@Ti3C2-MXene (243 mV), MXene-free CoP (303 mV) catalysts and many reported catalysts (Table S3). Fast kinetics of HER over CoP@3D Ti3C2-MXene catalyst is characterized by the smaller Tafel slope (58 mV dec-1) than that of CoP@Ti3C2-MXene (66 mV dec-1) and MXene-free CoP (80 mV dec-1) catalysts (Figure 7b). Under identical conditions, the Pt/C catalyst delivers a Tafel slope of 45 mV dec-1. By extrapolating the Tafel plot, the exchange current density of CoP@3D Ti3C2-MXene catalyst is determined to be 21.4 µA cm-2, much higher than that of CoP@Ti3C2-MXene (11.6 µA cm-2) and MXene-free CoP (5.1 µA cm-2) catalysts. After 2,000 CV cycles, the polarization curves of CoP@3D Ti3C2-MXene catalyst show negligible shift compared with the initial state at a scan rate of 50 mV s-1 in 1 M KOH, confirming good durability for HER in alkaline electrolyte (Figure S15). Overall, the CoP@3D Ti3C2-MXene catalyst exhibits comparable activity and kinetics to Pt/C catalyst but superior performance to MXene-free catalysts for HER in alkaline electrolyte. The excellent properties of CoP@3D Ti3C2-MXene catalyst for both OER and HER in alkaline electrolyte make it promising as a bifunctional catalyst for electrocatalytic water splitting. To this end, the working electrodes are made by loading CoP@3D Ti3C2-MXene or noble metal catalysts (e.g., RuO2, Pt/C) on CP with an average mass loading of 0.4 mg cm-2. In three-electrode system, the CoP@3D Ti3C2-MXene/CP electrodes exhibit much lower η j=10

(280 mV) than that of RuO2/CP electrode (342 mV) for OER and greatly improved η

j=10

(128 mV) for HER in 1 M KOH, respectively (Figure S16). The CP shows negligible activity towards OER and HER. The steady-state polarization curve for overall water splitting reveals that the voltage difference (∆V) between HER and OER is 1.58 V to reach a current density of 10 mA cm-2. It is lower than that of the device based on Pt/C/CP // RuO2/CP electrodes (1.62 13 ACS Paragon Plus Environment

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V) and outperforms many reported bifunctional catalysts (Figure 7c, Figure 7d and Table S4). The two-electrode devices are set up by using CoP@3D Ti3C2-MXene/CP electrodes with the same area as both the anode and cathode and 1 M KOH as the electrolyte. It can afford a current density of 10 mA cm-2 at an applied potential of 1.565 V, which is lower than that of the device with Pt/C/CP and RuO2/CP as the anode and cathode (1.59 V), respectively (Figure 7e and Figure S17a). The galvanostatic tests measured at a current density of 10 mA cm-2 shows that the operational potential is increased below 15 mV after continuous electrolysis for 10 h. In contrast, the device based on Pt/C/CP // RuO2/CP electrodes failed in 7 h with rapidly increased operational potential (Figure 7f). Gas chromatography (GC) confirms the production of the H2 and O2 with a ratio of 2 : 1 and nearly 100 % Faradaic efficiency over CoP@3D Ti3C2-MXene/CP electrodes (Figure S18). Even tested at 10 folds higher current density of 100 mA cm-2, the device made of CoP@3D Ti3C2-MXene/CP electrodes still manifests excellent long-term stability for overall water splitting. The potential is stabilized at around 1.71 V with a increment by only 25 mV after continuously electrolysis for 150 h. Benefited from high activity, the water electrolysis device made of CoP@3D Ti3C2MXene/CP electrodes could be driven by a solar cell with a nominal voltage of 1.571 V in 1 M KOH at room temperature (Figure S17b and Figure S17c), highlighting the potential in solar-driven water electrolysis. The electrocatalytic improvement of CoP@3D Ti3C2-MXene electrocatalyst is postulated to originate from several aspects. First, the 3D MXene architectures with aggregation-resistant properties and highly porous structure not only provides highly accessible electrodeelectrolyte-gas triple-phase interface for ionic exchange, but also lends isotropically broadened pathway to release the gas product during the electrocatalysis. Integrating ultrafine CoP nanoparticles on them further favors the exposure of extra active sites for reducing reaction barrier and accelerate the electrocatalysis. As a result, the CoP@3D Ti3C2-MXene catalyst can posse a 1.9 and 10.6 folds higher electrochemically active surface area (ECSA) 14 ACS Paragon Plus Environment

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than that of CoP@Ti3C2-MXene and MXene-free CoP catalyst, respectively, as determined by measuring the Cdl in a non-Faradaic region of CV with a linear relationship with ECSA (Figure S19). Second, the 3D MXene skeleton provides an interconnected network for fast yet continuous charge transfer in longer range order due to superior electrical contact of subunit sheets. In contrast, the individual MXene sheets are electrically isolated each other. The accelerated charge transfer is evidenced by the drastically reduced diameter of the semicircle at high-frequency region in electrochemical impedance spectroscopy (EIS) patterns. The CoP@3D Ti3C2-MXene catalyst exhibits the lowest charge-transfer impedance (Rct = 4.3 Ω) than that of CoP@3D rGO (Rct = 9.4 Ω) and CoP (Rct = 18.4 Ω) catalyst (Figure S20). Third, XPS analysis primarily reveals the prominent interaction between CoP and underneath MXene sheets. Compared with MXene-free CoP, the Co and P 2p peaks from CoP@3D Ti3C2-MXene structure are positively shifted by 0.6 eV and 0.49 eV, respectively. Accordingly, the Ti 2p peak from CoP@3D Ti3C2-MXene is negatively shifted by 0.58 eV with respect to 3D Ti3C2 MXene without loading CoP. This phenomenon implies the charge density transfer between CoP and Ti3C2 MXene (Figure S8). It gives rise to particle-MXene interactions that lead to synergistic behaviors, including inhibited particle sintering, strengthened interfacial junction and enhanced galvanic interactions for improved electrocatalysis.16, 20

Conclusions In summary, we report the aggregation of MXene nanosheets can be effectively mitigated by assembling them to 3D architecture with hierarchical structure and aggregation resistant properties. Topological structure of 3D MXene architecture not only prevents the intersheet aggregation inside, but also enables geometry-based resistance to aggregation in both solid state and solvents with different properties. The 3D MXene architectures manifest large surface area, high robustness, 3D conductive framework and better processability with respect to flat MXene sheets in addition to inheriting their attractive properties. It renders them very 15 ACS Paragon Plus Environment

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appealing as versatile host for engineering hierarchical hybrid systems with a broad spectrum of functionalities by coupling with the nanostructures of transitional and main-group metal oxides, perovskite-type metal oxide, metal phosphide, noble metals and sulfur, etc. In a proofof-concept trial, the CoP@3D Ti3C2-MXene hybrid architecture exhibits high electrocatalytic activity towards OER and HER in KOH due to high conductivity, enlarged active surface area, accelerated charge and mass transport through isotopically broadened pathway and possible particle-MXene synergy. The bifunctionally electrocatalytic activity of CoP@3D Ti3C2MXene catalysts allow them to be utilized for overall water splitting, exhibiting superior activity and durability to the combination of precious Pt and RuO2 catalysts. The work suggests not only a 3D MXene-based electrocatalyst that allows for good electrochemical characteristics, but also an effective way to address the fundamental difficulty in processing and application of MXene family.

Experimental Section Synthesis of 3D MXene architectures. The MXene were synthesized by etching the Ti3AlC2 MAX phase with HF, as reported elsewhere.25 For the synthesis of fluffy 3D MXene architectures, the Ti3C2 MXene colloid with certain concentration (0.5-5 mg mL-1) was ultrasonically nebulized to aerosol droplets by an ultrasonic atomizer (3.2 W, Siansonic DP30) with a feeding rate of 20 mL h-1. The aerosol droplets were subsequently flowed through a tube furnace connected to the ultrasonic atomizer with Ar as carrier gas. The tube furnace was preheated at 600

o

C before aerosolization. The sphere-like 3D MXene

architectures were produced by similar way except to add the polyvinyl pyrrolidone (PVP, 2 mg mL-1) in MXene colloids. The products were harvested by an electrostatic collector at the end of tube furnace. Synthesis of Co3O4@3D Ti3C2-MXene, SnO2@3D Ti3C2-MXene, MnTiO3/3D Ti3C2MXene, Pt@3D Ti3C2-MXene, Ag@3D Ti3C2-MXene. They were fabricated by similar 16 ACS Paragon Plus Environment

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approach for making 3D MXene architectures except to use MXene colloids (100 mL, 2 mg mL-1) dissolved with PVP (200 mg) and 2 mmol of Co(OAC)2•4H2O, SnCl4, Mn(CH3COO)2•4H2O, H2PtCl6•6H2O or AgNO3 for the aerosolization, respectively. Synthesis of S@3D Ti3C2-MXene. The S@3D Ti3C2-MXene was synthesized by mixing the 3D Ti3C2-MXene architectures with sublimed sulfur with a mass ratio of 1 : 2. The mixture was sealed in an Ar-filled autoclave and stored at 155 °C for 12 h. Synthesis of CoP@3D Ti3C2-MXene. Appropriate amount of Co3O4@3D Ti3C2-MXene was loaded in a porcelain boat that was placed at downstream side of a boat containing NaH2PO2·H2O powder in a tube furnace. The weight ratio between Co3O4@3D Ti3C2-MXene and NaH2PO2·H2O is 3: 25. After annealing at 300 °C for 2 h in Ar flow, the CoP@3D Ti3C2MXene was yielded. For compassion, the CoP@rGO was produced by similar approach except to replace the Ti3C2 MXene with GO of the same mass for aerosol spray drying. The GO was prepared by modified Hummers’ method reported elsewhere, which was reduced to rGO during annealing.[1] CoP@Ti3C2-MXene was made by annealing the freeze dried mixture of Ti3C2 MXene, PVP and Co(OAC)2•4H2O with a mass ratio of 1 : 1 : 25 at 600 oC for 1 h, followed by the similar phosphorization process at 300 °C for 2 h in Ar flow. The MXene-free CoP was obtained by the phosphorization of Co(OAC)2•4H2O and PVP with a mass ratio of 25 : 1 at 300 °C for 2 h in Ar flow. Material Characterization. The morphology of the samples was characterized with fieldemission scanning electron microscopy (FESEM, FEI NanoSEM 450) and transmission electron microscopy (TEM, FEI TF30). Powder X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance X-ray spectrometer equipped with 2D detector (Cu Kα, λ = 1.5406 Å). The surface characteristics of the samples were investigated using Thermo ESCALAB MK II X-ray photoelectron spectrometer (XPS). The textural properties of the samples were measured by Micrometrics ASAP 2020 Surface Area and Porosity Analyzer at 77 K. The weight ratio of Co, P and Ti element in the sample was measured by coupled 17 ACS Paragon Plus Environment

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plasma optical emission spectroscopy (ICP-OES, Optima 2000DV, PerkinElmer, Inc.). The FT-IR spectra were measured by a Nicolet-20DXB Fourier transform infrared spectrometer. OER test. The OER performance of the catalysts was evaluated on a CHI 760D electrochemical workstation with a standard three-electrode system in 1 M KOH. A glassycarbon (GC) rotating disk electrode (RDE) (d = 5.0 mm) was used as working electrode while a Pt foil and an Ag/AgCl electrode saturated with KCl solution was employed as counter and reference electrode, respectively. Prior to the test, the electrolyte was bubbled with O2 flow for 30 min, and continuous O2 flow was maintained during the measurement to ensure continuous gas saturation. The catalysts (4 mg) were ultrasonically dispersed in mixture of ethanol (500 µL), deionized water (485 µL) and Nafion (15 µL, 5.0 wt.%) to form a uniform suspension. A part of the catalyst ink (10 µL) was then loaded onto a GC electrode, yielding an average mass loading of around 0.2 mg cm-2 for all the samples. All potentials measured against an Ag/AgCl electrode were converted to potential vs. RHE according to Evs Ag/AgCl

RHE

= Evs

+ 0.059 pH + 0.197. The linear sweep voltammetry (LSV) were obtained at a rotating

rate of 1600 rpm with a scan rate of 10 mV s-1. EIS measurements were carried out in the same configuration at 1.60 V vs. RHE in a frequency range over 100 kHz to 1 Hz by applying an AC voltage with 5 mV amplitude. Chronopotentiometric measurements were recorded via applying a current density of 10 mA cm-2 on working electrode. The electrochemically active surface area was estimated from the electrochemical double layered capacitance (Cdl). The Cdl was determined from cyclic voltammograms (CV) measured in a non-Faradaic region at different scan rates (v) between 1.1 - 1.15 V vs. RHE in 1 M KOH. HER test. The HER performance of the electrocatalysts was evaluated on a CHI 760D electrochemical workstation with the same standard three-electrode system for OER tests. Prior to the test, the electrolyte was bubbled with Ar flow for 30 min, and continuous Ar flow was maintained during the measurement process. The working electrodes with a mass loading

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of ca. 0.2 mg cm-2 were made by similar way for OER test. The LSV were obtained at a rotating rate of 1600 rpm with a scan rate of 10 mV s-1. Overall water-splitting test. The overall water-splitting tests were performed in a twoelectrode system with 1 M KOH as the electrolyte. The catalysts (CoP@3D Ti3C2-MXene, Pt/C or RuO2) were casted onto a carbon paper (CP, TGP-H-060, Torray, 1 × 1 cm) with a mass loading of ca. 0.4 mg cm-2. The two-electrode devices were set up by using CoP@3D Ti3C2-MXene/CP with the same area as both the anode and cathode. As comparison, the devices with Pt/C as the anode and RuO2 as the cathode were also set up. The LSVs were conducted at a scan rate of 10 mV s-1 from 0.0 to 2.0 V. The faradaic efficiency for H2 production is studied by a gas chromatography (Agilent Technologies 7890N) in airtight cells with a two-electrode configuration. A static cell configuration was used to visualize the generation of H2 and O2 in the reaction headspace.55 It involved routing the gas outlet port of the GC system back into the cell, thereby only the volume of GC sample (0.5 mL) was lost with each sampling event. The GC system was calibrated for H2 using certified standards of H2 at a range of volume % in argon. Linear fits of volume % vs. peak area were obtained, which allowed peak areas to be converted into volume % of H2.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. More SEM, TEM, XPS, TGA, XRD, N2 adsorption-desorption isotherms and electrochemical data of the 3D MXene and related hybrid systems. A comparison of CoP@3D Ti3C2-MXene with recently reported electrocatalysts in the performance of OER, HER and overall water splitting in KOH. AUTHOR INFORMATION 19 ACS Paragon Plus Environment

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Corresponding Author *E-mail: [email protected], [email protected] ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (NSFC, No. 51522203, 51772040), Fok Ying Tung Education Foundation (No. 151047), the Fundermental Research Funds for the Central Universities (No. DUT18LAB19) and Xinghai Scholarship of Dalian University of Technology.

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43. Zhao, L.; Dong, B.; Li, S.; Zhou, L.; Lai, L.; Wang, Z.; Zhao, S.; Han, M.; Gao, K.; Lu, M.; Xie, X.; Chen, B.; Liu, Z.; Wang, X.; Zhang, H.; Li, H.; Liu, J.; Zhang, H.; Huang, X.; Huang, W., Interdiffusion Reaction-Assisted Hybridization of Two-Dimensional MetalOrganic Frameworks and Ti3C2Tx Nanosheets for Electrocatalytic Oxygen Evolution. ACS Nano 2017, 11, 5800-5807. 44. Luo, J.; Jang, H. D.; Sun, T.; Xiao, L.; He, Z.; Katsoulidis, A. P.; Kanatzidis, M. G.; Gibson, J. M.; Huang, J., Compression and Aggregation-Resistant Particles of Crumpled Soft Sheets. ACS Nano 2011, 5, 8943-8949. 45. Ma, X.; Zachariah, M. R.; Zangmeister, C. D., Crumpled Nanopaper from Graphene Oxide. Nano Lett. 2012, 12, 486-489. 46. Yan, J.; Ren, C. E.; Maleski, K.; Hatter, C. B.; Anasori, B.; Urbankowski, P.; Sarycheva, A.; Gogotsi, Y., Flexible MXene/Graphene Films for Ultrafast Supercapacitors with Outstanding Volumetric Capacitance. Adv. Funct. Mater. 2017, 27, 1701264. 47. Sun, Z.; Ma, T.; Tao, H.; Fan, Q.; Han, B., Fundamentals and Challenges of Electrochemical CO2 Reduction Using Two-Dimensional Materials. Chem. 2017, 3, 560587. 48. Nie, Y.; Li, L.; Wei, Z., Recent Advancements in Pt and Pt-free Catalysts for Oxygen Reduction Reaction. Chem. Soc. Rev. 2015, 44, 2168-2201. 49. Yang, H.; Zhang, Y.; Hu, F.; Wang, Q., Urchin-like CoP Nanocrystals as Hydrogen Evolution Reaction and Oxygen Reduction Reaction Dual-Electrocatalyst with Superior Stability. Nano Lett. 2015, 15, 7616-7620. 50. Burns, A. W.; Gaudette, A. F.; Bussell, M. E., Hydrodesulfurization Properties of Cobalt–Nickel Phosphide Catalysts: Ni-rich Materials are Highly Active. J. Catal. 2008, 260, 262-269. 51. Burke, M. S.; Kast, M. G.; Trotochaud, L.; Smith, A. M.; Boettcher, S. W., Cobalt-Iron (Oxy)Hydroxide Oxygen Evolution Electrocatalysts: the Role of Structure and Composition on Activity, Stability, and Mechanism. J. Am. Chem. Soc. 2015, 137, 36383648. 52. Chang, J.; Xiao, Y.; Xiao, M.; Ge, J.; Liu, C.; Xing, W., Surface Oxidized CobaltPhosphide Nanorods As an Advanced Oxygen Evolution Catalyst in Alkaline Solution. Acs Catal. 2015, 5, 6874-6878. 53. Ryu, J.; Jung, N.; Jang, J. H.; Kim, H.-J.; Yoo, S. J., In Situ Transformation of HydrogenEvolving CoP Nanoparticles: Toward Efficient Oxygen Evolution Catalysts Bearing Dispersed Morphologies with Co-Oxo/Hydroxo Molecular Units. Acs Catal. 2015, 5, 4066-4074. 54. Wang, J.; Xu, F.; Jin, H.; Chen, Y.; Wang, Y., Non-Noble Metal-Based Carbon Composites in Hydrogen Evolution Reaction: Fundamentals to Applications. Adv.Mater. 2017, 29, 1605838. 55. Symes, M. D.; Cronin, L., Decoupling Hydrogen and Oxygen Evolution during Electrolytic Water Splitting Using an Electron-Coupled-Proton Buffer. Nature Chem. 2013, 5, 403-409.


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Figure captions Figure 1. (a) Substantial merits of aggregation-resistant 3D MXene architecture; (b) capillary-forced assembly of MXene to 3D architecture and related hybrid systems by spray drying the aerosol droplet of MXene-containing colloids. Figure 2. (a, b) SEM and (c) TEM images of 3D Ti3C2 MXene architectures with fluffy shape and numerous ridges on rough surface; (d) TEM image revealing the inhibited aggregation of MXene nanosheets in 3D architecture; (e, f) SEM and (g) TEM images of sphere-like 3D Ti3C2 MXene architectures with loose interior; (h) size distribution of 3D Ti3C2 architecture made by varying the initial concentration of MXene colloids in a range of 0.5 to 5 mg mL-1. Figure 3. (a) XRD, (b) Ti 2p XPS and (c) FT-IR spectra of 3D Ti3C2 MXene architecture and pristine Ti3C2 MXene; (d) electrical conductivity of 3D Ti3C2 MXene architecture and pristine Ti3C2 MXene under a constant pressure of 20 MPa; (e) the 3D Ti3C2 MXene particles (top) can be dispersed in many solvents by hand shaking and keep stable for 4 h, while pristine Ti3C2 MXene rapidly aggregate (bottom) in most solvents even after ultrasonication. The solvents in 1 to 6 are water, ethanol, cyclohexane, ethyl acetate, NMP and pump oil, respectively. (f) N2 adsorption/desorption isotherms of 3D Ti3C2 MXene architecture, pristine Ti3C2 MXene and vacuum filtrated Ti3C2 MXene film; (g, h) SEM images of 3D Ti3C2MXene architecture and pristine Ti3C2 MXene after compressed under 150 MPa, respectively; (i) SEM image of vacuum filtrated Ti3C2 MXene film. The insets are optical images of these materials after ultrasonically dispersion in water. Figure 4. SEM and TEM images, elemental mapping analysis and XRD patterns of (a, b) Co3O4@3D Ti3C2-MXene; (c, d) SnO2@3D Ti3C2-MXene; (e, f) MnTiO3@3D Ti3C2-MXene; (g, h) Pt@3D Ti3C2-MXene; (i, j) Ag@3D Ti3C2-MXene and (k, l) S@3D Ti3C2-MXene. Figure 5. (a, b) SEM images of CoP@3D Ti3C2-MXene architecture; (c, d) TEM images of CoP@3D Ti3C2-MXene architecture; (e) TEM image revealing the uniform dispersion of ultrafine CoP nanoparticles on the surface of 3D MXene architecture; (f) HRTEM image of a CoP nanoparticle in CoP@3D Ti3C2-MXene architecture; (g) elemental mapping showing the homogenous distribution of C, Co, Ti and P element in CoP@3D Ti3C2-MXene architecture. Figure 6. (a) The iR-corrected LSV curves of CoP@3D Ti3C2-MXene catalysts with various CoP content, MXene-free CoP and 3D Ti3C2-MXene; (b) the iR-corrected LSV curves of CoP@3D Ti3C2-MXene, CoP@Ti3C2-MXene, CoP@3D rGO, CoP, 3D Ti3C2 MXene and RuO2 catalysts for OER; (c) a comparison of these catalysts in onset overpotential and η 24 ACS Paragon Plus Environment

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(d) Tafel plots and (e) chronopotentiometric response at a current density of 10.0 mA cm-2 of CoP@3D Ti3C2-MXene, CoP@Ti3C2-MXene, CoP@3D rGO, CoP and RuO2 catalysts; (f) comprehensive plots of catalytic activity and stability of CoP@3D Ti3C2-MXene, CoP@Ti3C2-MXene, CoP@3D rGO, MXene-free CoP and RuO2 catalysts, where the x-axis is η j=10 and y-axis is η j=10 at a reaction time of 2h. All the above tests were conducted at a scan rate of 10 mV s-1 in 1 M KOH at 1600 rpm. Figure 7. (a) The iR-corrected polarization curves and (b) Tafel plots of CoP@3D Ti3C2MXene, CoP@Ti3C2-MXene, CoP and Pt/C catalysts for HER. All the above tests were conducted at a scan rate of 10 mV s-1 in 1 M KOH at 1600 rpm. (c) Steady-state polarization curves of CoP@3D Ti3C2-MXene/CP, RuO2/CP, Pt/C/CP and CP electrodes for OER and HER in 1 M KOH; (d) a comparison of CoP@3D Ti3C2-MXene catalyst with the state-of-theart nonprecious bifunctional catalysts in the voltage required to reach 10 mA cm-2 for overall water splitting in KOH; (e) the polarization curves of a two-electrode system with CoP@3D Ti3C2-MXene loaded on CP as both the anode and cathode at a scan rate of 10 mV s-1 for overall water splitting in 1 M KOH. As compassion, the performance of a two-electrode system with RuO2/CP (+) and Pt/C/CP (-) electrodes is also tested under identical conditions. (f) Chronopotentiometric response of the devices with CoP@3D Ti3C2-MXene (+, -) and RuO2/CP (+)//Pt/C/CP (-) electrodes at a current density of 10 and 100 mA cm-2 in 1 M KOH.


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Aggregation-Resistant 3D MXene Based ... - ACS Publications

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