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Hierarchical design of NiOOH @ amorphous Ni-P bilayer on a 3D mesh substrate for high efficiency oxygen evolution reaction Xi Xu, Chaojiang Li, Jiahao Gwendolyn Lim, Yanqing Wang, Aaron Ong, Xinwei Li, Erwin Peng, and Jun Ding ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06730 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 19, 2018

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ACS Applied Materials & Interfaces

Hierarchical design of NiOOH @ amorphous Ni-P bilayer on a 3D mesh substrate for high efficiency oxygen evolution reaction Xi Xu †, Chaojiang Li †, Jiahao Gwendolyn Lim, Yanqing Wang, Aaron Ong, Xinwei Li, Erwin Peng*, and Jun Ding* Department of Materials Science and Engineering, Faculty of Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore. KEYWORDS 3D printing, robocasting, electroless nickel coating, oxygen evolution reaction, Ni-P alloy film

ABSTRACT. Recently, 3d metal phosphide and metal-phosphorus alloy have been intensively studied for oxygen evolution reaction (OER). Research work has indicated that the presence of phosphorus could lead to formation of a phosphide/ (hydro)-oxide core/shell structure. In this work, we have developed a fabrication technique for a robust NiOOH @ amorphous Ni-P bilayer on a zirconia mesh support through a collaboration of electroless deposition and robocasting. During the electroless deposition, fully amorphous structure can be obtained with certain phosphorus content (7-8 wt.%). Relatively thick films (in the order of 5 µm) had an excellent

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adhesion on mesh structure because of large curvature. A stable Ni oxy/hydroxide surface (~200 nm) can be formed in bilayer nature (NiOOH/Ni-P) due to the pre-activation. The combination of catalyst active sites on surface and high conductivity of metallic body enables good OER performance with an overpotential of 286 mV at a current density of 10 mA cm-2. Together with excellent chemical stability and mechanical strength of the ceramic substrate, this novel combination gives rather excellent adhesion and stability in alkaline solution and provides a different angle for the hierarchical design of corrosion resistant and high performance OER electrodes for industry.

INTRODUCTION. In the past few years, the development of efficient electro catalysts with good chemical stability for oxygen evolution reaction (OER) under different electrolyte environments has become a major topic in materials research. Although the activity that intensely relied on active sites can be tailored by altering defect density and nanostructures, the intrinsic overpotential at 10 mA.cm-2 for OER is still normally around 320mV in basic solution and around 0.75mV in acidic solution.1-3 Among all material categories adopted as promotors, catalysts of noble transition metals such as iridium, ruthenium and their different oxide forms have been reported as functional materials with high efficiency towards OER.4-5 Though these elements are excellent candidates, however their cost is still high for application on an industrial scale. There has been increasing interest in 3d transition metal-based catalysts for OER due to their much-reduced cost and promising behavior. 3d metal oxide/hydroxide and sulfide have been widely studied in this sense due to their stability. Among 3d metal elements, nickel (Ni) has drawn intensive attention due to high corrosion resistance. However, as it is well known, 3d metal oxide/hydroxide usually has poor conductivity. Since Budniok and Kupka’s first publication of transition metal phosphide for OER in 1989,6 metal phosphides have once again


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drawn increasing attention because of their superior activity towards electrolysis recently.7-11 For nickel phosphides, high performance of OER electrocatalyst is probably attributed to core-shell structure of nickel phosphide/NiOx due to in-situ oxidative transformation.12 This inspires us to develop a bilayer structure of NiOx/nickel phosphide. The oxide/hydroxide enables a good catalyst function on the surface, while the phosphide body can provide a good conductivity for charge transportation. A common way for fabrication of catalyst electrode is by deposition onto the surface of a conductive substrate. Based on published results, most successful phosphide electrodes for OER were fabricated through binding or growing phosphide nanostructures to form a surface layer on a conducting substrate (for example carbon) by spin casting and drop casting etc.13-14 The method may affect the measured performance as well as inheriting large resistance be, introduce unwanted polymer impurity into the solution, in the company of blocking the active sites.2 As a result, it is conceivably natural to target research outcome on finding an efficient design to fulfill a combination of continuous conductivity and sufficient active sites. Most recently, bulk amorphous FeNiP sheet was also reported by melt-spinning and exhibited excellent OER performance, showing clear advantages of the continuous amorphous structure compared to phosphide layers via binding using phosphide powder.2 The iron incorporation to Nickel based catalyst can increase the OER activity dramatically and is very essential in catalyst characterization.15-16 However, melt-spinning have the limitation for the fabrication of bulk electrode with required shape and size. The ribbons after melt-spinning might not have required mechanical strength for practical application. For practical application, suitable mechanical support would be needed.


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Previously, thick films of Ni-P alloy by electrodeposition have been studied by hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).17-18 Defects and structural inhomogeneity (presence of crystalline phases) were reported. In addition to this, poor film stability and adhesion to the copper foam substrate in corrosive electrolytes limits the use of the thick Ni-P alloy films for both HER and OER applications.19 As it is well known, electroless coating is an effective and economic method for stable nickel plating on metal for decorative use.20 The coated Ni-P layer is able to fully cover the support and has also been used for ceramic and polymer coating.21-22 A comparison of the performance between amorphous Ni oxyhydroxide and the spinel phase has been documented by Koper and co-workers.23 However, no detailed study has been focused on superiority of amorphous phase Ni-P alloy over crystalline phase towards water oxidation reaction. Also, it is interesting to explore if amorphous Ni-P coating derived by electroless deposition can exhibit high stability in harsh environment, sufficient bulk conductivity and plentiful active sites. In this research work, we documented amorphous Ni-P film in water splitting half-cell operation, from the consideration of hierarchical electrode design. Our strategy of Ni-P electrode design and preparation is shown in Figure 1. For the substrate, yttrium-stabilized zirconia (Ystabilized ZrO2) with excellent chemical stability, high mechanical hardness and toughness was selected. Relatively thick amorphous Ni-P layer will be deposited through electroless deposition technique. Based on reports of core/shell nickel oxide/hydroxide@nickel phosphide, we would form a thin surface layer of nickel oxide/hydroxide as OER catalyst, while the body of amorphous Ni-P will retain metallic for necessary electric conductivity.12 Therefore, the Ni-P amorphous layer will remain thick enough for necessary electronic transportation and surface oxidation (for formation of nickel oxyhydroxide) will be restricted only at the very surface. The


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pre-oxidation step of surface layer can be regarded as an activation step. Based on our preliminary study in this work, the adhesion of relatively thick amorphous Ni-P layer is relatively poor on Y-ZrO2 plate substrate if the thickness is in the range of µm. Therefore, YZrO2 round bar have been used as substrate instead of a flat surface, as reported previously that adhesion of a coated layer on substrate can be enhanced significantly, if the layer is coated on a surface with large curvature.24 In this concern, robocasting (a subset of 3D printing techniques) was adopted for the fabrication of Y-ZrO2. 3D printing techniques have been recently involved in designing water splitting platforms and with other energy and environmental usages.25-27 This technique enables design of more effective electrochemical electrode and creation of a macroscopic interconnected path for efficient electron motion.28-30 Among all these emerging techniques, robocasting or direct ink writing (DIW) can be ranked among the most efficient and easiest handling process to fabricate highly robust and designed structures.31-33 In general, a combination of two facile and rapid techniques, electroless nickel plating and direct paste writing (robocasting), was firstly adopted to functional hybrid three dimensional architectures for electrolysis to our knowledge. A class of bulk amorphous Ni-P materials on our self-fabricated substrate towards high OER reactivity in alkaline electrolyte was reported, from the viewpoint of- not only the catalyst type but also the electrode design, as functionality is also affected by the macroscopic local environments. Thus, this work takes the consideration to: (1) create a macroscopic interconnected path for efficient electron motion;(2) have designed surface roundness and filament spacing for sturdy coating; (3) give high electrochemically active surface area with respect to template surface area and catalyst itself. Detailed discussion regarding superiority of amorphous phase and coating time were also included. EXPERIMENTAL SECTION.


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Synthetic procedure of electrode: The Zirconia structures were fabricated via computer-aided robocasting of Zirconia pastes onto plastic substrates using a commercial robocasting system. (3Dison Multi; ROKIC Inc.) Prior to printing, it was fitted with 20 ml syringes and 400 microns nozzles. This extrusion-based robocasting strategy was then performed on a building plate following the slicing G-code. Representative zirconia samples were debinded at 500 °C for 5 hours at a heating rate of 1 °C/min, and subsequently sintered at 1400°C in air. The soak time was set to be 10 hours. The samples were cooled back to room temperature. The electroless nickel plating solution containing Nickel Chloride (30 wt.%), Sodium Hypophosphite (30%), Sodium Succinate (30%), and Hydrofluoric Acid (1%) purchased from Transene was used to plate printed surface at different time scales (3 min, 15 min, 60 min and 120 min). The zirconia structure surface lacks catalytic properties and therefore needs sensitization and activation steps to enable surface catalytic. One 4-layer mesh after 1hour electroless nickel plating was treated in vacuum tube furnace for 1 hour up to 600 °C with fully transformation to crystalline phase. Temperature was set accordingly based on literature.34 RESULTS AND DISCUSSION. Substrate Preparation: 3D mesh and plate structures were prepared using extrusion based direct ink writing technique. Figure 1 gives a schematic demonstration of the synthetic process. In order to get a better understanding of structure influence when serving as template for supporting catalyst, mesh structure and plate structure were first designed using Fusion 360 software. One important issue involved in robocasting that should be addressed here is shrinkage during the drying, debinding and sintering processes. For meshes, the inner struts should be 1.6 mm in XY thickness and about 0.4 mm in Z thickness (i.e. layer thickness) while the outer skirt should be 0.4 mm in XY thickness (equals to the nozzle size). The model for 7 x 7 mesh structure


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undergoes a linear shrinkage of about 15%-20% after sintering, as marked in Figure 2h. Hence for a requisite 10 mm x 10 mm apparent surface area, a 4-layer 5 x 5 porosity mesh model was used for further application. In this design (Figure S1), total volume was calculated to be 68.361 mm3 and porosity can be therefore determined to be the ratio of total volume to occupied volume 230.4 mm3. The materials were deposited in a layer by layer nature, with a rotation of 90° every two layers built with respected to previous patterns. The self-fabricated zirconia templates have been used as interconnected substrate material after sintering at 1400 °C for 5h with a debinding stage at 500 °C and became fully dense (Figure 2b).35-37 There are three major zirconia polymorphs including cubic, tetragonal and monoclinic. Pure zirconia at room temperature lies in monoclinic phase and transforms into tetragonal phase at elevated temperature of about 1173 °C. When temperature reaches about 2370 °C, the phase becomes cubic and eventually turns into liquid phase at 2690 °C. The cooling of sintered zirconia may introduce volume changes and this disruptive phase change can therefore cause cracks in the final product. In this concern, addition of other oxides can increase the temperature range where zirconia retains its stable cubic and tetragonal phase. Yttria-stabilized zirconia is more useful compared to pure zirconium dioxide phase due to more stabilized state. Upon heating to such high temperature, disruptive phase changes are eliminated by addition of 3-6% mole fraction of yittria and no additional m-ZrO2 was observed as indicated in X-ray powder diffraction (XRD) patterns shown in Figure 2i.38 The resulting materials have excellent mechanical and thermal properties as well as inert nature during further heat treatment compared with other metal or carbon-based substrate. Based on our separate study and unpublished results, Y-stabilized ZrO2 structure can possess density above 99% of its theoretical density and a fractural strength > 500 MPa after the debinding/sintering process.


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Ni-P Coating Characterization: The Ni-P alloy was introduced to the printed surface by immersing in an ammonia free coating solution for different time scale (3min, 15min, 60min and 120min). Electroless plating takes place simultaneously as the contact happens, which will not be affected by factors like uneven distributed current density as electrodeposition does. Since this process incorporates with no current flow, the rate of the deposition over areas should be equal provided uniform solution conditions were maintained. An oxidation-reduction reaction among nickel complex and hypophosphite ions was involved during electroless nickel plating process, which essentially regarded as a two-step procedure happening at meantime. Following equation is the overall reaction for introducing Ni-P alloy: 2Ni2+ + 4H2PO2- + H2O → 2Ni + P + 3H2PO3- + 2H+ + H2 (1) In addition, Ni3P formed improves corrosion resistance and here to incorporate active catalyst during transformation. It is an autocatalytic process following Ni-P binary phase. From the atomic percentage 0 – 25 at%, Ni-P phase diagram involves only Ni and Ni3P. At 19 at% P content, eutectic phase formed with a melting temperature of 880 degree. When P content increases, more metal phosphides emerge. These metastable phases of nickel phosphide will eventually transfer to Ni + Ni3P.39 For this procedure, the phosphorus content affects the deposit properties significantly such as corrosion resistance and more than a few research works have covered this part.40-41 Here we deposited nickel-phosphorus alloy with identified medium P content according to elemental analysis which was given by Energy-Dispersive X-ray (EDX) (Table S1). The nickel deposit is highly conductive, particularly after suitable heat treatment, and remains unstressed. As we can see from Figure 2b and Figure 2c, the coated phases are considered to follow the support materials surface feature. Table 1 listed out treatment condition for all samples. A trend can be observed from the comparison between NiP3 and NiP120 coating surface is the improvement of coating uniformity as well as the deposited particle shape.


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Figure 1. Schematic diagrams of the experimental procedure. (a) Robocating technology schematics and organic additives chemical structures; (b) Electroless plating process schematics; (c) Forming of stable activity layer on the working electrode surface.


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Table 1. Fabrication parameters of all samples

Sample Num.

Ni-P coating time /min

Heat treatment condition

Template structure

NiP3

3

-

mesh

NiP15

15

-

mesh

NiP60

60

-

mesh

HT-NiP60

60

600 °C; 60 min; vacuum

mesh

LHT-NiP60

60

280 °C; 60 min; vacuum

mesh

NiP60-P

60

-

plate

NiP120

120

-

mesh

X-ray diffraction patterns of as-deposited template and templates that subject to different heat treatments shown in Figure 2 demonstrated that the deposit consists of amorphous nickel and nickel phosphide phase. Several XRD studies on crystallization and phase transformation behavior of electroless nickel coating deposit suggested the existence of crystalline nickel phase with preference in certain orientation for medium phosphorus deposit.42 However, this preferred reflection direction was not observed in this content. No change in phase was recorded for low temperature treatment (280 °C) as suggested for crystallization starting temperature, nickel phosphide and nickel crystalline phase were generated after heating at vacuum till 600 °C as shown in Figure 2f and 2g. High resolution TEM images also match well with the XRD patterns. One conceptual replication shows the coating layer thickness is dependent on coating time (Figure 5). For longer plating time, normally a thicker deposit and better conductivity were obtained.


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Figure 2. (a) Scanning electron microscope (SEM) image of yittria stabilized zirconia starting powder; (b) SEM images of printed template surface after sintering showing fully dense structure (Black spot: Al2O3 additives for sintering); (c) SEM images of printed template surface after 1h plating; (d) HRTEM images of LHT-NiP60 and (e) SAED pattern; (f) HRTEM images of HTNiP60 and (g) SAED pattern; (h) Images of 7 x 7 test samples with dimensions before and after sintering. (Insert: 7 x 7 test meshes before and after electroless nickel plating); (i) X-Ray diffraction patterns of template (2theta from 20o to 80o). (Inset XRD pattern: whole data range of X-Ray diffraction patterns of all samples compared with JCPDS 80-0965 (t-ZrO2)); (J) High resolution (2theta from 30o to 60o) X-Ray diffraction patterns of coated template.


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Figure 3. (a) Polarization curves for Ni foam, NiP15, NiP60, HT-NiP60 and NiP120 after 20 scan cycles; (b) Tafel slopes of stated samples.

Figure 4. Cyclic voltammograms of (a) NiP15; (b) NiP60; (c) HT-NiP60 and (d) Ni foam; (e) Cathodic current measured for all samples at selected potential region as a function of scan rate. Excellent adhesion of deposit to template was observed under SEM even after water splitting cycles for printed mesh structure compared to plate template. A comparison of plate and mesh template surface morphology after polishing by silicon carbide abrasive paper was also conducted (Figure S2). Internal stress tend to change the shape of deposit to relieve force generated when the coating layer is getting thicker.24 For medium phosphorus content bath, the deposits are in a state of tensile stress from the solution. When the deposit forms in tensile state,


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it inclines to contract in order to relieve the stress. Round filament feature and fully dense structure enable this superior affinity. Deposits on plate structure therefore are more prone to crack due to larger range coverage especially after thousands of OER loops. Because of this overriding performance of mesh structure over plate structure fabricated by extrusion printer, further characterizations and tests were more focused on this designed porous feature. Electrochemical Measurement: The OER activities and performance of as-fabricated working electrodes were tested using three-electrode system. As indicated earlier, pre-activation of deposit has been done to obtain more reliable results. The iron impurities existed in the commercially available grade KOH was a possible source for pre-activation in 1M KOH. In order to exclude the contribution of iron species, special treatment was conducted for iron removal according to literature.16 Moreover, it can also be introduced by contaminated lab glassware and/or leaching from glass cell.43 New glassware with no pre-determination of iron species was adopted and cleaned three times with deionized water, detergent, 0.1% hydrochloric acid and boiled water before next use. The XPS and EDX results of representative sample after stability test was provided and no iron species was detected (Figure S5). Figure 3a shows the linear sweep voltammetry (LSV) of selected samples. Catalytic behavior was further summarized in Table S2. For comparison, the OER activity of commercial Ni foam with same apparent surface dimension was also recorded and displayed in the same figure. The electrolysis overpotential needed for hybrid electrodes of according time scale were enumerated in Figure 5. Among all materials, electroless plated mesh electrodes for 1 hour and 2 hours (NiP60 and NiP120) exhibit the best OER behavior, generating a geometric current density of 10 mA cm-2 at 326 mV and 330 mV. It requires only 286 mV after 10 hours stability test for NiP120.


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Figure 5. Resistance, thickness and overpotential @ 10 mA.cm-2 with coating period as independent variable (Error bar was given in percentage value). The performance difference between amorphous deposit and crystallized deposit (NiP60 and HT-NiP60, respectively) contradicts with the statement that activity of both crystalline and amorphous phosphide phases can be almost the same.6 In our work, the comparisons are conducted between electroless deposited films with the exact same geometric surface area. Also for Sample NiP60 and HT-NiP60, their starting mass loading were similar. For crystallized sample underwent vacuum annealing at 600 °C, mass loss is 0.4 mg as compared to untreated sample and the film thickness remained the same. The amorphism gives higher free energy and believed to contribute to their large OER active sites.23, 44 The larger peak exhibited for NiP60


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and NiP120 corresponds to transformation of surface species as compared to treated sample. Moreover, heat treatment up to high temperature, the resistivity of treated sample can also be decreased. However, as supported by the truth that behavior degrading for crystalline phase hybrid system and no significant performance increase was observed for NiP120, the conductivity required for electron motion could be enough. Additional growth in thickness may even cause performance degradation due to less efficiency in electron transfer. Another interesting observation is a stabilized oxidization layer was generated as shown in Figure S4. For most of the as-deposited samples, only one demonstrable peak exhibit during the scans. In the first cycle of mesh electrode coated for short period (NiP3), two forficate peaks displayed in the LSV curve (Figure S6). For phase transformation between Ni(OH)2 and NiOOH in a medium with pH=14, the voltage could be 0.5-1.0 V with respect to standard hydrogen electrode. The position of most oxidation peaks in this work assembles at 0.49-0.52 V vs. RHE, indicating oxidation takes place. The Ni species on the original deposit includes Ni, Ni3P and non-stabilized pre-oxidized species on the surface, as for NiP3, the coated layer is relatively thin and the amount of unstable surface oxidized layer can be comparable, forasmuch these peaks can be assigned to the oxidation of Ni and β-Ni(OH)2, and also further reaction to their higher valance products. The peaks stabilized after cycling and the area blow these peaks moved towards stabilization (Figure S6), suggesting the total active oxidized species remained almost constant afterwards. The passive films formed on the interface prevent further oxidation of inner atoms and impede diffusion from outer layer to inner part. This was supplementary supported by NiP120 results. For activation of NiP120 shown in Figure 7, ∆Ep (Epa − Epc) equals to 76.377 mV at the 20th cycle and 64.665 mV after 10-hour test, slightly higher than the expected value for a reversible system which is about 60 mV. It is rational to conclude that the phosphorus high


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valance state may alleviate other action. Moreover, the elemental P with appropriate electronegativity should in another sense stabilize the metallic phase inside this hybrid deposit to maintain the bulk conductivity and feasibly serve more OER active site.45-46 To obtain more information about reaction rate, the Tafel slopes are as well listed in Figure 3b and Table S2. Tafel slope reflects the charge transfer ability and lower the value indicates the higher charge transfer ability.47-48 For self-designed samples except sample 1, they showed competitive value with respect to Ni foam. Surface area can be another factor affecting the electro catalytic activity. To gain insights of aspects as various design and material in sense of catalytic activity, their double layer capacitances were measured (Figure 4) based on CV scans (Figure 4a-4d) at different scan rates. Generally, the mesh structures give a higher active surface area compared with porous nickel foam and printed plate structure. Nevertheless, among all mesh structure, variance exists. NiP3 acquires lowest effective surface area while for NiP60 and NiP120, they have up to six times higher value. Since a rather thin layer was formed on the surface compared to those coated for longer time scale, a reasonable deduction could be insufficient active site was generated. Therefore, it has lower electrochemically active surface area. While for HT-NiP60, the capacitance, proportional to effective surface area, is also smaller than as-deposited electrode for same period. The decrease in Cdl for samples subject to post treatment is likely related to the thermal diffusion of surface nickel and phosphorus atoms. As phosphorus has vacant 3d orbitals to accommodating surface electron and unshared electrons in 3p orbitals, the formation of PO34- can therefore accelerate the reaction step by providing more active states.


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Figure 6. High resolution XPS spectra of some representative mesh samples (a-d) before and (eg) after electrolysis. ((a-b) NiP60; (c-d) HT-NiP60; left column: Ni 2p region; right column: (b), (d) and (f): P 2p region; (g) O 1s region.))


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XPS measurement offered compositional transformation of electrodes before and after water splitting cycles. Some representative results were given in Figure 6. It is noteworthy that except NiP3, NiP60, NiP120 and HT-NiP60 exhibit weak peaks at 129-130 eV range of high-resolution P 2p region (Figure 6f and Figure S10) representing elemental P and phosphide existed in alloy phase. A dominant peak for phosphorus of all samples at around 133 eV can be related to oxidized P specie and agree with high valence P species such as PO43-.47 Under such circumstance, low valence P can be coupled into redox reaction to reduce the high valence Ni(III) to Ni(II).49 Since presence of high valance P(V) cannot be further oxidized, Ni(III) oxidized specie is therefore stabilized.50 Moreover, oxidized species will grow if oxygen can diffuse through the Niox and Pox layer. However, diffusion across oxide layer is controlled. A limited Pox layer was formed between Niox layer and the alloy. Further refinement of XPS results indicated that surface phosphorus concentration has decreased from about 8% to 3% for asdeposited electrodes after electrolysis, connected to the above discussion in terms of important roles that phosphorus played in this sense. This can be seen from Ni 2p patterns. Together with O 1s map, peaks for all samples in 531-532 eV range can be assigned to bond in oxyhydroxide, phosphate species and surface adsorbents.51 According to Siconolfi and co-worker’s results and XPS patterns in this work (Figure 6), a nickel oxy(hydroxide) layer formed on the surface as compared to those as-plated deposit.34 Stability tests were conducted in fixed current of 15 mA cm-2. Figure 7 shows potential change at certain current density for 10 hours operation for our NiP120 sample and Ni foam. For all selffabricated electrode, potential changes a little at the beginning and proceed towards stabilization even after 10h while for Ni foam, the potential increased through all the measurement indicating a not satisfying stability. The decrease at the early stage aligns well with Figure 7b. SEM cross-


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sectional images and EDX mapping of NiP120 before and after durability test confirmed a stabilized oxidized layer on the surface. No phase change was detected by XRD. XPS spectra showed an alignment of surface oxidized species with NiP60 as illustrated in the previous discussion. (Figure S10 and S11) The excellent catalytic behavior over commercial Ni foam is enabled by: 1) Potential drop at the first stage due to the continuous activation; 2) Good stability with minor variation 3.92mV over long time scale due to stabilized bilayer structure.

Figure 7. (a) Galvanostatic electrolysis on mesh sample surface against commercial nickel foam at constant current density of 15mA cm-2 in 1M KOH over 10h time frame (Insert: image of stability operation.); (b) Cyclic voltammograms for NiP120 after 20 cycles and after 10 hour stability test. CONCLUSION. In summary, a fabrication process has been developed for high-efficiency electrode for oxygen evolution reaction (OER). Robocasting (direct ink writing) has been used for Y-stabilized ZrO2 mesh with excellent chemical stability and high mechanical toughness. Thick fully amorphous Ni-P alloy layers were coated on zirconia substrate with a good adhesion because of large curvature. A highly stable nickel oxyhydroxide thin film was formed in electrolyte as effective OER catalyst, while the body remained metallic with high electric


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conductivity for effective charge transportation. Our result implies that phosphorus contributes to the formation of the stable oxide/hydroxide surface film (in the order of 200 nm) which can protect the further oxidation of the body under harsh chemical environment. After crystallization (heat treatment), there was no clear boundary between oxide and remaining body phase, indicating the importance of amorphous structure. Our experiments have demonstrated that a total layer thickness of 3-5 µm is enough for necessary conductivity to achieve high OER performance with an overpotential of 286 mV @ 10 mA cm-2 and a high effective capacity. Together with excellent chemical stability and mechanical strength, the robust electrode may have great potential in practical applications. Supporting Information. Detailed experimental information includes electrode fabrication, characterization and electrochemical measurement, Fusion360 model of 3d printed substrate, polarization curves of selected mesh electrode, images of sets of coated samples, EDX mapping, tables of catalytic parameters and comparison with reported work. This information is available free of charge at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Author * Corresponding author e-mail: [email protected], [email protected] (Professor Ding) Author Contributions † These authors contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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ACKNOWLEDGMENT The authors would like to thank Saint Gobain ZirPro for providing the zirconia powders. This project is financially supported by Saint Gobain (R-284-000-140-597), NUS Strategic Research Fund R-261-509-001-646, R-261-509-001-733 and NRF-CRP16-2015-01 (R-284-000-159-281). REFERENCES (1) McCrory, C. C.; Jung, S.; Ferrer, I. M.; Chatman, S. M.; Peters, J. C.; Jaramillo, T. F. Benchmarking hydrogen evolving reaction and oxygen evolving reaction electrocatalysts for solar water splitting devices. J. Am. Chem. Soc. 2015, 137, 4347-4357. (2) Hu, F.; Zhu, S.; Chen, S.; Li, Y.; Ma, L.; Wu, T.; Zhang, Y.; Wang, C.; Liu, C.; Yang, X.; Song, L.; Yang, X.; Xiong, Y. Amorphous Metallic NiFeP: A Conductive Bulk Material Achieving High Activity for Oxygen Evolution Reaction in Both Alkaline and Acidic Media. Adv. Mater. 2017, 29, 1606570-1606578. (3) Dresp, S.; Luo, F.; Schmack, R.; Kühl, S.; Gliech, M.; Strasser, P. An Efficient Bifunctional Two-component Catalyst for Oxygen Reduction and Oxygen Evolution in Reversible Fuel Cells, Electrolyzers and Rechargeable Air Electrodes. Energy Environ. Sci. 2016, 9, 2020-2024. (4) Rao, R. R.; Kolb, M. J.; Halck, N. B.; Pedersen, A. F.; Mehta, A.; You, H.; Stoerzinger, K. A.; Feng, Z.; Hansen, H. A.; Zhou, H.; Giordano, L.; Rossmeisl, J.; Vegge, T.; Chorkendorff, I.; Stephens, I. E. L.; Shao-Horn, Y. Towards identifying the active sites on RuO2(110) in catalyzing oxygen evolution. Energy Environ. Sci. 2017, 10, 2626-2637. (5) Linsey C. Seitz, C. F. D., Kazunori Nishio, Yasuyuki Hikita, Joseph Montoya, Andrew Doyle, Charlotte Kirk, Aleksandra Vojvodic, Harold Y. Hwang, Jens K. Norskov, Thomas F. Jaramillo. A Highly Active and Stable IrOx/SrIrO3 Catalyst for the Oxygen Evolution Reaction. Science 2016 353, 1011-1014.


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Hierarchical design of NiOOH @ amorphous Ni-P ... - ACS Publications

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