Nanoporous Nickel Phosphide Cathode for a High-Performance

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Nanoporous Nickel Phosphide Cathode for a High-Performance Proton Exchange Membrane Water Electrolyzer Jooyoung Kim, Junhyeong Kim, Hyunki Kim, and Sang Hyun Ahn* School of Chemical Engineering and Material Science, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul 06974, Republic of Korea

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ABSTRACT: Hydrogen production via a proton exchange membrane water electrolyzer (PEMWE) is an essential technology to complement discontinuity of renewable energies. Development of a high-efficiency and cost-effective gas diffusion electrode (GDE), which is a key component of this technology, remains a challenge. Here, we report a highperformance Ni phosphide GDE prepared by simple electrochemical methods. Selective leaching of excess Ni in electrodeposited NixP1−x enabled fabrication of a nanoporous NiP GDE with a large electrochemical surface area (ECSA). In half-cell tests, the nanoporous NiP GDE demonstrated a hydrogen-evolving current density of −10 mA/cm2 at an overpotential of 103 mV with good stability. In the single-cell tests, the PEMWE employing a nanoporous NiP cathode exhibited a current density of 1.47 A/cm2 at a cell voltage of 2.0 V, which was the competitive performance among state-of-the-art non-noble cathodes reported to date. KEYWORDS: transition metal phosphide, electrodeposition, selective leaching, nanoporous structure, hydrogen evolution reaction, proton exchange membrane water electrolyzer

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ratio.9,22,24 Ni phosphides exist in several phases, including Ni3P, Ni2P, Ni5P2, Ni12P5, and Ni5P4.22,24−26 It has been shown that the HER activity of Ni phosphides is increased by an increase of the P/Ni ratio. This can be explained by either the ensemble effect27 or ligand effect28,29 between Ni and P, which modify the hydrogen adsorption energy on the catalyst surface. In Ni phosphides, resistance to corrosion by acidic electrolytes is also related to the P/Ni ratio. Corrosion resistance is improved by increasing the P/Ni ratio because of the formation of much less soluble phosphate on the catalyst surface.14,22 For control of morphology, a high density of certain facets of nanostructured Ni2P results in high HER activity.9,25,27 Along with this, the enlarged electrochemical surface area (ECSA) of Ni2P obtained with highly roughened supports, such as Ni inverse opal30 or Ni foam,31 facilitates production of more active sites to reduce HER overpotential. There are two methods typically used to manufacture TMP electrocatalysts for the HER.32 For solution-phase synthesis, the metal precursors can be phosphorized at relatively high temperatures (∼300 °C).22,33 A substrate is then coated with a synthesized TMP electrocatalyst to form a catalyst layer.22,33 This process is time-consuming and may result in high contact resistance and poor adhesion. Another method for phospho-

INTRODUCTION Use of a proton exchange membrane water electrolyzer (PEMWE) for sustainable hydrogen production in connection with renewable energy is considered a potential candidate for the future energy supply, thereby complementing existing renewable energy technologies.1−3 For a PEMWE to be reasonably efficient and cost-effective for practical application, development of a highly active and stable gas diffusion electrode (GDE) is essential.3−5 Pt metal is regarded as the most effective electrocatalyst for cathodes, where the hydrogen evolution reaction (HER) occurs.6,7 Unfortunately, challenges remain for the development of earth-abundant electrocatalysts that can circumvent the scarcity and high cost of Pt.8−11 Most transition metals that demonstrate HER activity are theoretically unstable in part of the operating potential window for the HER.12−14 Various Ni-M alloys, where M = Fe,15 Co,16 Cu,17 Zn,18 Mo,15,19 and W,15 have been proposed as improved electrocatalysts for the HER. However, they may still undergo dissolution in acidic environments, resulting in problematic stability and start-up/shut-down issues during long-term operation of a PEMWE.20,21 In recent years, a great deal of research has been conducted on transition metal phosphide (TMP) electrocatalysts, in which composition and morphology can be controlled.10,11,22,23 Among them, Ni phosphides demonstrate superior HER catalytic activity and stability in acidic conditions, which are significantly influenced by the P/Ni © XXXX American Chemical Society

Received: May 9, 2019 Accepted: August 5, 2019 Published: August 5, 2019 A

DOI: 10.1021/acsami.9b08074 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

cathode, the performance of the PEMWE was improved by controlling several factors during single-cell fabrication. The optimized PEMWE demonstrated enhanced performance compared to current state-of-the-art PEMWEs.

rization of metals employs a gas−solid reaction, in which phosphine gas is used at elevated temperature (∼300 °C).34 These commonly used methods are disadvantageous for fabrication of electrodes, because they require multiple timeconsuming steps in processes performed at relatively high temperatures. They also involve costly equipment and/or and toxic gas. In recent, many efforts have been conducted to simplify the fabrication process using direct phosphorization of Ni35−37 or Co38 foam, which is also a feasible method to prepare Ni−P GDE.39 On the other hand, a more convenient electrodeposition process can be performed within a short time at room temperature and ambient pressure.5,40,41 With respect to preparing GDEs for a PEMWE, one-pot fabrication is feasible via direct electrodeposition of TMP catalyst on the gas diffusion layer (GDL), thereby minimizing contact resistance at the interface between the catalyst and the GDL.5,40,41 Electrodeposition is performed in a bath containing Ni and P precursors. A mixture of Ni metal and Ni phosphide results, indicating that metallic Ni is present in excess or is insufficient for electrodeposition compared to more thermodynamically stable Ni phosphides, such as Ni3P, Ni2P, Ni5P2, Ni12P5, and Ni5P4.42 Following electrodeposition, a P content lower than 15% eventually leads to corrosion problems during the HER in acidic environments.14 In order to overcome this, posttreatment with a selective leaching process has been introduced.43 Excess Ni metal is electrochemically dissolved at a positive potential where the Ni phosphides are stable, because they are more acid-resistant than Ni metal. In addition, the roughened surface generated after dissolution of Ni metal yields a larger ECSA for enhanced HER performance.44 Although much research has been performed to develop and evaluate transition metal-based electrocatalysts, most reports are from studies limited to HER activity measured in the half cell.9,22,24,25,33 For practical applications, performance of PEMWE single cells that employ the developed cathode should be demonstrated, because high HER activity in a halfcell test does not guarantee good performance in a single-cell test, even though the HER activities of several transition metalbased electrocatalysts approach that of commercial Pt/C catalyst.24,26,45 In several recent reports, PEMWE single cells that employed MoS2/C,20 MoSx,41 FeS2,46 Ni0.64Co0.36OxS0.14,40 Cu96.2Mo3.8,47 FeCoP/CF,48 Ni78P22,49 MoP|S−C,50 and Ni96W4/Cu NW51 cathodes demonstrated current density range between 0.35 and 1.79 A/cm2 at a cell voltage of 2.0 V. These values are obviously lower than the current densities of ∼1.7−2.7 A/cm2 obtained with the PEMWE single cell in which Pt/C was employed as the cathode catalyst.52−57 This implies that an electrode suitable for the single cell will be different than electrodes used for half cells. In the present study, a highly active and stable Ni phosphide GDE for PEMWE was fabricated by electrodeposition followed by a selective leaching process. For direct electrodeposition on a carbon paper (CP) substrate, Ni/P ratios in the deposits varied with the Ni/P precursor ratios in the deposition bath. A selective leaching process, in which leaching potentials and time were controlled, was then performed on the deposits. Under optimal leaching conditions, an electrochemically stable Ni phosphide cathode was obtained that had a large ECSA and modified electronic structure. The resulting Ni phosphide cathode demonstrated excellent HER activity and stability in acidic electrolyte in a half-cell test. While employing this

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EXPERIMENTAL SECTION

Electrodeposition of Nickel Phosphide (NixP1−x). A lab-made batch-type Teflon cell was connected to a potentiostat (Autolab PGSTAT302N, Metrohm) and used for NixP1−x electrodeposition. A conventional three-electrode system was used that consisted of a CP (TGPH-090, Toray) as a working electrode with an exposed area of 1.0 cm2; Pt wire as a counter electrode; and a saturated calomel electrode (SCE, KCl saturated) that served as a reference electrode. Prior to electrodeposition, the CP substrate was immersed in 30 wt % HNO3 (65.0%, JUNSEI) at 50 °C to obtain a hydrophilic surface.58 The deposition bath contained 100−250 mM NiCl2·6H2O (98.0%, WAKO) and 500 mM NaPO2H2·H2O (85.0%, DAEJUNG) as Ni and P precursors, respectively. The bath also contained 100 mM NaCl (99%, DAEJUNG) and 100 mM Na3C6H5O7 (99.5%, DAEJUNG) as supporting electrolytes. In order to purge dissolved oxygen, N2 bubbling was performed for 30 min with all electrolytes. Electrodeposition was carried out at a constant deposition potential of −1.5 VSCE for 600 s at room temperature under ambient pressure. Leaching of Electrodeposited NixP1−x/CP. The leaching procedure was also performed using the conventional three-electrode system in the lab-made batch-type Teflon cell. A graphite rod and a SCE were used as a counter electrode and a reference electrode, respectively. The electrodeposited NixP1−x/CP was carefully rinsed with deionized water and used as a working electrode. N2-purged 0.5 M H2SO4 (95%, JUNSEI) was employed as the leaching electrolyte. To determine the leaching potential, positive scans were performed by linear sweep voltammetry (LSV) over a potential range between −0.4 and 2.0 VSCE at a scan rate of 10 mV/s.43,59 All measured potentials were converted to reversible hydrogen electrode (RHE) potentials by ERHE = ESCE + 0.2412 + 0.0592·pH. Several leaching potentials were then applied in the range of −0.02 to 0.38 VRHE at intervals of 0.10 V by chronoamperometry (CA) varying leaching time. Characterization. Before and after the leaching procedure, the morphologies and bulk compositions of the NixP1−x/CPs were analyzed by field emission scanning electron microscopy (FESEM, SIGMA, Carl Zeiss) coupled with energy dispersive spectroscopy (EDS, Thermo, NORAN System 7). Their crystal and electron structures were determined by X-ray diffraction analysis (XRD, New D8-Advance, BRUKER) and X-ray photoelectron spectroscopy (XPS, K-alpha+, ThermoFisher Scientific), respectively. NixP1−x/CP surface morphologies were observed by transmission electron microscopy (TEM, JEOL Ltd., JEM-2100F), and their crystallinity was confirmed with selected area electron diffraction (SAED) and fast Fourier transformation (FFT) patterns. Double-layer capacitance (CDL), which reflected the ECSA, was measured by repeated CV over a non-Faradaic potential range in N2-purged 0.5 M H2SO4 electrolyte using the lab-made batch-type Teflon cell. The open circuit potential (EOCP) was held for 300 s to stabilize it prior to use, then a potential window of EOCP ± 0.05 V was used for repeated CV measurements at scan rates of 10, 25, 50, and 100 mV/s. The roughness factor (RF) was calculated by dividing CDL by the specific capacitance (CS). The CS was 0.035 mF/cm2, which is a typical value used for various metal electrodes in aqueous H2SO4 solutions, as previously reported.60,61 HER Activity and Stability Measurement. Catalytic activity was evaluated with the conventional three-electrode system in the labmade batch-type Teflon cell. A Pt wire and a SCE were used as the counter electrode and reference electrode, respectively. The NixP1−x/ CPs were used as working electrodes before and after leaching. In a potential range between −0.2 and −0.8 VSCE, negative scans with LSV at a scan rate of 10 mV/s were performed in N2-purged 0.5 M H2SO4 electrolyte. The obtained data was iR-corrected using a resistance of 2.23 Ω measured by electrochemical impedance spectroscopy (EIS, Iviumstat, Ivium Technology). Stability tests were performed with an H-type cell with a Nafion membrane (212, Dupont Co.). Selectively B


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Figure 1. (a) LSV curves of #NixP1−x/CP in 0.5 M H2SO4 electrolyte solution collected at a scan rate of 10 mV/s. Insets: FESEM images of #NixP1−x/CP. (b) Bulk Ni/P ratio of #NixP1−x/CP at various NiCl2 concentrations in the deposition bath.

Figure 2. (a) LSV curve of 200Ni0.91P0.09/CP in 0.5 M H2SO4 electrolyte solution at a scan rate of 10 mV/s. (b) Leaching process with CA on 200Ni0.91P0.09/CP. Inset: anodic charge densities. (c) LSV curves of 200Ni0.91P0.09/CP before and after leaching for 3600 s in 0.5 M H2SO4 electrolyte solution at various potentials at a scan rate of 10 mV/s. (d) Current and bulk Ni/P ratio before and after leaching process. CP cathode and an IrO2/CP anode (Figure S1). The IrO2/CP anode was prepared by electrodeposition with an IrO2 loading mass of 0.10 mg/cm2.56 As a reference, a Pt/C/CP cathode was prepared by spraying catalyst ink consisting of Pt/C (40 wt %, Premetek), Nafion ionomer (5 wt %, Alfa Aesar), isopropanol (99.5%, DAEJUNG), and deionized water in a weight ratio of 7:60:168:42. For assembly of the single cell, different gasket thicknesses of 155, 180, and 205 μm were evaluated for both the cathode and anode, and tests were performed at assembly pressures of 50, 60, and 70 N m. The active area of the

leached NixP1−x/CP was used as the working electrode in the catholyte portion of the cell, along with the SCE reference electrode. In the anolyte portion of the cell, Pt mesh served as a counter electrode. Chronopotentiometry was applied at −50 mA/cmgeo2 for 10 h, and the collected data was iR-corrected with a measured resistance of 4.30 Ω. PEMWE Single-Cell Operation. To fabricate a membrane electrode assembly (MEA), we sandwiched a commercial Nafion membrane (212, Dupont Co.) between a selectively leached NixP1−x/ C


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Figure 3. FESEM images of 200Ni0.91P0.09/CP (a) before and (b−f) after leaching at a potential of (b) −0.02 VRHE, (c) 0.08 VRHE, (d) 0.18 VRHE, (e) 0.28 VRHE, and (f) 0.38 VRHE for 3600 s. Insets: FESEM images at a higher magnification. PEMWE single cell was fixed at 1.0 cm2, and the operating temperature was maintained at 90 °C. Deionized water preheated to 90 °C was fed to the anode side at a constant flow rate of 15 mL/ min. Single-cell tests of NixP1−x/CPs were performed using a potentiostat in a cell voltage (Vcell) range between 1.50 and 2.00 Vcell at intervals of 0.05 V. Voltage was applied from 2.00 to 1.50 Vcell, and each voltage was maintained for 10 min. Meanwhile, single-cell tests with Pt/C/CP were performed in the cell potential range between 1.35 and 2.00 Vcell at intervals of 0.05 V. Each voltage was maintained for 1 min. Overpotential Analysis of PEMWE Single Cell. The ohmic resistance (ηohm) of the single cell was measured at 1.75 and 2.00 Vcell by EIS (Autolab FRA32M, Metrohm) in a frequency range of 50 kHz to 10 mHz. The ohmic overpotential (ηohm = iRohm) was calculated from the measured ohmic resistance to generate iR-corrected polarization curves. The kinetic overpotential (ηkin) was derived from the Tafel slope and exchange current density using the Tafel equation. The mass transfer overpotential (ηmass) was defined as the remaining overpotential according to equation: ηmass = E − E0 − ηkin − ηohm, where E0 is the theoretical potential for water electrolysis at 90 °C.

results of EDS analysis (Figure S3), it was determined that the bulk Ni/P ratio gradually increased from 4.6 (100Ni0.82P0.18/ CP) to 11.5 (250Ni0.92P0.08/CP), as shown in Figure 1b. This indicated that the excess metallic Ni was formed with Ni phosphide during electrodeposition. Figure 1a shows the LSV curves collected with #NixP1−x/CPs at a scan rate of 10 mV/s in the negative direction in a N2-purged 0.5 M H2SO4 electrolyte. For the bare CP, negligible current was obtained in the potential range (gray line). At the initial stage of the negative scans for #NixP1−x/CP, various anodic currents were observed, which indicated electrochemical oxidation in the potential range (e.g., Ni dissolution or oxide formation, Ni2+ + 2e− ↔ Ni, − 0.28 V vs RHE). Therefore, the cathodic currents measured in additional negative scans possibly represented electrochemical reduction of the oxidized NixP1−x deposits as well as that of the H+ in the electrolyte, which would inhibit accurate measurement of HER activity in the #NixP1−x samples. It should be noted that similar LSVs were found in reports from studies of acidic HER catalysis.14,62,63 Figure 2 demonstrates that further investigation on the oxidation behavior. LSV analysis was performed with the 200Ni0.91P0.09/CP sample in N2-purged 0.5 M H2SO4 electrolyte (Figure 2a). In the positive direction scans, two anodic peaks appeared, which corresponded to the oxidation of metallic Ni at 0.44 VRHE and NixP1−x at 1.46 VRHE.43,59 On the basis of the LSV curve, five potentials were chosen in the potential range of −0.02 to 0.38 VRHE with an interval of 0.10 V. For the leaching process, each potential is applied to 200Ni0.91P0.09/CP for 3600 s, as shown in Figure 2b. With the positive shift of applied potential, more time was required for saturation of the anodic current. The anodic charge density is

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RESULTS AND DISCUSSION The NixP1−x electrodes fabricated at various NiCl2 concentrations were designated #NixP1−x/CP, where # stands for the Ni precursor concentration in mM. Following electrodeposition, agglomerated spheres with continuous morphology were visible in all FESEM images, and a high degree of coverage was observed on the surfaces of individual carbon fibers. The FESEM images are shown in Figure 1a (insets) and Figure S2. With an increase of the Ni precursor concentration in the deposition bath, the sphere size increased, and the morphology of the deposits became smoother. From the D


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Figure 4. (a) LSV curves of 200Ni0.91P0.09/CP before and after leaching at various potentials for 3600 s in 0.5 M H2SO4 electrolyte at a scan rate of 10 mV/s. (b) Overpotentials recorded at a current density of −10 mA/cm2.

Figure 5. (a) LSV curves of 200Ni0.91P0.09/CP before and after leaching collected at a potential of 0.08 VRHE at a scan rate of 10 mV/s, depending on leaching time in 0.5 M H2SO4 electrolyte. (b) Current and bulk Ni/P ratios before and after the leaching process. (c) LSV curve of 200Ni0.91P0.09/CP before and after leaching at a potential of 0.08 VRHE, depending on leaching time in 0.5 M H2SO4 electrolyte at a scan rate of 10 mV/s. (d) Overpotentials recorded at a current density of −10 and −50 mA/cm2.

current densities continuously decreased for the second anodic peaks, demonstrating the electrochemical instability of NixP1−x deposits at relatively positive leaching potentials. In Figure 2d, the current ratios from the LSV tests (Figure 2c) and the bulk Ni/P ratios determined by EDS analysis (Figure S4) are plotted at potentials from −0.02 to 0.38 VRHE before and after

increased by a positive shift of leaching potential, as shown in the inset of Figure 2b. Figure 2c shows the LSV curves before and after leaching for 3600 s at each potential. With a positive shift of leaching potential, the first anodic peaks gradually decreased and then mostly vanished, indicating that the excess Ni metal was completely eliminated. On the other hand, the E


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Figure 6. Ni 2p and P 2p XPS spectra of (a) P−Ni0.63P0.37/CP and (b) L-Ni0.46P0.54/CP.

exhibited a potential of 147 ± 12 mV. After leaching at −0.02 and 0.08 VRHE, overpotentials for the HER were measured as 93 ± 9 mV and 104 ± 3 mV, respectively. With a positive shift in leaching potential, the measured potentials gradually increased, indicating less activity for the HER. However, it should be noted that the presence of slight anodic currents (Figure 4a) still inhibit accurate measurement of HER activity, and thus further modification is required. Leaching time was varied from 300 to 7200 s while a constant leaching potential of 0.08 VRHE was applied to the pristine 200Ni0.91P0.09/CP sample (Figure S5). The anodic current densities approached to zero value at 7200 s. Negligible changes in NixP1−x morphology were observed when leaching time was varied (Figure S6). LSV in the positive direction is then performed in N2-purged 0.5 M H2SO4 electrolyte, as illustrated in Figure 5a. The first anodic peaks continuously decreased as leaching time increased, whereas the second anodic peaks became less intense at 300 s relative to the pristine sample and indicated saturation after 600 s. The decay of the current ratio corresponded well with that of the bulk Ni/ P ratio determined by EDS (Figure S7), indicating that metallic Ni dissolution occurred continuously in the leaching time range of 300 to 7200 s (Figure 5b). Similarly, for LSV in the negative direction (Figure 5c), the anodic current is gradually decreased as a function of leaching time and then entirely vanished at 7200 s in the positive potential range. The onset potentials for cathodic currents were very similar after a leaching time of 600 s, indicating the intrinsic activities of the catalysts were similar. Figure 5d contains a summary of measured potentials at current densities of −10 and −50 mA/ cmgeo2. Potentials at both current densities gradually decreased and saturated as leaching time increased. Although the measured potentials are mostly saturated at longer leaching

leaching. For the pristine 200Ni0.91P0.09/CP sample, the current and bulk Ni/P ratios were 0.49 and 10.11, respectively. After leaching, both ratios gradually decreased and became saturated by the positive shift of leaching potential. Figure 3a shows FESEM images of pristine 200Ni0.91P0.09/ CP with agglomerated sphere morphology. After leaching at a potential of −0.02 VRHE (Figure 3b) and 0.08 VRHE (Figure 3c), several cracks appeared at the interfaces between spheres. Compared with the pristine sample, the sphere surfaces became smoother. This indicated that the excess deposited metallic Ni was located at the interfaces and surfaces of the spheres, and that it was selectively dissolved at these leaching potentials. At leaching potentials positively shifted to 0.18 and 0.28 VRHE (Figure 3d,e), the cracks became much wider, indicating that the dissolution of metallic Ni was possibly accelerated along with NixP1−x dissolution. Figure 3f shows that obvious damage on the deposits originated from a high degree of dissolution of both Ni and NixP1−x at a highly positive leaching potential of 0.38 VRHE. After leaching at various potentials, the HER activity is investigated with negative LSV scans, which are shown in Figure 4a. Compared with the pristine 200Ni0.91P0.09/CP, other samples demonstrated slight or zero anodic currents in the positive potential range. After leaching at −0.02 VRHE and 0.08 VRHE, similar catalytic activities for the HER were observed. The enhanced activity after leaching originated in the selective dissolution of less active Ni, whereas the highly active NixP1−x remained at these leaching potentials. In addition, several cracks enlarged ECSA for the HER compared to the pristine sample. On the other hand, with the positive shift of leaching potential, catalytic performance gradually decreased due to dissolution of NixP1−x. The measured potentials at −10 mA/ cmgeo2 are summarized in Figure 4b. The pristine sample F


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ACS Applied Materials & Interfaces time, the sample obtained at a leaching time of 7200 s was chosen as optimized one, because of the absence of anodic current in LSV (Figure 5c), to measure accurate HER activity. For this sample, the HER overpotentials at −10 and −50 mA/ cmgeo2 were 103 ± 1 and 149 ± 9 mV, respectively. XPS spectra for the pristine and optimally leached samples (at 0.08 VRHE for 7200 s) are shown in Figure 6. The Ni0 2p3/2 peak in the pristine sample was observed at 853.0 eV, which was positively shifted 0.2 eV from the pure Ni0 2p3/2 peak at 852.8 eV.33,64 The oxidized state of metallic Ni was typically observed with the electron transfer from Ni to P, which was induced by the difference between their electronegativities (1.9 and 2.2 for Ni and P, respectively). As a result, the peak binding energy (BE) for P 2p3/2 in the pristine sample was negatively shifted by 0.3 eV from the elemental P 2p3/2 BE of 130.2 eV,33,65 indicating that the electronic structure was reduced. The Ni and P molar compositions on the surface of the pristine sample were determined to be 62.8 and 37.2%, respectively. After leaching, the Ni0 2p3/2 peak BE of 853.3 eV and the P 2p3/2 peak BE of 129.6 eV indicated enhanced electron transfer from Ni to P, owing to the dissolution of deposited excess metallic Ni. The Ni and P surface compositions after leaching were 46.2 and 53.8%, respectively, reflecting the decrease of Ni on the sample surface. The surface Ni/P ratio of 0.86 measured by XPS was significantly different from the bulk Ni/P ratio of 2.85 measured by EDS (Figure 5b), which was due to the different analytic depths of XPS and EDS. This also indicated that the selective leaching of metallic Ni was carried out locally at the surface. In addition, the Ni2+ 2p3/2 peak was conceivably attributable to the native oxide formation of excess Ni, which was obviously decreased by leaching. The two samples were designated P−Ni0.63P0.37/CP (pristine) and L-Ni0.46P0.54/CP (optimally leached), based on their surface composition. The surface morphology of P−Ni0.63P0.37/CP (Figure 7a,b) and L-Ni0.46P0.54/CP (Figure 7c,d) is imaged by TEM analysis. At higher magnification, the nanoporous structure in the LNi0.46P0.54/CP (Figure 7d) was observed clearly at the edge position following the leaching process when compared with the smooth surface of P−Ni0.63P0.37/CP (Figure 7b). The numerous nanopores were generated by electrochemically selective dissolution of occupied Ni metals. It was expected that this highly roughened surface provided the enlarged ECSA, which is advantageous for the HER. Elemental mapping revealed uniform distribution of Ni and P in both L-Ni0.46P0.54/ CP (Figure 7e,f) and P−Ni0.63P0.37/CP (Figure S8), also confirmed by FESEM-EDS analysis at low magnification (Figure S9). At the edge site of L-Ni0.46P0.54/CP, three spots were selected to investigate the crystallinity of the sample (Figure S10a−d). Due to the absence of clear electron diffraction patterns, the amorphous nature of L-Ni0.46P0.54/CP was confirmed, which was analogous to the P−Ni0.63P0.37/CP (Figure S10e,f). In addition, XRD patterns obtained from LNi0.46P0.54/CP and P−Ni0.63P0.37/CP indicated each had an amorphous structure (Figure S11). The LSV curves of P−Ni0.63P0.37/CP and L-Ni0.46P0.54/CP are shown in Figure 8a. Even though the measured cathodic current for P−Ni0.63P0.37/CP was the sum of the HER and deposit reduction, the L-Ni0.46P0.54/CP curve shows a larger cathodic current without an anodic current, indicating that the leaching process facilitated accurate HER measurement and high HER catalytic activity. The Tafel slope of 53.4 mV/dec in the Tafel plot of Ni0.46P0.54/CP, shown in the inset of Figure

Figure 7. (a) Low- and (b) high-magnification TEM image of the P− Ni0.63P0.37/CP surface. (c) Low- and (d) high-magnification TEM image of the L-Ni0.46P0.54/CP surface. Corresponding elemental mapping images are shown in e and f.

8a, indicates that the HER occurred via the Volmer-Heyrovsky mechanism.8 The exchange current density was 117 μA/ cmgeo2, which was a reasonable value when compared to reported values for Ni3P (300.0 μA/cmgeo2),26 Ni12P5 (28.6 μA/cmgeo2),22 Ni2P (33.0 μA/cmgeo2),9 Ni2P (2.9 μA/ cmgeo2),14 Ni2P (3.8 μA/cmgeo2),66 Ni5P4 (57.0 μA/cmgeo2),22 and NiP2 (260.0 μA/cmgeo2).67 Figure S12 shows the CDL measurement results for L-Ni0.46P0.54/CP. From the repeated CVs in the non-Faradaic potential range (inset of Figure S12),60 the mean CDL was determined to be 37.6 mF/cmgeo2 and then, based on the CS value of 0.035 mF/cm2,60,61 the ECSA could be calculated as 1074.3 cm2 with the RF of 1074.3 cm2/cmgeo2, (geometric area: 1.0 cmgeo2). This huge value could be explained by the highly roughened surface of the nanoporous structure shown in Figure 7d. For comparison, the performances of other TMP catalysts reported in the literature are summarized in Table S1. Among them, the HER currents of Ni−P catalysts at −0.15 VRHE normalized by geometric area and RF are shown in Figure S13. The L-Ni0.46P0.54/CP demonstrates a moderate level of intrinsic HER activity compared with other Ni−P catalysts. The leaching process increases the surface P/Ni ratio via selective Ni dissolution, facilitating the ensemble effect27 or ligand effect28,29 at the surface of L-Ni0.46P0.54/CP to modify its hydrogen adsorption energy. Meanwhile, the leaching process also increases ECSA of L-Ni0.46P0.54/CP with its nanoporous structure, taking the morphological advantage with a number of active sites for the HER. As a result, synergy between high P/Ni ratio and G


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Figure 8. (a) LSV curves of P−Ni0.63P0.37/CP and L-Ni0.46P0.54/CP in 0.5 M H2SO4 electrolyte collected at a scan rate of 10 mV/s. Inset: Tafel plots of LSV curve for L-Ni0.46P0.54/CP. (b) Long-term stability test of L-Ni0.46P0.54/CP in an H-type cell at a current density of −50 mA/cm2 for 10 h. Insets: photographic images of the cell configuration and hydrogen bubbles on the surface of the working electrode.

Figure 9. (a) Polarization curves depending on gasket thickness for single-cell PEMWEs with a L-Ni0.46P0.54/CP cathode. (b) Overpotential subdivisions of a. (c) Tafel plots of the polarization curves for single-cell PEMWEs with Pt/C/CP and L-Ni0.46P0.54/CP cathodes. (d) Overpotential subdivisions of c.

measured potential fluctuated severely, because of the formation and detachment of hydrogen bubbles at the surface (Figure 8b inset). However, the mean potential value was maintained, indicating good stability of L-Ni0.46P0.54/CP. The LSV curves obtained before and after the stability test showed

enlarged ECSA shows high geometric HER current density, which is superior to that of other Ni−P catalysts. The stability of L-Ni0.46P0.54/CP in the H-type cell was tested by chronopotentiometry at an applied current density of −50 mA/cm2 for 10 h (Figure 8b). During the HER, the H


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dense catalyst layer formed by the conventional spray-coating method resulted in a trade-off between TPB formation (ηkin) and mass transfer (ηmass). In a recent study of ours, the GDE fabricated through the formation of Pt catalyst on the CP surface was only advantageous for PEMWE operating in the high current density region,5 which was identical to the nonnoble GDE. Thus, it can be suggested that the proper GDE structure for a high-performance PEMWE should have roughened catalyst on the CP surface, thereby maintaining the porosity of GDL. The PEMWE was operated at a constant current of 1.0 A/cm2 for 16 h, as shown in Figure S19, which illustrates the excellent stability of the L-Ni0.46P0.54/CP cathode. Compared to data in the literature from non-noble (Figure S20 and Table S2) and noble cathodes (Table S3), the L-Ni0.46P0.54/CP cathode demonstrated the competitive performance metrics for PEMWE operation, with an effective strategy for rapid and simple fabrication.

similar feature without significant change in morphology and surface composition (Figure S14). Figure 9a shows the polarization curves of the PEMWE equipped with an L-Ni0.46P0.54/CP cathode and an IrO2/CP anode. The gasket thickness for electrodes on both sides was tested at 155, 180, and 205 μm. The gasket thickness provided control of compressibility at both electrodes (catalyst deposits + GDL), allowed adjustment of contact at the membranecatalyst interface, and mass transfer of reactant/product streams. The performance of the PEMWE was maximized with a gasket thickness of 180 μm. The ohmic resistances measured by EIS were similar at ∼0.063 Ω cm2 (Figure S15a) under all conditions. These low ohmic resistances indicated good electrical conductivity by the L-Ni0.46P0.54/CP catalyst and lower contact resistance at the interface between the catalyst and membrane.4,68,69 To determine the effect of gasket thickness, overpotential breakdown was performed at current densities of 0.50, 0.75, and 1.00 A/cm2 as shown in Figure 9b. At each current density, the ηohm values calculated with the ohmic resistances were almost identical, indicating that contact between the catalyst and membrane was sufficient in all cases. On the other hand, the kinetic overpotential (ηkin) values derived from the exchange currents (Figure S16) were affected by gasket thickness. The ηkin value obtained in the 155 μm case exceeded the ηkin values obtained with the gaskets with 180 and 205 μm thicknesses. This could be because formation of the triple-phase boundary (TPB) might have differed at the respective interfaces.4 Interestingly, the ηmass value in the 180 μm case was smallest across the current density range, and its merit became obvious at higher current densities, where H2 and O 2 gases could be vigorously generated. Strong compression with the thinner gasket resulted in reduced GDL porosity, which inhibited the supply of the reactants as well as release of the products. Conversely, for the thicker gasket, weak compression might have increased the diffusion length for both reactants and products.70 Similar behavior was observed when the assembly pressure for the cathode was varied at a constant gasket thickness of 180 μm (Figures S15b and S17). Polarization curves (Figure S18a) were used to evaluate performance of the PEMWE equipped with either a commercial Pt/C/CP cathode or a L-Ni0.46P0.54/CP cathode with a gasket thickness of 215 μm. The resulting Tafel plots are shown in Figure 9c. The Pt/C/CP cathode demonstrated better performance than the L-Ni0.46P0.54/CP cathode. However, the Tafel plot of the Pt/C/CP cathode deviated significantly from the Tafel line at higher current densities, revealing the severe limitation by mass transfer. Overpotential breakdown yielded similar ηohm values (Figure S18b) at each current density, which is summarized in Figure 9d. The ηkin values obtained with the Pt/C/CP cathode were much smaller than those obtained with the L-Ni0.46P0.54/CP cathode, because accelerated TPB formation was enabled by the larger active sites and well-connected H+ network in the Pt/C catalyst layer supported by Nafion ionomer. However, the ηmass values obtained with the Pt/C/CP cathode were much larger than those obtained with the L-Ni0.46P0.54/CP cathode in the current density range of 0.50−1.25 A/cm2, and ηmass was gradually increased by an increase of current density, whereas the ηkin values remained mostly constant. Because of the dense Pt/C catalyst layer between the membrane and the GDL, mass transfer of the reactant and product was suppressed, especially in the higher current density region. This indicated that the

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CONCLUSION Nanoporous NiP was fabricated on CP surface by simple electrochemical methods consisted of electrodeposition and following selective leaching. After optimizing the leaching potential and time, high catalytic activity for HER in acidic electrolyte was obtained in a half-cell test because of the enlarged surface area and higher surface P/Ni ratio. The LNi0.46P0.54 electrocatalyst also demonstrates good stability. In a single-cell test, the MEA structure was optimized by controlling the compressibility of GDE by using gasket thickness and assembly pressure. It has been revealed that the moderately compressed cathode has advantages for enhanced mass transfer of reactants and products. Compared with the Pt/C/CP cathode, the MEA performance of LNi0.46P0.54/CP was lower; however, the smaller ηmass measured at the same current density provided the proper GDE structure, including the roughened catalyst selectively formed at the CP surface with maintained porosity of the GDL. It should be noted that the smaller ηmass of the cost-effective LNi0.46P0.54/CP cathode was advantageous, especially at higher current density, which was useful for practical application. Furthermore, the PEMWE with the L-Ni0.46P0.54/CP cathode demonstrated competitive performance among state-of-the-art non-noble cathodes with excellent stability reported recently.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b08074. Summary on performance of electrocatalyst and PEMWE, photo image and schematic illustration of PEMWE single-cell configuration, FESEM images, EDS spectra, CA data, TEM images, HRTEM images, XRD patterns, CDL measurements, Nyquist plots, Tafel plots, polarization curves for PEMWE single cell, overpotential subdivisions of polarization curves, stability test data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. Tel: +82-2-820-5287. Fax: +82-2824-3495. ORCID

Sang Hyun Ahn: 0000-0001-8906-5908 I


Research Article

ACS Applied Materials & Interfaces Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government MSIT (2018R1A4A1022647, 2018R1A1A1A05077634, and 2019M3E6A1064763). This research was supported by the Korea Institute of Energy Technology Evaluation and Planning funded by the Korea government MOTIE (2019281010007A).

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Nanoporous Nickel Phosphide Cathode for a High-Performance

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