Electrochemical fabrication of Fe-based binary and ternary phosphide cathodes for proton exchange membrane water electrolyzer

Journal of Alloys and Compounds 807 (2019) 148813

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Electrochemical fabrication of Fe-based binary and ternary phosphide cathodes for proton exchange membrane water electrolyzer Junhyeong Kim, Hyunki Kim, Jooyoung Kim, Jung Hwan Kim, Sang Hyun Ahn* School of Chemical Engineering and Material Science, Chung-Ang University, 84 Heukseokno, Dongjak-gu, Seoul 06974, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 September 2018 Received in revised form 10 December 2018 Accepted 12 December 2018 Available online 13 December 2018

The development of high-performance gas diffusion electrodes is essential for the fabrication of efficient proton exchange membrane water electrolyzers (PEMWEs) and thus for clean hydrogen production. Herein, we electrodeposited Fe-based binary and ternary phosphides on porous carbon paper (CP) as a substrate and demonstrated that, under optimized deposition conditions (i.e. when the P content was maximum), amorphous FeCoP showed the highest intrinsic activity for hydrogen evolution in an acidic medium. This behavior was ascribed to appropriate electronic structure modification and the alloying effect. Further enhancement of hydrogen evolution performance was achieved by increasing the electrochemical surface area of FeCoP by using a porous Cu foam (CF) support. In a half-cell test, the FeCoP/ CF/CP electrode featured an acceptably stable cathodic current of
10 mA/cm2 at an overpotential of
125 mV. A PEMWE single cell with an FeCoP/CF/CP cathode exhibited a current density of 0.95 A/cm2 at a cell voltage of 2.0 V, which is superior to or comparable with previously reported values. Thus, the developed electrode might be a promising alternative to Pt-based cathodes in practical PEMWE applications. © 2018 Elsevier B.V. All rights reserved.

Keywords: Electrode materials Chemical synthesis Electrodeposition Electrochemical reactions Hydrogen evolution reaction Proton exchange membrane water electrolyzer

1. Introduction Water electrolysis has been recognized as a promising technology for eco-friendly hydrogen production that can mitigate the problems of fossil fuel depletion and its associated environmental pollution [1e4]. Particularly, the selectivity of hydrogen and oxygen production can be increased using membrane electrode assembly (MEA)-based water electrolyzers. These electrolyzers comprise an ion-conducting membrane separating the cathode and anode [4] and feature a compact cell configuration. This allows the ohmic resistance to be minimized and thus overcomes the main disadvantage of conventional electrolyte-based water electrolyzers [4,5]. In addition, eco-friendly energy conversion systems can be established when the water electrolyzers are operated by electricity generated from renewable energy sources [6]. Among MEA-based water electrolyzers, those containing proton exchange membranes exhibit higher hydrogen production rates and energy efficiencies than those containing anion exchange membranes [7]. However, because of the acidic environment, large amounts of

* Corresponding author. E-mail address: [email protected] (S.H. Ahn). https://doi.org/10.1016/j.jallcom.2018.12.172 0925-8388/© 2018 Elsevier B.V. All rights reserved.

noble metals are currently used in the electrodes of protonexchange membrane water electrolyzers (PEMWEs), which hinders their large-scale use in commercial applications [8,9]. Considering the potential window of PEMWE operation and related stability issues, the utilization of noble metal oxides (e.g., IrOx and RuOx) in the anode to promote the oxygen evolution reaction cannot be avoided [10], whereas a much wider range of materials can be used in the cathode, where the hydrogen evolution reaction (HER) occurs [11]. In this sense, numerous first-row transition metals and their derivatives, such as phosphides [12e17], sulfides [18e21], carbides [22,23], and nitrides [24e26], have been tested as electrocatalysts for HER. However, only a few articles have reported the performance of non-noble cathodes for PEMWE single-cell operation [18,27e29]. Transition metal phosphides (TMPs; CoP [30e33], NiP [34e36], FeP [12e14,17], and MoP [37e39]) are viewed as promising replacements of Pt because of their high HER activity in acidic media and good stability. Additionally, further improvement of intrinsic HER activity can be realized by employing more than one transition metal [40,41]. For example, FeCoP shows a near-optimal hydrogen adsorption free energy (DGH) that is similar to that of Pt [41] and is significantly affected by the Fe/Co ratio. Specifically, the opposing effects of the strong hydrogen adsorption on FeP and weak

2

J. Kim et al. / Journal of Alloys and Compounds 807 (2019) 148813

hydrogen adsorption on CoP are optimally balanced at a Fe/Co ratio of ~1 [41], as supported by several recent reports on FeCoP catalysts [42,43]. A similar enhancement of intrinsic HER activity has been observed in FeNiP [44,45] and NiCoP [46] catalysts. The modified electronic structure of two metals complementing each other shows a synergetic effect from the metallic alloy structures [44,46]. Furthermore, it has been reported that the intrinsic HER activity of bimetallic sulfides such as CoMoSx [47], FeNiSx [48], WxMo1
xS2 [49], and CoxW1
xS2 is significantly affected by the ratio of the two metals [50]. Alternatively, HER activity can be increased by elevating the electrochemical surface area (ECSA), which can be achieved by nanostructure engineering of the catalyst itself [51e53] or catalyst preparation on hierarchical metal supports such as Ni foam and Cu foam (CF), which are cost-effective [54e57]. In addition, the electrical conductivity of the metallic support should be considered as an important factor for determining the electron transfer to the active site [58]. Thus, the above approach overcomes the low intrinsic activity of TMPs (compared to that of Pt) by significantly increasing the number of HER sites. TMP catalysts are commonly prepared by gas-phase phosphidation [59,60]. However, this requires the use of high temperatures (~300  C), long process times, and a toxic gas atmosphere [60]. Alternatively, TMPs can be prepared by solution-phase techniques typically used for nanoparticle formation [35,61] that also require relatively high temperatures (~300  C) but exhibit the advantages of day-scale process times and the elimination of ligands on the catalyst surface after synthesis [36,61]. The as-prepared catalysts are obtained as powders, and an additional coating process is therefore required to deposit them on the gas diffusion layer (GDL) to fabricate electrode structures suitable for PEMWEs [62,63]. The above disadvantages can be circumvented using electrodeposition, which is a simple process allowing TMP catalysts to be prepared in a short time at room temperature and ambient pressure [64,65]. Particularly, the morphology and composition of TMP catalysts can be easily controlled by varying the applied deposition potential, time, and electrolyte composition. Furthermore, when the GDL is used as an electrodeposition substrate, the direct formation of the TMP catalyst allows for a one-pot fabrication of a proper PEMWE electrode structure and for minimization of the contact resistance of the TMP catalyst/GDL surface interface. Herein, Fe-based binary and ternary phosphides were directly fabricated on carbon paper (CP) by electrodeposition. After optimization of deposition conditions, the HER activities of FeP/CP, FeCoP/CP, and FeNiP/CP composites were tested in an acidic electrolyte. Based on the results of double-layer capacitance (Cdl, a measure of ECSA) measurements, the order of HER intrinsic activity was determined as FeCoP/CP > FeNiP/CP z FeP/CP, i.e., the introduction of Co into FeP/CP modified the electronic structure of the latter and resulted in increased HER activity. Subsequently, the ECSA of FeCoP was increased by supporting it on lab-made CF electrodeposited on CP. Half-cell tests revealed that the FeCoP/CF/ CP cathode showed an overpotential of 125 mV at
10 mA/cm2 and an acceptable long-term stability over 8 h. Furthermore, a single PEMWE cell employing the FeCoP/CF/CP cathode exhibited a current density of 0.95 A/cm2 at 2.0 V that could be mostly maintained for 8 h.

electrodeposition, the CP substrate was treated by 30 wt% HNO3 at 50  C for 30 min to increase surface hydrophilicity [66]. The deposition electrolyte contained transition metal precursors (FeSO4$7H2O, CoSO4$7H2O, and NiSO4$7H2O), a phosphorus precursor (NaH2PO2$H2O), and supporting electrolytes (C6H8O7 and (NH4)2SO4) (Table S1). All electrolytes were purged with N2 gas for 30 min before proceeding with the experiment. Fe-based TMPs were electrodeposited onto pretreated CP at various potentials and times controlled by a potentiostat (Autolab, PGSTAT032F, Metrohm). In addition, CP coated with lab-made CF was used as a metallic support to increase the active area of the Fe-based TMPs.

2.2. Bubble-templated electrodeposition of CF onto CP Cu was chronopotentiometrically electrodeposited on pretreated CP by two steps (
0.3 A/cm2 for 1 s and
1.7 A/cm2 for 5 s) in the electrolyte containing 0.12 M CuSO4$5H2O, 1.2 M (NH4)2SO4, 0.4 mM 1,2,3-benzotriazole, and 0.7 M H2SO4 [67]. The as-prepared composite was washed with deionized water, electrochemically etched in N2-purged 0.1 M H2SO4 at
5.0 mA/cm2 for 300 s to eliminate the native oxide on the CF surface, and used as a working electrode for TMP electrodeposition.

2. Experimental 2.1. Electrodeposition of Fe-based TMPs Fe-based TMP catalysts were fabricated by electrodeposition. The three-electrode cell system comprised CP, a saturated calomel electrode (SCE, KCl saturated), and Pt wire as the working, reference, and counter electrodes, respectively. Before

Fig. 1. (a) LSV curves of the CP substrate recorded at a scan rate of 10 mV/s in transition metal precursor-containing electrolytes without (scattered points) and with (solid lines) the P source. (b) Bulk P content of electrodeposited TMPs at a deposition time of 600 s as a function of deposition potential.

J. Kim et al. / Journal of Alloys and Compounds 807 (2019) 148813

3

Fig. 2. Low- and (inset) high-magnification FESEM images of (a) pretreated CP and (b) FeP/CP, (c) FeCoP/CP, and (d) FeNiP/CP electrodeposited on CP.

Fig. 3. (a) FESEM image of FeCoP/CP. Elemental mapping images of FeCoP/CP: (b) Fe, (c) Co, (d) P. (e) TEM image and (inset) SAED pattern of a single FeCoP particle. STEM and elemental mapping images of a single FeCoP particle: (f) Fe, (g) Co, (h) P.

4

J. Kim et al. / Journal of Alloys and Compounds 807 (2019) 148813

2.3. Characterization The surface morphology of as-prepared TMP catalysts was observed by field-emission scanning electron microscopy (FESEM; SIGMA, Carl Zeiss), and crystal structures were investigated by Xray diffraction (XRD; New D8-Advance, BRUKER). XRD analysis was performed in the 2q angle range of 20 e80 at a scan rate of 5 /min. Bulk and surface compositions were probed by energy-dispersive spectroscopy (EDS; Thermo NORAN System 7) and X-ray photoelectron spectroscopy (XPS; K-alphaþ, ThermoFisher Scientific), respectively. Transmission electron microscopy (TEM; JEOL Ltd., JEM-2100F) and scanning transmission electron microscopy (STEM; bright-field, lattice resolution ¼ 0.2 nm) were employed to examine the shape and composition of a single TMP catalyst particle detached from CP by ultrasonication. The crystal structure and elemental distribution of the single particle were analyzed by selected area electron diffraction (SAED) and EDS (Oxford Instruments, AZtecTEM), respectively.

and then exhibited small fluctuations, which indicated that the reduction of Hþ on as-deposited TMPs occurred at a potential more negative than
1.40 VSCE. In P precursor-containing electrolytes, TMP electrodeposition on the pretreated CP substrate was conducted by applying various constant potentials (from
1.0 to
2.0

2.4. Electrochemical measurements Electrochemical properties were measured in N2-purged 0.5 M H2SO4 as an electrolyte using a three-electrode system connected to a potentiostat (Autolab PGSTAT302N, Metrohm). Electrodeposited Fe-based TMP catalysts, SCE, and Pt wire were used as the working, reference, and counter electrodes, respectively. Their HER activity was characterized by cyclic voltammetry (CV) measurements in a potential range of
0.35 to
0.80 VSCE at a scan rate of 50 mV/s. The obtained CV curves were iR-corrected using the electrolyte resistance measured by electrochemical impedance spectroscopy (EIS): 1.60 U for FeP/CP, 1.48 U for FeCoP/CP, and 1.53 U for FeNiP/CP. Cdl values representing ECSA were determined by repeated CV measurements in a non-faradaic potential range at scan rates of 10e100 mV/s. All measured potentials were referenced to the reversible hydrogen electrode (RHE). 2.5. PEMWE single cell operation PEMWE single cell performance was evaluated by employing electrodeposited FeCoP/CF/CP as the cathode and electrodeposited IrO2/CP (loading ¼ 0.1 mg/cm2) as the anode [8]. The MEA with an active area of 1 cm2 was fabricated by sandwiching a Nafion 212 membrane (Dupont Co.) between the above electrodes. Preheated deionized water (95  C) was injected into the anode via a serpentine-type flow channel, and the PEMWE single cell (maintained at 90  C) was operated in the potential range of 2.00e1.50 Vcell at an interval of 0.05 V with each potential maintained for 30 min. 3. Results and discussion Deposition potentials were determined by recording linear sweep voltammetry (LSV) curves of the pretreated CP substrate in transition metal precursor-containing electrolytes with and without the P precursor at a scan rate of 10 mV/s (Fig. 1a). In the absence of the P precursor, the onset potentials appeared at
1.05,
0.96, and
1.01 VSCE in the 500 mM Fe2þ, 250 mM Fe2þ þ 250 mM Co2þ, and 250 mM Fe2þ þ 250 mM Ni2þ electrolytes, respectively. In the presence of the P precursor, all onset potentials were positively shifted and appeared at approximately
0.93 VSCE, which indicated that the reduction of transition metal ion-citric acid complexes involved the P precursor. In addition, the P precursor acted as a supporting electrolyte and lowered the ohmic resistance of the deposition electrolyte (Fig. S1). As the potential shifted to negative values, the current density gradually increased

Fig. 4. P 2p XPS spectra of (a) FeP/CP, (b) FeCoP/CP, and (c) FeNiP/CP.

J. Kim et al. / Journal of Alloys and Compounds 807 (2019) 148813

VSCE at an interval of 0.2 V) for 600 s. Fig. 1b shows the EDSdetermined bulk P content of TMP/CP as a function of deposition potential, revealing that at a deposition potential of
1.0 VSCE, the P content of FeP/CP, FeCoP/CP, and FeNiP/CP ranged from 25 to 30 at%. As the applied potential became more negative, the P content continuously decreased and became saturated after
1.6 VSCE. M2þ þ 2e
/ M (M ¼ Fe, Co, Ni)

(1)

þ
H2PO
2 þ 2H þ e / P þ 2H2O

(2)

2Hþ þ 2e
/ H2

(3)

Contrary to the TMP electrodeposition at low potentials, where most of the Hþ could be used for P precursor reduction (reaction (2)) [68], the Hþ used for HER on as-deposited TMP became dominant at a high potential range (reaction (3)), indicating that reaction (3) occurred more readily than reaction (2). As a result, TMPs electrodeposited at more negative potentials featured lower P contents [68] with accelerated HER. Based on the results of LSV and EDS analyses, the deposition potential was determined as
1.20 VSCE, because it is generally accepted that a higher P content in TMPs demonstrates a higher HER catalytic activity [35,37,69]. Fig. 2a shows a representative FESEM image of the CP substrate pretreated in 30 wt% HNO3 at 50  C for 30 min, and Fig. 2bed and S1bed demonstrate that three kinds of Fe-based TMPs were

5

formed on the CP surface with high coverage and uniform distribution after a deposition potential of
1.20 VSCE was applied for 10 min. Thus, proper pretreatment significantly increased the hydrophilicity of the CP surface. In addition, it should be noted that the surfaces of the inner carbon fibers were less or not coated by Febased TMPs due to the insufficient mass transfer of the metal and P precursors. High-magnification FESEM imaging revealed that the TMPs were comprised of partially agglomerated spherical particles, which was ascribed to the dominance of charge transfer from the CP surface under the employed deposition conditions (i.e., at a less negative deposition potential). EDS analysis demonstrated that Fe, Co, and P elements were uniformly distributed in FeCoP/CP (Fig. 3aed) and the single FeCoP particle (Fig. 3eeh). The single FeCoP particle imaged by TEM had a size of ~200 nm (Fig. 3e), which corresponded well to the results of FESEM imaging (inset of Fig. 2c). The SAED pattern of the FeCoP particle did not feature a diffraction ring (inset of Fig. 3e). This indicated an amorphous nature, which is in good agreement with the results of the XRD analysis (Fig. S3). Similarly, FeP/CP and FeNiP/CP also exhibited amorphous structures (Figs. S3 and S4) and uniform distributions of constituent elements (Fig. S5). The electronic structures and surface compositions of Fe-based TMPs were probed by XPS. Fig. 4 shows the P 2p spectra of FeP/ CP, FeCoP/CP, and FeNiP/CP. The spectrum of FeP/CP could be deconvoluted into three peaks at 129.5, 130.4, and 132.8 eV (Fig. 4a). The peaks at 129.5 and 130.4 eV were ascribed to the P 2p3/ 2 and 2p1/2 transitions of FeP, respectively, and featured binding

Fig. 5. (a) Polarization curves of the CP substrate and TMP/CP catalysts recorded at a scan rate of 50 mV/s. (b) Tafel plots corresponding to polarization curves. (c) Results of Cdl measurements for TMP/CP catalysts. (d) HER current densities and scaled currents at
0.20 VRHE. All electrochemical measurements were performed in N2-purged 0.5 M H2SO4.

6

J. Kim et al. / Journal of Alloys and Compounds 807 (2019) 148813

energies more negative than that of the P 2p3/2 peak of elemental P (130.1 eV) [70]. In addition, the peak at 132.8 eV reflected the occurrence of partial P oxidation caused by the exposure of the catalyst surface to air. Similarly, three separate peaks were observed in the P 2p spectra of FeCoP/CP and FeNiP/CP (Fig. 4b and c, respectively). Figs. S6 and S7 show the compositional analyses of the Fe 2p, Co 2p, and Ni 2p spectra of TMP/CP catalysts. All 2p3/2 peaks were positively shifted relative to those of metallic Fe (706.8 eV) [15], Co (778.1 eV) [71], and Ni (852.5 eV) [72], which confirms the occurrence of electron transfer from the transition metal to P due to electronegativity differences. The change in electronic structure was expected to result in high HER activity, since separating the roles of transition metals as hydride acceptors and P as a proton acceptor has been reported to accelerate HER on TMPs in acidic solutions [73]. In addition, the activity of TMP catalysts is influenced by the kind of transition metal and elemental composition [74]. The HER activities of the CP substrate and TMP/CP catalysts were measured by CV in N2-purged 0.5 M H2SO4 (Fig. S8). Fig. 5a shows representative first-cycle negative scans after iR-correction, revealing that the HER activity of the CP substrate was negligible. The averaged overpotentials at
10 mA/cm2 increased in the order of 195 ± 5 mV (FeCoP/CP) < 212 ± 3 mV (FeP/CP) < 248 ± 14 mV (FeNiP/CP) and were much larger than that of commercial Pt/C/CP (18.1 mV). For all TMPs, the Tafel slopes were between 60 and 74 mV/dec (Fig. 5b), i.e., the HER mechanism involved the Volmer reaction (proton adsorption; commonly viewed as the ratedetermining step) followed by the Heyrovsky reaction

(electrochemical desorption) [75]. To investigate the ECSAs of Febased TMP catalysts, their Cdl values were determined by repeated CV measurements in a non-faradaic potential range at scan rates of 10e100 mV/s (Fig. S9). Fig. 5c shows the mean anodic and cathodic currents as functions of scan rate, where the slope corresponds to Cdl [42]. To evaluate the intrinsic activity of Fe-based TMP catalysts, HER current densities obtained from polarization curves were normalized by Cdl (Fig. S10). At
0.20 VRHE, the scaled current representing intrinsic activity decreased in the order of FeCoP/CP > FeNiP/CP > FeP/CP (Fig. 5d). Since all Fe-based TMP catalysts exhibited an amorphous nature and similar particle shapes, the intrinsic activity was concluded to depend only on elemental composition, and the highest HER intrinsic activity of FeCoP/CP was ascribed to the decrease of the hydrogen atom adsorption (Had þ Had) energy barrier caused by the introduction of Co [38,76,77]. The mechanism of HER charge transfer was additionally probed by EIS measurements. In the corresponding Nyquist plots (Fig. S11), the smallest semicircle diameter was observed for FeCoP/CP, which indicated that this catalyst featured the lowest charge transfer resistance (Rct). Although the FeCoP/CP catalyst showed the highest HER intrinsic activity, its performance could be further enhanced by adopting a roughened structure with enlarged ECSA. To achieve this goal, a metallic support was prepared by bubble-templated CF electrodeposition [67] on CP. The Cu support was fabricated by two steps of galvanostatic electrodeposition. The first step at a relatively low current of
0.3 A/cm2 was the Cu nucleation process on the surface of the CP substrate to increase adhesion between CP and CF.

Fig. 6. Low- and (inset) high-magnification FESEM images of (a) lab-made CF on CP and FeCoP electrodeposited on CF/CP at
1.2 VSCE and deposition times of (b) 30 s, (c) 100 s, (d) 200 s, (e) 300 s, and (f) 600 s.

J. Kim et al. / Journal of Alloys and Compounds 807 (2019) 148813

Then, in the second step, a high current of
1.7 A/cm2 facilitated the formation of a three-dimensional Cu structure by reducing Cu2þ and Hþ ions at the same time, thus forming a Cu network assisted by hydrogen bubble generation [67]. Fig. 6a shows the FESEM images of lab-made CF, demonstrating its porous morphology with a number of uniformly distributed 3e5 mm pores. Highmagnification FESEM imaging revealed that Cu dendrites were formed in the interconnected structure. Subsequently, FeCoP was electrodeposited on CF/CP at a constant potential of
1.2 VSCE. The coverage of spherical FeCoP particles on the dendritic Cu surface was shown to increase with deposition time in the range of 30e300 s (Fig. 6bef). The morphologies of FeCoP on CP and CF/CP substrates were mostly similar (Figs. 2 and 6). At deposition times longer than 300 s, the CF surface was completely covered by FeCoP. Decreases in FeCoP agglomeration and CF pore size were also observed. The coverage of CF/CP by FeCoP was confirmed by LSV measurements in N2-purged 0.1 M KOH at a scan rate of 5 mV/s (Fig. 7a). For CF/CP, the anodic peak at ~0.1 VSCE corresponded to Cu oxidation in alkaline solution (2Cu þ 2OH
/ Cu2O þ H2O þ 2e
) [78]. As the FeCoP deposition time increased, the anodic peak gradually lost intensity and completely disappeared after 200 s. On the other hand, the current density at
0.5 VSCE for the oxidation of FeCoP was increased by increasing deposition time, indicating an increase of surface area for the as-deposited FeCoP. It should be noted that the above current density decreased at deposition times longer than 600 s because of the surface area decrease caused by the aggregation of deposited FeCoP particles, which agrees with the results of FESEM imaging (Fig. 6f). The Cdl of FeCoP-300/CF/CP (measured as 3.90 mF/cm2) was ~16 times larger than that of FeCoP/CP (Figs. 7b and S12). Therefore, compared to FeCoP/CP, FeCoP-300/CF/CP showed a lower HER overpotential of 125 mV at a current density of
10 mA/cm2 (Fig. 7c) with acceptable stability (Fig. S13). Fig. 8a shows the polarization curves of a PEMWE with the FeCoP-300/CF/CP cathode, which showed the highest catalytic performance in the half-cell test. At 2.0 Vcell, the achieved current density (0.95 A/cm2) exceeded those reported for MoS2 (0.35 A/ cm2) [27], FeS2 (0.61 A/cm2) [28], Ni0.64Co0.36OxS0.14 (0.72 A/cm2) [18], and Cu93.7Mo6.3 (0.73 A/cm2) [29] cathodes. Compared with the commercial Pt/C/CP cathode, the cell performance was much lower. However, the cost-effectiveness of the non-noble catalyst should be considered. In the Nyquist plot measured at 2.0 Vcell, the ohmic resistance of PEMWE was 0.0817 U/cm2 (Fig. S14), indicating that the good conductivity of the electrode and small contact resistance between the electrode surface and membrane contributed to the high performance of the PEMWE [79e81]. Fig. 8b shows the Tafel plot of the corresponding polarization curve after iRcorrection, from which the Tafel slope and exchange current density were determined as 136 mV/dec and 6.0  10
8 A/cm2, respectively. Based on these values, the overpotentials were subdivided at current densities of 0.35, 0.55, 0.75, and 0.95 A/cm2, as shown in Fig. 8c. The kinetic overpotentials (hkin) were similar (~620 mV) at all current densities, whereas the ohmic overpotential (hohm) gradually increased with increasing current density. Interestingly, compared to hkin, the contribution of mass transfer overpotential (hmass) was minor, even at a high current density. Compared to the case of spray-coated cathodes [27,28], the selective formation of TMP/CF on the carbon fiber surface preserved the GDL porous structure and enhanced reactant (H2O) and product (H2) mass transfer [82]. The stability of PEMWE operation was confirmed by 8-h chronoamperometric characterization at 2.0 Vcell (Fig. 8d). Although the enlarged ECSA of CF-supported FeCoP and the preserved GDL porous structure resulted in acceptable hkin and

7

Fig. 7. (a) LSV curves of FeCoP/CF/CP recorded in N2-purged 0.1 M KOH at a scan rate of 5 mV/s. (b) Comparison of FeCoP/CP and FeCoP-300/CF/CP Cdl values. (c) Polarization curves of CP, CF/CP, FeCoP/CP, FeCoP-300/CF/CP, and commercial Pt/C/CP recorded at a scan rate of 50 mV/s in N2-purged 0.5 M H2SO4.

lower hmass, the performance of the PEMWE with the FeCoP/CF/CP cathode was still inferior to that of the Pt/C cathode (1.46e2.71 A/ cm2) [8,83e86], although the former electrode exhibited the advantages of low price and facile fabrication.

8

J. Kim et al. / Journal of Alloys and Compounds 807 (2019) 148813

Fig. 8. (a) Polarization curve of a PEMWE single cell with FeCoP-300/CF/CP and commercial Pt/C/CP cathode, and an IrO2/CP anode [9]. (b) Tafel plot corresponding to the above polarization curve. (c) Overpotential subdivisions at current densities of 0.35, 0.55, 0.75, and 0.95 A/cm2. (d) Results of PEMWE full cell stability testing performed at 2.0 Vcell for 8 h.

4. Conclusions

References

In summary, we successfully electrodeposited Fe-based binary and ternary phosphides onto CP and maximized the P content of TMPs by deposition potential control. Among the thus-prepared catalysts, amorphous FeCoP exhibited the highest intrinsic HER activity, which was ascribed to its modified electronic structure and the alloying effect. The use of CF prepared by bubble-assisted electrodeposition on CP as a metallic support for FeCoP allowed the ECSA of this catalyst to be increased by a factor of 16.3. In the half-cell test, the FeCoP/CF/CP cathode exhibited a HER current density of
10 mA/cm2 at an overpotential of 125 mV. Furthermore, a stable (for 8 h) current density of 0.95 A/cm2 at a cell voltage of 2.0 V was observed in the full-cell PEMWE test.

[1] Z. Kang, J. Mo, G. Yang, S.T. Retterer, D.A. Cullen, T.J. Toops, J.B. Green Jr., M.M. Mench, F.-Y. Zhang, Investigation of thin/well-tunable liquid/gas diffusion layers exhibiting superior multifunctional performance in lowtemperature electrolytic water splitting, Energy Environ. Sci. 10 (2017) 166e175. [2] M. Wang, Z. Wang, X. Gong, Z. Guo, The intensification technologies to water electrolysis for hydrogen production e a review, Renew. Sustain. Energy Rev. 29 (2014) 573e588. [3] J.A. Turner, Sustainable hydrogen production, Science 305 (2004) 972. [4] M. Carmo, D.L. Fritz, J. Mergel, D. Stolten, A comprehensive review on PEM water electrolysis, Int. J. Hydrogen Energy 38 (2013) 4901e4934. [5] K. Zeng, D. Zhang, Recent progress in alkaline water electrolysis for hydrogen production and applications, Prog. Energy Combust. Sci. 36 (2010) 307e326. [6] C.C. Pavel, F. Cecconi, C. Emiliani, S. Santiccioli, A. Scaffidi, S. Catanorchi, M. Comotti, Highly efficient platinum group metal free based membraneelectrode assembly for anion exchange membrane water electrolysis, Angew. Chem. Int. Ed. 53 (2013) 1378e1381. [7] F.M. Sapountzi, J.M. Gracia, C.J. Weststrate, H.O.A. Fredriksson, J.W. Niemantsverdriet, Electrocatalysts for the generation of hydrogen, oxygen and synthesis gas, Prog. Energy Combust. Sci. 58 (2017) 1e35. [8] B.-S. Lee, S.H. Ahn, H.-Y. Park, I. Choi, S.J. Yoo, H.-J. Kim, D. Henkensmeier, J.Y. Kim, S. Park, S.W. Nam, K.-Y. Lee, J.H. Jang, Development of electrodeposited IrO2 electrodes as anodes in polymer electrolyte membrane water electrolysis, Appl. Catal. B Environ. 179 (2015) 285e291. [9] J.D. Holladay, J. Hu, D.L. King, Y. Wang, An overview of hydrogen production technologies, Catal. Today 139 (2009) 244e260. , S. Siracusano, N. Briguglio, V. Baglio, A. Di Blasi, V. Antonucci, [10] A.S. Arico Polymer electrolyte membrane water electrolysis: status of technologies and potential applications in combination with renewable power sources, J. Appl. Electrochem. 43 (2013) 107e118. [11] X. Zou, Y. Zhang, Noble metal-free hydrogen evolution catalysts for water splitting, Chem. Soc. Rev. 44 (2015) 5148e5180. [12] C.Y. Son, I.H. Kwak, Y.R. Lim, J. Park, FeP and FeP2 nanowires for efficient

Acknowledgements This research was supported by the Chung-Ang University Graduate Research Scholarship in 2017 and by the National Research Foundation of Korea (NRF) grant funded by the Korean government MSIT (2018R1A4A1022647). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2018.12.172.

J. Kim et al. / Journal of Alloys and Compounds 807 (2019) 148813

[13] [14]

[15]

[16]

[17]

[18]

[19]

[20]

[21]

[22]

[23] [24]

[25]

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

electrocatalytic hydrogen evolution reaction, Chem. Commun. 52 (2016) 2819e2822. L. Tian, X. Yan, X. Chen, Electrochemical activity of iron phosphide nanoparticles in hydrogen evolution reaction, ACS Catal. 6 (2016) 5441e5448. X. Yang, A.-Y. Lu, Y. Zhu, S. Min, M.N. Hedhili, Y. Han, K.-W. Huang, L.-J. Li, Rugae-like FeP nanocrystal assembly on a carbon cloth: an exceptionally efficient and stable cathode for hydrogen evolution, Nanoscale 7 (2015) 10974e10981. C. Lv, Z. Peng, Y. Zhao, Z. Huang, C. Zhang, The hierarchical nanowires array of iron phosphide integrated on a carbon fiber paper as an effective electrocatalyst for hydrogen generation, J. Mater. Chem. A 4 (2016) 1454e1460. D. Li, Q. Liao, B. Ren, Q. Jin, H. Cui, C. Wang, A 3D-composite structure of FeP nanorods supported by vertically aligned graphene for the high-performance hydrogen evolution reaction, J. Mater. Chem. A 5 (2017) 11301e11308. Y. Yan, X.R. Shi, M. Miao, T. He, Z.H. Dong, K. Zhan, J.H. Yang, B. Zhao, B.Y. Xia, Bio-inspired design of hierarchical FeP nanostructure arrays for the hydrogen evolution reaction, Nano Res. 11 (2018) 3537e3547. H. Kim, J. Kim, S.-K. Kim, S.H. Ahn, A transition metal oxysulfide cathode for the proton exchange membrane water electrolyzer, Appl. Catal. B Environ. 232 (2018) 93e100. P. Cao, J. Peng, J. Li, M. Zhai, Highly conductive carbon black supported amorphous molybdenum disulfide for efficient hydrogen evolution reaction, J. Power Sources 347 (2017) 210e219. J. Jiang, L. Zhu, H. Chen, Y. Sun, W. Qian, H. Lin, S. Han, Highly active and stable electrocatalysts of FeS2ereduced graphene oxide for hydrogen evolution, J. Mater. Sci. 54 (2019) 1422e1433. N. Kornienko, J. Resasco, N. Becknell, C.-M. Jiang, Y.-S. Liu, K. Nie, X. Sun, J. Guo, S.R. Leone, P. Yang, Operando spectroscopic analysis of an amorphous cobalt sulfide hydrogen evolution electrocatalyst, J. Am. Chem. Soc. 137 (2015) 7448e7455. H.B. Wu, B.Y. Xia, L. Yu, X.-Y. Yu, X.W. Lou, Porous molybdenum carbide nanooctahedrons synthesized via confined carburization in metal-organic frameworks for efficient hydrogen production, Nat. Commun. 6 (2015) 6512. D. Wang, T. Liu, J. Wang, Z. Wu, N, P (S) Co-doped Mo2C/C hybrid electrocatalysts for improved hydrogen generation, Carbon 139 (2018) 845e852. Z. Lv, M. Tahir, X. Lang, G. Yuan, L. Pan, X. Zhang, J.-J. Zou, Well-dispersed molybdenum nitrides on a nitrogen-doped carbon matrix for highly efficient hydrogen evolution in alkaline media, J. Mater. Chem. A 5 (2017) 20932e20937. H.-W. Liang, S. Brüller, R. Dong, J. Zhang, X. Feng, K. Müllen, Molecular metaleNx centres in porous carbon for electrocatalytic hydrogen evolution, Nat. Commun. 6 (2015) 7992. Z. Chen, Y. Ha, Y. Liu, H. Wang, H. Yang, H. Xu, Y. Li, R. Wu, In situ formation of cobalt nitrides/graphitic carbon composites as efficient bifunctional electrocatalysts for overall water splitting, ACS Appl. Mater. Interfaces 10 (2018) 7134e7144. s, A. Urakawa, MoS2-based materials as T. Corrales-S anchez, J. Ampurdane alternative cathode catalyst for PEM electrolysis, Int. J. Hydrogen Energy 39 (2014) 20837e20843.  Reyes-Carmona, A. Coursier, S. Nowak, J.M. Grene che, C.D. Giovanni, A. re, D. Jones, J. Peron, M. Giraud, C. Tard, Low-cost H. Lecoq, L. Mouton, J. Rozie nanostructured iron sulfide electrocatalysts for PEM water electrolysis, ACS Catal. 6 (2016) 2626e2631. H. Kim, E. Hwang, H. Park, B.-S. Lee, J.H. Jang, H.-J. Kim, S.H. Ahn, S.-K. Kim, Non-precious metal electrocatalysts for hydrogen production in proton exchange membrane water electrolyzer, Appl. Catal. B Environ. 206 (2017) 608e616. L. Tian, X. Yan, X. Chen, L. Liu, X. Chen, One-pot, large-scale, simple synthesis of CoxP nanocatalysts for electrochemical hydrogen evolution, J. Mater. Chem. A 4 (2016) 13011e13016. Y. Ji, L. Yang, X. Ren, G. Cui, X. Xiong, X. Sun, Nanoporous CoP3 nanowire array: acid etching preparation and application as a highly active electrocatalyst for the hydrogen evolution reaction in alkaline solution, ACS Sustain. Chem. Eng. 6 (2018) 11186e11189. W. Gao, M. Yan, H.-Y. Cheung, Z. Xia, X. Zhou, Y. Qin, C.-Y. Wong, J.C. Ho, C.R. Chang, Y. Qu, Modulating electronic structure of CoP electrocatalysts towards enhanced hydrogen evolution by Ce chemical doping in both acidic and basic media, Nano Energy 38 (2017) 290e296. T. Wu, M. Pi, X. Wang, W. Guo, D. Zhang, S. Chen, Hierarchical cobalt polyphosphide hollow spheres as highly active and stable electrocatalysts for hydrogen evolution over a wide pH range, Appl. Surf. Sci. 427 (2018) 800e806. J. Chang, K. Li, Z. Wu, J. Ge, C. Liu, W. Xing, Sulfur-doped nickel phosphide nanoplates arrays: a monolithic electrocatalyst for efficient hydrogen evolution reactions, ACS Appl. Mater. Interfaces 10 (2018) 26303e26311. Y. Pan, Y. Liu, J. Zhao, K. Yang, J. Liang, D. Liu, W. Hu, D. Liu, Y. Liu, C. Liu, Monodispersed nickel phosphide nanocrystals with different phases: synthesis, characterization and electrocatalytic properties for hydrogen evolution, J. Mater. Chem. A 3 (2015) 1656e1665. Y. Pan, Y. Liu, C. Liu, Nanostructured nickel phosphide supported on carbon nanospheres: synthesis and application as an efficient electrocatalyst for hydrogen evolution, J. Power Sources 285 (2015) 169e177. M. Hou, X. Teng, J. Wang, Y. Liu, L. Guo, L. Ji, C. Cheng, Z. Chen, Multiscale porous molybdenum phosphide of honeycomb structure for highly efficient hydrogen evolution, Nanoscale 10 (2018) 14594e14599.

9

[38] J.M. McEnaney, J.C. Crompton, J.F. Callejas, E.J. Popczun, A.J. Biacchi, N.S. Lewis, R.E. Schaak, Amorphous molybdenum phosphide nanoparticles for electrocatalytic hydrogen evolution, Chem. Mater. 26 (2014) 4826e4831. [39] A. Adam, M.H. Suliman, H. Dafalla, A.R. Al-Arfaj, M.N. Siddiqui, M. Qamar, Rationally dispersed molybdenum phosphide on carbon nanotubes for the hydrogen evolution reaction, ACS Sustain. Chem. Eng. 6 (2018) 11414e11423. [40] J. Yu, Q. Li, Y. Li, C.-Y. Xu, L. Zhen, V.P. Dravid, J. Wu, Ternary metal phosphide with triple-layered structure as a low-cost and efficient electrocatalyst for bifunctional water splitting, Adv. Funct. Mater. 26 (2016) 7644e7651. [41] J. Kibsgaard, C. Tsai, K. Chan, J.D. Benck, J.K. Nørskov, F. Abild-Pedersen, T.F. Jaramillo, Designing an improved transition metal phosphide catalyst for hydrogen evolution using experimental and theoretical trends, Energy Environ. Sci. 8 (2015) 3022e3029. [42] X. Zhang, X. Zhang, H. Xu, Z. Wu, H. Wang, Y. Liang, Iron-doped cobalt monophosphide nanosheet/carbon nanotube hybrids as active and stable electrocatalysts for water splitting, Adv. Funct. Mater. 27 (2017) 1606635. [43] Y. Du, H. Qu, Y. Liu, Y. Han, L. Wang, B. Dong, Bimetallic CoFeP hollow microspheres as highly efficient bifunctional electrocatalysts for overall water splitting in alkaline media, Appl. Surf. Sci. 465 (2019) 816e823. [44] Y. Feng, C. Xu, E. Hu, B. Xia, J. Ning, C. Zheng, Y. Zhong, Z. Zhang, Y. Hu, Construction of hierarchical FeP/Ni2P hollow nanospindles for efficient oxygen evolution, J. Mater. Chem. A 6 (2018) 14103e14111. [45] W.L. Kwong, C.C. Lee, J. Messinger, Scalable two-step synthesis of nickeleiron phosphide electrodes for stable and efficient electrocatalytic hydrogen evolution, J. Phys. Chem. C 121 (2017) 284e292. [46] C. Huang, T. Ouyang, Y. Zou, N. Li, Z.-Q. Liu, Ultrathin NiCo2Px nanosheets strongly coupled with CNTs as efficient and robust electrocatalysts for overall water splitting, J. Mater. Chem. A 6 (2018) 7420e7427. [47] J. Staszak-Jirkovský, Christos D. Malliakas, Pietro P. Lopes, N. Danilovic, Subrahmanyam S. Kota, K.-C. Chang, B. Genorio, D. Strmcnik, Vojislav R. Stamenkovic, M.G. Kanatzidis, N.M. Markovic, Design of active and stable CoeMoeSx chalcogels as pH-universal catalysts for the hydrogen evolution reaction, Nat. Mater. 15 (2015) 197. [48] X. Long, G. Li, Z. Wang, H. Zhu, T. Zhang, S. Xiao, W. Guo, S. Yang, Metallic ironenickel sulfide ultrathin nanosheets as a highly active electrocatalyst for hydrogen evolution reaction in acidic media, J. Am. Chem. Soc. 137 (2015) 11900e11903.  pez, A. Laura [49] Y. Lei, S. Pakhira, K. Fujisawa, X. Wang, O.O. Iyiola, N. Perea Lo Elías, L. Pulickal Rajukumar, C. Zhou, B. Kabius, N. Alem, M. Endo, R. Lv, J.L. Mendoza-Cortes, M. Terrones, Low-temperature synthesis of heterostructures of transition metal dichalcogenide alloys (WxMo1exS2) and graphene with superior catalytic performance for hydrogen evolution, ACS Nano 11 (2017) 5103e5112. [50] T.A. Shifa, F. Wang, K. Liu, K. Xu, Z. Wang, X. Zhan, C. Jiang, J. He, Engineering the electronic structure of 2D WS2 nanosheets using Co incorporation as CoxW(1-x)S2 for conspicuously enhanced hydrogen generation, Small 12 (2016) 3802e3809. [51] H. Du, S. Gu, R. Liu, C.M. Li, Highly active and inexpensive iron phosphide nanorods electrocatalyst towards hydrogen evolution reaction, Int. J. Hydrogen Energy 40 (2015) 14272e14278. [52] X. Li, W. Liu, M. Zhang, Y. Zhong, Z. Weng, Y. Mi, Y. Zhou, M. Li, J.J. Cha, Z. Tang, H. Jiang, X. Li, H. Wang, Strong metalephosphide interactions in coreeshell geometry for enhanced electrocatalysis, Nano Lett. 17 (2017) 2057e2063. [53] P. Wang, Z. Pu, Y. Li, L. Wu, Z. Tu, M. Jiang, Z. Kou, I.S. Amiinu, S. Mu, Irondoped nickel phosphide nanosheet arrays: an efficient bifunctional electrocatalyst for water splitting, ACS Appl. Mater. Interfaces 9 (2017) 26001e26007. [54] Z. Zhang, J. Hao, W. Yang, J. Tang, Iron triad (Fe, Co, Ni) trinary phosphide nanosheet arrays as high-performance bifunctional electrodes for full water splitting in basic and neutral conditions, RSC Adv. 6 (2016) 9647e9655. [55] Y. Li, C. Zhao, Iron-doped nickel phosphate as synergistic electrocatalyst for water oxidation, Chem. Mater. 28 (2016) 5659e5666. [56] X. Ma, Y. Chang, Z. Zhang, J. Tang, Forest-like NiCoP@Cu3P supported on copper foam as a bifunctional catalyst for efficient water splitting, J. Mater. Chem. A 6 (2018) 2100e2106. [57] Q. Liu, S. Gu, C.M. Li, Electrodeposition of nickelephosphorus nanoparticles film as a Janus electrocatalyst for electro-splitting of water, J. Power Sources 299 (2015) 342e346. [58] C. Zhang, Y. Xie, H. Deng, C. Zhang, J.-W. Su, Y. Dong, J. Lin, Ternary nickel iron phosphide supported on nickel foam as a high-efficiency electrocatalyst for overall water splitting, Int. J. Hydrogen Energy 43 (2018) 7299e7306. [59] J. Tian, Q. Liu, A.M. Asiri, X. Sun, Self-supported nanoporous cobalt phosphide nanowire arrays: an efficient 3D hydrogen-evolving cathode over the wide range of pH 0e14, J. Am. Chem. Soc. 136 (2014) 7587e7590. [60] X. Yang, A.-Y. Lu, Y. Zhu, M.N. Hedhili, S. Min, K.-W. Huang, Y. Han, L.-J. Li, CoP nanosheet assembly grown on carbon cloth: a highly efficient electrocatalyst for hydrogen generation, Nano Energy 15 (2015) 634e641. [61] Y. Pan, Y. Lin, Y. Chen, Y. Liu, C. Liu, Cobalt phosphide-based electrocatalysts: synthesis and phase catalytic activity comparison for hydrogen evolution, J. Mater. Chem. A 4 (2016) 4745e4754. , Nanosized [62] S. Siracusano, N. Van Dijk, E. Payne-Johnson, V. Baglio, A.S. Arico IrOx and IrRuOx electrocatalysts for the O2 evolution reaction in PEM water electrolysers, Appl. Catal. B Environ. 164 (2015) 488e495. [63] V.K. Puthiyapura, S. Pasupathi, H. Su, X. Liu, B. Pollet, K. Scott, Investigation of supported IrO2 as electrocatalyst for the oxygen evolution reaction in proton

10

[64]

[65]

[66]

[67]

[68]

[69]

[70]

[71]

[72]

[73]

[74]

[75]

J. Kim et al. / Journal of Alloys and Compounds 807 (2019) 148813 exchange membrane water electrolyser, Int. J. Hydrogen Energy 39 (2014) 1905e1913. N. Jiang, B. You, M. Sheng, Y. Sun, Electrodeposited cobalt-phosphorousderived films as competent bifunctional catalysts for overall water splitting, Angew. Chem. 127 (2015) 6349e6352. N. Jiang, B. You, M. Sheng, Y. Sun, Bifunctionality and mechanism of electrodeposited nickelephosphorous films for efficient overall water splitting, ChemCatChem 8 (2015) 106e112. V. Vosoughi, S. Badoga, A.K. Dalai, N. Abatzoglou, Effect of pretreatment on physicochemical properties and performance of multiwalled carbon nanotube supported cobalt catalyst for FischereTropsch synthesis, Ind. Eng. Chem. Res. 55 (2016) 6049e6059. Y. Li, W.-Z. Jia, Y.-Y. Song, X.-H. Xia, Superhydrophobicity of 3D porous copper films prepared using the hydrogen bubble dynamic template, Chem. Mater. 19 (2007) 5758e5764. F.H. Saadi, A.I. Carim, E. Verlage, J.C. Hemminger, N.S. Lewis, M.P. Soriaga, CoP as an acid-stable active electrocatalyst for the hydrogen-evolution reaction: electrochemical synthesis, interfacial characterization and performance evaluation, J. Phys. Chem. C 118 (2014) 29294e29300. D.-H. Ha, B. Han, M. Risch, L. Giordano, K.P.C. Yao, P. Karayaylali, Y. Shao-Horn, Activity and stability of cobalt phosphides for hydrogen evolution upon water splitting, Nano Energy 29 (2016) 37e45. W. Cui, Q. Liu, Z. Xing, A.M. Asiri, K.A. Alamry, X. Sun, MoP nanosheets supported on biomass-derived carbon flake: one-step facile preparation and application as a novel high-active electrocatalyst toward hydrogen evolution reaction, Appl. Catal. B Environ. 164 (2015) 144e150. T. Wu, M. Pi, D. Zhang, S. Chen, 3D structured porous CoP3 nanoneedle arrays as an efficient bifunctional electrocatalyst for the evolution reaction of hydrogen and oxygen, J. Mater. Chem. A 4 (2016) 14539e14544. J. Luo, H. Wang, G. Su, Y. Tang, H. Liu, F. Tian, D. Li, Self-supported nickel phosphosulphide nanosheets for highly efficient and stable overall water splitting, J. Mater. Chem. A 5 (2017) 14865e14872. C. Lv, Q. Yang, Q. Huang, Z. Huang, H. Xia, C. Zhang, Phosphorus doped single wall carbon nanotubes loaded with nanoparticles of iron phosphide and iron carbide for efficient hydrogen evolution, J. Mater. Chem. A 4 (2016) 13336e13343. J. Kim, H. Kim, S.-K. Kim, S.H. Ahn, Electrodeposited amorphous CoePeB ternary catalyst for hydrogen evolution reaction, J. Mater. Chem. A 6 (2018) 6282e6288. C.G. Morales-Guio, L.-A. Stern, X. Hu, Nanostructured hydrotreating catalysts for electrochemical hydrogen evolution, Chem. Soc. Rev. 43 (2014)

6555e6569. [76] J. Li, M. Yan, X. Zhou, Z.-Q. Huang, Z. Xia, C.-R. Chang, Y. Ma, Y. Qu, Mechanistic insights on ternary Ni2
xCoxP for hydrogen evolution and their hybrids with graphene as highly efficient and robust catalysts for overall water splitting, Adv. Funct. Mater. 26 (2016) 6785e6796. [77] D.-Y. Wang, M. Gong, H.-L. Chou, C.-J. Pan, H.-A. Chen, Y. Wu, M.-C. Lin, M. Guan, J. Yang, C.-W. Chen, Y.-L. Wang, B.-J. Hwang, C.-C. Chen, H. Dai, Highly active and stable hybrid catalyst of cobalt-doped FeS2 nanosheetsecarbon nanotubes for hydrogen evolution reaction, J. Am. Chem. Soc. 137 (2015) 1587e1592. [78] J.C. Hamilton, J.C. Farmer, R.J. Anderson, In situ Raman spectroscopy of anodic films formed on copper and silver in sodium hydroxide solution, J. Electrochem. Soc. 133 (1986) 739e745. [79] J. Mo, Z. Kang, S.T. Retterer, D.A. Cullen, T.J. Toops, J.B. Green, M.M. Mench, F.Y. Zhang, Discovery of true electrochemical reactions for ultrahigh catalyst mass activity in water splitting, Sci. Adv. 2 (2016). [80] Z. Kang, G. Yang, J. Mo, Y. Li, S. Yu, D.A. Cullen, S.T. Retterer, T.J. Toops, G. Bender, B.S. Pivovar, J.B. Green, F.-Y. Zhang, Novel thin/tunable gas diffusion electrodes with ultra-low catalyst loading for hydrogen evolution reactions in proton exchange membrane electrolyzer cells, Nano Energy 47 (2018) 434e441. [81] Z. Kang, J. Mo, G. Yang, Y. Li, D.A. Talley, S.T. Retterer, D.A. Cullen, T.J. Toops, M.P. Brady, G. Bender, B.S. Pivovar, J.B. Green, F.-Y. Zhang, Thin film surface modifications of thin/tunable liquid/gas diffusion layers for high-efficiency proton exchange membrane electrolyzer cells, Appl. Energy 206 (2017) 983e990. [82] H. Kim, S. Choe, H. Park, J.H. Jang, S.H. Ahn, S.-K. Kim, An extremely low Pt loading cathode for a highly efficient proton exchange membrane water electrolyzer, Nanoscale 9 (2017) 19045e19049. [83] L. Ma, S. Sui, Y. Zhai, Investigations on high performance proton exchange membrane water electrolyzer, Int. J. Hydrogen Energy 34 (2009) 678e684. [84] C. Rozain, E. Mayousse, N. Guillet, P. Millet, Influence of iridium oxide loadings on the performance of PEM water electrolysis cells: part Iepure IrO2-based anodes, Appl. Catal. B Environ. 182 (2016) 153e160. [85] V.K. Puthiyapura, M. Mamlouk, S. Pasupathi, B.G. Pollet, K. Scott, Physical and electrochemical evaluation of ATO supported IrO2 catalyst for proton exchange membrane water electrolyser, J. Power Sources 269 (2014) 451e460. [86] H.-S. Oh, H.N. Nong, T. Reier, M. Gliech, P. Strasser, Oxide-supported Ir nanodendrites with high activity and durability for the oxygen evolution reaction in acid PEM water electrolyzers, Chem. Sci. 6 (2015) 3321e3328.