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Proceedings of the National Academy of Sciences of the United States of America logoLink to Proceedings of the National Academy of Sciences of the United States of America
. 2017 Aug 7;114(34):8986–8990. doi: 10.1073/pnas.1710443114

Electrocatalytic hydrogen evolution reaction activity comparable to platinum exhibited by the Ni/Ni(OH)2/graphite electrode

Manjeet Chhetri a, Salman Sultan a, C N R Rao a,1
PMCID: PMC5576835  PMID: 28784781

Significance

The successful utilization of solar energy to economically produce green fuel should involve facile and inexpensive means for electrolysis of water. To do so, it is necessary to replace the platinum catalyst with an in situ electrode fabrication process involving active catalyst with readily available materials. We have been successful in synthesizing an inexpensive Ni/Ni(OH)2/graphite electrode whose performance is as good as Pt. By a suitable choice of the relative proportion of Ni and Ni(OH)2, we obtain high current density at low overpotentials. The sequential galvanostatic and potentiostatic pulses used for the electrodeposition of Ni on the graphite rod provide control over the morphology and composition and the improved electrochemical performance.

Keywords: hydrogen evolution reaction, dual-pulse plating, nickel/nickel hydroxide interface, graphite rod electrode

Abstract

Electrochemical dual-pulse plating with sequential galvanostatic and potentiostatic pulses has been used to fabricate an electrocatalytically active Ni/Ni(OH)2/graphite electrode. This electrode design strategy to generate the Ni/Ni(OH)2 interface on graphite from Ni deposits is promising for electrochemical applications and has been used by us for hydrogen generation. The synergetic effect of nickel, colloidal nickel hydroxide islands, and the enhanced surface area of the graphite substrate facilitating HO–H cleavage followed by H(ad) recombination, results in the high current density [200 mA/cm2 at an overpotential of 0.3 V comparable to platinum (0.44 V)]. The easy method of fabrication of the electrode, which is also inexpensive, prompts us to explore its use in fabrication of solar-driven electrolysis.

To render electrochemical generation of H2 from water ecofriendly, we could use electricity from solar photovoltaic devices. A major limitation would still be the use of Pt as the catalyst. In the last few years, there has been great interest in replacing Pt by inexpensive, readily available catalysts. Several catalysts have been studied in recent times for the electrochemical hydrogen evolution reaction (HER) including transition metal-based heterostructures (1–6) and certain metal-free catalysts (7–11). Of these, Ni-based catalysts such as Ni2P (12, 13), NiFeP (14), NiFe layered double hydroxide (15–17), and Ni/NiO/carbon nanotube (18) seem to be more promising for water splitting. It has been shown recently that activation of a Ni-carbon–based catalyst through the application of an electrochemical potential results in HER activity comparable to Pt in acidic medium (6). We have been investigating the use of Ni along with Ni(OH)2 as a potential catalyst for the purpose, since Ni(OH)2 clusters with Pt and other transition metals (19–23) generally exhibit good HER activity and Ni itself is only next to Pt in activity. With this purpose, we have used the dual-pulse–plating (PP) method (24) to generate the Ni/Ni(OH)2 interface embedded in graphene sheets on a graphite electrode. Amazingly, the Ni/Ni(OH)2/graphite electrode prepared by us gives a current density of ∼200 mA/cm2 [at −0.30 V vs. reversible hydrogen electrode (RHE)] and an overpotential of ∼190 mV required to sustain a current density of 20 mA/cm2 over long periods. For a current density of 200 mA/cm2, this electrode beats the activity of the Pt wire by a factor of ∼1.5 in terms of the overpotential.

The outstanding performance of the Ni/Ni(OH)2/graphite electrode is due to the dual-PP method adopted by us to give rise to colloidal hydroxide inclusions in the electrodeposits (25–30) (Methods). While fabricating the Ni/Ni(OH)2 interface, the galvanostatic pulses shift the cathodic potential in the negative direction to such an extent that the ensuing water splitting yielding hydrogen is followed by the simultaneous incorporation of colloidal nickel hydroxide (Movie S1 and Fig. S1) (3). Thus, the use of dual PP affords in obtaining Ni and Ni(OH)2 from the sequential galvanostatic and potentiostatic conditions. While only Ni deposition is expected in the potentiostatic pulse, both Ni and Ni(OH)2 get deposited in the galvanostatic pulse due to variable potentials. The reduction of hydronium ions at the catholyte leads to the codeposition of Ni(OH)2 along with Ni. The origin of Ni(OH)2 generation has been explained earlier (25, 29, 31). Compared with direct current deposition (DP), cycling potentiostatic and galvanostatic pulses in PP changes the morphology of nickel deposits from pure nickel to layered Ni/colloidal Ni(OH)2 deposits on the graphite substrate to a greater extent (Fig. S1 A and B and Methods). The fresh deposits of Ni on graphite referred to as “fresh Ni-Gr” get converted into “active Ni-Gr” during electrochemical HER in linear sweep voltammetry (LSV). Fresh Ni-Gr consists of walnut-shaped particles distributed throughout the electrode surface as observed in field emission scanning electron microscopy (FESEM) image (Fig. 1A), and this morphology is lost after activation of the electrode at HER-4. The active Ni-Gr electrode surface consists of Ni/Ni(OH)2 embedded in a sea of graphite sheets (Fig. 1B and Fig. S1C) as confirmed by energy-dispersive X-ray analysis (EDAX) (Inset of Fig. 1 A and B), high-resolution TEM (HRTEM) images, inductively coupled plasma spectrometry–optical emission spectrometry (ICP-OES) analysis, and the dimethylglyoxime (DMG) test. HRTEM images (Fig. 1 C and D) of fresh and active electrodes reveal the presence of Ni/Ni(OH)2 interfaces throughout the catalyst. An analysis of the composition and morphology is provided in the supporting information (Figs. S2–S4). X-ray photoelectron spectroscopy (XPS) substantiates the generation of colloidal Ni(OH)2 embedded within the fresh Ni deposit and graphene sheets.

Fig. S1.

Fig. S1.

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Fabrication of electrode. A and B are the potentiostatic (P) and galvanostatic (G) graphs during the electrochemical fabrication of electrode. The five sequential steps involve P-G-P-G-P. (1) and (2) are the photographs of the polished graphite electrode before and after Ni pulse-plated electrodeposition, respectively. (3) Autocatalytic generation of fine H2 bubbles during Ni electrodeposition. (C) Cross-sectional FESEM images of electrodes: C1 and C2 are low and high magnified images of “fresh Ni-Gr.” The interface of Ni deposits and graphene sheets is marked by red dots. (C3) Magnified image of “active Ni-Gr” showing Ni/Ni(OH)2 embedded within the graphene sheets.

Fig. 1.

Fig. 1.

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Morphology study of the electrode. (A) FESEM image of Ni-deposited graphite electrodes. Inset shows the magnified image and EDAX data for the deposit. (B) The FESEM images of active Ni-Gr electrode. The Inset shows cross-sectional view of the electrode surface. The elemental composition is also depicted. The active electrode consists of Ni/Ni(OH)2 embedded in the graphite sheets. C and D are HRTEM images of active Ni-Gr electrode illustrating Ni, Ni(OH)2, and graphite interfaces.

Fig. S2.

Fig. S2.

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The compositional analysis of fresh Ni-Gr at different places on electrode surface. The places are marked by arrows, and the corresponding EDAX elemental mapping plot is given accordingly.

Fig. S4.

Fig. S4.

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(A) The normalized XAFS spectra of Ni-K edge of the fresh Ni-Gr electrode, Ni foil, and Ni(OH)2 as reference samples, showing the presence of Ni(OH)2/Ni interface on the fresh Ni-Gr electrode. (B) The DMG test of electrolyte after the HER-4 shows the presence of etched Ni. (C) The DMG test done with cotton plugs, dipped in DMG solution rubbed on the surface of (1) polished graphite electrode, (2) active Ni-Gr, and (3) fresh Ni-Gr. (4) shows the photograph of active Ni-Gr electrode.

Fig. S3.

Fig. S3.

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The compositional analysis of cross-section of active Ni-Gr at different depths. The places are marked as numbers, and the corresponding EDAX elemental mapping plot is given accordingly. The variation of Ni and O percentage from substrate surface to electrode surface indicates the presence of Ni(OH)2 along with Ni.

We have studied the catalytic activity of the Ni/Ni(OH)2/graphite electrode by means of LSV plots (Fig. 2A) in comparison with a Pt wire. The overpotentials required for obtaining current densities of 100 and 200 mA/cm2 are 270 and 299 mV for active Ni-Gr, while for Pt it was 243 and 442 mV, respectively (Fig. 2B), making active Ni-Gr a competitive, ready-to-use electrode in commercial electrolyzers. The tafel slopes are close to 116 mV/dec (Fig. S5A), suggesting adsorption of hydronium ions to be the rate-determining step (32). Due to the burst of gas bubbles (Fig. 2C and Movie S2), the LSV curve is noisy and we have not attempted to smoothen the plot. To check the efficacy of our method of electrode fabrication, a comparative study of the HER activity was carried out with an electrode fabricated through conventional DP of Ni. The results showed much higher current density (12 times increment at 300 mV) with the electrode prepared by the PP method (Fig. S5B). This significant difference is ascribed to the absence of the active Ni/Ni(OH)2 interface embedded within the graphene sheets in DP-fabricated Ni-Gr. Presence of the polished graphite electrode is crucial for the successful fabrication of active Ni-Gr electrode as confirmed by controlled experiments using a conducting carbon fiber (Fig. S6). We propose that, during HER in acidic medium, dissolution of the nickel deposit gives rise to or expose Ni/Ni(OH)2 interfaces, which then catalyze hydrogen evolution. Formation of Ni(OH)2 gets enhanced with successive HER tests due to electrogeneration of the base in the catholyte by the reduction of hydrogen ions.

Fig. 2.

Fig. 2.

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Electrochemical hydrogen evolution activity test. (A) Hydrogen evolution activity (HER) tests of the electrode by linear sweep voltammetry (LSV). HER-1 to HER-4 represents successive LSV runs. (B) The comparison of HER activity of Pt wire (0.5-mm diameter × 30.25-mm length; CH Instruments, Pt counter electrode CHI 115) and active Ni-Gr. (C) Photograph of hydrogen evolution (bubbles) during the HER tests through LSV. (D) Stability test of “active Ni-Gr electrode” before and after 200 CV cycles between −0.18 and −0.28 V vs. RHE at a scan rate of 5 mV/s. For HER testing, 0.5 M HCl was the electrolyte, and LSV was run at a scan rate of 5 mV/s. Pt wire and Ag/AgCl were used as counter and reference electrodes, respectively. The potentials were reported with respect to RHE according to following equation: E(RHE) = E (vs. Ag/AgCl) + E°(Ag/AgCl) + 0.059*pH.

Fig. S5.

Fig. S5.

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(A) Comparison of table slope of electrode obtained from successive LSV runs. (B) Comparison of HER activity of electrode fabricated by DP and PP methods. (C) The amperometric I–t stability curve for active Ni-Gr showing current density vs. time at −0.23 V for different time intervals.

Fig. S6.

Fig. S6.

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A and B are the photograph of HER activity test and LSV graph of Ni deposited on carbon fiber, respectively. C and D are the photographs of HER test of polished graphite rod and its corresponding LSV graph.

We have examined the stability of active Ni-Gr by cyclic voltammetry (Fig. 2D), as well as chronopotentiometric (Fig. 3A) and chronoamperometric studies (Fig. S5C). An activity retention of ∼96% was observed up to 200 cyclic voltammetry (CV) cycles between −0.18 and −0.28 V at a scan rate of 5 mV/s (Fig. 2D). Active Ni-Gr can sustain a current density of 20 mA/cm2 for 24 h requiring overpotential of only 190 mV (Fig. 3A). Since the adsorption of hydronium ions is the rate-determining step, we performed electrochemical impedance spectroscopic studies at onset potential to estimate the resistance involved in charge transfer (Rct) between the electrode and the electrolyte. Fig. 3B shows the Nyquist plot for the Ni-Gr and the equivalent circuit used to fit the data in the Inset, giving the value of Rct as 58 Ω.

Fig. 3.

Fig. 3.

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Electrochemical properties: (A) Chronopotentiometric V-t graph showing the overpotential required to sustain a current density of 20 mA/cm2 over long time. (B) Nyquist plot of active Ni-Gr at onset potential and the equivalent circuit.

XPS studies show a drastic change occurs in relative proportion of Ni and Ni(OH)2 on the active Ni-Gr electrode surface during successive LSV runs in HER tests. We see a greater fraction of Ni(OH)2 on the surface of active Ni-Gr electrode (Fig. 4) as corroborated by EDAX analysis (Inset of Fig. 1 A and B). X-ray absorption near-edge structure (XANES) region of the XAS spectra at the Ni-K edge shows a gradual evolution of Ni(OH)2 with increasing HER cycles (Fig. 5A). The coordination environment of Ni changes as evident from the intensity of the white line. The relative composition of Ni and Ni(OH)2 on the electrodes estimated by a linear combination fit (LCF) method (Fig. 5 B–D) gives the optimum ratio to be 14.3:64.3 at the Ni/Ni(OH)2 interface responsible for the burst of hydrogen evolution activity observed in our experiment (Table 1).

Fig. 4.

Fig. 4.

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Analysis of oxidation state by XPS. A and B are the Ni-2p core level XPS plots for fresh Ni-Gr and the active Ni-Gr electrodes, respectively. The peaks at 853.01, 855.77, and 862.07 eV can be assigned to 3p3/2 and peaks at 870.47, 873.61, and 881.23 eV to 3p1/2 for Ni, Ni(OH)2, and Ni(OH)2 satellite peaks, respectively. The XPS analysis hints to two inferences: First, generation of colloidal Ni(OH)2 embedded within the fresh Ni deposit and graphene sheets during the synthesis, and second, drastic change in relative proportion of Ni and Ni(OH)2 on the active electrode surface during HER tests. We see greater fraction of Ni(OH)2 on the surface of active Ni-Gr electrode, also corroborated by EDAX analysis (Inset of Fig. 1 A and B) and XANES analysis.

Fig. 5.

Fig. 5.

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Analysis of oxidation state and relative proportion of Ni and Ni(OH)2. (A) Normalized XAFS spectra of Ni-K edge of the electrodes as a function of successive LSV runs depicted as HER numbers showing the evolution of Ni(OH)2/Ni interface. (B–D) The linear combination fitted normalized XAFS analysis of the electrodes to determine the percentage of Ni metal and Ni(OH)2 with increasing HER numbers. All probable Ni species viz. Ni foil, NiO, Ni(OH)2, and Ni2+ surrounded by water molecules were taken as reference material for fitting purpose. The collected data at Ni-K edge for all these samples along with the electrodes were then analyzed by LCF method. Fitting was done in Athena software. The fitting range was selected up to maximum limit possible, that is, from −20 to 200 eV. In all cases, it was observed that the principal contributions are from Ni foil and Ni(OH)2 species, and hence NiO and “Ni2+ in water” data were not included.

Table 1.

LCF data parameters of fresh and active Ni-Gr

Sample Ni metal, % Ni(OH)2, % R factor χ2 Reduced χ2
Fresh Ni-Gr 82.9 17.1 0.0011842 0.0324 0.000040
HER-1 75.5 24.5 0.0006258 0.0170 0.000022
HER-2 73.1 26.9 0.0006483 0.0185 0.000023
HER-3 59.2 40.8 0.0022793 0.0615 0.000083
HER-4* 14.3 64.3 0.007684 0.3161 0.000398
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*

The percentage sum is not equal to 100 since 21.3% is contributed by Ni2+ in aqueous solution. This is similar to Ni(OH)2 environment with one hydrogen less. Hence the presence of Ni2+ surrounded by OH− ion is inferred. The exact coordination number and nature of coordination shell are beyond the scope of discussion for this study.

Atomic force microscopy (AFM)-assisted topographic analysis of active Ni-Gr and fresh Ni-Gr electrodes provide insight to HER activity (Fig. 6). The 3D topography of these electrodes is contrastingly different in terms of surface roughness (Fig. 6 C and D, and Figs. S7 and S8). Spikes and corrugations on the active Ni-Gr electrode surface give rise to excess surface area of the activated graphite surface. The sharp edges favor the increased electronic charge density during LSV, thus aiding the high activity.

Fig. 6.

Fig. 6.

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Surface roughness and topographic analysis: atomic force microscopy (AFM) image of (A) fresh Ni-Gr surface, (B) active Ni-Gr electrode surface. The Inset shows the height profile of the electrode surface along white line in the AFM image. Three-dimensional topographic AFM images (20 × 20 μm2 in size) of fresh Ni-Gr and active Ni-Gr electrodes are given in C and D. The increased surface area and the induced surface mesoporosity (by H2 evolution as a function of HER numbers) along with difference in the population of catalytically active interface are evident from the 3D images.

Fig. S7.

Fig. S7.

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Comparison of the 3D AFM images of electrodes highlighting the difference in their surface morphology. Enhanced population of nanospikes with resultant increase in surface area is noticeable for active Ni-Gr in comparison with other two electrode surfaces.

Fig. S8.

Fig. S8.

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Comparison of the height profiles of electrodes highlighting the difference of their surface morphology. (A and B) Polished graphite electrode. (C) Active Ni-Gr. (D and E) Fresh Ni-Gr electrode. The height profiles are given in E along with the representative line numbers to represent the area in D taken for height profiling. Since the graphite base is not well defined due to continuous deposition of Ni on graphite, the height may not truly represent the actual thickness of deposits. However, a clear picture is given by cross-sectional FESEM in Fig. 1.

Conclusions

In summary, we have discovered that nanoscale Ni/Ni(OH)2/graphite is an outstanding catalyst with high HER activity comparable to that of Pt exhibiting high current density over a range of overpotentials. The design and electrode fabrication strategy to generate the highly catalytic Ni/Ni(OH)2 interface on graphite from Ni deposits (Fig. S9, the coaction effect of nickel, colloidal nickel hydroxide islands, and the enhanced surface area of the graphite substrate facilitating HO–H cleavage followed by H(ad) recombination, results in the high current density) used by us are unique and unprecedented. To the best of our knowledge, dual-PP synthesis has not been studied for improving the HER activity on graphite. There is a need for an optimum ratio between Ni and Ni(OH)2 for high activity. In situ growth of the catalyst on the graphite surface eliminates cumbersome electrode fabrication procedures. The catalyst can advantageously be reused, thus making the process economical. The current density of ∼200 mA/cm2 (at −0.3 V vs. RHE) and a retention of activity (overpotential of ∼190 mV required to sustain a current density of 20 mA/cm2) for long term (24 h) is noteworthy. In comparison with Pt wire electrode, Ni/Ni(OH)2/graphite requires 144 mV less overpotential to produce a current density of 200 mA/cm2. Based on the results of the present study, it would be desirable to examine the fabrication of acid/alkaline prototype electrolyzers operating at a low voltage and combining them with solar driven electrolyzers. It must be noted that PP is not only a reproducible, inexpensive, and easy method, but it also avoids the pitfalls of the drop-casting method of electrode fabrication technique.

Fig. S9.

Fig. S9.

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The mechanistic understanding of HER. Ni acts as Had sites and Ni/Ni(OH)2 interface facilitates in splitting of water. The combination of Ni/colloidal Ni(OH)2 interfaces, mesoporous graphite, and enhanced surface area with nanospikes plays a synergetic role to transform nickel-deposited graphite into a remarkably active catalyst.

Methods

The essentials of dual PP are as follows. The ends of 5-cm graphite rods were cut into square shape. One side was polished with different grades of silicon carbide paper and sequentially cleaned/degreased by sonicating in water and acetone. Leaving an area of 0.6 cm2, the rest of graphite rod was electrically insulated (Fig. S1). The 0.3 M nickel acetate solution in 5 mol% N-methylformamide–water mixture was used as an electrolyte for Ni deposition using Ni strip as counter electrode, Ag/AgCl as the reference, and the polished/cleaned graphite rod as working electrode. Nickel electrodeposition was done through successive potentiostatic/galvanostatic pulses (P/G PP with CHI760E). For potentiostatic pulse, voltage ranged between −0.9 and −1.2 V with respective hold times as 10 and 100 s while in the galvanostatic pulse current densities were fixed at 34 and 8.4 mA/cm2. Five segments consisting of P-G-P-G-P were used during electrodeposition (Fig. S1 and Movie S1). The activation of Ni-Gr electrode was observed in successive LSV cycles during hydrogen evolution activity test in 0.5 M HCl with the appearance of fresh Ni-Gr turning black (Fig. S4C).

Characterizations

Transmission electron microscopy (TEM) (Technai F30 UHR, 200 kV) was used to study the morphology, and field emission scanning electron microscopy (FESEM) (FEI Quanta operated at 15 kV, equipped with EDAX) was used to investigate the composition, morphology, and thickness of electrodes. The elemental ratios of the as-prepared electrodes were confirmed by optical emission spectrometry–inductively coupled plasma spectrometry (OES−ICP) (Perkin-Elmer; Optima 7000 DV). X-ray photoelectron spectroscopy (XPS) (Mg–Kα X-ray source, 1,253.6 eV) was recorded to analyze the composition of samples. Atomic force microscopy (AFM) was carried out on scanning probe microscope in tapping mode in air under ambient conditions using silicon cantilevers (Bruker Innova). Typical image sizes are 20 × 20 μm2 at a scan rate of 40 μm/s with 256 lines per image. Electrochemical synthesis was performed by CHI760E electrochemical workstation (CH Instruments).

Graphite Rod as the Electrode Substrate Material

HER activity has been generally tested in three-electrode systems, where the catalyst film formed on a glassy carbon electrode (GCE) or Ni foam is used as the working electrode. Although highly active catalysts are reported by this means, there is instant demand for easy fabrication of electrodes from powder samples in practical electrolyzers. It has become necessary to identify a reusable cheap, durable, high-surface area conducting electrode for the growth of an active catalyst, required for electrochemical applications. At present, one uses substrates like Ni foam (13, 18), GCE (9, 33), carbon cloth (34, 35), etc. However, firm adherence with the substrate electrode material for long-term applications is not provided by available methods, mainly involving drop-drying. It is, therefore, important to explore a suitable in situ electrode fabrication process to obtain good films of the catalyst on a substrate, for use as a working electrode. Electrodeposition would be an obvious choice. After a detailed examination of the literature and control experiments on metal strips and carbon fiber, we have chosen graphite rod as the electrode substrate. Graphite rod is advantageous in the sense that multiple uses are possible by simple repolishing. It is a sturdy, self-supporting, and inexpensive electrode. We have electrodeposited nickel on the clean graphite substrate.

XPS Analysis

The core level XPS spectrum is shown in Fig. 5. We observed the characteristic multiplet splitting and satellite peaks at higher binding energies (BEs) with broad maxima for Ni-3p in both samples. The peaks at 853.01, 855.77, and 862.07 eV can be assigned to 3p3/2 and peaks at 870.47, 873.61, and 881.23 eV to 3p1/2 for Ni, Ni(OH)2, and Ni(OH)2 satellite peaks, respectively. The assigned peaks are in accordance with previously reported results (36–39). The XPS analysis prompted us toward two inferences: First, as stated earlier, generation of colloidal Ni(OH)2 embedded within the fresh Ni deposit and graphene sheets during the synthesis, and second, drastic change in relative proportion of Ni and Ni(OH)2 on the active electrode surface during HER tests. We see greater fraction of Ni(OH)2 on the surface of active Ni-Gr electrode (Fig. 5), as also corroborated by EDAX analysis (Inset of Fig. 1 A and B). As discussed earlier, these heterointerfaces are crucial for electrochemical applications, and therefore, it was important to find the exact ratio of these two species on the catalyst surface as a function of successive HER cycles (see electrochemical HER discussion). To confirm this, we carried out XANES studies.

XANES Analysis: LCF Method

All probable Ni species viz. Ni foil, NiO, Ni(OH)2, and Ni2+ surrounded by water molecules were taken as reference material for fitting purpose. The collected data at Ni-K edge for all of these samples along with the electrodes were then analyzed by LCF method. Fitting was done in Athena software. Best fit was selected in terms of the values of R factor, χ2 and reduced χ2, and visual observation of the fit between the graphs (Fig. 3D and Fig. S4A). The fitting range was selected up to maximum limit possible, that is, from −20 to 200 eV. In all cases, it was observed that the principal contributions are from Ni foil and Ni(OH)2 species, and hence NiO and “Ni2+ in water” data were not included.

Supplementary Material

Supplementary File
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Supplementary File
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Acknowledgments

We thank the Jawaharlal Nehru Centre for Advanced Scientific Research for managing the X-ray absorption fine-structure measurement. We acknowledge Synchrotron SOLEIL for provision of synchrotron radiation facilities at beamline SAMBA (Proposal 20160052). M.C. thanks the University Grants Commission of India for a fellowship. We thank the Department of Science and Technology, India (SR/NM/Z-07/2015), for financial support.

Footnotes

The authors declare no conflict of interest.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1710443114/-/DCSupplemental.

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