Highly Efficient Electrocatalytic Hydrogen Production over Carbon Nanotubes Loaded with Platinum Nanoparticles Using Solution Processing

Proton exchange membrane (PEM) electrolyzers are used for hydrogen (H2) production by water electrolysis. The commercial cathodic electrocatalyst for this process is typically mechanically mixed platinum on carbon (Pt/C). However, aggregation of the platinum (Pt) makes high loading of the catalyst difficult. Therefore, a method for the homogeneous combination of Pt and carbon materials is required. Herein, the first example of a highly efficient single‐walled carbon nanotube (SWCNT) cathodic H2‐production electrocatalyst that is loaded with platinum nanoparticles (PtNPs) using a newly developed suspension method is reported. Combining SWCNTs lapped with a water‐soluble, thiol‐functionalized polymer with PtNPs in water yields a PtNP‐conjugated SWCNT suspension. The electrocatalyst exhibits a low overpotential of 47 mV at a current density of 10 mA cm−2 toward H2 evolution in 0.5 m sulfuric acid. A PEM electrolyzer fabricated using the optimally prepared electrocatalyst with the low loading of 15 µgPt cm−2 shows a high mass activity of 27 200 A gPt−1, which is 80 times that of Pt/C with a loading amount of 2.8 mgPt cm−2 (324 A gPt−1). In addition, the PEM electrolyzer produces H2 at a Faradaic efficiency of 97% and operates stably for 150 h at 100 mA cm−2.


Introduction
The depletion of fossil fuels and worsening global environmental problems have led to a growing worldwide interest in carbon-free energy. Hydrogen (H 2 ), which emits no environmental pollutants upon combustion, is attracting particular attention as a carbon-free energy resource. Water electrolysis is promising as an environmentally friendly method of producing H 2 . [1][2][3][4] However, H 2 is produced industrially through Research into inexpensive and abundant catalysts to replace Pt for the HER is widely underway. Accordingly, numerous promising HER catalysts have been evaluated; [9][10][11][12][13][14][15][16] however, none rival Pt in terms of intrinsic catalytic activity. [17] Pt on carbon (Pt/C) is a commercial Pt-activated-carbon composite that has become a benchmark HER catalyst. It consists of Pt immobilized on activated carbon and exhibits HER activity associated with water electrolysis on the Pt surface. However, Pt in Pt/C tends to aggregate, and since protons cannot access the interior of aggregated Pt particles, HER efficiency is decreased. As a result, Pt/C requires Pt of the order of milligrams per cm 2 for the cathode in the PEM electrolyzer as the Pt/C coverage on the electrode.
The use of carbon nanotubes (CNTs) has been investigated as a means to improve HER efficiency. [18][19][20][21][22][23] CNTs are generally known for their high electron conductivity and durability compared with conventional activated carbon. Furthermore, the high surface area of CNTs should promote the HER activity of an immobilized catalyst supported thereon. However, CNTs do not disperse readily in solvents because of aggregation due to van der Waals interactions and their hydrophobic properties, resulting in precipitation. This inevitable aggregation leads to poor processability and negatively affects the intrinsic properties of CNTs.
Recently, we have developed functional dispersants for the solution processing of single-walled CNTs (SWCNTs) through noncovalent functionalization. [24] This functionalization is termed a "soft" dispersion methodology because the dispersant is adsorbed onto the nanotube surface in the solvent through π-π and/or hydrophobic interactions rather than through "hard" covalent functionalization, producing an SWCNT suspension. The resulting SWCNT suspensions can be applied to solution-processed energy conversion devices for photoelectric conversion, [25] thermoelectric conversion, [26] and enzymatic biosensing. [27] Accordingly, we expected that SWCNTs modified by noncovalent functionalization and the chemical immobilization of Pt nanoparticles (PtNPs) would produce suspensions of PtNP-conjugated SWCNTs. This methodology would decrease the structural aggregation of both PtNPs and SWCNTs, allowing deposition using solution processing. This suppression of aggregation induces uniform loading of PtNPs on nanotube surfaces and good electrical conductivity of SWCNTs, which leads to an efficient HER with a minimum amount of Pt catalyst. Because SWCNTs have a simple 1D structure comprising rolled graphene sheets, they typically produce more uniform nanocomposites than those afforded using other carbon materials.
Here, we report nanocomposite electrocatalysts comprising PtNP-conjugated SWCNTs for the fabrication of solutionprocessed PEM electrolyzers (Figure 1). A water-soluble thiol (SH)-functionalized polymer acts as the SWCNT dispersant, providing dispersed SWCNTs in water through noncovalent functionalization. PtNPs synthesized by chemical reduction are immobilized on the SWCNTs by Ptthiolate (S) bonding. The resulting PtNP/polymer/SWCNT suspension can be used as an ink to fabricate cathodes for PEM electrolyzers by spray coating. Our results show that a PEM electrolyzer fabricated with the optimal PtNP-conjugated SWCNT electrocatalyst at a low loading of 15 µg Pt cm −2 delivers a high mass activity of 27 200 A g Pt , 80-times that of commercial Pt/C at a loading amount of 2.8 mg Pt cm −2 . Furthermore, the electrocatalyst has a high Faradaic efficiency (FE) of 97% and shows stable HER performance for 150 h at 100 mA cm −2 with a low operation cost of $US 4.9 per kg for H 2 production. Figure 2a shows the chemical structure of the SH-functionalized water-soluble polymer HA29 explored in this study. An SH group was introduced by the amidation of a carboxyl group www.advmatinterfaces.de in glucuronic acid with cysteamine hydrochloride (CSA·HCl) (Table S1, Supporting Information). The introduction of the SH group was confirmed by 1 H-NMR from the signals for the methylene group of CSA at 2.74 and 2.78 ppm (Figures S1 and S2, Supporting Information). The SH-modification percentage for HA29 is 29%, and water solubility does not change significantly after SH modification.

SH-Functionalized Polymer/SWCNT Suspensions through Noncovalent Functionalization
HA29 acts as the dispersant for SWCNTs through noncovalent functionalization. The absorption spectra reveal the suspension behavior of polymer/SWCNT (Figure 2b). The suspension in water presents features corresponding to the first (S 11 , 940-1300 nm) and second (S 22 , 620-940 nm) optical transitions of semiconducting SWCNTs, as well as for the first optical transition (M 11 ) of metallic SWCNTs at ≈400-620 nm. [28] For more detailed information, dispersion stability was investigated for suspensions with different HA29/SWCNT ratios. Figure S3, Supporting Information, shows their absorption spectra just after preparation and after 1 week of storage. The absorption spectra do not change during storage for HA29/ SWCNT ratios of 1:1 and 1.5:1 (w/w). However, HA29/SWCNT = 2:1 (w/w) significantly decreases in intensity over 1 week of storage ( Figure S3i, Supporting Information). This decrease in absorbance is attributed to the aggregation and precipitation of SWCNTs in water owing to the excess HA29 in the suspension. The dispersion stability of the HA29/SWCNT suspension (1.5:1, w/w) was confirmed by zeta potential (ζ) analysis. The ζ value of −39 mV shows almost no change, even after 1 week of storage ( Figure S4, Supporting Information). Accordingly, we used HA29/SWCNT = 1.5:1 (w/w) suspensions for all subsequent experiments (Figure 2c).
The degree of SWCNT aggregation is proportional to the full width at half maximum (FWHM) of an absorption peak. Therefore, suspension properties were evaluated from the FWHM value at 1002 nm in the S 11 region. The FWHM value for the HA29/SWCNT suspension is 57 meV, which is comparable to that using the typical dispersant sodium dodecyl sulfate (49 meV). [29] High-resolution transmission electron microscopy (HR-TEM) observation ( Figure 2d) further indicated that HA29/SWCNT exhibits a good dispersion with little aggregation. The composite exists as small bundles with an average diameter of 7 nm and an average length of 2.3 µm. The diameters of the single SWCNTs are 0.7-1.4 nm; thus, the composite bundles appear to comprise 5 or 6 entangled SWCNTs. Raman spectroscopy indicated that HA29/SWCNT has few structural defects ( Figure 2e). The disordered (D) band at 1300 cm −1 is associated with sp 3 carbon and therefore represents structural defects in the otherwise sp 2 carbon in SWCNTs. The graphitic (G) band is attributed to vibrations of the sp 2 carbon in the graphite layer; the G − -mode at 1566 cm −1 corresponds to the vibration of carbon atom along the circumferential direction, while the G +mode at 1594 cm −1 corresponds to the vibration of carbon atom along the SWCNT axis. The intensity ratios of the D band to the G band (I D /I G s) for as-prepared SWCNT and HA29/SWCNT are 0.05 and 0.07, respectively, indicating that HA29/SWCNT remains almost free of structural defects upon noncovalent functionalization.
X-ray photoelectron spectroscopy (XPS) was used to explore the chemical bonding between PtNP and SH29. The highresolution spectrum shows a Pt 4f 7/2 peak at 75.6 eV, which is associated with Pt 4+ in H 2 PtCl 6 ·6H 2 O ( Figure S5i, Supporting Information). [30] Upon reduction of H 2 PtCl 6 ·6H 2 O in the HA29/SWCNT suspension, the synthesized PtNPs conjugate with the SH groups of HA29, as confirmed by the 4f 7/2 peak for PtS bonds at 74.2 eV ( Figure S5ii, Supporting Information). [31] The HR-TEM image directly evidences the conjugation of PtNP onto HA29/SWCNT (Figure 3b). The diameter of the HA29/SWCNT bundles increases to an average of 22 nm; however, the morphological properties of the 1D structure remain unchanged upon the conjugation of PtNPs. The diameters of the PtNPs change little with the amount of H 2 PtCl 6 ·6H 2 O used in the synthesis, being 3.2, 2.9, and 3.3 nm for H 2 PtCl 6 ·6H 2 O:HA29/SWCNT = 1:8, 1:4, and 1:2 (w/w), respectively ( Figure S6, Supporting Information). As the amount of H 2 PtCl 6 6H 2 O is increased, the distance between PtNPs becomes smaller, resulting in their aggregation.
The uniform distribution of PtNPs on the HA29/SWCNT surface was confirmed by scanning electron microscopy (SEM) observation and the corresponding energy dispersive X-ray spectroscopy (EDS) elemental mapping image (Figure 3c). These results demonstrate that using our new methodology, PtNPs with small particle size and narrow particle size distribution can be conjugated to the HA29/SWCNT surface uniformly. We expected that the PtNP/HA29/SWCNT nanostructure would provide good electron transport owing to the SWCNTs and excellent exposure of the active PtNP sites for the HER.

PEM Electrolyzers Fabricated with PtNP/HA29/SWCNT by Solution Processing
Mixtures of PtNP/HA29/SWCNT suspensions in 5% Nafion solution were used as spray-coating inks for composites Com x (x: 15,33,40). To estimate the loading of PtNPs in the ink, thermogravimetric analysis measurements were performed ( Figure S7 and Table S2, Supporting Information). The PtNP loading was adjusted by changing the amount of the Pt precursor H 2 PtCl 6 ·6H 2 O. Since HA29, SWCNT, and Nafion are www.advmatinterfaces.de thermally decomposed when heated to 1000 °C, the PtNPs could be determined from the residue. Table 1 shows that the PtNP loadings for Com 15 , Com 33 , and Com 40 are 15, 33, and 40 µg Pt cm −2 , respectively, hence the labeling. In contrast, the Pt content for commercial Pt/C is 2.8 mg Pt cm −2 . The electrochemical activity of Com 15 toward the HER was evaluated using the three-electrode configuration in 0.5 m sulfuric acid (H 2 SO 4 ; pH: 0.4) electrolyte. This measurement revealed that Com 15 exhibits a low overpotential, so it was expected to exhibit efficient HER catalysis. The sample was prepared by dipping the carbon paper of a lab-fabricated working electrode into a Com 15 ink ( Figure S8, Supporting Information). From a practical point of view, we evaluated the overpotential at a current density of 10 mA cm −2 . The overpotential of Com 15 is 47 mV, comparable with that of commercial Pt/C (43 mV; Figure 4a). The Tafel slope of Com 15 (49 mV dec −1 ) is almost the same as that of Pt/C (48 mV dec −1 ), indicating that the electrochemical performance of Com 15 is comparable with that of commercial Pt/C (Figure 4b).
To fabricate a PEM electrolyzer, a cathode and anode were prepared by spray-coating Com 15 and iridium oxide (IrO x ) inks onto opposing sides of a Nafion membrane (see Supporting Information). Cross-sectional SEM images revealed that the Com 15 cathode has a dense structure (Figure 4c). In contrast, the spray-coated IrO x anode shows an uneven surface. The differences in the surface morphologies of these electrodes are due to the suspension behaviors of their respective inks. As described above, the cathode ink of Com 15 is a homogeneous suspension (Figure 3b). Conversely, the IrO x anode ink becomes a non-uniform suspension in water, resulting in a rougher surface of the anode.
The PEM electrolyzer was fabricated by sandwiching the Nafion membrane coated with the cathode and anode between Pt-coated titanium meshes and silicon sheets ( Figure S9, Supporting Information). The resulting PEM was fixed using stainless-steel end plates and the water flowed at a flow rate of 100 mL min −1 . Figure 4d shows the current-voltage characteristics of the composites in the PEM electrolyzers at 25 °C. Each composite shows a sigmodal curve, and the current densities for Com 15 , Com 33 , and Com 40 reach 410, 521, and 593 mA cm −2 , respectively, at 2 V. Thus, the current densities increase with PtNP loading, which is due to the increased availability of active PtNP sites. Furthermore, the effect of cathode thickness on current density was investigated for each PtNP loading ( Figure S10, Supporting Information). In all cases, the current density increases with decreasing thickness. The high current density can be attributed to the smooth proton transfer from the anode through the Nafion membrane, resulting in efficient reduction at the cathode.
The efficiency of the reduction reaction using our composite was confirmed by mass activity analysis. The mass activity for Com 15 is 27 200 A g Pt −1 at 2 V, which is more than 80 times that of commercial Pt/C (324 A g Pt −1 ) (Figure 4e). The mass activity for Com 15 is higher than those for Com 33 and Com 40 , indicating that Com 15 is the most efficient nanocomposite electrocatalyst in terms of HER activity ( Figure S11, Supporting Information). We compared this result with the maximum mass activities achieved for PEM electrolyzers prepared with other Pt-based catalysts reported in previous studies (Figure 4f and Table S3, Supporting Information). [32][33][34][35][36][37][38][39][40][41][42][43] The data clearly show that Com 15 achieves a higher HER activity, even though the Pt loading is 1/100 of those of conventional Pt electrocatalysts. As a control, as-prepared SWCNTs show no catalytic activity.

HER Behavior Com 15 in PEM Electrolyzer
Finally, we confirmed the HER behavior of a PEM electrolyzer fabricated with Com 15 as a nanocomposite electrocatalyst. Figure 5a shows a 5-h monitoring of H 2 and oxygen (O 2 ) generated from the PEM electrolyzer at 25 °C. Since it cannot be guaranteed that currents are due to water electrolysis, it is essential to confirm the generation of H 2 and O 2 by electrochemical water splitting. Therefore, we performed water splitting using Com 15 and determined that the resulting current is derived from the HER, as shown in Figure 4d. The yields reached 15.5 mmol h −1 for H 2 and 7.85 mmol h −1 for O 2 , confirming a stoichiometric ratio of 2:1 (mol/mol). This result indicates that the observed current originates from HER by water electrolysis.
FE is a quantitative indicator of how many electrons are transported from the external circuit to the electrodes for the HER. Therefore, FE is the fraction of electrons applied from the electrode that is used for the HER. FE was 97% (Figure 5b). This value is comparable to that of commercial Pt/C (100%). However, the Pt loading of Pt/C is 2.8 mg Pt cm −2 , while that of Com 15 is only 15 µg Pt cm −2 .
The PEM electrolyzer fabricated with the Com 15 electrocatalyst showed no obvious performance degradation after 150 h of operation at 100 mA cm −2 , producing 4.65 × 10 −3 kg H 2 (52.1 L) at the cathode and 37.6 × 10 −3 kg O 2 (26.4 L) at the anode at ambient pressure and 25 °C (Figure 5c). Nanocomposite electrocatalysts consisting of Com 33 and Com 40 with different PtNP concentrations also showed comparable device stability with that of Com 15 ( Figure S12, Supporting Information). Remarkably, the operating cost of our Com 15 PEM electrolyzer is just US$ 4.9 per kg H 2 (see, Supporting Information). This value is comparable to that (US$ 3.7) of the state-of-the-art PEM electrolyzer built by the US Department of Energy using a Pt loading of 2 mg Pt cm −2 . [44] Currently, H 2 produced by reforming fossil fuels costs US$ 1.3-1.5 per kg. In contrast, water electrolysis using renewable energy costs over US$ 4 per kg H 2 , so that needs to fall to US$ 2 per kg to be cost competitive. [45] Thus, the present study demonstrates that Com 15 not only exhibits HER at low PtNP loading but also has techno-economic advantages. Further development of our method, such as controlling the size of the PtNPs and the nanostructure of the SWCNTs, will make it possible to fabricate a PEM electrolyzer with stable and high-performance HER activity under practical conditions.  www.advmatinterfaces.de

Conclusions
We have demonstrated a PtNP-conjugated SWCNT electrocatalyst for a PEM electrolyzer that exhibits excellent activity and stability for the HER. The enhanced catalytic activity is attributed to the chemical immobilization of PtNPs on SWCNTs through noncovalent functionalization. Most remarkably, the PtNPs used in the electrocatalyst show sufficient HER catalysis at microgram loadings, so this methodology will contribute to the reduction of precious-metal consumption. The electrocatalyst allows water splitting in a PEM electrolyzer under cost-effective conditions. Thus, our findings not only introduce a useful strategy for fabricating nanostructured catalysts with enhanced activities in electrochemical reactions but also present a highly efficient HER catalyst that may facilitate the industrial application of PEM electrolyzers.
Electrochemical Characterization: Electrochemical properties were investigated using a potentiostat (ALS/CH instruments Model 700C, scan rate: 20 mV s −1 , BAS Inc., Tokyo, Japan); a lab-fabricated working electrode prepared with carbon paper (active area: 0.5 cm 2 ); an Ag/ AgCl reference electrode (RE-1B, 3 m sodium chloride, BAS); and a Pt wire counter electrode (0.5 mm diameter, BAS). All experiments were performed using nitrogen (N 2 )-saturated electrolyte (0.5 m sulfuric acid) with a constant flow of N 2 from the back of the working electrode. All the potentials reported in this study are relative to the reversible hydrogen electrode (RHE) using the following equation The Tafel slope was obtained using the Tafel equation where η, b, and j are the overpotential, Tafel slope, and current density. The polarization curve of the Tafel plot was obtained at a scan rate of 20 mV s −1 . Stability tests were performed using a chronopotentiometer equipped with data loggers (GL840, Graphtec Co., Kanagawa, Japan) and a digital power supply (ZX-S-400LAN, Takasago Ltd., Kanagawa, Japan). The constant applied current corresponded to a current density of 10 or 100 mA cm −2 .
Faradaic Efficiency: H 2 and O 2 evolution by water electrolysis was measured using a mass flow meter (S48-32, Horiba Stec Co., Ltd., Kyoto, Japan) and a power supply (BP4610, NF Corporation, Kanagawa, Japan). FE was determined from the relationship between the amount of H 2 gas generated and the applied voltage as follows where V H2 is the volume of generated H 2 gas (L), V m is the molar volume of H 2 gas (L mol −1 ), i is the applied current (A), t is the total time of operation (s), and F is the Faraday constant (96 485 C mol −1 ).

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.