The limited abundance of conventional fossil fuels in the Earth’s crust and environmental pollution concerns have increased recent social and industrial interest in many renewable and alternative energy sources. One of the most promising next-generation energy sources of hydrogen energy can be easily obtained through the electrochemical splitting of water, an environmentally friendly resource; this mechanism has the strong advantages of being non-polluting, renewable, and free from the generation of carbon dioxide, a greenhouse gas with world-wide production restrictions [1]. The basic principle of electrochemical water splitting can be explained by the two distinctive reactions of the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode, which entail the reduction and oxidation of water, respectively [[2], [3], [4], [5]]:

In the HER, which is directly related to hydrogen production, the use of alternative catalytic materials such as transition metal sulfides and phosphides, including cobalt sulfide, molybdenum disulfide, and nickel phosphide, has been continuously proposed because of the high cost of typical platinum-based catalysts, despite their excellent efficiency [[6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]]. However, their intrinsic catalytic activity toward HER is inferior to that of Pt, and the development of new catalysts remains an urgent issue.

With the development of nanochemistry, unique physicochemical properties derived from the quantum confinement effect, which are not observed in conventional bulk materials but arise in nanoscale materials, have presented numerous possibilities for the application of the latter in various fields [[25], [26], [27]]. Beyond simple solid nanostructures, the high complexity of morphologies such as hollow nanoshells, nanocages, and porous structures provides them with significant advantages as heterogeneous catalysts because of the abundance of exposed catalytically active sites they contain as a result of their high surface-to-volume ratios [28]. However, because manufacturing such complex nanostructures often requires environmentally toxic surfactants or additional labor-intensive processes like etching or calcination, extensive efforts have been made to develop facile, robust, and safe synthetic approaches to create these [29]. Also, the electrocatalytic performance of a material is strongly related to its exposed active crystal planes. However, the preparation of a type of electrocatalyst with a large proportion of exposed active crystal planes with enhanced mass activity for HER is still highly desirable.

In the present study, we manufactured and applied HM-PtNPs with coral-like rough surface morphologies using environmentally friendly and biocompatible fucoidan (Fu) biopolymer as a surface engineering compound. The brown seaweed extract Fu is widely used in nanomaterial synthesis and has promising applications, including antibacterial materials and cancer treatment [30,31]. However, although numerous biomedical applications have been investigated because of Fu’s abundant negatively charged functional groups, morphological specificity, and environmental benefits, Fu has not yet been studied in the areas of energy, catalysis, and sensors. Based on the advantageous effects of the surface irregularity and porosity of nanostructures on electrocatalytic efficiency toward water splitting, we confirmed the applicability of the HM-PtNPs formed without toxic surfactants.

Fu extract powder was purchased from Okinawa Agent (Okinawa, Japan). Potassium tetrachloroplatinate (II) hydrate was purchased from Sigma-Aldrich (St. Louis, MO, USA). l-ascorbic acid was purchased from Junsei (Tokyo, Japan). Commercial 20 % Pt on carbon black (commercial Pt/C) and Nafion (5 wt % in a mixture of lower aliphatic alcohols and water) were purchased from Alfa Aesar (Haverhill, USA). Carbon black (Vulcan XC-72R) was purchased from Cabot Corporation (Boston, USA).

A Fu aqueous stock solution was prepared by adding 300 mg of Fu powder to 20 mL of deionized (DI) water in a conical tube. The mixture was completely dissolved by ultrasonication and vigorous vortexing. To optimize the synthesis of hierarchical mesoporous-Pt nanoparticles (denoted as HM-PtNPs), 1.25 mL of Fu stock solution and 8.75 mL of DI water were added to a 20-mL transparent glass vial, representing an eight-fold dilution. Then 50 μL of 240.91 mM aqueous K₂PtCl₄ and 120 μL of 100 mM l-ascorbic acid were sequentially injected into the Fu solution at ambient conditions. The mixture was incubated for 2 h in a 60 °C water bath without disturbance. After the reaction, the solution was cooled to room temperature and purified by centrifugation at 9000 rpm for 10 min. The HM-PtNPs were washed with DI water three times and finally redispersed in 5 mL of DI water for further use. In addition, nonporous-Pt nanoparticles (denoted as N-PtNPs) was made from K₂PtCl₄ without fucoidan (Fu) biopolymer as a control.

An energy-filtering transmission electron microscope LIBRA 120 (Carl Zeiss, Germany) was used to obtain TEM images of the synthesized nanoparticles. The morphologies and corresponding energy dispersive X-ray spectroscopy (EDS) of the catalysts were obtain via a field emission-scanning electron microscope (FE-SEM) (Hitachi SU 8010, Hitachi, Japan) operating at 15 kV. A UV-vis spectrophotometer Lambda 465 (PerkinElmer, USA) and SynergyMx (Biotek, UK) were used to measure the UV-vis-NIR extinction spectra. The mass ratios of Pt in catalysts were characterized by Bruker Aurora M90 inductively coupled plasma mass spectrometer (ICP-MS). The wettability of nanoparticle-coated FTO glasses was characterized by the measurement of water contact angles on their surfaces (FM 40, KRUSS Gmblt Germany). The volume of the distilled water droplet used for the contact angle determination was 10 ml.

Electrochemical measurements were performed in a three-electrode system using a potentiostat (Compactstat.h, Ivium Technologies). To prepare the working electrode of HM-PtNPs, 3.2 mg carbon black was dispersed in 970 μL HM-PtNPs and 30 μL 5 wt% Nafion and sonicated for 30 min. As a control, the same procedures were conducted using commercial Pt/C and ethanol instead of the carbon black and HM-PtNPs dispersion solution, respectively. The 5 μL as-prepared catalyst ink was drop-casted onto a glassy carbon electrode (GCE; diameter: 3 mm) and dried at room temperature. The amount of catalyst loaded on the GCE was 0.23 mg/cm². A carbon rod and an Ag/AgCl (3 M NaCl) electrode were used as the counter and reference electrodes, respectively. The electrolyte was 0.5 M H₂SO₄ aqueous solution purged with N₂ gas. The reference electrode potential was converted to that of a reversible hydrogen electrode (RHE) using the following equation:

Before the electrochemical measurements, cyclic voltammetry (CV) was performed for at least 10 cycles using a potential range of 0.5 V to -0.1 V vs. RHE at a scan rate of 50 mV/s; this was done to activate the electrode and remove impurities from near the surface. Linear sweep voltammetry (LSV) curves were generated for a range of 0.5 V to -0.2 V vs. RHE at 5 mV/s. Electrochemical impedance spectroscopy (EIS) was measured at -0.01 V vs RHE in a frequency range from 100 kHz to 0.1 Hz with a 5 mV amplitude. Long-term stability was tested by obtaining LSV curves after 1000 accelerated-degradation CV cycles in the potential window from 0.3 V to -0.15 V vs. RHE at a scan rate of 50 mV/s. The electrochemical surface area (ECSA) was calculated using the underpotential deposition of hydrogen (H-UPD) based on previous reports [32,33]. All the data presented include no iR compensation.

The mass activity (A/mg) of different samples were calculated from the electrocatalyst loading m (0.23 mg/cm²) and the measured current density j (mA/cm²) at η = -0.025 V, -0.05 V, and -0.1 V:

Mass activity = j/m

The synthesis of the HM-PtNPs was accomplished by reducing the platinate ions in the presence of Fu (Scheme 1). Briefly, highly negatively charged Fu side chains with abundant sulfonate functional groups contributed to binding Pt(II) cations, followed by the formation of local-concentrated precursor environment within the dimensions of the polymeric dynamic length. During the formation of small Pt nanoseeds and the subsequent seed-mediated growth, the Fu polymer concentrated Pt(II) and functioned as a morphology-regulating biocompatible surfactant. The contribution of Fu polymer to particle formation process and electrocatalysis may be derived from higher negatively charged functional group density compared to the PVP polymer applied to the control group. In case of synthetic process, the negatively charged sulfate functional groups induced the binding of Pt2+ cation in solution, leading to effective nanostructure formation as mentioned. In addition, the volumetric space occupied by the Fu polymeric chain during this process forms a porous channel embedded hierarchical mesoporous structure. In case of electrocatalysis, aside from the properties due to the large surface-to-volume ratio, the accessibility of H + cation for hydrogen evolution is expected to be improved from the formation of negatively charged nanostructured originated from the adsorption of hydrophilic and negatively charged Fu polymer. There is no direct observation on this speculation, but it is classically expected that the local concentration is increased due to improved access of the cation to the negatively charged surface [34,35].

Transmission electron microscopy (TEM) images of the manufactured HM-PtNPs exhibited the successful formation of a 34 ± 2.5 nm sized homogeneous coral-like structure with a rough surface, as shown in Fig. 1(a, b), and S1(a). In case of control nanoparticles, N-PtNPs, exhibited average size distribution with 10-60 nm with solid morphology, as shown in Fig. 1(c), and S1 (b). NP-PtNPs would not be appropriate for electrochemical applications if only predicted by nanostructural aspects due to their low surface area resulting from lack of porosities. It was worth noting that negatively charged Fu biopolymer played a pivotal role as a surface engineering compound to control the agglomeration of Pt precursor during the chemical process from the comparison with N-PtNPs. SEM in conjunction with energy dispersive X-ray spectroscopy (SEM-EDS) was carried out to determine the morphology of catalyst and to give an indication of the elements present on HM-PtNPs. (Figure S3) The Pt, C, and S elements are uniformly distributed among the nanoparticle shape, demonstrating the formation of the HM-PtNPs are derived from biocompatible Fu biopolymer as a surface engineering compound. It is worth mentioning that after the stability tests (1000 cycles) under HER conditions, the composition and morphology of the HM-PtNPs remains unchanged. (Figure S3)

Ultraviolet-visible-near infrared (UV-Vis-NIR) spectra indicated the existence of Fu on the HM-PtNPs. HM-PtNPs exhibited enhanced extinction under the 500-nm region with an inflection point at 254 nm that is clearly observed in the Fu aqueous solution, as shown in Fig. 2. This indicates that the chemical process did not alter the chemical structure of Fu biopolymer significantly. None of the characteristic extinction peaks were visible for N-PtNPs, with the exception of a broad band under 500-nm attributable to the chemical bonding in Pt.

The electrocatalytic HER activities were characterized using a three-electrode system in N₂-saturated 0.5 M H₂SO₄. Before characterization, the HM-PtNPs and N-PtNPs were loaded on the CB. Fig. 3(a) shows the LSV curves without any iR compensation of the GCEs deposited with HM-PtNPs, N-PtNPs, commercial Pt/C, and carbon black. The HM-PtNPs, N-PtNPs, and commercial Pt/C have overpotentials (η) of 33 mV, 45 mV, and 29 mV, respectively, while producing a current density of 10 mA^-2. Moreover, the onset potential of the HM-PtNPs was similar to that of the commercial Pt/C, whereas that of the N-PtNPs was substantially higher. This indicates that the HM-PtNPs exhibit HER activity higher than that of the N-PtNPs and comparable to that of commercial Pt/C. Although commercial Pt/C showed enhanced HER activity compared to the HM-PtNPs, the Pt content of the HM-PtNPs catalyst ink (0.033 mg mL^-1) was approximately 20 times lower than that of the commercial Pt/C (0.64 mg mL^-1). The Tafel slope of the HM-Pt NPs-deposited GCE (33 mV dec^-1) was comparable to those of the commercial Pt/C (30 mV dec^-1), as shown in Fig. 3(b). These results suggest that HER may occur through a Tafel-Heyrovsky mechanism. As shown in Fig. 3d, the polarization curves obtained after 1000 continuous cyclic voltammograms represented a negligible decrease in current density compared with the initial curve, indicating the excellent catalytic stability of HM-PtNPs. Such a high HER performance was attributed to the improved electrochemical surface area and well-organized mesoporous structure of HM-PtNPs for the enhancement of the electrocatalytic activity, reduction of resistance at the electrode/electrolyte interface, and the facile penetration of the electrolytes inside the electrode. The non iR-corrected LSV curves as well as mass normalized activity maintaining equivalent loading of Pt amount on electrode surface for both HM-PtNPs, and commercial Pt/C have also been shown in Figure S4 and their mass activities are summarized in Figure S5 as a function of the overpotential. At -0.025 V versus RHE, the HM-PtNPs demonstrates 2.7 A/mg mass activity 15 times higher compared to commercial Pt/C. And, at the potential of -0.05, -0.1 V versus RHE, HM-PtNPs produces mass activities of 9.4 A/mg, 26.9 A/mg, respectively, which are about 15 and 16 times better than commercial Pt/C. All these results demonstrate that HM-PtNPs are highly active for HER with an extremely large mass activity. The mass activity results are also extremely encouraging, showing that these HM-PtNPs rival the performance of pure Pt metal catalyst and outperform the currently reported catalysts, as shown in Figure S6 and Table S2.

Fig. 4(a) shows the EIS analysis, while Fig. 4(b) shows the equivalent circuit of the electrochemical system. Electrochemical parameters, i.e. resistance values (R_s, R_p, and R_ct) are summarized in Table S1. R_s is the series resistance, which can be determined by the point of the x-axis intersecting with the semicircle; R_p is related to the porosity of the electrode in the high-frequency region; R_ct is the charge-transfer resistance in the low-frequency region; and CPE1 and CPE2 represent the double-layer capacitance. [36,37] The R_p is strongly related to the diffusion of electrolyte through electrode pores, in general, it is omitted in most electrodes. [38] Therefore, the commercial Pt/C, N-PtNPs, and CB electrodes presented simple one-semicircle spectra while the HM-PtNPs present two semicircles. EIS spectra provide good support for the mesoporous nature of the HM-PtNPs. The R_p + R_ct value of the HM-PtNPs (13.3 Ω) and the R_ct values of commercial Pt/C (12.0 Ω) and N-PtNPs (16.3 Ω) show good agreement with their relative HER activities.

To evaluate the electrocatalytic activity of the HM-PtNPs, NP-PtNPs, and commercial Pt/C, CV tests were performed in N₂-saturated 0.5 M H₂SO₄ aqueous solution with scanning rate of 50 mV s^-1. The HM-PtNPs had an ECSA of ~12 % only less than that of the commercial Pt/C despite having 20 times less Pt, as shown in Fig. 5. It was noteworthy that the enhanced electrocatalytic activity of HM-PtNPs compared to the commercial Pt/C could be originated from larger electrochemical surface area, which was related to the surface morphology, nanostructure and the available electrocatalytic active sites.

The surface wettability of the catalyst electrode significantly affects the water-splitting activity because high surface wettability favors electrolyte-electrode interaction and enhances HER performance. [39,40] The relative wettability of the electrodes were obtained using water contact angle (CA) measurements as shown in Fig. 6. The CA measurements were conducted using the catalyst inks spin-coated onto the conductive side of fluorine-doped tin oxide (FTO). The HM-PtNPs/FTO showed the lowest CA, suggesting that the existence of hydrophilic Fu on the HM-PtNPs/FTO further enhanced wettability. Taken together, the series of results suggest that the HM-PtNPs exhibit more favorable HER kinetics than N-PtNPs because of their unique mesoporous structures and good surface wettability. In addition, the hydrophilic Fu polymer in the HM-PtNPs may reduce resistance at the electrode/electrolyte interface and facilitate electrolyte access to the inner pores of the electrode.

In the present study, we utilized the environmental friendly and biocompatible Fu biopolymer as a surface engineering compound in the preparation of HM-PtNPs with coral-like rough surface morphologies by one-pot process. The unique morphology of HM-PtNPs endowed them with both geometric and surface electronic benefits to enhance the catalytic reaction kinetics of the HER. The prepared HM-PtNPs showed an overpotential of 33 mV at 10 mA cm^-2 for the HER, which is highly comparable to that of the state-of-the-art commercial Pt/C catalysts (29 mV), and the loading of noble metal was 20 times lower than that of commercial Pt/C. The HM-PtNPs also showed excellent stability even after 1000 cycles. The excellent HER performances of the high activity and cycling stability of HM-PtNPs are attributed to the combination of improved electrochemical surface area, highly porous structure, and good surface wettability which results in enhanced electrocatalytic activity, decreased resistance at the electrode/electrolyte interface, and facile penetration of the electrolytes inside the electrode. This work not only provides a deep understanding of the effects of morphology and structure of the electrocatalyst, but also broadens our horizons to construct cost-efficient novel metal electrocatalysts for practical application in combination with biopolymer in nanocatalyst field.

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2019R1C1C1010283 and NRF-2019R1C1C1002305).

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jmst.2020.01.036.