Development of High Performance Electrocatalyst for Water Splitting Application

SummaryEfficient electrocatalyst toward hydrogen evolution/oxidation reactions (HER/HOR) and oxygen reduction reaction (ORR) is desirable for water splitting, fuel cells, etc. Herein, we report an advanced platinum phosphide (PtP2) material with only 3.5 wt % Pt loading embedded in phosphorus and nitrogen dual-doped carbon (PNC) layer (PtP2@PNC). The obtained catalyst exhibits robust HER, HOR, and ORR performance. For the HER, a much low overpotential of 8 mV is required to achieve the current density of 10 mA cm−2 compared with Pt/C (22 mV). For the HOR, its mass activity (MA) at an overpotential of 40 mV is 2.3-fold over that of the Pt/C catalyst. Interestingly, PtP2@PNC also shows exceptional ORR MA which is 2.6 times higher than that of Pt/C and has robust stability in alkaline solutions. Undoubtedly, this work reveals that PtP2@PNC can be employed as nanocatalysts with an impressive catalytic activity and stability for broad applications in electrocatalysis.

Efficient hydrogen production of phosphonitrilic-bridged metal porphyrin porous covalent organic polymer with a push-pull motif

Thin Films Fabricated by Pulsed Laser Deposition for Electrocatalysis

The hydrogen evolution reaction (HER) as part of water splitting is a fundamental electrocatalytic process and plays a crucial role in hydrogen-based energy conversion. Although platinum and its group metals rank among the most active electrocatalysts for the HER, the slow reaction kinetics under alkaline media conditions has hindered their application in alkaline water electrolyzers. Therefore, much effort has been devoted to exploring highly active electrocatalysts for the alkaline HER. Transition-metal hydroxides/oxides are efficient for the prior water-dissociation step of alkaline HER, and therefore, combining these materials with Pt may be promising candidates for alkaline HER catalysts. Here, we demonstrate that Pt nanoparticles supported on interconnected NiFe oxide particles can serve as enhanced catalysts for alkaline HER.The NiFe oxide was synthesized by the flame pyrolysis method.1,2 The Pt nanoparticles were loaded on the NiFe oxide by a modified colloidal method.3,4 The as-prepared Pt/NiFe oxide sample was reduced in 5% H2 (N2 balance) at 100 oC for 1h. The Pt loading was determined to be 28.3 wt% by inductively coupled plasma-optical emission spectroscopy (ICP-OES). The scanning transmission electron microscopy (STEM) image was shown in Figure 1 ((a) secondary electron (SE) image and (b) transmission electron (TE) image). It can be seen that Pt nanoparticles (some interconnected to form rod/wire-like structures) are dispersed on the NiFe oxide support. Figure 1c shows the HER polarization curves of Pt/NiFe oxide compared with a commercial catalyst Pt/C (TEC10E50E, TKK). The HER specific activity (j s) and mass activity (MA) were summarized in Figure 1d. The HER activity is shown to be greatly enhanced on the Pt/NiFe oxide catalyst, with the specific activity and mass activity (@-0.1 V) being approximately 3.9 and 2.4 times higher than that of Pt/C, respectively. The high mass activity of Pt/NiFe oxide would make it possible to lower the Pt usage, and thus cost, when implemented as a cathode in a practical alkaline water electrolyzer. In addition, the Pt/NiFe oxide catalyst was further heat-treated at high temperatures to obtain different nanostructures and elucidate the structure-activity relationship, achieving optimal HER activity at a moderate hydrogen binding energy. These results are expected to aid in the design of highly efficient HER catalysts for hydrogen production by alkaline water electrolysis. Acknowledgement This work was partially supported by the JSPS KAKENHI (23H02059) and the projects from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References K. Kakinuma, M. Uchida, T. Kamino, H. Uchida, and M. Watanabe, Electrochim. Acta, 56, 2881 (2011).G. Shi, T. Tano, D. A. Tryk, M. Yamaguchi, A. Iiyama, M. Uchida, K. Iida, C. Arata, S. Watanabe, and K. Kakinuma, ACS Catal., 12, 14209 (2022).G. Shi, T. Tano, D. A. Tryk, A. Iiyama, M. Uchida, and K. Kakinuma, ACS Catal., 11, 5222 (2021).G. Shi, T. Tano, D. A. Tryk, T. Uchiyama, A. Iiyama, M. Uchida, K. Terao, M. Yamaguchi, K. Tamoto, Y. Uchimoto, and K. Kakinuma, ACS Catal., 13, 12299 (2023). Figure 1

The effective utilization of clean energy and finding alternatives to fossil resources are highly important to ensure the sustainability of human society and are always among the major goals of both chemistry and material science research. Advanced electrochemical devices, such as fuel cells, water electrolysers and metal-air batteries, represent the most promising strategies for clean-energy utilization. In an electrochemical device, the redox reactions are spatially separated by a membrane, allowing direct extraction/transfer of electrons at an electrode-electrolyte interface, which leads to higher intrinsic energy conversion efficiencies, milder process conditions, easy product separation and excellent design features for coupling to renewable energy infrastructure. The performance of such electrochemical processes is fundamentally determined by the physicochemical properties of the electrochemical interfaces, encompassing both the electrocatalyst and the structure of the adjacent electrochemical double layer. Specifically, electrocatalysts play key roles in electrochemical reactions and often limit the performance of entire systems due to their insufficient activity, low durability or high cost. Ideally, the rate, efficiency, and selectivity of the above electrochemical reactions can be substantially improved by developing high-performance electrocatalyst. One of the central tasks for chemists and material scientists is to design and fabricate the high-efficient efficiency but low-cost electrocatalysts systems. The current promising electrochemical reactions mainly focus on the realization of the reversible conversion between chemical and electricity energy, e.g., the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen oxidation reaction (HOR), and hydrogen evolution reaction (HER). Coupling of the above electrochemical reactions provide a solid foundation for various essential electrochemical devices, such as direct hydrogen fuel cells (HOR + ORR); electrolysers (OER + HER); rechargeable zinc (Zn)-air battery (ORR + OER). Therefore, this thesis aims to design and synthesize high-performance electrocatalysts for HER, ORR and OER based on earth-abundant materials with proper hierarchical 2D or 3D nanostructures. Combined with the advanced characterization techniques and density functional theory (DFT) calculations, the relationship between the electrochemical activity and active sites of these earth-abundant electrocatalysts were detailedly explored and confirmed. Furthermore, to emphasize the hierarchical 2D or 3D nanostructures, the actual performance of these electrocatalysts was all evaluated in practical devices including Zn-air battery and proton exchange membrane fuel cell (PEMFC), specifically as follows: (1) The vast majority of the reported HER electrocatalysts performs poorly under alkaline conditions due to the sluggish water dissociation kinetics. In the first work, a hybridization catalyst construction concept is presented to dramatically enhance the alkaline HER activities of catalysts based on 2D transition metal dichalcogenides (TMDs) (MoS2 and WS2). A series of ultrathin 2D-hybrids are synthesized via facile controllable growth of 3d metal (Ni, Co, Fe, Mn) hydroxides on the monolayer 2D-TMD nanosheets. The resultant Ni(OH)2 and Co(OH)2 hybridized ultrathin MoS2 and WS2 nanosheet catalysts exhibit significantly enhanced alkaline HER activity and stability compared to their bare counterparts. The combined theoretical and experimental studies confirm that the formation of the heterostructured boundaries by suitable hybridization of the TMD and 3d metal hydroxides is responsible for the improved alkaline HER activities because of the enhanced water dissociation step and lowers the corresponding kinetic energy barrier by the hybridized 3d metal hydroxides. (2) Nitrogen-coordinated iron atoms on carbon matrix (Fe-N-C) materials are the most active Pt-group-metal-free ORR catalysts but still suffering their low stability and relatively lower activity compared to platinum-based materials. In the second work, Fe and Ni dual sites atomically dispersed in hierarchically ordered macroporous carbon support (Fe-Ni/N-HOMC) was designed and successfully prepared. Isolated atomic Fe- N4 and Ni-N4 active sites were confirmed via various characterizations. The ORR activity and stability of Fe-Ni/N-HOMC in both acid and alkaline electrolyte were much higher than commercial Pt/C and the mono-Fe doping counterpart, which was among the state-of-the-art ORR electrocatalysts. In addition, this 3D ordered interconnected macroporous structure with abundant mesopores and micropores could greatly increase the accessible ORR active site and also enhance the mass transport during the ORR process. When employed as cathodes for PEMFC, we found the excellent ORR activity of Fe-Ni/N-HOMC was completely translated to the cathode in the fuel cell. (3) High-performance bifunctional electrocatalysts with ORR and OER activity is the key to developing efficient rechargeable Zn-air batteries. In the third work, a high-performance bifunctional electrocatalysts for both OER and ORR were synthesized via further hybridizing as-prepared Fe-Ni/N-HOMC with NiFe layer double hydroxides (LDHs). Layered double hydroxides (LDHs) have been reported to be promising OER electrocatalysts with ultrahigh OER performances. The as-synthesized new composites exhibited almost the same ORR activity as Fe-Ni/N-HOMC, revealing that hybridization of NiFe-LDHs would not deteriorate the initial ORR activity. Moreover, the remarkable enhancement of OER activity was observed after the hybridization, which was attributed to the strong coupling of uniformly dispersed small NiFe-LDH nanoparticles with the carbon substrate. The prototype Zn-air battery was assembled using these new composites, which displayed the ultralow voltage gap and long-term stability. (4) Compared with Fe-N-C or Co-N-C based ORR electrocatalysts, the Cu-nitrogen-carbon composites were attracted little attention. However, the natural multicopper oxidases (MCOs) enzymes, such as laccase, can serve as efficient ORR catalyst with almost no overpotential. Inspired by their tris-copper centers in MCO, one novel Cu-nitrogen-carbon composite (Cu SAs/N-CS) with atomic Cu coordination sites were synthesized via the pyrolysis of the Cu-involved metal-organic-framework. The copper contents in Cu SAs/N-CS reaches as high as 3.17 wt.%, and the average distances of adjacent copper sites was around only 3.1 Å. Due to the synergetic effect of abundant single atomic copper active sites with closer distance and ultrathin carbon nanosheet structure, Cu SAs/N-CS exhibited superior ORR activity exceeding commercial Pt/C catalyst, methanol tolerance, and long-term stability in both alkaline and neutral electrolyte. In summary, four kinds of new composites were successfully designed and prepared as high-performance electrocatalysts for HER, ORR and OER. Multi-dimensional heterostructures, atomic metal coordination sites and 3D hierarchically porous structure were designed and observed, which contributed greatly to improve activities of these composites. This thesis suggests several new viewpoints in the design of electrocatalysts based on earth-abundant materials: (i) offering new strategies for the preparation of novel 2D and 3D heterostructures as electrocatalysts; (ii) expanding methods for the synthesis of atomic metal coordination sites and evaluating their activities for ORR; (iii) evaluating the practical performances of achieved electrocatalysts in proton exchange membrane fuel cell and Zn-air battery; (iv) attempting to explain reaction mechanisms of some electrocatalysts by DFT calculation.

Nickel (Ni)-based materials are among the most promising platinum group metal-free (PGM-free) electrocatalysts for hydrogen evolution reaction (HER) in alkaline media. With the growing demand for green hydrogen production, the HER activity of Ni-based catalysts must be enhanced to meet the goal of low-cost and sustainable hydrogen production.1 Carbon-supported Ni-based catalysts are typically prepared through the heat-treatment of Ni-based precursors with carbon supports.2 However, the catalysts obtained by this approach suffer from high content of inactive carbon and a non-uniform distribution of Ni-based nanoparticles, leading to low HER activity. These issues amplify the need for novel Ni-based catalysts with a well-controlled catalyst structure designed for maximizing catalyst utilization and minimizing electrode thickness.In this talk, we will present our recent progress in developing Ni-based HER catalysts derived from metal-organic framework (MOF) precursors. Taking advantage of the characteristics of well-defined structure, high porosity, and flexible chemistry of MOF precursors3, the MOF-derived catalysts exhibit very promising activity towards HER allowing to reach 10 mA/cm2 at an overpotential of less than 100 mV in alkaline media. We will discuss how the ligand in MOF precursors and heat treatment conditions influence the structure and HER performance of Ni-based catalysts. This understanding promises to provide much needed guidance for the advancement of Ni-based catalysts for HER in alkaline media. Acknowledgement This work has been partially supported by the Center for Alkaline Based Energy Solutions (CABES) funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award DE-SC0019445.

The hydrogen evolution reaction (HER) is involved in energy-intensive water- and chlor-alkali electrolyzers, and thus, highly active and stable HER electrocatalysts in alkaline media are needed. Titanates, a family of representative two-dimensional materials with negatively charged main layers, are chemically and structurally stable under strongly basic conditions, but they have never been shown to have electrocatalytic activity for HER. Herein, we report that intercalating 3d metal cations, including Fe3+, Co2+, Ni2+, and Cu2+ ions, into the interlayer regions of titanates yields efficient and robust electrocatalysts for the alkaline HER. The intercalation of 3d metal cations in titanates is achieved by rapid cation-exchange reaction between Na+-containing titanates and 3d metal cations at room temperature. Among the 3d metal-intercalated titanates we synthesize, the Co2+-containing material is found to show the best electrocatalytic activity. Experimental and theoretical results reveal that the strong electronic interaction between 3d metal cations and negatively charged main [TiO6]∞ layers renders good catalytic activity to the outermost oxygen atoms in the [TiO6]∞ layer, further making 3d metal-intercalated titanate an efficient electrocatalyst for the HER.

Novel electrocatalyst of nickel sulfide boron coating for hydrogen evolution reaction in alkaline solution

Water splitting by electrocatalysis into hydrogen via the hydrogen evolution reaction (HER) offers a promising solution for green renewable energy generation1. In past several years, Pt and Pt based nanomaterials are the mostly served as an efficient electrocatalyst for HER; however, due to high-cost and low abundancy of Pt limits its vast utility for the commercial HER application. Therefore, it's highly needed to develop nonprecious metal based electrocatalyst for HER towards cost effective commercial application2. Over the past decades, tremendous efforts have been made for replacing the Pt based electrocatalysts for HER. Recent advances in carbon nanomaterials (RGO, CNT, etc.,) have shown their promising future in energy-related electrocatalytic reactions, such as ORR, OER, and HER. After heteroatom (such as N, B, P, and S) doping, the enhanced catalysis activity of carbon nanomaterials has been widely illustrated for ORR. Among all the reported electrocatalysts, the co-doping of trace transition metals on heteroatoms doped carbon materials leads to form metal complexes (Ex. Co-Nx), showing promising HER activity in the aqueous electrolyte. In addition, the reported density functional theory (DFT) simulation shows that when combining both coordinations into one complex, the optimized charge distribution results in an ideal value of ΔGH in Metal-Carbon-Nitrogen (M-C-N) hybrid system and which is much better than the single or mixture system of M-C and M-N3. In this presentation, we will demonstrate a new HER electrocatalyst based on N-Co-C system. The N-Co-C catalysts are prepared by a one-step pyrolysis process at the high temperature in which vitamin-B12 used as a metal precursor together with reduced graphene oxide (RGO). We observed that the electrocatalytic activities of N-Co-C catalysts are strongly related with the pyrolysis temperature and metal loading. The N-Co-C catalyst pyrolyzed at 800oC showed the highest HER activity with lowest overpotential of 110 mV at a current density of 10 mA cm-2. It shows the optimum Tafel slope of 65 mV decade-1 and excellent stability over 20 h in acidic condition. The as synthesized catalyst was characterized by XRD, XPS and XAS. The results indicate that Co-corrin complexes (Vitamin B12) together with RGO have been decomposed at the high temperature to form N-Co-C structure. This conjugation induces that, downshift of the d-band center of cobalt, and thereby decreases its hydrogen binding energy. This, in turn, favors the electrochemical desorption of Hads and leads to a relatively moderate Co−H binding strength, resulting in the enhancement of the hydrogen evolution reaction. The comparison of HER activity and stability of N-Co-C electrocatalyst in alkaline electrolytes will be discussed at the meeting.

Improved HER activity of Ni and stainless steel electrodes activated by NiCoMo ionic activator – A combined DFT and experimental study

Accelerating water dissociation kinetics on Ni3S2 nanosheets by P-induced electronic modulation

Fabrication of Te@NiTe2/NiS heterostructures for electrocatalytic hydrogen evolution reaction

A review on recent advances and progress in Mo2C@C: A suitable and stable electrocatalyst for HER

Here, we report an efficient hydrogen evolution reaction (HER) electrocatalyst (Fe@Fe2P/NCNT) fabricated from the phosphorization of partially oxidized iron nanorod encapsulated in nitrogen doped carbon nanotubes (Fe@Fe2O3/NCNT) synthesized from iron trichloride and melamine, which only requires ∼0 mV and 78.2 mV overpotentials to achieve cathodic current densities of 1 mA cm−2 and 10 mA cm−2 with Tafel slope of 52.2 mV dec−1 in acidic media, which exhibits higher HER electrocatalytic activity compared to Fe/NCNT requiring a overpotential of 118.5 mV to attain 10 mA cm−2 with Tafel slope of 92.3 mV dec−1. Due to the phosphorization process, additional active sites coming from Fe2P boost the electrocatalytic activity of Fe@Fe2P/NCNT resulting in decrement in overpotentials by 40.3 mV and 186 mV for 10 mA cm−2 and 50 mA cm−2 compared to Fe/NCNT electrocatalyst, respectively. Meanwhile, Fe@Fe2P/NCNT exhibits ignorable degradation in HER activity after 6000 potential cycles suggesting that the Fe@Fe2P/NCNT with superior HER activity and stability could potentially replace the benchmark Pt/C (overpotential@10 mA cm−2: 31 mV) as efficient HER electrocatalyst for water splitting.

Hydrogen is one of the most promising energy carriers, especially if produced from renewable energy sources, replacing fossil fuels for both portable and stationary applications [1]. Although hydrogen is one of the most abundant elements on Earth, it does not exist in nature in molecular form. Currently, most of the hydrogen is produced from fossil fuel-based technologies, such as gas reforming. Only 5% of worldwide hydrogen production comes from water electrolysis. The main problem of this process is the high overpotentials necessary to split the water molecule into hydrogen and oxygen and, consequently, the high energy consumption. To overcome this problem, different electrode materials such as Pt, Ni, Co, Ir and Rh, have been tested as cathodes for water electrolyzers. Pt has high activity and good stability for the hydrogen evolution reaction (HER), but its limited resources and high price prevent its use on industrial scale [2]. Alkaline water electrolysis is a mature technology that typically operates at temperatures around 60 – 80 ºC and uses cathodes based on Ni, Raney Ni and Co [3]. These cathodes are less expensive than Pt, but show good activity (in alkaline media) and corrosion resistance. A common approach used in electrocatalysis to minimize the utilization of precious metals involves employing an electrocatalyst support material. The support material has several functions, as to increase the catalytic electrochemically surface area, to stabilize and to accommodate the metal particles and also to increase the catalyst’s conductivity [4-6]. Carbon-based supports are the mostly used, as they can offer all the aforementioned properties [6]. The present work combines the advantages of using highly active Pt together with low cost transition metals, like Ni, Co and Cu, with the employment of reduced graphene oxide (rGO) as an efficient carbon-based electrocatalyst support. Thus, Pt and three different PtM alloys supported on rGO, i.e., Pt/rGO, PtCo/rGO, PtCu/rGO and PtNi/rGO, were tested as electrocatalysts for hydrogen production by alkaline water electrolysis. rGO has adequate features to enhance and stabilize the metal particles, with rGO-supported catalysts having recently demonstrated promising results in alkaline media [7]. To prepare the working electrodes, 5 mg of each of the fours electrocatalysts was dispersed in 125 µL of 5 wt.% solution of polyvinylidene fluoride (PVDF, Alfa Aesar) in N-methyl-2-pyrrolidone (NMP, Sigma Aldrich) followed by ultrasonic treatment for ca. 30 min. The working electrodes were prepared by pipetting 80 μL of the corresponding catalytic ink onto a polished glassy carbon (GC) electrode (A = 4 cm2). The electrodes were dried at 80 ºC for 4 hours. A conventional three-electrode setup was used for the electrochemical characterization. The rGO-supported PtM (M = Co, Cu, Ni) electrode was used as working electrode, the counter electrode was a Pt mesh (Johnson Matthey) of 25 cm2 area, and a calomel electrode (Hanna Instruments, HI-5412, 3.5 M KCl) was used as the reference. All recorded potentials were converted to RHE scale using the equation ERHE = Ecal + 0.250 + 0.059 pH. The evaluation of HER kinetics at the PtM/rGO composites was done by linear scan voltammetry measurements at scan rate of 1 mV s-1, starting from the open circuit potential up to -0.17 V in 8 M KOH (AnalaR NORMAPUR, 87 wt.%) solutions. Higher current densities were achieved for the PtM/rGO electrocatalysts, in comparison with the non-alloyed material (Pt/rGO). It was also observed that increasing the temperature up to 65 ºC leads to a substantial increase of HER current densities at all electrocatalysts. Furthermore, long-term stability of the four electrocatalysts’ activity for HER was evaluated by chronoamperometry studies at 25 and 65 ºC, revealing better stability at lower temperature. It is shown that PtM/rGO nanocomposites are good candidates for application as novel electrocatalysts for the HER in alkaline media. [1] J.A.S.B. Cardoso, B. Šljukić, M. Erdem, C.A.C. Sequeira, D.M.F. Santos, Catalysts 50, 8 (2018). [2] D.M.F. Santos, B. Šljukić, C.A.C. Sequeira, D. Macciò, A. Saccone, J.L. Figueiredo, Energy 50, 486 (2013). [3] D.S.P. Cardoso, L. Amaral, D.M.F. Santos, B. Šljukić, C.A.C. Sequeira, D. Macciò, A. Saccone, Int. J. Hydrogen Energy 40, 4295 (2015). [4] M. Martins, B. Šljukić, C.A.C. Sequeira, O. Metin, M. Erdem, T. Şener, D.M.F. Santos, Int. J. Hydrogen Energy 41, 10914 (2016). [5] R.C.P. Oliveira, V. Vasić, D.M.F. Santos, B. Babić, R. Hercigonja, C.A.C. Sequeira, B. Šljukić, Electrochim. Acta 269, 517 (2018). [6] E. Antolini, Appl. Catal. B 88, 1 (2009). [7] M. Martins, B. Šljukić, O. Metin, M. Sevim, C.A.C. Sequeira, T. Şener, D.M.F. Santos, J. Alloys Compd. 718, 204 (2017).