Hydrogen Evolution Reaction Pdf

[Francisca Noggles]

Hydrogenevolution reaction (HER) is a chemical reaction that yields H2.[1] The conversion of protons to H2 requires reducing equivalents and usually a catalyst. In nature, HER is catalyzed by hydrogenase enzymes. Commercial electrolyzers typically employ supported platinum as the catalyst at the anode of the electrolyzer. HER is useful for producing hydrogen gas, providing a clean-burning fuel.[2] HER, however, can also be an unwelcome side reaction that competes with other reductions such as nitrogen fixation, or electrochemical reduction of carbon dioxide[3] or chrome plating.

HER is a key reaction which occurs in the electrolysis of water for the production of hydrogen for both industrial energy applications,[4] as well as small-scale laboratory research. Due to the abundance of water on Earth, hydrogen production poses a potentially scalable process for fuel generation. This is an alternative to steam methane reforming[5] for hydrogen production, which has significant greenhouse gas emissions, and as such scientists are looking to improve and scale up electrolysis processes that have fewer emissions.

Both of these mechanisms can be seen in industrial practices at the anode side of the electrolyzer where hydrogen evolution occurs. In acidic conditions, it is referred to as proton exchange membrane electrolysis or PEM, while in alkaline conditions it is referred to simply as alkaline electrolysis. Historically, alkaline electrolysis has been the dominant method of the two, though PEM has recently began to grow due to the higher current density that can be achieved in PEM electrolysis.[7]

The HER process is driven forward by electricity and requires a large energy input without a highly efficient catalyst, which is a chemical which lowers the activation energy of a reaction without being consumed. In alkaline electrolyzers, Nickel and Iron based catalysts for HER are typically used at the anode.[8] The alkalinity of the electrolyte in these processes enables the use of less expensive catalysts[4] In PEM electrolyzers, the standard catalyst for HER is platinum supported on carbon, or Pt/C,[8] used at the anode. The performance of a catalyst can be characterized by the level of adsorption of hydrogen into binding sites of the metal surface, as well as the overpotential of the reaction as current density increases.[4]

The high cost and energy input from water electrolysis poses a challenge to the large scale implementation of hydrogen power. While alkaline electroysis is commonly used, its limited current density capacity requires large electrical input, which poses both a cost and environmental concern due to the high carbon content of electricity in the many countries, including the United States[9] The electrocatalysts used for electrolysis of PEM electrolyzers currently account for about 5% of the total process cost, however, as this process is scaled up, it is predicted that catalysts costs will rise due to scarcity and become a huge factor in the cost of producing hydrogen.[10] As such, low-cost, high-efficiency, and scalable alternative materials for the HER catalysts in PEM electrolyzers are a point of research interest for scientists.

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Platinum (Pt)-based catalysts are generally considered to be the most effective electrocatalysts for the hydrogen evolution reaction (HER)5,7. Unfortunately, Pt is expensive and scarce, limiting the commercial potential for such catalysts. The development of active, stable and inexpensive electrocatalysts for water splitting is a key step in the realization of a hydrogen economy, which is based on the use of molecular hydrogen for energy storage. Significant effort has been devoted to the search of non-precious-metal-based HER catalysts, including sulfide-based materials8,9,10,11, and C3N4 (refs 12, 13, 14). Although these candidate materials show promising activities for the HER, the activities of these catalysts in their present form are insufficient for industrial applications15.

To overcome the challenges associated with the Pt HER catalysts and to drive the cost of H2 production from water electrolysis down, it is very important to markedly decrease the Pt loading and increase the Pt utilization efficiency. Currently, supported Pt nanoparticles (NPs) are typically used to promote Pt activity towards the HER. Unfortunately, the geometry of the NPs limit the majority of the Pt atoms to the particle core, deeming them ineffective, as only surface atoms are involved in the electrochemical reaction16. Reducing the size of the Pt NPs to clusters or even single atoms could significantly decrease the noble metal usage and increase their catalytic activity, which is highly desirable to enhance the Pt utilization and decrease the cost of the electrocatalysts17. It has been shown that single Pt atoms dispersed on an FeOx surface have a higher catalytic activity for CO oxidation compared with the corresponding Pt NPs18. Moreover, the single-atom catalysts also exhibited a significantly improved catalytic activity towards methanol oxidation, up to 10 times greater than the state-of-the-art commercial carbon-supported Pt (Pt/C) catalysts19.

Controlled and large-scale synthesis of stable single atoms and clusters remains a considerable challenge due to the natural tendency for metal atoms to diffuse and agglomerate, resulting in the formation of larger particles20,21. In practical applications, it is required that the single atoms not only have a high activity but also exhibit a satisfactory stability17,22,23. Moreover, it is also desired to produce a high density of single atoms to meet the practical applications. Consequently, an ideal single-atom catalyst must have a high activity, a high stability and a high density. Thus, we need to discover an effective means to synthesize this ideal single-atom catalyst. In this paper, the atomic layer deposition (ALD) technique was utilized, as it has been proven to be a powerful tool for large-scale synthesis of stable single-atom and cluster catalysts19,24. ALD has the ability to precisely control the size and distribution of particles on a substrate by using sequential and self-limiting surface reactions25,26,27.

In this work, we fabricate single platinum atoms and clusters supported on nitrogen-doped graphene nanosheets (NGNs) for the HER using the ALD technique, resulting in the utilization of nearly all the Pt atoms. The size and density of the Pt catalysts on the NGNs are precisely controlled by simply adjusting the number of ALD cycles. The Pt atoms and clusters on the NGNs show much greater activity for the HER in comparison with conventional Pt NP catalysts.

X-ray absorption spectroscopy was used to study the local electronic structure of the Pt catalysts and their interaction with the support material38,39. The normalized X-ray absorption near edge structure (XANES) spectra for both the Pt L3- and L2-edges of the ALDPt/NGNs and Pt/C catalysts are shown in Fig. 3 with comparison to a standard Pt foil. It can be seen in Fig. 3 that the threshold energy (E0) and the maximum energy (Epeak) of the Pt L3-edge for the ALDPt/NGNs are similar to those of the corresponding metal foil, thus confirming the metallic nature of the Pt atoms and clusters on the ALDPt/NGNs samples. Furthermore, detailed examination of the spectra was conducted by qualitative and quantitative analysis of the Pt L2 and L3 white line (WL) edges. It has been shown that the area under the WL of the L2,3-edge of the Pt metal is directly related to the unoccupied density of states of the Pt 5d orbitals. This in turn has been used to correlate the catalytic activity of Pt-based electrocatalysts to changes in their local electronic structure. Close examination reveals that the intensity of the Pt L3 WL exhibits small differences for the ALDPt/NGNs and the Pt/C catalysts. The magnitude of the Pt WL intensity at the Pt L3-edge appears to increase in the order of Pt foil

To fully understand the effect of the unoccupied densities of 5d states of the Pt catalysts, quantitative WL intensity analysis has been conducted on the basis of a reported method to determine the occupancy of the 5d states in each sample38,40,41 (see details in Methods). The Pt L3- and Pt L2-edge threshold and WL parameters were summarized in Table 1. The results indicate that the ALD50Pt/NGNs catalysts have the highest total unoccupied density of states of Pt 5d character, while the Pt/C sample has the lowest. It has been demonstrated in literature that the vacant d-orbitals of individual atoms play a vital role in the activity of catalysts and account for the excellent catalytic activity of single-atom catalysts18,42.

To understand the stabilization mechanism of Pt atoms on N-doped graphene, we applied density functional theory (DFT) calculations. The DFT calculations have been carried out using a graphene (5 5 1) supercell containing 48 C atoms and 1 N atom to identify the distribution of single Pt atoms on the NGNs. All inequivalent Pt-adsorption configurations around the N atom have been considered, and the most stable configurations are shown in Supplementary Fig. 14. It can be seen that in site III, Pt is located closest to the N atom, while in site I, II, IX, X and XI the Pt atom is situated further away from the N-dopant. To determine the most stable adsorption site for the Pt atom, the adsorption energies (Ea) for each site in Supplementary Fig. 14 were calculated by the following equation:

The chemical bonding of Pt with the N-doped graphene has also led to unique electronic properties of single Pt atoms with respect to Pt NPs, due to the charge transfer required for bond formation. The single metal atoms still carry a charge after adsorption onto the N-doped graphene substrate, which can be verified by various spectral measurements and computational modelling of the catalysts18,49,50,51. In our study, it was found that the discrete 5d-orbitals of the single Pt atoms are mixed with the N-2p orbitals around the Fermi level (Fig. 4). The calculated Bader charges (Table 2) show that the single Pt atoms are positively charged, where the N atom obtains the electron. In this case, the single Pt atoms on the N-doped graphene contain more unoccupied 5d densities of states. On H chemisorption (Fig. 4b), the 5d orbitals of the Pt atoms interact strongly with the 1s orbital of the H atoms, leading to electron pairing and hydride formation. In addition, more Pt (5d) states are found above the Fermi level, which is consistent with the calculated charge transfer from the Pt atoms to the H atoms. To fully understand the unique electronic properties of the single Pt atom catalysts for the HER, Pt clusters were also examined. As shown in Supplementary Fig. 16, the electronic properties for H adsorption on Pt clusters was investigated using a typical cluster of Pt44 (ref. 52). On the basis of the partial density of states and Bader charge analysis of both H adsorption on Pt44 and on the single Pt atoms/NGNs system (Table 2, Fig. 4 and Supplementary Fig. 16) it was found that the electron transfer from each surface Pt atom of Pt44 to H is

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