Platinum single-atom and cluster catalysis of the hydrogen evolution reaction

Abstract

Platinum-based catalysts have been considered the most effective electrocatalysts for the hydrogen evolution reaction in water splitting. However, platinum utilization in these electrocatalysts is extremely low, as the active sites are only located on the surface of the catalyst particles. Downsizing catalyst nanoparticles to single atoms is highly desirable to maximize their efficiency by utilizing nearly all platinum atoms. Here we report on a practical synthesis method to produce isolated single platinum atoms and clusters using the atomic layer deposition technique. The single platinum atom catalysts are investigated for the hydrogen evolution reaction, where they exhibit significantly enhanced catalytic activity (up to 37 times) and high stability in comparison with the state-of-the-art commercial platinum/carbon catalysts. The X-ray absorption fine structure and density functional theory analyses indicate that the partially unoccupied density of states of the platinum atoms' 5d orbitals on the nitrogen-doped graphene are responsible for the excellent performance.

Figure 1. ADF STEM images and schematic illustration of the Pt ALD mechanism on NGNs.

ADF STEM images of ALDPt/NGNs samples with (a,b) 50 and (c,d) 100 ALD cycles. Scale bars, 10 nm (a,c); 5 nm (b,d). (e) Schematic illustration of the Pt ALD mechanism on NGNs. The ALD process includes the following: the Pt precursor (MeCpPtMe₃) first reacts with the N-dopant sites in the NGNs (i). During the following O₂ exposure, the Pt precursor on the NGNs is completely oxidized to CO₂ and H₂O, creating a Pt containing monolayer (ii). These two processes (i and ii) form a complete ALD cycle. During process (ii), a new layer of adsorbed oxygen forms on the platinum surface, which provides functional groups for the next ALD cycle process (iii).

Figure 2. Electrocatalytic properties.

(a) The HER polarization curves for ALDPt/NGNs and Pt/C catalysts were acquired by linear sweep voltammetry with a scan rate of 2 mV s⁻¹ in 0.5 M H₂SO₄ at room temperature. N₂ was purged before the measurements. The inset shows the enlarged curves at the onset potential region of the HER for the different catalysts. (b) Mass activity at 0.05 V (versus RHE) of the ALDPt/NGNs and the Pt/C catalysts for the HER. (c) Durability measurement of the ALD50Pt/NGNs. The polarization curves were recorded initially and after 1,000 cyclic voltammetry sweeps between +0.4 and −0.15 V (versus RHE) at 100 mV s⁻¹ in 0.5 M H₂SO₄ at a scan rate of 2 mV s⁻¹. (d) ADF STEM images of ALD50Pt/NGNs samples after ADT; scale bar, 20 nm.

Figure 3. X-ray absorption studies.

(a) The normalized XANES spectra at the Pt L₃-edge of the ALDPt/NGNs, Pt/C catalysts and Pt foil. The inset shows the enlarged spectra at the Pt L₃-edge. (b) The normalized XANES spectra at the Pt L₂-edge of ALDPt/NGNs, Pt/C catalysts and Pt foil. The inset shows the enlarged spectra at the Pt L₂-edge WL.

Figure 4. The electronic structure of a single Pt atom before and after hydrogen adsorption.

Partial density of states (PDOS) of (a) non-H and (b) two H atoms adsorbed on a single Pt atom of ALDPt/NGNs. The Fermi level is shifted to zero. The upper part of the panel shows the PDOS of graphene, the middle part of the panel gives the PDOS of the N atom and the lower part of the panel exhibits the PDOS of the d orbital of Pt.