Hf₂B₂Ir₅: A Self-Optimizing Catalyst for the Oxygen Evolution Reaction

Abstract

The ternary compound Hf 2 B 2 Ir 5 was assessed as an electrocatalyst for the oxygen evolution reaction (OER) in 0.1 M H 2 SO 4 . The oxidative environment restructures the studied material in the near-surface region, creating cavities in which agglomerates of IrO x (OH) y (SO 4 ) z particles are incorporated. These in situ generated particles result from the oxidation of secondary phases in the matrix as well as from self-controlled near-surface oxidation of the ternary compound itself. The oxidation is controlled by the structural and chemical bonding features of Hf 2 B 2 Ir 5 . The cage-like motif, exhibiting mostly ionic interactions between positively charged Hf atoms and a covalently bonded Ir−B network, selectively controls the extent and kinetics of the transformation process induced during the operation of the electrocatalyst. The resulting self-optimized composite material, formed by a Hf 2 B 2 Ir 5 matrix surrounding IrO x (OH) y (SO 4 ) z particles, was used in the OER over 240 h at 100 mA cm −2 current density. The chemical changes, as well as the OER performance, were studied via a combination of bulk-and surface-sensitive experimental techniques as well as by employing a quantum-chemical bonding analysis.

Intermetallic compounds M2Pt (M = Al, Ga, In, Sn) in the oxygen evolution reaction

“…18 Also, the intermetallic compound Hf 2 B 2 Ir 5 with a cage-like type of structure self-improves its OER activity through near-surface oxidation, including Hf leaching and the controlled dynamic in situ formation of IrO x (OH) y (SO 4 ) z particles. 77 These thoughts agree with the electrochemical results and the elemental analysis of the electrolyte (Figure 5 The in situ-formed more active Pt species participate in two concurrent processes: (i) as active sites for OER, revealed by the activation of the material with electrochemical treatment and increasing surface area of the active sites, and (ii) in oxidation at high anodic potentials yielding the Pt oxides. This gives rise to a dynamic state of the surface, composed of the intermetallic Pt, freshly formed more active Pt of the solid solution M x Pt 1-x and platinum oxides.…”

Perspective on intermetallics towards efficient electrocatalytic water-splitting

“…131 The core-shell Cu 1−x NNi 3−y /FeNiCu, reported by Shao's group described the FeNiCu-(oxy)hydroxide as the active species in the HER process, that retained structural integrity post 24 h of OER. 99 The group of Grin observed that the bulk Hf 2 B 2 Ir 5 and major volume close to the surface did not 112 Zhang and co-workers showed that the surface oxidation of NiIrRuAl led to an improvement of catalytic activity. By tuning the atomic ratio of Ir/Ru, it was demonstrated that a higher Ru content improves the intrinsic catalytic activity while more Ir effectively generates Ir oxides on the surface to retain the catalytic stability.…”

“… 111 In a similar approach, the noble metal-based Hf 2 B 2 Ir 5 phase was obtained via a two-step process in which the metals were first arc melted in Ar at about 1200 °C for several weeks and then applied through spark plasma sintering (SPS) to retrieve the electrode material for OER catalysis. 112 Moreover, in a unique strategy, Jian and co-workers synthesized nanoporous hybrid Cu 12− x − y Co x Mo y Al 88 ( x = 0 or 3, y = 0 or 1) electrodes by arc melting pure Cu, Al, Co, and Mo metals in an Ar atmosphere and successively chemically-etched them in a N 2 -purged 6 M KOH electrolyte at 70 °C for 3 h. The synthesized electrodes were successfully applied for overall water splitting, while the OER electrodes were additionally electrochemically oxidized in a dealloying step at 1.57 V vs. RHE for 20 min under alkaline conditions. 113 …”

“…They concluded that the chemical bonding features of the Hf 2 B 2 Ir 5 compound with a cage-like Ir–B framework, hosting Hf atoms, only allow near-surface oxidation and inhibit deep Ir leaching. 112 Zhang and co-workers showed that the surface oxidation of NiIrRuAl led to an improvement of catalytic activity. By tuning the atomic ratio of Ir/Ru, it was demonstrated that a higher Ru content improves the intrinsic catalytic activity while more Ir effectively generates Ir oxides on the surface to retain the catalytic stability.…”

Structural Changes of Intermetallic Catalysts under Reaction Conditions

“…The near‐surface region is shown to undergo restructurtion during OER in 0.1 m H 2 SO 4 , creating cavities in the Hf 2 B 2 Ir 5 matrix, in which Hf 2 B 2 Ir 5 is oxidized to IrO x (OH) y (SO 4 ) z particles. [ 55 ] The resultant self‐optimized composite material shows long‐term stability (over 240 h) with a relatively high activity (100 mA cm −2 current density).…”