Real-Time Detection of Acetaldehyde in Electrochemical CO Reduction on Cu Single Crystals

Copper is known to be versatile in producing various products from electrochemical CO2 reduction reaction (eCO2RR), and the product preference depends on reaction environments. The literature has reported that alkaline electrolytes favor acetate production and proposed hypotheses on reaction pathways accordingly. However, our work shows acetate can also come from the fast non-Faradaic chemical oxidation of acetaldehyde in alkaline environments. This adds uncertainties into measurements of both acetaldehyde and acetate production and leads to untrustful investigations on the reaction mechanism as a consequence. With an electrochemistry-mass spectrometry combined (EC-MS) system, we not only demonstrate why and how the imprecise acetaldehyde and acetate production occurs in previous research but also present immediate detection of acetaldehyde as a function of applied potential on single crystal Cu electrodes during electrochemical CO reduction reaction (eCORR). Moreover, the quantified acetaldehyde-to-ethylene production rate ratio provides insightful information on the acetaldehyde-to-ethylene bifurcation point in eCO2RR and thus helps understand the reaction pathways

Tuning Surface Reactivity and Electric Field Strength via Intermetallic Alloying

Many electrosynthesis reactions, such as CO2 reduction to multicarbon products, involve the formation of dipolar and polarizable transition states during the rate-determining step. Systematic and independent control over surface reactivity and electric field strength would accelerate the discovery of highly active electrocatalysts for these reactions by providing a means of reducing the transition state energy through field stabilization. Herein, we demonstrate that intermetallic alloying enables independent and systematic control over d-band energetics and work function through the variation of alloy composition and oxophilic constituent identity, respectively. We identify several intermetallic phases exhibiting properties that should collectively yield higher intrinsic activity for CO reduction compared to conventional Cu-based electrocatalysts. However, we also highlight the propensity of these alloys to segregate in air as a significant roadblock to investigating their electrocatalytic activity

Using TiO₂ as a Conductive Protective Layer for Photocathodic H₂ Evolution

Surface passivation is a general issue for Si-based photoelectrodes because it progressively hinders electron conduction at the semiconductor/electrolyte interface. In this work, we show that a sputtered 100 nm TiO₂ layer on top of a thin Ti metal layer may be used to protect an n⁺p Si photocathode during photocatalytic H₂ evolution. Although TiO₂ is a semiconductor, we show that it behaves like a metallic conductor would under photocathodic H₂ evolution conditions. This behavior is due to the fortunate alignment of the TiO₂ conduction band with respect to the hydrogen evolution potential, which allows it to conduct electrons from the Si while simultaneously protecting the Si from surface passivation. By using a Pt catalyst the electrode achieves an H₂ evolution onset of 520 mV vs NHE and a Tafel slope of 30 mV when illuminated by the red part (λ > 635 nm) of the AM 1.5 spectrum. The saturation photocurrent (H₂ evolution) was also significantly enhanced by the antireflective properties of the TiO₂ layer. It was shown that with proper annealing conditions these electrodes could run 72 h without significant degradation. An Fe²⁺/Fe³⁺ redox couple was used to help elucidate details of the band diagram

Protection of p⁺‑n-Si Photoanodes by Sputter-Deposited Ir/IrO_<i>x</i> Thin Films

Sputter deposition of Ir/IrO_<i>x</i> on p⁺-n-Si without interfacial corrosion protection layers yielded photoanodes capable of efficient water oxidation (OER) in acidic media (1 M H₂SO₄). Stability of at least 18 h was shown by chronoamperomety at 1.23 V versus RHE (reversible hydrogen electrode) under 38.6 mW/cm² simulated sunlight irradiation (λ > 635 nm, AM 1.5G) and measurements with quartz crystal microbalances. Films exceeding a thickness of 4 nm were shown to be highly active though metastable due to an amorphous character. By contrast, 2 nm IrO_<i>x</i> films were stable, enabling OER at a current density of 1 mA/cm² at 1.05 V vs. RHE. Further improvement by heat treatment resulted in a cathodic shift of 40 mV and enabled a current density of 10 mA/cm² (requirements for a 10% efficient tandem device) at 1.12 V vs. RHS under irradiation. Thus, the simple IrO_<i>x</i>/Ir/p⁺-n-Si structures not only provide the necessary overpotential for OER at realistic device current, but also harvest ∼100 mV of free energy (voltage) which makes them among the best-performing Si-based photoanodes in low-pH media

Comparison of the Performance of CoP-Coated and Pt-Coated Radial Junction n⁺p‑Silicon Microwire-Array Photocathodes for the Sunlight-Driven Reduction of Water to H₂(g)

The electrocatalytic performance for hydrogen evolution has been evaluated for radial-junction n⁺p-Si microwire (MW) arrays with Pt or cobalt phosphide, CoP, nanoparticulate catalysts in contact with 0.50 M H₂SO₄(aq). The CoP-coated (2.0 mg cm^–2) n⁺p-Si MW photocathodes were stable for over 12 h of continuous operation and produced an open-circuit photovoltage (<i>V</i>_oc) of 0.48 V, a light-limited photocurrent density (<i>J</i>_ph) of 17 mA cm^–2, a fill factor (ff) of 0.24, and an ideal regenerative cell efficiency (η_IRC) of 1.9% under simulated 1 Sun illumination. Pt-coated (0.5 mg cm^–2) n⁺p-Si MW-array photocathodes produced <i>V</i>_oc = 0.44 V, <i>J</i>_ph = 14 mA cm^–2, ff = 0.46, and η = 2.9% under identical conditions. Thus, the MW geometry allows the fabrication of photocathodes entirely comprised of earth-abundant materials that exhibit performance comparable to that of devices that contain Pt