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Electrochemical Impedance Spectroscopy (EIS)

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Electrochemical impedance spectroscopy applies a small AC perturbation and records how current lags voltage as frequency is swept. Nyquist or Bode plots are then fit to resistances, capacitances and Warburg elements that stand in for charge transfer, films and diffusion. Double-layer capacitance from those spectra (or from CV charging) is often converted to electrochemical surface area with an assumed specific capacitance. In this library EIS is thinly quoted: one HER paper extracts capacitance/ECSA with 40 μF cm⁻², while a sodium-ion composite and an LDH OER catalyst are indexed here mainly for DC electrochemistry — capacity retention and Ni³⁺ ratios — not for published Rct values.

Electrochemists reach for EIS when they need to separate kinetic, film and transport contributions that a single overpotential hides. It answers 'which equivalent-circuit piece is dominating at this frequency?' Its main limitation in these summaries is missing Nyquist numbers: you must not invent an Rct, and an assumed 40 μF cm⁻² turns capacitance into a geometric factor, not a solved mechanism or a device lifetime.

Studies

3

Findings

3

3 supporting · 0 challenging · 0 qualifying citations

Open tensions

2

Currently

What we know

  1. Ultrathin porous CoP nanosheets are the paper that actually ties impedance-style capacitance to a rate. In 0.5 M H₂SO₄, 56 and 131 mV reach 10 and 100 mA cm⁻²; the Tafel slope is 44 mV dec⁻¹; mass activity is 151 A g⁻¹ at 100 mV, with ~100% Faradaic efficiency and 20 h stability. ECSA uses an assumed 40 μF cm⁻² specific capacitance. P-terminated (200) is preferred computationally. Device-scale alkaline or seawater HER is untested.
  2. A natural-stibnite/sulfur-doped-carbon anode is the kind of composite EIS is often asked to explain (volume swing, contact loss), but the quoted numbers are galvanostatic. Sb₂S₃/SCS keeps 455.8 mA h g⁻¹ after 100 cycles at 0.1 A g⁻¹ (70.8% retention) versus 190.1 mA h g⁻¹ and 30.7% for raw stibnite. Theoretical capacity is 946 mA h g⁻¹ via conversion plus alloying (12 Na); carbon buffering targets a ~390% volume swing. Half-cell versus sodium metal, not a full pack; the summary does not quote an Rct.
  3. NaBH₄-treated NiFe LDH reports a surface-valence descriptor rather than an impedance fit. Ni³⁺/Ni²⁺ rises from 0.39 to 1.32 and Fe²⁺/Fe³⁺ from 0.40 to 1.49; hydride deprotonates Ni–OH and leaves oxygen vacancies near Fe. O₂ faradaic efficiency is 95.9% (7.48×10⁻⁵ mol O₂ at 50 mA for 10 min). A full electrolyzer lifetime and a complete microkinetic OER cycle are not provided — and no Nyquist Rct is in the teaching summary.

Largest unresolved question

Capacitance-derived ECSA, galvanostatic capacity, and an XPS Ni³⁺ ratio are three different electrochemical claims that students lump as 'impedance showed it was better.' CoP uses 40 μF cm⁻² to normalise HER current (56 mV at 10 mA cm⁻²). Stibnite quotes 455.8 versus 190.1 mA h g⁻¹ after 100 cycles. LDH quotes Ni³⁺/Ni²⁺ 0.39 → 1.32 and 95.9% O₂ FE. Those are not replications of one EIS equivalent circuit.

Common misconceptions

  • ECSA from capacitance is a measured true surface area.

    The CoP paper converts capacitance to ECSA with an assumed 40 μF cm⁻². Change that specific capacitance and the area — and every A g⁻¹ or mA cm⁻²ECSA figure — moves. Geometry-normalised 56/131 mV overpotentials still depend on how the electrode was loaded.

  • If a battery paper is indexed under EIS, the 70.8% retention is an impedance result.

    455.8 mA h g⁻¹ after 100 cycles at 0.1 A g⁻¹ is galvanostatic cycling versus sodium metal. EIS might have been used in the full paper to discuss charge transfer after a ~390% volume swing, but the teaching summary does not quote Rct, so you cannot cite an ohmic number from it.

  • A higher Ni³⁺/Ni²⁺ XPS ratio is the charge-transfer resistance.

    0.39 → 1.32 is a surface valence ratio correlated with faster OER and 95.9% O₂ FE. XPS is not EIS. The atomic OER cycle and electrolyzer lifetime remain unsolved.

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