her-overpotential

name	her-overpotential
description	Use when the user asks about HER (hydrogen evolution reaction), hydrogen adsorption free energy, or volcano plot descriptor for HER catalysts.

HER Overpotential Calculation

Theory

The hydrogen evolution reaction has a single key intermediate:

* + H+ + e- --> *H    (Volmer step)
*H + H+ + e- --> H2   (Heyrovsky step)
   or
2 *H --> H2            (Tafel step)

Sabatier Criterion

The optimal HER catalyst has:

dG_H* = G(*H) - G(*) - 0.5 * G(H2) ~ 0 eV

• dG_H* < 0: H binds too strongly (poisoned surface)
• dG_H* > 0: H binds too weakly (low coverage, slow Volmer)
• dG_H* ~ 0: optimal (top of volcano plot)

pH Correction

At non-zero pH, the proton-transfer step is corrected by:

dG_H*(pH) = dG_H* - 0.059 * pH   (eV, at 298 K)

This shifts the free energy of the (H+ + e-) transfer by -0.059 eV per pH unit (Nernst relation). At pH 0, no correction is needed.

Overpotential

eta_HER = |dG_H*| / e

A perfect catalyst has eta_HER = 0 V. Pt(111) gives dG_H* ~ -0.09 eV.

Important: All G values must be Gibbs free energies (from geo_opt + freq + gibbs_energy chain), NOT raw DFT electronic energies. Using E_DFT instead of G omits ZPE and entropy, leading to errors of ~0.2 eV.

Discussion Checkpoints

🔴 Must discuss with user:

• Surface choice — Miller index and termination determine H binding site and dG_H*; e.g., Pt(111) fcc hollow vs MoS2 S-edge give very different results
• Functional — must be consistent between H slab, clean slab, and gas-phase H2; PBE vs SCAN can shift dG_H by 0.1-0.3 eV
• Competing reactions — on surfaces active for OER/ORR, H adsorption may compete; always check if HER or OER dominates at the operating potential

🟡 Recommend confirming:

• Coverage effects — at high H coverage, lateral interactions shift dG_H*; consider testing 1/4 ML vs 1/2 ML vs 1 ML
• Zero-point energy correction — ZPE contributes ~0.04 eV to dG_H*; always include freq + gibbs_energy chain rather than using raw DFT energies
• Adsorption site — test ontop, bridge, and hollow; report the most stable site (lowest |dG_H*|)

🟢 Safe defaults:

• Single intermediate (*H)
• dG_H* = G(H) - G() - 0.5*G(H2)
• eta_HER = |dG_H*| / e

MCP Workflow

1. Create workflow

{"tool": "catgo_workflow_engine", "arguments": {
  "action": "create", "name": "HER on Pt(111)"
}}

2. Build structures

Fetch bulk, cut slab, place H adsorbate:

{"tool": "catgo_fetch", "arguments": {
  "action": "crystal", "formula": "Pt", "source": "mp"
}}

{"tool": "catgo_structure", "arguments": {
  "action": "slab", "miller_index": [1,1,1],
  "min_slab_size": 12.0, "min_vacuum_size": 15.0
}}

{"tool": "catgo_structure", "arguments": {
  "action": "add_atom", "element": "H",
  "position": [2.77, 1.60, 14.2]
}}

3. Clean slab branch

{"tool": "catgo_workflow_engine", "arguments": {
  "action": "add_task", "workflow_id": "wf_her",
  "task_type": "geo_opt",
  "params": {"software": "vasp", "ENCUT": 520, "system_name": "clean_slab"}
}}

4. *H branch: geo_opt --> freq --> gibbs

{"tool": "catgo_workflow_engine", "arguments": {
  "action": "add_task", "workflow_id": "wf_her",
  "task_type": "geo_opt",
  "params": {"software": "vasp", "ENCUT": 520, "system_name": "*H"}
}}

{"tool": "catgo_workflow_engine", "arguments": {
  "action": "add_task", "workflow_id": "wf_her",
  "task_type": "freq", "depends_on": "task_h_opt",
  "params": {"software": "vasp", "freeze_mode": "layers", "freeze_layers": 4,
             "system_name": "*H"}
}}

{"tool": "catgo_workflow_engine", "arguments": {
  "action": "add_task", "workflow_id": "wf_her",
  "task_type": "gibbs_energy",
  "depends_on": ["task_h_opt", "task_h_freq"],
  "params": {"phase": "adsorbed", "system_name": "*H"}
}}

5. Gas-phase H2 reference

{"tool": "catgo_workflow_engine", "arguments": {
  "action": "add_task", "workflow_id": "wf_her",
  "task_type": "gibbs_energy",
  "depends_on": ["task_h2_opt", "task_h2_freq"],
  "params": {"phase": "gas", "system_name": "H2(g)"}
}}

6. Submit

{"tool": "catgo_workflow_engine", "arguments": {
  "action": "submit", "workflow_id": "wf_her"
}}

Python API

from catgo.workflow import Workflow

wf = Workflow("HER on Pt(111)")

# Clean slab
slab_inp = wf.add_task("structure_input", structure=clean_slab_json)
slab_opt = wf.add_task("geo_opt", structure=slab_inp.output.structure,
                        software="vasp", ENCUT=520)

# *H on slab
h_inp = wf.add_task("structure_input", structure=slab_h_json)
h_opt = wf.add_task("geo_opt", structure=h_inp.output.structure,
                     software="vasp", ENCUT=520)
h_frq = wf.add_task("freq", structure=h_opt.output.structure,
                     software="vasp", freeze_mode="layers", freeze_layers=4)
h_gib = wf.add_task("gibbs_energy", energy=h_opt.output.energy,
                     frequencies=h_frq.output.frequencies, phase="adsorbed")

# Gas-phase H2
h2_inp = wf.add_task("structure_input", structure=h2_json)
h2_opt = wf.add_task("geo_opt", structure=h2_inp.output.structure,
                      software="vasp")
h2_frq = wf.add_task("freq", structure=h2_opt.output.structure,
                      software="vasp")
h2_gib = wf.add_task("gibbs_energy", energy=h2_opt.output.energy,
                      frequencies=h2_frq.output.frequencies, phase="gas")

wf.submit()

DAG Structure

clean_slab --> geo_opt
*H   --> geo_opt --> freq --> gibbs_energy (adsorbed)
H2   --> geo_opt --> freq --> gibbs_energy (gas)

Three independent branches, 7 total tasks.

Interpreting Results

dG_H* (eV)	Interpretation	Action
-0.5 to -0.1	Strong binding, decent catalyst	May need surface modification
-0.1 to +0.1	Near optimal (volcano peak)	Excellent HER catalyst
+0.1 to +0.5	Weak binding, moderate activity	Consider alloying or doping
> +0.5	Too weak, poor HER catalyst	Different material needed

Adsorption Sites for H

Surface Type	Preferred H Site	Typical dG_H*
Pt(111)	fcc hollow	-0.09 eV
MoS2 edge	S-edge top	+0.08 eV
Graphene + N-doped	C adjacent to N	varies

Common Pitfalls

1. H is small -- use tight EDIFFG (-0.01 eV/A) to ensure proper relaxation.
2. Only freeze bottom slab layers in freq, not the H atom itself.
3. For alloy surfaces, test multiple adsorption sites (top, bridge, hollow) and report the most stable one (lowest dG_H*).
4. Always verify H does not migrate subsurface during geo_opt -- check the final structure with catgo_view.
5. For MoS2 and 2D materials, the "slab" is the monolayer itself with vacuum. Set freeze_layers=0 and use freeze_mode="none" in freq.