@mhchem, could you confirm whether this is expected mhchem behaviour?

In an arrow label, ^ after \overbrace{…} and _ after \underbrace{…} are treated differently by mhchemParser (MathJax and our PHP port give the same TeX):

\ce{H2O ->[+\overbrace{237.2 \ kJ / mol}^{\ce{electricity}}][+\underbrace{48.6 \ kJ /mol}_{\ce{heat}}] H2}

• Upper label: \overbrace{237.2 \ kJ / mol}{\vphantom{A}}^{\mathrm{electricity}} The content stays plain TeX, and the ^ becomes a chemical superscript on a \vphantom instead of attaching to the brace.
• Lower label: \underbrace{48.6\ k\,\mathrm{J}~/\mathrm{mol}}_{\mathrm{heat}} The content is converted as chemistry.

The LaTeX package mhchem (v4.10) treats both labels alike. The attached comparison shows mhchem in LaTeX next to the TeX that WikiTexVC generates:

Related: \ce{\underbrace{a}} gives "Extra close brace or missing open brace" in MathJax, while LaTeX mhchem typesets it.

Workaround until then, which renders as intended in MathJax and in WikiTexVC: switch the labels to math mode.
\ce{H2O (l) ->[$+\overbrace{\pu{237.2 kJ/mol}}^{\text{electricity}}$][$+\underbrace{\pu{48.6 kJ/mol}}_{\text{heat}}$] H2 + 1/2 O2}