Hydrogen adsorption and subsurface absorption on <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si79.svg" display="inline" id="d1e2582"> <mml:mrow> <mml:mi mathvariant="normal">Ni</mml:mi> <mml:mrow> <mml:mo>(</mml:mo> <mml:mn>001</mml:mn> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> : Atomic-scale insights toward efficient and stable HER catalysts