Table S1.

Volumetric current densities and specific activities of several representative Type I and II catalysts

Source	Fig. no.	Sample identifier	Catalyst type (I or II)	NPMC loading, mg/cm2	O2/H2 absolute pressure, bar	Reported original mass (A/g) or areal (mA/cm2) or volumetric (A/cm3) current density at 0.8 V	Reported extrapolated mass (A/g) or areal (mA/cm2) or volumetric (A/cm3) current density at 0.8 V	Normalized volumetric current density (measured at 0.8ViR-free, H2/O2 is 1 bar), A/cm3	Normalized volumetric activity (extrapolated at 0.8ViR-free, H2/O2 is 1bar), A/cm3
8	8	Impregnation NCR2	I	1	1.5/1.5	16†	25	3.5‡	5.5
10	2	A 100% NH3	I	1	1.5/1.5	22†	30	5.2‡	7.1
11	8	BP_CV1	I	1	2.56/2.56	39†	88	4.6‡	10.5
15	6	(D)	I	1	1.5/1.5	3†	3	0.7‡	0.7
16	6	(d)	I	3.8	1.5/1.5	24§	24	1.5¶	1.5
19	2	Large open circles	I	1	1/1	18‖	99	18‖	99
8	10	Ball milling NCR2	II	1	1.5/1.5	66†	187	16‡	44
17	6		II	1.6	2/2	12‖	18	4.9††	7.4
20	3	Stars	II	1	1/1	37‖	230	37‖	230
21	3		II	4	1/1	20‖	130	20‖	130
22	S6	Zn(eIm)2TPI	II	2.2	1/1	52‖	146	52‖	146
27	S5	10/90	II	1	1.5/1.5	132†	262	31‡	62
This work	1		II	2	1/1	60‖	450	60‖	450

All of the volumetric current densities and activities are derived from peer-reviewed literature. The values are normalized to 1-bar reference pressure if not measured at 1 bar. The fuel cells were operated at 80 °C and 100% RH. The equation to convert mass activity, I_m measured under pressures P_O2 and P_H2 into volumetric activity, I^*_v, under 1 bar of O₂ and H₂ pressures is as follows (ref. 19, equation S1):

$$I_v^{\ast }=I_m\mathrm{\cdot }{\rho }_{\text{eff}}\mathrm{\cdot }{\left(\frac{P_{O2}^{\ast }}{P_{O2}}\right)}^{0.79}\mathrm{\cdot }{\left(\frac{P_{H2}^{\ast }}{P_{H2}}\right)}^{{\alpha }_{\text{C}}/2},$$

where P^*_O2 and P^*_H2 are the reference pressures (1 bar). ρ_eff is the effective density of carbon-based, porous NPMC electrode, which is set at 0.4 g/cm³ (19). α_c is the cathodic transfer coefficient and is assumed to be 1.

^†Reported mass activity.

^‡Normalized volumetric current density is converted from the reported mass activity based on the equation above.

^§Reported areal activity.

^¶Normalized volumetric current density is converted from the reported areal activity based on the equation above.

^‖Reported volumetric activity.

^††Normalized volumetric current density is converted from the reported data based on the equation above.